K. Reygers

Glauber Modeling in High-Energy Nuclear Collisions

We review the theoretical background, experimental techniques, and phenomenology of what is known in relativistic heavy ion physics as the Glauber model, which is used to calculate geometric quantities. A brief history of the original Glauber model is presented, with emphasis on its development into the purely classical, geometric picture used for present-day data analyses. Distinctions are made between the optical limit and Monte Carlo approaches, which are often used interchangeably but have some essential differences in particular contexts. The methods used by the four RHIC experiments are compared and contrasted, although the end results are reassuringly similar for the various geometric observables. Finally, several important RHIC measurements are highlighted that rely on geometric quantities, estimated from Glauber calculations, to draw insight from experimental observables. The status and future of Glauber modeling in the next generation of heavy ion physics studies is briefly discussed.

Charged-Particle Multiplicity in Proton-Proton Collisions

This review summarizes and critically reviews measurements of charged-particle multiplicity distributions and pseudorapidity densities in p + p(P) collisions between √s = 23.6 GeV and √s = 1.8 TeV. Related theoretical concepts are briefly introduced. Moments of multiplicity distributions are presented as a function of √s. Feynman scaling, KNO scaling as well as the description of multiplicity distributions with a single negative binomial distribution and with combinations of two or more negative binomial distributions is discussed. Moreover, similarities between the energy dependence of charged-particle multiplicities in p + p(p) and e + e - collisions are studied. Finally, various predictions for pseudorapidity densities, average multiplicities in full phase space and multiplicity distributions of charged particles in p + p(P) collisions at the LHC energies of √s = 7 TeV, 10 TeV and 14 TeV are summarized and compared.

Pulse height measurements and electron attachment in drift chambers operated with Xe, CO2 mixtures

We present pulse height measurements in drift chambers operated with Xe,CO2 gas mixtures. We investigate the attachment of primary electrons on oxygen and SF6 contaminants in the detection gas. The measurements are compared with simulations of properties of drifting electrons. We present two methods to check the gas quality: gas chromatography and 55 Fe pulse height measurements using monitor detectors.

Characteristics of parton energy loss studied with high pt particle spectra from PHENIX

In the first three years of the physics program at the relativistic heavy ion collider (RHIC) a picture was established in which the suppression of hadrons at high transverse momenta (pT) in central Au+Au collisions is explained by energy loss of quark and gluon jets in a medium of high color-charge density. Measurements of single-particle spectra for a smaller nucleus (Cu), for different center-of-mass energies and with higher statistics were performed in the subsequent years and are used to test predictions and assumptions of jet quenching models in more detail. The measurements presented here are consistent with a parton energy loss scenario so that these models can be used to relate the observed suppression to properties of the created medium.

Production of neutral pions and eta-mesons in pp collisions measured with ALICE

Invariant cross sections for neutral pions and η mesons in pp collisions at , 2.76 and 7 TeV were measured by the ALICE detector at the LHC. Next-to-leading order perturbative QCD calculations describe the π0 and η spectra at 0.9 TeV, but overestimate the measured cross sections at 2.76 and 7 TeV. The measured η/π0 ratio is consistent with mT scaling at 2.76 TeV. At 7 TeV, indications for a violation of mT scaling were found.

Direct-photon production in heavy-ion collisions from SPS to RHIC energies

Direct photons are an important tool for the detection of the quark-gluon plasma in ultrarelativistic nucleus-nucleus collisions. Direct-photon measurements were made in Pb+Pb collisions at √ sNN = 17.2 GeV and in Au+Au collisions at √ sNN = 200 GeV. These results are reviewed and compared with model calculations. PACS. PACS-key describing text of that key – PACS-key describing text of that key

Pion-pion cross section from proton-proton collisions at the LHC

The zero-degree calorimeters (ZDC) installed in the ALICE, ATLAS and CMS experiments at the LHC, make possible simultaneous detection of forward-backward leading neutrons, pp=>nXn. Such data with sufficiently high statistics could be a source of information about the pion-pion total cross section at high energies, provided that the absorption corrections, which are expected to be strong, are well understood. Otherwise, making a plausible assumption about the magnitude of the pion-pion cross section, one can consider such measurements as a way to study the absorption effects, which is the main focus of the present paper. These effects introduced at the amplitude level, are found to be different for the pion fluxes, which either conserve or flip the nucleon helicity. The pion fluxes from both colliding protons are essentially reduced by absorption, moreover, there is a common absorption suppression factor, which breaks down the factorized form of the cross section. We also evaluate the feed-down corrections related to the initial/final state inelastic processes possessing a rapidity gap, and found them to be small in the kinematic range under consideration. The contribution of other iso-vector Reggeons, spin-flip natural parity rho and a2, and spin non-flip unnatural parity a1 are also evaluated and found to be rather small.

J/Psi in high-multiplicity pp collisions: lessons from pA

Gluons at small x in high-energy nuclei overlap in the longitudinal direction, so the nucleus acts as a single source of gluons, like higher Fock components in a single nucleon, which contribute to inelastic collisions with a high multiplicity of produced hadrons. This similarity helps to make a link between nuclear effects in pA and high-multiplicity pp collisions. Such a relation is well confirmed by data for the J/Psi production rate in high-multiplicity pp events measured recently in the ALICE experiment. Broadening of J/Psi transverse momentum is predicted for high-multiplicity pp collisions.

Results on photon production in Au+Au collisions at RHIC.

Abstract The status of the search for direct photon in Au+Au collisions at S N N = 130 and S N N = 200 with the PHENIX experiment is presented. Within errors, no excess of direct photons was found in a first analysis pass done on a limited data set. Significantly reduced systematic and statistical uncertainties are expected in future analyses.

How brightly does the Glasma shine? Photon production off-equilibrium

We present a parametric estimate of photon production at early times in heavy-ion collisions based on a consistent weak coupling thermalization scenario. We quantify the contribution of the off-equilibrium Glasma phase relative to that of a thermalized Quark-Gluon Plasma. Taking into account the constraints from charged hadron multiplicity data, the Glasma contribution is found to be significant especially for large values of the saturation scale.