The Laser of the ALICE Time Projection Chamber

The large TPC ($95 \mathrm{m}^3$) of the ALICE detector at the CERN LHC was commissioned in summer 2006. The first tracks were observed both from the cosmic ray muons and from the laser rays injected into the TPC. In this article the basic principles of operating the $266 \mathrm{nm}$ lasers are presented, showing the installation and adjustment of the optical system and describing the control system. To generate the laser tracks, a wide laser beam is split into several hundred narrow beams by fixed micro-mirrors at stable and known positions throughout the TPC. In the drift volume, these narrow beams generate straight tracks at many angles. Here we describe the generation of the first tracks and compare them with simulations.Comment: QM06 poster proceedings, 6 pages, 4 figure

The Laser Calibration System of the ALICE Time Projection Chamber

A Large Ion Collider Experiment (ALICE) is the only experiment at the Large Hadron Collider (LHC) dedicated to the study of heavy ion collisions. The Time Projection Chamber (TPC) is the main tracking detector covering the pseudo rapidity range $|\eta|< 0.9$. It is designed for a maximum multiplicity \dNdy = 8000. The aim of the laser system is to simulate ionizing tracks at predifined positions throughout the drift volume in order to monitor the TPC response to a known source. In particular, the alignment of the read-out chambers will be performed, and variations of the drift velocity due to drift field imperfections can be measured and used as calibration data in the physics data analysis. In this paper we present the design of the pulsed UV laser and optical system, together with the control and monitoring systems.Comment: 4 pages, 3 figure

Coulomb Effects on Particle Spectra in Relativistic Nuclear Collisions

Coulomb effects on $\pi^\pm$ and $K^\pm$ spectra in relativistic nuclear collisions are investigated. At collision energies around 1 GeV the ratio of at ultrarelativistic energies. We describe the ratios at SIS, AGS and SPS energies with simple analytic models as well as more elaborate numerical models incorporating the expansion dynamics. The Coulomb effect depends on the properties of the source after the violent collision phase and provides information on source sizes, freeze-out times, and expansion velocities. Comparison with results from HBT analyses are made. Predictions for $\pi^\pm$ and $K^\pm$ at RHIC and LHC energies are given

Morphology of High-Multiplicity Events in Heavy Ion Collisions

We discuss opportunities that may arise from subjecting high-multiplicity events in relativistic heavy ion collisions to an analysis similar to the one used in cosmology for the study of fluctuations of the Cosmic Microwave Background (CMB). To this end, we discuss examples of how pertinent features of heavy ion collisions including global characteristics, signatures of collective flow and event-wise fluctuations are visually represented in a Mollweide projection commonly used in CMB analysis, and how they are statistically analyzed in an expansion over spherical harmonic functions. If applied to the characterization of purely azimuthal dependent phenomena such as collective flow, the expansion coefficients of spherical harmonics are seen to contain redundancies compared to the set of harmonic flow coefficients commonly used in heavy ion collisions. Our exploratory study indicates, however, that these redundancies may offer novel opportunities for a detailed characterization of those event-wise fluctuations that remain after subtraction of the dominant collective flow signatures. By construction, the proposed approach allows also for the characterization of more complex collective phenomena like higher-order flow and other sources of fluctuations, and it may be extended to the characterization of phenomena of non-collective origin such as jets.Comment: Matches version accepted for publication in Physical Review C. 13 pages, 9 figure

Freeze-Out Time in Ultrarelativistic Heavy Ion Collisions from Coulomb Effects in Transverse Pion Spectra

The influence of the nuclear Coulomb field on transverse spectra of $\pi^+$ and $\pi^-$ measured in $Pb+Pb$ reactions at 158 A GeV has been investigated. Pion trajectories are calculated in the field of an expanding fireball. The observed enhancement of the $\pi^-/\pi^+$ ratio at small momenta depends on the temperature and transverse expansion velocity of the source, the rapidity distribution of the net positive charge, and mainly the time of the freeze-out.Comment: 11 pages including 2 figure

