875 research outputs found
Some exact analytical results and a semi-empirical formula for single electron ionization induced by ultrarelativistic heavy ions
The delta function gauge of the electromagnetic potential allows
semiclassical formulas to be obtained for the probability of exciting a single
electron out of the ground state in an ultrarelativistic heavy ion reaction.
Exact formulas have been obtained in the limits of zero impact parameter and
large, perturbative, impact parameter. The perturbative impact parameter result
can be exploited to obtain a semi-empirical cross section formula of the form,
sigma = A ln(gamma) + B, for single electron ionization. A and B can be
evaluated for any combination of target and projectile, and the resulting
simple formula is good at all ultrarelativistic energies. The analytical form
of A and B elucidates a result previously found in numerical calculations:
scaled ionization cross sections decrease with increasing charge of the nucleus
being ionized. The cross section values obtained from the present formula are
in good agreement with recent CERN SPS data from a Pb beam on various nuclear
targets.Comment: 14 pages, latex, revtex source, no figure
Microlensing of gamma ray bursts by stars and MACHOs
The microlensing interpretation of the optical afterglow of GRB 000301C seems
naively surprising, since a simple estimate of the stellar microlensing rate
gives less than one in four hundred for a flat Omega_Lambda=0.7 cosmology,
whereas one event was seen in about thirty afterglows. Considering baryonic
MACHOs making up half of the baryons in the universe, the microlensing
probability per burst can be roughly 5% for a GRB at redshift z=2. We explore
two effects that may enhance the probability of observing microlensed gamma-ray
burst afterglows: binary lenses and double magnification bias. We find that the
consideration of binary lenses can increase the rate only at the ~15% level. On
the other hand, because gamma-ray bursts for which afterglow observations exist
are typically selected based on fluxes at widely separated wavebands which are
not necessarily well correlated (e.g. localization in X-ray, afterglow in
optical/infrared), magnification bias can operate at an enhanced level compared
to the usual single-bias case. We find that existing estimates of the slope of
the luminosity function of gamma-ray bursts, while as yet quite uncertain,
point to enhancement factors of more than three above the simple estimates of
the microlensing rate. We find that the probability to observe at least one
microlensing event in the sample of 27 measured afterglows can be 3-4% for
stellar lenses, or as much as 25 Omega_lens for baryonic MACHOs. We note that
the probability to observe at least one event over the available sample of
afterglows is significant only if a large fraction of the baryons in the
universe are condensed in stellar-mass objects. (ABRIDGED)Comment: 22 pages, 4 figures, 2 table
Coulomb corrections and multiple e+e- pair production in ultra-relativistic nuclear collisions
We consider the problem of Coulomb corrections to the inclusive cross
section. We show that these corrections in the limiting case of small charge
number of one of the nuclei coincide with those to the exclusive cross section.
