Recently, the first ever lattice computation of the γ W -box radiative correction to the rate of the semileptonic pion decay allowed for a reduction of the theory uncertainty of that rate by a factor ...of ∼ 3 . A recent dispersion evaluation of the γ W -box correction on the neutron also led to a significant reduction of the theory uncertainty, but shifted the value of V u d extracted from the neutron and superallowed nuclear β decay, resulting in a deficit of the Cabibbo-Kobayashi-Maskawa (CKM) unitarity in the top row. A direct lattice computation of the γ W -box correction for the neutron decay would provide an independent cross-check for this result but is very challenging. Before those challenges are overcome, we propose a hybrid analysis, converting the lattice calculation on the pion to that on the neutron by a combination of dispersion theory and phenomenological input. The new prediction for the universal radiative correction to free and bound neutron β -decay reads Δ V R = 0.02477 ( 24 ) , in excellent agreement with the dispersion theory result Δ V R = 0.02467 ( 22 ) . Combining with other relevant information, the top-row CKM unitarity deficit persists.
We analyze the recoil corrections in superallowed beta decays of T=1, J^{P}=0^{+} nuclei by fixing the mean square charged weak radius model independently using the data of multiple charge radii ...across the nuclear isotriplet. By comparing to model estimations, we argue that the existing theory uncertainty in the statistical rate function f might have been substantially underestimated. We discuss the implications of our proposed strategy for precision tests of the standard model, including a potential alleviation of the first-row CKM unitarity deficit, and motivate new experiments for charge radii measurements.
We propose a novel dispersive treatment of the so-called inner radiative correction to the neutron and nuclear β decay. We show that it requires knowledge of the parity-violating structure function ...F3(0) that arises from the interference of the axial vector charged current and the isoscalar part of the electromagnetic current. By isospin symmetry, we relate this structure function to the charged current inelastic scattering of neutrinos and antineutrinos. Applying this new data-driven analysis we obtain a new, more precise evaluation for the universal radiative correction ΔRV,new=0.02467(22) that supersedes the previous estimate by Marciano and Sirlin, ΔRV=0.02361(38). The substantial shift in the central value of ΔRV reflects in a respective shift of Vud and a considerable tension in the unitarity constraint on the first row of the Cabibbo-Kobayashi-Maskawa matrix which is used as one of the most stringent constraints on new physics contributions in the charged current sector. We also point out that dispersion relations offer a unifying tool for treating hadronic and nuclear corrections within the same framework. We explore the potential of the dispersion relations for addressing the nuclear structure corrections absorbed in the Ft values, a crucial ingredient alongside ΔRV in extracting Vud from superallowed nuclear decays. In particular, we estimate the quenching of the free neutron Born contribution in the nuclear environment, corresponding to a quasielastic single-nucleon knockout, and find a significantly stronger quenching effect as compared to currently used estimates based on the quenching of spin operators in nuclear transitions. This observation suggests that the currently used theoretical uncertainties of Ft values might be underestimated and require a renewed scrutiny, while emphasizing the importance of new, more precise measurements of the free neutron decay where nuclear corrections are absent.
A
bstract
The measurements of
V
us
in leptonic (
K
μ
2
) and semileptonic (
K
l
3
) kaon decays exhibit a 3
σ
disagreement, which could originate either from physics beyond the Standard Model or some ...large unidentified Standard Model systematic effects. Clarifying this issue requires a careful examination of all existing Standard Model inputs. Making use of a newly-proposed computational framework and the most recent lattice QCD results, we perform a comprehensive re-analysis of the electroweak radiative corrections to the
K
e
3
decay rates that achieves an unprecedented level of precision of 10
−
4
, which improves the current best results by almost an order of magnitude. No large systematic effects are found, which suggests that the electroweak radiative corrections should be removed from the “list of culprits” responsible for the
K
μ
2
–
K
l
3
discrepancy.
A
bstract
We introduce a useful framework for high-precision studies of the neutron beta decay by merging the current algebra description and the fixed-order effective field theory calculation of the ...electroweak radiative corrections to the neutron axial form factor. We discuss the advantages of this hybrid method and show that it only requires a minimal amount of lattice QCD inputs to achieve a 10
−4
theory accuracy for the Standard Model prediction of the neutron lifetime and the axial-to-vector coupling ratio
λ
, both important to the search for physics beyond the Standard Model.
We lay out a novel formalism to connect the isospin-symmetry breaking correction to the rates of superallowed nuclear beta decays, δC, to the isospin-breaking sensitive combinations of electroweak ...nuclear radii that can be accessed experimentally. We individuate transitions in the superallowed decay chart where a measurement of the neutron skin of a stable daughter even at a moderate precision could already help discriminating between models used to compute δC. We review the existing experimental situation and make connection to the existing and future experimental programs.
A
bstract
We construct several classes of hadronic matrix elements and relate them to the low-energy constants in Chiral Perturbation Theory that describe the electromagnetic effects in the ...semileptonic beta decay of the pion and the kaon. We propose to calculate them using lattice QCD, and argue that such a calculation will make an immediate impact to a number of interesting topics at the precision frontier, including the outstanding anomalies in |
V
us
| and the top-row Cabibbo-Kobayashi-Maskawa matrix unitarity.
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