A. Placci

We report the results of two searches made on data recorded at the CERN SPS Proton-Antiproton Collider: one for isolated large-ET electrons, the other for large-ET neutrinos using the technique of missing transverse energy. Both searches converge to the same events, which have the signature of a two-body decay of a particle of mass ∼80 GeV/c2. The topology as well as the number of events fits well the hypothesis that they are produced by the proces p̄+p→W±+X, with W± →e±+ν; where W± is the Intermediate Vector Boson postulated by the unified theory of weak and electromagnetic inter- actions.

We report the observation of four electron-positron pairs and one muon pair which have the signature of a two-body decay of a particle of mass ∼ 95 GeV/c2. These events fit well the hypothesis that they are produced by the process p̄+ p→Z0+X(with Z0→ ℓ++ ℓ−), where Z0 is the Intermediate Vector Boson postulated by the electroweak theories as the mediator of weak neutral currents.

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.

We have measured transverse momentum spectra up to 10 GeV/c for charged particles produced centrally in proton-antiproton collisions at 540 GeV in the centre of mass at the CERN collider. Our results are compared with data at ISR energies and with the predictions of a QCD model. The charged particle spectrum shows a clear dependence on charged track multiplicity.

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.

The Small Area Tracking system of the COMPASS experiment at CERN includes a set of 20 large area, fast position-sensitive Gas Electron Multiplier detectors, designed to reliably operate in the harsh radiation environment of the experiment. We describe in detail the design, choice of materials, assembly procedures and quality controls used to manufacture the devices. The test procedure in the laboratory, the performance in test beams and in the initial commissioning phase in the experiment are presented and discussed.

We report the observation of five events in which a missing transverse energy larger than 40 GeV is associated with a narrow hadronic jet and of two similar events with a neutral electromagnetic cluster (either one or more closely spaced photons). We cannot find an explanation for such events in terms of backgrounds or within the expectations of the Standard Model.

Invariant cross-sections are presented for the inclusive reaction p + p → πo + anything, Measurements of large transverse momentum πo's (2.5 GeV/c<p⊥<9 GeV/c) were made near 90° at the CERN ISR at five centre-of-mass energies (√s = 23.5, 30.6, 44.8, 52.7 and 62.4 GeV. At large p⊥, the invariant cross-sections are seem to vary with s and p⊥, in good agreement with a fit of the form Ap⊥−n F(p⊥/√s), with n≈8 and F(p⊥/√s)≈exp(−26p⊥/√s).

A clear signal is observed for the production of an isolated large-transverse-momentum lepton in association with two or three centrally produced jets. The two-jet events cluster around the W± mass, indicating a novel decay of the Intermediate Vector Boson. The rate and features of these events are not consistent with expectations of known quark decays (charm, bottom). They are, however, in agreement with the process W→ tb followed by t → bℓv, where t is the sixth quark (top) of the weak Cabibbo current. If this is indeed so, the bounds on the mass of the top quark are 30 GeV/c2 < mt <550 GeV/c2.

A sample of 52 Intermediate Vector Boson decays in the (vee) channel is described. They were produced at the CERN SPS Collider for an integrated luminosity of 0.136 pb−1. Both production and decay properties fit well with expectations from the Standard Model of weak interactions. An improved value for the W mass is given and compared with the previously published value for the Z0 mass.

With a segmented total absorption calorimeter of large acceptance, we have measured the total transverse energy spectrum for pp̄ collisions at s12 = 540 GeV up to ΣET= 130 GeV in the pseudo-rapidity range |η|< 1.5. Using two different algorithms, we have looked for localized depositions of transverse energy (jets). For ΣET > 40 GeV, the fraction of events with two jets increases with ΣET; this event structure is dominant for ΣET > 100 GeV. We measure the inclusive jet cross section up to ET(jet) = 60 GeV and the two-jets mass distribution to 120 GeV/c2. The measured cross sections are compatible with the predictions of hard scattering models based on QCD.

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.

We present a detailed study of hadronic jets obtained in a data sample taken in the UA1 detector with a localized transverse energy trigger. We discuss the average shape of jets in terms of energy and charged particle content, and compare this to data generated in Monte Carlo programs. We further extend the previously reported inclusive jet cross section to the region of ET = 100 GeV. A comparison with theoretical models of cross sections for events with more than two jets is also given.

The two-jet cross section measured in the UA1 apparatus at the CERN pp Collider has been analysed in terms of the centre-of-mass scattering angle θ and the scaled longitudinal parton momenta x1 and x2. The angular distribution dσ/d cos σ rises rapidly as cos → 1, independent of x2 and x2, as expected in vector gluon theories (QCD). The differential cross section in x1 and x2 is consistent with factorization and provides a measurement of the proton structure function F(x) = G(x) + 49[Q(x) + Q(x)] at values of the four-momentum transfer squared, -t̂ ≈ 2000 GeV2. Over the range x = 0.10−0.80 the structure function shows an exponential x dependence and may be parametrized by the form F(x) = 6.2 exp (−9.5x).

Track information from the central detector in the UA1 experiment at the CERN proton-antiproton collider (√s = 540 GeV) is used to determine the charged particle multiplicity distribution for the pseudorapidity range ∣η∣ < 1.3. The mean value for this multiplicity per unit of η is 3.9 ± 0.3 for events having at least one track in this range. Data from the central electromagnetic and hadron calorimeters are used to examine the correlation between the transverse energy measured in the collisions and the multiplicity in the central region.

