757 research outputs found
Direct numerical computation of disorder parameters
In the framework of various statistical models as well as of mechanisms for
color confinement, disorder parameters can be developed which are generally
expressed as ratios of partition functions and whose numerical determination is
usually challenging. We develop an efficient method for their computation and
apply it to the study of dual superconductivity in 4d compact U(1) gauge
theory.Comment: 5 pages, 6 figures. Final revised version published in PR
Two flavor QCD and Confinement
We argue that the order of the chiral transition for N_f=2 is a sensitive
probe of the QCD vacuum, in particular of the mechanism of color confinement. A
strategy is developed to investigate the order of the transition by use of
finite size scaling analysis. An in-depth numerical investigation is performed
with staggered fermions on lattices with N_t=4 and N_s=12,16,20,24,32 and quark
masses am_q ranging from 0.01335 to 0.307036. The specific heat and a number of
susceptibilities are measured and compared with the expectations of an O(4)
second order and of a first order phase transition. A second order transition
in the O(4) and O(2) universality classes are excluded. Substantial evidence
emerges for a first order transition. A detailed comparison with previous works
is performed.Comment: 46 pages, 20 eps figures, 9 tables, REVTeX
SXP 7.92: A Recently Rediscovered Be/X-ray Binary in the Small Magellanic Cloud, Viewed Edge On
We present a detailed optical and X-ray study of the 2013 outburst of the Small Magellanic Cloud Be/X-ray binary SXP 7.92, as well as an overview of the last 18 years of observations from OGLE (Optical Gravitational Lensing Experiment), RXTE, Chandra and XMM-Newton. We revise the position of this source to RA(J2000) = 00:57:58.4, Dec(J2000) = â72:22:29.5 with a 1Ï uncertainty of 1.5 arcsec, correcting the previously reported position by Coe et al. by more than 20 arcmin. We identify and spectrally classify the correct counterpart as a B1Ve star. The optical spectrum is distinguished by an uncharacteristically deep narrow Balmer series, with the Hα line in particular having a distinctive shell profile, i.e. a deep absorption core embedded in an emission line. We interpret this as evidence that we are viewing the system edge on and are seeing self-obscuration of the circumstellar disc. We derive an optical period for the system of 40.0 ±â0.3 d, which we interpret as the orbital period, and present several mechanisms to describe the X-ray/optical behaviour in the recent outburst, in particular the âflares'and âdipsâ seen in the optical light curve, including a transient accretion disc and an elongated precessing disc
Chiral Transition of N=4 Super Yang-Mills with Flavor on a 3-Sphere
We use the AdS/CFT correspondence to perform a numerical study of a phase
transition in strongly-coupled large-Nc N = 4 Super-Yang-Mills theory on a
3-sphere coupled to a finite number Nf of massive N = 2 hypermultiplets in the
fundamental representation of the gauge group. The gravity dual system is a
number Nf of probe D7-branes embedded in AdS_5 x S^5. We draw the phase diagram
for this theory in the plane of hypermultiplet mass versus temperature and
identify for temperatures above the Hawking-Page deconfinement temperature a
first-order phase transition line across which the chiral condensate jumps
discontinuously.Comment: 23 pages, 15 figures, minor corrections, reference added, nine
figures improved, no change in results or conclusions; expanded discussion of
fixing finite counterterms in section 3.3, figures updated, no changes to
conclusion
The distribution of equivalent widths in long GRB afterglow spectra
The extreme brightness of gamma-ray burst (GRB) afterglows and their simple
spectral shape make them ideal beacons to study the interstellar medium of
their host galaxies through absorption line spectroscopy. Using 69
low-resolution GRB afterglow spectra, we conduct a study of the rest-frame
equivalent width (EW) distribution of features with an average rest-frame EW
larger than 0.5 A. To compare an individual GRB with the sample, we develop EW
diagrams as a graphical tool, and we give a catalogue with diagrams for the 69
spectra. We introduce a line strength parameter (LSP) that allows us to
quantify the strength of the absorption features as compared to the sample by a
single number. Using the distributions of EWs of single-species features, we
derive the distribution of column densities by a curve of growth (CoG) fit. We
find correlations between the LSP and the extinction of the GRB, the UV
brightness of the host galaxies and the neutral hydrogen column density.
