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.
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.
I consider the γW-box correction to superallowed nuclear β decays in the framework of dispersion relations. I address a novel effect of a distortion of the emitted electron energy spectrum by nuclear ...polarizabilities and show that this effect, while neglected in the literature, is sizable. The respective correction to the β^{+} spectrum is estimated to be Δ_{R}(E)=(1.6±1.6)×10^{-4}E/MeV assuming a conservative 100% uncertainty. The effect is positive definite and can be observed if a high-precision measurement of the positron spectrum is viable. If only the full rate is observed, it should be included in the calculated Ft values of nuclear decays. I argue that this novel effect should be included in the analyses of nuclear beta decay experiments to ensure the correct extraction of V_{ud} from decay rates, and of the Fierz interference term from precision measurements of decay spectra.
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 present the first and complete dispersion relation analysis of the inner radiative corrections to the axial coupling constant
g
A
in the neutron
β
-decay. Using experimental inputs from ...the elastic form factors and the spin-dependent structure function
g
1
, we determine the contribution from the
γW
-box diagram to a precision better than 10
−
4
. Our calculation indicates that the inner radiative corrections to the Fermi and the Gamow-Teller matrix element in the neutron
β
-decay are almost identical, i.e. the ratio
λ
=
g
A
/g
V
is almost unrenormalized. With this result, we predict the bare axial coupling constant to be
g
∘
A
=
−
1.2754
13
exp
2
RC
based on the PDG average
λ
=
−
1
.
2756(13).
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.
Abstract We fill up the missing piece in our own re-analysis of the long-distance electromagnetic radiative corrections to semileptonic kaon decays by performing a rigorous study in the K → πμ + ν μ ...(γ) channels. With appropriate experimental and lattice inputs, we achieve a precision level of 10 −4 in these channels. This is comparable to our previous analysis in the K → πe + ν e (γ) channels. With this new result, we present an updated global analysis to extract the Standard Model parameter |V us | from semileptonic kaon decays. We obtain |V us | = 0.22308(55) and 0.22356(73), using the lattice average of the K 0 → π − transition form factor at N f = 2 + 1 + 1 and N f = 2 + 1, respectively.
We report a high-precision calculation of the Standard Model electroweak radiative corrections in the K→πe+ν(γ) decay as a part of the combined theory effort to understand the existing anomaly in the ...determinations of Vus. Our new analysis features a chiral resummation of the large infrared-singular terms in the radiative corrections and a well-under-control strong interaction uncertainty based on the most recent lattice QCD inputs. While being consistent with the current state-of-the-art results obtained from chiral perturbation theory, we reduce the existing theory uncertainty from 10−3 to 10−4. Our result suggests that the Standard Model electroweak effects cannot account for the Vus anomaly.
We present a lattice QCD calculation of the axial γW-box diagrams relevant for the kaon semileptonic decays. We utilize a recently proposed method, which connects the electroweak radiative ...corrections in Sirlin's representation to that in chiral perturbation theory. It allows us to use the axial γW-box correction in the SU(3) limit to obtain the low energy constants for chiral perturbation theory. From first principles our results confirm the previously used low energy constants provided by the minimal resonance model with a significant reduction in uncertainties.
It was recently estimated that the strong isospin-symmetry breaking (ISB) corrections to the Fermi matrix element in free neutron decay could be of the order 10−4, one order of magnitude larger than ...the naïve estimate based on the Behrends-Sirlin-Ademollo-Gatto theorem. To investigate this claim, we derive a general expression of the leading ISB correction to hadronic Fermi matrix elements, which takes the form of a four-point correlation function in lattice gauge theory and is straightforward to compute from first principles. Our formalism paves the way for the first determination of such correction in the neutron sector with fully-controlled theory uncertainties.
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