Proton-antiproton elastic scattering at CM energy 540 GeV has been studied in the t-range 0.04 < −t < 0.45 GeV2. The data are well fitted by the form exp (bt) with b = 17.1 ± 1.0 GeV−2 for |t| = 0.04 − 0.18 GeVsu2 and b = 13.7 ± 0.2 ± 0.2 GeV−2 for |t| = 0.21−0.45 GeV2. A luminosity measurement combined with the optical theorem gives σtot = 67.6 ± 5.9 ± 2.7 mb and σe1/σtot = 0.209 ± 0.018 ± 0.008.

An experiment to measure the muon nuclear capture rate in ultrapure gaseous hydrogen (8 atm, 293\ifmmode^\circ\else\textdegree\fi{}K) has been performed using a special target in which a system of gas proportional counters, working with the pure hydrogen of the target itself, were operating. Neutrons from the capture reactions were detected using a scintillation-counter technique, and the $\ensuremath{\gamma}$-ray background was eliminated by pulse-shape discrimination. The working conditions ensured that the captures were taking place in $\ensuremath{\mu}p$ atomic systems in a singlet total-spin state. The experimental result is ${\ensuremath{\Lambda}}_{\mathrm{expt}}=651\ifmmode\pm\else\textpm\fi{}57$ ${\mathrm{sec}}^{\ensuremath{-}1}$, which has to be compared with the theoretical rate ${\ensuremath{\Lambda}}_{s, \mathrm{theor}}=626\ifmmode\pm\else\textpm\fi{}26$ ${\mathrm{sec}}^{\ensuremath{-}1}$. From the experimental capture rate, and within the framework of the currently accepted theory, we have obtained for the induced pseudoscalar coupling constant ${g}_{p}=(\ensuremath{-}7.3\ifmmode\pm\else\textpm\fi{}3.7){g}_{V}$. The results of the present experiment are analyzed, together with results obtained from stopping negative muons in liquid hydrogen.

Results from the ντ appearance search in a neutrino beam using the full NOMAD data sample are reported. A new analysis unifies all the hadronic τ decays, significantly improving the overall sensitivity of the experiment to oscillations. The “blind analysis” of all topologies yields no evidence for an oscillation signal. In the two-family oscillation scenario, this sets a 90% CL allowed region in the sin22θμτ–Δm2 plane which includes sin22θμτ<3.3×10−4 at large Δm2 and Δm2< 0.7 eV2/c4 at sin22θμτ=1. The corresponding contour in the νe→ντ oscillation hypothesis results in sin22θeτ<1.5×10−2 at large Δm2 and Δm2<5.9 eV2/c4 at sin22θeτ=1. We also derive limits on effective couplings of the τ lepton to νμ or νe.

We describe the development and operation of the Gas Electron Multiplier (GEM), a thin insulating foil metal-clad on both sides and perforated by a regular pattern of small holes. The mesh can be incorporated into the gas volume of an active detector to provide a first amplification channel for electrons, or used as stand alone. We report on the basic properties of GEMs manufactured with different geometries and operated in several gas mixtures as well as on their long-term stability after accumulation of charge equivalent to several years of operation in high-luminosity experiments. Optimized GEMs reach gains close to 10 000 at safe operating voltages, permitting the detection of ionizing tracks, without other amplifying elements, on a simple Printed Circuit Board (PCB), opening new possibilities for detector design.

Results on charged particle production in pp̄ collision at s12 = 540 GeV are presented. The data were obtained at the CERN pp̄ collider using the UA1 detector, operated without magnetic field. The central particle density is 3.3 + - 0.2 per unit o pseudo-rapidity for non-diffractive events. KNO scaling of the multiplicity distributions withresults from ISR energies is observed.

We summarize here the results of the TARC experiment whose main purpose is to demonstrate the possibility of using Adiabatic Resonance Crossing (ARC) to destroy efficiently Long-Lived Fission Fragments (LLFFs) in accelerator-driven systems and to validate a new simulation developed in the framework of the Energy Amplifier programme. An experimental set-up was installed in a CERN PS proton beam line to study how neutrons produced by spallation at relatively high energy (En⩾1MeV) slow down quasi-adiabatically with almost flat isolethargic energy distribution and reach the capture resonance energy of an element to be transmuted where they will have a high probability of being captured. Precision measurements of energy and space distributions of spallation neutrons (using 2.5 and 3.5 GeV/c protons) slowing down in a 3.3 m×3.3 m×3 m lead volume and of neutron capture rates on LLFFs 99Tc, 129I, and several other elements were performed. An appropriate formalism and appropriate computational tools necessary for the analysis and understanding of the data were developed and validated in detail. Our direct experimental observation of ARC demonstrates the possibility to destroy, in a parasitic mode, outside the Energy Amplifier core, large amounts of 99Tc or 129I at a rate exceeding the production rate, thereby making it practical to reduce correspondingly the existing stockpile of LLFFs. In addition, TARC opens up new possibilities for radioactive isotope production as an alternative to nuclear reactors, in particular for medical applications, as well as new possibilities for neutron research and industrial applications.

Muons of high transverse momentum pμT have been observed in the large drift chambers surrounding the UA1 detector at the CERN 540 GeV pp̄ collider. For an integrated luminosity of 108 nb−1, 14 isolated muons have been found with pT > 15 GeV/c. They are correlated with a large imbalance in total transverse energy, and show a kinematic behaviour consistent with the muonic decay of the Intermediate Vector Boson W± of weak interactions. The partial cross section is in agreement with previous measurements for electronic decays and with muon-electron universality. The W mass is determined to be mW = 81+6−7 GeV/c2.

Results of an experiment to measure the 2P3/2−2S1/2 energy difference δ1 in the muonic (μ−4H e)+ ion are presented; we found δ1exp = 1.5274 ± 0.0009 eV. Comparison with QED predictions shows that there is agreement between theory and experiment within 0.0087 eV; this limitation is mainly due to the uncertainty with which the r.m.s. charge radius of the 4He nucleus is known.

