Theodoros Geralis

Hypothetical low-mass particles, such as axions, provide a compelling explanation for the dark matter in the universe. Such particles are expected to emerge abundantly from the hot interior of stars. To test this prediction, the CERN Axion Solar Telescope (CAST) uses a 9 T refurbished Large Hadron Collider test magnet directed towards the Sun. In the strong magnetic field, solar axions can be converted to X-ray photons which can be recorded by X-ray detectors. In the 2013–2015 run, thanks to low-background detectors and a new X-ray telescope, the signal-to-noise ratio was increased by about a factor of three. Here, we report the best limit on the axion–photon coupling strength (0.66 × 10−10 GeV−1 at 95% confidence level) set by CAST, which now reaches similar levels to the most restrictive astrophysical bounds. Axions are hypothetical light particles that could explain the dark matter. They could be produced in the interior of the Sun and the CERN Axion Solar Telescope sets the best limit on how strongly axions can interact with light.

Hypothetical axionlike particles with a two-photon interaction would be produced in the sun by the Primakoff process. In a laboratory magnetic field ("axion helioscope"), they would be transformed into x-rays with energies of a few keV. Using a decommissioned Large Hadron Collider test magnet, the CERN Axion Solar Telescope ran for about 6 months during 2003. The first results from the analysis of these data are presented here. No signal above background was observed, implying an upper limit to the axion-photon coupling g(agamma)<1.16x10(-10) GeV-1 at 95% C.L. for m(a) less, similar 0.02 eV. This limit, assumption-free, is comparable to the limit from stellar energy-loss arguments and considerably more restrictive than any previous experiment over a broad range of axion masses.

The International Axion Observatory (IAXO) will be a forth generation axion helioscope. As its primary physics goal, IAXO will look for axions or axion-like particles (ALPs) originating in the Sun via the Primakoff conversion of the solar plasma photons. In terms of signal-to-noise ratio, IAXO will be about 4–5 orders of magnitude more sensitive than CAST, currently the most powerful axion helioscope, reaching sensitivity to axion-photon couplings down to a few × 10−12 GeV−1 and thus probing a large fraction of the currently unexplored axion and ALP parameter space. IAXO will also be sensitive to solar axions produced by mechanisms mediated by the axion-electron coupling gae with sensitivity — for the first time — to values of gae not previously excluded by astrophysics. With several other possible physics cases, IAXO has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade. In this paper we present the conceptual design of IAXO, which follows the layout of an enhanced axion helioscope, based on a purpose-built 20 m-long 8-coils toroidal superconducting magnet. All the eight 60cm-diameter magnet bores are equipped with focusing x-ray optics, able to focus the signal photons into ∼ 0.2 cm2 spots that are imaged by ultra-low-background Micromegas x-ray detectors. The magnet is built into a structure with elevation and azimuth drives that will allow for solar tracking for ∼ 12 h each day.

We have searched for solar axions or similar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup with improved conditions in all detectors. From the absence of excess X-rays when the magnet was pointing to the Sun, we set an upper limit on the axion-photon coupling of 8.8 x 10^{-11} GeV^{-1} at 95% CL for m_a <~ 0.02 eV. This result is the best experimental limit over a broad range of axion masses and for m_a <~ 0.02 eV also supersedes the previous limit derived from energy-loss arguments on globular-cluster stars.

We study the feasibility of a new generation axion helioscope, the most ambitious and promising detector of solar axions to date. We show that large improvements in magnetic field volume, x-ray focusing optics and detector backgrounds are possible beyond those achieved in the CERN Axion Solar Telescope (CAST). For hadronic models, a sensitivity to the axion-photon coupling of gaγ ≳ few × 10−12 GeV−1 is conceivable, 1–1.5 orders of magnitude beyond the CAST sensitivity. If axions also couple to electrons, the Sun produces a larger flux for the same value of the Peccei-Quinn scale, allowing one to probe a broader class of models. Except for the axion dark matter searches, this experiment will be the most sensitive axion search ever, reaching or surpassing the stringent bounds from SN1987A and possibly testing the axion interpretation of anomalous white-dwarf cooling that predicts ma of a few meV. Beyond axions, this new instrument will probe entirely unexplored ranges of parameters for a large variety of axion-like particles (ALPs) and other novel excitations at the low-energy frontier of elementary particle physics.

We have searched for solar axions or other pseudoscalar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we previously have reported results from CAST with evacuated magnet bores (Phase I), setting limits on lower mass axions, here we report results from CAST where the magnet bores were filled with 4He gas (Phase II) of variable pressure. The introduction of gas generates a refractive photon mass mγ, thereby achieving the maximum possible conversion rate for those axion masses ma that match mγ. With 160 different pressure settings we have scanned ma up to about 0.4 eV, taking approximately 2 h of data for each setting. From the absence of excess x-rays when the magnet was pointing to the Sun, we set a typical upper limit on the axion-photon coupling of gaγ≲2.2 × 10−10 GeV−1 at 95% CL for ma≲0.4 eV, the exact result depending on the pressure setting. The excluded parameter range covers realistic axion models with a Peccei-Quinn scale in the neighborhood of fa ∼ 107 GeV. Currently in the second part of CAST Phase II, we are searching for axions with masses up to about 1.2 eV using 3He as a buffer gas.

The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using 3He as a buffer gas. At T=1.8 K this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with 4He. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39 eV < m_a < 0.64 eV. From the absence of excess X-rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of g_ag < 2.3 x 10^{-10} GeV^{-1} at 95% CL, the exact value depending on the pressure setting. KSVZ axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In future we will extend our search to m_a < 1.15 eV, comfortably overlapping with cosmological hot dark matter bounds.

A new Micromegas manufacturing technique, based on kapton etching technology, has been developed recently, resulting in further improvement of the characteristics of the detector, such as uniformity and stability. Excellent energy resolution has been obtained, reaching 11% FWHM for the 5.9 keV photon peak of the55Fe X-ray source and 1.8% FWHM (with possible evidence of less than 1%) for the 5.5 MeV alpha peak of the 241Am source. The new Microbulk detector shows several advantages like flexible structure, low material and high radio-purity, opening thus new possibilities for both accelerator and low counting-rate experiments. The detector has already been used in CAST and n-TOF, while it is being tested for future neutrinoless double-beta decay experiments like NEXT. Details of the production of several types of Microbulk detectors will be described. First benchmark results will be presented, demonstrating the enhanced performance of Microbulk detectors.

An event sample enriched in semileptonic decays of b hadrons is selected using an inclusive lepton selection from approximately 3.0 million hadronic Z0 decays collected with the OPAL detector. This sample is used to investigate B meson oscillations by reconstructing a proper decay time for the parent of each lepton, using a jet charge method to estimate the production flavour of this parent, and using the lepton charge to tag the decay flavour. We measure the mass difference between the two B0 d mass eigenstates

In non-hadronic axion models, which have a tree-level axion-electron interaction, the Sun produces a strong axion flux by bremsstrahlung, Compton scattering, and axio-recombination, the ``BCA processes.'' Based on a new calculation of this flux, including for the first time axio-recombination, we derive limits on the axion-electron Yukawa coupling gae and axion-photon interaction strength gaγ using the CAST phase-I data (vacuum phase). For ma≲10 meV/c2 we find gaγ gae < 8.1 × 10−23 GeV−1 at 95% CL. We stress that a next-generation axion helioscope such as the proposed IAXO could push this sensitivity into a range beyond stellar energy-loss limits and test the hypothesis that white-dwarf cooling is dominated by axion emission.

The inclusive production rates of π±,K ± andp $$\bar p$$ inZ 0 decays have been measured with the OPAL detector at LEP. Using the energy loss measurement in the jet chamber, the momentum range up to the beam energy (45.6 GeV/c) has been covered. Differential cross sections and total particle yields are given. Comparisons of the inclusive momentum spectra and the total rates with predictions of the JETSET and the HERWIG Monte Carlo model are presented. The total single rates are found to be 17.05±0.43 π±, 2.42±0.13K ± and 0.92±0.11p $$\bar p$$ per hadronic event. Predictions of JETSET for cross sections and total rates agree very well for π±; however, for momenta greater than 4 GeV/c,K ± rates are underestimated and $$\bar p$$ rates are overestimated. Combined with data of other particle species there is evidence that the peak positions in the ξ=ln(1/x p ) distributions show a different mass dependence for mesons and baryons. However, both JETSET and HERWIG Monte Carlo predictions agree with the observed data.

The longitudinal polarization, the transverse polarization, and the forward-backward asymmetry of $\Lambda$ baryons, have been measured using a sample of 4.34 million hadronic $\mathrm{Z}^0$ decays collected with the OPAL detector at LEP between 1990 and 1995. These results are important as an aid to the understanding of hadronization mechanisms. Significant longitudinal polarization has been observed at intermediate and high momentum. For $x_E$ ( $\equiv 2 E_{\Lambda}/\sqrt{s}) >$ 0.3, the longitudinal polarization has been measured to be $-32.9 \pm 5.5 $ (stat) $\pm$ 5.2 (syst)%. We have observed no transverse polarization. A significant forward-backward asymmetry has been measured and can be described by a JETSET model.

detectors and shielding, we returned to 4 He in 2012 to investigate a narrow m a range around 0.2 eV ("candidate setting" of our earlier search) and 0.39-0.42eV, the upper axion mass range reachable with 4 He, to "cross the axion line" for the KSVZ model.We have improved the limit on the axion-photon coupling to g aγ < 1.47 × 10 -10 GeV -1 (95% C.L.), depending on the pressure settings.Since 2013, we have returned to the vacuum and aim for a significant increase in sensitivity.

