Framework for maximum likelihood analysis of neutronβdecay observables to resolve the limits of theV−Alaw

Abstract

We assess the ability of future neutron β decay measurements of up to O(10 −4 ) precision to falsify the standard model, particularly the V − A law, and to identify the dynamics beyond it. To do this, we employ a maximum likelihood statistical framework which incorporates both experimental and theoretical uncertainties. Using illustrative combined global fits to Monte Carlo pseudodata, we also quantify the importance of experimental measurements of the energy dependence of the angular correlation coefficients as input to such efforts, and we determine the precision to which ill-known "second-class" hadronic matrix elements must be determined in order to exact such tests.

Isospin Breaking in the Nucleon Mass and the Sensitivity ofβDecays to New Physics

“…We have g V ¼ 1 up to second order isospinbreaking corrections [29], whereas the axial charge has been accurately measured in β decays, g A ¼ 1.2701ð25Þ × g V [30]. Lastly, the subleading contributions coming from the so-called "induced" form factorsg i are known in the limit of isospin symmetry, a safe approximation at the current level of experimental precision [13,31]. The description of nuclear β decays requires the introduction of the Fermi and Gamow-Teller nuclear matrix elements that play an analogous role to g V and g A in the neutron decay.…”

Spin flip loss in magnetic confinement of ultracold neutrons for neutron lifetime experiments

“…To solve (A10) we substitute the WKB approximation for the partial derivatives on the RHS, α (3) x ′ = ik x ′ α (3) , α (A13) The solution of (A13) has been outlined in [20,21]. The phase factor for the wave function β(z ′ ) is the same as for α(z ′ ): e iX ′ + (z ′ ).…”

“…Superscript (3) indicates that α (3) (x, y, z), for the spin-up wave (relative to the local magnetic field direction), and β (3) (x, y, z), for the spindown wave, are functions of the three spatial coordinates.…”

“…τ n can be combined with the neutron β-decay (n → p + e − +ν e ) correlation coefficients to determine the universal weak interaction vector and axial-vector coupling constants whose values allow searches for semi-leptonic scalar and tensor currents beyond the Standard Model [1][2][3][4]. A τ n of reliable precision is also needed for calculations of the neutrino flux expected from solar and reactor sources, including detection efficiencies [5][6][7], as well as in Big Bang nucleosynthesis calculations.…”

An overview of neV probes of PeV scale physics – and of what's in between

“…Although all the mentioned matrix elements could be computed in lattice QCD, not all of the precise matrix elements needed have been -and we have already noted the problem with g A . Consequently, to resolve the limits of the V − A law in decay we must fit for SM physics, specifically for ≡ g A /g V , and BSM physics simultaneously [37]. There are poorly known recoil-order matrix elements, notably g 2 and f 3 ; they enter in recoil order and can mimic the appearance of scalar and tensor effects.…”