The construction of a large (25 m3, 23 000 wires) drift chamber assembly with image readout to be used at the CERN p-p collider facility is reported. In this contribution the emphasis is mainly directed towards the mechanical and electrostatic design and on the implementation of the various control procedures.

The observation of an apparent excess of radiative Z0 decays into e+e−γ and μ+μ−γ has prompted the search for massive eνγ and μνγ final states containing an energetic photon. No events were found other than those consistent with QED radiative effects in leptonic W decays. The data sample corresponds to an integrated luminosity of 0.136 pb−1 produced at the CERN super proton synchrotron (SPS) collider. An upper limit on the occurrence of such events is given.

The electronics systems for the readout of a large drift chamber (25 m3, 6110 sense wires) with image readout, to be used at the CERN p-p collider, is described. The system uses a flash analog-to-digital converter and is able to measure directly the drift time, the charge division, and the energy losses for many tracks on each wire. The results obtained with chamber and electronics prototypes are reported.

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.

We describe recent developments of the gas electron multiplier (GEM), a thin composite mesh acting as proportional avalanche amplifier in gas counters. In beam tests we have verified the excellent efficiency, time resolution and localization accuracy for a GEM with micro-strip read-out. Efficiency, localization accuracy and operation in strong magnetic fields has been verified; operation at rates above 10/sup 6/ Hz/mm/sup 2/ and lifetimes corresponding to at least 10 mC/cm of collected charge have been demonstrated. Refinements in the manufacturing technology have permitted the realization of large size detectors (27 by 25 cm/sup 2/), to be used in conjunction with microstrip gas chambers. With an improved design, stable gains above two thousand have been reached (GEM2000); larger gains can be obtained increasing the thickness of the foils, cascading two GEMs at some distance or in electrical contact. Further developments of the technology and prospective applications are discussed.

The method developed for the calculation of the flux and composition of the West Area Neutrino Beam used by NOMAD in its search for neutrino oscillations is described. The calculation is based on particle production rates computed using a recent version of FLUKA and modified to take into account the cross-sections measured by the SPY and NA20 experiments. These particles are propagated through the beam line taking into account the material and magnetic fields they traverse. The neutrinos produced through their decays are tracked to the NOMAD detector. The fluxes of the four neutrino flavours at NOMAD are predicted with an uncertainty of about 8% for νμ and νe, 10% for ν̄μ, and 12% for ν̄e. The energy-dependent uncertainty achieved on the νe/νμ prediction needed for a νμ→νe oscillation search ranges from 4% to 7%, whereas the overall normalization uncertainty on this ratio is 4.2%.

We report the observation of five muonic Z0 decays. The mass and cross section times branching ratio is consistent with the previous measurements of Z0→e+e−. Three of the muonic decays have unexpected features. One event is of the type Z0→μ+μ−γ. Two of the Z0→μ+μ− decays are accommpanied by several (>4) energetic (ET > 10 GeV) jets which are difficult to explain within the framework of standard QCD corrections.

A search for events having the characteristics of cosmic ray Centauros has been made in 540 GeV centre of mass proton-antiproton collisions, using information on charged particle multiplicities and transverse momenta from our central detector image chamber, together with energy deposition in our calorimeters. No such events were found in 48 000 low bias events.

We report the results of an experimental investigation on the properties of the (μ 4He) 2S + metastable muonic system, which was performed stopping negative muons in a pure helium target at pressures ranging from 10 to 50 atm, and observing the total yield and the differential time distribution of the X-rays released in delayed coincidence with the arrival of muons. At each pressureP, information was obtained on the following quantities:a) the total disappearance rate λ2S,tot(P) of the (μ 4He) 2S + system,b) the disappearance rates λA(P) and λSt(itP) of the (μ 4He) 2S + system for external Auger effect and Stark-mixing collisions,c) the fractionε 2S of muons stopped in helium which form the (μ 4He) 2S + system. The results obtained on λ2S,tot(P) are in agreement with the results of a cascade calculation, which was performed requiring consistency with previous experimental results; the present measurements show that the variation ofε 2S (P) with pressure in the range (7÷50) atm is small. Good agreement was found between the values for the two-quantum decay rate λ2X of the (μ 4He) 2S + system derived from the present data and the theoretical predictions. The results obtained on λ2S,tot(P), instead, are somewhat in conflict with the results of a recent calculation on λ2St(P). The possible reasons of such disagreement are discussed, referring to possible clustering effects of the helium atoms around the (μHe+ ion.

An apparatus for the detection of high-energy electrons and photons is described. Lead-glass Cherenkov counters measure the energies, and charged particle tracks are reconstructed using an array of wire spark chambers. The mode of operation, calibration, and stability of the system are described, and details are given of the ability to discriminate electrons from hadrons.

Using a proportional gas counter, de-excitation muonic X-rays from a μp atom, formed by stopping negative muons in a hydrogen gas target, have been detected.

The NOMAD Collaboration presents a study of opposite sign dimuon events in the framework of Leading Order QCD. A total of 2714 neutrino- and 115 antineutrino-induced opposite sign dimuon events with Eμ1,Eμ2>4.5 GeV, 15<Eν<300 GeV and Q2>1(GeV/c)2 are observed in the Front-Calorimeter of NOMAD during the 1995 and 1996 runs. The analysis yields a value for the charm quark mass of mc=1.3+0.3+0.3−0.3−0.3GeV/c2 and for the average semileptonic branching ratio of Bc=0.095+0.007+0.014−0.007−0.013. The ratio of the strange to non-strange sea in the nucleon is measured to be κ=0.48+0.09+0.17−0.07−0.12. The measured rate of charm-induced dimuon relative to single muon, as a function of neutrino energy, is consistent with the slow rescaling hypothesis of heavy quark production.

