3,929 research outputs found
Staggered Chiral Perturbation Theory for Heavy-Light Mesons
We incorporate heavy-light mesons into staggered chiral perturbation theory,
working to leading order in 1/m_Q, where m_Q is the heavy quark mass. At first
non-trivial order in the chiral expansion, staggered taste violations affect
the chiral logarithms for heavy-light quantities only through the light meson
propagators in loops. There are also new analytic contributions coming from
additional terms in the Lagrangian involving heavy-light and light mesons.
Using this heavy-light staggered chiral perturbation theory, we perform the
one-loop calculation of the B (or D) meson leptonic decay constant in the
partially quenched and full QCD cases. In our treatment, we assume the validity
both of the "fourth root trick" to reduce four staggered tastes to one, and of
the prescription to represent this trick in the chiral theory by insertions of
factors of 1/4 for each sea quark loop.Comment: 48 pages, 6 figures. v3: Some clarifying comments/caveats added;
typos fixed. Corresponds to published versio
Heavy-Light Semileptonic Decays in Staggered Chiral Perturbation Theory
We calculate the form factors for the semileptonic decays of heavy-light
pseudoscalar mesons in partially quenched staggered chiral perturbation theory
(\schpt), working to leading order in , where is the heavy quark
mass. We take the light meson in the final state to be a pseudoscalar
corresponding to the exact chiral symmetry of staggered quarks. The treatment
assumes the validity of the standard prescription for representing the
staggered ``fourth root trick'' within \schpt by insertions of factors of 1/4
for each sea quark loop. Our calculation is based on an existing partially
quenched continuum chiral perturbation theory calculation with degenerate sea
quarks by Becirevic, Prelovsek and Zupan, which we generalize to the staggered
(and non-degenerate) case. As a by-product, we obtain the continuum partially
quenched results with non-degenerate sea quarks. We analyze the effects of
non-leading chiral terms, and find a relation among the coefficients governing
the analytic valence mass dependence at this order. Our results are useful in
analyzing lattice computations of form factors and when the
light quarks are simulated with the staggered action.Comment: 53 pages, 8 figures, v2: Minor correction to the section on finite
volume effects, and typos fixed. Version to be published in Phys. Rev.
Discretization effects and the scalar meson correlator in mixed-action lattice simulations
We study discretization effects in a mixed-action lattice theory with
domain-wall valence quarks and Asqtad-improved staggered sea quarks. At the
level of the chiral effective Lagrangian, discretization effects in the
mixed-action theory give rise to two new parameters as compared to the lowest
order Lagrangian for staggered fermions -- the residual quark mass, m_res, and
the mixed valence-sea meson mass-splitting, Delta_mix. We find that the size of
m_res is approximately four times smaller than our lightest valence quark mass
on our coarser lattice spacing, and comparable to that of simulations by RBC
and UKQCD. We also find that the size of Delta_mix is comparable to the
smallest of the staggered meson taste-splittings measured by MILC. Because
lattice artifacts are different in the valence and sea sectors of the
mixed-action theory, they give rise to unitarity-violating effects that
disappear in the continuum limit. Such effects are expected to be mild for many
quantities of interest, but are significant in the case of the isovector scalar
(a_0) correlator. Specifically, once m_res, Delta_mix, and two other parameters
that can be determined from the light pseudoscalar spectrum are known, the
two-particle intermediate state "bubble" contribution to the scalar correlator
is completely predicted within mixed-action chiral perturbation theory
(MAChPT). We find that the behavior of the scalar meson correlator is
quantitatively consistent with the MAChPT prediction; this supports the claim
that MAChPT describes the dominant unitarity-violating effects in the
mixed-action theory and can be used to remove lattice artifacts and recover
physical quantities.Comment: 33 pages, 12 figure
Phase structure of SU(3) gauge theory with two flavors of symmetric-representation fermions
We have performed numerical simulations of SU(3) gauge theory coupled to Nf=2
flavors of symmetric representation fermions. The fermions are discretized with
the tadpole-improved clover action. Our simulations are done on lattices of
length L=6, 8, and 12. In all simulation volumes we observe a crossover from a
strongly coupled confined phase to a weak coupling deconfined phase.
