PhD Dissertation Defense

Hadronic Effects in Parity Violating Electron Scattering

Gian Franco Sacco
Physics Department

Since their advent, Parity Violating experiments have played a crucial role in testing the standard model (SM) of electroweak interactions. In recent years, these experi­ments have reached such a high degree of accuracy that they might test for physics beyond the SM. When making high precision measurements, a recurrent problem is how to properly include hadronic corrections. In this study I consider possible hadronic effects that appear in such experiments.

In the first part, I consider the possible badronic corrections to the deep inelastic parity violating (PV) asymmetry, ARL, stemming from sea quarks, perturbative QCD (PQCD), target mass (TM) and higher twist (HT) corrections. To estimate the first three effects, I use available parametrizations of the leading twist parton distribution functions (PDF). For the HT, which are not well known, I suggest using the MIT bag model to give some reasonable estimate. HT corrections could be relevant at the kinematics in the proposed experiments [2,3] where the weak mixing angle τw may be measured by using ARL. In this part of the study I consider the implications of these corrections for the extraction of sin2 τw.

In the second part I study the contribution to ARL of the bremsstrahlung radiation in electron-proton scattering. Such a process constitutes a background in the SLAC experi­ment E-158. In the experiment, which uses polarized M˙ller scattering to measure ARL at the 8% level, it was noticed that the inelastic electron-proton (EP) scattering background amounted for 40% of the total asymmetry, which implies that, in order to reach the pre­scribed precision in the ARL, one has to know the EP background to a 20% level. I suggest to use a simple model, consisting in only two resonances, to investigate which fraction of the bremmstrahlung radiation in the proton electro-excitation contributes to the total back­ground and if the lack of knowledge in the resonance form factors might spoil the prescribed accuracy of the experiment.

In the last part of the study I estimate the contribution to ARL in the electro-excitation of the (1232) resonance in chiral perturbation theory. I specifically focus on the chiral corrections of the terms stemming from the electric-dipole, the anapole and the d-wave chi­ral effective lagrangian and give some reasonable range of values for their contribution to ARL in the low Q2 region. In particular the dipole (or Siegert) contribution assumes great importance at very low Q2 since it does not vanish at the photon point, while all the other contributions do.

Thursday, September 23, 2004
10:30 AM
Gant Science Complex
Materials Science Building
Room IMS20

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