We present a detailed description of the drift chambers used as an active target and a tracking device in the NOMAD experiment at CERN. The main characteristics of these chambers are a large area, a self supporting structure made of light composite materials and a low cost. A spatial resolution of 150 microns has been achieved with a single hit efficiency of 97%.

One of the three x-ray detectors of the CERN Axion Solar Telescope (CAST) experiment searching for solar axions is a time projection chamber (TPC) with a multi-wire proportional counter (MWPC) as a readout structure. Its design has been optimized to provide high sensitivity to the detection of the low intensity x-ray signal expected in the CAST experiment. A low hardware threshold of 0.8 keV is set to a safe level during normal data taking periods, and the overall efficiency for the detection of photons coming from conversion of solar axions is 62%. Shielding has been installed around the detector, lowering the background level to 4.10 × 10−5 counts cm−2 s−1 keV−1 between 1 and 10 keV. During phase I of the CAST experiment the TPC has provided robust and stable operation, thus contributing with a competitive result to the overall CAST limit on axion–photon coupling and mass.

We have measured the nuclear capture rate of negative muons by deuterons in $\ensuremath{\mu}d$ atoms. The experiment was performed by stopping negative muons in a target of ultrapure gaseous hydrogen at 7.6 atm with 5.2% deuterium and looking at the outgoing neutrons. The result is ${\ensuremath{\Lambda}}_{\mathrm{expt}}=451\ifmmode\pm\else\textpm\fi{}70$ ${\mathrm{sec}}^{\ensuremath{-}1}$.

Energy and space distributions of spallation neutrons (from 2.5 and 3.57 GeV/c CERN proton beams) slowing down in a 3.3 × 3.3 × 3 m3 lead volume and neutron capture rates on long-lived fission fragments 99Tc and 129I demonstrate that Adiabatic Resonance Crossing (ARC) can be used to eliminate efficiently such nuclear waste and validate innovative simulation.

The small-area tracker of COMPASS, a high-luminosity fixed target experiment at CERN's SPS, includes a set of 20 large-size (31 /spl times/ 31 cm/sup 2/) gas electron multiplier (GEM) detectors. Based on gas amplification in three cascaded GEM foils, these devices permit to obtain high gain and good spatial resolution even at very high particle fluxes. A two-coordinate projective readout yields, for each track, highly correlated signal amplitudes on both projections, allowing to resolve multiple hits in high occupancy regions close to the central deactivated area of 5 cm diameter. At the same time, the material exposed to the beam is minimized. Splitting the amplification in three cascaded stages permits to achieve a gain of /spl sim/ 8000, necessary for efficient (> 98%) detection of minimum ionizing particles on both coordinates, already at relatively moderate voltages across individual GEM foils. As a consequence, the probability of a gas discharge to occur when a heavily ionizing particle enters the detector volume, is reduced by more than one order of magnitude at a given gain compared to the initially foreseen double GEM structure. In conjunction with other strategies resulting from extensive R&D on discharge phenomena, we were able to further reduce both the energy and the probability of such breakdowns. In order to completely exclude permanent damage to the front-end chip by the rare event of a discharge fully propagating to the readout strips, an external electronic protection circuit is used. The operational characteristics of these detectors were examined both in the laboratory and in the beam, where a spatial resolution for minimum ionizing particles of (46 /spl plusmn/ 3) /spl mu/m and a time resolution of /spl sim/ 15 ns were achieved. For the 2001 run of COMPASS, a total of 14 triple GEM detectors have been installed. First results from the commissioning phase in the high-intensity /spl mu/ beam are presented.

Inclusive fragmentation of jets into charged particles has been studied in the UA1 experiment at the CERN Super Proton Synchrotron (SPS) pp̄ Collider at s = 540 GeV for jets having a transverse energy above 30 GeV. The observed fragmentation function is very similar to that seen in e+e−jets and is therefore indicative of similar hadronization processes for gluons and quarks. Scaling with the energy of the jet is valid to a good approximation, although slight deviations are suggested by the data. The average transverse momentum with respect to the jet axis for jets with ET > 30 GeV is 600 MeV/c for particles having z > 0.1 and increases slowly with the jet energy.

The ratio $B$ between the transfer rates ${\ensuremath{\lambda}}_{\ensuremath{\mu}p,\mathrm{Xe}}$ and ${\ensuremath{\lambda}}_{\ensuremath{\mu}d,\mathrm{Xe}}$ of muons from $\ensuremath{\mu}p$ and $\ensuremath{\mu}d$ muonic atoms to xenon has been directly measured by stopping negative muons in a gaseous target containing deuterated hydrogen and small xenon admixtures at a total pressure of 6 atm abs. and at 293 \ifmmode^\circ\else\textdegree\fi{}K. The results were obtained by analyzing the differential time distribution of the decay electrons coming from muons stopped within the gaseous mixture. In this way one gets $B=1.98\ifmmode\pm\else\textpm\fi{}0.12$, which supports the dependence of the transfer rates on the mass of the primary muonic atom within 6%. More precise values for ${\ensuremath{\lambda}}_{\ensuremath{\mu}p,\mathrm{Xe}}$ and ${\ensuremath{\lambda}}_{\ensuremath{\mu}d,\mathrm{Xe}}$ are also given, i.e., ${\ensuremath{\lambda}}_{\ensuremath{\mu}p,\mathrm{Xe}}=(4.53\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{11}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ and ${\ensuremath{\lambda}}_{\ensuremath{\mu}d,\mathrm{Xe}}=(2.30\ifmmode\pm\else\textpm\fi{}0.17)\ifmmode\times\else\texttimes\fi{}{10}^{11}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. A lower limit for the scattering cross section $\ensuremath{\sigma}$ of $\ensuremath{\mu}d$ atoms against xenon is obtained, i.e., $\ensuremath{\sigma}\ensuremath{\ge}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$.

