495 research outputs found

### Charged-Particle Decay at Finite Temperature

Radiative corrections to the decay rate of charged fermions caused by the
presence of a thermal bath of photons are calculated in the limit when
temperatures are below the masses of all charged particles involved. The
cancellation of finite-temperature infrared divergences in the decay rate is
described in detail. Temperature-dependent radiative corrections to a two-body
decay of a hypothetical charged fermion and to electroweak decays of a muon are
given. We touch upon possible implications of these results for charged
particles in the early Universe.Comment: 18 pages, 6 figures. v2: typos corrected, bibliography revised,
content matches published versio

### A relativistic non-relativistic Goldstone theorem: gapped Goldstones at finite charge density

We adapt the Goldstone theorem to study spontaneous symmetry breaking in
relativistic theo- ries at finite charge density. It is customary to treat
systems at finite density via non-relativistic Hamiltonians. Here we highlight
the importance of the underlying relativistic dynamics. This leads to seemingly
new results whenever the charge in question is spontaneously broken and does
not commute with other broken charges. We find that that the latter interpolate
gapped excitations. In contrast, all existing versions of the Goldstone theorem
predict the existence of gapless modes. We derive exact non-perturbative
expressions for their gaps, in terms of the chemical potential and of the
symmetry algebra.Comment: 5 pages. v2: minor modifications, matches the PRL versio

### Non-relativistic bound states at finite temperature (II): the muonic hydrogen

We illustrate how to apply modern effective field theory techniques and
dimensional regularization to factorise the various scales which appear in QED
bound states at finite temperature. We focus here on the muonic hydrogen atom.
Vacuum polarization effects make the physics of this atom at finite temperature
very close to that of heavy quarkonium states. We comment on the implications
of our results for these states in the quark gluon plasma. In particular, we
estimate the effects of a finite charm quark mass in the dissociation
temperature of bottomonium.Comment: 22 pages, 8 figures. Journal version, reference adde

### Gamma flashes from relativistic electron-positron plasma droplets

Ultra-intense lasers are expected to produce, in near future, relativistic
electron-positron plasma droplets. Considering the local photon production rate
in complete leading order in quantum electrodynamics (QED), we point out that
these droplets are interesting sources of gamma ray flashesComment: 4 pages, 6 figures; Text has been revised and new refs. are adde

### Momentum Broadening of a Fast Parton in a Perturbative Quark-Gluon Plasma

The average transverse momentum transfer per unit path length to a fast
parton scattering elastically in a perturbative quark-gluon plasma is related
to the radiative energy loss of the parton. We first calculate the momentum
transfer coefficient $\hat q$ in terms of a classical Langevin problem and then
define it quantum-mechanically through scattering matrix element. After
treating the well known case of a quark-gluon plasma in equilibrium we consider
an off-equilibrium unstable plasma. As a specific example, we treat the
two-stream plasma with unstable modes of longitudinal chromoelectric field. In
the presence of the instabilities, $\hat q$ is shown to exponentially grow in
time.Comment: Updated version containing an analysis of insufficiencies in previous
calculations of momentum broadening in unstable plasma

### The Wake of a Heavy Quark in Non-Abelian Plasmas : Comparing Kinetic Theory and the AdS/CFT Correspondence

We compute the non-equilibrium stress tensor induced by a heavy quark moving
through weakly coupled QCD plasma at the speed of light and compare the result
to N = 4 Super Yang Mills theory at strong coupling. The QCD Boltzmann equation
is reformulated as a Fokker-Planck equation in a leading log approximation
which is used to compute the induced stress. The transition from nonequilibrium
at short distances to equilibrium at large distances is analyzed with first and
second order hydrodynamics. Even after accounting for the obvious differences
in shear lengths, the strongly coupled theory is significantly better described
by hydrodynamics at sub-asymptotic distances. We argue that this difference
between the kinetic and AdS/CFT theories is related to the second order
hydrodynamic coefficient $\tau_\pi$. $\tau_\pi$ is numerically large in units
of the shear length for theories based on the Boltzmann equation.Comment: 31 pages, 8 figure

### Thermal Dimuon Yields at NA60

Dilepton emission rates from a hadronic gas at finite temperature and baryon
density are completely constrained by broken chiral symmetry in a density
expansion. The rates can be expressed in terms of vacuum correlations which are
measured in $e^+e^-$ annihilation, $\tau$ decays and photo-reactions on
nucleons and nuclei. In this paper, the theoretical results are summarized and
the total dimuon yield is calculated by integrating the dimuon rates over the
space-time history of a fireball based on hydrodynamic calculations with CERN
SPS conditions. The resulting dimuon yield is in good agreement with the recent
measurements reported by NA60.Comment: 9 pages, 8 figure

### The effect of an imaginary part of the Schwinger-Dyson equation at finite temperature and density

We examined the effect of an imaginary part of the ladder approximation
Schwinger-Dyson equation. We show the imaginary part enhances the effect of the
first order transition, and affects a tricritical point. In particular, a
chemical potential at a tricritical point is moved about 200(MeV). Thus, one
should not ignore the imaginary part. On the other hand, since an imaginary
part is small away from a tricritical point, one should be able to ignore an
imaginary part. In addition, we also examined the contribution of the wave
function renormalization constant.Comment: 12 pages, 14 figure

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