1,800 research outputs found
A Quantitative Analysis of Charmonium Suppression in Nuclear Collisions
Data from J/psi and psi' production in p-A collisions are used to determine
the cross section for absorption of pre-resonance charmonium in nuclear matter.
The J/psi suppression in O-Cu, O-U and S-U collisions is fully reproduced by
the corresponding nuclear absorption, while Pb-Pb collisions show an additional
suppression increasing with centrality. We study the onset of this change in
terms of hadronic comover interactions and conclude that so far no conventional
hadronic description can consistently account for all data. Deconfinement,
starting at a critical point determined by central S-U collisions, is in accord
with the observed suppression pattern.Comment: 37 pages, 12 figures, uses epsfig style, LaTe
Mesonic correlation functions at finite temperature and density in the Nambu-Jona-Lasinio model with a Polyakov loop
We investigate the properties of scalar and pseudo-scalar mesons at finite
temperature and quark chemical potential in the framework of the
Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (PNJL model) with
the aim of taking into account features of both chiral symmetry breaking and
deconfinement. The mesonic correlators are obtained by solving the
Schwinger-Dyson equation in the RPA approximation with the Hartree (mean field)
quark propagator at finite temperature and density. In the phase of broken
chiral symmetry a narrower width for the sigma meson is obtained with respect
to the NJL case; on the other hand, the pion still behaves as a Goldstone
boson. When chiral symmetry is restored, the pion and sigma spectral functions
tend to merge. The Mott temperature for the pion is also computed.Comment: 24 pages, 9 figures, version to appear in Phys. Rev.
Delay performance in random-access grid networks
We examine the impact of torpid mixing and meta-stability issues on the delay
performance in wireless random-access networks. Focusing on regular meshes as
prototypical scenarios, we show that the mean delays in an toric
grid with normalized load are of the order . This
superlinear delay scaling is to be contrasted with the usual linear growth of
the order in conventional queueing networks. The intuitive
explanation for the poor delay characteristics is that (i) high load requires a
high activity factor, (ii) a high activity factor implies extremely slow
transitions between dominant activity states, and (iii) slow transitions cause
starvation and hence excessively long queues and delays. Our proof method
combines both renewal and conductance arguments. A critical ingredient in
quantifying the long transition times is the derivation of the communication
height of the uniformized Markov chain associated with the activity process. We
also discuss connections with Glauber dynamics, conductance and mixing times.
Our proof framework can be applied to other topologies as well, and is also
relevant for the hard-core model in statistical physics and the sampling from
independent sets using single-site update Markov chains
Kaon and production vs Participants in Nuclear Collisions
Data on kaon and production in nuclear collisions as a function of
centrality are analysed both at AGS and SPS energy range. We compare the
results of several experiments, looking for common trend in `participant
scaling' of production yields. We find a smooth description of scaled kaon and
yields as a function of participant density. We also show a participant
density dependence of kaons and produced in the forward hemisphere for
proton-nucleus collisions.Comment: Proceedings of the International Conference on Strangeness in Quark
Matter, 20-25 July 2000, Berkeley, CA. To appear in Journal of Physics G:
Nuclear and Particle Physic
Signals of Unconventional E Models at Colliders
Generation dependent discrete symmetries often appear in models derived from
superstring theories. In particular, in the framework of E models the
presence of such symmetries is required in order to allow for the radiative
generation of naturally small neutrino masses. Recently it was shown that by
imposing suitable generation dependent discrete symmetries, a class of models
can be consistently constructed in which the three sets of known fermions in
each generation do not have the same assignments with respect to the {\bf 27}
representation of E. In this scenario, the different embedding in the gauge
group of the three generations implies in particular that the known charged
leptons couple in a non--universal way to the new neutral gauge bosons
present in these models. We exploit this fact to study the
signature of this class of models at present and future colliders. We
show that some signals of deviation from lepton universality as well as some
other discrepancies with the standard model predictions which have been
observed at the TRISTAN collider in the production rate of and ,
can be accounted for if the mass is not much heavier than 300 GeV. We
also study the discovery limits for lepton universality violation of this type
at LEP-2 and at the 500 GeV Next Linear Collider (NLC). We show that
