Dezsö Horváth

Physical laws are believed to be invariant under the combined transformations of charge, parity and time reversal (CPT symmetry). This implies that an antimatter particle has exactly the same mass and absolute value of charge as its particle counterpart. Metastable antiprotonic helium (pHe(+)) is a three-body atom consisting of a normal helium nucleus, an electron in its ground state and an antiproton (p) occupying a Rydberg state with high principal and angular momentum quantum numbers, respectively n and l, such that n ≈ l + 1 ≈ 38. These atoms are amenable to precision laser spectroscopy, the results of which can in principle be used to determine the antiproton-to-electron mass ratio and to constrain the equality between the antiproton and proton charges and masses. Here we report two-photon spectroscopy of antiprotonic helium, in which p(3)He(+) and p(4)He(+) isotopes are irradiated by two counter-propagating laser beams. This excites nonlinear, two-photon transitions of the antiproton of the type (n, l) → (n - 2, l - 2) at deep-ultraviolet wavelengths (λ = 139.8, 193.0 and 197.0 nm), which partly cancel the Doppler broadening of the laser resonance caused by the thermal motion of the atoms. The resulting narrow spectral lines allowed us to measure three transition frequencies with fractional precisions of 2.3-5 parts in 10(9). By comparing the results with three-body quantum electrodynamics calculations, we derived an antiproton-to-electron mass ratio of 1,836.1526736(23), where the parenthetical error represents one standard deviation. This agrees with the proton-to-electron value known to a similar precision.

A femtosecond optical frequency comb and continuous-wave pulse-amplified laser were used to measure 12 transition frequencies of antiprotonic helium to fractional precisions of $(9--16)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$. One of these is between two states having microsecond-scale lifetimes hitherto unaccessible to our precision laser spectroscopy method. Comparisons with three-body QED calculations yielded an antiproton-to-electron mass ratio of ${M}_{\overline{p}}/{m}_{e}=1836.152\text{ }674(5)$.

Exotic molecule tests fundamental symmetry Spectroscopy of exotic molecules can offer insight into fundamental physics. Hori et al. studied the transition frequencies of an unusual helium atom in which one of the two electrons was substituted by an antiproton, the negatively charged antiparticle partner of the proton (see the Perspective by Ubachs). The antiprotonic helium was cooled down to low temperatures to allow the frequencies to be measured with high precision. The extracted mass of the antiproton (relative to the electron mass) was in good agreement with previous measurements of the proton mass. This finding is in keeping with the implications of the combined charge, parity, and time-reversal symmetry of physical laws. Science , this issue p. 610 ; see also p. 546

We have observed the first laser-induced resonant transitions in antiprotonic helium atoms. These occur between metastable states and Auger dominated short lived states, and show that the anomalous longevity of antiprotons previously observed in helium media results from the formation of high-n high-l atomic states of p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{He}}^{+}$. The observed transition with vacuum wavelength 597.259\ifmmode\pm\else\textpm\fi{}0.002 nm is tentatively assigned to (n,l)=(39,35)\ensuremath{\rightarrow}(38,34).

This paper describes topological searches for neutral scalar bosons S0 produced in association with a Z0 boson via the Bjorken process $e^+e^- \to{\rm S}^0{}{\rm Z}^0$ at centre-of-mass energies of 91 GeV and 183-209 GeV. These searches are based on studies of the recoil mass spectrum of ${\rm Z}^0 \to e^+e^-$ and $\mu^+ \mu^-$ events and on a search for ${\rm S}^0{\rm Z}^0$ with ${\rm Z}^0 \to \nu\bar{\nu}$ and S $^0 \to e^+ e^-$ or photons. They cover the decays of the S0 into an arbitrary combination of hadrons, leptons, photons and invisible particles as well as the possibility that it might be stable. No indication for a signal is found in the data and upper limits on the cross section of the Bjorken process are calculated. Cross-section limits are given in terms of a scale factor k with respect to the Standart Model cross section for the Higgs-strahlung process $e^+ e^-\to{\rm H}_{\rm SM}{\rm Z}^0$ . These results can be interpreted in general scenarios independently of the decay modes of the S0. The examples considered here are the production of a single new scalar particle with a decay width smaller than the detector mass resolution, and for the first time, two scenarios with continuous mass distributions, due to a single very broad state or several states close in mass.

A radio frequency quadrupole decelerator and achromatic momentum analyzer were used to decelerate antiprotons and produce $\overline{p}^{4}\mathrm{H}\mathrm{e}^{+}$ and $\overline{p}^{3}\mathrm{H}\mathrm{e}^{+}$ atoms in ultra-low-density targets, where collision-induced shifts of the atomic transition frequencies were negligible. The frequencies at near-vacuo conditions were measured by laser spectroscopy to fractional precisions of $(6--19)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$. By comparing these with QED calculations and the antiproton cyclotron frequency, we set a new limit of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ on possible differences between the antiproton and proton charges and masses.

We review recent progress in the laser and microwave spectroscopy of antiprotonic helium atoms carried out at CERN's Antiproton Decelerator facility (AD). Laser transitions were here induced between Rydberg states (n, ℓ) and (n ± 1, ℓ − 1) of (n ∼ 40 and ℓ ≲ n − 1 being the principal and orbital angular momentum quantum numbers of the antiproton orbit). Successive refinements in the experimental techniques improved the fractional precision on the frequencies from 3 parts in 106 to ∼1 part in 108. These included a radiofrequency quadrupole decelerator, which reduced the energy of the antiprotons from 5.3 MeV (the energy of the beam emerging from AD) to ∼100 keV. This enabled the production of in ultra-low density targets, where collisional effects with other helium atoms are negligible. A continuous wave pulse-amplified dye laser, stabilized against a femtosecond optical frequency comb, was then used to measure the frequencies with ppb-scale precision. This progress in the experimental field was matched by similar advances in computing methods for evaluating the expected transition frequencies in three-body QED calculations. The comparison of experimental (νexp) and theoretical (νth) frequencies for seven transitions in and five in yielded an antiproton-to-electron mass ratio of . This agrees with the known proton-to-electron mass ratio at the level of ∼2 × 10−9. The experiment also set a limit on any CPT-violating difference between the antiproton and proton charges and masses, to a 90% confidence level. If on the other hand we assume the validity of the CPT invariance, the result can be taken to be equal to mp/me. This can be used as an input to future adjustments of fundamental constants. The hyperfine structure of a state in has also been measured by microwave spectroscopy to a precision of 3 × 10−5. This corresponds to the accuracy of the most precise three-body QED calculations. Further increases in the experimental precision may soon yield an improvement in the value of the antiproton magnetic moment.

Six laser-resonant transitions have been detected in metastable antiprotonic helium atoms produced at the CERN Antiproton Decelerator. They include UV transitions from the last metastable states in the $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}n\ensuremath{-}\ensuremath{\ell}\ensuremath{-}1\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ and 1 cascades. Zero-density frequencies were obtained from measured pressure shifts with fractional precisions between $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ and $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. By comparing these with QED calculations and the antiproton cyclotron frequency, we deduce that the antiproton and proton charges and masses agree to within $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ with a confidence level of $90%$.

.Approximately 438 pb-1 of e + e- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 GeV, are analyzed to search for evidence of chargino pair production, \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1\), or neutralino associated production, \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1\). Limits are set at the 95% confidence level on the product of the cross-section for the process \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1\) and its branching ratios to topologies containing jets and missing energy, or jets with a lepton and missing energy, and on the product of the cross-section for \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1\) and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the \(\tilde\chi^{\pm}_1, \tilde\chi^0_1\) and \(\tilde\chi^0_2\), and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on \(\sigma({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1)\) with the assumption that each chargino decays via a W boson, and on \(\sigma({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1)\) with the \(\tilde\chi^0_2\) assumed to decay via a Z0.

