Gas Purity effect on GEM Performance in He and Ne at Low Temperatures

The performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne, He+H2 and Ne+H2 was studied at temperatures in the range of 3-293 K. This paper reports on previously published measurements and additional studies on the effects of the purity of the gases in which the GEM performance is evaluated. In He, at temperatures between 77 and 293 K, triple-GEM structures operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved through the Penning effect as a result of impurities in the gas. At lower temperatures the gain-voltage characteristics are significantly modified probably due to the freeze-out of these impurities. Double-GEM and single-GEM structures can operate down to 3 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 re-established in Penning mixtures of Ne+H2: very high gains, exceeding 104, have been obtained in these mixtures at 30-77 K, at a density of 9.2 g/l which corresponds to saturated Ne vapor density at 27 K. The addition of small amounts of H2 in He also re-establishes large GEM gains above 30 K but no gain was observed in He+H2 at 4 K and a density of 1.7 g/l (corresponding to roughly one-tenth of the saturated vapor density). These studies are, in part, being pursued in the development of two-phase He and Ne detectors for solar neutrino detection.Comment: 4 pages, 7 figure

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 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-GEM 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 10000, 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.Comment: 13 pages, 14 figures. Accepted for publishing in Nucl. Instr. and Meth.

Two-Proton Correlations near Midrapidity in p+Pb and S+Pb Collisions at the CERN SPS

Correlations of two protons emitted near midrapidity in p+Pb collisions at 450 GeV/c and S+Pb collisions at 200A GeV/c are presented, as measured by the NA44 Experiment. The correlation effect, which arises as a result of final state interactions and Fermi-Dirac statistics, is related to the space-time characteristics of proton emission. The measured source sizes are smaller than the size of the target lead nucleus but larger than the sizes of the projectiles. A dependence on the collision centrality is observed; the source size increases with decreasing impact parameter. Proton source sizes near midrapidity appear to be smaller than those of pions in the same interactions. Quantitative agreement with the results of RQMD (v1.08) simulations is found for p+Pb collisions. For S+Pb collisions the measured correlation effect is somewhat weaker than that predicted by the model simulations, implying either a larger source size or larger contribution of protons from long-lived particle decays.Comment: 10 pages (LaTeX) text, 4 (EPS) figures; accepted for publication in Phys. Lett.

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

Strange Meson Enhancement in PbPb Collisions

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.Comment: 11 pages, including 4 figures, LATE

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

3pi- 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 PbPb than for SPb. The two-dimensional three-particle correlation function is also measured and the longitudinal extension of the source is larger than the transverse extension

Two-kaon correlations in central Pb + Pb collisions at 158 A GeV/c

Two-particle interferometry of positive kaons is studied in Pb + Pb collisions at mean transverse momenta $\approx 0.25$ and 0.91 GeV/c. A three-dimensional analysis was applied to the lower $p_T$ data, while a two-dimensional analysis was used for the higher $p_T$ data. We find that the source size parameters are consistent with the $m_T$ scaling curve observed in pion correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.Comment: 4 pages incl. 1 table and 3 fig's; RevTeX; accepted for publication in PR

Position resolution and particle identification with the ATLAS EM calorimeter

In the years between 2000 and 2002 several pre-series and series modules of the ATLAS EM barrel and end-cap calorimeter were exposed to electron, photon and pion beams. The performance of the calorimeter with respect to its finely segmented first sampling has been studied. The polar angle resolution has been found to be in the range 50-60 mrad/sqrt(E (GeV)). The neutral pion rejection has been measured to be about 3.5 for 90% photon selection efficiency at pT=50 GeV/c. Electron-pion separation studies have indicated that a pion fake rate of (0.07-0.5)% can be achieved while maintaining 90% electron identification efficiency for energies up to 40 GeV.Comment: 32 pages, 22 figures, to be published in NIM

Hadron Energy Reconstruction for the ATLAS Calorimetry in the Framework of the Non-parametrical Method

This paper discusses hadron energy reconstruction for the ATLAS barrel prototype combined calorimeter (consisting of a lead-liquid argon electromagnetic part and an iron-scintillator hadronic part) in the framework of the non-parametrical method. The non-parametrical method utilizes only the known $e/h$ ratios and the electron calibration constants and does not require the determination of any parameters by a minimization technique. Thus, this technique lends itself to an easy use in a first level trigger. The reconstructed mean values of the hadron energies are within $\pm 1$ of the true values and the fractional energy resolution is $[(58\pm3)/\sqrt{E}+(2.5\pm0.3)$. The value of the $e/h$ ratio obtained for the electromagnetic compartment of the combined calorimeter is $1.74\pm0.04$ and agrees with the prediction that $e/h > 1.7$ for this electromagnetic calorimeter. Results of a study of the longitudinal hadronic shower development are also presented. The data have been taken in the H8 beam line of the CERN SPS using pions of energies from 10 to 300 GeV.Comment: 33 pages, 13 figures, Will be published in NIM

Single hadron response measurement and calorimeter jet energy scale uncertainty with the ATLAS detector at the LHC

The uncertainty on the calorimeter energy response to jets of particles is derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the calorimeter response to single isolated charged hadrons is measured and compared to the Monte Carlo simulation using proton-proton collisions at centre-of-mass energies of sqrt(s) = 900 GeV and 7 TeV collected during 2009 and 2010. Then, using the decay of K_s and Lambda particles, the calorimeter response to specific types of particles (positively and negatively charged pions, protons, and anti-protons) is measured and compared to the Monte Carlo predictions. Finally, the jet energy scale uncertainty is determined by propagating the response uncertainty for single charged and neutral particles to jets. The response uncertainty is 2-5% for central isolated hadrons and 1-3% for the final calorimeter jet energy scale.Comment: 24 pages plus author list (36 pages total), 23 figures, 1 table, submitted to European Physical Journal