Energy loss of a heavy quark produced in a finite-size quark-gluon plasma

We study the energy loss of an energetic heavy quark produced in a high temperature quark-gluon plasma and travelling a finite distance before emerging in the vacuum. While the retardation time of purely collisional energy loss is found to be of the order of the Debye screening length, we find that the contributions from transition radiation and the Ter-Mikayelian effect do not compensate, leading to a reduction of the zeroth order (in an opacity expansion) energy loss.Comment: QM2006 Proceedings; caption of fig 1 and ref [7] modified in v

Structure Functions are not Parton Probabilities

We explain why contrary to common belief, the deep inelastic scattering structure functions are not related to parton probabilities in the target.Comment: 4 pages. Invited talk presented during the `International Light-Cone Workshop', Trento, ECT, September 3-11, 2001. Updated Report-Number

Energy Loss of a Heavy Quark Produced in a Finite Size Medium

We study the medium-induced energy loss $-\Delta E_0(L_p)$ suffered by a heavy quark produced at initial time in a quark-gluon plasma, and escaping the plasma after travelling the distance $L_p$. The heavy quark is treated classically, and within the same framework $-\Delta E_0(L_p)$ consistently includes: the loss from standard collisional processes, initial bremsstrahlung due to the sudden acceleration of the quark, and transition radiation. The radiative loss {\it induced by rescatterings} $-\Delta E_{rad}(L_p)$ is not included in our study. For a ultrarelativistic heavy quark with momentum p \gsim 10 {\rm GeV}, and for a finite plasma with L_p \lsim 5 {\rm fm}, the loss $-\Delta E_0(L_p)$ is strongly suppressed compared to the stationary collisional contribution $-\Delta E_{coll}(L_p) \propto L_p$. Our results support that $-\Delta E_{rad}$ is the dominant contribution to the heavy quark energy loss (at least for L_p \lsim 5 {\rm fm}), as indeed assumed in most of jet-quenching analyses. However they might raise some question concerning the RHIC data on large $p_{\perp}$ electron spectra.Comment: 18 pages, 3 figures. New version clarified and simplified. A critical discussion added in section 2, and previous sections 3 and 4 have been merged together. Main results are unchange

Radiative energy loss of high energy quarks and gluons in a finite volume quark-gluon plasma

The medium induced energy loss spectrum of a high energy quark or gluon traversing a hot QCD medium of finite volume is studied. We model the interaction by a simple picture of static scattering centres. The total induced energy loss is found to grow as $L^2$, where $L$ is the extent of the medium. The solution of the energy loss problem is reduced to the solution of a Schr\"odinger-like equation whose ``potential'' is given by the single-scattering cross section of the high energy parton in the medium. These resuls should be directly applicable to a quark-gluon plasma.Comment: 29 pages, LaTeX2e, 43 figure

Induced Gluon Radiation in a QCD Medium

Soft gluon radiation induced by multiple scattering of a fast quark or gluon propagating through QCD matter is discussed. After revisiting the Landau-Pomeranchuk-Migdal effect in QED we show that large formation times of bremsstrahlung quanta determine the QCD radiation intensity (in analogy to QED) and derive the gluon energy spectrum. Coherent suppression takes place as compared to the Bethe-Heitler situation of independent emissions. As a result the energy loss of fast partons in a QCD medium depends on the incident energy $E$ similarly to QED, $-dE/dz \propto \sqrt{E}$.Comment: 13 pages, 3 included figures, latex fil

The heavy fermion damping rate puzzle

: We examine again the problem of the damping rate of a moving heavy fermion in a hot plasma within the resummed perturbative theory of Pisarski and Braaten. The ansatz for its evaluation which relates it to the imaginary part of the fermion propagator pole in the framework of a self-consistent approach is critically analyzed. As already pointed out by various authors, the only way to define the rate is through additional implementation of magnetic screening. We show in detail how the ansatz works in this case and where we disagree with other authors. We conclude that the self-consistent approach is not satisfactory.Comment: 17 page

Retardation Effect for Collisional Energy Loss of Hard Partons Produced in a QGP

We study the collisional energy loss suffered by an energetic parton travelling the distance L in a high temperature quark-gluon plasma and initially produced in the medium. We find that the medium-induced collisional loss -Delta E(L) is strongly suppressed compared to previous estimates which assumed the collisional energy loss rate -dE/dx to be constant. The large L linear asymptotic behaviour of -Delta E(L) sets in only after a quite large retardation time. The suppression of -Delta E(L) is partly due to the fact that gluon bremsstrahlung arising from the initial acceleration of the energetic parton is reduced in the medium compared to vacuum. The latter radiation spectrum is sensitive to the plasmon modes of the quark-gluon plasma and has a rich angular structure.Comment: 19 pages, 4 figures. Substantial changes have been made in version 2, especially in section 3. In particular, in section 3.2 the dependence on a spurious ultraviolet cut-off has been removed. As a consequence numerical results are quantitatively modified, but the retardation effect is qualitatively unchanged. The physical origin of the largeness of the retardation time is also emphasized, and two Appendices have been adde