A study of b quark hadronisation is presented using inclusively reconstructed B hadrons in about four million hadronic Z decays recorded in 1992-2000 with the OPAL detector at LEP. The data are compared to different theoretical models, and fragmentation function parameters of these models are fitted. The average scaled energy of weakly decaying B hadrons is determined to be <xe>=0.7193+-0.0016(stat)+0.0036-0.0031(syst)

We have performed laser spectroscopy of metastable antiprotonic helium atoms $(\overline{p}{\mathrm{He}}^{+})$ formed in helium media of 0.2--8.0 bars at 5.8--6.3 K and have observed a density dependence of the resonance vacuum wavelengths for the known transitions $(n,l)=(39,35)\ensuremath{\rightarrow}(38,34)$ and $(37,34)\ensuremath{\rightarrow}(36,33).$ They showed linear redshifts of $0.61\ifmmode\pm\else\textpm\fi{}0.01$ GHz and $0.22\ifmmode\pm\else\textpm\fi{}0.02$ GHz per 1 g/l, respectively. With the shift parameters above, the transition vacuum wavelengths were extrapolated to zero-density limits, yielding ${\ensuremath{\lambda}}_{0}=597.2570\ifmmode\pm\else\textpm\fi{}0.0003$ nm and ${\ensuremath{\lambda}}_{0}=470.7220\ifmmode\pm\else\textpm\fi{}0.0006$ nm, respectively. These values, with a 0.5-ppm precision, were compared with the result of recent theoretical calculations on the energy of the Coulombic three-body system, including relativistic corrections and the Lamb shift. The agreement between our experimental values and the calculations has become as good as $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}.$ This excellent agreement in turn provides a precise value of the antiproton Rydberg constant that surpasses the currently known precision and sets a severe constraint on the antiproton charge $(\ensuremath{-}{Q}_{\overline{p}})$ and the mass ${(M}_{\overline{p}})$ that both $|{Q}_{\mathit{p}}\ensuremath{-}{Q}_{\overline{p}}|/e$ and $|{M}_{\mathit{p}}\ensuremath{-}{M}_{\overline{p}}|{/M}_{p}$ be less than $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7},$ when a more precisely known constraint on the charge-to-mass ratio is combined. Thus we have opened a possibility of determining fundamental constants of the antiproton.

We have used a radio frequency quadrupole decelerator to decelerate antiprotons emerging from the CERN Antiproton Decelerator from MeV- to keV-scale energy, and collected five decelerated pulses in a multiring trap. Some $5\ifmmode\times\else\texttimes\fi{}{10}^{6}$ antiprotons were stacked in this way. Cooling of the trapped antiprotons by a simultaneously trapped electron plasma was studied nondestructively via shifts in plasma mode frequencies. We have also demonstrated the first step in extracting a 10--500 eV antiproton beam from the trap.

The first page of this article is displayed as the abstract.

The process $\mathrm{e}^+\mathrm{e}^- \to\gamma\gamma(\gamma)$ is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3 pb-1 at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross-sections are determined and found to be in good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type $\mathrm{e}^+\mathrm{e}^- \gamma\gamma$ , gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance $X\to\gamma\gamma$ is found and limits are placed on the product of the $\rm X \gamma$ production cross-section and branching ratio.

Laser studies of metastable antiprotonic helium atoms, which we recently initiated by observing a sharp increase of the antiproton annihilation rate induced by laser-stimulated resonant transitions, have been extended. With a single laser tuned to the resonance already found at 597.26 nm, we have now established the time dependence of the upper state population. With two lasers ignited at variable time separation, we also studied the feeding of the upper state from higher atomic levels. The initial populations and level lifetimes of excited exotic atoms were determined.

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or fractional charges of 2/3, 4/3, and 5/3 is reported using data collected by the OPAL detector at LEP, at centre-of-mass energies from 130 to 209 GeV. These particles are assumed to be pair-produced in e+e− collisions and not to interact strongly. No evidence for the production of these particles was observed. Model-independent upper limits on the production cross-section between 0.005 and 0.028 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the Constrained Minimal Supersymmetric Standard Model (CMSSM), this implies a lower limit of 98.0 (98.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged heavy leptons and charginos are excluded for masses below 102.0 GeV. For particles with fractional charge ±2/3, ±4/3 and ±5/3, the upper limit on the production cross-section varies between 0.005 and 0.020 pb. All mass and cross-section limits are derived at the 95% confidence level and are valid for particles with lifetimes longer than 10−6 s.

Using a newly developed laser-microwave-laser resonance method, we observed a pair of microwave transitions between hyperfine levels of the $(n,L)=(37,35)$ state of antiprotonic helium. This experiment confirms the quadruplet hyperfine structure arising from the interaction of the antiproton orbital angular momentum, the electron spin and the antiproton spin as predicted by Bakalov and Korobov. The measured frequencies of ${\ensuremath{\nu}}_{\mathrm{H}\mathrm{F}}^{+}=12.895\text{ }96\ifmmode\pm\else\textpm\fi{}0.000\text{ }34\text{ }\mathrm{G}\mathrm{H}\mathrm{z}$ and ${\ensuremath{\nu}}_{\mathrm{H}\mathrm{F}}^{\ensuremath{-}}=12.924\text{ }67\ifmmode\pm\else\textpm\fi{}0.000\text{ }29\text{ }\mathrm{G}\mathrm{H}\mathrm{z}$ agree with recent theoretical calculations on a level of $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.

We report a determination of the antiproton magnetic moment, measured in a three-body system, independent of previous experiments. We present results from a systematic study of the hyperfine (HF) structure of antiprotonic helium where we have achieved a precision more than a factor of 10 better than our first measurement. A comparison between the experimental results and three-body quantum electrodynamic (QED) calculations leads to a new value for the antiproton magnetic moment μsp¯=−2.7862(83)μN, which agrees with the magnetic moment of the proton within 2.9×10−3.

We report here the radial compression of a large number of antiprotons ( approximately 5 x 10(5)) in a strong magnetic field under ultrahigh vacuum conditions by applying a rotating electric field. Compression without any resonant structures was demonstrated for a range of frequencies from the sideband frequency of 200 kHz to more than 1000 kHz. The radial compression achieved is a key technique for synthesizing and manipulating antihydrogen atoms and antiprotonic atoms.

Hadronic event shape distributions from e+e- annihilation measured by the OPAL experiment at centre-of-mass energies between 91 GeV and 209 GeV are used to determine the strong coupling {\alpha}S. The results are based on QCD predictions complete to the next-to- next-to-leading order (NNLO), and on NNLO calculations matched to the resummed next-to-leading-log-approximation terms (NNLO+NLLA). The combined NNLO result from all variables and centre-of-mass energies is {\alpha}S(mZ0) = 0.1201 {\pm} 0.0008(stat.) {\pm} 0.0013(exp.) {\pm} 0.0010(had.) {\pm} 0.0024(theo.). while the combined NNLO+NLLA result is {\alpha}S(mZ0) = 0.1189 {\pm} 0.0008(stat.) {\pm} 0.0016(exp.) {\pm} 0.0010(had.) {\pm} 0.0036(theo.). The completeness of the NNLO and NNLO+NLLA results with respect to missing higher order contributions, studied by varying the renormalization scale, is improved compared to previous results based on NLO or NLO+NLLA predictions only. The observed energy dependence of {\alpha}S agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.

A laser-induced resonant transition in metastable antiprotonic helium atoms has been found at a wavelength of 470.724\ifmmode\pm\else\textpm\fi{}0.002 nm and assigned to the transition (n,l)=(37,34)\ensuremath{\rightarrow}(36,33). From the time evolution of the laser resonance intensity, the decay rate of the (37,34) state was determined to be 1.17\ifmmode\pm\else\textpm\fi{}0.09 \ensuremath{\mu}${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, about 60% larger than the theoretical radiative decay rate. The sum of the initial populations in the v=n-l-1=2 decay chain was 23\ifmmode\pm\else\textpm\fi{}4% of the total number of metastable atoms formed.

Initial distributions of metastable antiprotonic (4)He and (3)He atoms over principal (n) and angular momentum (l) quantum numbers have been deduced using laser spectroscopy experiments. The regions n = 37-40 and n = 35-38 in the two atoms account for almost all of the observed fractions [(3.0 +/- 0.1)% and (2.4 +/- 0.1)%] of antiprotons captured into metastable states.

A precise scan of the previously discovered laser-induced transition (n, l) = (37, 35) → (38, 34) in p̄He+ revealed a doublet structure with a separation of ΔνHF = 1.70 ± 0.05 GHz. This new type of “hyperfine” splitting is ascribed to the interaction of the antiproton orbital angular momentum and the electron spin.

A search for the single production of doubly-charged Higgs bosons is performed using e+e− collision data collected by the OPAL experiment at centre-of-mass energies between 189 GeV and 209 GeV. No evidence for the existence of H±± is observed. Upper limits are derived on hee, the Yukawa coupling of the H±± to like-signed electron pairs. A 95% confidence level upper limit of hee< 0.071 is inferred for M(H±±)<160 GeV assuming that the sum of the branching fractions of the H±± to all lepton flavour combinations is 100%. Additionally, indirect constraints on hee from Bhabha scattering at centre-of-mass energies between 183 GeV and 209 GeV, where the H±± would contribute via t-channel exchange, are derived for M(H±±)<2 TeV. These are the first results both from a single production search and on constraints from Bhabha scattering reported from LEP.

Cold, two-body antiprotonic helium ions p 4He2+ and p 3He2+ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ni=28-32 and li=ni-1 have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature T approximately 10 K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p 3He2+ against annihilation induced by collisions were shorter than those of p 4He2+, and decreased for larger-ni states.

