[Jlab-seminars] Theory Center Seminar

[Mary Fox]

*Theory Center Seminar*
Wed., Oct. 26, 2011
2:00 p.m. (coffee at 1:45 p.m.)
CEBAF Center, Room L104

*Aspects of Nucleon Structure From Lattice QCD: Electric Polarizabilities
and Strangeness Content (Lecture 2)*

Michael Engelhardt
New Mexico State University

In the course of two sessions, I will report on lattice QCD studies of 
two aspects of nucleon structure.
The primary focus will lie on electric polarizabilities of the neutron. 
One particular difficulty encountered
in investigating these polarizabilities is the evaluation of 
disconnected contributions; this motivated a
secondary study of the strange quark content of the nucleon, which 
counts among the simplest contexts
in which disconnected contributions arise.

Polarizabilities encode the response of hadrons to external fields. A 
calculational scheme, based on a
four-point function approach to the background field method, is 
developed for obtaining both the static
and the spin electric polarizabilities of the neutron in fully dynamical 
lattice QCD. Particular emphasis is
placed on the physical role of spatially constant external gauge fields 
on a finite lattice; the presence of
these fields implies isospin-breaking Bloch momenta, the effects of 
which have to be carefully disentangled
from the polarizabilities themselves. Calculations are carried out in a 
mixed action scheme employing domain
wall valence quarks and quark loops on MILC asqtad dynamical fermion 
ensembles. The results of these
lattice calculations are confronted with chiral effective theory.

In order to gain further insight into the behavior of disconnected 
contributions to nucleon correlators within
a simpler context, both the scalar strange content of the nucleon as 
well as the strange quark contribution to
nucleon spin are investigated within the same mixed action scheme. The 
convergence behavior of stochastic
estimation is seen to depend strongly on the matrix element. Nonzero 
signals are obtained for both quantities;
neither result is unnaturally large to an extent which would impact 
either dark matter searches or the nucleon
spin budget.

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