1,912 research outputs found
Screening of heavy quark free energies at finite temperature and non-zero baryon chemical potential
We analyze the dependence of heavy quark free energies on the baryon chemical
potential (mu_b) in 2-flavour QCD using improved (p4) staggered fermions with a
bare quark mass of m/T = 0.4. By performing a 6th order Taylor expansion in the
chemical potential which circumvents the sign problem. The Taylor expansion
coefficients of colour singlet and colour averaged free energies are calculated
and from this the expansion coefficients for the corresponding screening masses
are determined. We find that for small mu_b the free energies of a static quark
anti-quark pair decrease in a medium with a net excess of quarks and that
screening is well described by a screening mass which increases with increasing
mu_b. The mu_b-dependent corrections to the screening masses are well described
by perturbation theory for T > 2 T_c. In particular, we find for all
temperatures above T_c that the expansion coefficients for singlet and colour
averaged screening masses differ by a factor 2.Comment: 14 page
Towards a controlled study of the QCD critical point
The phase diagram of QCD, as a function of temperature T and quark chemical
potential mu, may contain a critical point (mu_E,T_E) whose non-perturbative
nature makes it a natural object of lattice studies. However, the sign problem
prevents the application of standard Monte Carlo techniques at non-zero baryon
density. We have been pursuing an approach free of the sign problem, where the
chemical potential is taken as imaginary and the results are Taylor-expanded in
mu/T about mu=0, then analytically continued to real mu.
Within this approach we have determined the sensitivity of the critical
chemical potential mu_E to the quark mass, d(\mu_E)^2/dm_q|_{\mu_E=0}. Our
study indicates that the critical point moves to {\em smaller} chemical
potential as the quark mass {\em increases}. This finding, contrary to common
wisdom, implies that the deconfinement crossover, which takes place in QCD at
mu=0 when the temperature is raised, will remain a crossover in the mu-region
where our Taylor expansion can be trusted. If this result, obtained on a coarse
lattice, is confirmed by simulations on finer lattices now in progress, then we
predict that no {\em chiral} critical point will be found for mu_B \lesssim 500
MeV, unless the phase diagram contains additional transitions.Comment: 4 pages, 6 figures, proceedings of Quark Matter 2008, Jaipur (India),
Feb. 2008, to appear in J. Phys.
QCD at non-zero temperature and density from the lattice
The study of systems as diverse as the cores of neutron stars and heavy-ion
collision experiments requires the understanding of the phase structure of QCD
at non-zero temperature, T, and chemical potential, mu_q. We review some of the
difficulties of performing lattice simulations of QCD with non-zero mu_q, and
outline the re-weighting method used to overcome this problem. This method is
used to determine the critical endpoint of QCD in the (mu_q,T) plane. We study
the pressure and quark number susceptibility at small mu_q.Comment: 5 pages, talk presented by C.R. Allton at the QCD Downunder
Conference, Barossa Valley and Adelaide, March 200
The QCD phase transition at high temperature and low density
We study the thermal properties of QCD in the presence of a small quark
chemical potential . Derivatives of the phase transition point with
respect to are computed at for 2 and 3 flavors of p-4 improved
staggered fermions on a lattice. Moreover we contrast the case of
isoscalar and isovector chemical potentials, quantify the effect of
on the equation of state, and comment on the screening effect by dynamical
quarks and the complex phase of the fermion determinant in QCD with
.Comment: Lattice2002(nonzerot), 3 pages, 2 figure
Technicolor and Beyond: Unification in Theory Space
The salient features of models of dynamical electroweak symmetry breaking are
reviewed. The ideal walking idea is introduced according to which one should
carefully take into account the effects of the extended technicolor dynamics on
the technicolor dynamics itself. The effects amount at the enhancement of the
anomalous dimension of the mass of the techniquarks allowing to decouple the
Flavor Changing Neutral Currents problem from the one of the generation of the
top mass. Precision data constraints are reviewed focussing on the latest
crucial observation that the S-parameter can be computed exactly near the upper
end of the conformal window (Conformal S-parameter) with relevant consequences
on the selection of nature's next strong force. We will then introduce the
Minimal Walking Technicolor (MWT) models. In the second part of this review we
consider the interesting possibility to marry supersymmetry and technicolor.
The reason is to provide a unification of different extensions of the standard
model. For example, this means that one can recover, according to the
parameters and spectrum of the theory distinct extensions of the standard
model, from supersymmetry to technicolor and unparticle physiscs. A surprising
result is that a minimal (in terms of the smallest number of fields)
supersymmetrization of the MWT model leads to the maximal supersymmetry in four
dimensions, i.e. N=4 SYM.Comment: Extended version of the PASCOS10 proceedings for the Plenary Tal
The QCD phase diagram at nonzero quark density
We determine the phase diagram of QCD on the \mu-T plane for small to
moderate chemical potentials. Two transition lines are defined with two
quantities, the chiral condensate and the strange quark number susceptibility.
