[Jlab-seminars] Theory Center Seminar

[Mary Fox]

Theory Center Seminar
Mon., March 19, 2012
CEBAF Center, Room L104
1:00 p.m. (coffee at 12:45 p.m.)

Harald W. Griesshammer
George Washington University
Washington DC

*Compton Scattering and the Energy-Dependence of Nucleon Polarizabilities *

Compton scattering from protons and neutrons provides important insight 
into the structure of the nucleon. We present a new extraction of the 
static electric and magnetic dipole polarizabilities of the proton and 
neutron from all published elastic data below 300~MeV in Chiral 
Effective Field Theory. Including the $\Delta(1232)$ as explicit degrees 
of freedom is particularly important for the proton above about 
$100$~MeV and for deuteron Compton scattering at $\gtrsim90$~MeV as 
measured at SAL and MAXlab. Consistency arguments dictate including the 
$np$-rescattering states and automatically render the correct Thomson 
limit, shedding new light on Weinberg's power-counting of nuclear 
forces. We show that the static electric and magnetic scalar 
polarizabilities of the proton and neutron are identical within the 
accuracy of available data. In view of ongoing efforts at HI$\gamma$S, 
MAMI and MAXlab, we address in detail single- and doubly-polarized 
observables with linearly or circularly polarized photons on both un-, 
vector- and tensor-polarized deuterons. Several observables can be used 
to extract not only spin-independent nucleon polarizabilities, but also 
the so-far practically undetermined spin-dependent polarizabilities 
which parameterize the stiffness of the nucleon spin in external 
electro-magnetic fields. Amongst the questions addressed are convergence 
of the expansion when including the $\Delta$ and the r\^ole of 
$np$-rescattering in few-nucleon systems.
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