Undergraduate Research at Jefferson Lab

Analyzing the Shift in Detector Signal for Polarized Protons in Matter

Student: Austin Blitstein

School: University of North Carolina at Chapel Hill

Mentored By: Dr. Bogdan Wojtsekhowski

Much of what is currently known about the proton has been ascertained from scattering experiments. Recently, the parity violation present in the elastic scattering of polarized electrons incident on protons has been used to gather information regarding the pairs of virtual strange quarks inside the proton. It is thought that the strange form factor at a momentum transfer Q² of 3 GeV² could be large, a discovery that would shed light on the proton's constituents. Typically, elastic scattering data is obtained from a measurement of the outgoing electron's energy using high resolution small solid angle magnetic spectrometers. Nevertheless, this technique is not practical at large momentum transfer due to the low counting rate in spectrometers. Alternatively, measurements with large acceptance detectors are complicated by the predominance of inelastic scattering events. To isolate the elastic scattering events, these detectors should measure the angular correlation between the scattered electrons and protons. However, such measurements are complicated by the shift in detector signal induced by the interaction of the polarized protons in the detector. If the shift is too large, the cuts made in data analysis cease to be polarization independent. This investigation aimed to find an estimate for the magnitude of this shift. In particular, the Geant4 toolkit was used to simulate the relevant physics via Monte Carlo techniques. Custom modification was included to account for proton polarization, taking the form of an empirical asymmetry in the outgoing azimuthal distribution of protons quasi-elastically scattered from nuclei in the detector. The energy deposited by the outgoing protons and their showers were detected with a spatial segmentation of 5 cm in a 100 cm long sampling calorimeter with 40 layers composed of 1.5 cm of iron and 1 cm of plastic scintillator. The shift in the mean center of detector signal for 2.5 GeV protons with a typical side polarization of 0.075 was determined to be 14±1 μm. This shift is relatively small compared to the spatial resolution of 2-5 cm typical in hadronic calorimeters, so only slight modifications in specification of data cuts are required. If these computational predictions can be verified in a follow-up experiment at JLab, everything will be in place for a precise measurement of the strange form factor in elastic polarized electron-proton scattering, which could result in a potential discovery if shown to be large.

[Watch the presentation on YouTube]