523 research outputs found
Topologically induced local P and CP violation in hot QCD
Very stringent experimental bounds exist on the amount of P and CP violation
in strong interactions. Nevertheless, the presence of non-Abelian topological
solutions and the axial anomaly make the issue of CP invariance in QCD
non-trivial ("the strong CP problem"). Even in the absence of a global P and CP
violation the fluctuations of topological charge in the QCD vacuum are expected
to play an important role in the breaking of chiral symmetry, and in the mass
spectrum and other properties of hadrons. Here I argue that topological
fluctuations in hot QCD matter can become directly observable in the presence
of a very intense external magnetic field by inducing local P- and CP- odd
effects. These local parity-violating phenomena can be described by using the
Maxwell-Chern-Simons, or axion, electrodynamics as an effective theory. Local P
and CP violation in hot QCD matter can be observed in experiment through the
"chiral magnetic effect" - the separation of electric charge along the axis of
magnetic field that is created by the colliding relativistic ions. There is a
recent evidence for the electric charge separation relative to the reaction
plane of heavy ion collisions from the STAR Collaboration at RHIC.Comment: 10 pages, 2 figures; presented at the 25th Winter Workshop on Nuclear
Dynamics, Big Sky, Montana, February 1-8, 200
D-instantons and multiparticle production in N=4 SYM
N=4 Super-symmetric Yang-Mills theory (N=4 SYM) in the strong coupling regime
has been successfully applied (through the AdS/CFT correspondence) to the
description of strongly coupled plasma which is a multiparticle state. Yet, the
high-energy scattering in the strong coupling limit of N=4 SYM is purely
elastic, so this multiparticle final state can never be produced: this is
because in this limit the theory is dual to weak supergravity, and the dominant
interaction is the elastic graviton exchange. Here we propose a resolution of
this dilemma by considering the contribution of D-instantons in bulk
space to the scattering amplitude. We argue that D-instantons coupled to
dilatons and axions are responsible for multiparticle production in strongly
coupled N=4 SYM, and the corresponding cross section increases with energy. We
evaluate the intercept and the slope of the corresponding Pomeron trajectory in
terms of the typical size of the D-instanton, and argue that the resulting
physical picture may resemble the real world.Comment: 22 pp and 11 figures in the eps forma
High energy nuclear interactions and QCD: an introduction
The goal of these lectures, oriented towards the students just entering the
field, is to provide an elementary introduction to QCD and the physics of
nuclear interactions at high energies. We first introduce the general structure
of QCD and discuss its main properties. Then we proceed to Glauber multiple
scattering theory which lays the foundation for the theoretical treatment of
nuclear interactions at high energies. We introduce the concept of Gribov's
inelastic shadowing, crucial for the understanding of quantum formation
effects. We outline the problems facing Glauber approach at high energies, and
discuss how asymptotic freedom of QCD helps to resolve them, introducing the
concepts of parton saturation and color glass condensate.Comment: 43 pp., 19 figures, based on lectures given by D. Kharzeev at the
School on "New States of Matter in Hadronic Interactions" of the Pan American
Advanced Study Institute (PASI 2002) in Campos do Jordao, Sao Paulo, Brazil,
on January 7-18, 200
Quark fragmentation in the -vacuum
The vacuum of Quantum Chromodynamics is a superposition of degenerate states
with different topological numbers that are connected by tunneling (the
-vacuum). The tunneling events are due to topologically non-trivial
configurations of gauge fields (e.g. the instantons) that induce local \p-odd
domains in Minkowski space-time. We study the quark fragmentation in this
topologically non-trivial QCD background. We find that even though QCD globally
conserves \p and \cp symmetries, two new kinds of \p-odd fragmentation
functions emerge. They generate interesting dihadron correlations: one is the
azimuthal angle correlation usually referred to as
the Collins effect, and the other is the \p-odd correlation that vanishes in the cross section summed over many events, but
survives on the event-by-event basis. Using the chiral quark model we estimate
the magnitude of these new fragmentation functions. We study their experimental
manifestations in dihadron production in collisions, and comment on
the applicability of our approach in deep-inelastic scattering, proton-proton
and heavy ion collisions.Comment: 4 pages, 2 figure
suppression in heavy ion collsions and the QCD phase transition
We suggest that the new regime of suppression in Pb-Pb collisions
found by the NA50 experiment at CERN is the result of non-trivial space-time
evolution due to specific behavior of the Equation of State (EOS) near the QCD
phase transition. We also study another suppression channel, the conversion of
into during the late cool hadronic stage, and find it rather
inefficient.Comment: 10 pages, 3 figure
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