658 research outputs found
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 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
Rethinking the Properties of the Quark-Gluon Plasma at
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 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 . As a result, the Coulomb phase
supports weakly bound Coulombic s-wave , light quark and even
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 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 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
fermions but in an infinitesimal box in a world 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
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