Flavor-changing light bosons with accidental longevity

Abstract

We consider a model with a complex scalar field that couples to (e, μ) or (μ, τ) within the “longevity” window: $$ \left[|{m}_{l_1}-{m}_{l_2}|,{m}_{l_1}+{m}_{l_2}\right] $$ m l 1 − m l 2 m l 1 + m l 2 in which l1 and l2 are the two different charged leptons. Within such a mass window, even a relatively large coupling (e.g. of the size commensurate with the current accuracy/discrepancy in the muon g − 2 experiment) leads to long lifetimes and macroscopic propagation distance between production and decay points. We propose to exploit several existing neutrino experiments and one future experiment to probe the parameter space of this model. For the μ − e sector, we exploit the muonium decay branching ratio and the production and decay sequence at the LSND experiment, excluding the parametric region suggested by gμ − 2 anomaly. For the τ − μ sector, we analyze three main production mechanisms of scalars at beam dump experiments: the Drell-Yan process, the heavy meson decay, and the muon scattering. We explore the constraints from the past CHARM and NuTeV experiments, and evaluate sensitivity for the proposed beam dump experiment, SHiP. The latter can thoroughly probe the parameter space relevant for the gμ− 2 anomaly.

Sensitivity potential to a light flavor-changing scalar boson with DUNE and NA64$$\mu $$

“…The model proposed by [14] considers a new complex scalar field, φ, with a mass window [m τ − m μ , m τ + m μ ] that couples to (μ, τ ). With such a mass range, long lifetimes can be achieved with propagation distances on the order of tens of kilometers.…”

“…where 2 is the form factor, with d = 0.164 GeV 2 A −2/3 , and the limits are given in the appendix of [14]. We calculate the analytical expression for χ using Mathematica [34] and obtain…”

“…To calculate the lifetime of the φ boson, τ φ , we use Equations 3.2−3.5 in [14] adapted to the (μ, τ ) case. We calculate the cross section and decay width using the GNU Scientific Library [36].…”

“…In DUNE, a signal could be detected in the near detector from the decay of the φ boson that was produced by the muons hitting the stainless-steel dump. We consider the decay channel φ → μ + μ − ν μ ν τ with a branching ratio of 17% [14], motivated by the dimuon results from NuTeV [16]. Possible backgrounds leading to a dimuon signature include deep inelastic scattering (DIS) and resonance production of mesons in charged-current (CC) muon-neutrino (ν μ ) interactions with a target nucleus.…”

“…In this work, we study the sensitivity potential of muon-ontarget experiments to new physics using a CLFV benchmark model introduced in a recent work [14], which uses a light, scalar boson associated with μ−τ conversion. While the previous work derived constraints from data in existing beamdump experiments (LSND [15], NuTeV [16], CHARM [17]) and for the future SHiP experiment [18], here we focus on two further experiments: the coming neutrino experiment, DUNE [12] deploying a high-intensity beam from the Long-Baseline Neutrino Facility (LBNF) [19], and a fixed-target experiment, NA64μ [20,21], which searches for new physics with the high-energy muon beam from the CERN Super Proton Synchrotron (SPS) accelerator.…”