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 438 pb−1 at centre-of-mass energies of s=192–209 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level lower limit on the scalar top quark mass is 97.6 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. When the scalar top quark decouples from the Z0 boson, the lower limit is 95.7 GeV. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. The lower limit on the scalar top quark mass is 96.0 GeV for this decay mode, if the mass difference between the scalar top quark and the scalar neutrino is greater than 10 GeV and if the mixing angle of the scalar top quark is zero. From a search for the scalar bottom quark, a mass limit of 96.9 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 10 GeV.

Total cross sections, angular, and mass distributions for the reaction ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{0}$${\mathrm{\ensuremath{\pi}}}^{0}$n have been measured for ${\mathit{p}}_{\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$(lab)=7--140 MeV/c above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, \ensuremath{\xi}, of -0.98\ifmmode\pm\else\textpm\fi{}0.52. The \ensuremath{\pi}\ensuremath{\pi} scattering lengths ${\mathit{a}}_{\mathit{I}}$ for isospin I=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are ${\mathit{a}}_{0}$=(0.207\ifmmode\pm\else\textpm\fi{}0.028)${\mathit{m}}_{\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}1}$ and ${\mathit{a}}_{2}$=(-0.022\ifmmode\pm\else\textpm\fi{}0.011)${\mathit{m}}_{\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}1}$. These values are consistent with chiral symmetry broken by the Weinberg \ensuremath{\pi}\ensuremath{\pi} interaction and the effects of the ${\mathit{f}}_{0}$(975) scalar meson.

.Using the OPAL detector at LEP, the running of the effective QED coupling α(t) is measured for space-like momentum transfer from the angular distribution of small-angle Bhabha scattering. In an almost ideal QED framework, with very favourable experimental conditions, we obtain: \( \Delta \alpha {\left( { - 6.07\;{\text{GeV}}^{2} } \right)} - \Delta \alpha {\left( { - 1.81\;{\text{GeV}}^{2} } \right)} = {\left( {440 \pm 58 \pm 43 \pm 30} \right)} \times 10^{{ - 5}} ,\) where the first error is statistical, the second is the experimental systematic and the third is the theoretical uncertainty. This agrees with current evaluations of α(t). The null hypothesis that α remains constant within the above interval of -t is excluded with a significance above 5σ. Similarly, our results are inconsistent at the level of 3σ with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurement where the running α(t) is probed differentially within the measured t range.

A laser spectroscopic method was developed to directly observe the lifetimes of individual states in metastable antiprotonic helium atoms $(\overline{p}{}^{4}{\mathrm{He}}^{+}).$ Collisional effects with surrounding helium atoms were studied in cryogenic gas targets at pressures between 0.1 and 9.0 bars and at temperatures between 5.5 and 7.0 K. The metastable state $(n,\mathcal{l})=(39,35)$ showed a lifetime of 1.5 $\ensuremath{\mu}\mathrm{s}$, regardless of density, whereas the lifetime of the energetically lower-lying state $(37,34)$ was found to shorten from 1.3 to 0.1 $\ensuremath{\mu}$s with increasing density. The lifetimes of the states $(38,34)$ and $(36,33)$ were determined to be 10 ns and 5 ns, respectively, showing the effects of the internal Auger process. When extrapolated to zero density, the measured lifetimes agreed with the results of calculations for the radiative and Auger widths. Using the fact that the state $(37,34)$ becomes short lived at high density, the resonance $(38,35)\ensuremath{\rightarrow}(37,34)$ at 529.62 nm was detected.

The time evolution of the state populations in the $v\ensuremath{\equiv}n\ensuremath{-}\ensuremath{\ell}\ensuremath{-}1=2$ and 3 metastable cascades of antiprotonic helium (${\overline{p}}^{4}{\mathrm{He}}^{+}$) atoms were studied using laser spectroscopy. The antiprotonic states $(n,\ensuremath{\ell})=(39,35)$, (40,36), and (41,37) in the $v=3$ cascade were estimated to initially contain, respectively, $(0.28\ifmmode\pm\else\textpm\fi{}0.04)%$, $(0.06\ifmmode\pm\else\textpm\fi{}0.02)%$, and $(0.02\ifmmode\pm\else\textpm\fi{}0.01)%$ of the antiprotons stopped in the helium target, while the corresponding values for states $(37,34)$, $(38,35)$, and $(39,36)$ in the $v=2$ cascade were $(0.21\ifmmode\pm\else\textpm\fi{}0.04)%$, $(0.49\ifmmode\pm\else\textpm\fi{}0.07)%$, and $(0.19\ifmmode\pm\else\textpm\fi{}0.07)%$. The shortening of the state lifetimes due to collisions between $\overline{p}{\mathrm{He}}^{+}$ and helium atoms was studied. As the atomic density of the target was increased from $\ensuremath{\rho}=2\ifmmode\times\else\texttimes\fi{}{10}^{20}$ to $2\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the lifetime of state $(37,34)$ decreased by an order of magnitude, whereas the higher states $(38,35)$ and $(39,35)$ were relatively unaffected. Between densities $\ensuremath{\rho}=6\ifmmode\times\else\texttimes\fi{}{10}^{20}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, a short-lived component with a time constant $\ensuremath{\tau}\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ appeared at early times in the delayed annihilation time spectrum of $\overline{p}{\mathrm{He}}^{+}$, while the total fraction of long-lived antiprotons decreased from 3.0 to $2.5%$. These effects were caused by the lifetime-shortening of specific $\overline{p}{\mathrm{He}}^{+}$ states such as $(37,34)$.

The photon spectrum from ${K}^{\mathrm{\ensuremath{-}}}$ stopping in liquid hydrogen has been measured with a high-resolution (1.5% FWHM at 300 MeV) NaI(Tl) detector. The branching ratios for ${K}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}\ensuremath{\Lambda}\ensuremath{\gamma} (${E}_{\ensuremath{\gamma}}$=281.4 MeV) and ${K}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}${\ensuremath{\Sigma}}^{0}$\ensuremath{\gamma} (${E}_{\ensuremath{\gamma}}$=219.5 MEV) were obtained. The results are ${\mathit{R}}_{\mathrm{\ensuremath{\Lambda}}\ensuremath{\gamma}}$=(0.86\ifmmode\pm\else\textpm\fi{}0.${07}_{\mathrm{\ensuremath{-}}0.08}^{+0.1}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ and ${\mathit{R}}_{\mathrm{\ensuremath{\Sigma}}0\ensuremath{\gamma}}$=(1.44\ifmmode\pm\else\textpm\fi{}0.${20}_{\mathrm{\ensuremath{-}}0.10}^{\ifmmode\pm\else\textpm\fi{}0.12}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ where the first error is statistical and the second systematic. These results are not in good agreement with published predictions or with previous \ensuremath{\Lambda}\ensuremath{\gamma} measurements.

We have analysed the data collected by OPAL at centre-of-mass energies between 189 and 209 GeV searching for Higgs boson candidates from the process $\mathrm {e^+e^-} \to \mathrm {h^0}\mathrm {Z}^0$ followed by the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$ where $\mathrm {A^0}$ is the CP-odd Higgs boson. The search is done in the region where the $\mathrm {A^0}$ mass, $m_{\mathrm{A}}$ , is below the production threshold for $\mathrm {b \bar b}$ , and the CP-even Higgs boson mass $m_{\mathrm{h}}$ is within the range 45-86 GeV/c 2 . In this kinematic range, the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$ may be dominant and previous Higgs boson searches have very small sensitivities. This search can be interpreted within any model that predicts the existence of at least one scalar and one pseudoscalar Higgs boson. No excess of events is observed above the expected Standard Model backgrounds. Model-independent limits on the cross-section for the process $\mathrm {e^+e^-}\rightarrow \mathrm {h^0} \mathrm {Z}^0$ are derived assuming 100% decays of the $\mathrm {h^0}$ into $\mathrm {A^0} \mathrm {A^0}$ and 100% decays of the $\mathrm {A^0} \mathrm {A^0}$ into each of the following final states: $\mathrm {c \bar c} \mathrm {c \bar c}$ , $\mathrm {gggg}$ , $\tau^+ \tau^- \tau^+ \tau^-$ , $\mathrm {c \bar c} \mathrm {gg}$ , $\mathrm {gg} \tau^+ \tau^-$ and $\mathrm {c \bar c} \tau^+ \tau^-$ . The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region $45 \le m_{\mathrm{h}} \le 85 {\mathrm{GeV}}/c^2$ and $2\le m_{\mathrm{A}} \le 9.5 {\mathrm{GeV}}/c^2$ is excluded.

