Prospects for Neutrino Observation and Mass Measurement from Binary Neutron Star Mergers

TL;DR

This work reevaluates the prospects for observing neutrinos from binary neutron star mergers and for using such detections to constrain the absolute neutrino mass. By incorporating updated binary merger rates from LVK O4 and realistic neutrino emission from recent simulations, it shows that current and near-term detectors are unlikely to observe BNS neutrinos, pushing the requirement to megaton-scale detectors with low ~10 MeV thresholds. It introduces an energy- and distance-dependent background mitigation framework that leverages the GW–neutrino coincidence, demonstrating that background contamination can be drastically reduced with realistic time windows and potentially modest background reductions. Beyond detection, the paper shows that a single BNS neutrino event could yield competitive, if not leading, constraints on the lightest neutrino mass via time-of-flight analyses, with sensitivity improving as the neutrino energy increases or the emission window and distance uncertainties are tightened, potentially surpassing the current KATRIN and some cosmological bounds in favorable scenarios.

Abstract

Over the next decade, $\mathcal{O}(100)$ diffuse supernova neutrino background (DSNB) events are expected in Hyper-Kamiokande. Another neutrino source that has received far less attention is binary neutron star mergers. Including the data from recent simulations, we find that detection in current and near-future neutrino experiments is not feasible, and a megaton-scale detector with $\mathcal{O}(10)$ MeV threshold, such as the proposed Deep-TITAND, MEMPHYS, or MICA, will be required. This is due to the updated binary neutron star merger rate and the time-of-flight delay caused by the nonzero neutrino mass. Regarding the former, recent results from LIGO, Virgo, and KAGRA has significantly lowered the upper limit on the neutron star merger rate. As for the latter, neutrino events from neutron star mergers are expected to be recorded shortly after the gravitational wave signal. Limiting the analysis to such short time windows can significantly reduce background rates. While this approach has been qualitatively discussed in the literature, the effect of the time delay caused by neutrino mass, which can substantially extend the observation windows, has been disregarded. We present a refined analysis employing energy-dependent time windows and luminosity distance cuts for the mergers and provide realistic estimates of the detector runtime required to record neutrinos from binary neutron star mergers with small background contamination. The relative timing between the neutrino and gravitational wave signals can also be employed to probe the scale of neutrino mass. We find that the sensitivity to the lightest neutrino mass exceeds both the most stringent terrestrial bounds from KATRIN and the projections based on galactic supernovae. This level of sensitivity may become particularly relevant in the future if terrestrial and supernova constraints are not significantly improved.

Figures (8)

• Figure 1: Number of neutrino events from binary neutron star mergers expected in JUNO, DUNE, Hyper-Kamiokande (HK), $1$ Mt and $5$ Mt water Cherenkov detector (WCD) as a function of the maximum redshift considered for such mergers. We adopt the neutrino luminosities from Ref. Fujibayashi:2020dvr. The sensitivities of current and upcoming gravitational-wave interferometers are also shown as vertical lines, indicating the redshift at which a merger can be detected with 90% efficiency.
• Figure 2: The number of expected neutrino events from binary neutron star mergers up to redshift $z=1$, in the $(16, 50)$ MeV neutrino energy interval at a $5$ Mt WCD over 20 years of data taking. The results are shown as a function of the time-integrated electron antineutrino luminosity and the average energy of electron antineutrinos. The star indicates the values adopted in \ref{['fig:events']}.
• Figure 3: Schematic illustration of the uniform distribution of background events over time, as well as the signal events contained within the observational time window of length $x$, measured from the coalescence time.
• Figure 4: Energy bin-dependent $z_\text{cut}$ as a function of neutrino energy and the lightest neutrino mass for a 5 Mt WCD with the models taken from Ref. Fujibayashi:2020dvr.
• Figure 5: Values of the maximum merger redshift $z_{\text{cut}}$ as a function of neutrino energy for two neutrino emission durations of 6 s (left panel) and 0.6 s (right panel) are shown in black. We also show, in red, the corresponding average value of $\Delta t_{\text{tot}}$. Solid lines show results using the full background expected for a 5 Mt WCD, while dashed lines represent the case of a 50% background reduction.