Directional recoil detection for CEvNS measurements with light nuclei at the Spallation Neutron Source

Abstract

The coherent elastic scattering of neutrinos on nuclei, also known as CEvNS, has been studied for several years by the COHERENT program of experiments using neutrinos from stopped-pion decays produced at the Spallation Neutron Source (SNS). We propose a new approach for CEvNS measurements at the SNS that aims to complement the COHERENT experiments in two main ways: by reconstructing the angular distribution of CEvNS-induced recoils, and by measuring CEvNS on much lighter target nuclei such as helium, carbon, and fluorine. The proposed detector would employ a gaseous time-projection chamber with a highly segmented charge readout to enable the spatial reconstruction of $\sim$10-500 keV ionisation tracks created by CEvNS-induced recoils. This would enable the simultaneous measurement of the CEvNS recoil energy and scattering angle, thereby allowing event-by-event reconstruction of the neutrino energy. We estimate that a 60:40 He:CF$_4$ gas mixture at atmospheric pressure offers a good trade-off between total target mass and good directionality and could deliver a detection of the angular distribution of CEvNS, even under pessimistic background conditions. We project the sensitivity of 1 and 10 m$^3$-scale detectors in the context of several physics cases, including: the measurement of the Standard Model CEvNS cross section, reconstruction of the flavour-dependent neutrino fluxes, observing the neutrino-induced Migdal effect, constraints on beyond-Standard Model neutrino interactions, and probing 10-eV-scale sterile neutrinos.

Figures (17)

• Figure 1: The SNS neutrino flux model adopted in this work, which consists of two continuous fluxes of delayed $\nu_e$ and $\bar{\nu}_\mu$ as well as the mono-energetic line at $E_\nu \approx 30$ MeV corresponding to the prompt flux of $\nu_\mu$. The assumed effective source-to-detector distance is 12 metres.
• Figure 2: Left: Expected number of events per year in a $V = 1$ m$^3$ experiment as a function of the recoil energy threshold $E_{\rm th}$, defined by integrating Eq. (\ref{['eq:d2RdEdOmega']}) over all angles and for energies $E_r>E_{\rm th}$. Right: Distribution of events in the same gas mixture as the left-hand panel, as a function of the scattering angle, $\theta$. This is calculated by integrating Eq. (\ref{['eq:d2RdEdOmega']}) over energies in the range $E_r \in [10,1500]$ keV, and then changing variables to $\theta$ as opposed to $\cos{\theta}$. In both panels, as well as the total yearly event rate (black dashed line), we also show the number of events attributed to each nucleus in our fiducial 60:40 He:CF$_4$ gas mixture.
• Figure 3: Diagram of the anticipated dimensions of a 10 m$^3$ directional gas TPC. The drift axis would be kept short, limiting the maximum drift distance to 50 cm so as to reduce the impact of diffusion, as this has the largest impact in washing out the directionality of the charge track (discussed further in Fig. \ref{['fig:angular_resolution']}). An initial 1 m$^3$ version would have the same design but with the readout planes scaled down.
• Figure 4: Angular resolution model for our conceptual detector design as a function of nuclear recoil energy, $E_r$, and for five pressures of pure CF$_4$ gas. This model is expressed mathematically in Eq. (\ref{['eq:angularresolution']}). The pressure-independent contribution from nuclear straggling alone is shown as a black dashed line. On the upper horizontal axis, we also show the fraction of $^{19}$F recoils lying above the corresponding recoil energy shown on the bottom axis.
• Figure 5: Fraction of events with angular resolution better than some "directionality threshold", $\sigma_\theta<\sigma_\theta^{\rm th}$. We show this fraction as a function of the CF$_4$ pressure since the angular resolution degrades for higher gas densities. We show four example choices for $\sigma_\theta^{\rm th}$. If we wish the majority of the sample of events to have angular resolution better than $\sigma_\theta<26$--$30^\circ$ (twice the standard deviation of the true underlying recoil angle distribution), then a gas pressure lower than 0.4--0.5 atmospheres is required.