We report the results of an experiment performed to measure the muon nuclear capture rate by free deuterons. The muons were slowed down in ultrapure gaseous hydrogen at 7.6 atm and 293 \ifmmode^\circ\else\textdegree\fi{}K, containing 5% of deuterium. A special target was used, in which a system of gas proportional counters, working with the (${\mathrm{H}}_{2}$ + ${\mathrm{D}}_{2}$) gaseous mixture itself, was operating. Neutrons from the capture reactions were detected using liquid scintillation counters, and the $\ensuremath{\gamma}$-ray background was eliminated by pulse-shape discrimination. The experimental result is ${\ensuremath{\Lambda}}_{\mathrm{exp}}=(445\ifmmode\pm\else\textpm\fi{}60)$ ${\mathrm{sec}}^{\ensuremath{-}1}$, which is consistent with muon-electron universality and with the assumption that the nuclear capture proceeds from the doublet spin state of the $\ensuremath{\mu}d$ muonic atoms. Combining the present experimental value with a previous result obtained with a liquid-hydrogen deuterated target, one obtains a ratio between the axial-vector and vector coupling constants given by $\frac{{g}_{A,\ensuremath{\mu}}}{{g}_{V,\ensuremath{\mu}}}=\ensuremath{-}1.35\ifmmode\pm\else\textpm\fi{}0.1$.

We present a measurement of the polarization of Λ̄ hyperons produced in νμ charged current interactions. The full data sample from the NOMAD experiment has been analyzed using the same V0 identification procedure and analysis method reported in a previous paper [NOMAD Collaboration, Nucl. Phys. B 588 (2000) 3] for the case of Λ hyperons. The Λ̄ polarization has been measured for the first time in a neutrino experiment. The polarization vector is found to be compatible with zero.

A study of strange particle production in νμ charged current interactions has been performed using the data from the NOMAD experiment. Yields of neutral strange particles (K0s,Λ,Λ̄) have been measured. Mean multiplicities are reported as a function of the event kinematic variables Eν, W2 and Q2 as well as of the variables describing particle behaviour within a hadronic jet: xF, z and pT2. Decays of resonances and heavy hyperons with identified K0s and Λ in the final state have been analyzed. Clear signals corresponding to K★±, Σ★±, Ξ− and Σ0 have been observed.

We report evidence for the production of the charged D∗ mesons in pp̄ collisions at s = 540 GeV. The search was confined to the charged particle fragments of hadronic jets, which are expected to be predominantly gluon jets in this experiment. The fragmentation function and production rate for D∗ in jets of average transverse momentum of 28 GeV/c are given.

3roton-antiproton elastic scattering at cm energy 540 GeV has been studied in the t range 0.14 ⩽ −t ⩽ 0.26 GeV2. The data is well fitted by an exponential form exp(bt) with b = 13.3 ± 1.5 GeV−2.

In events at centre of mass energy 540 GeV from the CERN pp collider, we have found clear evidence for correlations in rapidity and azimuthal angle between high transverse momentum charged particles. These correlations increase with transverse momentum and are much stronger than the general two-particle correlations in minimum bias events. By analogy to ISR results, a qualitative interpretation in terms of hard scattering and fragmentation of partons is discussed.

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.

A system of NE 213 liquid scintillator neutron counters working with a straightforward neutron-gamma discrimination technique and a digitized data output is described. The apparatus was built up using only standard electronics, and has been working at CERN in recent muon capture experiments. The performances of the neutron counters are reported with particular reference to the neutron-gamma discrimination features and to the energy resolution. The calculation of the efficiency of the described system is also discussed.

Pionic X-rays have been observed when negative pions are stopped in a hydrogen gas target. The intensity of the K lines has been found to be about 0.12 X-ray/pion stopped; moreover we found Kα/K = 0.53 ± 0.04.

A transition radiation detector to identify electrons at 90% efficiency with a rejection factor against pions of 103 on an area of 2.85 × 2.85 m2 has been constructed for the NOMAD experiment. Each of its 9 modules includes a 315 plastic foil radiator and a detector plane of 176 vertical straw tubes filled with a xenon-methane gas mixture. Details of the design, construction and operation of the detector are given.

In the R&D programme for the ICARUS experiment, we have developed a practical procedure to purify liquid argon in the liquid phase. Extreme purity is obtained, corresponding to an electronegative impurity concentration below 0.1 ppb of 02 equivalent. This corresponds to an electron lifetime in the range of several milliseconds equivalent to attenuation length of a few metres. The procedure has been tested up to flows of the order of 1000 litres of liquid per hour. The new technique makes it possible to operate very large amounts of ultrapure liquid argon as foreseen for the multikiloton ICARUS experiment at Gran Sasso.

We report on measurements of correlated $$b\bar b$$ production in $$p\bar p$$ collisions at $$\sqrt s = 630GeV$$ , using dimuon data to tag both theb and $$\bar b$$ quarks. Starting from an inclusive dimuon sample we obtain improved cross-sections for single inclusive beauty production and confirm our earlier results on $$B^0 - \bar B^0$$ mixing. From a study of $$b\bar b$$ correlations we derive explicit cross-sections for semi-differential $$b\bar b$$ production. We compare the measured cross-sections and correlations to $$\mathcal{O}\left( {\alpha _s^3 } \right)$$ QCD predictions and find good quantitative agreement. From the measured angular distributions we establish a size-able contribution from higher order QCD processes with a significance of about seven standard deviations. A large nonperturbative contribution to these higher order corrections is excluded.

