R-charge thermodynamical spectral sum rule in N=4 Yang-Mills theory

A sum rule related to the R-current correlator at vanishing three-momentum is derived in the N=4 supersymmetric Yang-Mills field theory at infinite 't Hooft coupling. For reference it is compared to the one in the free field theory, i.e. at the one loop perturbative approximation.Comment: 5 page

On radiation by a heavy quark in N = 4 SYM

A short note on radiation by a moving classical particle in N = 4 supersymmetric Yang-Mills theoryComment: 11 pages, conclusion corrected and extende

Particle Correlations in Saturated QCD Matter

We study quantitatively angular correlations in the two-particle spectrum produced by an energetic probe scattering off a dense hadronic target with sizeable saturation momentum. To this end, two parton inclusive cross sections for arbitrary projectiles with small color charge density are derived in the eikonal formalism. Our results are the following: For large momenta of the observed particles, the perturbative limit with characteristic back-to-back correlation is recovered. As the trigger momenta get closer to the saturation scale Q_s, the angular distribution broadens. When the momenta are significantly smaller than Q_s, the azimuthal distribution is broad but still peaked back-to-back. However, in a narrow momentum range (0.5 - 1.5) Q_s, we observe that the azimuthal correlation splits into a double peak with maxima displaced away from 180 degree. We argue that it is the soft multiple scattering physics that is responsible for the appearance of this shift in the angle of maximal correlation. We also point out that when the physical size of the projectile is particularly small, the double peak structure persists in a significantly wider range of final state momenta.Comment: 28 pages LaTex, 7 eps-figure

Hadronic production of J/Psi and Upsilon: transverse momentum distributions

Baier R, Rückl R. Hadronic production of J/Psi and Upsilon: transverse momentum distributions. Physics Letters B. 1981;102(5):364-370

Effects of soft gluon emission on the opposite-side acollinearity distributions in e+e- annihilation

Baier R, Fey K. Effects of soft gluon emission on the opposite-side acollinearity distributions in e+e- annihilation. Nuclear physics B. 1981;179(1):49-61

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

Correlation with large transverse momentum photons and the gluon structure function

Baier R, Engels J, Petersson B. Correlation with large transverse momentum photons and the gluon structure function. Zeitschrift für Physik, C: Particles and Fields. 1980;6(4):309-316

Light-by-light scattering in the presence of magnetic fields

Baier R, Rebhan A, Wödlinger M. Light-by-light scattering in the presence of magnetic fields. Physical Review D. 2018;98(5): 056001.The low-energy light-by-light cross section as determined by the nonlinear Euler-Heisenberg QED Lagrangian is evaluated in the presence of constant magnetic fields in the center-of-mass system of the colliding photons. This cross section has a complicated dependence on directions and polarizations. The overall magnitude decreases as the magnetic field is increased from zero, but this trend is reversed for ultrastrong magnetic fields B≳Bc, where the cross section eventually grows quadratically with the magnetic field strength perpendicular to the collision axis. This effect is due to interactions involving the lowest Landau level of virtual Dirac particles; it is absent in scalar QED. An even more dramatic effect is found for virtual charged vector mesons where the one-loop cross section diverges at the critical field strength due to an instability of the lowest Landau level and the possibility of the formation of a superconducting vacuum state. We also discuss (the absence of) implications for the recent observation of light-by-light scattering in heavy-ion collisions

Dissipative Hydrodynamics and Heavy Ion Collisions

Recent discussions of RHIC data emphasized the exciting possibility that the matter produced in nucleus-nucleus collisions shows properties of a near-perfect fluid. Here, we aim at delineating the applicability of fluid dynamics, which is needed to quantify the size of corresponding dissipative effects. We start from the equations for dissipative fluid dynamics, which we derive from kinetic theory up to second order (Israel-Stewart theory) in a systematic gradient expansion. In model studies, we then establish that for too early initialization of the hydrodynamic evolution (\tau_0 \lsim 1 fm/c) or for too high transverse momentum (p_T \gsim 1 GeV) in the final state, the expected dissipative corrections are too large for a fluid description to be reliable. Moreover, viscosity-induced modifications of hadronic transverse momentum spectra can be accommodated to a significant degree in an ideal fluid description by modifications of the decoupling stage. We argue that these conclusions, drawn from model studies, can also be expected to arise in significantly more complex, realistic fluid dynamics simulations of heavy ion collisions.Comment: 18 pages, 5 figures, uses revtex4; v2: references added, typos correcte