Radiative corrections to neutron and nuclear beta decays revisited

Abstract

The universal radiative corrections common to neutron and super-allowed nuclear beta decays (also known as "inner" corrections) are revisited in light of a recent dispersion relation study that found +2.467(22)%, i.e. about 2.4σ larger than the previous evaluation. For comparison, we consider several alternative computational methods. All employ an updated perturbative QCD four-loop Bjorken sum rule (BjSR) defined QCD coupling supplemented with a nucleon form factor based Born amplitude to estimate axial-vector induced hadronic contributions. In addition, we now include hadronic contributions from low Q 2 loop effects based on duality considerations and vector meson resonance interpolators. Our primary result, 2.426(32)% corresponds to an average of a Light Front Holomorphic QCD approach and a three resonance interpolator fit. It reduces the dispersion relation discrepancy to approximately 1.1σ and thereby provides a consistency check. Consequences of our new radiative correction estimate, along with that of the dispersion relation result, for CKM unitarity are discussed. The neutron lifetime-gA connection is updated and shown to suggest a shorter neutron lifetime < 879 s. We also find an improved bound on exotic, non-Standard Model, neutron decays or oscillations of the type conjectured as solutions to the neutron lifetime problem, BR(n → exotics) < 0.16%.

A new theory framework for the electroweak radiative corrections in Kl3 decays

“…This is, however, not yet conclusive and needs to be further scrutinized. Further discussions along this line have also led to the identification of well-defined steps toward the further reduction of the |V ud | uncertainty: first, next-generation neutrino experiments at the Long-Baseline Neutrino Facility (LBNF) at Fermilab are expected to provide much more precise neutrino data input to the dispersion integral [11], second, a direct lattice approach to the γW -box diagram is recently suggested [12], alternative computational methods of the box diagram are also proposed in comparison to the dispersive method [13], and finally, there are plans to study the above-mentioned new nuclear effects with ab-initio methods [6]. In short, one can be optimistic that the theoretical uncertainty in the |V ud | extraction will be further reduced in the near future.…”

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.…”

A new angle on an old problem: helicity approach to neutron beta decay in the Standard Model

“…As concerns the error propagation, the rate measurement is 2.5 times better than the best asymmetry measurement. However, the extraction of λ from the rate measurement requires additional input in the form of the value of V ud and the size of the radiative corrections [58]. In contrast to this the asymmetry measurements are independent of the value of V ud .…”

“…This opens the opportunity for further measurements of the form factor F A 1 from other observables on top of the usual determination of F A 1 from the rate measurement (see the discussion in Ref. [42,58]). Particularly well suited for such a measurement of F A 1 would be the three observables A PFB , A Pconv and A χ2 which are the most sensitive asymmetries for a determination of F A 1 .…”