We report the observation of 21 dimuon events at the CERN pp Collider with the UA1 detector. The events range in invariant dimuon mass from 2 to 22 GeV / c2. The properties of these events are given. The bulk of the events are consistent with heavy-flavour production (mainly bb) with a few candidates for Drell-Yan production. There remain a few events which are difficult to interpret in terms of these processes, in particular two events with isolated, like-sign muons.

All hadron beams of the CERN SPS are being equipped with a mass identification facility. The project involves the construction of twelve differential Cerenkov counters specialized either for the energy range of 15 GeV/c to 150 GeV/c (CEDAR-W) or 30 GeV/c to 340 GeV/c (CEDAR-N). The performances of four W-type and one N-type CEDAR are reported.

Abstract We report here on a semi-automatic system for the control and measurement of the wire tension in a large detector (23 000 wires) to be used by the UA1 Collaboration in an experiment at the CERN p p collider. The method consists in feeding an alternating current of variable frequency to the wire in the presence of a small magnetic field. The wire resonance frequency, which is simply related to the mechanical tension, is found by requiring a 90° phase shift between the driving current and the oscillation of the wire. The control procedure is computer controlled and can be applied to any large system of wire chambers where a precise (

The Transmutation by Adiabatic Resonance Crossing (TARC) experiment was carried out as PS211 at the CERN PS from 1996 to 1999. Energy and space distributions of spallation neutrons (produced by 2.5 and 3.57 GeV/c CERN proton beams) slowing down in a 3.3×3.3×3 m3 lead volume and neutron capture rates on long-lived fission fragments 99Tc and 129I demonstrate that Adiabatic Resonance Crossing (ARC) can be used to eliminate efficiently such nuclear waste and validate innovative simulation.

The Central Detector of the UA1 experiment at the CERN proton-antiproton Collider underwent a first physics run at the end of 1981. The detector consists of a large drift chamber assembly (25 m3, ~ 6000 sense wires). An electronics readout with multi-hit capability simultaneously digitizes the time and the analog information used for charge division and energy measurement. The initial performance of the readout and control system will also be presented. The detector was tested in two cosmic-ray runs, and is now fully operational for the second physics run; this started at the beginning of October 1982.

A search has been made for electron pairs of large invariant mass produced in high-energy proton collisions at the CERN ISR. Data are presented for a centre-of-mass energy (√s) of 52.7 GeV. All electron pair candidates are believed to be consistent with background. Upper limits are obtained for the existence of massive objects decaying into electron pairs.

ŽWe carried out a model-independent search for light scalar or pseudoscalar particles a's an example of which is the .axion that couple to two photons by using a photon-regeneration method at high energies allowing a substantial increase in the sensitivity to eV masses.The experimental set-up is based on elements of the CERN West Area Neutrino Facility Ž .WANF beam line and the NOMAD neutrino detector.The new particles, if they exist, could be produced through the Primakoff effect in interactions of high energy photons, generated by the 450 GeV protons in the CERN SPS neutrino target, with virtual photons from the WANF horn magnetic field.The particles would penetrate the downstream shielding and would be observed in the NOMAD neutrino detector through their re-conversion into real high energy photons by interacting with the virtual photons from the magnetic field of the NOMAD dipole magnet.From the analysis of the data collected during the 1996 run with 1.08 = 10 19 protons on target, 312 candidate events with energy between 5 and 140 GeV were found.This number is in general agreement with the expectation of 272 " 18 background events from standard neutrino processes.A 90 % CL upper limit on the agg-coupling g -1.5 = 10 y4 GeV y1 agg for a masses up to 40 eV is obtained.

A search for multifractal structures, in analogy with multifractal theories, is performed on UA1 minimum bias events. A downward concave multifractal spectral function,f(α) (where α is the Lipschitz-Hölder exponent), indicates that there are self-similar cascading processes, governing the evolution from the quark to the hadron level, in the final states of hadronic interactions.f(α) is an accurate measure of the bin to bin fluctuations of any observable. It is shown that the most sensitive comparison between data and the Monte Carlo models, GENCL and PYTHIA 4.8 can be made usingf(α). It is found that these models do not fully reproduce the behaviour of the data.

We describe the construction and performance of the improved muon detection system of the UA1 experiment. The new position detectors, that complement the original muon detection system based on large planar drift chambers, are limited-streamer chambers for a total surface of 800 m2. The coordinate parallel to the wires is readout through the especially developed STAR electronics that integrates and digitizes the signal from the strips. The intrinsic spatial resolution, determined in a test beam, is 400 μm, that achieved so far in the real system is 1.2 mm. The efficiency is evaluated to be (94.9±0.6)%.

An apparatus is described in which a ruby-pumped dye laser is used to induce transitions from the 2S to the 2P levels of the muonic ion (μHe)+. The dye laser supplies infra-red radiation pulses in the wavelengths (8040÷8180) Å, at typical repetition rates of 1 pulse every 4 s, with an energy release per pulse of 300 mJ for 1.2 J pumping

We have measured the response to electrons and protons of a large-size NE-213 scintillation detector. The result is discussed referring to preceding experimental data.

Backward proton and π− production has been studied in νμCC interactions with carbon nuclei. Detailed analyses of the momentum distributions, of the production rates, and of the general features of events with a backward going particle, have been carried out in order to understand the mechanism producing these particles. The backward proton data have been compared with the predictions of the reinteraction and the short range correlation models.

