311 research outputs found
Jet quenching and broadening: the transport coefficient in an anisotropic plasma
The jet quenching parameter is analyzed for a quark jet propagating
in an anisotropic plasma. The momentum anisotropy is calculated at high
temperature of the underlying quark-gluon plasma. is explicitly
estimated in leading-logarithmic approximation by the broadening of the
massless quark interacting via gluon exchange. A plasma instability is present.
Strong indications are found that is increasing with increasing
anisotropy. Possible implications for the saturation scale in
collisions are pointed out.Comment: 1
Duality between QCD Perturbative Series and Power Corrections
We elaborate on the relation between perturbative and power-like corrections
to short-distance sensitive QCD observables. We confront theoretical
expectations with explicit perturbative calculations existing in literature. As
is expected, the quadratic correction is dual to a long perturbative series and
one should use one of them but not both. However, this might be true only for
very long perturbative series, with number of terms needed in most cases
exceeding the number of terms available. What has not been foreseen, the
quartic corrections might also be dual to the perturbative series. If
confirmed, this would imply a crucial modification of the dogma. We confront
this quadratic correction against existing phenomenology (QCD (spectral) sum
rules scales, determinations of light quark masses and of
\alpha_s from \tau-decay).
We find no contradiction and (to some extent) better agreement with the data
and with recent lattice calculations.Comment: shorten version to appear in PL
On the equivalence between the Boltzmann equation and classical field theory at large occupation numbers
We consider a system made up of exictations of a neutral scalar field, \phi,
having a \lambda\phi^4 interaction term. Starting from an ensemble where the
occupation number f is large, but \lambda f is small, we develop a classical
field theory description of the evolution of the system toward equilibrium. A
Boltzmann equation naturally emerges in this description and we show by
explicit calculation that the collision term is the same as that coming from
elastic scattering. This shows the equivalence of a Boltzmann equation
description and a classical field theory description of the same system.Comment: 15 pages, 7 figure
On Renormalons and Landau Poles in Gauge Field Theories
It is shown that the commonly accepted relationship between the Landau
singularity in the running coupling constant of QED or QCD and the renormalon
singularities in the Borel sums of perturbation theory expansions is only a
particular feature of the restriction of the perturbative --function to
the one loop level.Comment: 11 pages, latex. One comment and one reference adde
Multi-solitary waves for the nonlinear Klein-Gordon equation
International audienceWe consider the nonlinear Klein-Gordon equation in . We call multi-solitary waves a solution behaving at large time as a sum of boosted standing waves. Our main result is the existence of such multi-solitary waves, provided the composing boosted standing waves are stable. It is obtained by solving the equation backward in time around a sequence of approximate multi-solitary waves and showing convergence to a solution with the desired property. The main ingredients of the proof are finite speed of propagation, variational characterizations of the profiles, modulation theory and energy estimates
Silicon intercalation into the graphene-SiC interface
In this work we use LEEM, XPEEM and XPS to study how the excess Si at the
graphene-vacuum interface reorders itself at high temperatures. We show that
silicon deposited at room temperature onto multilayer graphene films grown on
the SiC(000[`1]) rapidly diffuses to the graphene-SiC interface when heated to
temperatures above 1020. In a sequence of depositions, we have been able to
intercalate ~ 6 ML of Si into the graphene-SiC interface.Comment: 6 pages, 8 figures, submitted to PR
Instanton--anti-instanton pair induced contributions to and
The instanton--anti-instanton pair induced asymptotics of perturbation theory
expansion for the cross section of electron--positron pair annihilation to
hadrons and hadronic width of -lepton was found. For the
nonperturbative instanton contribution is finite and may be calculated without
phenomenological input. The instanton induced peturbative asymptotics was shown
to be enhanced as and in the intermediate region may exceed
the renormalon contribution. Unfortunately, the analysis of
corrections shows that for the obtained asymptotic expressions are
at best only the order of magnitude estimate. The asymptotic series for , though obtained formally for , is valid
up to energies Gev. The instanton--anti-instanton pair nonperturbative
contribution to blows up. On the one hand, this
means that instantons could not be considered {\it ab--initio} at such
energies. On the other hand, this result casts a strong doubt upon the
possibility to determine the from the --lepton width.Comment: 22 pages, latex, no figure
Parton energy loss in a classical strongly coupled QGP
We investigate the energy loss of heavy quarks in the gas, liquid and solid
phase of a classical quark-gluon plasma (cQGP) using molecular dynamics
simulations. The model consists of massive quarks and gluons interacting as a
classical non-relativistic colored Coulomb gas. We show that the electric force
decorrelates on a short time scale causing the energy loss to be mostly
diffusive and langevin-like in the cQGP. We find that the drag coefficient
changes with the heavy quark mass, while the diffusion constant does not. The
fractional collisional energy loss is much larger than the leading order
estimates from a wQGP because of the core repulsion. Following recent
suggestions, we show how the cQGP results can be translated to the sQGP results
in the range.Comment: 17 pages, 13 figures, corrected minor typo
Light Quark Masses 99
I give a short historical and a critical review of the determinations of
light quark masses from QCD at dawn of the next millennium. QCD spectral sum
rules combined with ChPT give, to order , the world average for the
running masses: (2 GeV)= MeV,
MeV, MeV
and the corresponding values of the invariant masses given in Eq. 24. Lower and
upper bounds derived from the positivity of spectral moments are presented in
Tables 2 and 3. For a comparison, we critically review the recent lattice
results (section 8 and Table 5) and attempt to deduce the present {\it QCD
grand average} determination (Table 6): (2 GeV)=
MeV, to be used with a great care. Then, we deduce the value:
and the lower bound (resp. ), for the
combination of the penguin operators, governing the CP-violating parameters
without (resp. with) the inclusion of the final state
interaction effects. The result signals a possible deviation from the leading
1/N prediction by about , which should be tested using accurate
non-perturbative calculations.Comment: Latex2e two-column 13 pages 5 Tables + 1 figure. Review talk given at
QCD 99 (Montpellier 7-13th July 1999) and plenary talk at QCD Confinement
2000 (Osaka 7-10th March 2000) Report-n0: Montpellier preprint PM/99-50.
Updated version and application to epsilon'/epsilo
Next-to-next-to-leading order QCD analysis of the Gross-Llewellyn Smith sum rule and the higher twist effects
We present the next-to-next-to-leading order QCD analysis of the
Gross-Llewellyn Smith (GLS) sum rule in deep inelastic lepton-nucleon
scattering, taking into account dimension-two, twist-four power correction. We
discuss in detail the renormalization scheme dependence of the perturbative QCD
approximations, propose a procedure for an approximate treatment of the quark
mass threshold effects and compare the results of our analysis to the recent
experimental data of the CCFR collaboration. From this comparison we extract
the value of the strong coupling constant . We
stress the importance of an accurate measurement of the GLS sum rule and in
particular of its dependence.Comment: Latex 19 pages and 4 Postscript figures appended at the end of this
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