Degeneracies in screening masses, plus the behavior of the pseudoscalar decay
constant, indicate that the deconfined phase is also a phase in which chiral
symmetry is restored. The movement of the confinement transition as the volume
is changed is consistent with avoidance of the basin of attraction of an
infrared fixed point of the massless theory.Comment: 12 pages, 11 figure
Effects of electron-phonon interactions on the electron tunneling spectrum of PbS quantum dots
We present a tunnel spectroscopy study of single PbS Quantum Dots (QDs) as
function of temperature and gate voltage. Three distinct signatures of strong
electron-phonon coupling are observed in the Electron Tunneling Spectrum (ETS)
of these QDs. In the shell-filling regime, the degeneracy of the
electronic levels is lifted by the Coulomb interactions and allows the
observation of phonon sub-bands that result from the emission of optical
phonons. At low bias, a gap is observed in the ETS that cannot be closed with
the gate voltage, which is a distinguishing feature of the Franck-Condon (FC)
blockade. From the data, a Huang-Rhys factor in the range is
obtained. Finally, in the shell tunneling regime, the optical phonons appear in
the inelastic ETS .Comment: 5 pages, 5 figure
Magnetic field-induced quantum superconductor-insulator transition in
A study of magnetic-field tuned superconductor-insulator transitions in
amorphous thin films shows that quantum
superconductor-insulator transitions are characterized by an unambiguous
signature -- a kink in the temperature profile of the critical magnetic field.
Using this criterion, we show that the nature of the magnetic-field tuned
superconductor-insulator transition depends on the orientation of the field
with respect to the film. For perpendicular magnetic field, the transition is
controlled by quantum fluctuations with indications for the existence of a Bose
insulator; while for parallel magnetic field, the transition is classical,
driven by the breaking of Cooper pairs at the temperature dependent critical
field .Comment: 5 pages, 4 figure
A length scale for the superconducting Nernst signal above T in NbSi
We present a study of the Nernst effect in amorphous superconducting thin
films of NbSi. The field dependence of the Nernst coefficient
above T displays two distinct regimes separated by a field scale set by
the Ginzburg-Landau correlation length. A single function , with the
correlation length as its unique argument set either by the zero-field
correlation length (in the low magnetic field limit) or by the magnetic length
(in the opposite limit), describes the Nernst coefficient. We conclude that the
Nernst signal observed on a wide temperature () and field () range is exclusively generated by short-lived Cooper pairs.Comment: 4 pages, 4 figure
Delta-baryon electromagnetic form factors in lattice QCD
We develop techniques to calculate the four Delta electromagnetic form
factors using lattice QCD, with particular emphasis on the sub-dominant
electric quadrupole form factor that probes deformation of the Delta. Results
are presented for pion masses down to approximately 350 MeV for three cases:
quenched QCD, two flavors of dynamical Wilson quarks, and three flavors of
quarks described by a mixed action combining domain wall valence quarks and
dynamical staggered sea quarks. The magnetic moment of the Delta is chirally
extrapolated to the physical point and the Delta charge density distributions
are discussed.Comment: 4 pages, 5 figure
The constraint equations for the Einstein-scalar field system on compact manifolds
We study the constraint equations for the Einstein-scalar field system on
compact manifolds. Using the conformal method we reformulate these equations as
a determined system of nonlinear partial differential equations. By introducing
a new conformal invariant, which is sensitive to the presence of the initial
data for the scalar field, we are able to divide the set of free conformal data
into subclasses depending on the possible signs for the coefficients of terms
in the resulting Einstein-scalar field Lichnerowicz equation. For many of these
subclasses we determine whether or not a solution exists. In contrast to other
well studied field theories, there are certain cases, depending on the mean
curvature and the potential of the scalar field, for which we are unable to
resolve the question of existence of a solution. We consider this system in
such generality so as to include the vacuum constraint equations with an
arbitrary cosmological constant, the Yamabe equation and even (all cases of)
the prescribed scalar curvature problem as special cases.Comment: Minor changes, final version. To appear: Classical and Quantum
Gravit
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