705,069 research outputs found
Spontaneous chiral symmetry breaking in QCD:a finite-size scaling study on the lattice
Spontaneous chiral symmetry breaking in QCD with massless quarks at infinite
volume can be seen in a finite box by studying, for instance, the dependence of
the chiral condensate from the volume and the quark mass. We perform a
feasibility study of this program by computing the quark condensate on the
lattice in the quenched approximation of QCD at small quark masses. We carry
out simulations in various topological sectors of the theory at several
volumes, quark masses and lattice spacings by employing fermions with an exact
chiral symmetry, and we focus on observables which are infrared stable and free
from mass-dependent ultraviolet divergences. The numerical calculation is
carried out with an exact variance-reduction technique, which is designed to be
particularly efficient when spontaneous symmetry breaking is at work in
generating a few very small low-lying eigenvalues of the Dirac operator. The
finite-size scaling behaviour of the condensate in the topological sectors
considered agrees, within our statistical accuracy, with the expectations of
the chiral effective theory. Close to the chiral limit we observe a detailed
agreement with the first Leutwyler-Smilga sum rule. By comparing the mass, the
volume and the topology dependence of our results with the predictions of the
chiral effective theory, we extract the corresponding low-energy constant.Comment: 24 pages, 8 figure
A note on supersymmetric Yang-Mills thermodynamics
The thermodynamics of supersymmetric Yang-Mills theories is studied by
computing the two-loop correction to the canonical free energy and to the
equation of state for theories with 16, 8 and 4 supercharges in any dimension
, and in two dimensions at finite volume. In the
four-dimensional case we also evaluate the first non-analytic contribution in
the 't Hooft coupling to the free energy, arising from the resummation of ring
diagrams. To conclude, we discuss some applications to the study of the
Hagedorn transition in string theory in the context of Matrix strings and
speculate on the possible physical meaning of the transition.Comment: 19 pages, harvmac, epsf. 1 figure included. Minor changes: typos
corrected; references, a footonote and a note adde
Exact holography and black hole entropy in N=8 and N=4 string theory
We compute the exact entropy of one-eighth and one-quarter BPS black holes in
N=8 and N=4 string theory respectively. This includes all the N=4 CHL models in
both K3 and T^4 compactifications. The main result is a measure for the finite
dimensional integral that one obtains after localization of supergravity on
AdS_2xS^2. This measure is determined entirely by an anomaly in supersymmetric
Chern-Simons theory on local AdS_3 and takes into account the contribution from
all the supergravity multiplets. In Chern-Simons theory on compact manifolds
this is the anomaly that computes a certain one-loop dependence on the volume
of the manifold. For one-eighth BPS black holes our results are a first
principles derivation of a measure proposed in arXiv:1111.1161, while in the
case of one-quarter BPS black holes our result computes exactly all the
perturbative or area corrections. Moreover, we argue that instantonic
contributions can be incorporated and give evidence by computing the measure
which matches precisely the microscopics. Along with this, we find an unitary
condition that truncates the answer to a finite sum of instantons in perfect
agreement with a microscopic formula. Our results solve a number of puzzles
related to localization in supergravity and constitute a larger number of
examples where holography can be shown to hold exactly.Comment: 49 pages; clarifications in the discussion of section 4.3, results
unchanged; other minor typos correcte
On supersymmetric D6‒D6 systems with magnetic fields
We study systems of D6 and D6 branes with non-zero world-volume magnetic fields in the weak coupling limit. We find two configurations for which the conditions for absence of tachyons in the spectra coincide exactly with those found in the low energy effective theory approach, for the systems to preserve 1/8 of the supersymmetries of the type IIA string theory vacuum. These conditions give rise to a four-parameter family of solutions in each case. We present further evidence of the stability of these systems by computing the lowest order interaction amplitude, verifying the no force condition as well as the supersymmetric character of the spectrum.Facultad de Ciencias ExactasInstituto de Física La Plat
On supersymmetric D6‒D6 systems with magnetic fields
We study systems of D6 and D6 branes with non-zero world-volume magnetic fields in the weak coupling limit. We find two configurations for which the conditions for absence of tachyons in the spectra coincide exactly with those found in the low energy effective theory approach, for the systems to preserve 1/8 of the supersymmetries of the type IIA string theory vacuum. These conditions give rise to a four-parameter family of solutions in each case. We present further evidence of the stability of these systems by computing the lowest order interaction amplitude, verifying the no force condition as well as the supersymmetric character of the spectrum.Facultad de Ciencias ExactasInstituto de Física La Plat
On supersymmetric - systems with magnetic fields
We study systems of and branes with non zero world-volume
magnetic fields in the weak coupling limit. We find two configurations for
which the conditions for absence of tachyons in the spectra coincide exactly
with those found in the low energy effective theory approach, for the systems
to preserve 1/8 of the supersymmetries of the Type string theory vacuum.
These conditions give rise to a four-parameter family of solutions in each
case. We present further evidence of the stability of these systems by
computing the lowest order interaction amplitude, verifying the no force
condition as well as the supersymmetric character of the spectrum.Comment: Latex file, no figures, 11 pages, references added, to be published
in Physics Letters
Frequency-splitting estimators of single-propagator traces
Single-propagator traces are the most elementary fermion Wick contractions
which occur in numerical lattice QCD, and are usually computed by introducing
random-noise estimators to profit from volume averaging. The additional
contribution to the variance induced by the random noise is typically orders of
magnitude larger than the one due to the gauge field. We propose a new family
of stochastic estimators of single-propagator traces built upon a frequency
splitting combined with a hopping expansion of the quark propagator, and test
their efficiency in two-flavour QCD with pions as light as 190 MeV. Depending
on the fermion bilinear considered, the cost of computing these diagrams is
reduced by one to two orders of magnitude or more with respect to standard
random-noise estimators. As two concrete examples of physics applications, we
compute the disconnected contributions to correlation functions of two vector
currents in the isosinglet omega channel and to the hadronic vacuum
polarization relevant for the muon anomalous magnetic moment. In both cases,
estimators with variances dominated by the gauge noise are computed with a
modest numerical effort. Theory suggests large gains for disconnected three and
higher point correlation functions as well. The frequency-splitting estimators
and their split-even components are directly applicable to the newly proposed
multi-level integration in the presence of fermions.Comment: 26 pages, 8 figures, LaTe
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