This Letter describes a direct search for pair produced magnetic monopoles in e+e− collisions. The analysis is based on 62.7 pb−1 of data collected with the OPAL detector at an average centre-of-mass energy of s=206.3GeV. The monopole signal was assumed to be characterized by two back-to-back particles with an anomalously high ionization energy loss dE/dx in the tracking chambers. No evidence for production of monopoles was observed. Upper limits were obtained on the magnetic monopole pair-production cross-section (σ) in the mass range 45GeV/c2<mM<102GeV/c2. The average limit is σ<0.05pb and is essentially independent of the magnetic monopole mass. The cross-section limit is derived at the 95% confidence level and is valid for spin-1/2 magnetic monopoles.

We report the initial results from a systematic study of the hyperfine (HF) structure of antiprotonic helium (n, l) = (37, 35) carried out at the antiproton decelerator (AD) at CERN. We performed laser–microwave–laser resonance spectroscopy using a continuous wave (CW) pulse-amplified laser system and microwave cavity to measure the HF transition frequencies. Improvements in the spectral linewidth and stability of our laser system have increased the precision of these measurements by a factor of 5 and reduced the linewidth by a factor of 3 compared to our previous results. A comparison of the experimentally measured transition frequencies with three-body QED calculations can be used to determine the antiproton spin magnetic moment, leading towards a test of CPT invariance.

The Atomic Spectroscopy and Collisions Using Slow Antiprotons experiment at the Antiproton Decelerator (AD) facility of CERN constructed segmented scintillators to detect and track the charged pions which emerge from antiproton annihilations in a future superconducting radiofrequency Paul trap for antiprotons. A system of 541 cast and extruded scintillator bars were arranged in 11 detector modules which provided a spatial resolution of 17 mm. Green wavelength-shifting fibers were embedded in the scintillators, and read out by silicon photomultipliers which had a sensitive area of 1 × 1 mm2. The photoelectron yields of various scintillator configurations were measured using a negative pion beam of momentum p ≈ 1 GeV/c. Various fibers and silicon photomultipliers, fiber end terminations, and couplings between the fibers and scintillators were compared. The detectors were also tested using the antiproton beam of the AD. Nonlinear effects due to the saturation of the silicon photomultiplier were seen at high annihilation rates of the antiprotons.

Laser resonance transitions between normally metastable states of antiprotonic helium atoms were observed making use of state dependent quenching effects caused by small admixtures of ${\mathrm{H}}_{2}$ molecules. By selectively shortening the lifetimes of states with higher principal quantum number $n$ as compared to those of lower $n$, this method for the first time provides access to all initially populated metastable states of $\overline{p}{\mathrm{He}}^{+}$ atoms. This was demonstrated by observing the transitions $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(38,l)\ensuremath{\rightarrow}(39,l+1)$, $l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}35,36,37$ and $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(37,l)\ensuremath{\rightarrow}(38,l+1)$, $l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}34,35,36$.

Laser-induced resonant transitions in metastable antiprotonic $^{3}\mathrm{He}$ atoms have been found. The observed transitions at wavelengths 593.388\ifmmode\pm\else\textpm\fi{}0.001 nm and at 463.947\ifmmode\pm\else\textpm\fi{}0.002 nm have been respectively ascribed to the $(n, l)=(38, 34)\ensuremath{\rightarrow}(37, 33)$ and the (36,33)\ensuremath{\rightarrow}(35,32) transitions, showing excellent agreements to 50 ppm with recent predictions of Korobov.

Very weak laser resonances of unfavored transitions (n,l)\ensuremath{\rightarrow}(n+1,l-1) have been observed in metastable p-|${\mathrm{Am}}^{4}$${\mathrm{He}}^{+}$ atoms by using a pulsed antiproton beam. The observed wavelengths (713.578\ifmmode\pm\else\textpm\fi{}0.006 nm for l=34 and 726.095\ifmmode\pm\else\textpm\fi{}0.004 nm for l=35), as well as those of the known favoured transitions, are to a few ppm accounted for by a recent variational calculation of Korobov [Phys. Rev. A 54, R1749 (1996); and (unpublished)] which includes the relativistic correction to the motion of the electron.

Another laser-induced resonant transition in the v=n-l-1=2 metastable cascade of antiprotonic $^{4}\mathrm{He}$ atoms has been found by using a double-resonance technique. This was done by setting the first laser to the already known 470.724-nm resonance [(n,l)=(37,34)\ensuremath{\rightarrow}(36,33)], while the (38,35)\ensuremath{\rightarrow}(37,34) transition was searched for with the second laser. The resonant transition was found at a wavelength of 529.622\ifmmode\pm\else\textpm\fi{}0.003 nm, showing excellent agreement with a recent prediction of Korobov.

A search was performed for charginos with masses close to the mass of the lightest neutralino in e+e- collisions at centre-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP. Events were selected if they had an observed high-energy photon from initial state radiation, reducing the dominant background from two-photon scattering to a negligible level. No significant excess over Standard Model expectations has been observed in the analysed data set corresponding to an integrated luminosity of 570pb-1. Upper limits were derived on the chargino pair-productin cross-section, and lower limits on the chargino mass were derived in the context of the Minimal Supersymmetric Extension of the Standard Model for the gravity and anomaly mediated Supersymmetry breaking scenarios.

Non-commutative QED would lead to deviations from the Standard Model depending on a new energy scale ΛNC and a unique direction in space defined by two angles η and ξ. In this analysis, η is defined as the angle between the unique direction and the rotation axis of the earth. The predictions of a tree level calculation for the process e+e−→γγ are evaluated for the specific orientation of the OPAL detector and compared to the measurements. Distributions of the polar and azimuthal photon angles are used to extract limits on the energy scale ΛNC depending on the model parameter η. It is shown that the time dependence of the total cross-section could be used to determine the model parameter ξ if there were a detectable signal. This is the first experimental study of non-commutative QED at an e+e− collider.

Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are measured in e+e- annihilations from data collected at centre-of-mass energies of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are defined by hemispheres of inclusive hadronic events, while the biased jet measurements are based on three-jet events selected with jet algorithms. Several methods are employed to extract the fragmentation functions over a wide range of scales. Possible biases are studied in the results are obtained. The fragmentation functions are compared to results from lower energy e+e- experiments and with earlier LEP measurements and are found to be consistent. Scaling violations are observed and are found to be stronger for the fragmentation functions of gluon jets than for those of quarks. The measured fragmentation functions are compared to three recent theoretical next-to-leading order calculations and to the predictions of three Monte Carlo event generators. While the Monte Carlo models are in good agreement with the data, the theoretical predictions fail to describe the full set of results, in particular the b and gluon jet measurements.

Systematic experimental study has been carried out on the atomic capture of negative pions by 3He in binary gas mixtures of 3He + Z, where Z is Ne, Ar, Kr, Xe, N2, O2, CO2 and sf6. The results are analysed in the framework of a phenomenological model. It is shown that there is no pion transfer from the 3Heπ− mesic atoms to the heavier Z-atoms. The probabilities of pion capture in the various atoms of the mixtures are found to be proportional to the atomic concentrations, thereby excluding the possibility of a concentration dependence in the atomic capture ratio A(Z/3He). In contradiction to previous assumptions the probability of pion capture into an atomic orbit is not proportional to the stopping power of the components of the mixture. The atomic capture ratio of pions in a 3He + 4He mixture is A(4He/3He) = 0.75 ± 0.13, which might be the indication of an isotopic effect. The branching ratio for the charge-exchange reaction at rest (π− + 3He → π0 + 3H) is found to be 0.128 ± 0.012.

A search is performed for Higgs bosons decaying into invisible final states, produced in association with a Z0 boson in e+e− collisions at energies between 183 and 209 GeV. The search is based on data samples collected by the OPAL detector at LEP corresponding to an integrated luminosity of about 660 pb−1. The analysis aims to select events containing the hadronic decay products of the Z0 boson and large missing momentum, as expected from Higgs boson decay into a pair of stable weakly interacting neutral particles, such as the lightest neutralino in the Minimal Supersymmetric Standard Model. The same analysis is applied to a search for nearly invisible Higgs boson cascade decays into stable weakly interacting neutral particles. No excess over the expected background from Standard Model processes is observed. Limits on the production of invisibly decaying Higgs bosons produced in association with a Z0 boson are derived. Assuming a branching ratio BR(h0→invisible)=1, a lower limit of 108.2 GeV is placed on the Higgs boson mass at the 95% confidence level. Limits on the production of nearly invisibly decaying Higgs bosons are also obtained.