We have searched for lepton flavour violating Z0→eμ, Z0→eτ and Z0→μτ decays in a sample of 4.0×106 visible Z0 decays collected with the OPAL detector at LEP during 1991 to 1994. No candidates are found for Z0→eμ. The samples of selected Z0→eτ and Z0→μτ candidates are consistent with the expected background. The following limits are set at 95% confidence level: $$\begin{gathered} BR(Z^0 \to e\mu ){\text{ }}< {\text{ 1}}{\text{.7 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to e\tau ){\text{ }}< {\text{ 9}}{\text{.8 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to \mu \tau ){\text{ }}< {\text{ 17}}{\text{. }} \times {\text{ 10}}^{ - {\text{6}}} . \hfill \\ \end{gathered}$$

Evidence is presented for kinematic and charge correlations of B mesons with charged pions and kaons. Using a new technique, a sample of over 80 000 partially reconstructed B mesons is obtained in 3.5·106 hadronic Z0 decays recorded using the OPAL detector at LEP. The invariant mass distributions of B+π− and B+K− combinations show enhancements consistent with the decays of P-wave resonances of a b antiquark and a light quark. We observe an excess of 1738±195 B+π− pairs with invariant masses in the range 5.60-5.85 GeV and an excess of 149±31 B+K− pairs with invariant masses in the range 5.80–6.00 GeV. Labeling the observed enhancements generically as B** we find $$\begin{gathered} \frac{{BR(Z^0 \to \bar b \to B^{**0} \to B^{(*) + } \pi ^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.18 \pm 0.04, \hfill \\ \frac{{BR(Z^0 \to \bar b \to B_s^{**0} \to B^{(*) + } K^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.026 \pm 0.008, \hfill \\ \end{gathered} $$ where B(*)+ indicates the sum of B+ and B*+ and the errors include statistical and systematic contributions. From a study of π-B charge-flavor correlations we conclude that the production flavor of a B meson can be tagged with the charge of a pion in an appropriately chosen kinematic region, and that the performance of this flavor tag compares favorably in the $$Z^0 \to q\bar q$$ environment with lepton-based tags.

We have studied hadronic events produced at LEP at centre-of-mass energies of 130 and 136 GeV. Distributions of event shape observables, jet rates, momentum spectra and multiplicities are presented and compared to the predictions of several Monte Carlo models and analytic QCD calculations. From fits of event shape and jet rate distributions to $${\mathcal{O}}(\alpha _s^2 ) + NLLA$$ QCD calculations, we determineα s (133 GeV)=0.110±0.005(stat.)±0.009(syst.). We measure the mean charged particle multiplicity 〈n ch〉=23.40±0.45(stat.) ±0.47(syst.) and the position ζ0 of the peak in the ζ p = ln(1/x p ) distribution ζ0=3.94±0.05(stat.)±0.11(syst.). These results are compared to lower energy data and to analytic QCD or Monte Carlo predictions for their energy evolution.

We describe a cone-based jet finding algorithm (similar to that used in $$\bar p$$ p experiments), which we have applied to hadronic events recorded using the OPAL detector at LEP. Comparisons are made between jets defined with the cone algorithm and jets found by the “JADE” and “Durham” jet finders usually used ine + e − experiments. Measured jet rates, as a function of the cone size and as a function of the minimum jet energy, have been compared with O(α 2 ) calculations, from which two complementary measurements $$\alpha _s \left( {M_{Z^0 } } \right)$$ have been made. The results are $$\alpha _s \left( {M_{Z^0 } } \right)$$ =0.116±0.008 and $$\alpha _s \left( {M_{Z^0 } } \right)$$ =0.119±0.008 respectively, where the errors include both experimental and theoretical uncertainties. Measurements are presented of the energy flow inside jets defined using the cone algorithm, and compared with equivalent data from $$\bar p$$ p interactions, reported by the CDF collaboration. We find that the jets ine + e − are significantly narrower than those observed in $$\bar p$$ p. The main contribution to this effect appears to arise from differences between quark- and gluon-induced jets.

A low-background Micromegas detector has been operating in the CAST experiment at CERN for the search for solar axions during the first phase of the experiment (2002–2004). The detector, made out of low radioactivity materials, operated efficiently and achieved a very high level of background rejection (5 × 10−5 counts keV−1 cm−2 s−1) without shielding.

The production rates of ${\rm D}^{*\pm}$ mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary ${\rm c\bar c}$ pairs in hadronic ${\rm Z}^0$ decays have been measured at LEP using almost 4.4 million hadronic ${\rm Z}^0$ decays collected with the OPAL detector between 1990 and 1995. Using a combination of several charm quark tagging methods based on fully and partially reconstructed ${\rm D}^{*\pm}$ mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into ${\rm D}^{*\pm}$ mesons have been found to be \[ {\rm f(b \to D^{*+}}X) = 0.173 \pm 0.016 \pm 0.012\quad {\rm and }\quad {\rm f (c \to D^{*+}}X) = 0.222 \pm 0.014 \pm 0.014\; . \] The fraction of ${\rm c\bar c}$ events in hadronic ${\rm Z}^0$ decays, $\Gamma_{\rm c\bar c}/\Gamma_{\rm had}=\Gamma({\rm Z}^0\to{\rm c\bar c}) / \Gamma({\rm Z}^0\to\rm hadrons)$ , is determined to be \[ \Gamma_{\rm c\bar c}/\Gamma_{\rm had} = 0.180\pm 0.011 \pm 0.012 \pm 0.006 \ . \] In all cases the first error is statistical, and the second one systematic. The last error quoted for $\Gamma_{\rm c\bar c}/\Gamma_{\rm had}$ is due to external branching ratios.

A measurement of the single photon production cross-section is presented based on a data-sample of 40.5 pb−1 collected with the OPAL detector at centre-of-mass energies within 3 GeV of the Z0 mass. Single photon events arise from initial state radiation and the production of an "invisible" final state consisting of neutrinos or possibly particles such as sneutrinos or photinos. The single photon topology is also sensitive to new Z0 decays such as $$Z^0 \to \bar vv* \to \bar vv\gamma $$ orZ 0→γX, X→invisible particles. A total of 447 single photon candidates were observed with energy exceeding 1.75 GeV in the polar angle region |cosθ|<0.7. The estimated background from processes with visible reaction products, mainly e+e−→e+e−γ, is 37±6 events. Interpreting the cross-sections as being solely due to Z0 decay to invisible particles and the expected W-contributions, the Z0 invisible with is determined to be 539±26±17 MeV corresponding toN v=3.23±0.16±0.10 light neutrino generations. The differential cross-section with photon energy is presented. Upper limits are set on additional invisible contributions to the Z0 width, on possible non-resconant processes, and on Z0 decays to single photons. The energy spectra are used to constrain exotic sources of high energy single photons. In particular, the radiative twobody decay of the Z0 to a new particle X, with mass below 64 GeV and an invisible signature, has a Z0 branching ratio of less than 4.3×10−6 at 95% confidence level.

We have searched for 14.4 keV solar axions or more general axion-like particles (ALPs), that may be emitted in the M1 nuclear transition of 57Fe, by using the axion-to-photon conversion in the CERN Axion Solar Telescope (CAST) with evacuated magnet bores (Phase I). From the absence of excess of the monoenergetic X-rays when the magnet was pointing to the Sun, we set model-independent constraints on the coupling constants of pseudoscalar particles that couple to two photons and to a nucleon gaγ|−1.19gaN0+gaN3| < 1.36 × 10−16 GeV−1 for ma < 0.03 eV at the 95% confidence level.

Axion helioscopes aim at the detection of solar axions through their conversion into x-rays in laboratory magnetic fields. The use of low background x-ray detectors is an essential component contributing to the sensitivity of these searches. Here we review the recent advances on Micromegas detectors used in the CERN Axion Solar Telescope (CAST) and proposed for the future International Axion Observatory (IAXO). The most recent Micromegas setups in CAST have achieved background levels of 1.5 × 10−6 keV−1 cm−2 s−1, a factor of more than 100 lower than the ones obtained by the first generation of CAST detectors. This improvement is due to the development of active and passive shielding techniques, offline discrimination techniques allowed by highly granular readout patterns, as well as the use of radiopure detector components. The status of the intensive R&D to reduce the background levels will be described, including the operation of replica detectors in test benches and the detailed Geant4 simulation of the detector setup and the detector response, which has allowed the progressive understanding of background origins. The best levels currently achieved in a test setup operating in the Canfranc Underground Laboratory (LSC) are as low as ∼ 10−7 keV−1 cm−2 s−1, showing the good prospects of this technology for application in the future IAXO.

Using radiative Z0→ τ+τ−γevents collected with the OPAL detector at LEP at s=MZ during 1990–95, a direct study of the electromagnetic current at the τγ vertex has been performed in terms of the anomalous magnetic form factor F2 of the τ lepton. The analysis is based on a data sample of 1429 e+e−→τ+τ−γevents which are examined for a deviation from the expectation with F2=0. From the non-observation of anomalous τ+τ−γproduction a limit of−0.068<F2<0.065is obtained. This can also be interpreted as a limit on the electric dipole form factor F3 as|eF3|<3.7×10−16ecm.The above ranges are valid at the 95% confidence level.

We have studied the production of D*± mesons in a sample of 1.25 million multihadronic decays of the Z0, in which 1969 candidates have been identified. We have determined the total multiplicity of charged D* mesons in multihadronic Z0 decays to be

The observation and measurement of the time-dependence of B0d↔B0d mixing are described. The B0d meson is reconstructed in final states that contain a D∗− and an ℓ+, where the b flavour of the B0d at decay time is tagged by the electric charge of the lepton. A new and efficient method, using a jet charge technique, is developed for identifying the b flavour of the produced B0d. From a sample of 556 D∗±ℓ∓ candidates reconstructed in the OPAL data collected during 1990–1993, the B0d↔B0d oscillation frequency is measured to be Δmd = 0.508 ± 0.075 (stat) ±0.025(syst) ps−1 giving an oscillation parameter of χd = 0.73 ± 0.11 (stat) ±0.08 (syst), where 0.076 of the systematic error on χd arises from the uncertainty on the B0d lifetime.

Micromegas detectors are an optimum technological choice for the detection of low energy x-rays. The low background techniques applied to these detectors yielded remarkable background reductions over the years, being the CAST experiment beneficiary of these developments. In this document we report on the latest upgrades towards further background reductions and better understanding of the detectors' response. The upgrades encompass the readout electronics, a new detector design and the implementation of a more efficient cosmic muon veto system. Background levels below 10−6keV-1cm-2s-1 have been obtained at sea level for the first time, demonstrating the feasibility of the expectations posed by IAXO, the next generation axion helioscope. Some results obtained with a set of measurements conducted in the x-ray beam of the CAST Detector Laboratory will be also presented and discussed.

