Hosted by The College of William and Mary
Department of Physics
August 15-17, 2005
Lodging and location information below
Elastic electron-nucleon scattering in the one-photon exchange approximation is a time-honored tool to access information on the structure of hadrons. New experimental techniques exploiting polarization observables have made possible precision measurements of hadron structure quantities, such as its electroweak form factors, parity violating effects, N to Delta transition form factors, and the measurement of spin dependent structure functions.
One surprising experimental result from polarization experiments of elastic electron-nucleon scattering has been the ratio of electric to magnetic proton form factors, which is clearly at variance with unpolarized measurements using the Rosenbluth separation technique. The understanding of this puzzle has generated a lot of activity recently. It has been suggested in various calculations and analyses that contributions from two-photon exchange amplitudes at the level of a few percent can help resolve this puzzle.
A systematic study of two-photon exchange effects is justified, both experimentally and theoretically, in order to push the precision frontier further in electron scattering. The real (dispersive) part of the two-photon exchange amplitude can be accessed through the difference between elastic electron and positron scattering off a nucleon. The imaginary (absorptive) part of the two-photon exchange amplitude on the other hand can be accessed through a single spin asymmetry (SSA) in elastic electron-nucleon scattering, when either the target or beam spin is polarized normal to the scattering plane. In the case of a polarized lepton beam, asymmetries of the order of a few parts per million (ppm) are currently accessible in parity violating (PV) elastic electron-nucleon scattering experiments. While the PV asymmetry measurements involve a beam spin polarized along its momentum, the SSA for an electron beam spin normal to the scattering plane can be accessed using the same experimental apparatus. First measurements of this beam normal SSA at beam energies below 1 GeV have yielded values around 10 ppm (SAMPLE, A4), and at higher beam energies, the beam normal SSA is being analysed in ongoing PV elastic electron-nucleon scattering experiments (E158,Happex,G0).
The aim of the workshop is to bring together researchers working in the broad field of precision electroweak physics to address how two-photon exchange processes enter into precision electromagnetic processes. Access to new physics topics provided by possible future measurements in upgraded experimental facilties will be explored. Particular emphasis will be put on approaches connecting different kinematical regimes.
Some of the topics include:
The workshop is part of a series of joint workshops sponsored by the Institute for Nuclear Theory at the University of Washington and by the Thomas Jefferson National Acclerator Facility.
Location and lodging
The workshop will be held on the campus, in the physics building, of The College of William and Mary in Virginia. Coffee breaks and lunch will be provided. There will be one evening reception.
We have reserved a block of rooms at the Days Inn downtown, in Williamsburg. It is an easy walk from the Days Inn to the William and Mary physics building (formally, the William Small Physical Laboratory). Maps can be found here. The Days Inn is at 902 Richmond Road, just off the right hand edge of the campus map, and the physics building is building number 74 on the same map.