Jet Tomography of Hot and Cold Nuclear Matter

Medium modification of parton fragmentation functions induced by multiple scattering and gluon bremsstrahlung is shown to describe the recent HERMES data in deeply inelastic scattering (DIS) very well, providing the first evidence of $A^{2/3}$-dependence of the modification. The energy loss is found to be $\approx 0.5$ GeV/fm for a 10-GeV quark in a $Au$ nucleus. Including the effect of expansion, analysis of the $\pi^0$ spectra in central $Au+Au$ collisions at $\sqrt{s}=130$ GeV yields an averaged energy loss equivalent to $\approx 7.3$ GeV/fm in a static medium. Predictions for central $Au+Au$ collisions at $\sqrt{s}=200$ GeV are also given.Comment: 4 pages in RevTex with 3 ps figures, final version published in Phys. Rev. Letter

High Pt hadron-hadron correlations

We propose the formulation of a dihadron fragmentation function in terms of parton matrix elements. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order (LO) in e+ e- annihilation is shown to factorize into a short distance parton cross section and the long distance dihadron fragmentation function. We also derive the DGLAP evolution equation of this function at leading log. The evolution equation for the non-singlet and singlet quark fragmentation function and the gluon fragmentation function are solved numerically with the initial condition taken from event generators. Modifications to the dihadron fragmentation function from higher twist corrections in DIS off nuclei are computed. Results are presented for cases of physical interest.Comment: 7 pages, 8 figures, Latex, Proceedings of Hot Quarks 2004, July 18-24, Taos, New Mexic

Do Search for Dibaryonic De - Excitations in Relativistic Nuclear Reactions

Some odd characteristics are observed in the single particle distributions obtained from $He + Li$ interactions at $4.5 AGeV/c$ momenta which are explained as the manifestation of a new mechanism of strangeness production via dibaryonic de-excitations. A signature of the formation of hadronic and baryonic clusters is also reported. The di-pionic signals of the dibaryonic orbital de-excitations are analyzed in the frame of the MIT - bag Model and a Monte Carlo simulation.The role played by the dibaryonic resonances in relativistic nuclear collisions could be a significant one. Key words: Relativistic nuclear interactions negative pions, negative kaons, di-pions , streamer chamber, dibaryons, MIT - bag model PACS codes: 25.75.+r,14.40.Aq,14.20.Pt,12.40.AsComment: 17 pages,LATEX, preprint ICTP -243 1993,figures available by reques

Perturbative Gluon Shadowing in Heavy Nuclei

We study how much gluon shadowing can be perturbatively generated through the modified QCD evolution in heavy nuclei. The evolution of small-$x$ gluons is investigated within the semiclassical approximation. The method of characteristics is used to evaluate the shadowed distributions in low-$Q$ and small-$x$ region. In solving the modified evolution equation, we model in simultaneously fusions from independent constituents and from the same constituent, both in a proton and in a large loosely bound nucleus of $A\sim 200$. In addition to the actual distributions at small $x$, we study the ratios of the distributions at an initial scale $Q_0 = 2$ GeV, and show that a strong nuclear shadowing can follow from the modified QCD evolution.Comment: 9 pages in LATEX with 2 postscript figures in a separate uuencoded file, LBL-3415

Parton Energy Loss with Detailed Balance

Stimulated gluon emission and thermal absorption in addition to induced radiation are considered for an energetic parton propagating inside a quark- gluon plasma. In the presence of thermal gluons, stimulated emission reduces while absorption increases the parton's energy. The net effect is a reduction of the parton energy loss. Though decreasing asymptotically as $T/E$ with the parton energy, the relative reduction is found to be important for intermediate energies. The modified energy dependence of the energy loss will affect the shape of suppression of moderately high $p_T$ hadrons due to jet quenching in high-energy heavy-ion collisions.Comment: 4 pages in RevTex with 1 postscript figure. Some typos are corrected and new comments are adde

Studying minijets via the pTp_T dependence of two-particle correlation in azimuthal angle ϕ\phi

Following my previous proposal that two-particle correlation functions can be used to resolve the minijet contribution to particle production in minimum biased events of high energy hadronic interactions, I study the $p_T$ and energy dependence of the correlation. Using HIJING Monte Carlo model, it is found that the correlation $c(\phi_1,\phi_2)$ in azimuthal angle $\phi$ between two particles with $p_T>p_T^{cut}$ resembles much like two back-to-back jets as $p_T^{cut}$ increases at high colliding energies due to minijet production. It is shown that $c(0,0)-c(0,\pi)$, which is related to the relative fraction of particles from minijets, increases with energy. The background of the correlation for fixed $p_T^{cut}$ also grows with energy due to the increase of multiple minijet production. Application of this analysis to the study of jet quenching in ultrarelativistic heavy ion collisions is also discussed.Comment: 11 pages Latex text and 8 ps figures, LBL-3349

Kinetic equation with exact charge conservation

We formulate the kinetic master equation describing the production of charged particles which are created or destroyed only in pairs due to the conservation of their Abelian charge.Our equation applies to arbitrary particle multiplicities and reproduces the equilibrium results for both canonical (rare particles) and grand canonical (abundant particles) systems. For canonical systems, the equilibrium multiplicity is much lower and the relaxation time is much shorter than the naive extrapolation from the grand canonical ensemble results. Implications for particle chemical equilibration in heavy-ion collisions are discussed.Comment: 4 Pages in RevTe

Hard Parton Rescatterings and Minijets in Nuclear Collisions at LHC

The average number of minijets and the corresponding transverse energy produced in heavy ion collisions are evaluated by including explicitly semi-hard parton rescatterings in the dynamics of the interaction. At the LHC semi-hard rescatterings have a sizable effect on global characteristics of the typical inelastic event. An interesting feature is that the dependence on the cutoff which separates soft and hard parton interactions becomes less critical after taking rescatterings into account.Comment: 14 pages, 3 figures. Enlarged discussion in sect.1 and 4; 1 figure added. To be published in Phys.Rev.

Dileptons from Disoriented Chiral Condensates

Disoriented chiral condensates or long wavelength pionic oscillations and their interaction with the thermal environment can be a significant source of dileptons. We calculate the yield of such dilepton production within the linear sigma model, both in a quantal mean-field treatment and in a semi-classical approximation. We then illustrate the basic features of the dilepton spectrum in a schematic model. We find that dilepton yield with invariant mass near and below $2m_{\pi}$ due to the soft pion modes can be up to two orders of magnitude larger than the corresponding equilibrium yield.Comment: 22 pages, 8 figures, uses epsf-styl