Gluon jets are identified in ${\rm e^+e^-}$ hadronic annihilation events by tagging two quark jets in the same hemisphere of an event. The gluon jet is defined inclusively as all the particles in the opposite hemisphere. Gluon jets defined in this manner have a close correspondence to gluon jets as they are defined for analytic calculations, and are almost independent of a jet finding algorithm. The charged particle multiplicity distribution of the gluon jets is presented, and is analyzed for its mean, dispersion, skew, and curtosis values, and for its factorial and cumulant moments. The results are compared to the analogous results found for a sample of light quark (uds) jets, also defined inclusively. We observe differences between the mean, skew and curtosis values of gluon and quark jets, but not between their dispersions. The cumulant moment results are compared to the predictions of QCD analytic calculations. A calculation which includes next-to-next-to-leading order corrections and energy conservation is observed to provide a much improved description of the data compared to a next-to-leading order calculation without energy conservation. There is agreement between the data and calculations for the ratios of the cumulant moments between gluon and quark jets.

A study of b quark fragmentation at LEP is presented using a sample of semileptonic B decays containing a fully reconstructed charm meson. The data are compared to several theoretical models for heavy quark fragmentation; the free parameters in these models are fitted and the sensitivity of the model parameters to the rate of P-wave B meson production is studied. The mean scaled energy fraction of B0 and B+ mesons has been determined to be <xE> = 0.695 ± 0.006 ± 0.003 ± 0.007, where the errors are statistical, systematic and model dependence respectively. This result is consistent with previous, less direct measurements from inclusive leptonic B decays. Also presented is a model independent fit to the shape of the energy distribution of weakly decaying B mesons at LEP.

This letter describes the first observation of W boson pair production at a centre-of-mass energy s=161GeV in the OPAL detector at LEP. The analysis is sensitive to all expected W+W− decay channels. A total of 28 events have been selected for an integrated luminosity of 9.89±0.06 pb−1. This is consistent with the Standard Model expectation, including signal and background contributions. The W pair production cross-section is measured to be σWW = 3.62−0.82+0.93±0.16 pb. An analysis of the predicted MW dependence of the accepted cross-section, taking into account interference in the four-fermion production processes, yields MW = 80.40−0.41−0.10+0.44+0.09±0.10 GeV, where the first and second uncertainties are statistical and systematic, respectively, and the third arises form the beam energy uncertainty.

Gluon jets with about 39 GeV energy are identified in hadronic Z0 decays by tagging two jets in the same hemisphere of an event as quark jets. Identifying the gluon jet to be all the particles observed in the hemisphere opposite to that containing the two tagged jets yields an inclusive gluon jet definition corresponding to that used in analytic calculations, allowing the first direct test of those calculations. In particular, this jet definition yields results which are only weakly dependent on a jet finding algorithm. We find rch.=1.552±0.0041 (stat) ±0.061 (syst.) for the ratio of the mean charged particle multiplicity in gluon jets to that in light quark uds jets, where the uds jets are identified using an inclusive jet definition similar to that used for the gluon jets. Our result is in general agreement with the prediction of a recent analytic calculation which incorporates energy conservation into the parton shower branching processes, but is considerably smaller than analytic predictions which do not incorporate energy conservation.

MICROMEGAS detectors have been running in the CERN Axion Solar Telescope (CAST) experiment since 2002. The detector, constructed of low radioactivity materials, operated efficiently exploiting its good spatial and energy resolution of the detector as well as the time information contained in the pulse shape of the events. Last year Microbulk detectors were installed in the experiment achieving very low background levels thanks to the improved performances of the detector as well as the upgraded shielding. The performance during 2008 data-taking and recent background studies will be presented.

We report on a new search for solar chameleons with the CERN Axion Solar Telescope (CAST). A GridPix detector was used to search for soft X-ray photons in the energy range from 200 eV to 10 keV from converted solar chameleons. No signiffcant excess over the expected background has been observed in the data taken in 2014 and 2015. We set an improved limit on the chameleon photon coupling, $\beta_\gamma < 5.7\times10^{10}$ for $1<\beta_\mathrm{m}<10^6$ at 95% C.L. improving our previous results by a factor two and for the first time reaching sensitivity below the solar luminosity bound for tachocline magnetic fields up to $12.5\,\mathrm{T}$.

Cross-sections for hadronic and leptonic two-fermion events, and leptonic forward-backward asymmetries, have been measured in e+e− collisions at a centre-of-mass energy of 161 GeV, using the OPAL detector at LEP. Results are presented both including and excluding the dominant production of radiative γZ0 events. We have measured Rb, the ratio of the number of bb to all multihadronic events at 161 GeV, and compared it to the result obtained at 130–136 GeV. All results agree well with the Standard Model expectations. In a model-independent fit to the Z0 lineshape, the data presented here give an improved precision on the γZ0-interference term. The data have also been used to obtain new limits on extensions of the Standard Model described by effective four-fermion contact interactions.

New measurements are presented of the photon structure function F2γ(x,Q2) at four values of Q2 between 9 and 59 GeV2 based on data collected with the OPAL detector at centre-of-mass energies of 161–172 GeV, with a total integrated luminosity of 18.1 pb−1. The evolution of F2γ with Q2 in bins of x is determined in the Q2 range from 1.86 to 135 GeV2 using data taken at centre-of-mass energies of 91 GeV and 161–172 GeV. F2γ is observed to increase with Q2 with a slope of α−1dF2γdlnQ2 = 0.10−0.03+0.05 measured in the range 0.1 < x < 0.6.

Using the OPAL data accumulated in 1991–1993 amounting to 74 pb−1 of integrated luminosity, corresponding to 1.64×106 selected multi-hadronic events, a search has been performed for charged particles with unusual mass or unusual charge. The mass was determined from a combination of momentum and ionization energy loss measurements. No isolation criteria were applied to the tracks examined, so that both isolated particles and particles produced in jets were valid candidates. For particles with chargeQ/e=−1, one candidate with a mass of approximately 4.2 GeV/c2 was found, which is compatible with the background rate expected according to a Monte Carlo simulation. The implications of this search for the mass limits of a conjectured stable or quasi-stable charged gluino composite $$(\tilde gq\bar q')^ \pm $$ are discussed. Limits are also presented for the production of fractionally-charged particles withQ/e=±2/3 and ±4/3 as well as for particles withQ/e=±2.

Three jet events are selected from hadronic Z0 decays with a symmetry such that the two lower energy jets are produced with the same energy and in the same jet environment. In some of the events, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet, with an estimated purity of about 93%. Comparing these gluon jets to the inclusive sample of lower energy jets from the symmetric data set yields direct, model independent measurements of quark and gluon jet properties, which have essentially no bias except from the jet definition. Results are reported using both thek ⊥ and cone jet definitions. For thek ⊥ algorithm, we find $$\frac{{\left\langle {n_{k_ \bot }^{ch.} } \right\rangle gluon}}{{\left\langle {n_{k_ \bot }^{ch.} } \right\rangle quark}} = 1.25 \pm 0.02(stat.) \pm 0.03(syst.)$$ for the ratio of the mean charged particle multiplicity of gluon to quark jets, while for the cone algorithm, we find $$\frac{{\left\langle {n_{cone}^{ch.} } \right\rangle gluon}}{{\left\langle {n_{cone}^{ch.} } \right\rangle quark}} = 1.10 \pm 0.02(stat.) \pm 0.02(syst.)$$ using a cone size of 30°. We also report measurements of the angular distributions of particle energy and multiplicity around the jet directions, and of the fragmentation functions of the jets. Gluon jets are found to be substantially broader and to have a markedly softer fragmentation function than quark jets, in agreement with our earlier observations.

This paper presents updated measurements of the lifetimes of the Bs0 meson and the Λb0 baryon using 4.4 million hadronic Z0 decays recorded by the OPAL detector at LEP from 1990 to 1995. A sample of Bs0 decays is obtained using Ds−ℓ+ combinations, where the Ds− is fully reconstructed in the φπ−, K∗0K− and K−K0S decay channels and partially reconstructed in the φℓ−ν̄X decay mode. A sample of Λb0 decays is obtained using Λc+ℓ− combinations, where the Λc+ is fully reconstructed in its decay to a pK−π+ final state and partially reconstructed in the Λℓ+νX decay channel. From 172±28 Ds−ℓ+ combinations attributed to Bs0 decays, the measured lifetime is τ(Bs0)=1.50+0.16−0.15±0.04ps, where the errors are statistical and systematic, respectively. From the 129±25 Λc+ℓ− combinations attributed to Λb0 decays, the measured lifetime is τ(Λb0)=1.29+0.24−0.22±0.06ps, where the errors are statistical and systematic, respectively.

Helicity density matrix elements for inclusive K∗(892)0 mesons from hadronic Z0 decays have been measured over the full range of K∗ 0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum xp values above 0.3, with the matrix element ϱ00 rising to 0.66 ± 0.11 for xp > 0.7. The values of the real part of the off-diagonal element ϱ1 - 1 are negative at large xp, with a weighted average value of −0.09 ± 0.03 for xp > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq system from the Z0 decay. All other helicity density matrix elements measured are consistent with zero over the entire xp range. The K∗ 0 fragmentation function has also been measured and the total rate determined to be 0.74 ± 0.02 ± 0.02 K∗(892)0 mesons per hadronic Z0 decay.

A search for decays of the Bc meson was performed using data collected from 1990–1995 with the OPAL detector on or near the Z0 peak at LEP. The decay channels Bc+→J/ψπ+, Bc+→J/ψa1+ and Bc+→J/ψℓ+ν were investigated, where ℓ denotes an electron or a muon. Two candidates are observed in the mode Bc+→J/ψπ+, with an estimated background of (0.63±0.20) events. The weighted mean of the masses of the two candidates is (6.32±0.06) GeV/c2, which is consistent with the predicted mass of the Bc meson. One candidate event is observed in the mode Bc+→J/ψℓ+ν, with an estimated background of (0.82±0.19) events. No candidate events are observed in the Bc+→J/ψa1+ decay mode, with an estimated background of (1.10±0.22) events. Upper bounds at the 90% confidence level are set on the production rates for these processes.