Six resonance transitions of the antiprotonic helium atom in helium gas at densities of $3\ifmmode\times\else\texttimes\fi{}{10}^{20}--3\ifmmode\times\else\texttimes\fi{}{10}^{21} {\mathrm{cm}}^{\ensuremath{-}3}$ were studied at the antiproton decelerator (AD) of CERN. The decay rates of the daughter states of these transitions were determined either from the time distributions of the resonance spikes or from the widths of the resonance lines. Whereas most of the observed decay rates agree with theoretical calculations of Auger rates, two states, $(n,l)=(37,33)$ and (32,31), were found to have decay rates two orders of magnitude larger than predicted by these calculations. The effect of coupling with near-lying electron-excited states is considered to be the reason for the anomaly of the (37, 33) state, as pointed out by Kartavtsev et al. [Phys. Rev. A $61,$ 062507 (2000)].

.A search for pair-produced leptoquarks is performed using \(\mbox{e}^ + \mbox{e}^-\) collision events collected by the OPAL detector at LEP at centre-of-mass energies between 189 and 209 GeV. The data sample corresponds to a total integrated luminosity of 596 pb-1. The leptoquarks are assumed to be produced via couplings to the photon and the Z0. For a given search channel only leptoquark decays involving a single lepton generation are considered. No evidence for leptoquark pair production is observed. Lower limits on masses for scalar and vector leptoquarks are calculated. The results improve most of the LEP limits derived from previous searches for the pair production process by 10-25 GeV, depending on the leptoquark quantum numbers.

Various models of atomic capture of negative mesonic particles are tested against 321 experimental Coulomb-capture ratios, measured on binary systems: gas mixtures, alloys and simple compounds. The comparison has shown that the general agreement between theory and experiment is not satisfactory. We tried to improve on the models by introducing adjustable parameters to be estimated by fitting the experimental data. The predictions of the model proposed by Schneuwly, Pokrovsky and Ponomarev (SPP) are closest to the atomic capture ratios for alloys and compounds while the data measured in gas mixtures are better approximated by the empirical formula of Vasilyev et al. The theoretical formula of Daniel fails in describing the capture in the light elements.

The influence of molecular structure on the nuclear capture probability of stopped negative pions has been observed by comparing the ${\ensuremath{\pi}}^{0}$ gamma-ray spectrum from ${\ensuremath{\pi}}^{\ensuremath{-}}$ mesons stopped in HD gas to that from a mixture of equal amounts of ${\mathrm{H}}_{2}$ plus ${\mathrm{D}}_{2}$. The fraction of stopped pions that are captured by a proton in the ${\mathrm{H}}_{2}$+${\mathrm{D}}_{2}$ mixture is ${f}_{{\mathrm{H}}_{2}{\mathrm{D}}_{2}}=0.417\ifmmode\pm\else\textpm\fi{}0.004$, while for HD it is ${f}_{\mathrm{HD}}=0.338\ifmmode\pm\else\textpm\fi{}0.008$, independent of the gas pressure between 6 and 90 atm. The ratio, $\frac{{f}_{{\mathrm{H}}_{2}{\mathrm{D}}_{2}}}{{f}_{\mathrm{HD}}}$, of the fractions is 1.23\ifmmode\pm\else\textpm\fi{}0.03.

A study of W+W− events accompanied by hard photon radiation, Eγ>2.5 GeV, produced in e+e− collisions at LEP is presented. Events consistent with being two on-shell W-bosons and an isolated photon are selected from 681 pb−1 of data recorded at 180 GeV<s<209 GeV. From the sample of 187 selected W+W−γ candidates with photon energies greater than 2.5 GeV, the W+W−γ cross-section is determined at five values of s. The results are consistent with the Standard Model expectation. Averaging over all energies, the ratio of the observed cross-section to the Standard Model expectation is R(data/SM)=0.99±0.09±0.04, where the errors represent the statistical and systematic uncertainties respectively. These data provide constraints on the related O(α) systematic uncertainties on the measurement of the W-boson mass at LEP. Finally, the data are used to derive 95% confidence level upper limits on possible anomalous contributions to the W+W−γγ and W+W−Z0γ vertices: −0.020 GeV−2<a0Λ2<0.020 GeV−2,−0.053 GeV−2<acΛ2<0.037 GeV−2,−0.16 GeV−2<anΛ2<0.15 GeV−2, where Λ represents the energy scale for new physics and a0, ac and an are dimensionless coupling constants.

The delayed annihilation time spectra (DATS) of antiprotonic helium atoms have been studied in isotopically pure low-temperature $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ gas at various densities. The DATS taken at 5.8 K and 400 mbar are very similar in shape except for (i) a small difference in the time scale and (ii) the presence of a distinct fast decay component in the case of $^{3}\mathrm{He}$. The ratio of overall trapping times (mean lifetimes against annihilation), R=${\mathit{T}}_{\mathrm{trap}}$${(}^{4}$He)/${\mathit{T}}_{\mathrm{trap}}$${(}^{3}$He), has been determined to be 1.144 \ifmmode\pm\else\textpm\fi{} 0.009, which is in good agreement with a theoretical estimate yielding R=[${\mathit{M}}^{\mathrm{*}}$(p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{}}^{4}$He)/ ${\mathit{M}}^{\mathrm{*}}$(p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{}}^{3}$He)${]}^{2}$=1.14, where ${\mathit{M}}^{\mathrm{*}}$ denotes the reduced mass of the p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{He}}^{2+}$system. The presence of a short-lived component with a lifetime of 0.154\ifmmode\pm\else\textpm\fi{}0.007 \ensuremath{\mu}s in the case of $^{3}\mathrm{He}$ suggests that the p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{}}^{3}$${\mathrm{He}}^{+}$atom has a state of intermediate lifetime on the border between a metastable zone and an Auger-dominated short-lived zone. The fraction of antiprotons trapped in metastable states at 5.8 K and 400 mbar is lower by 22.2(4)% for $^{3}\mathrm{than}$ for $^{4}\mathrm{He}$. All the data can be fitted fairly well with simple three-level and four-level cascade models. \textcopyright{} 1996 The American Physical Society.

.Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from hadronic Z0 decay events produced in e + e- annihilations. A subsample of these jets is identified which exhibits a large gap in the rapidity distribution of particles within the jet. After imposing the requirement of a rapidity gap, the gluon jet purity is 86%. These jets are observed to demonstrate a high degree of sensitivity to the presence of color reconnection, i.e. higher order QCD processes affecting the underlying color structure. We use our data to test three QCD models which include a simulation of color reconnection: one in the Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection models can describe our gluon jet measurements only if very large values are used for the cutoff parameters which serve to terminate the parton showers, and that the description of inclusive Z0 data is significantly degraded in this case. We conclude that color reconnection as implemented by these two models is disfavored. The signal from the Herwig color reconnection model is less clear and we do not obtain a definite conclusion concerning this model. In a separate study, we follow recent theoretical suggestions and search for glueball-like objects in the leading part of the gluon jets. No clear evidence is observed for these objects.

A search is performed for production of short-lived particles in e+e−→XY, with X→γγ and Y→ff̄, for scalar X and scalar or vector Y. Model-independent limits in the range of 25–60 femtobarns are presented on σ(e+e−→XY)×B(X→γγ)×B(Y→ff̄) for centre-of-mass energies in the range 205–207 GeV. The data from all LEP centre-of-mass energies 88–209 GeV are also interpreted in the context of fermiophobic Higgs boson models, for which a lower mass limit of 105.5 GeV is obtained for a “benchmark” fermiophobic Higgs boson.

A beam line for the transportation of slow antiprotons from a multiring electrode trap to an experimental chamber is described. The beam line is equipped with a three-stage differential pumping system in order to maintain a pressure lower than 1×10−12 Torr in the trap region while simultaneously having a pressure of around 1×10−6 Torr in the chamber. Tests have shown that 105 positive ions per trapping cycle were successfully extracted at 250 eV from the trap positioned in a superconducting solenoid. The ions were then further transported through three small apertures to the target area located 3.5 m downstream of the trap. Results from the first delivery of a 250 eV antiproton beam are described.

The production of short-lived isomeric states in 18F, 22Na, 40K, 85Rb and 92Mo was investigated by 14.7 MeV fast neutrons. A combined α-particle and pulsed-beam method was used to measure the time distribution of γ-rays produced in the de-excitation process. The spin cut-off factors were obtained from the experimental cross sections and other known values by means of the Huizenga-Vandenbosch method. Half-lives of the isomeric levels of the nuclei were also deduced.

We investigate the potential of short-baseline experiments in order to measure the dispersion relation of the (muon) neutrino, with a prospect of eventually measuring the neutrino mass. As a byproduct, the experiment would help to constrain parameters of Lorentz-violating effects in the neutrino sector. The potential of a high-flux laser-accelerated proton beam (e.g., at the upcoming ELI facility), incident on a thick target composed of a light element to produce pions, with a subsequent decay to muons and muon-neutrinos, is discussed. We find a possibility for a muon neutrino mass measurement of unprecedented accuracy.

