Hadronic Probes of Non-Standard Neutrino Interactions

Abstract

In this work, we study leptonic decays of hadrons as probes of light neutrinophilic scalars that mediate enhanced neutrino self-interactions. Such scalars can be emitted in processes involving neutrinos, turning two-body decays into three-body final states and producing characteristic spectral distortions. We compute these effects for charged pion decay and nuclear electron capture decay, including both on-shell and off-shell scalar emission, as well as the loop-induced renormalization required to cancel divergences. Using these results, we derive the projected sensitivity of PIONEER and assess the current and future reach of BeEST. The resulting low-energy spectral tails provide a characteristic signal for light neutrinophilic scalars, making upcoming hadron decay experiments powerful probes of light mediators of non-standard neutrino self-interactions.

Figures (4)

• Figure 1: Differential branching fraction of the positron energy distribution for the SM (black), muon decay-in-flight background (gray), and the neutrinophilic scalar with $\lambda_{e \ell} = 0.3$ for different $\phi$ masses. We use two $\mu$ decay-in-flight background hypotheses (gray): (i) $\mu\text{DIF}_{\text{nom.}}$ which is what is obtained with an average $\mu$ stopping time of $12~{\rm ps}$ and no further suppression and (ii) $\mu\text{DIF}_{\text{1\%}}$ where this background is suppressed to 1% of $\mu\text{DIF}_{\text{nom.}}$, which aligns with the precision target of PIONEER. See text for details.
• Figure 2: Projected $95\%$ CL sensitivity of PIONEER to neutrinophilic scalars coupled to electron neutrinos, shown in the $(m_\phi,\lambda_{e\ell})$ plane with $\Lambda=500$ GeV and $\xi_{UV}=1$. The solid (dashed) pink curve corresponds to the $\mu\text{DIF}_{\text{1\%}}$ ($\mu\text{DIF}_{\text{nom.}}$) background assumptions. Existing laboratory constraints from PIENU, kaon decays, invisible $Z$ and Higgs decays are shown for comparison, together with indicative constraints from neutrino experiments and cosmology. The shaded teal region indicates the parameter space favored by recent CMB analyses of enhanced neutrino self-interactions, while the vertical teal line shows the approximate lower bound from BBN, assuming standard cosmology. We also show the future sensitivity for DUNE Kelly:2019wow and the Forward Physics Facility (FPF) Kelly:2021mcd for comparison.
• Figure 3: Differential Branching fraction of the recoil energy distribution for the SM (black) and the neutrinophilic scalar with $\lambda_{e\ell} = 0.3$. In dashed gray, we show the Gaussian fits of the four decay peaks used for the analysis.
• Figure 4: Projected sensitivity of BeEST to neutrinophilic scalars coupling to electron neutrinos. The curves correspond to increasing experimental exposure and capability: a single pixel-day, the current Phase III configuration, a projected Phase IV exposure, and Phase IV with photon-coincidence tagging of decays to the excited nuclear state. Existing laboratory constraints and the projected PIONEER sensitivity are shown for comparison.