We present the results of a search for a high-energy axion emission signal from 7Li (0.478 MeV) and D(p,gamma)3He (5.5 MeV) nuclear transitions using a low-background gamma-ray calorimeter during Phase I of the CAST experiment. These so-called "hadronic axions" could provide a solution to the long-standing strong-CP problem and can be emitted from the solar core from nuclear M1 transitions. This is the first such search for high-energy pseudoscalar bosons with couplings to nucleons conducted using a helioscope approach. No excess signal above background was found.

MICROMEGAS detectors have been running in the CERN Axion Solar Telescope (CAST) experiment since 2002. The detector, constructed of low radioactivity materials, operated efficiently exploiting its good spatial and energy resolution of the detector as well as the time information contained in the pulse shape of the events. Last year Microbulk detectors were installed in the experiment achieving very low background levels thanks to the improved performances of the detector as well as the upgraded shielding. The performance during 2008 data-taking and recent background studies will be presented.

A search for scalar particles and neutralinos such as those predicted by supersymmetric models has been performed using a data sample of 4.4 million hadronic Z0 decays recorded by the OPAL detector at the e+e− collider LEP. The production of such particles typically leads to event topologies consisting of an acoplanar pair of jets, or of a mono-jet, accompanied by sizeable missing energy owing to neutrinos and other undetectable neutral particles. Limits are obtained, at the 95% confidence level, on the masses and production rates of scalar particles produced in association with the Z0. Limits are also placed on neutralino production, for which an additional possible signature is also studied, events containing a single observed photon. Within the Minimal Supersymmetric Standard Model, the mass of the lightest neutralino is found to be larger than 12.5 GeV/c2 at 95% C.L., provided that tan β is larger than 1.5.

A new measurement of the τ lifetime is presented. It uses data collected with the Opal detector during 1994, which almost doubles the size of the Opal τ sample. Two statistically independent techniques are used: an impact parameter analysis of one-prong decay tracks and a fit to the decay length distribution of three-prong decays. The lifetime obtained from the 1994 data by combining the results of these methods is ττ = 289.7 ± 2.5 (stat)± 1.5 (sys) fs. When combined with the previous Opal τ lifetime measurement the improved τ lifetime is ττ = 289.2 ± 1.7 (stat.) ± 1.2 (sys.) fs.

The results of a study of charm meson production in semileptonic B hadron decays are presented. Based on a sample of 1.72 million hadronic Z0 decays the following product branching ratios (averaged over electrons and muons) are obtained: $$\begin{gathered} B(b \to D^ + \ell X) \cdot B(D^ + \to K^ - \pi ^ + \pi ^ + ) \hfill \\ = (1.82 \pm 0.20 \pm 0.12) \times 10^{ - 3} , \hfill \\ B(b \to D^0 \ell X) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (2.52 \pm 0.14 \pm 0.17) \times 10^{ - 3} \hfill \\ B(b \to D^* \ell X) \cdot B(D^{* + } \to D^0 \pi ^ + ) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (7.53 \pm 0.47 \pm 0.56) \times 10^{ - 4} \hfill \\ \end{gathered} $$ where the first error is statistical and the second systematic. The results of a search for the narrow P-wave charmed mesons, (D J ), in semileptonic B decays are also reported. Using the decay channels D 0 →D+π−, D 0 →D*+π− and D + →D0π+ we observe a total signal of 44±8 −7 +3 D 0 and 48±10 −6 +3 D + events. This is the first evidence for semileptonic B decays to charged P-wave charm states. The observed signals also provide evidence for the production of both pseudovector and tensor P-wave charmed mesons in semileptonic B decay. Together these modes are estimated to make up 34±7% of charmed semileptonic B0 and B+ decays.

Measurements of helicity density matrix elements have been made for the φ(1020), D*± and B* vector mesons in multihadronic Z0 decays in the OPAL experiment at LEP. Results for inclusive φ produced with high energy show evidence for production preferentially in the helicity zero state, with ρ00 = 0.54 ± 0.08, compared to the value of 1/3 expected for no spin alignment. The corresponding element for the D*± has a value of 0.40 ± 0.02, also suggesting a deviation from 1/3. The B* result, with ρ00 = 0.36 ± 0.09, is consistent with no spin alignment. Off-diagonal elements have been measured for the f and D* mesons; for the D* the element Re ρ1−1 is non-zero, indicating non-independent fragmentation of the primary quarks.

We report a study of forward-backward multiplicity correlations and a measurement of the dependence on charged multiplicity of the mean transverse momentum of charged hadrons, measured with respect to the thrust axis. The study was performed on a high statistics sample of Z0 decays to multihadronic final states collected by the OPAL Collaboration at LEP. The positive forward-backward multiplicity correlation observed in our inclusive sample can be understood in terms of a superposition of distinct event topologies characterized by a different amount of hard gluon radiation (2-, 3- and 4-jet events) and with different mean multiplicities. The residual positive correlation that we see in a clean 2-jet sample can be interpreted in terms of fragmentation properties of different quark flavours and of the production and decay of resonances. We have compared the observed effects with the predictions of QCD-based parton shower models. The data are well described by the Jetset 7.3 Monte Carlo, while Herwig 5.5 does not satisfactorily reproduce the measured correlations. Hard gluon radiation is also shown to be responsible for the observed increase of about 40% in the mean transverse momentum of produced charged hadrons in the multiplicity range from 10 to 30. The comparison with the results obtained in an analysis of a sample enriched in Z0 → bb̄ events, shows that the presence of heavy flavours does not contribute significantly to the observed effect.

The fragmentation function for the process e+e−→h+X, whereh represents a hadron, may be decomposed into transverse, longitudinal and asymmetric contributions by analysis of the distribution of polar production angles. A number of new tests of QCD have been proposed using these fragmentation functions, but so far no data have been published on the separate components. We have performed such a separation using data on charged particles from hadronic Z0 decays atOpal, and have compared the results with the predictions of QCD. By integrating the fragmentation functions, we determine the average charged particle multiplicity to be $$\overline {n_{ch} } = 21.05 \pm 0.20$$ . The longitudinal to total cross-section ratio is determined to be σ L /σ tot =0.057±0.005. From the longitudinal fragmentation function we are able to extract the gluon fragmentation function. The connection between the asymmetry fragmentation function and electroweak asymmetrics is discussed.

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 2.6 pb−1 at a centre-of-mass energy of S=130 GeV and 2.6 pb−1 at 136 GeV collected with the OPAL detector at LEP during November 1995. No candidate events were observed. The 95% C.L. lower limit on the lightest chargino mass in the Minimal Supersymmetric Standard Model is 65.4 GeV if the universal scalar mass m0 is greater than 1 TeV, and 58.7 GeV for the smallest m0 compatible with slepton and sneutrino mass limits obtained at centre-of-mass energies near the Z peak. These limits were obtained under the conditions that the lightest chargino is heavier than the lightest neutralino by more than 10 GeV and tan β is larger than 1.5. The results of a model independent search for charginos and neutralinos are also given.

From 1 105 045 hadronic Z0 decays observed with the OPAL detector at the LEP e+e− collider, 21 732 four-jet events are selected. A simultaneous fit of three selected angular variables from these events by the second order QCD matrix element calculation yieldsC A /C F =2.11±0.16(stat.)±0.28(syst.)T F /C F =0.40±0.11(stat.)±0.14(syst.) for the ratios of colour factors, in agreement with SU(3) expectations ofC A /C F =9/4 andT F /C F =3/8.

Abstract Micromegas detectors have been taking data in the CAST experiment since 2002, occupying one opening (out of the two looking for sunrise axions) of the magnet and showing good performance and stability. Currently, three of the four X-ray detectors used in the experiment are micromegas. The new detectors are of the Microbulk technology, which have attracted a lot of attention because of the advantages they present, among them the low-material construction, high radiopurity and good energy resolution. Here, their performance during the last year will be commented. In particular, the low background levels reached in some detectors have triggered a set of studies in order to understand the effect.

Micromegas, as a proportional and compact gaseous detector, is well suited for sampling calorimetry. The limitation of occasional sparking has now been lifted by means of resistive electrodes but at the cost of current-dependent charge-up effects. These effects are studied in this contribution, with an emphasis on gain variations during operation at high particle rate and under heavy ionisation. Results are reproduced by a simple model of charging-up which will be used for detector design optimisation in the future.

We present a new measurement of the KL-KS mass difference (Δm) using semileptonic decays of neutral kaons. The measurement yields Δm = (0.5274 ± 0.0029stat. ± 0.0005syst.) × 1010ħ/s.

Deep inelastic electron-photon scattering is studied using e+e− data collected by the OPAL detector at centre-of-mass energies s=MZ0. The photon structure function F2γ(x,Q2) is explored in a Q2 range of 1.1 to 6.6 GeV2 at lower x values than ever before. To probe this kinematic region events are selected with a beam electron scattered into one of the OPAL luminosity calorimeters at scattering angles between 27 and 55 mrad. A measurement is presented of the photon structure function F2γ(x,Q2) at 〈Q2〉 = 1.86 GeV2 and 3.76 GeV2 in five logarithmic x bins from 0.0025 to 0.2.

We have used data from the OPAL detector at LEP to reconstruct D∗ mesons and secondary vertices in jets. We have studied the hemispheres of the events opposite these jets and obtain values of the hemisphere charged particle multiplicity in Z0 → uu, dd, ss, Z0 → cc and Z0 → bb events of nuds = 10.41 ± 0.06 ± 0.09 ± 0.19; nc = 10.76 ± 0.20 ± 0.14 ± 0.19; nb = 11.81 ± 0.01 ± 0.12 ± 0.21 where the first errors are statistical, the second systmatic and the third a common scale uncertainty. We find the difference in total charged particle multiplicity between c and b quark events and light (u, d, s) quark events to be δcl = 0.69 ± 0.51 ± 0.35; δbl = 2.79 ± 0.12 ± 0.27. These results are compared to the predictions of various models and QCD based calculations.

