[Paper Review] Parity-Violating Electroweak Asymmetry in (E)Over-Right-Arrowp Scattering

K. Aniol, Edgar E. Kooijman|arXiv (Cornell University)|Jan 1, 2006

Atomic and Subatomic Physics ResearchPhysics and Astronomy3 references83 citations

TL;DR

This study measures the parity-violating electroweak asymmetry in polarized e⁻p scattering at ⟨θlab⟩ = 12.3° and ⟨Q²⟩ = 0.477 (GeV/c)², yielding A = −15.05 ± 0.98(stat) ± 0.56(syst) ppm. It extracts the linear combination of strange form factors GEs + 0.392GMs = 0.014 ± 0.020 ± 0.010, providing constraints on nucleon strangeness using novel experimental techniques including a strained GaAs crystal and laser-Compton polarimeter.

ABSTRACT

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from protons. Significant contributions to this asymmetry could arise from the contributions of strange form factors in the nucleon. The measured asymmetry isA=−15.05±0.98(stat)±0.56(syst)ppmat the kinematic point⟨θlab⟩=12.3°and⟨Q2⟩=0.477(GeV∕c)2. Based on these data as well as data on electromagnetic form factors, we extract the linear combination of strange form factorsGEs+0.392GMs=0.014±0.020±0.010, where the first error arises from this experiment and the second arises from the electromagnetic form factor data. This paper provides a full description of the special experimental techniques employed for precisely measuring the small asymmetry, including the first use of a strained GaAs crystal and a laser-Compton polarimeter in a fixed target parity-violation experiment.

Motivation & Objective

• To measure the parity-violating electroweak asymmetry in elastic scattering of polarized electrons from protons with high precision.
• To probe the contribution of strange quarks to the nucleon's electromagnetic form factors via the asymmetry measurement.
• To constrain the linear combination of strange form factors GEs + 0.392GMs using experimental data and electromagnetic form factor information.
• To demonstrate and apply novel experimental techniques, including a strained GaAs crystal and laser-Compton polarimeter, in a fixed-target parity-violation experiment.

Proposed method

• The experiment employs a beam of longitudinally polarized electrons incident on a proton target at a lab angle of 12.3° and momentum transfer Q² = 0.477 (GeV/c)².
• A strained GaAs crystal is used as an electron spin polarizer, enabling precise control and measurement of electron beam polarization.
• A laser-Compton polarimeter is implemented for the first time in a fixed-target parity-violation experiment to measure the electron beam polarization with high accuracy.
• The parity-violating asymmetry is extracted from the differential cross-section asymmetry between left- and right-handed electron beams.
• The measured asymmetry is combined with existing data on electromagnetic form factors to extract the linear combination of strange form factors.
• Systematic uncertainties are evaluated through detailed calibration and control of beam parameters, target density, and detector response.

Experimental results

Research questions

• RQ1 What is the precise value of the parity-violating electroweak asymmetry in e⁻p scattering at Q² ≈ 0.477 (GeV/c)² and θlab ≈ 12.3°?
• RQ2 To what extent do strange quark contributions to the nucleon's form factors contribute to the measured asymmetry?
• RQ3 How can the linear combination GEs + 0.392GMs be constrained using this asymmetry and electromagnetic form factor data?
• RQ4 What is the impact of the novel experimental techniques—strained GaAs and laser-Compton polarimetry—on the precision of the asymmetry measurement?

Key findings

• The measured parity-violating asymmetry is A = −15.05 ± 0.98(stat) ± 0.56(syst) ppm at ⟨θlab⟩ = 12.3° and ⟨Q²⟩ = 0.477 (GeV/c)².
• The linear combination of strange form factors is determined as GEs + 0.392GMs = 0.014 ± 0.020 (statistical) ± 0.010 (electromagnetic form factor uncertainty).
• The result is consistent with zero within the combined uncertainties, indicating no significant evidence for large strange quark contributions to the nucleon's electromagnetic properties at this Q².
• The use of a strained GaAs crystal enables high-purity electron beam polarization, enhancing the precision of the asymmetry measurement.
• The laser-Compton polarimeter provides a new, high-accuracy method for beam polarization measurement in fixed-target experiments.
• The experimental techniques demonstrate feasibility and high precision for future parity-violation experiments probing nucleon structure.