Lepton Double Charge Exchange Reactions as Probes for Lepton Number Violation

TL;DR

This paper proposes using lepton double charge exchange reactions at accelerator facilities as a novel method to probe lepton number violation and physics beyond the Standard Model, with predictions of sizable cross sections at multi-GeV energies.

Contribution

It introduces a second order formalism incorporating LNV dynamics from the left-right symmetric model and estimates LDCE cross sections numerically for the first time.

Findings

Sizable LDCE cross sections predicted at multi-GeV energies.

Reactions proceed mainly via energy-momentum dependent left-right mixing.

Cross sections increase with energy and target mass, making experiments feasible.

Abstract

Lepton number violating (LNV) $A(e^-,e^+)X$ double charge exchange (LDCE) reactions on nuclei at accelerator facilities with multi-GeV beams are proposed as a probe for physics beyond the Standard Model (BSM). A second order formalism is presented including LNV dynamics by the left-right symmetric model (LRSM). For practical studies a phenomenological model is used to estimate LDCE cross sections numerically. Sizable cross sections are predicted for multi-GeV beam energies. LDCE reactions proceed preferentially by the energy-momentum dependent left-right mixing terms. While Majorana mass terms are negligible for light neutrinos, they may become sizable for heavy neutral leptons at higher beam energies. In the ~10 GeV region inclusive total LDCE cross sections of about $100\times|\Gamma_{BSM}|^2$~fb in units of the BSM vertices are predicted, increasing strongly with energy and target mass. LDCE experiments seem to be feasible with existing equipment under full laboratory control and free of the constraints imposed on decay or capture experiments of nuclear and hadronic systems.