George Fai

Discovery of jet quenching at RHIC and the opacity of the produced gluon plasma

The predicted quenching of jets in A+A at RHIC energies has been discovered by STAR and PHENIX in preliminary data reported at this conference. We apply the GLV theory of QCD radiative energy loss to estimate the opacity, L/\lambda_g, of the gluon plasma produced in Au+Au collisions at 130 AGeV. We show that (in contrast to the factor of two Cronin enhancement of \pi^0 found at the SPS by WA98) the factor of 5-8 suppression of p_T \sim 2-4 GeV \pi^0 reported by PHENIX can be accounted for with an effective static plasma opacity L/\lambda_g \approx 3-4.

FREESCO: Statistical event generator for nuclear collisions☆

Abstract Title of program: FREESCO Catalogue number: AALV Program obtainable from: CPC Program Library, Queens University of Belfast, N. Ireland (see application form in this issue) Computers: CDC-6600, -7600, VAX-750, -780, -8600, CRAY-1,… Programming language used: Fortran 77 High speed storage required: 71 680 bytes Number of bits per word: 32 Peripherals required: output device for error messages, mass storage device for nuclear data input and recording of generated events Number of lines in program: 1779

On the observation of a transition from fusion to multifragmentation in high multiplicity 16O induced reactions

Abstract We have studied high multiplicity 16 O + Br and 16 O + Ag collisions from 10 A to 220 A MeV on an event-by-event basis. The collisions with the highest multiplicity are fusion-like for energies below 35 A MeV whereas at higher energies the onset of multifragmentation (two or more medium mass fragments) is observed with a charge distribution which gets less and less wide with increasing energy. Correlations between high moments of the charge distribution and the maximum charge or the total multiplicity have been studied. These correlations are generally well described by statistical models. Predicted signals of critical behaviour are however not observed.

Decisive role of fragmentation functions in hard hadron production

It is demonstrated that the fragmentation functions at large momentum fraction play a key role in hard hadron production from relativistic proton-proton collisions. We find that this region of the fragmentation functions is not strongly constrained by the electron-positron data. This freedom can be used (together with the transverse-momentum distribution of partons) to reproduce hard pion-to-proton ratio data in relativistic proton-proton collisions.

THE CHROMO-DIELECTRIC MODEL: CONFINEMENT, EFFECTIVE COUPLING AND CHIRAL INVARIANCE

Abstract The chromo-dielectric mechanism of absolute confinement is studied in the nontopological soliton model. The model lagrangian is chirally invariant, since it contains no direct coupling between the quarks and the scalar field. The static chromo-electric gluon propagator is calculated in medium in the one-loop approximation, and the ultraviolet divergence in the self-energy of fixed quarks is regulated by a form factor. Effective quark-scalar coupling emerges through the self-energy of the quarks in the dielectric medium, which is a function of the scalar field.

Spectator recoil in intermediate-energy heavy-ion collisions

Abstract Recoil momenta of heavy spectators in intermediate-energy heavy-ion collisions of the order of 50–500 MeV per nucleon have been investigated within a statistical transfer model. Special emphasis was put on the role of fluctuations in defining the reaction geometry from correlation experiments involving recoil momenta and masses.

Jets and produced particles in pp collisions from SPS to RHIC energies for nuclear applications

Higher-order perturbative QCD corrections play an important role in the reproduction of data at high transverse momenta in the energy range 20 GeV≤(s)1/2≤200 GeV. Recent calculations of photon and pion production in pp collisions yield detailed information on the next-to-leading-order (NLO) contributions. However, the application of these results in proton-nucleus and nucleus-nucleus collisions is not straightforward. The study of nuclear effects requires a simplified understanding of the output of these computations. Here we summarize our analysis of recent calculations, aimed at handling the NLO results by introducing process and energy-dependent K factors.

Thermal photon production in high-energy nuclear collisions.

We use a boost-invariant one-dimensional (cylindrically symmetric) fluid dynamics code to calculate thermal photon production in the central rapidity region of S+Au and Pb+Pb collisions at SPS energy ([radical][ital s] =20 GeV/nucleon). We assume that the hot matter is in thermal equilibrium throughout the expansion, but consider deviations from chemical equilibrium in the high temperature (deconfined) phase. We use equations of state with a first-order phase transition between a massless pion gas and quark gluon plasma, with transition temperatures in the range 150[le][ital T][sub [ital c]][le]200 MeV.

Pion production in dAu collisions at RHIC energy

Abstract. We present our results on neutral pion (π0) production in pp and dAu collisions at RHIC energy. Pion spectra are calculated in a next-to-leading order (NLO) perturbative QCD-based model. The model includes the transverse component of the initial parton distribution (“intrinsic kT”). We compare our results to the available experimental data from RHIC, and fit the data with high precision. The calculation tuned this way is repeated for the dAu collision, and used to investigate the interplay of shadowing and multiple scattering at RHIC. The centrality dependence of the nuclear modification factor shows a measurable difference between different shadowing parameterizations.

Cronin effect at different rapidities at RHIC

Calculations of the nuclear modification factor, R dAu , for π 0 production in dAu collisions at √S NN = 200 GeV are presented. The applied pQCD-improved parton model incorporates intrinsic k T . Nuclear multiscattering and nuclear shadowing are considered in the Au nucleus. Theoretical results are displayed for midrapidity and high pseudorapidity, η, and compared to preliminary PHENIX and BRAHMS data.