We have measured the multiplicity of charm quark pairs arising from gluon splitting in a sample of about 3.5 million hadronic Z0 decays. By selecting a 3-jet event topology and tagging charmed hadrons in the lowest energy jet using leptons, we established a signature of heavy quark pair production from gluons. The average number of gluons splitting into a cc pair per hadronic event was measured to be ng→cc=(2.27±0.28±0.41) × 10−2.

Bose-Einstein correlations between like charged track pairs have been studied using a sample of approximately 3.6 million multihadronic Z0 decays collected with the OPAL detector at LEP. The radius of the emitting sourceR and the chaoticity parameterλ were studied using two parametrisations, the Goldhaber (G) parametrisation and the one-dimensional Kopylov-Podgoretskii (KP) parametrisation. The radiiR G andR KP are found to increase linearly with the average observed charged multiplicityn ch , with changes with respect to a unit increase inn ch of $$\begin{gathered} \frac{1}{{\langle R_G \rangle }}\frac{{\Delta R_G }}{{\Delta n_{ch} }} = (3.6 \pm 0.6) \cdot 10^{ - 3} and \hfill \\ \frac{1}{{\langle R_{KP} \rangle }}\frac{{\Delta R_{KP} }}{{\Delta n_{ch} }} = (3.4 \pm 1.0) \cdot 10^{ - 3} . \hfill \\ \end{gathered} $$ where the 〈R〉 are the radius values measured in the inclusive event sample. The chaoticity parametersλ G andλ KP decrease with increasing charged multiplicity. It is shown that the increase ofR with multiplicity may be connected with differences between two- and three-jet events.

The production of neutral kaons in e+e− annihilation at centre-of-mass energies in the region of the Z0 mass and their Bose-Einstein correlations are investigated with the OPAL detector at LEP. A total of about 1.26×106 Z0 hadronic decay events are used in the analysis. The production rate of K0 mesons is found to be 1.99±0.01±0.04 per hadronic event, where the first error is statistical and the second systematic. Both the rate and the differential cross section for K0 production are compared to the predictions of Monte Carlo generators. This comparison indicates that the fragmentation is too soft in bothJetset andHerwig. Bose-Einstein correlations in K s 0 K s 0 pairs are measured through the quantityQ, the four momentum difference of the pair. A threshold enhancement is observed in K s 0 K s 0 pairs originating from a mixed sample of $$K^0 \bar K^0$$ and K0K0 ( $$\bar K^0 \bar K^0$$ ) pairs. For the strength of the effect and for the radius of the emitting source we find values of λ=1.14±0.23±0.32 andR 0=(0.76±0.10±0.11) fm respectively. The first error is statistical and the second systematic.

The lifetime of the Λb0 baryon has been measured using 3.6 million hadronic Z0 decays recorded by the OPAL detector at LEP from 1990 to 1994. A sample of Λb0 decays is obtained using partially reconstructed semileptonic decays involving Λc+ℓ− combinations, where the Λc+ is reconstructed from its decay to a pK−π+ final state. From the 69 ± 13Λc+ℓ− combinations attributed to Λb0 decays in this data sample, we measure τ(Λb0) = 1.14−0.19+0.22 ± 0.07 ps, where the errors are statistical and systematic, respectively.

A low background Micromegas detector has been operating in the CAST experiment at CERN for the search of solar axions since the start of data taking in 2002. The detector, made out of low radioactivity materials, operated efficiently and achieved a very low level of background (5×10-5 keV-1-cm-2-s-1) without any shielding. New manufacturing techniques (Bulk/Microbulk) have led to further improvement of the characteristics of the detector such as uniformity, stability and energy resolution. These characteristics, the implementation of passive shielding and the improvement of the analysis algorithms have dramatically reduced the background level (2×10-7 keV-1-cm-2∣s-1), improving thus the overall sensitivity of the experiment and opening new possibilities for future searches.

Micromegas technology is a promising candidate to replace Atlas forward muon chambers -tracking and trigger- for future HL-LHC upgrade of the experiment. The LHC accelerator luminosity will be ten times the nominal one, increasing background and pile-up event probability in the same proportion. This requires detector performances which are currently under studies in intensive RD activities.

We present measurements of triple gauge boson coupling parameters using data recorded by the OPAL detector at LEP2 at a centre-of-mass energy of 172 GeV. A total of 120 W-pair candidates has been selected in the qqqq, qq`ν` and `ν`` ′ ν`′ decay channels, for an integrated luminosity of 10.4 pb. We use these data to determine several different anomalous coupling parameters using the measured cross-section and the distributions of kinematic variables. We measure αBφ=0.35 +1.29 −1.07 ± 0.38, αWφ=0.00 +0.30 −0.28 ± 0.11, αW=0.18 +0.49 −0.47 ± 0.23, ∆g z 1=−0.03 +0.40 −0.37 ± 0.14, ∆κ γ =0.03 +0.55 −0.51± 0.20, and ∆κ=0.03 +0.49 −0.46± 0.21. Combining the αWφ result with our previous result obtained from the 161 GeV data sample we measure αWφ=−0.08 +0.28 −0.25 ± 0.10. All of these measurements are consistent with the Standard Model. (To be submitted to Zeitschrift fur Physik C.) The OPAL Collaboration K. Ackerstaff, G. Alexander, J. Allison, N. Altekamp, K.J. Anderson, S. Anderson, S. Arcelli, S. Asai, D. Axen, G. Azuelos, A.H. Ball, E. Barberio, R.J. Barlow, R. Bartoldus, J.R. Batley, S. Baumann, J. Bechtluft, C. Beeston, T. Behnke, A.N. Bell, K.W. Bell, G. Bella, S. Bentvelsen, S. Bethke, O. Biebel, A. Biguzzi, S.D. Bird, V. Blobel, I.J. Bloodworth, J.E. Bloomer, M. Bobinski, P. Bock, D. Bonacorsi, M. Boutemeur, B.T. Bouwens, S. Braibant, L. Brigliadori, R.M. Brown, H.J. Burckhart, C. Burgard, R. Burgin, P. Capiluppi, R.K. Carnegie, A.A. Carter, J.R. Carter, C.Y. Chang, D.G. Charlton, D. Chrisman, P.E.L. Clarke, I. Cohen, J.E. Conboy, O.C. Cooke, M. Cuffiani, S. Dado, C. Dallapiccola, G.M. Dallavalle, R. Davis, S. De Jong, L.A. del Pozo, K. Desch, B. Dienes, M.S. Dixit, E. do Couto e Silva, M. Doucet, E. Duchovni, G. Duckeck, I.P. Duerdoth, D. Eatough, J.E.G. Edwards, P.G. Estabrooks, H.G. Evans, M. Evans, F. Fabbri, M. Fanti, A.A. Faust, F. Fiedler, M. Fierro, H.M. Fischer, I. Fleck, R. Folman, D.G. Fong, M. Foucher, A. Furtjes, D.I. Futyan, P. Gagnon, J.W. Gary, J. Gascon, S.M. Gascon-Shotkin, N.I. Geddes, C. Geich-Gimbel, T. Geralis, G. Giacomelli, P. Giacomelli, R. Giacomelli, V. Gibson, W.R. Gibson, D.M. Gingrich, D. Glenzinski, J. Goldberg, M.J. Goodrick, W. Gorn, C. Grandi, E. Gross, J. Grunhaus, M. Gruwe, C. Hajdu, G.G. Hanson, M. Hansroul, M. Hapke, C.K. Hargrove, P.A. Hart, C. Hartmann, M. Hauschild, C.M. Hawkes, R. Hawkings, R.J. Hemingway, M. Herndon, G. Herten, R.D. Heuer, M.D. Hildreth, J.C. Hill, S.J. Hillier, P.R. Hobson, R.J. Homer, A.K. Honma, D. Horvath, K.R. Hossain, R. Howard, P. Huntemeyer, D.E. Hutchcroft, P. Igo-Kemenes, D.C. Imrie, M.R. Ingram, K. Ishii, A. Jawahery, P.W. Jeffreys, H. Jeremie, M. Jimack, A. Joly, C.R. Jones, G. Jones, M. Jones, U. Jost, P. Jovanovic, T.R. Junk, D. Karlen, V. Kartvelishvili, K. Kawagoe, T. Kawamoto, P.I. Kayal, R.K. Keeler, R.G. Kellogg, B.W. Kennedy, J. Kirk, A. Klier, S. Kluth, T. Kobayashi, M. Kobel, D.S. Koetke, T.P. Kokott, M. Kolrep, S. Komamiya, T. Kress, P. Krieger, J. von Krogh, P. Kyberd, G.D. Lafferty, R. Lahmann, W.P. Lai, D. Lanske, J. Lauber, S.R. Lautenschlager, J.G. Layter, D. Lazic, A.M. Lee, E. Lefebvre, D. Lellouch, J. Letts, L. Levinson, S.L. Lloyd, F.K. Loebinger, G.D. Long, M.J. Losty, J. Ludwig, A. Macchiolo, A. Macpherson, M. Mannelli, S. Marcellini, C. Markus, A.J. Martin, J.P. Martin, G. Martinez, T. Mashimo, P. Mattig, W.J. McDonald, J. McKenna, E.A. Mckigney, T.J. McMahon, R.A. McPherson, F. Meijers, S. Menke, F.S. Merritt, H. Mes, J. Meyer, A. Michelini, G. Mikenberg, D.J. Miller, A. Mincer, R. Mir, W. Mohr, A. Montanari, T. Mori, M. Morii, U. Muller, S. Mihara, K. Nagai, I. Nakamura, H.A. Neal, B. Nellen, R. Nisius, S.W. O’Neale, F.G. Oakham, F. Odorici, H.O. Ogren, A. Oh, N.J. Oldershaw, M.J. Oreglia, S. Orito, J. Palinkas, G. Pasztor, J.R. Pater, G.N. Patrick, J. Patt, M.J. Pearce, R. Perez-Ochoa, S. Petzold, P. Pfeifenschneider , J.E. Pilcher, J. Pinfold, D.E. Plane, P. Poffenberger, B. Poli, A. Posthaus, D.L. Rees, D. Rigby, S. Robertson, S.A. Robins, N. Rodning, J.M. Roney, A. Rooke, E. Ros, A.M. Rossi, P. Routenburg, Y. Rozen, K. Runge, O. Runolfsson, U. Ruppel, D.R. Rust, R. Rylko, K. Sachs, T. Saeki, E.K.G. Sarkisyan, C. Sbarra, A.D. Schaile, O. Schaile, F. Scharf, P. Scharff-Hansen, P. Schenk, J. Schieck, P. Schleper, B. Schmitt, S. Schmitt, A. Schoning, M. Schroder, H.C. Schultz-Coulon, M. Schumacher, C. Schwick, W.G. Scott, T.G. Shears, B.C. Shen, C.H. Shepherd-Themistocleous , 1 P. Sherwood, G.P. Siroli, A. Sittler, A. Skillman, A. Skuja, A.M. Smith, G.A. Snow, R. Sobie, S. Soldner-Rembold, R.W. Springer, M. Sproston, K. Stephens, J. Steuerer, B. Stockhausen, K. Stoll, D. Strom, P. Szymanski, R. Tafirout, S.D. Talbot, S. Tanaka, P. Taras, S. Tarem, R. Teuscher, M. Thiergen, M.A. Thomson, E. von Torne, S. Towers, I. Trigger, Z. Trocsanyi, E. Tsur, A.S. Turcot, M.F. Turner-Watson, P. Utzat, R. Van Kooten, M. Verzocchi, P. Vikas, E.H. Vokurka, H. Voss, F. Wackerle, A. Wagner, C.P. Ward, D.R. Ward, P.M. Watkins, A.T. Watson, N.K. Watson, P.S. Wells, N. Wermes, J.S. White, B. Wilkens, G.W. Wilson, J.A. Wilson, G. Wolf, T.R. Wyatt, S. Yamashita, G. Yekutieli, V. Zacek, D. Zer-Zion School of Physics and Space Research, University of Birmingham, Birmingham B15 2TT, UK Dipartimento di Fisica dell’ Universita di Bologna and INFN, I-40126 Bologna, Italy Physikalisches Institut, Universitat Bonn, D-53115 Bonn, Germany Department of Physics, University of California, Riverside CA 92521, USA Cavendish Laboratory, Cambridge CB3 0HE, UK 6 Ottawa-Carleton Institute for Physics, Department of Physics, Carleton University, Ottawa, Ontario K1S 5B6, Canada Centre for Research in Particle Physics, Carleton University, Ottawa, Ontario K1S 5B6, Canada CERN, European Organisation for Particle Physics, CH-1211 Geneva 23, Switzerland Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago IL 60637, USA Fakultat fur Physik, Albert Ludwigs Universitat, D-79104 Freiburg, Germany Physikalisches Institut, Universitat Heidelberg, D-69120 Heidelberg, Germany Indiana University, Department of Physics, Swain Hall West 117, Bloomington IN 47405, USA Queen Mary and Westfield College, University of London, London E1 4NS, UK Technische Hochschule Aachen, III Physikalisches Institut, Sommerfeldstrasse 26-28, D-52056 Aachen, Germany University College London, London WC1E 6BT, UK Department of Physics, Schuster Laboratory, The University, Manchester M13 9PL, UK Department of Physics, University of Maryland, College Park, MD 20742, USA Laboratoire de Physique Nucleaire, Universite de Montreal, Montreal, Quebec H3C 3J7, Canada University of Oregon, Department of Physics, Eugene OR 97403, USA Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel Department of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel International Centre for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo 113, and Kobe University, Kobe 657, Japan Brunel University, Uxbridge, Middlesex UB8 3PH, UK Particle Physics Department, Weizmann Institute of Science, Rehovot 76100, Israel Universitat Hamburg/DESY, II Institut fur Experimental Physik, Notkestrasse 85, D-22607 Hamburg, Germany University of Victoria, Department of Physics, P O Box 3055, Victoria BC V8W 3P6, Canada University of British Columbia, Department of Physics, Vancouver BC V6T 1Z1, Canada 2 University of Alberta, Department of Physics, Edmonton AB T6G 2J1, Canada Duke University, Dept of Physics, Durham, NC 27708-0305, USA Research Institute for Particle and Nuclear Physics, H-1525 Budapest, P O Box 49, Hungary Institute of Nuclear Research, H-4001 Debrecen, P O Box 51, Hungary Ludwigs-Maximilians-Universitat Munchen, Sektion Physik, Am Coulombwall 1, D-85748 Garching, Germany a and at TRIUMF, Vancouver, Canada V6T 2A3 b and Royal Society University Research Fellow c and Institute of Nuclear Research, Debrecen, Hungary d and Department of Experimental Physics, Lajos Kossuth University, Debrecen, Hungary e and Department of Physics, New York University, NY 1003, USA