The calculations are carried out on N_t =6,8 and 10 lattices generated with a
Symanzik improved gauge and stout-link improved 2+1 flavor staggered fermion
action using physical quark masses. After carrying out the continuum
extrapolation we find that both quantities result in a similar curvature of the
transition line. Furthermore, our results indicate that in leading order the
width of the transition region remains essentially the same as the chemical
potential is increased.Comment: 12 pages, 6 figure
Cognitive Computation sans Representation
The Computational Theory of Mind (CTM) holds that cognitive processes are essentially computational, and hence computation provides the scientific key to explaining mentality. The Representational Theory of Mind (RTM) holds that representational content is the key feature in distinguishing mental from non-mental systems. I argue that there is a deep incompatibility between these two theoretical frameworks, and that the acceptance of CTM provides strong grounds for rejecting RTM. The focal point of the incompatibility is the fact that representational content is extrinsic to formal procedures as such, and the intended interpretation of syntax makes no difference to the execution of an algorithm. So the unique 'content' postulated by RTM is superfluous to the formal procedures of CTM. And once these procedures are implemented in a physical mechanism, it is exclusively the causal properties of the physical mechanism that are responsible for all aspects of the system's behaviour. So once again, postulated content is rendered superfluous. To the extent that semantic content may appear to play a role in behaviour, it must be syntactically encoded within the system, and just as in a standard computational artefact, so too with the human mind/brain - it's pure syntax all the way down to the level of physical implementation. Hence 'content' is at most a convenient meta-level gloss, projected from the outside by human theorists, which itself can play no role in cognitive processing
Latent heat in the chiral phase transition
The chiral phase transition at finite temperature and density is discussed in
the framework of the QCD-like gauge field theory. The thermodynamical potential
is investigated using a variational approach. Latent heat generated in the
first-order phase transition is calculated. It is found that the latent heat is
enhanced near the tricritical point and is more than several hundred MeV per
quark.Comment: 6 pages, 3 figure
Indirect ELISAs based on recombinant and affinity-purified glycoprotein E of Aujeszky's disease virus to differentiate between vaccinated and infected animals
Two indirect ELISAs for the detection of antibodies against glycoprotein E (gE) of Aujeszky's disease virus (ADV) in sera have been developed. The rec-gE-ELISA is based on theE. coli-expressed recombinant protein containing the N-terminal sequences of gE (aa 1-125) fused with the glutathione S-transferase fromSchistosoma japonicum. The affi-gE-ELISA is based on native gE, which was purified from virions by affinity chromatography. The tests were optimised and compared with each other, as well as with the recently developed blocking gE-ELISA (Morenkov et al., 1997b), with respect to specificity and sensitivity. The rec-gE-ELISA was less sensitive in detecting ADV-infected animals than the affi-gE-ELISA (sensitivity 80% and 97%, respectively), which is probably due to the lack of conformation-dependent immunodominant epitopes on the recombinant protein expressed inE. coli. The specificity of the rec-gE-ELISA and affi-gE-ELISA was rather moderate (90% and 94%, respectively) because it was necessary to set such cut-off values in the tests that provided a maximum level of sensitivity, which obviously increased the incidence of false positive reactions. Though the indirect ELISAs detect antibodies against many epitopes of gE, the blocking gE-ELISA, which detects antibodies against only one immunodominant epitope of gE, showed a better test performance (specificity 99% and sensitivity 98%). This is most probably due to rather high dilutions of the sera used in the indirect gE-ELISAs (1:30) as compared to the serum dilution in the blocking gE-ELISA (1:2). We conclude that the indirect gE-ELISAs are sufficiently specific and sensitive to distinguish ADV-infected swine from those vaccinated with gE-negative vaccine and can be useful, in particularly affi-gE-ELISA, as additional tests for the detection of antibodies to gE
The quark mass and dependence of the QCD chiral critical point
In order to study the QCD chiral critical point we investigate Binder
Cumulants of the chiral condensate. The results were obtained from simulations
of 3 and 2+1 flavors of standard staggered fermions and 3 flavors of p4
improved staggered fermions. The quark masses used are close to the physical
quark mass. To extract the dependence on quark mass and chemical potential we
apply a new reweighting technique based on a Taylor expansion of the action.
The reweighting accuracy is for the standard and ,
for the p4 action.Comment: 3 pages, 6 figures, Lattice2002(nonzerot
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