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Search for light massive gauge bosons as an explanation of the $(g-2)_μ$ anomaly at MAMI

H. Merkel, P. Achenbach, C. Ayerbe Gayoso, T. Beranek, J. Beričič, J. C. Bernauer, R. Böhm, D. Bosnar, L. Correa, L. Debenjak, A. Denig, M. O. Distler, A. Esser, H. Fonvieille, I. Friščić, M. Gómez Rodríguez de la Paz, M. Hoek, S. Kegel, Y. Kohl, D. G. Middleton, M. Mihovilovič, U. Müller, L. Nungesser, J. Pochodzalla, M. Rohrbeck, G. Ron, S. Sánchez Majos, B. S. Schlimme, M. Schoth, F. Schulz, C. Sfienti, S. Širca, M. Thiel, A. Tyukin, A. Weber, M. Weinriefer

TL;DR

This paper searches for a light dark photon—an additional U(1) gauge boson—as a possible explanation for the muon g‑2 anomaly, using fixed-target electron scattering at the A1 setup at Mainz-MAMI. The authors perform a high-resolution search for a sharp e+e− invariant-mass peak in the 40–300 MeV range, calibrating the detector response with Ta ground-state transitions and elastic scattering, and normalizing potential signals to the well-understood QED background. No significant signal is observed, and 2σ exclusion limits on the kinetic-mixing parameter ε extend significantly over existing bounds, particularly in the muon g‑2–relevant region, thereby constraining the dark-photon explanation. The results, while leaving a small low-mass gap, substantially restrict the parameter space for dark photons in this mass range and guide future low-mass searches.

Abstract

A massive, but light abelian U(1) gauge boson is a well motivated possible signature of physics beyond the Standard Model of particle physics. In this paper, the search for the signal of such a U(1) gauge boson in electron-positron pair-production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron (MAMI) is described. Exclusion limits in the mass range of 40 MeV up to 300 MeV with a sensitivity in the mixing parameter of down to $ε^2 = 8\times 10^{-7}$ are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U(1) gauge boson.

Search for light massive gauge bosons as an explanation of the $(g-2)_μ$ anomaly at MAMI

TL;DR

This paper searches for a light dark photon—an additional U(1) gauge boson—as a possible explanation for the muon g‑2 anomaly, using fixed-target electron scattering at the A1 setup at Mainz-MAMI. The authors perform a high-resolution search for a sharp e+e− invariant-mass peak in the 40–300 MeV range, calibrating the detector response with Ta ground-state transitions and elastic scattering, and normalizing potential signals to the well-understood QED background. No significant signal is observed, and 2σ exclusion limits on the kinetic-mixing parameter ε extend significantly over existing bounds, particularly in the muon g‑2–relevant region, thereby constraining the dark-photon explanation. The results, while leaving a small low-mass gap, substantially restrict the parameter space for dark photons in this mass range and guide future low-mass searches.

Abstract

A massive, but light abelian U(1) gauge boson is a well motivated possible signature of physics beyond the Standard Model of particle physics. In this paper, the search for the signal of such a U(1) gauge boson in electron-positron pair-production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron (MAMI) is described. Exclusion limits in the mass range of 40 MeV up to 300 MeV with a sensitivity in the mixing parameter of down to are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U(1) gauge boson.

Paper Structure

This paper contains 5 sections, 1 equation, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experiment
  3. Data Analysis
  4. Results and Interpretation
  5. Acknowledgments

Figures (3)

  • Figure 1: Radiative production of a $\gamma'$ in final (a) and initial state (b) on a heavy target nucleus $Z$. The subsequent decay of the $\gamma'$ to an electron-positron pair would be the unique signal of such a $\gamma'$ with a sharp mass distribution.
  • Figure 2: (color online). Mass distribution of the individual settings (color/shaded) and of the sum (black). The experiment probes the invariant mass region between 40 and $300~\mathrm{MeV}/c^2$.
  • Figure 3: (color online). Exclusion limits in terms of mixing parameter $\epsilon$. The yellow (light shaded) area marked with A1 is excluded by this experiment.