A novel MicroMegas detector based on microbulk technology with an embedded XY strip structure was developed, obtained by segmenting both the mesh and the anode in perpendicular directions. This results in a very low-mass device with good energy and spatial resolution capabilities. Such a detector is practically "transparent" to neutrons, being ideal for in-beam neutron measurements and can be used as a quasi-online neutron beam profiler at neutron time-of-flight facilities. A dedicated front end electronics and acquisition system has been developed and used. The first studies of this new detection system are presented and discussed.

The fraction of $$Z^0 \to b\bar b$$ ; events in hadronicZ 0 decays has been measured using the data collected by OPAL in 1992 and 1993. The presence of electrons or muons from semileptonic decays of bottom hadrons and the detection of bottom hadron decay vertices were used together to obtain an event sample enriched in $$Z^0 \to b\bar b$$ decays. To reduce the systematic error on the measurement of the $$Z^0 \to b\bar b$$ fraction, the efficiency of the $$b\bar b$$ event tagging was obtained from the data by comparing the numbers of events having a bottom signature in either one or both thrust hemispheres. A value of $$\frac{{\Gamma (Z^0 \to b\bar b)}}{{\Gamma (Z^0 \to hadrons)}} = 0.2171 \pm 0.0021 \pm 0.0021$$ was obtained, where the first error is statistical and the second systematic. The uncertainty on the decay width $$\Gamma (Z^0 \to c\bar c)$$ is not included in these errors. A fractional variation of this width by ±8% about its Standard Model prediction would result in a variation of the measured $$Z^0 \to b\bar b$$ fraction of ±0.0015.

Measurements have been made in the OPAL experiment at LEP of the inclusive production of strange vector φ(1020) and K*(892)0 mesons, and the tensor meson K 2 * (1430)0. The overall production rates per hadronic Z0 decay have been determined to be 0.100±0.004stat.±0.007syst. φ(1020) mesons, 0.74±0.03stat.±0.03syst. K*(892)0 mesons and (forx E <0.3) 0.19±0.04stat.±0.06syst. K 2 * (1430)0 mesons. The measurements for the vector states update previously published results based on lower statistics, while the K 2 * (1430)0 rate represents the first direct measurement of a strange tensor state in Z0 decay. For the vector states, both the overall production rates and normalised differential cross sections, with respect to the scaled energy variablex E , have been compared to JETSET and HERWIG predictions. The peak positions in the ζ=ln(1/x p ) distributions have been measured and compared to measurements of other hadron states.

The spin composition of ΛΛ, ΛΛ and ΛΛ pairs at low invariant mass values has been measured for the first time in multihadronic Z0 decays with the OPAL detector at LEP. No single spin state has been observed in the ΛΛ sample, verifying that the low mass enhancement in this sample, attributed to local baryon number compensation, is not a resonance state. The fraction of the spin 1 contribution to the ΛΛ pairs was found to be consistent with the value 0.75, as expected from a statistical spin mixture. This may be the net effect of many different QCD processes which contribute to the hyperon anti-hyperon pair production. The spin composition of the identical ΛΛ and ΛΛ pairs, well above threshold, is found to be similar to that of the ΛΛ sample. A ΛΛ emitter dimension is estimated from the data assuming the onset of the Pauli exclusion principle near threshold.

A search for the Minimal Standard Model Higgs boson (H0) has been performed with data from e+e− collisions collected by the OPAL detector at LEP. The search was made for events of the types e+e−→(e+e−,μ+μ− or νν)H0, H0→qq̄ and was based on approximately 78 pb−1 of data taken at center-of-mass energies between 88 and 95 GeV in the years 1990–1993. The present study, combined with previous OPAL publications, excludes the existence of a Minimal Standard Model Higgs boson with mass below 56.9 GeV at the 95% confidence level.

This paper describes a search for the Standard Model Higgs boson using data from ${\rm e^+e^-}$ collisions collected at center-of-mass energies of 161, 170 and 172 GeV by the OPAL detector at LEP. The data collected at these energies correspond to integrated luminosities of 10.0, 1.0 and 9.4 pb $^{-1}$ , respectively. The search is sensitive to the main final states from the process in which the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electron, muon or tau leptons. One candidate event is observed, in agreement with the Standard Model background expectation. In combination with previous OPAL searches at center-of-mass energies close to the Z $^0$ resonance and the revised previous OPAL searches at 161 GeV , we derive a lower limit of 69.4 GeV for the mass of the Standard Model Higgs boson at the 95% confidence level.

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or 2/3, pair-produced in e+e− collisions at centre-of-mass energies from 130 to 183 GeV, is reported by the OPAL collaboration at LEP. No evidence for production of these particles was observed in a mass range between 45 and 89.5 GeV. Model-independent upper limits on the production cross-section between 0.05 and 0.19 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the minimal supersymmetric model (MSSM), this implies a lower limit of 82.5 (83.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged leptons and charginos are excluded for masses below 89.5 GeV. For particles with charge ±2/3 the upper limits on the production cross-section vary between 0.05 and 0.2 pb. All limits, on masses and on cross-sections, are valid at the 95% confidence level for particles with lifetimes longer than 10−6 s.

