[Paper Review] An Active-to-Sterile Neutrino Transition Dipole Moment and the XENON1T Excess

Ian M. Shoemaker, Yu-Dai Tsai+1 more|arXiv (Cornell University)|Jul 10, 2020

Cosmology and Gravitation Theories11 citations

TL;DR

This paper proposes that an active-to-sterile neutrino transition magnetic dipole moment can explain the XENON1T electron recoil excess without violating astrophysical constraints. By introducing a sterile neutrino with mass ~260 keV or in the 500–800 keV range, the model evades bounds from SN1987A and stellar energy loss, while the low reheating temperature of the early universe alleviates big bang nucleosynthesis (BBN) constraints.

ABSTRACT

In this short letter, we find that a magnetic transition dipole moment between tau and sterile neutrinos can account for the XENON1T excess events. Unlike the ordinary neutrino dipole moment, the introduction of the new sterile mass scale allows for astrophysical bounds to be suppressed. Interestingly, the best-fit regions that are compatible with the SN1987A imply either boron-8 or CNO neutrinos as the source flux. We find that sterile neutrinos of either $\sim$ 260 keV or in the $\sim$(500 - 800) keV mass range are capable of evading astrophysical constraints while being able to successfully explain the XENON1T event rate. The sterile neutrino in the best fit parameter space may have significant effects on big bang nucleosynthesis (BBN). We show the region in which a low reheating temperature of the Universe may allow the BBN constraints to be alleviated.

Motivation & Objective

• To explain the XENON1T electron recoil excess using a novel neutrino dipole portal interaction.
• To evade stringent astrophysical constraints (e.g., from SN1987A and stellar energy loss) that typically rule out such models.
• To identify viable sterile neutrino masses and coupling strengths consistent with the XENON1T data and cosmological bounds.
• To explore how a low reheating temperature in the early universe can alleviate big bang nucleosynthesis (BBN) constraints on the sterile neutrino.
• To assess the viability of the dipole portal model under thermalization and decay processes in the early universe.

Proposed method

• The model introduces a transition magnetic dipole moment between active (tau) and sterile neutrinos via the interaction Lagrangian: $\mathcal{L} \supset d(\bar{\nu}_L \sigma_{\mu\nu} F^{\mu\nu} N) + \text{h.c.}$, with $d$ as the dipole coupling.
• The sterile neutrino is assigned a Dirac mass $m_4$, and the model assumes negligible mixing with active neutrinos to avoid strong constraints.
• The analysis computes event rates in XENON1T using solar neutrino fluxes (pp, CNO, and $^8$B) and compares them to the observed excess, including detector efficiency.
• Thermalization rates for $N$ production via $\nu + \gamma \to N$ and $\nu + e^- \to N$ are calculated using thermal-averaged cross-sections and compared to the Hubble rate.
• The parameter space is constrained by requiring that thermalization rates do not exceed the Hubble rate below 2–5 MeV, to avoid overpopulating $N$ and violating BBN.
• The model evaluates the impact of low reheating temperature ($T_{\rm RH} \sim 2{-}5$ MeV) on thermalization and BBN, showing that such scenarios can evade strong bounds.

Experimental results

Research questions

• RQ1 Can a transition magnetic dipole moment between active and sterile neutrinos explain the XENON1T electron recoil excess?
• RQ2 How can such a model evade astrophysical bounds from SN1987A and stellar energy loss?
• RQ3 What sterile neutrino masses and dipole couplings are consistent with XENON1T data and cosmological constraints?
• RQ4 Can a low reheating temperature in the early universe alleviate big bang nucleosynthesis (BBN) constraints on the sterile neutrino?
• RQ5 What role do inverse decay processes ($\nu + \gamma \to N$) play in thermalizing the sterile neutrino and affecting cosmological bounds?

Key findings

• The XENON1T excess can be explained by a transition dipole moment between tau neutrinos and a sterile neutrino with mass ~260 keV or in the 500–800 keV range.
• The model evades SN1987A and stellar energy loss bounds due to the suppression of astrophysical constraints by the new sterile neutrino mass scale.
• The best-fit regions for $^8$B and CNO neutrinos are compatible with low reheating temperature scenarios ($T_{\rm RH} \sim 2{-}5$ MeV), which suppress thermalization and alleviate BBN constraints.
• The $^8$B neutrino flux leads to a best-fit region near $m_4 \sim 640$ keV and $d \sim 2.2 \times 10^{-9}~\mu_B$, matching the observed XENON1T signal spectrum.
• The $\nu + \gamma \to N$ inverse decay process plays a key role in thermalization, with a resonance at $T \sim m_4/2$, and can dominate over $e^- + \nu \to N$ in certain regions.
• The parameter space where thermalization rates exceed the Hubble rate below 2 MeV or 5 MeV is identified, and viable regions lie below these curves, indicating potential BBN safety under low $T_{\rm RH}$.