We have employed a laser resonance method to study the interactions of individual states of metastable antiprotonic atomcules pHe+ with surrounding H2 molecules. We have found that the lifetimes of the (n, 1) = (37,34) and (39,35) states are shortened by small admixtures of H2 molecules in quite different ways; the observed quenching cross section for the upper (39,35) state in helium medium at 1 bar and 30 K is (2.4 ± 1.0) × 10−15 cm2, a factor of 24 larger than that for the lower (37,34) state.

We have studied the reaction p+p\ensuremath{\rightarrow}p+p+${\mathrm{\ensuremath{\pi}}}^{0}$ from 320 to 500 MeV by detecting the \ensuremath{\gamma} rays from ${\mathrm{\ensuremath{\pi}}}^{0}$ decay in coincidence in two large NaI crystals. The obtained differential and total cross sections of the pions are in agreement with previous measurements but are of a higher precision. We have also measured the pion analyzing powers; the first measurement of this observable. Our results are compared with those for the reactions np\ensuremath{\rightarrow}NN${\mathrm{\ensuremath{\pi}}}^{\ifmmode\pm\else\textpm\fi{}}$ with confusing conclusions concerning the contribution of ${\mathrm{\ensuremath{\sigma}}}_{01}$, the zero isospin pion production channel.

Elements of the spin density matrix for W bosons in e+e−→W+W−→qq̄′ℓνℓ events are measured from data recorded by the OPAL detector at LEP. This information is used to calculate polarised differential cross-sections and to search for CP-violating effects. Results are presented for W bosons produced in e+e− Collisions with centre-of-mass energies between 183 GeV and 209 GeV. The average fraction of W bosons that are longitudinally polarised is found to be (23.9±2.1±1.1)% compared to a Standard Model prediction of (23.9±0.1)%. All results are consistent with CP conservation.

Di-jet production is studied in collisions of quasi-real photons at e + e -centre-of-mass energies √ s ee from 189 to 209 GeV at LEP.The data were collected with the OPAL detector.Jets are reconstructed using an inclusive k ⊥ -clustering algorithm for all cross-section measurements presented.A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms.The inclusive di-jet cross-section is measured as a function of the mean transverse energy Ējet T of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, x γ , for different regions of Ējet T .Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space.Furthermore the inclusive di-jet cross-section as a function of |η jet | and |∆η jet | is presented, where η jet is the jet pseudo-rapidity.Different regions of the x + γ -x - γ -space are explored to study and control the influence of an underlying event.The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.

This bibliography on exotic atoms covers the years 1939 till 1982. The annual entries are headed by an introduction describing the state of affairs of the branch of science and listing the main applications in quantum electrodynamics, particle physics, nuclear physics, atomic physics, chemical physics and biological sciences. The bibliography includes an author index and a subject index.

A method is described by which muons are decelerated from MeV kinetic energies to energies below 50 keV inside a magnetic focusing device (cyclotron trap) and then extracted by an electric field into a magnetic guiding channel. Test measurements were performed which resulted in about 104/s extracted negative muons, in reasonable agreement with Monte-Carlo simulations. Using this method, muon beams in the energy range of 10–50 keV with intensities up to 105 μ−/s will be available in the near future.

We have established a technique for laser resonance spectroscopy of metastable antiprotonic helium atoms (pHe+) by successfully observing laser-induced annihilation. In this paper, we describe the full instrumentation of the experiment in detail, namely, the particle detectors, the laser system, the cryostat and the helium gas target, the data acquisition and analysis method. For an effective laser triggering selective to 3% metastable antiprotons, a highly efficient detection system for antiproton annihilation was required, with a suppressed inefficiency of less than a percent. Guided by simulations, we designed a system of seven lead-scintillator sandwich counters to detect the charged pions and π0-decay gamma rays from antiproton annihilation, and achieved 99.7±0.1% efficiency for the detection of annihilation. We employed two identical sets of excimer-pumped dye laser systems with 2–5 mJ output energy per pulse, which were triggered by every metastable antiproton formed in a helium gas target of 0.3–1.0 bar at 4.5–10 k. The resonant deexcitation of the metastable states was detected as a spike-like forced annihilation in the time spectrum, thus revealing the level structure of this exotic atom. This powerful technique enabled us to study also the lifetimes and populations of specific metastable states, by changing the timing and the wavelengths of the laser pulses.

A search for charged excited leptons decaying into a lepton and photon has been performed using approximately 680 pb-1 of e+e- collision data collected by the OPAL detector at LEP at centre-of-mass energies between 183 GeV and 209 GeV. No evidence for their existence was found. Upper limits on the product of the cross-section and the branching fraction are inferred. Using results from the search for singly produced excited leptons, upper limits on the ratio of the excited lepton coupling constant to the compositeness scale are calculated. From pair production searches, 95% confidence level lower limits on the masses of excited electrons, muons and taus are determined to be 103.2 GeV.

The transfer of negative pions from pionic hydrogen to deuterium has been investigated in gas mixtures of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ as a function of the ${\mathrm{D}}_{2}$ concentration (C). The concentration dependence of the transfer rate was fitted using a phenomenological model with two parameters. For C\ensuremath{\rightarrow}\ensuremath{\infty} (32\ifmmode\pm\else\textpm\fi{}3)% of the pions undergo transfer. The fitted parameters reflect the ratio of pion capture to pion transfer in collisions of pionic hydrogen with protons or deuterons. No pressure dependence for pion transfer was found.

A systematic analysis is presented of the possible elementary processes which determine the fate of negative pions stopped in hydrogen-containing samples with use of a phenomenological description in comparison with the available experimental information on pion capture by hydrogen in gas mixtures.

We present the first experimental results based on the jet boost algorithm, a technique to select unbiased samples of gluon jets in ${e}^{+}{e}^{\ensuremath{-}}$ annihilations, i.e. gluon jets free of biases introduced by event selection or jet finding criteria. Our results are derived from hadronic ${Z}^{0}$ decays observed with the OPAL detector at the LEP ${e}^{+}{e}^{\ensuremath{-}}$ collider at CERN. First, we test the boost algorithm through studies with HERWIG Monte Carlo events and find that it provides accurate measurements of the charged particle multiplicity distributions of unbiased gluon jets for jet energies larger than about 5 GeV, and of the jet particle energy spectra (fragmentation functions) for jet energies larger than about 14 GeV. Second, we apply the boost algorithm to our data to derive unbiased measurements of the gluon jet multiplicity distribution for energies between about 5 and 18 GeV, and of the gluon jet fragmentation function at 14 and 18 GeV. In conjunction with our earlier results at 40 GeV, we then test QCD calculations for the energy evolution of the distributions, specifically the mean and first two nontrivial normalized factorial moments of the multiplicity distribution, and the fragmentation function. The theoretical results are found to be in global agreement with the data, although the factorial moments are not well described for jet energies below about 14 GeV.

The hadronic structure function of the photon F2γ(x,Q2) is measured as a function of Bjorken x and of the photon virtuality Q2 using deep-inelastic scattering data taken by the OPAL detector at LEP at e+e− centre-of-mass energies from 183 to 209 GeV. Previous OPAL measurements of the x dependence of F2γ are extended to an average Q2 of 〈Q2〉=780 GeV2 using data in the kinematic range 0.15<x<0.98. The Q2 evolution of F2γ is studied for 12.1<〈Q2〉<780 GeV2 using three ranges of x. As predicted by QCD, the data show positive scaling violations in F2γ with F2γ(Q2)/α=(0.08±0.02+0.05−0.03)+(0.13±0.01+0.01−0.01)lnQ2, where Q2 is in GeV2, for the central x region 0.10–0.60. Several parameterisations of F2γ are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data.

The effects of the final state interaction phenomenon known as colour reconnection are investigated at centre-of-mass energies in the range $\sqrt s$ ≃ 189–209 using the OPAL detector at LEP. Colour reconnection is expected to affect observables based on charged particles in hadronic decays of . Measurements of inclusive charged particle multiplicities, and of their angular distribution with respect to the four jet axes of the events, are used to test models of colour reconnection. The data are found to exclude extreme scenarios of the Sjöstrand-Khoze Type I () model and are compatible with other models, both with and without colour reconnection effects. In the context of the model, the best agreement with data is obtained for a reconnection probability of 37%. Assuming no colour reconnection, the charged particle multiplicity in hadronically decaying W bosons is measured to be ${\left\langle {n^{{{\text{qq}}}}_{{{\text{ch}}}} } \right\rangle }$ =19.38±0.05(stat.)±0.08(syst.).

