10,142 research outputs found
Development of Innovative Heat Sinks for Power Electronics Cooling within the More Electrical Aircraft
Peer ReviewedPostprint (published version
Neutron electric polarizability from unquenched lattice QCD using the background field approach
A calculational scheme for obtaining the electric polarizability of the
neutron in lattice QCD with dynamical quarks is developed, using the background
field approach. The scheme differs substantially from methods previously used
in the quenched approximation, the physical reason being that the QCD ensemble
is no longer independent of the external electromagnetic field in the dynamical
quark case. One is led to compute (certain integrals over) four-point
functions. Particular emphasis is also placed on the physical role of constant
external gauge fields on a finite lattice; the presence of these fields
complicates the extraction of polarizabilities, since it gives rise to an
additional shift of the neutron mass unrelated to polarizability effects. The
method is tested on a SU(3) flavor-symmetric ensemble furnished by the MILC
Collaboration, corresponding to a pion mass of m_pi = 759 MeV. Disconnected
diagrams are evaluated using stochastic estimation. A small negative electric
polarizability of alpha =(-2.0 +/- 0.9) 10^(-4) fm^3 is found for the neutron
at this rather large pion mass; this result does not seem implausible in view
of the qualitative behavior of alpha as a function of m_pi suggested by Chiral
Effective Theory.Comment: 36 pages, 11 figures. Note added concerning analytic continuation in
the external electric field; some notation made more precis
Nucleon axial charge from quenched lattice QCD with domain wall fermions
We present a quenched lattice calculation of the nucleon isovector vector and
axial-vector charges gV and gA. The chiral symmetry of domain wall fermions
makes the calculation of the nucleon axial charge particularly easy since the
Ward-Takahashi identity requires the vector and axial-vector currents to have
the same renormalization, up to lattice spacing errors of order O(a^2). The
DBW2 gauge action provides enhancement of the good chiral symmetry properties
of domain wall fermions at larger lattice spacing than the conventional Wilson
gauge action. Taking advantage of these methods and performing a high
statistics simulation, we find a significant finite volume effect between the
nucleon axial charges calculated on lattices with (1.2 fm)^3 and (2.4 fm)^3
volumes (with lattice spacing, a, of about 0.15 fm). On the large volume we
find gA = 1.212 +/- 0.027(statistical error) +/- 0.024(normalization error).
The quoted systematic error is the dominant (known) one, corresponding to
current renormalization. We discuss other possible remaining sources of error.
This theoretical first principles calculation, which does not yet include
isospin breaking effects, yields a value of gA only a little bit below the
experimental one, 1.2670 +/- 0.0030.Comment: 38 pages, 12 figures, 9 tables, Revtex. Version accepted for
publication in Physical Review
Electromagnetic structure of charmed baryons in Lattice QCD
As a continuation of our recent work on the electromagnetic properties of the
doubly charmed baryon, we compute the charge radii and the magnetic
moments of the singly charmed , and the doubly charmed
baryons in 2+1 flavor Lattice QCD. In general, the charmed
baryons are found to be compact as compared to the proton. The charm quark acts
to decrease the size of the baryons to smaller values. We discuss the mechanism
behind the dependence of the charge radii on the light valence- and sea-quark
masses. The magnetic moments are found to be almost stable with respect to
changing quark mass. We investigate the individual quark sector contributions
to the charge radii and the magnetic moments. The magnetic moments of the
singly charmed baryons are found to be dominantly determined by the light quark
and the role of the charm quark is significantly enhanced for the doubly
charmed baryons.Comment: Updated results, improved analysis. Version to appear in JHE
Update of from the form factor at zero recoil with three-flavor lattice QCD
We compute the zero-recoil form factor for the semileptonic decay
(and modes related by isospin and charge
conjugation) using lattice QCD with three flavors of sea quarks. We use an
improved staggered action for the light valence and sea quarks (the MILC
\asqtad\ configurations), and the Fermilab action for the heavy quarks. Our
calculations incorporate higher statistics, finer lattice spacings, and lighter
quark masses than our 2008 work. As a byproduct of tuning the new data set, we
obtain the and hyperfine splittings with few-MeV accuracy. For the
zero-recoil form factor, we obtain , where the
first error is statistical and the second is the sum in quadrature of all
systematic errors. With the latest HFAG average of experimental results and a
cautious treatment of QED effects, we find . The
QCD error is now commensurate with the experimental error.Comment: 53 pages, 12 figures; expanded discussion of correlator fits, typos
corrected, conforms to version published in PR
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