Production of events with multihadronic and leptonic final states has been measured in e+e− collisions at centre-of-mass energies significantly above the Z0 mass, using the OPAL detector at LEP. A substantial production rate of radiative Z0γ events was observed, as expected, together with events with less energetic initial-state photons. The cross-sections and leptonic forward-backward asymmetries were measured and compared with Standard Model expectations. In a model-independent fit to the Z0 lineshape, the hadronic cross-section and lepton asymmetries presented here provide constraints on the size of the γZ0-interference term which are complementary to those afforded by LEP data accumulated at the Z0 resonance.

The last generation of micromegas, called microbulk, are ahead of classical gas detectors (or even other kind of micro-pattern gas detectors) in gain stability, efficiency (by operation at high pressure), simplicity, robustness, energy resolution, readout features and radiopurity. This makes them a competent solution in the field of Rare Event Searches, a field where low background is the most appreciated feature of a detector. The CAST (CERN Axion Solar Telescope) experiment is the best example of their application in the X-rays range. In CAST, these detectors have achieved background rates as low as 6 × 10−6 counts keV−1 cm−2 s−1. Beyond this nominal operation, there have been several periods where the background has been reduced to a level of 2 × 10−7 counts keV−1 cm−2 s−1, due to reasons which are under investigation. The CAST experiment will be presented, paying special attention to their microbulk micromegas, as well as the procedures to achieve low background. Latest news about the operation of these kinds of detectors for the first time in underground conditions will be advanced here. Copyright © 2011 John Wiley & Sons, Ltd.

The increase in luminosity, and consequent higher backgrounds, of the LHC upgrades require improved rejection of fake tracks in the forward region of the ATLAS Muon Spectrometer. The New Small Wheel upgrade of the Muon Spectrometer aims to reduce the large background of fake triggers from track segments that are not originated from the interaction point. The New Small Wheel employs two detector technologies, the resistive strip Micromegas detectors and the "small" Thin Gap Chambers, with a total of 2.45 Million electrodes to be sensed. The two technologies require the design of a complex electronics system given that it consists of two different detector technologies and is required to provide both precision readout and a fast trigger. It will operate in a high background radiation region up to about 20 kHz/cm$^{2}$ at the expected HL-LHC luminosity of $\mathcal{L}$=7.5$\times10^{34}$cm$^{-2}$s$^{-1}$. The architecture of the system is strongly defined by the GBTx data aggregation ASIC, the newly-introduced FELIX data router and the software based data handler of the ATLAS detector. The electronics complex of this new detector was designed and developed in the last ten years and consists of multiple radiation tolerant Application Specific Integrated Circuits, multiple front-end boards, dense boards with FPGA's and purpose-built Trigger Processor boards within the ATCA standard. The New Small Wheel has been installed in 2021 and is undergoing integration within ATLAS for LHC Run 3. It should operate through the end of Run 4 (December 2032). In this manuscript, the overall design of the New Small Wheel electronics is presented.

We report a measurement of the Cabibbo-Kobayashi-Maskawa matrix element |Vcb|. From approximately 4.2 million hadronic Z0 decays recorded with the OPAL deteector, a sample is selected containing 1251 ± 125B0 → D∗+ℓ−νℓ candidates, where ℓ is either an electron or a muon. Using Heavy Quark Effective Theory calculations for the decay form factor at zero recoil of the D∗+ meson in the B0 rest frame, we derive |Vcb| = [36.0 ± 2.1 (stat) ± 2.4 (syst) ± 1.2 (theory)] × 10−3.

This letter presents an updated measurement of the lifetime of the Bs0 meson using 3.6 million hadronic Z0 decays recorded by the OPAL detector at LEP from 1990 to 1994. A sample of Bs0 decays is obtained using Ds−ℓ+ combinations, where the Ds− is reconstructed in either the φπ− or K∗0K− decay mode. From 79 ± 13 Dxℓ combinations attributed to Bs0 decays in this data sample, we measure τ(Bs0) = 1.54−0.21+0.25 ± 0.06 ps, where the errors are statistical and systematic, respectively.

Charginos and neutralinos, predicted by supersymmetric theories, have been searched for in e+e− collisions with an intergrated luminosity of 10.0 pb−1 at a centre-of-mass energy of s= 161GeV with the OPAL detector at LEP. Two candidate events are selected; consistent with the total background estimate of 0.7 ± 0.2 events. The 95% C.L. lower limit on the lightest chargino mass in the Minimal Supersymmetric Standard Model is 78.5 GeV if the universal scalar mass m0 is greater than 1 TeV, and 62.0 GeV for the smallest m0 compatible with slepton and sneutrino mass limits obtained at centre-of-mass energies near the Z peak. These limits were obtained under the conditions that the lightest chargino is heavier than the lightest neutralino by more than 10 GeV and tan β = 1.5. The new exclusion limits significantly improve on the results obtained at s= 130 and 136 GeV.

Data collected with the OPAL detector during 1990–1994 are used to measure the time dependence of $$B_d^0 \leftrightarrow \bar B_d^0 $$ mixing. A sample of 348 D*± candidates with a lepton in the opposite hemisphere are reconstructed, of which 167±25 are expected to be from B d 0 decays. The B d 0 oscillation frequency is measured to be $$\Delta m_d = 0.567 \pm 0.089(stat)_{ - 0.023}^{ + 0.029} (syst) ps^{ - 1} .$$ . A previously published analysis ofΔm d using D*± and lepton candidates in the same hemisphere and jet charge is also updated with a larger data sample. From 1200 D*±ℓ∓ candidates, of which 778±84 are expected to be from B d 0 decays, we find a value of: $$\Delta m_d = 0.539 \pm 0.060(stat) \pm 0.024(syst) ps^{ - 1} .$$ . The combined result of these two analyses is $$\Delta m_d = 0.548 \pm 0.050(stat)_{ - 0.019}^{ + 0.023} (syst) ps^{ - 1} .$$ .

This letter describes a measurement of one of the anomalous triple gauge boson couplings using the first data recorded by the OPAL detector at LEP2. A total of 28 W-pair candidates have been selected for an integrated luminosity of 9.89±0.06 pb−1 recorded at a centre-of-mass energy of 161 GeV. We use these data to place constraints upon the coupling parameter αWφ. We analyse the predicted variation of the total cross-section for all observed channels and the distribution of kinematic variables in the semileptonic decay channels. We measure αWφ to be −0.61−0.610.73±0.35, which is consistent with the Standard Model expectation of zero.

Micromegas readouts are an attractive option for many of the rare event searches, due to their performance regarding energy resolution, gain stability, homogeneity and material budget.The T-REX project aims at developing further these novel readout techniques for Time Projection Chambers and their potential use in experiments searching for rare events.Here we will refer to the latest results regarding the use and prospects of Micromegas read-outs in axion physics (CAST and the future helioscope), as well as the R&D carried out within NEXT, to search for the neutrinoless double-beta decay.

Updated measurements of the B0 and B+ meson lifetimes are presented. From a data sample of 1.72 million hadronic Z0 decays recorded during the period 1991 to 1993, a sample of approximately 1000 semileptonic B meson decays containing a D0, D+ or D*+ has been isolated. From the distribution of decay times in the different samples the lifetimes of the B0 and B+ mesons are determined to be 1.53±0.12±0.08 ps and 1.52±0.14±0.09 ps, respectively, where the first error is statistical and the second systematic. The ratio of the B+ to B0 lifetimes is measured to be 0.99±0.14 −0.04 +0.05 , confirming expectations that the lifetimes are similar.

Data collected with the OPAL detector at LEP during 1990–1993 are used to measure the time dependence of Bd0↔Bd0 mixing. From a sample of 153 ± 12 events with a charged D∗ and a lepton in the opposite hemisphere, we measure the Bd0↔Bd0 oscillation frequency to be Δmd = 0.57±0.11(stat.)±0.02(syst.) ps−1. This corresponds to an oscillation parameter of xd = ΔmdτBd0 = 0.82±0.16(stat.) ± 0.03(syst.) ± 0.09(syst. τB0d), where the second systematic uncertainty is due to the error in the measured Bd0 lifetime.

We present an update of our measurement of the τ lepton lifetime, using data taken during 1992 and 1993 with the OPAL detector at LEP. The lifetime is determined from analyses of the impact parameters of tracks from τ decays to a single charged particle, and the reconstructed decay lenghts from the τ decays to three charged particles. With the added statistics (which increase the τ pair event sample size by more than a factor of four over our 1990 and 1991 data sample), the updated lifetime measurement is: ττ = 288.8 ± 2.2 (stat) ± 1.4 (sys) fs. When combined with world-average measurements of the tau leptonic branching fractions (assuming e-μ universality), the ratio of charged-current couplings is: (gτgμ) = 1.005 ± 0.007, in agreement with the hypothesis of τ-μ charged-current universality.

A search is described to detect charged Higgs bosons via the process Z0 → H+H−, using data collected by the OPAL detector at LEP which correspond to an integrated luminosity of approximately 110 pb−1. It is assumed that the H+ boson decays only to τ+ντ and cs final states. From the negative outcome of this search a lower bound of 44.1 GeV (95% CL) is derived for the mass of the charged Higgs boson.

The time dependence of B meson oscillations is studied using hadronic Z0 decays with identified leptons in both thrust hemispheres. Decay times are reconstructed for each of the semileptonic B decays by forming vertices which include the lepton and by estimating the B meson momentum. The mass difference of the two mass eigenstates in the B d 0 system, Δm d, is measured to be $$0.462_{ - 0.053 - 0.035}^{ + 0.040 + 0.052} ps^{ - 1}$$ , where the first error is statistical and the second systematic. For the B s 0 system, a lower limit of Δm s >2.2 ps−1 at 95% C.L. is derived.