Updated results from the appearance searches for νμ→ντ and νe→ντ oscillations in the full NOMAD data sample are reported. The increased data and the use of more refined kinematic schemes for the ντ CC selection allow a significant improvement of the overall sensitivity. The “blind analysis” of both the deep-inelastic and the low multiplicity samples yields no evidence for an oscillation signal. In the two-family oscillation scenario, this sets a 90% C.L. region in the sin22θμτ−Δm2 plane which includes sin22θμτ<4.4×10−4 at large Δm2 and Δm2<0.8 eV2/c4 at sin22θμτ=1. The corresponding contour in the νe→ντ oscillation hypothesis results in sin22θeτ<2.2×10−2 at large Δm2 and Δm2<6.5 eV2/c4 at sin22θeτ=1.

The inclusive production of the meson resonances ρ0(770), f0(980) and f2(1270) in neutrino–nucleus charged current interactions has been studied with the NOMAD detector exposed to the wide band neutrino beam generated by 450 GeV protons at the CERN SPS. For the first time the f0(980) meson is observed in neutrino interactions. The statistical significance of its observation is 6 standard deviations. The presence of f2(1270) in neutrino interactions is reliably established. The average multiplicity of these three resonances is measured as a function of several kinematic variables. The experimental results are compared to the multiplicities obtained from a simulation based on the Lund model. In addition, the average multiplicity of ρ0(770) in antineutrino–nucleus interactions is measured.

Preliminary results of an experiment to determine the rate of the capture process μ− + p → n + vμ are given.

For the small-area tracking of particles within COMPASS (common muon and proton apparatus for structure and spectroscopy), a new fixed target experiment at CERN/SPS, several large-size(31/spl times/31 cm/sup 2/) detectors based on the gas electron multiplier (GEM), have been built. These new devices, consisting of several GEM amplification stages with a two-coordinate readout, combine good spatial resolution with high rate capability, which is required by the large particle flux near the beam. At the same time, the material exposed to the beam is minimized in order not to spoil the mass resolution of the spectrometer. The first detectors out of a total of 20 were subject to extensive tests in the beam and in the laboratory, showing that full (i.e., >98%) detection efficiency for minimum ionizing particles can be achieved at a total effective gain of 6000. Ongoing research work focuses on discharges triggered by heavily ionizing particles entering the detectors. Systematic studies of the energy released in discharges and their probability of occuring at all as a function of a variety of parameters suggest several means to minimize their impact on detector performance. First results of the operational characteristics of these detectors in the real COMPASS beam are presented.

A special assembly of high pressure wire proportional counters is described. Their operation defines a volume in a gaseous target, the effective limits of which are set mainly by the electric fields of the counters and not by a continuous material wall. The apparatus, used in a recent experiment — at the CERN 600 MeV synchrocyclotron — while filled with extremely purified hydrogen, is fitted to work with any gas, or mixture of gases, proper to fill gas proportional counters. The device seems suitable to be used in different fields of high-energy physics.

The UA1 Collaboration has recently improved its measurement of the beauty production cross-section by including explicit measurements of bb correlations. Using these data we have determined the strong coupling constant αs. The comparison of the measured cross-section for 2-body final states with O(αs3) QCD predictions yields a measurement of αs(20 GeV) = 0.145−0.010 exp −0.016 th+0.012 +0.013, corresponding to αs(Mz) = 0.113−0.006 exp −0.009 th+0.007 +0.008. This is the first theoretically well-defined measurement of αs from a purely hadronic production process. Evaluating αs from cross-sections at different Q2-values we find that the running of αs is needed for internal consistency of the UA1 data.

A uranium tetramethylpentane hadronic calorimeter has been tested using electrons and pions of 7 GeV/c momentum with electric fields varying from 0.8 to 16.0 kV/cm; the e/π charge collection ratio has been measured as a function of the electric field within this range. It is observed that the e/π ratio can be tuned acting on the electric field without spoiling the energy resolution for electrons. At the same time the energy resolution for pions is improved. The effect was cross-checked using a second module and 40 GeV/c electron and pion beams.

In the context of a two-flavour approximation we reinterpret the published NOMAD limit on νμ→ντ oscillations in terms of νe→ντ oscillations. At 90% C.L. we obtain sin22θeτ<5.2×10−2 for large Δm2, while for sin22θeτ=1 the confidence region includes Δm2<11 eV2/c4.

We report on a direct search in NOMAD for a new 33.9 MeV/c2 neutral particle (X) produced in pion decay in flight, π→μX followed by the decay X→νe+e−. Both decays are postulated to occur to explain the time anomaly observed by the KARMEN experiment. From the analysis of the data collected during the 1996–1998 runs with 4.1×1019 protons on target, a single candidate event consistent with background expectations was found. The search is sensitive to a pion branching ratio BR(π→μX)>3.7×10−15, significantly smaller than previous experimental limits.

Abstract The small area tracking system of COMPASS, a new high-luminosity fixed target experiment at CERN's SPS accelerator, comprises 20 large-size triple GEM (Gas Electron Multiplier) detectors. It was completed and fully operational for the first time for the 2002 muon run. We report on the performance of these chambers in the high-intensity beam and give first results on efficiencies as well as spatial and time resolutions measured for the full set of detectors under various beam conditions.

At √s = 52.7 GeV, no electron events were observed for E > 7.0 GeV where one observed event corresponds to d2 σ/dE dω = 4 × 10−36 cm2/(srGeV). The observed events for 2.3 < E < 7.0 GeV are consistent with background. Upper limits to d2 σ/dE dω are given, and their relevance to the production of massive mesons is discussed.

We discuss the main operating features of GEM detectors, optimized for use as trackers in a high radiation environment. The construction, tests and performances of large prototypes for the COMPASS experiment are also described, as well as the results of an exposure to very high intensity beams.

