Probing TeV scale physics in precision UCN decays

TL;DR

This paper calculates neutron-proton matrix elements for scalar, axial, and tensor charges using lattice QCD to explore potential new physics interactions at the TeV scale in neutron beta-decay.

Contribution

It provides the first lattice QCD determination of these matrix elements on $N_f=2+1+1$ HISQ configurations with valence clover fermions, addressing systematic errors.

Findings

Matrix elements computed at $a=0.12$ fm for two pion masses.

Analysis of excited state contributions and renormalization constants.

Results inform constraints on scalar and tensor interactions beyond the Standard Model.

Abstract

We present the calculation of matrix elements of iso-vector scalar, axial and tensor charges between a neutron and a proton state on dynamical $N_f=2+1+1$ HISQ configurations generated by the MILC Collaboration using valence clover fermions. These matrix elements are needed to probe novel scalar and tensor interactions in neutron beta-decay that can arise in extensions to the Standard Model at the TeV scale. Results are presented at one value of the lattice spacing, $a=0.12$ fm, and two values of light quarks corresponding to $M_\pi=310$ and $220$ MeV. We discuss two sources of systematic errors, contribution of excited states to these matrix elements and the renormalization constants, and the efficacy of methods used to control them.