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a data sample of 57.7 pb−1 collected at an average center-of-mass energy of 182.6 GeV with the OPAL detector at LEP. No evidence for contributions from non-Standard Model physics processes was observed. The observed candidates are used to place limits on B(H0→γγ) assuming a Standard Model production rate for Higgs boson masses up to 92 GeV, and on the production cross section for a scalar resonance decaying into di-photons up to a mass of 170 GeV. Upper limits on the product of cross section and branching ratios, σ(e+e−→XY)×B(X→γγ)×B(Y→ff̄), as low as 70 fb are obtained over the range 10<MX<170 GeV for the case where 10<MY<160 GeV and MX+MY>90 GeV, independent of the nature of Y provided it decays to a fermion pair and has negligible width. Higgs scalars which couple only to gauge bosons at Standard Model strength are ruled out up to a mass of 90.0 GeV at the 95% confidence level. Limits are also placed on non-minimal Higgs sectors having triplet representations.

With the tenfold luminosity increase envisaged at the HL-LHC, the background (i.e., photons, neutrons, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\ldots$</tex> </formula> ) and the event pileup probability are expected to highly increase in proportion in the different experiments, especially in the forward regions, for instance the muons chambers of the ATLAS detector. Detectors based on the Micromegas principle should be good alternatives for the detector upgrade in the HL-LHC framework because of its good spatial <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(&lt; 100\mu{\hbox{m}})$</tex></formula> and time (few ns) resolutions, high-rate capability, radiation hardness, good robustness, and the possibility to build large areas. The aim of this study is to demonstrate that it is possible to reduce the discharge probability and protect the electronics by using a resistive anode plane in a high-flux hadrons environment. Several prototypes of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10 \times 10~{\hbox{cm}}^{2}$</tex></formula> , with different pitches (0.5–2 mm) and different resistive layers have been tested at CERN <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(\pi^{+}@{\rm SPS})$</tex></formula> . Several tests have been performed with a telescope at different voltages to assess the performance of the detectors in terms of position resolution and efficiency. The spark behavior in these conditions has also been evaluated. Resistive coating has been shown to be a successful method to reduce the effect of sparks on the efficiency of micromegas. A good spatial resolution <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(\sim 80\ \mu{\hbox{m}})$</tex></formula> can be reached with a resistive strip coating detector of 1-mm pitch, and a high efficiency <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$( &gt; 98\%)$</tex></formula> can be achieved with resistive-anode micromegas detector. An X-rays irradiation has also been performed, showing no ageing effect after more than 21 days exposure and an integrated charge of almost 1C.

Time Projection Chambers are widely used since many years for tracking and identification of charged particles in high energy physics. We present here a new R&D project, called FIDIAS, meant to investigate the feasibility of a Micromegas TPC for low energy heavy ions detection. In this framework, a TPC prototype based on Micromegas bulk technique has been extensively tested with spontaneous fission source. A deep analysis of the experimental results has been realized leading to a full characterization of the prototype in terms of gain, energy resolution and track reconstruction as a function of three working gas: helium, neon and argon. The encouraging results have also been compared to simulations, showing the Micromegas TPC is a very well suited detector for the detection of heavy ions in nuclear reactions at low energy.

The differential cross-sections for e+e− → e+e−, e+e− → μ+μ− and e+e− → τ+τ−, and the total cross-section for e+e− → qq̄ at centre-of-mass energies of 130–140 GeV were studied using about 5 pb−1 of data collected with the OPAL detector at LEP in October and November 1995. The results are in agreement with the Standard Model predictions. Four-fermion contact interaction models were fitted to the data and lower limits were obtained on the energy scale Λ at the 95% confidence level.

The production of J/psi mesons in Z(0) decays is studied using 3.6 million hadronic events recorded by the OPAL detector at LEP. The inclusive Z(0) to J/psi and b-quark to J/psi branching ratios are measured from the total yield of J/psi mesons, identified from their decays into lepton pairs. The J/psi momentum distribution is used to study the fragmentation of b-quarks. The production rate of psi' mesons, identified from their decays into a J/psi and a pi(+)pi(-) pair, is measured as well. The following results are obtained:Br(Z(0)-->J/psi X) = (3.9 +/- 0.2 +/- 0.3). 10(-3) and Br(Z(0)-->psi' X) = (1.6 +/- 0.3 +/- 0.2). 10(-3),where the first error is statistical and the second systematic. Finally the J/psi sample is used to reconstruct exclusive b-hadron decays and calculate the corresponding b-hadron branching ratios and masses.

Abstract We report on the definition and construction of the trigger system for the CP violation experiment with tagged K 0 (PS195) at LEAR, the low energy antiproton machine at CERN. The beam is assumed to have a continuous intensity of 2 × 10 6 antiprotons per second. The requirements for fast and efficient rejection are stringent.

The production of Δ++ baryons has been measured using 3.5 million hadronic Z0 decays collected with the OPAL detector at LEP. The production rate and fragmentation function are presented. A total of 0.22 ± 0.04 ± 0.04 Δ++ + (Δ)−− per hadronic Z0 decay is observed. The fragmentation function is found to be softer than that predicted by the JETSET and HERWIG Monte Carlo event generators. With this measurement of Δ++ production, at least one baryon of each strangeness level in the lightest baryon decuplet has now been measured at LEP.

We report on a measurement of the tau lepton polarization and its forward-backward asymmetry at the Z0 resonance using the OPAL detector. This measurement is based on analyses of the $$\tau \to e\bar v_e v_\tau $$ , $$\tau \to \mu \bar v_\mu v_\tau $$ , τ→π(K)v τ and τ→ρ v τ from a sample of 30663 e+e+→τ+τ- events collected during the years 1990 to 1992. Assuming that the tau lepton decays according to V-A theory, we measure the average τ polarization to be <P τ>=(−14.9±1.9±1.3)% and the τ polarization forward-backward asymmetry to be A =(−8.9±2.2±0.9)%, where the first error is statistical and the second systematic. These results are consistent with the hypothesis of lepton universality. When combined under the assumption of universality, our results can be interpreted as a measurement of sin2 θ eff lept =0.2321±0.0023 within the context of the Standard Model.

A search for the neutral Higgs bosons h0 and A0, predicted by the Minimal Supersymmetric Extension of the Standard Model (MSSM), has been performed by the OPAL Collaboration at LEP. The analysis was based on approximately 75 pb−1 of data taken at centre-of-mass energies in the vicinity of the Z0 resonance. No Higgs boson signals have been detected. Using, in addition, an upper limit on the contribution of non-Standard Model processes to the Z0 boson width, almost the entire MSSM parameter space that can be reached at present LEP energies has been excluded. In particular, at the 95% confidence level, our results imply that $$m_{h^0 } > 44.5$$ GeV/c2 and $$m_{A^0 } > 24.3$$ GeV/c2, for tan β≥1. The sensitivity to this assumption is discussed and the search results are also interpreted in the context of general two-doublet models.

The forward-backward asymmetry of $$e^ + e^ - \to Z^0 \to b\bar b$$ has been measured using approximately 2.15 million hadronicZ 0 decays collected at the LEP e+e− collider with the OPAL detector. A lifetime tag technique was used to select an enriched $$b\bar b$$ event sample. The measurement of the $$b\bar b$$ asymmetry was then performed using a jet charge algorithm to determine the direction of the primary quark. Values of: $$\begin{gathered} A_{FB}^b = 0.062 \pm 0.034 \pm 0.002 - 0.082\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 89.52GeV, \hfill \\ A_{FB}^b = 0.0963 \pm 0.0067 \pm 0.0038 - 0.471\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 91.25GeV, \hfill \\ A_{FB}^b = 0.172 \pm 0.028 \pm 0.007 - 0.055\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 92.94GeV, \hfill \\ \end{gathered}$$ were measured where, in each case, the first error is statistical, the second is systematic and the third term gives the variation due to a change $$\Delta (\Gamma _{b\bar b} /\Gamma _{had} )$$ in the value of $$\Gamma _{b\bar b} /\Gamma _{had} = 0.216$$ assumed. The dependence on the assumed charm asymmetry at the same energy is Δ(A FB b )≈+0.07Δ(A FB c ). Assuming the Standard Model form for the couplings, these measurements correspond to an effective weak mixing angle of: $$\sin ^2 \theta _W^{eff,e} = 0.2313 \pm 0.0012 \pm 0.0006$$ giving Mtop=196 −38−19 +33+16 GeV/c 2, where the first error is statistical and the second is systematic. The Higgs mass assumed is 300 GeV/c 2. A variation in the assumed mass of the Higgs boson between 60 and 1000 GeV/c 2 corresponds to an uncertainty in sin2θ W eff,e of ±0.00006 and on Mtop of −26 +20 GeV/c 2.

Evindence for the production of Y mesons in hadronic Z0 decays is presented. Using a sample of 3.7 million hadronic events, eight Y candidates are identified from their decays into e+e− and μ+μ− pairs. The estimated background in the signal region is 1.6 ± 0.3 events. Based on existing theoretical models for inclusive Y production, the following branching ratio, summed over the three lightest Y states, is obtained: Br(Z0 → Y + X) = (1.0 ± 0.4 ± 0.1 ± 0.2) × 10−4 where the first error is statistical, the second systematic and the third error accounts for uncertainties in the production mechanism.

Cross-sections and angular distributions for the production of events with single and multiple photons are measured from data recorded with the OPAL detector at the recently upgraded LEP collider. The measured cross-sections are generally consistent with Standard Model expectations for the e+e−→ νvγ(γ) and e+e− → γγ(γ) processes. Six events with an acoplanar photon pair and large missing mass are found. The observed number of events is larger than expected from e+e−→ ννγγ; however, the missing mass distribution is compatible with the Z0 resonance. Deviations from QED are constrained by the data on e+e− → γγ(γ). Lower limits are set at 95% confidence level on the QED cut-off parameters Λ+ and Λ− of 152 GeV and 142 GeV, respectively, and also on the mass of an excited electron of 147 GeV.

A search for a supersymmetric partner of the top quark (t̃1) has been performed by the OPAL experiment in e+e- collisions at LEP. The integrated luminosity of the data sample analysed was 69.1 pb−1, which corresponds to 1.68 × 106 produced Z0 → qq̄ events. No t̃1 candidates have been found. This study excludes the existence of the t̃1 with a mass below 45.1 GeV at 95% C.L., where the mixing angle of left- and right-handed partners is smaller than 0.85 rad or greater than 1.15 rad, and the mass difference between the t̃1 and the lightest neutralino is greater than 5 GeV.