A systematic study of the decay rates of antiprotonic helium (${\overline{p}}^{4}{\mathrm{He}}^{+}$ and ${\overline{p}}^{3}{\mathrm{He}}^{+}$) at CERN Antiproton Decelerator has been made by a laser spectroscopic method. The decay rates of some of its short-lived states, namely those for which the Auger rates ${\ensuremath{\gamma}}_{A}$ are much larger than their radiative decay rates $({\ensuremath{\gamma}}_{\mathrm{rad}}\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{s}^{\ensuremath{-}1})$, were determined from the time distributions of the antiproton annihilation signals induced by laser beams, and the widths of the atomic resonance lines. The magnitude of the decay rates, especially their relation with the transition multipolarity, is discussed and compared with theoretical calculations.

From a total data sample of 701.1 pb-1 recorded with e+e- centre-of-mass energies of $\sqrt{s} =$ 161–209 GeV with the OPAL detector at LEP, 11693 W-pair candidate events are selected. These data are used to obtain measurements of the W-pair production cross sections at 10 different centre-of-mass energies. The ratio of the measured cross sections to the standard model expectation is found to be: ${\text{data}}/{{\text{SM}}} = 1.002\pm0.011 ({\text{stat.}}) \pm0.007 ({\text{syst.}}) \pm0.005 ({\text{theory}})$ , where the uncertainties are statistical, experimental systematics and theory systematics respectively. The data are used to determine the W boson branching fractions, which are found to be consistent with lepton universality of the charged current interaction. Assuming lepton universality, the branching ratio to hadrons is determined to be 67.41±0.37(stat.)±0.23(syst.)%, from which the CKM matrix element |Vcs| is determined to be 0.969±0.017(stat.)±0.012(syst.). The differential cross section as a function of the W- production angle is measured for the qqeν and qqμν final states. The results described in this paper are consistent with the expectations from the standard model.

A study of Bose–Einstein correlations in pairs of identically charged pions produced in e+e- annihilations at the Z0 peak has been performed for the first time assuming a non-static emitting source. The results are based on the high statistics data obtained with the OPAL detector at LEP. The correlation functions have been analyzed in intervals of the average pair transverse momentum and of the pair rapidity, in order to study possible correlations between the pion production points and their momenta (position–momentum correlations). The Yano–Koonin and the Bertsch–Pratt parameterizations have been fitted to the measured correlation functions to estimate the geometrical parameters of the source as well as the velocity of the source elements with respect to the overall centre-of-mass frame. The source rapidity is found to scale approximately with the pair rapidity, and both the longitudinal and transverse source dimensions are found to decrease for increasing average pair transverse momenta.

We report on the first experimental results for microwave spectroscopy of the hyperfine structure of p¯3He+. Due to the helium nuclear spin, p¯3He+ has a more complex hyperfine structure than p¯4He+, which has already been studied before. Thus a comparison between theoretical calculations and the experimental results will provide a more stringent test of the three-body quantum electrodynamics (QED) theory. Two out of four super-super-hyperfine (SSHF) transition lines of the (n,L)=(36,34) state were observed. The measured frequencies of the individual transitions are 11.12559(14) GHz and 11.15839(18) GHz, less than 1 MHz higher than the current theoretical values, but still within their estimated errors. Although the experimental uncertainty for the difference of these frequencies is still very large as compared to that of theory, its measured value agrees with theoretical calculations. This difference is crucial to be determined because it is proportional to the magnetic moment of the antiproton.

The resonance transition (n, l) = (40, 36) ? (41, 35) of the antiprotonic helium (He+) isotope at a wavelength of 1154.9?nm was detected by laser spectroscopy. The population of He+ occupying the resonance parent state (40, 36) was found to decay at a rate of 0.45 ? 0.04 ?s?1, which agreed with the theoretical radiative rate of this state. This implied that very few long-lived He+ are formed in the higher-lying states with principal quantum number n ? 41, in agreement with the results of previous experiments.

In this work, we describe the latest results for the measurements of the hyperfine structure of antiprotonic 3He. Two out of four measurable super–super-hyperfine (SSHF) transition lines of the (n, L) = (36, 34) state of antiprotonic 3He were observed. The measured frequencies of the individual transitions are 11.125 48(08) GHz and 11.157 93(13) GHz, with the increased precisions of about 43% and 25%, respectively, compared to our first measurements with antiprotonic 3He (Friedreich et al 2011 Phys. Lett. B 700 1–6). They are less than 0.5 MHz higher with respect to the most recent theoretical values, still within their estimated errors. Although the experimental uncertainty for the difference of 0.032 45(15) GHz between these frequencies is large as compared to that of theory, its measured value also agrees with theoretical calculations. The rates for collisions between antiprotonic helium and helium atoms have been assessed through comparison with simulations, resulting in an elastic collision rate of γe = 3.41 ± 0.62 MHz and an inelastic collision rate of γi = 0.51 ± 0.07 MHz.

We have studied the reaction p+p\ensuremath{\rightarrow}p+p+${\mathrm{\ensuremath{\pi}}}^{0}$, near threshold, at five energies between 320 and 500 MeV, by detecting the decay \ensuremath{\gamma} rays of the ${\mathrm{\ensuremath{\pi}}}^{0}$ in coincidence in two large NaI crystals. The ${\mathrm{\ensuremath{\pi}}}^{0}$ energy and angular distributions were measured from which we determine that the s-wave contribution to the total cross section is ${\mathrm{\ensuremath{\sigma}}}_{\mathit{T}\mathit{s}}^{\mathit{S}}$(\ensuremath{\mu}b)=(15.2\ifmmode\pm\else\textpm\fi{}3.0)${\mathrm{\ensuremath{\eta}}}_{0}^{2}$. This is somewhat lower than results of previous experiments.

Positron lifetime measurements were performed on eight different amino acids, τ1, τ2 and I2 values were observed at around 0.35 nsec, 0.7–1.2 nsec and 20–30%, respectively. The long-lived component seems to be produced by positronium states trapped in the crystalline lattice or by positronium atoms bound to molecules.

Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching effects caused by trace admixtures of H2 to the target helium gas. With this method of “H2-assisted inverse resonances” the decay rates of the states (n,l)=(39,l),l=36,37,38, and (38,l),l=35,36,37 of p̄He+ were determined as a function of the H2 admixture. The quenching cross sections at 30 K deduced therefrom for the states with n=39 were found to be of the order of the geometrical cross section for p̄He+–H2 collisons (2⋅10−15 cm2), with a moderate decrease with increasing l. Within a given cascade with constant v=n−l−1, the quenching cross sections for states with n=38 are smaller by a factor of 4–6 than those for states with n=39.

An analog detection system has been developed to measure delayed antiproton annihilation time spectra for laser resonance spectroscopy of metastable antiprotonic helium atoms using the high-intensity pulsed beam of antiprotons from LEAR at CERN.

The capture probability of stopped π− mesons by hydrogen atoms of ammonia increases with temperature in the liquid phase but in the supercritical phase it is temperature-independent. This can be attributed to the temperature breaking of hydrogen bonds. Rough estimates are given for the fraction of broken hydrogen bonds at various temperatures.

The τ−→μ−ν̄μντ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B(τ−→μ−ν̄μντ)=0.1734±0.0009(stat)±0.0006(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios gμ/ge=1.0005±0.0044 and gτ/ge=1.0031±0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η=0.004±0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, mH±>1.28tanβ, in the Minimal Supersymmetric Standard Model.

We present a determination of the LEP beam energy using “radiative return” fermion-pair events recorded at centre-of-mass energies from 183 to 209 GeV. We find no evidence of a disagreement between the OPAL data and the LEP Energy Working Group's standard calibration. Including the energy-averaged 11 MeV uncertainty in the standard determination, the beam energy we obtain from the OPAL data is higher than that obtained from the LEP calibration by 0±34(stat.)±27(syst.)MeV.

For the precise measurement of the hyperfine structure of antiprotonic helium, microwave radiation of 12.9 GHz frequency is needed, tunable over ±100MHz. A cylindrical microwave cavity is used whose front and rear faces are meshed to allow the antiprotons and laser beams to enter. The cavity is embedded in a cryogenic helium gas target. Frequency tuning of ∼300MHz with Q values of 2700–3000 was achieved using over-coupling and an external triple stub tuner. We also present Monte-Carlo simulations of the stopping distribution of antiprotons in the low-density helium gas using the GEANT4 package with modified energy loss routines.

