New physics searches in nuclear and neutron β decay

Abstract

The status of tests of the standard electroweak model and of searches for new physics in allowed nuclear β decay and neutron decay is reviewed including both theoretical and experimental developments. The sensitivity and complementarity of recent and ongoing experiments are discussed with emphasis on their potential to look for new physics. Measurements are interpreted using a model-independent effective field theory approach enabling to recast the outcome of the analysis in many specific new physics models. Special attention is given to the connection that this approach establishes with high-energy physics. A new global fit of available β-decay data is performed incorporating, for the first time in a consistent way, superallowed 0 + → 0 + transitions, neutron decay and nuclear decays. The constraints on exotic scalar and tensor couplings involving left-or right-handed neutrinos are determined while a constraint on the pseudoscalar coupling from neutron decay data is obtained for the first time as well. The values of the vector and axial-vector couplings, which are associated within the standard model to V ud and g A respectively, are also updated. The ratio between the axial and vector couplings obtained from the fit under standard model assumptions is C A /C V = −1.27510(66). The relevance of the various experimental inputs and error sources is critically discussed and the impact of ongoing measurements is studied. The complementarity of the obtained bounds with other low-and high-energy probes is presented including ongoing searches at the Large Hadron Collider.

Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

“…The hadronic uncertainty of the forward γZ-box correction has recently raised a significant interest [28][29][30][31][32][33][34][35][36][37][38][39][40] in the context of a precision determination of the weak mixing angle with parity-violating electron scattering [41,42]. Similarly, recent works on reducing hadronic and nuclear uncertainties of the γWbox correction [43][44][45][46][47] prove central in extracting the V ud element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix and testing the CKM unitarity, a sensitive probe of the SM extensions [48]. Experimental observables explicitly sensitive to the TPE mechanism are instrumental in developing a dispersion-theoretical framework for TPE.The lepton-proton scattering amplitude in presence of TPE can be parameterized in terms of six generalized form factorsG E (Q 2 , ε),G M (Q 2 , ε) andF i (Q 2 , ε), i = 3, ..., 6 [49], where Q 2 is the negative four-momentum transfer squared, and ε = (1 + 2(1 + Q 2 4M 2 ) tan 2 θ 2 ) −1 , with M and θ being the nucleon mass and the lepton scattering angle, respectively.…”

Lattice QCD Inputs for nuclear double beta decay

“…This calculation was performed without the inclusion of electromagnetism, at a single lattice spacing and volume, and at the SU(3) flavor-symmetric point with degenerate up, down and strange quark masses corresponding to a larger-than-physical pion mass of m π ∼ 806 MeV. While all of these caveats are possibly important, the key qualitative result of that work was to reveal the potential significance of an operator that contributes to the ββ-decay of nuclei, but not to single-β decays, namely the isotensor axial polarizability, β (2) A , of the 1 S 0 two-nucleon system. This polarizability is defined from M 2ν GT by subtracting the term corresponding to an intermediate deuteron state, i.e., the 'Born' term as in forward Compton scattering:…”

“…The coefficient of the term linear in t in Eq. (46), which determines the isotensor axial polarizability β (2) A defined in Eq. (41), can be extracted from the large-time limit of…”

“…The study of nuclear beta decays has been instrumental in the development of the modern theory of electroweak interactions encoded in the Standard Model (SM) [1]. Single beta decay of the neutron and nuclei with sufficiently high precision has been used to test the SM and probe new physics in chargedcurrent electroweak interactions [2]. Double beta decay [3] (DBD) is a rare nuclear process, observable only in certain nuclei with even numbers of protons and neutrons (even-even nuclei) for which single beta decay is energetically forbidden.…”

Improved limits on Fierz interference using asymmetry measurements from the Ultracold Neutron Asymmetry (UCNA) experiment

“…There are many past and current experiments to measure these decay parameters such as lifetimes, angular/spin correlations, and energy spectra, to name a few [1][2][3][4][5][6][7]. The Fierz interference term is one such decay parameter in the neutron β-decay rate (explained below) which vanishes in the Standard Model but would serve as a probe for beyond Standard Model physics in scalar and tensor couplings [1,4,8,9].In this publication, the UCNA collaboration presents a measurement of the Fierz interference term, b n , using the energy-dependence of the neutron β-decay asymmetry for the 2010 [10], and 2011-2013 data-taking runs [11]. For completeness, the electron energy spectra from the 2011-2013 data-taking runs are also used to extract b n arXiv:1911.05829v1 [nucl-ex]…”