Scattering Through QCD Sphalerons

Diffractive parton-parton scattering in the soft pomeron regime can be explained by the production of a QCD sphaleron. Sphaleron production results into the emission of $3+2N_F$ gluons and quarks. At RHIC we expect many sphalerons to be released thereby affecting most prompt processes.Comment: Contribution to quark matter 200

The Fate of the Initial State Fluctuations in Heavy Ion Collisions. III The Second Act of Hydrodynamics

Hydrodynamical description of the "Little Bang" in heavy ion collisions is surprisingly successful, mostly due to the very small viscosity of the Quark-Gluon plasma. In this paper we systematically study the propagation of small perturbations, also treated hydrodynamically. We start with a number of known techniques allowing for analytic calculation of the propagation of small perturbations on top of the expanding fireball. The simplest approximation is the "geometric acoustics", which substitutes the wave equation by mechanical equations for the propagating "phonons". Next we turn to the case in which variables can be separated, in which case one can obtain not only the eikonal phases but also amplitudes of the perturbation. Finally, we focus on the so called Gubser flow, a particular conformal analytic solution for the fireball expansion, on top of which one can derive closed equations for small perturbations. Perfect hydrodynamics allows all variables to be separated and all equations to be solved in terms of known special functions. We can thus collect the analytical expression for all the harmonics and reconstruct the complete Green function of the problem. In the viscous case the equations still allow for variable separation, but one of the equations has to be solved numerically. We still can collect all the harmonics and show real-time perturbation evolution, observing viscosity-induced changes in the spectra and the correlation functions of secondaries. We end up by comparing the calculated angular shape of the correlation function to the STAR experimental data, and find, for sufficiently large viscosity, a surprisingly good agreement.Comment: The paper was changed after PRC referee report. It was resubmitted in this for

Instanton-induced Azimuthal Spin Asymmetry in Deep Inelastic Scattering

It is by now well understood that spin asymmetry in deep inelastic scattering (DIS) can appear if two things are both present: (i) a chirality flip of the struck quark; (ii) a nonzero T-odd phase due to its final state interaction. So far (i) was attributed to a new structure/wave function of the nucleon and (ii) to some gluon exchanges. We propose a new mechanism utilizing strong vacuum fluctuations of the gluon field described semiclasically by instantons, and show that both (i) and (ii) are present. The magnitude of the effect is estimated using known parameters of the instanton ensemble in the QCD vacuum and known structure and fragmentation functions, without any new free parameters. The result agrees in sign and (roughly) in magnitude with the available data on single particle inclusive DIS. Furthermore, our predictions uniquely relate effects for longitudinally and transversely polarized targets.Comment: version 2 includes few refs and new fig.5 which contains comparison to recent dat

Evidence for the Role of Instantons in Hadron Structure from Lattice QCD

Cooling is used as a filter on a set of gluon fields sampling the Wilson action to selectively remove essentially all fluctuations of the gluon field except for the instantons. The close agreement between quenched lattice QCD results with cooled and uncooled configurations for vacuum correlation functions of hadronic currents and for density-density correlation functions in hadronic bound states provides strong evidence for the dominant role of instantons in determining light hadron structure and quark propagation in the QCD vacuum.Comment: 26 pages in REVTeX, plus 10 figures, uuencoded. Submitted to Physical Review D. MIT-CTP-226

The Instanton Density at Finite Temperatures

For {\it low} T new strict results for the instanton density n(T) are reported. Using the PCAC methods, we express n(T) in terms of {\it vacuum} average values of certain operators, times their {\it calculated} T-dependence. At high T, we discuss the {\it applicability} limits of the perturbative results. We further speculate about possible behaviour of n(T) at $T\sim T_c$

Rethinking the Properties of the Quark-Gluon Plasma at T∼TcT\sim T_c

We argue that although at asymptotically high temperatures the QGP in bulk behaves as a gas of weakly interacting quasiparticles (modulo long-range magnetism), at temperatures up to few times the critical temperature $T_c$ it displays different properties. If the running of the QCD coupling constant continues in the Coulomb phase till the screening length scale, it reaches the strong coupling treshold $\alpha_s(m_D)\sim 1$. As a result, the Coulomb phase supports weakly bound Coulombic s-wave $\bar c c$, light quark and even $gg$ states. The existence of shallow bound states dramatically increases the quasiparticle rescattering at low energies, reducing the viscosity and thereby explaining why heavy ion collisions at RHIC exhibit robust collective phenomena. In conformal gauge theories at finite temperature the Coulomb binding persists further in the strong coupling regime, as found for ${\cal N}=4$ SUSY YM in the Maldacena regime.Comment: v2 version have one more figure and one more reference, v3 is the same as v2 except a double-page format (the v2 had corrupted last lines on the page

Screening of the topological charge in a correlated instanton vacuum

Screening of the topological charge due to he fermion-induced interactions is an important phenomenon, closely related with the resolution of the strong CP and U(1) problems. We study the mechanism of such screening in a 'correlated instanton vacuum', as opposed to the 'random' one. Both scalar and pseudoscalar gluonic correlators are analyzed by means of an observable that minimizes finite size effects. Screening of the topological charge is established. This allows us to calculate the $\eta'$ mass without having to invert the Dirac operator. We suggest that this method might be used in lattice QCD calculations as well. Our results for the screening of the topological charge are in agreement with the chiral Ward identities, and the scalar gluonic correlator satisfies a low energy theorem first derived by Novikov et al. \cite{Novikov-etal}. We also propose to evaluate the topological susceptibility in the Witten-Veneziano formula not in an infinite box in an world $without$ fermions but in an infinitesimal box in a world $with$ fermions.Comment: 22 pages + 5 postscript figures, SUNY-NTG/94-25. Corrected LATEX erro

Massless Three Dimensional Quantum Electrodynamics and Thirring Model Constrained by Large Flavor Number

We explicitly prove that in three dimensional massless quantum electrodynamics at finite temperature, zero density and large number of flavors the number of infrared degrees of freedom is never larger than the corresponding number of ultraviolet. Such a result, strongly dependent on the asymptotic freedom of the theory, is reversed in three dimensional Thirring model due to the positive derivative of its running coupling constant

The Shear Viscosity to Entropy Density Ratio of Trapped Fermions in the Unitarity Limit

We extract the shear viscosity to entropy density ratio \eta/s of cold fermionic atoms in the unitarity limit from experimental data on the damping of collective excitations. We find that near the critical temperature \eta/s is roughly equal to 1/2 in units of \hbar/k_B. With the possible exception of the quark gluon plasma, this value is closer to the conjectured lower bound 1/(4\pi) than any other known liquid.Comment: published versio