M. Leltchouk

Energy linearity and resolution of the ATLAS electromagnetic barrel calorimeter in an electron test-beam

A module of the ATLAS electromagnetic barrel liquid argon calorimeter was exposed to the CERN electron test-beam at the H8 beam line upgraded for precision momentum measurement. The available energies of the electron beam ranged from 10 to 245 GeV. The electron beam impinged at one point corresponding to a pseudo-rapidity of eta=0.687 and an azimuthal angle of phi=0.28 in the ATLAS coordinate system. A detailed study of several effects biasing the electron energy measurement allowed an energy reconstruction procedure to be developed that ensures a good linearity and a good resolution. Use is made of detailed Monte Carlo simulations based on Geant which describe the longitudinal and transverse shower profiles as well as the energy distributions. For electron energies between 15 GeV and 180 GeV the deviation of the measured incident electron energy over the beam energy is within 0.1%. The systematic uncertainty of the measurement is about 0.1% at low energies and negligible at high energies. The energy resolution is found to be about 10% sqrt(E) for the sampling term and about 0.2% for the local constant term.

Identified pion interferometry in heavy-ion collisions at CERN

Abstract π + π + correlations from S + Pb collisions at 200 GeV/ c per nucleon, measured by the focusing spectrometer of the NA44 experiment at CERN, are presented. The large data set, particularly at small values of Q inv , allows determination of the detailed shape of the correlation function. We construct the background by event mixing, and correct for resolution, Coulomb interaction, and distortion of single-particle spectra which produce effects large compared to the statistical errors at small Q inv . An exponential, rather than gaussian, source distribution is slightly favoured, but both are statistically acceptable.

GEM operation in helium and neon at low temperatures

We study the performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne and Ne+H2 at temperatures in the range of 2.6–293 K. In He, at temperatures between 62 and 293 K, the triple-GEM structures often operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved by the Penning effect in the gas impurities released by outgassing. At lower temperatures, the gain–voltage characteristics are significantly modified probably due to the freeze-out of impurities. In particular, the double- and single-GEM structures can operate down to 2.6 K at gains reaching only several tens at a gas density of about 0.5 g/l; at higher densities the maximum gain drops further. In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in Ne at low temperatures can be reestablished in Penning mixtures of Ne+H2: very high gains, exceeding 10 4 , have been obtained in these mixtures at 50–60 K, at a density of 9.2 g/l corresponding to that of saturated Ne vapor near 27 K. The results obtained are relevant in the fields of two-phase He and Ne detectors for solar neutrino detection and electron avalanching at low temperatures.

Mid-rapidity protons in 158A·GeV Pb+Pb collisions

Abstract Proton distributions at mid-rapidity (2 ≤ y ≤ 3) have been measured for 158A·GeV Pb+Pb collisions in the focusing spectrometer experiment NA44 at CERN. From baryon number conservation and by comparing the experimentally measured d N d y distribution with the transport model RQMD, we conclude that a rather high degree of nuclear stopping has been reached for the truly heavy-ion collisions at these energies. Transverse mass distributions exhibit characteristically thermal shapes and the slope parameters increase with the mass of the colliding system.

Three Pion Correlations in Sulphur Lead Collisions at the CERN SPS

Abstract π + π + π + correlations from sulphur-lead collisions at 200 GeV/c per nucleon are presented as measured by the focusing spectrometer of experiment NA44 at CERN. We have investigated the three-pion correlation function at mid-rapidity and found that a genuine three-body correlation is suppressed. A possible interpretation of this result is that the emission of particles is partially coherent.

Strange meson enhancement in PbPb collisions

Abstract The NA44 Collaboration has measured yields and differential distributions of K + , K − , π + , π − in transverse kinetic energy and rapidity, around the center-of-mass rapidity in 158 A GeV/ c Pb+Pb collisions at the CERN SPS. A considerable enhancement of K + production per π is observed, as compared to p + p collisions at this energy. To illustrate the importance of secondary hadron rescattering as an enhancement mechanism, we compare strangeness production at the SPS and AGS with predictions of the transport model RQMD.The NA44 Collaboration has measured yields and differential distributions of K+, K-, pi+, pi- in transverse kinetic energy and rapidity, around the center-of-mass rapidity in 158 A GeV/c Pb+Pb collisions at the CERN SPS. A considerable enhancement of K+ production per pi is observed, as compared to p+p collisions at this energy. To illustrate the importance of secondary hadron rescattering as an enhancement mechanism, we compare strangeness production at the SPS and AGS with predictions of the transport model RQMD.

One and two-dimensional analysis of 3π correlations measured in Pb+Pb interactions

Abstract π−π−π− correlations from Pb+Pb collisions at 158 GeV/c per nucleon are presented as measured by the focusing spectrometer of the NA44 experiment at CERN. The three-body effect is found to be stronger for Pb+Pb than for S+Pb. The two-dimensional three-particle correlation function is also measured and the longitudinal extension of the source is larger than the transverse extension.

Coulomb effect in single particle distributions

Abstract Single particle distributions from heavy-ion collisions show the effect of Coulomb interactions on the final state. While a rather strong effect is seen in the ratio π − π + from central 158A·GeV/c Pb+Pb collisions, at most a small enhancement is found in the ratios from S+S and S+Pb collisions at 200A·GeV/c.

Directional dependence of the pion source in high-energy heavy-ion collisions

Abstract π + π + and π − π − correlations from 200 GeV per nucleon S + Pb collisions and π + π + correlations from 450 GeV p + Pb collisions measured by the focussing spectrometer of CERN experiment NA44 are presented. The large data set which emphasizes small values of momentum difference allows multi-dimensional analysis along with the more traditional one-dimensional parameterization to characterize the pion emission source. It is found that the three radius parameters are similar and large compared to the projectile radius. This can be explained by pion scattering in the final state hadronic system.

Kaon interferometry in heavy ion collisions at the CERN SPS

K+K+ and K−K− correlations from S+Pb collisions at 200 GeV/c per nucleon and K+K+ correlations from p+Pb collisions at 450 GeV/c per nucleon, are presented as measured by the focusing spectrometer of the NA44 experiment at CERN. Multidimensional fits are performed in order to characterize the kaon-emission volume, which is found to be smaller than the pion-emission volume.