[Paper Review] Nuclear $β$ decay as a probe for physics beyond the Standard Model

M. Brodeur, N. Buzinsky+44 more|arXiv (Cornell University)|Jan 10, 2023

Neutrino Physics Research4 citations

TL;DR

This white paper advocates for enhanced experimental and theoretical nuclear β decay research to probe physics beyond the Standard Model (BSM), leveraging precision measurements of β and electron capture decays in rare isotopes to search for sub-MeV sterile neutrinos and new physics. It highlights that advanced many-body theories and next-generation experiments like BeEST and HUNTER now enable the most stringent laboratory limits on sterile neutrinos, with BeEST already setting the best constraints in the 100–850 keV range.

ABSTRACT

This white paper was submitted to the 2022 Fundamental Symmetries, Neutrons, and Neutrinos (FSNN) Town Hall Meeting in preparation for the next NSAC Long Range Plan. We advocate to support current and future theoretical and experimental searches for physics beyond the Standard Model using nuclear $β$ decay.

Motivation & Objective

• To advance nuclear β decay as a sensitive probe for physics beyond the Standard Model (BSM), particularly for sub-MeV sterile neutrinos.
• To address the need for improved theoretical calculations of electroweak radiative corrections and nuclear matrix elements in forbidden decays.
• To support experimental programs that measure low-Q β and electron capture decays with high precision to detect light and heavy neutrino masses.
• To resolve the reactor antineutrino anomaly by improving β-shape function measurements and background modeling.
• To explore rare decay modes as signatures of new physics, such as dark decay branches in neutron-rich nuclei.

Proposed method

• Utilize precision measurements of nuclear recoil and decay products in electron capture and β decay to reconstruct the missing neutrino four-momentum and mass.
• Apply advanced many-body nuclear theory methods with controlled uncertainties to calculate spectral shapes and matrix elements in forbidden decays.
• Focus on neutron-deficient nuclei like 7Be and 131Cs, where 3-body β decay is forbidden, simplifying neutrino mass reconstruction.
• Implement high-resolution momentum and energy measurements of decay products in experiments such as BeEST and HUNTER to probe neutrino mass and mixing.
• Combine experimental data with theoretical models to constrain sterile neutrino mixing in the eV–MeV range.
• Use rare decay branching ratios—such as β-decay to 10Be in 11Be or neutron decay from 5He in 6He—to search for dark decay signatures.

Experimental results

Research questions

• RQ1 Can precision nuclear β and electron capture decay measurements detect sub-MeV sterile neutrinos with improved sensitivity?
• RQ2 To what extent can many-body nuclear theory reduce uncertainties in electroweak radiative corrections and nuclear matrix elements for BSM searches?
• RQ3 Can rare decay modes such as dark decay branches in 11Be or 6He provide evidence for new physics beyond the Standard Model?
• RQ4 How do precise β-shape function measurements in fission fragments impact reactor antineutrino flux predictions and the reactor anomaly?
• RQ5 What constraints do low-Q decay experiments place on light neutrino masses and sterile neutrino mixing in the 100–850 keV range?

Key findings

• The BeEST experiment currently sets the most stringent laboratory limits on sterile neutrinos in the 100–850 keV mass range.
• Projected sensitivities from HUNTER and BeEST will significantly improve constraints on sub-MeV sterile neutrinos, surpassing previous experimental limits.
• Neutron-deficient nuclei like 7Be and 131Cs enable clean neutrino mass measurements due to the suppression of 3-body β decay, simplifying missing energy reconstruction.
• Theoretical progress in many-body methods now allows for controlled uncertainties in spectral shape calculations, essential for BSM sensitivity.
• Measurements of 11Be’s (β,p) decay via a near-threshold resonance in 11B confirm a rare decay mode, but the presence of a dark decay branch remains inconclusive due to conflicting data on 10Be yield.
• A very low upper limit was found for a dark neutron decay in 6He, suggesting no significant branching into unbound 5He states.