A decommissioned LHC test magnet is being prepared as the CERN Axion Solar Telescope (CAST) experiment. The magnet has a field of 9.6 Tesla and length of 10 meters. It is being mounted on a platform to track the sun over ±8° vertically and ±45°, horizontally. A sensitivity in axion-photon coupling gαγγ < 5 × 10−11GeV−1 can be reached for mα ≤ 10−2eV, and with a gas filled tube-can reach gαγγ ≤ 10−10GeV−1 for axion masses mα < 2eV.

The total nuclear capture rates of muons by argon and neon have been measured. The experiment was performed by stopping negative muons in a target of ultrapure gaseous deuterium (at 6 atm pressure and 293\ifmmode^\circ\else\textdegree\fi{}K) separately contaminated by small amounts of argon and neon, and observing the differential time distribution of the decay electrons coming from muons stopped within the gaseous mixture. The results are ${\ensuremath{\lambda}}_{c,\mathrm{Ar}}=(1.20\ifmmode\pm\else\textpm\fi{}0.08)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ and ${\ensuremath{\lambda}}_{c,\mathrm{Ne}}=(0.30\ifmmode\pm\else\textpm\fi{}0.02)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${\mathrm{sec}}^{\ensuremath{-}1}$.

We present results from the barrel depleted uranium/TMP calorimeter modules constructed by the UA1 Collaboration. Electromagnetic and hadronic energy resolutions have been measured using electron and pion beams with momenta in the range 7 to 70 GeV/c. Results on the energy linearity and the spatial uniformity of response are reported. The electromagnetic shower position resolution has been measured as a function of energy using a fine grained position detector placed at a depth of − 3.5χ0. The noise arising both from the electronics chain and from the uranium radioactivity is compared with 70 GeV/c muon signals. The ratio of the electron to pion response has been measured both as a function of the energy and of the electric field. The high lateral and longitudinal granularity of the calorimeter and the presence of a position detector have been used to determine the electron pion separation as a function of energy.

Abstract Extruded plastic tubes coated with a resistive material, having a 1 cm 2 profile and central 100 μm anode wire are operated in limited streamer mode. The charge induced on an external layer of cathode strips aligned perpendicularly to the anode wire is measured and used to compute the streamer coordinate. Test results both with an X-ray source and a particle beam are presented. Spatial resolution of the other of 400 μm is observed over the 40 cm 2 area studied.

The CERN Axion Solar Telescope (CAST), a 10 meter long LHC, 9 Tesla, test magnet is mounted on a moving platform that tracks the sun about 1.5 hours during sunrise, again during sunset. It moves ±80 vertically and ±400 horizontally. It has been taking data continuously since July 10, 2003. Data analyzed thus far yield an upper bound on the photon-axion coupling constant, gaγγ ⩽ 3 × 10−10 GeV−1 for axion masses less than 5 × 10−2 eV.

The CAST experiment is being mounted at CERN. It will make use of a decommissioned LHC test magnet to look for solar axions through its conversion into photons inside the magnetic field. The magnet has a field of 9.6 Tesla and length of 10 m and is installed in a platform which allows to move it ±8° vertically and ±40° horizontally. According to these numbers we expect a sensitivity in axion-photon coupling gαγγ ⪅ 5 × 10−11 GeV−1 for ma ⪅ 0.02 eV, and with a gas filled tube gaγγ ⪅ 10−10

We descril'e here an assembly target-X-ray detector that is particularly suitable for detecting muonic and pionic X-rays in the range 1.3–10 keV. Some of the main features of the system are described.

1. European Organization for Nuclear Research (CERN), Gen`eve, Switzerland2. DAPNIA, Centre d’´Etudes Nucl´eaires de Saclay (CEA-Saclay), Gif-sur-Yvett e, France3. Technische Universita¨t Darmstadt, IKP, Darmstadt, Germany4. Max-Planck-Institut fu¨r extraterrestrische Physik, Garching, Germany5. Instituto de F´isica Nuclear y Altas Energ´ias, Universidad de Zaragoza, Zaragoza, Spain6. Enrico Fermi Institute and KICP, University of Chicago, Chicago, IL, USA7. Aristotle University of Thessaloniki, Thessaloniki, Greece8. National Center for Scientific Research “Demokritos”, Athens, Greece9. Albert-Ludwigs-Universita¨t Freiburg, Freiburg, Germany10. Institute for Nuclear Research (INR), Russian Academy of Sciences, Moscow, Russia11. Department of Physics and Astronomy, University of British Columbia, Department of Physics, Vancouver,Canada12. Johann Wolfgang Goethe-Universita¨t, Institut fu¨r Angewandte Physik, Frankfurt am Main, Germany13. Max-Planck-Institut fu¨r Physik (Werner-Heisenberg-Institut), Munich, Germany14. Rudjer Boˇskovi´c Institute, Zagreb, Croatia15. Physics Department, University of Patras, Patras, Greece16. Lawrence Livermore National Laboratory, Livermore, CA, USA17. Dogus University, Istanbul, Turkey18. Instituto Nazionale di Fisica Nucleare (INFN), Sezione di Trieste and Universita` di Trieste, Trieste, Italy19. Max-Planck-Institut fu¨r Aeronomie, Katlenburg-Lindau, Germany20. National Technical University of Athens, Athens, Greece

A muon position detector based on limited streamer tubes has been built for the UA1 experiment at the CERN pp collider. The tubes are arranged in chambers equipped with electronics to allow for analog readout of the induced signals on strips. The electronics module for charge amplification and conversion (STAR), the data acquisition system based on the VME standard and the performance of the detector will be described.