Beyond the Standard Model: Coherent Neutrino Scattering at the European Spallation Source

Introduction

Coherent Elastic Neutrino-Nucleus Scattering (CEvNS), a recently discovered process of neutrino interaction, provides numerous opportunities to search for physics beyond the Standard Model (SM). The CEvNS cross-section, much larger than for any other coupling, results in a dramatic miniaturization of neutrino detectors. However, the faint signals generated (few-keV nuclear recoils, NRs) require advanced detector technologies. The unprecedented neutrino flux from the upcoming European Spallation Source (ESS) provides the opportunity for a definitive exploration of all phenomenological applications of CEvNS.

Detector Technology

The centerpiece of this proposal is the development of a new CEvNS detector technology, cryogenic undoped CsI scintillators monitored by innovative sensors (the largest avalanche photodiodes produced to date), in combination with state-of-the-art waveshifters (nanostructured organosilicon luminophores).

1. An array of seven CsI crystals at the ESS, operating at 80 K and adding up to just 32 kg, will provide an exceptionally high signal rate of 12,000 CEvNS events per year.
2. This rate significantly surpasses the sensitivity of ton-scale detectors at present-day spallation sources.
3. The signal output per energy deposited in this new hybrid device is the highest ever achieved with scintillators, providing sensitivity to the lowest NR energies expected from CEvNS, where deviations from the SM are most evident.

Sensitivity to New Physics

This combination of detector technology and neutrino source will achieve the best foreseeable sensitivity to new particle and nuclear physics via CEvNS. A first investigation of the response of high-pressure xenon detectors to low-energy NRs will also be performed.

Comparison of Technologies

For reasons of nuclear structure, CsI and Xe are essentially identical in their response to CEvNS. However, the technologies and their expected systematics are entirely different. Their simultaneous use at the ESS will provide a unique, robust confirmation of any signatures of new physics encountered.