[Paper Review] Beam Normal Single Spin Asymmetry Measurements from QWeak

B. Waidyawansa, Collaboration, the QWeak|arXiv (Cornell University)|Apr 15, 2016

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents a preliminary 3.2% measurement of the beam normal single spin asymmetry ($B_n$) in elastic electron-proton scattering at 1.155 GeV and 7.8° lab angle using the Q weak experiment at Jefferson Lab. The measurement, which favors theoretical models with multi-pion intermediate states in the two-photon exchange amplitude, provides critical constraints on $B_n$ in the forward-angle regime and sets the stage for future in-depth studies of inelastic and nuclear scattering processes.

ABSTRACT

The Q weak experiment has made several interesting beam normal single spin asymmetry measurements. Preliminary result from a 3.2% measurement of the beam normal single spin asymmetry in elastic e+p scattering at E = 1.155 GeV and θ lab = 7.8(deg) is presented. We have also made measurements of this asymmetry in elastic and inelastic scattering in the Delta resonance region from Hydrogen, Aluminum and Carbon targets and e+e scattering from Hydrogen target. Some initial results from these measurements are also presented.

Motivation & Objective

• To measure the beam normal single spin asymmetry ($B_n$) in elastic electron-proton scattering at forward angles with high precision.
• To test theoretical models of two-photon exchange amplitudes by comparing $B_n$ measurements with predictions based on different intermediate nucleon states.
• To extend $B_n$ measurements to inelastic scattering in the $Δ$ resonance region and to elastic scattering from carbon and aluminum targets.
• To probe the role of multi-pion excitations in the two-photon exchange process by comparing data with models that include or exclude such contributions.
• To provide new constraints on $B_n$ in spin-0 nuclei between mass numbers A = 12 and A = 208, addressing discrepancies in existing theory-experiment comparisons.

Proposed method

• Utilized a transversely polarized electron beam (150–180 $μ$A) at 1.16 GeV, with helicity flipped at 960 Hz using a Pockels cell.
• Measured detector asymmetries $A_{det} = (Y^+ - Y^-)/(Y^+ + Y^-)$ in eight azimuthal octants using Cerenkov detectors with ~50% acceptance.
• Applied a half-wave plate insert to periodically flip beam helicity independently of the Pockels cell, reducing helicity-correlated beam systematics.
• Fitted the octant-dependent asymmetry using $A_{det}(φ_{det}) = A_{exp} \sin(\phi_{det} - \phi_s)$ to extract the experimental asymmetry $A_{exp}$.
• Corrected $A_{exp}$ for beam polarization, kinematics, and backgrounds (e.g., target window, inelastic contamination) using $B_n = R \left( \frac{A_{exp}/P - \sum A_{bkg}^i f_i}{1 - \sum f_i} \right)$.
• Used multiple linear regression to remove residual helicity-correlated beam asymmetries during analysis.

Experimental results

Research questions

• RQ1 What is the magnitude and sign of the beam normal single spin asymmetry ($B_n$) in elastic electron-proton scattering at 1.155 GeV and 7.8°?
• RQ2 Which theoretical models of two-photon exchange—those with single-pion or multi-pion intermediate states—best describe the measured $B_n$?
• RQ3 How does $B_n$ behave in inelastic electron scattering from the $Δ$ resonance region, and does it show the expected positive asymmetry at forward angles?
• RQ4 What is the value of $B_n$ in elastic scattering from carbon and aluminum targets, and how does it vary with nuclear mass number?
• RQ5 Can $B_n$ measurements in e+e scattering at the Moller peak provide a clean test of two-photon exchange models in a purely leptonic process?

Key findings

• A preliminary measurement of $B_n$ in elastic e+p scattering yields $-5.35 \pm 0.07$ (stat) $\pm 0.15$ (syst) ppm at 1.155 GeV and 7.8°, achieving a 3.2% precision.
• The measured $B_n$ value favors theoretical models that include multi-pion intermediate states in the two-photon exchange amplitude over those limited to single-pion excitations.
• The asymmetry in inelastic e+p scattering at the $Δ$ resonance region shows a non-zero amplitude and is opposite in phase to the elastic asymmetry, consistent with theoretical expectations.
• Initial fits to elastic e+Al scattering data show a non-zero experimental asymmetry, providing a new data point to constrain $B_n$ in spin-0 nuclei between A = 12 and A = 208.
• Statistical error bars from e+e scattering data at 0.877 GeV indicate a precision approaching a few ppm, consistent with theory predictions of ~few ppm at the Moller peak.
• The analysis of $B_n$ from carbon and aluminum targets is ongoing, with progress dependent on theoretical inputs for inelastic contributions and target contamination effects.