Nuclear-level effective theory of 𝜇 → 𝑒 conversion: Inelastic process

Journal Article · 06 February 2025 · Physical Review. C

DOI: https://doi.org/10.1103/PhysRevC.111.025501 · OSTI ID:2512410

Haxton, W. C.  ^[1]; Rule, Evan  ^[2]

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1. University of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). AmeriFlux
2. University of California, Berkeley, CA (United States); Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Mu2e and COMET will search for electrons produced via the neutrinoless conversion of stopped muons bound in 1s atomic orbits of ²⁷Al, improving existing limits on charged lepton flavor violation (CLFV) by roughly four orders of magnitude. Conventionally, 𝜇 → 𝑒 conversion experiments are optimized to detect electrons originating from transitions where the nucleus remains in the ground state, thereby maximizing the energy of the outgoing electron. Clearly, detection of a positive signal in forthcoming experiments would stimulate additional work—including subsequent conversion experiments using complementary nuclear targets—to further constrain the new physics responsible for CLFV. Here we argue that additional information can be extracted without the need for additional experiments, by considering inelastic conversion in ²⁷Al. Transitions to low-lying nuclear excited states can modify the near-endpoint spectrum of conversion electrons, with the ratio of the elastic and inelastic responses being sensitive to the underlying CLFV operator. We extend the nuclear effective theory of 𝜇 → 𝑒 conversion to the inelastic case, which adds five new response functions to the six that arise for the elastic process. We evaluate these nuclear response functions in ²⁷Al and calculate the resulting conversion-electron signal, taking into account the resolution anticipated in Mu2e/COMET. We find that ²⁷Al is an excellent target choice from the perspective of the new information that can be obtained from inelastic 𝜇 → 𝑒 conversion.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Los Alamos National Laboratory (LANL), Los Alamos, NM (United States); University of California, Berkeley, CA (United States)
• Sponsoring Organization: Heising-Simons Foundation; National Science Foundation (NSF); Simons Foundation; USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC)
• Grant/Contract Number: 89233218CNA000001; AC02-05CH11231; SC0004658; SC0023663
• OSTI ID: 2512410
• Report Number(s): LA-UR--24-28651
• Journal Information: Physical Review. C, Journal Name: Physical Review. C Journal Issue: 2 Vol. 111; ISSN 2469-9985; ISSN 2469-9993
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English