A search for the Higgsstrahlung process e+e−→hZ is described, where the neutral Higgs boson h is assumed to decay into hadronic final states. In order to be sensitive to a broad range of models, the search is performed independent of the flavour content of the Higgs boson decay. The analysis is based on e+e− collision data collected by the OPAL detector at energies between 192 and 209 GeV. The search does not reveal any significant excess over the Standard Model background prediction. Results are combined with previous searches at energies around 91 and at 189 GeV. A limit is set on the product of the cross-section and the hadronic branching ratio of the Higgs boson, as a function of the Higgs boson mass. Assuming the hZ coupling predicted by the Standard Model, and a Higgs boson decaying only into hadronic final states, a lower bound of 104 GeV/c2 is set on the mass at the 95% confidence level.

Data from e+e- annihilation into hadrons at centre-of-mass energies between 91 GeV and 209 GeV collected with the OPAL detector at LEP, are used to study the four-jet rate as a function of the Durham algorithm resolution parameter ycut. The four-jet rate is compared to next-to-leading order calculations that include the resummation of large logarithms. The strong coupling measured from the four-jet rate is alphas(Mz0)= 0.1182+-0.0003(stat.)+-0.0015(exp.)+-0.0011(had.)+-0.0012(scale)+-0.0013(mass) in agreement with the world average. Next-to-leading order fits to the D-parameter and thrust minor event-shape observables are also performed for the first time. We find consistent results, but with significantly larger theoretical uncertainties.

Abstract Progress in the field of exotic atoms seems to increase proportionally with the number of exotic atoms produced and the increase in energy resolution with which the transition energies are determined. Modern experiments use high resolution crystal spectrometers or even aim at laser spectroscopy. The accuracy of these methods is limited by the interaction of the exotic atoms with their surroundings. The most important source of errors is the energy shift caused by the not well known status of the atomic electron shell. A novel method to eliminate these sources of error is presented and the possibilities for further high precision experiments is outlined.

This paper describes measurements of the hyperfine structure of two antiprotonic atoms that are planned at the Antiproton Decelerator (AD) at CERN. The first part deals with antiprotonic helium, a three-body system of α-particle, antiproton and electron that was previously studied at LEAR. A measurement will test existing three-body calculations and may— through comparison with these theories— determine the magnetic moment $$ \mu _{\overline {\text{p}} } $$ of the antiproton more precisely than currently available, thus providing a test of CPT invariance. The second system, antihydrogen, consisting of an antiproton and apositron, is planned to be produced at thermal energies at the AD. A measurement of the ground-state hyperfine splitting $$ v_{{\text{HF}}} \left( {\overline {\text{H}} } \right) $$ , which for hydrogen is one of the most accurately measuredp hysical quantities, will directly yielda precise value for $$ \mu _{\overline {\text{p}} } $$ , andalso compare the internal structure of proton andan tiproton through the contribution of the magnetic size of the $$ \overline {\text{p}} {\mathbf{ }}{\text{to}}{\mathbf{ }}\nu _{{\text{HF}}} \left( {\overline {\text{H}} } \right) $$ .

The production of charm quarks is studied in deep-inelastic electron–photon scattering using data recorded by the OPAL detector at LEP at nominal e+e− centre-of-mass energies from 183 to 209 GeV. The charm quarks have been identified by full reconstruction of charged D★ mesons using their decays into D0π with the D0 observed in two decay modes with charged particle final states, Kπ and Kπππ. The cross-section σD★ for production of charged D★ in the reaction e+e−→e+e−D★X is measured in a restricted kinematical region using two bins in Bjorken x, 0.0014<x<0.1 and 0.1<x<0.87. From σD★ the charm production cross-section σ(e+e−→e+e−cc̄X) and the charm structure function of the photon F2,cγ are determined in the region 0.0014<x<0.87 and 5<Q2<100 GeV2 . For x>0.1 the perturbative QCD calculation at next-to-leading order agrees perfectly with the measured cross-section. For x<0.1 the measured cross-section is 43.8±14.3±6.3±2.8 pb with a next-to-leading order prediction of 17.0+2.9−2.3 pb.

We observe Bose–Einstein correlations in π0 pairs using back-to-back two jet hadronic events from Z0 decays in the data sample collected by the OPAL detector at LEP 1 from 1991 to 1995. Using a static Gaussian picture for the pion emitter source, we obtain the chaoticity parameter λ=0.55±0.10±0.10 and the source radius R=(0.59±0.08±0.05) fm. According to the JETSET and HERWIG Monte Carlo models, the Bose–Einstein correlations in our data sample largely connect π0s originating from the decays of different hadrons. Prompt pions formed at string break-ups or cluster decays only form a small fraction of the sample.

A novel method of determining the mass of the W boson in the ${\rm W^+ W^-} \to \ell \nu \ell^\prime \nu^\prime$ channel is presented and applied to 667 pb-1 of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP. The measured energies of charged leptons and the results of a new procedure based on an approximate kinematic reconstruction of the events are combined to give: M_W = 80.41\pm 0.41\pm 0.13 GeV, where the first error is statistical and the second is systematic. The systematic error is dominated by the uncertainty on the lepton energy, which is calibrated using data, and the parameterization of the variables used in the fitting, which is obtained using Monte Carlo events. Both of these are limited by statistics.

A measurement of the forward–backward asymmetries of e+e−→bb̄ and e+e−→cc̄ events using electrons and muons produced in semileptonic decays of bottom and charm hadrons is presented. The outputs of two neural networks designed to identify b→ℓ− and c→ℓ+ decays are used in a maximum likelihood fit to a sample of events containing one or two identified leptons. The b and c quark forward–backward asymmetries at three centre-of-mass energies s and the average B mixing parameter χ̄ are determined simultaneously in the fit. Using all data collected by OPAL near the Z resonance, the asymmetries are measured to be: AFBbb̄=(4.7±1.8±0.1)%,AFBcc̄=(−6.8±2.5±0.9)%at〈s〉=89.51 GeV,AFBbb̄=(9.72±0.42±0.15)%,AFBcc̄=(5.68±0.54±0.39)%at〈s〉=91.25 GeV,AFBbb̄=(10.3±1.5±0.2)%,AFBcc̄=(14.6±2.0±0.8)%at〈s〉=92.95 GeV. For the average B mixing parameter, a value of: χ̄=(13.12±0.49±0.42)% is obtained. In each case the first uncertainty is statistical and the second systematic. These results are combined with other OPAL measurements of the b and c forward–backward asymmetries, and used to derive a value for the effective electroweak mixing angle for leptons sin2θeffℓ of 0.23238±0.00052.

The inclusive production of charged hadrons in the collisions of quasi-real photons (e+e−→e+e−+X) has been measured using the OPAL detector at LEP. The data were taken at e+e− centre-of-mass energies from 183 to 209GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant αs. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed.

Inclusive jet production (e+e- -> e+e- +jet+X) is studied in collisions of quasi-real photons radiated by the LEP beams at e+e- centre-of-mass energies sqrt see from 189 to 209 GeV. Jets are reconstructed using the kp jet algorithm. The inclusive differential cross-section is measured as a function of the jet transverse momentum, ptjet, in the range 5 <ptjet < 40 GeV for pseudo-rapidities, etaj, in the range -1.5 < etaj < 1.5. The results are compared to predictions of perturbative QCD in next-to-leading order in the strong coupling constant.

The PMTs of the CMS Hadron Forward calorimeter were found to generate a large size signal when their windows were traversed by energetic charged particles. This signal, which is due to Čerenkov light production at the PMT window, could interfere with the calorimeter signal and mislead the measurements. In order to find a viable solution to this problem, the response of four different types of PMTs to muons traversing their windows at different orientations is measured at the H2 beam-line at CERN. Certain kinds of PMTs with thinner windows show significantly lower response to direct muon incidence. For the four anode PMT, a simple and powerful algorithm to identify such events and recover the PMT signal using the signals of the quadrants without window hits is also presented. For the measurement of PMT responses to Čerenkov light, the Hadron Forward calorimeter signal was mimicked by two different setups in electron beams and the PMT performances were compared with each other. Superior performance of particular PMTs was observed.

The branching ratio for the \ensuremath{\Lambda} weak radiative decay has been measured to be B(\ensuremath{\Lambda}\ensuremath{\rightarrow}n+\ensuremath{\gamma})/(\ensuremath{\Lambda}\ensuremath{\rightarrow}anything) =[1.78\ifmmode\pm\else\textpm\fi{}0.24(stat)${\ifmmode\pm\else\textpm\fi{}}_{0.16}^{0.14}$(syst)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$. A low-energy kaon beam was used to produce the \ensuremath{\Lambda} hyperons via the reaction ${\mathit{K}}^{\mathrm{\ensuremath{-}}}$+p\ensuremath{\rightarrow}\ensuremath{\Lambda}+${\mathrm{\ensuremath{\pi}}}^{0}$ at rest. Photons from the signal channel and ${\mathrm{\ensuremath{\pi}}}^{0}$ decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.