Jet Quenching

A short summary of the physics underlying jet quenching is givenComment: 10 pages, LaTex, 11 postscript figures, Proceedings of Quark Matter 2002, Nantes, France, July 18-24, 200

Perturbative Hot Gauge Theories - Recent Results

Current results in high temperature gauge theories obtained in the context of the perturbative method of resumming hard thermal loops are reviewed. Beyond leading order properties of the gluon excitation, and the recent (controversial) calculations of the damping rates are discussed. QCD predictions on plasma signatures are exemplified by the thermal production rates of energetic as well as soft photons. [Talk given at the 3rd Workshop on Thermal Field Theories and their Applications, August 15--27, 1993, Banff, Alberta, Canada]Comment: 11 pages, typeset using LATEX, BI-TP 93/6

Stopping distance for high energy jets in weakly-coupled quark-gluon plasmas

We derive a simple formula for the stopping distance for a high-energy quark traveling through a weakly-coupled quark gluon plasma. The result is given to next-to-leading-order in an expansion in inverse logarithms ln(E/T), where T is the temperature of the plasma. We also define a stopping distance for gluons and give a leading-log result. Discussion of stopping distance has a theoretical advantage over discussion of energy loss rates in that stopping distances can be generalized to the case of strong coupling, where one may not speak of individual partons.Comment: 20 pages, 4 figures [change from v1: fixed embarrassing reference error

Deciphering the properties of the medium produced in heavy ion collisions at RHIC by a pQCD analysis of quenched large p⊥p_{\perp} π0\pi^0 spectra

We discuss the question of the relevance of perturbative QCD calculations for analyzing the properties of the dense medium produced in heavy ion collisions. Up to now leading order perturbative estimates have been worked out and confronted with data for quenched large $p_{\perp}$ hadron spectra. Some of them are giving paradoxical results, contradicting the perturbative framework and leading to speculations such as the formation of a strongly interacting quark-gluon plasma. Trying to bypass some drawbacks of these leading order analysis and without performing detailed numerical investigations, we collect evidence in favour of a consistent description of quenching and of the characteristics of the produced medium within the pQCD framework.Comment: 10 pages, 3 figure

On the string-inspired approach to QED in external field

Strassler's formulation of the string-derived Bern-Kosower formalism is extended to consider QED processes in homogeneous constant external field. A compact expression for the contribution of the one-loop with arbitrary number of external photon lines is given for scalar QED. Extension to spinor QED is shortly discussed.Comment: 14 pages,Revtex,no figure

Comparison of theory with experiment for positron production from high-energy electrons moving along crystal axes

Various positron distributions are obtained using an approach developed earlier for the description of electron-photon showers in axially aligned single crystals. Based on these distributions, characteristics of the positron yield measured in recent experiments are calculated. Theoretical estimations display a rather good agreement with experimental results obtained using 3 to 10 GeV electrons aligned to the - axis of the tungsten crystals.Comment: 10 pages, 6 Postscript figure

On the Angular Dependence of the Radiative Gluon Spectrum

The induced momentum spectrum of soft gluons radiated from a high energy quark produced in and propagating through a QCD medium is reexamined in the BDMPS formalism. A mistake in our published work (Physical Review C60 (1999) 064902) is corrected. The correct dependence of the fractional induced loss $R(\theta_{{\rm cone}})$ as a universal function of the variable $\theta^2_{{\rm cone}} L^3 \hat q$ where $L$ is the size of the medium and $\hat q$ the transport coefficient is presented. We add the proof that the radiated gluon momentum spectrum derived in our formalism is equivalent with the one derived in the Zakharov-Wiedemann approach.Comment: LaTex, 5 pages, 1 figur

Angular Dependence of the Radiative Gluon Spectrum and the Energy Loss of Hard Jets in QCD Media

The induced momentum spectrum of soft gluons radiated from a high energy quark propagating through a QCD medium is derived in the BDMPS formalism. A calorimetric measurement for the medium dependent energy lost by a jet with opening angle $\theta_{{\rm cone}}$ is proposed.The fraction of this energy loss with respect to the integrated one appears to be the relevant observable.It exhibits a universal behaviour in terms of the variable $\theta^2_{{\rm cone}} L^3 \hat q$ where $L$ is the size of the medium and $\hat q$ the transport coefficient. Phenomenological implications for the differences between cold and hot QCD matter are discussed.Comment: 13 pages and 7 figures, RevTe

On p_T-broadening of high energy partons associated with the LPM effect in a finite-volume QCD medium

We study the contributions from radiation to $p_{\perp}$-broadening of a high energy parton traversing a QCD medium with a finite length $L$. The interaction between the parton and the medium is described by decorrelated static multiple scattering. Amplitudes of medium-induced gluon emission and parton self-energy diagrams are evaluated in the soft gluon limit in the BDMPS formalism. We find both the double-logarithmic correction from incoherent scattering, which is parametrically the same as that in single scattering, and the logarithmic correction from the LPM effect. Therefore, we expect a parametrically large correction from radiation to the medium-induced $p_\perp$-broadening in perturbative QCD.Comment: 19 pages, focusing only on calculations about the medium-induced diagrams, origin for double-log reinterpreted, final version to appear in JHE

How does transverse (hydrodynamic) flow affect jet-broadening and jet-quenching ?

We give the modification of formulas for $p_{\perp}$-broadening and energy loss which are necessary to calculate parton interactions in a medium with flow. Arguments are presented leading to the conclusion that for large $p_{\perp}$-spectra observed in heavy ion collisions at RHIC, the influence of transverse flow on the determination of the "quenching power" of the produced medium is small. This leaves open the question of the interpretation of data in a consistent perturbative framework.Comment: 8 pages and 4 figures; version v2 includes more consistent notations and correction of fig.2; version v3 contains a misprint correction, an added sentence in section 3C and in the Introduction as well as in section 4 a rephrasing of comments about references [31,32,35