New Physics Searches via Beam Normal Spin Asymmetry in Bhabha Scattering

Abstract

We examine the sensitivity of the beam normal spin asymmetry in Bhabha scattering to beyond the Standard Model (BSM) mediators, in the context of the JLab polarized positron program. A key property of this observable is that the Standard Model contribution exhibits a zero crossing at a fixed scattering angle, providing a clean, effectively background-free point for these searches. We consider scalar, vector and axial-vector mediators and present projected bounds, finding scalar and vector scenarios allowing to extend their search ranges significantly beyond existing constraints.

Figures (3)

• Figure 1: Feynman diagrams contributing to $B_n$ at one-loop order (a-h), and the BSM contribution (i). The red vertical line shows the physical-region cuts.
• Figure 2: Comparison of the angular dependence of the QED and BSM contributions to the Bhabha beam normal spin asymmetry $B_n$ for indicated values of BSM coupling strengths, for an 11 GeV $e^+$ beam at JLab. The QED zero-crossing point is marked with a red dot.
• Figure 3: Constraints on the allowed BSM parameter space from $(g-2)_e$ and from existing experiments looking for visible $e^+e^-$ decays (shaded exclusion regions), for the scalar (left) and vector/axial-vector (right) scenarios, together with the projected sensitivity of a future JLab polarized-positron experiment (corresponding with a 1 ppm precision on $B_n$) shown as dashed lines.