Laser and Electron Beam Technology for Parity Violating Electron Scattering Measurements

Palatchi, Caryn, Physics - Graduate School of Arts and Sciences, University of Virginia
Paschke, Kent, AS-Physics, University of Virginia

Parity violating electron scattering (PVES) is a precision tool used in a broad program of experiments which include studying the structure of protons and nuclei and searching for new Beyond the Standard Model (BSM) physics. In parity violation electron scattering experiments, a longitudinally polarized electron beam is incident on an unpolarized target. The sign of the longitudinal polarization is changed (making a parity transformation), and the fractional rate difference between right and left helicity states is measured. An interference between the electromagnetic and weak amplitudes, gives rise to a parity violating asymmetry Apv, defined by the fractional rate difference between right and left helicity states. Apv can be measured to extraordinarily high precision and is proportional to the ratio of the weak and electromagnetic amplitudes.

This thesis highlights three such PVES experiments as well as a new polarized beam source which will improve their precision. The PVES experiments are PREX-II, MOLLER, and a 12C transverse asymmetry measurement at Mainz. Chapters 1-3 of this thesis cover introductions to MOLLER and PREX-II, Chapter 4 contains the results of the 12C transverse asymmetry measurement at Mainz, and Chapters 5-6 contain the original Pockels cell work performed by this author. PREX-II, the Lead Radius Experiment, measures the weak skin of the 208Pb nucleus, provides a clean measurement of Rn, the RMS radius of neutrons in a heavy nucleus, and constrains the equation of state (EOS) of highly dense matter which is important for describing neutron star structure, heavy ion collisions, and atomic parity violation experiments. The 12C transverse asymmetry measurement at Mainz addresses an important systematic correction for PREX and other PVES experiments preformed with heavy-nuclei. MOLLER (Measurement Of Lepton Lepton Electroweak Reaction) is an extremely precise PVES experiment searching for new neutral currents in electron-electron scattering. It’s called MOLLER because it will measure Apv in the Moller ee scattering process, and will infer the weak charge of the electron to extremely high precision. MOLLER is sensitive to new Beyond the Standard Model physics at MeV and multi-TeV scales and will serve as an indirect complementary measurement to direct searches at high energy colliders. To achieve high precision measurements on Apv for MOLLER, we have developed an innovative Rubidium Titanyl Phosphate (RTP) Pockels cell in the polarized source to satisfy both statistical and systematic requirements as regards the electron beam produced. This new ultra-fast RTP cell design uses electric field gradients to provide unprecedented control over helicity correlated beam asymmetries and has been demonstrated to be capable of producing precisely controlled polarized electron beam at Jefferson Laboratory, controlling beam steering down to the nm-level. The precision reached with the RTP cell offers sufficient control over and minimization of helicity correlated beam asymmetries to perform PREXII. The RTP Pockels cell system will provide fast flipping and suitable control helicity correlated beam asymmetries and parity quality beam for the future MOLLER experiment, providing an unprecedented precision on the electron weak charge and electroweak mixing angle.

PHD (Doctor of Philosophy)
Parity Violation, PREX, MOLLER, Pockels Cell, Rubidium Titanyl Phosphate, RTP, Lead Radius, Weak Mixing Angle
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