Enhanced active-sterile neutrino polarizability at the intensity frontier

Abstract

Electromagnetic probes of neutrinos can provide insights into physics beyond the Standard Model. Among the possible electromagnetic interactions of neutrinos is neutrino polarizability, a dimension-7 effective operator that couples two neutrinos to two photons. In this manuscript, we study a realization of the neutrino polarizability operator in which one of the active neutrinos is replaced by a sterile neutrino. We derive new constraints on this active-sterile neutrino polarizability from its contribution to neutrino-nucleus scattering with a single photon in the final state at neutrino experiments. We show that a realization of this operator via a light mediator can explain the MiniBooNE low-energy excess while remaining consistent with other experimental constraints. Finally, we comment on additional model realizations of this higher-dimensional operator.

Figures (9)

• Figure 1: The polarizability effective vertex between two photons, one active neutrino, and one sterile neutrino.
• Figure 2: The differential cross section, divided by the incoming neutrino energy $E_\nu$, for the coherent $\nu_a \mathcal{A} \to N \mathcal{A} \gamma$ scattering off an atomic target ${\cal A}$ with respect to the final state photon energy $E_\gamma$, for massless incoming neutrinos and benchmark choices for the heavy neutrino mass $m_{N}$. For low energies, the differential cross section decreases and shifts to lower photon energies as $m_{N}$ approaches threshold due to kinematic phase space alone (dashed lines). At higher energies, as $m_{N}$ grows the increased momentum transfer suppresses the coherence via the nuclear form factor; hence, the $m_{N}=250, \, 400$ MeV cases are upscaled by $10$ and $10^3$, respectively.
• Figure 3: Heat maps of the two-dimensional differential $\nu$IIP coherent cross section, in the heavy EFT picture (Eq. \ref{['eq:effopt']}), over the outgoing photon energy $E_\gamma$ and cosine of the angle with respect to the parent neutrino direction $\cos\theta_\gamma$; the vertical axis is shown over $(1-\cos\theta_\gamma)/2$ to instead show the very-forward region. Below the gray dashed line indicates where $\cos\theta_\gamma > 0.9$ corresponding to the very-forward region.
• Figure 4: Neutrino-induced inverse Primakoff ($\nu$IIP) coherent scattering on atomic targets (top), incoherent scattering on individual fermions $f=e^-, p$ (middle), and deep inelastic scattering (bottom) each become dominant in the soft ($q^2 \lesssim 0.1$ GeV$^2$), high energy ($q^2 \sim$ GeV$^2$), and very high energy regimes, respectively.
• Figure 5: Exclusion region and future sensitivity from different neutrino experiments in the plane of the mass of the sterile state and the effective couplings to muon neutrinos and photons $C_{\mu j}/\Lambda^3$ (upper plot) and electron neutrinos and photons $C_{e j}/\Lambda^3$ (lower plot). Future experimental projections are shown as dashed lines.