24 research outputs found
Variational techniques in non-perturbative QCD
We review attempts to apply the variational principle to understand the
vacuum of non-abelian gauge theories. In particular, we focus on the method
explored by Ian Kogan and collaborators, which imposes exact gauge invariance
on the trial Gaussian wave functional prior to the minimization of energy. We
describe the application of the method to a toy model -- confining compact QED
in 2+1 dimensions -- where it works wonderfully and reproduces all known
non-trivial results. We then follow its applications to pure Yang-Mills theory
in 3+1 dimensions at zero and finite temperature. Among the results of the
variational calculation are dynamical mass generation and the analytic
description of the deconfinement phase transition.Comment: 71 pages, 1 figure. To be published in the memorial volume "From
Fields to Strings: Cirvumnavigating Theoretical Physics", World Scientific,
2004. Dedicated to the memory of Ian Koga
Forward-Backward rapidity correlations at all rapidities
We discuss forward-bacward rapidity correlations in the general situation of
asymmetrical collisions, asymmetric rapidity windows, higher rapidities and
higher energy. We give predictions for RHIC and LHC.Comment: 6 pages, 3 figure
Variational analysis of the deconfinement phase transition
We study the deconfining phase transition in 3+1 dimensional pure SU(N)
Yang-Mills theory using a gauge invariant variational calculation. We
generalize the variational ansatz of Phys. Rev. D52, 3719 (1995) to mixed
states (density matrices) and minimize the free energy. For N > 3 we find a
first order phase transition with the transition temperature of T_C = 450 Mev.
Below the critical temperature the Polyakov loop has vanishing expectation
value, while above T_C, its average value is nonzero. According to the standard
lore this corresponds to the deconfining transition. Within the accuracy of our
approximation the entropy of the system in the low temperature phase vanishes.
The latent heat is not small but, rather, is of the order of the
nonperturbative vacuum energy.Comment: 15 pages, correction of minor typos only, submitted to JHE
Testing collinear factorization and nuclear parton distributions with pA collisions at the LHC
Global perturbative QCD analyses, based on large data sets from
electron-proton and hadron collider experiments, provide tight constraints on
the parton distribution function (PDF) in the proton. The extension of these
analyses to nuclear parton distributions (nPDF) has attracted much interest in
recent years. nPDFs are needed as benchmarks for the characterization of hot
QCD matter in nucleus-nucleus collisions, and attract further interest since
they may show novel signatures of non- linear density-dependent QCD evolution.
However, it is not known from first principles whether the factorization of
long-range phenomena into process-independent parton distribution, which
underlies global PDF extractions for the proton, extends to nuclear effects. As
a consequence, assessing the reliability of nPDFs for benchmark calculations
goes beyond testing the numerical accuracy of their extraction and requires
phenomenological tests of the factorization assumption. Here we argue that a
proton-nucleus collision program at the LHC would provide a set of measurements
allowing for unprecedented tests of the factorization assumption underlying
global nPDF fits.Comment: 4 pages, 5 figure
Vector potential versus colour charge density in low-x evolution
We reconsider the evolution equations for multigluon correlators derived in
hep-ph/9709432. We show how to derive these equations directly in terms of
vector potentials (or colour field strength) avoiding the introduction of the
concept of colour charge density in the intermediate steps. The two step
procedure of deriving the evolution of the charge density correlators followed
by the solution of classical equations for the vector potentials is shown to be
consistent with direct derivation of evolution for vector potentials. In the
process we correct some computational errors of hep-ph/9709432 and present the
corrected evolution equations which have a somewhat simpler appearance.Comment: 15 pages, 1 figure, changes made referee report, to be published in
Phys. Rev
Jet Quenching via Jet Collimation
The ATLAS Collaboration recently reported strong modifications of dijet
properties in heavy ion collisions. In this work, we discuss to what extent
these first data constrain already the microscopic mechanism underlying jet
quenching. Simple kinematic arguments lead us to identify a frequency
collimation mechanism via which the medium efficiently trims away the soft
components of the jet parton shower. Through this mechanism, the observed dijet
asymmetry can be accomodated with values of that lie in the
expected order of magnitude.Comment: 6 pages, 4 figure
The contribution of medium-modified color flow to jet quenching
Multiple interactions between parton showers and the surrounding QCD matter
are expected to underlie the strong medium-modifications of jet observables in
ultra-relativistic heavy ion collisions at RHIC and at the LHC. Here, we note
that such jet-medium interactions alter generically and characteristically the
color correlations in the parton shower. We characterize these effects in a
color-differential calculation of the medium-induced gluon radiation spectrum
to first and second order in opacity. By interfacing simple branching histories
of medium-modified color flow with the Lund hadronization model, we analyze how
the medium modification of color correlations can affect the distribution of
hadronic fragments in jets. Importantly, we observe that jet-medium
interactions give rise to the medium-induced color decoherence of gluons from
the parton shower. Since hadronization respects color flow and since each color
singlet in a parton shower is hadronized separately, this medium-induced color
decoherence leaves characteristic signatures in the jet fragmentation pattern.
In particular, it can contribute to the quenching of leading hadron spectra.
Moreover, it can increase strongly the yield of soft hadronic fragments from a
jet, while the distribution of more energetic hadrons follows naturally the
shape of a vacuum-like fragmentation pattern of lower total energy