However, we see no significant evolution of the LSP with the redshift. There is
a weak correlation between the ionisation of the absorbers and the energy of
the GRB, indicating that, either the GRB event is responsible for part of the
ionisation, or that galaxies with high-ionisation media produce more energetic
GRBs. Spectral features in GRB spectra are, on average, 2.5 times stronger than
those seen in QSO intervening damped Lyman-alpha (DLA) systems and slightly
more ionised. In particular we find larger excess in the EW of CIV1549 relative
to QSO DLAs, which could be related to an excess of Wolf-Rayet stars in the
environments of GRBs. From the CoG fitting we obtain an average number of
components in the absorption features of GRBs of 6.00(-1.25,+1.00). The most
extreme ionisation ratios in our sample are found for GRBs with low neutral
hydrogen column density, which could be related to ionisation by the GRB
emission.Comment: 37 pages, 31 figures, 15 tables. Accepted for publication in Astonomy
and Astrophysic
Holographic Roberge-Weiss Transitions
We investigate N=4 SYM coupled to fundamental flavours at nonzero imaginary
quark chemical potential in the strong coupling and large N limit, using
gauge/gravity duality applied to the D3-D7 system, treating flavours in the
probe approximation. The interplay between Z(N) symmetry and the imaginary
chemical potential yields a series of first-order Roberge-Weiss transitions. An
additional thermal transition separates phases where quarks are bound/unbound
into mesons. This results in a set of Roberge-Weiss endpoints: we establish
that these are triple points, determine the Roberge-Weiss temperature, give the
curvature of the phase boundaries and confirm that the theory is analytic in
mu^2 when mu^2~0.Comment: 37 pages, 13 figures; minor comments added, to appear in JHE
Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310
We model the time-variable absorption of FeII, FeIII, SiII, CII and CrII
detected in UVES spectra of GRB 080310, with the afterglow radiation exciting
and ionizing the interstellar medium in the host galaxy at a redshift of
z=2.42743. To estimate the rest-frame afterglow brightness as a function of
time, we use a combination of the optical VRI photometry obtained by the
RAPTOR-T telescope array -- which are presented in this paper -- and Swift's
X-Ray Telescope observations. Excitation alone, which has been successfully
applied for a handful of other GRBs, fails to describe the observed
column-density evolution in the case of GRB 080310. Inclusion of ionization is
required to explain the column-density decrease of all observed FeII levels
(including the ground state 6D9/2) and increase of the FeIII 7S3 level. The
large population of ions in this latter level (up to 10% of all FeIII) can only
be explained through ionization of FeII, whereby a large fraction of the
ionized FeII ions -- we calculate 31% using the Flexible Atomic (FAC) and Cowan
codes -- initially populate the 7S3 level of FeIII rather than the ground
state. This channel for producing a significant FeIII 7S3 level population may
be relevant for other objects in which absorption lines from this level -- the
UV34 triplet -- are observed, such as BAL quasars and Eta Carinae. This
provides conclusive evidence for time-variable ionization in the circumburst
medium, which to date has not been convincingly detected. However, the best-fit
distance of the neutral absorbing cloud to the GRB is 200--400 pc, i.e. similar
to GRB-absorber distance estimates for GRBs without any evidence for
ionization. We find that the presence of time-varying ionization in GRB 080310
is likely due to a combination of the super-solar iron abundance ([Fe/H]=+0.2)
and the low HI column density (log N(HI)=18.7). [abridged]Comment: 14 pages, 6 figures; accepted for publication in A&A (on August 8,
2012
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
Dynamical Symmetry Breaking in Models with the Yukawa Interaction
We discuss models with a massless fermion and a self-interacting massive
scalar field with the Yukawa interaction. The chiral condensate and the fermion
mass are calculated analytically. It is shown that the models have a phase
transition as a function of the squared mass of the scalar field.Comment: 7 pages, no figures, in Eqs. (7) and (11) one coefficient was change
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