models predicting unconventional assignments for the leptons will give an
unmistakable signature, when the mass is as heavy as GeV
(LEP-2) and TeV (NLC).Comment: Plain Tex, 20 pages. 4 PostScript figures (uses `epsf.tex'). Modified
file-format. No changes in the tex
R_b and New Physics: A Comprehensive Analysis
We survey the implications for new physics of the discrepancy between the LEP
measurement of and its Standard Model prediction. Two broad classes of
models are considered: () those in which new Z\bbar b couplings arise at
tree level, through or -quark mixing with new particles, and ()
those in which new scalars and fermions alter the Z \bbar b vertex at one
loop. We keep our analysis as general as possible in order to systematically
determine what kinds of features can produce corrections to of the right
sign and magnitude. We are able to identify several successful mechanisms,
which include most of those which have been recently been proposed in the
literature, as well as some earlier proposals (\eg\ supersymmetric models). By
seeing how such models appear as special cases of our general treatment we are
able to shed light on the reason for, and the robustness of, their ability to
explain .Comment: 60 pages, 8 figures, plain tex, uses epsf. Final version to appear in
Phys. Rev. D; propgating sign error corrected in eqs. 78, 87, 88, 89, 98, and
107; results unchange
Leptogenesis from Soft Supersymmetry Breaking (Soft Leptogenesis)
Soft leptogenesis is a scenario in which the cosmic baryon asymmetry is
produced from a lepton asymmetry generated in the decays of heavy sneutrinos
(the partners of the singlet neutrinos of the seesaw) and where the relevant
sources of CP violation are the complex phases of soft supersymmetry-breaking
terms. We explain the motivations for soft leptogenesis, and review its basic
ingredients: the different CP-violating contributions, the crucial role played
by thermal corrections, and the enhancement of the efficiency from lepton
flavour effects. We also discuss the high temperature regime GeV in
which the cosmic baryon asymmetry originates from an initial asymmetry of an
anomalous -charge, and soft leptogenesis reembodies in -genesis.Comment: References updated. Some minor corrections to match the published
versio
High Energy Neutrinos From Superheavy Dark Matter Annihilation
Superheavy ( GeV) particles produced during inflation may be the
dark matter, independent of their interaction strength. Strongly interacting
superheavy particles will be captured by the sun, and their annihilation in the
center of the sun will produce a flux of energetic neutrinos that should be
detectable by neutrino telescopes. Depending on the particle mass, event rates
in a cubic-kilometer detector range from several per hour to several per year.
The signature of the process is a predominance of tau neutrinos, with a
relatively flat energy spectrum of events ranging from 50 GeV to many TeV, and
with the mean energy of detected tau neutrinos about 3 TeV.Comment: 24 pages, 7 figure
Small steps towards Grand Unification and the electron/positron excesses in cosmic-ray experiments
We consider a small extension of the standard model by adding two Majorana
fermions; those are adjoint representations of the SU(2)_L and SU(3)_c gauge
groups of the standard model. In this extension, the gauge coupling unification
at an energy scale higher than 10^{15} GeV is realized when the masses of the
triplet and the octet fermions are smaller than 10^4 GeV and 10^{12} GeV,
respectively. We also show that an appropriate symmetry ensures a long lifetime
of the neutral component of the triplet fermion whose thermal relic density
naturally explains the observed dark matter density. The electron/positron
excesses observed in recent cosmic-ray experiments can be also explained by the
decay of the triplet fermion.Comment: 11 pages, 5 figure
Research Project as Boundary Object: negotiating the conceptual design of a tool for International Development
This paper reflects on the relationship between who one designs for and what one designs in the unstructured space of designing for political change; in particular, for supporting âInternational Developmentâ with ICT. We look at an interdisciplinary research project with goals and funding, but no clearly defined beneficiary group at start, and how amorphousness contributed to impact. The reported project researched a bridging tool to connect producers with consumers across global contexts and show players in the
supply chain and their circumstances. We explore how both the nature of the research and the toolâs function became contested as work progressed. To tell this tale, we invoke
the idea of boundary objects and the value of tacking back and forth between elastic meanings of the projectâs artefacts and processes. We examine the projectâs role in India, Chile and other arenas to draw out ways that it functioned as a catalyst and how absence of committed design choices acted as an unexpected strength in reaching its goals
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