Evidence for the Jacobi shape transition in hot 46Ti

The gamma-rays from the decay of the GDR in 46Ti compound nucleus formed in the 18O+28Si reaction at bombarding energy 105 MeV have been measured in an experiment using a setup consisting of the combined EUROBALL IV, HECTOR and EUCLIDES arrays. A comparison of the extracted GDR lineshape data with the predictions of the thermal shape fluctuation model shows evidence for the Jacobi shape transition in hot 46Ti. In addition to the previously found broad structure in the GDR lineshape region at 18-27 MeV caused by large deformations, the presence of a low energy component (around 10 MeV), due to the Coriolis splitting in prolate well deformed shape, has been identified for the first time.Comment: 8 pages, 4 figures, proceedings of the COMEX1 conference, June 2003, Paris; to be published in Nucl. Phys.

Giant-dipole Resonance and the Deformation of Hot, Rotating Nuclei

The development of nuclear shapes under the extreme conditions of high spin and/or temperature is examined. Scaling properties are used to demonstrate universal properties of both thermal expectation values of nuclear shapes as well as the minima of the free energy, which can be used to understand the Jacobi transition. A universal correlation between the width of the giant dipole resonance and quadrupole deformation is found, providing a novel probe to measure the nuclear deformation in hot nuclei.Comment: 6 pages including 6 figures. To appear in Phys. Rev. Lett. Revtex

Angular distribution of photons from the delay of the GDR in hot and rotating light Yb nuclei from exclusive experiments

Abstract Angular distributions of photons associated with the damping of excited-state giant dipole resonances (GDR) in hot and rotating 161,162 Yb nuclei have been measured in exclusive experiments using the HECTOR array. In reactions with heavy ions ( 48 T) angular distributions are determined as a function of the angular momentum of the compound nuclei. In reactions with lighter ions ( 17,18 O) a difference method is applied to isolate GDR decays originating from specific excitation regions. The systematics of the measured angular distributions as a function of excitation energy and angular momentum are compared to theories taking into account fluctuations of the shape and orientation of the excited nuclei

Rapidity and centrality dependence of particle production for identified hadrons in Cu+Cu collisions at sNN=200\sqrt{s_{NN}} = 200 GeV

The BRAHMS collaboration has measured transverse momentum spectra of pions, kaons, protons and antiprotons at rapidities 0 and 3 for Cu+Cu collisions at $\sqrt{s_{NN}} = 200$ GeV. As the collisions become more central the collective radial flow increases while the temperature of kinetic freeze-out decreases. The temperature is lower and the radial flow weaker at forward rapidity. Pion and kaon yields with transverse momenta between 1.5 and 2.5 GeV/c are suppressed for central collisions relative to scaled $p+p$ collisions. This suppression, which increases as the collisions become more central is consistent with jet quenching models and is also present with comparable magnitude at forward rapidity. At such rapidities initial state effects may also be present and persistence of the meson suppression to high rapidity may reflect a combination of jet quenching and nuclear shadowing. The ratio of protons to mesons increases as the collisions become more central and is largest at forward rapidities.Comment: 19 pages, 11 figures and 6 table

Rapidity dependence of deuteron production in Au+Au collisions at sNN\sqrt{s_{NN}} = 200 GeV

We have measured the distributions of protons and deuterons produced in high energy heavy ion Au+Au collisions at RHIC over a very wide range of transverse and longitudinal momentum. Near mid-rapidity we have also measured the distribution of anti-protons and anti-deuterons. We present our results in the context of coalescence models. In particular we extract the "volume of homogeneity" and the average phase-space density for protons and anti-protons. Near central rapidity the coalescence parameter $B_2(p_T)$ and the space averaged phase-space density $(p_T)$ are very similar for both protons and anti-protons. For protons we see little variation of either $B_2(p_T)$ or the space averaged phase-space density as the rapidity increases from 0 to 3. However both these quantities depend strongly on $p_T$ at all rapidities. These results are in contrast to lower energy data where the proton and anti-proton phase-space densities are different at $y$=0 and both $B_2$ and $f$ depend strongly on rapidity.Comment: Document updated after proofs received from PR