M. Spegel

The TIC — a multi-particle threshold imaging Cherenkov detector

Abstract The Threshold Imaging Cherenkov (TIC) was built as part of an upgrade programme for the NA44 spectrometer in view of 160 GeV·A lead-ion beams from the CERN SPS. The detector is used to distinguish pion tracks from those of heavier particles (K, p, d) in the momentum range of 4–8 GeV/c. Cherenkov photons produced by pions in a 1 m long isobutane radiator are converted to electrons in TMAE at 25°C and detected in a MWPC with 2-dimensional cathode pad readout. A threshold is set on the number of pads hit within a fiducial zone on the cathode plane around a projected external track.

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.

Proton and Antiproton Distributions at Mid-Rapidity in Proton-Nucleus and Sulphur-Nucleus Collisions

Experiment NA44 has measured proton and antiproton distributions at mid-rapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon respectively. The inverse slopes of transverse mass distributions increase with system size for both protons and antiprotons but are slightly lower for antiprotons. This could happen if antiprotons are annihilated in the nuclear medium. The antiproton yield increases with system size and centrality and is largest at mid-rapidity. The proton yield also increases with system size and centrality, but decreases from backward rapidity to mid-rapidity. The stopping of protons at these energies lies between the full stopping and nuclear transparency scenarios. The data are in reasonable agreement with RQMD predictions except for the antiproton yields from sulphur-nucleus collisions. PACS numbers: 25.75.-q 13.85.-t 13.60.Rj Typeset using REVTEX 2

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.

Particle ratios from central Pb + Pb collisions at the CERN SPS

We will address the physics of K?/K+ and /p ratios measured in 158 A GeV Pb + Pb collisions as a function of centrality and transverse momentum. We observe little significant centrality dependence in neither K?/K+ nor /p ratios and they are almost constant as a function of PT. In addition to the chemical freeze-out temperature Tch, we extract the chemical potentials for both light and strange quarks (?q and ?s) by comparing the present data with simple model predictions. Tch is greater than the thermal freeze-out temperature which is extracted from the transverse momentum distribution of charged hadrons.

Ageing studies for the ATLAS-monitored drift tubes

Abstract The lifetime of drift tubes operated with Ar–CH 4 –N 2 91–5–4, Ar–CH 4 –N 2 –CO 2 94–3–2–1, Ar–CF 4 –N 2 –CO 2 94.5–0.5–2–3 and Ar–CO 2 80–20 is studied under various conditions. H 2 O and ethanol were successfully tested as a remedy against ageing. The method of glow discharge in argon is used to clean the cathode and inefficient wires and first results of this treatment are presented.

A THRESHOLD IMAGING CHERENKOV DETECTOR WITH CSI PHOTOCATHODES

Abstract A Threshold Imaging Cherenkov (TIC) detector, in conjunction with a tracking device and a time-of-flight system, has been developed to allow pion, kaon and proton identification in the 3–8GeV/ c range of momenta. The system allows spatial identification of the photons of particles above the Cherenkov threshold and their correlation to a particular track. The TIC detector uses a MWPC detector with a CsI coated photocathode for photon conversion. The results obtained in ultrarelativistic lead–lead collisions at the CERN SPS accelerator are presented.

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.Abstract Correlations of two protons emitted near midrapidity in p + Pb collisions at 450 GeV/ c and S + Pb collisions at 200 A 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.

Measuring the space-time extent of nuclear collisions using interferometry

Abstract Hanbury-Brown and Twiss (HBT) interferometry is used to study the space-time evolution of nuclear collisions at CERN energies. Pairs of identified pions, kaons and protons are studied by NA44 for a variety of collisions systems ranging from p-Pb to PbPb. The pion and kaon correlation data from the 1994 and 1995 Pb runs are compared to published data. The systematic increase in radius parameter with system size is continued in the Pb data sample. Model comparision stress the importance of scattering in the evolution of the collision.