723 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
Implications of muon anomalous magnetic moment for supersymmetric dark matter
The anomalous magnetic moment of the muon has recently been measured to be in
conflict with the Standard Model prediction with an excess of 2.6 sigma. Taking
the excess at face value as a measurement of the supersymmetric contribution,
we find that at 95% confidence level it imposes an upper bound of 500 GeV on
the neutralino mass and forbids higgsinos as being the bulk of cold dark
matter. Other implications for the astrophysical detection of neutralinos
include: an accessible minimum direct detection rate, lower bounds on the
indirect detection rate of neutrinos from the Sun and the Earth, and a
suppression of the intensity of gamma-ray lines from neutralino annihilations
in the galactic halo.Comment: 4 pages, 2 figures, revised version accepted for publication in
Physical Review Letter
Directional detection of Dark Matter
Among the many experimental techniques available, those providing directional
information have the potential of yielding an unambiguous observation of WIMPs
even in the presence of insidious backgrounds. A measurement of the
distribution of arrival direction of WIMPs can also discriminate between
Galactic Dark Matter halo models. In this article, I will discuss the
motivation for directional detectors and review the experimental techniques
used by the various experiments. I will then describe one of them, the DMTPC
detector, in more detail.Comment: 17 pages, 11 postscript figures, mini-review submitted to Modern
Physics Letters A (MPLA). Submitted to Modern Physics Letters A (MPLA
Impact parameter dependence of heavy ion e+ e- pair production to all orders in Z alpha
The heavy ion probability for continuum e+ e- pair production has been
calculated to all orders in Z alpha as a function of impact parameter. The
formula resulting from an exact solution of the semiclassical Dirac equation in
the ultrarelativistic limit is evaluated numerically. In a calculation of gamma
= 100 colliding Au ions the probability of e+ e- pair production is reduced
from the perturbation theory result throughout the impact parameter range.Comment: 20 pages, latex, revtex, 6 eps figures. Revised Phys. Rev. C version
with minor additions, one figure added, and added reference
Process 3 -> 3 and crossing symmetry violation
Using the Sudakov technique we sum the perturbation series for the process
and obtain the compact analytical expression for the amplitude of this
process, which takes into account all possible Coulomb interactions between
colliding particles. Compare it with the amplitude of the lepton pair
production in heavy ion collision i.e. in the process , we show that
crossing symmetry between this processes holds only if one neglects the
interaction of produced pair with ions (i.e. in the approximation
).Comment: LaTeX2e, 10 pages, 5 eps figure
Determination of Dark Matter Properties at High-Energy Colliders
If the cosmic dark matter consists of weakly-interacting massive particles,
these particles should be produced in reactions at the next generation of
high-energy accelerators. Measurements at these accelerators can then be used
to determine the microscopic properties of the dark matter. From this, we can
predict the cosmic density, the annihilation cross sections, and the cross
sections relevant to direct detection. In this paper, we present studies in
supersymmetry models with neutralino dark matter that give quantitative
estimates of the accuracy that can be expected. We show that these are well
matched to the requirements of anticipated astrophysical observations of dark
matter. The capabilities of the proposed International Linear Collider (ILC)
are expected to play a particularly important role in this study.Comment: 124 pages, 62 figures; corrections and new material in Section 2.6
(direct detection); misc. additional correction
Spin polarization versus lifetime effects at point contacts between superconducting niobium and normal metals
Point-contact Andreev reflection spectroscopy is used to measure the spin
polarization of metals but analysis of the spectra has encountered a number of
serious challenges, one of which is the difficulty to distinguish the effects
of spin polarization from those of the finite lifetime of Cooper pairs. We have
recently confirmed the polarization-lifetime ambiguity for Nb-Co and Nb-Cu
contacts and suggested to use Fermi surface mismatch, the normal reflection due
to the difference of Fermi wave vectors of the two electrodes, to solve this
dilemma. Here we present further experiments on contacts between
superconducting Nb and the ferromagnets Fe and Ni as well as the noble metals
Ag and Pt that support our previous results. Our data indicate that the Nb -
normal metal interfaces have a transparency of up to about 80 per cent and a
small, if not negligible, spin polarization.Comment: 7 pages, 2 figures, submitted to Proceedings of the 26th Conference
on Low Temperature Physic
Luminosity Function of Faint Globular Clusters in M87
We present the luminosity function to very faint magnitudes for the globular
clusters in M87, based on a 30 orbit \textit{Hubble Space Telescope (HST)}
WFPC2 imaging program. The very deep images and corresponding improved false
source rejection allow us to probe the mass function further beyond the
turnover than has been done before. We compare our luminosity function to those
that have been observed in the past, and confirm the similarity of the turnover
luminosity between M87 and the Milky Way. We also find with high statistical
significance that the M87 luminosity function is broader than that of the Milky
Way. We discuss how determining the mass function of the cluster system to low
masses can constrain theoretical models of the dynamical evolution of globular
cluster systems. Our mass function is consistent with the dependence of mass
loss on the initial cluster mass given by classical evaporation, and somewhat
inconsistent with newer proposals that have a shallower mass dependence. In
addition, the rate of mass loss is consistent with standard evaporation models,
and not with the much higher rates proposed by some recent studies of very
young cluster systems. We also find that the mass-size relation has very little
slope, indicating that there is almost no increase in the size of a cluster
with increasing mass.Comment: 22 pages, 5 figures, Accepted for publication in Ap
- …