1,134 research outputs found
Chemical equilibrium study in nucleus-nucleus collisions at relativistic energies
We present a detailed study of chemical freeze-out in nucleus-nucleus
collisions at beam energies of 11.6, 30, 40, 80 and 158A GeV. By analyzing
hadronic multiplicities within the statistical hadronization approach, we have
studied the strangeness production as a function of centre of mass energy and
of the parameters of the source. We have tested and compared different versions
of the statistical model, with special emphasis on possible explanations of the
observed strangeness hadronic phase space under-saturation. We show that, in
this energy range, the use of hadron yields at midrapidity instead of in full
phase space artificially enhances strangeness production and could lead to
incorrect conclusions as far as the occurrence of full chemical equilibrium is
concerned. In addition to the basic model with an extra strange quark
non-equilibrium parameter, we have tested three more schemes: a two-component
model superimposing hadrons coming out of single nucleon-nucleon interactions
to those emerging from large fireballs at equilibrium, a model with local
strangeness neutrality and a model with strange and light quark non-equilibrium
parameters. The behaviour of the source parameters as a function of colliding
system and collision energy is studied. The description of strangeness
production entails a non-monotonic energy dependence of strangeness saturation
parameter gamma_S with a maximum around 30A GeV. We also present predictions of
the production rates of still unmeasured hadrons including the newly discovered
Theta^+(1540) pentaquark baryon.Comment: 36 pages, 14 figures. Revised version published in Phys. Rev. C:
title changed, one paragraph added in section 2, other typos correcte
Third generation sfermions decays into Z and W gauge bosons: full one-loop analysis
The complete one-loop radiative corrections to third generation scalar
fermions into gauge bosons Z and W^\pm is considered. We focus on \wt{f}_2 \to
Z \wt{f}_1 and \wt{f}_i \to W^\pm \wt{f'}_j (f,f'=t,b). We include both
SUSY-QCD, QED and full electroweak corrections. It is found that the
electroweak corrections can be of the same order as the SUSY-QCD corrections.
The two sets of corrections interfere destructively in some region of parameter
space. The full one loop correction can reach 10% in some SUGRA scenario, while
in model independent analysis like general MSSM, the one loop correction can
reach 20% for large \tan\beta and large trilinear soft breaking terms A_b.Comment: Latex file, 18 pages, 8 figures, version to appear in PR
Fixed block configuration group divisible designs with block size six
AbstractWe present constructions and results about GDDs with two groups and block size six. We study those GDDs in which each block has configuration (s,t), that is in which each block has exactly s points from one of the two groups and t points from the other. We show the necessary conditions are sufficient for the existence of GDD(n,2,6;λ1,λ2)s with fixed block configuration (3,3). For configuration (1,5), we give minimal or near-minimal index examples for all group sizes n≥5 except n=10,15,160, or 190. For configuration (2,4), we provide constructions for several families of GDD(n,2,6;λ1,λ2)s
Lorentz angle measurements in irradiated silicon detectors between 77 K and 300 K
Future experiments are using silicon detectors in a high radiation
environment and in high magnetic fields. The radiation tolerance of silicon
improves by cooling it to temperatures below 180 K. At low temperatures the
mobility increases, which leads to larger deflections of the charge carriers by
the Lorentz force. A good knowledge of the Lorentz angle is needed for design
and operation of silicon detectors. We present measurements of the Lorentz
angle between 77 K and 300 K before and after irradiation with a primary beam
of 21 MeV protons.Comment: 13 pages, 9 figures, submitted to ICHEP2000, Osaka, Japa
Ultra-high energy cosmic rays from Quark Novae
We explore acceleration of ions in the Quark Nova (QN) scenario, where a
neutron star experiences an explosive phase transition into a quark star (born
in the propeller regime). In this picture, two cosmic ray components are
isolated: one related to the randomized pulsar wind and the other to the
propelled wind, both boosted by the ultra-relativistic Quark Nova shock. The
latter component acquires energies while
the former, boosted pulsar wind, achieves ultra-high energies
eV. The composition is dominated by ions present in the pulsar wind in the
energy range above eV, while at energies below eV the
propelled ejecta, consisting of the fall-back neutron star crust material from
the explosion, is the dominant one. Added to these two components, the
propeller injects relativistic particles with Lorentz factors , later to be accelerated by galactic supernova shocks. The
QN model appears to be able to account for the extragalactic cosmic rays above
the ankle and to contribute a few percent of the galactic cosmic rays below the
ankle. We predict few hundred ultra-high energy cosmic ray events above
eV for the Pierre Auger detector per distant QN, while some thousands
are predicted for the proposed EUSO and OWL detectors.Comment: 20 pages, 1 figure. Major revisions in the text. Accepted for
publication in the Astrophysical Journa
Transport in holographic superfluids
We construct a slowly varying space-time dependent holographic superfluid and
compute its transport coefficients. Our solution is presented as a series
expansion in inverse powers of the charge of the order parameter. We find that
the shear viscosity associated with the motion of the condensate vanishes. The
diffusion coefficient of the superfluid is continuous across the phase
transition while its third bulk viscosity is found to diverge at the critical
temperature. As was previously shown, the ratio of the shear viscosity of the
normal component to the entropy density is 1/(4 pi). As a consequence of our
analysis we obtain an analytic expression for the backreacted metric near the
phase transition for a particular type of holographic superfluid.Comment: 45 pages + appendice
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