Constraints on New Physics in the Electron g-2 from a Search for Invisible Decays of a Scalar, Pseudoscalar, Vector, and Axial Vector

TL;DR

This study searches for new light bosons in electron scattering experiments, setting stringent limits on their couplings and contributions to the electron g-2, surpassing previous experimental sensitivities.

Contribution

The paper provides the first direct experimental constraints on scalar, pseudoscalar, vector, and axial vector bosons from electron scattering, improving bounds on their couplings and effects on the electron g-2.

Findings

No evidence for new bosons was observed.

Set new upper bounds on boson-electron couplings.

Constraints on boson contributions to electron g-2 are an order of magnitude more sensitive.

Abstract

We performed a search for a new generic $X$ boson, which could be a scalar ($S$), pseudoscalar ($P$), vector ($V$) or an axial vector ($A$) particle produced in the 100 GeV electron scattering off nuclei, $e^- Z \to e^- Z X$, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such process was found in the full NA64 data set of $2.84\times 10^{11}$ electrons on target. We place new bounds on the $S, P, V, A$ coupling strengths to electrons, and set constraints on their contributions to the electron anomalous magnetic moment $a_e$, $|\Delta a_{X}| \lesssim 10^{-15} - 10^{-13}$ for the $X$ mass region $m_X\lesssim 1$ GeV. These results are an order of magnitude more sensitive compared to the current accuracy on $a_e$ from the electron $g-2$ experiments and recent high-precision determination of the fine structure constant.