Within our approach we also obtain the Coulomb corrections for the case of
large charge numbers of both nuclei.Comment: 7 pages, REVTeX
A light-fronts approach to electron-positron pair production in ultrarelativistic heavy-ion collisions
We perform a gauge-transformation on the time-dependent Dirac equation
describing the evolution of an electron in a heavy-ion collision to remove the
explicit dependence on the long-range part of the interaction. We solve, in an
ultra-relativistic limit, the gauged-transformed Dirac equation using
light-front variables and a light-fronts representation, obtaining
non-perturbative results for the free pair-creation amplitudes in the collider
frame. Our result reproduces the result of second-order perturbation theory in
the small charge limit while non-perturbative effects arise for realistic
charges of the ions.Comment: 39 pages, Revtex, 7 figures, submitted to PR
Strong suppression of Coulomb corrections to the cross section of e+e- pair production in ultrarelativistic nuclear collisions
The Coulomb corrections to the cross section of pair production in
ultrarelativistic nuclear collisions are calculated in the next-to-leading
approximation with respect to the parameter
( are the Lorentz factors of colliding nuclei). We found
considerable reduction of the Coulomb corrections even for large
due to the suppression of the production of pair
with the total energy of the order of a few electron masses in the rest frame
of one of the nuclei. Our result explains why the deviation from the Born
result were not observed in the experiment at SPS.Comment: 4 pages, RevTe
Bound-free pair production in ultra-relativistic ion collisions at the LHC collider: Analytic approach to the total and differential cross sections
A theoretical investigation of the bound-free electron-positron pair
production in relativistic heavy ion collisions is presented. Special attention
is paid to the positrons emitted under large angles with respect to the beam
direction. The measurement of these positrons in coincidence with the
down--charged ions is in principle feasible by LHC experiments. In order to
provide reliable estimates for such measurements, we employ the equivalent
photon approximation together with the Sauter approach and derive simple
analytic expressions for the differential pair--production cross section, which
compare favorably to the results of available numerical calculations. Based on
the analytic expressions, detailed calculations are performed for collisions of
bare Pb ions, taking typical experimental conditions of the LHC
experiments into account. We find that the expected count rate strongly depends
on the experimental parameters and may be significantly enhanced by increasing
the positron-detector acceptance cone.Comment: 10 pages, 4 figure
On the nature of Coulomb corrections to the e^+e^- pair production in ultrarelativistic heavy-ion collisions
We manifest the origin of the wrong conclusion made by several groups of
authors on the absence of Coulomb corrections to the cross section of the
e^+e^- pair production in ultrarelativistic heavy-ion collisions. The source of
the mistake is connected with an incorrect passage to the limit in the
expression for the cross section. When this error is eliminated, the Coulomb
corrections do not vanish and agree with the results obtained within the
Weizs\"acker-Williams approximation.Comment: 7 pages, LaTe
Coulomb Effects on Electromagnetic Pair Production in Ultrarelativistic Heavy-Ion Collisions
We discuss the implications of the eikonal amplitude on the pair production
probability in ultrarelativistic heavy-ion transits. In this context the
Weizs\"acker-Williams method is shown to be exact in the ultrarelativistic
limit, irrespective of the produced particles' mass. A new equivalent
single-photon distribution is derived which correctly accounts for the Coulomb
distortions. As an immediate application, consequences for unitarity violation
in photo-dissociation processes in peripheral heavy-ion encounters are
discussed.Comment: 13 pages, 4 .eps figure
Colliders as a simultaneous probe of supersymmetric dark matter and Terascale cosmology
Terascale supersymmetry has the potential to provide a natural explanation of
the dominant dark matter component of the standard lambda-CDM cosmology.
However once we impose the constraints on minimal supersymmetry parameters from
current particle physics data, a satisfactory dark matter abundance is no
longer prima facie natural. This Neutralino Tuning Problem could be a hint of
nonstandard cosmology during and/or after the Terascale era. To quantify this
possibility, we introduce an alternative cosmological benchmark based upon a
simple model of quintessential inflation. This benchmark has no free
parameters, so for a given supersymmetry model it allows an unambiguous
prediction of the dark matter relic density. As a example, we scan over the
parameter space of the CMSSM, comparing the neutralino relic density
predictions with the bounds from WMAP. We find that the WMAP--allowed regions
of the CMSSM are an order of magnitude larger if we use the alternative
cosmological benchmark, as opposed to lambda-CDM. Initial results from the CERN
Large Hadron Collider will distinguish between the two allowed regions.Comment: 14 pages, 8 figure
Probing the central black hole in M87 with gamma-rays
Recent high-sensitivity observation of the nearby radio galaxy M87 have
provided important insights into the central engine that drives the large-scale
outflows seen in radio, optical and X-rays. This review summarizes the
observational status achieved in the high energy (HE;<100 GeV) and very high
energy (VHE; >100 GeV) gamma-ray domains, and discusses the theoretical
progress in understanding the physical origin of this emission and its relation
to the activity of the central black hole.Comment: Invited compact review to be published in Modern Physics Letters A;
19 pages, 4 figure
- …