Leptonic Decay Rates Research Articles

Isospin-breaking corrections to light-meson leptonic decays from lattice simulations at physical quark masses

The decreasing uncertainties in theoretical predictions and experimental measurements of several hadronic observables related to weak processes, which in many cases are now smaller than O(1%), require theoretical calculations to include subleading corrections that were neglected so far. Precise determinations of leptonic and semi-leptonic decay rates, including QED and strong isospin-breaking effects, can play a central role in solving the current tensions in the first-row unitarity of the CKM matrix. In this work we present the first RBC/UKQCD lattice calculation of the isospin-breaking corrections to the ratio of leptonic decay rates of kaons and pions into muons and neutrinos. The calculation is performed at fixed lattice spacing (a−1 ≃ 1.730 GeV) on a 483× 96 volume with Nf = 2 + 1 dynamical quarks close to the physical point and domain wall fermions in the Möbius formulation are employed. Long-distance QED interactions are included according to the QEDL prescription and the crucial role of finite-volume electromagnetic corrections in the determination of leptonic decay rates, which produce a large systematic uncertainty, is extensively discussed. Finally, we study the different sources of uncertainty on |Vus|/|Vud| and observe that, if finite-volume systematics can be reduced, the error from isospin-breaking corrections is potentially sub-dominant in the final precision of the ratio of the CKM matrix elements.

New phenomena in laser-assisted leptonic decays of the negatively charged boson W −

The majority of studies and experiments performed at electron-positron colliders over the last two decades have focused on studying W and Z weak force-carrying bosons and accurately measuring all their properties, not only because they play an important role in establishing Standard Model theory and providing an accurate test of its predictions of particle interactions, but also because they are a unique tool for probing manifestations of the new physics beyond the standard model. Therefore, it would be particularly important to discuss some of the new phenomena and changes that can arise in these bosons when their decay occurs under an external electromagnetic field. In a recent paper, we investigated the laser effect on the final products of Z boson decay and found that laser had an unprecedented effect on branching ratios. In this work and within the standard Glashow-Weinberg-Salam model of electroweak interactions, we study theoretically the leptonic decay of the W −-boson in the presence of a circularly polarized electromagnetic field and we examine the laser effect, in terms of its field strength and frequency, on the leptonic decay rate and the phenomenon of multiphoton processes. The calculations are carried out using the exact relativistic wave functions of charged particles in an electromagnetic field. It was found that the laser significantly contributed to reducing the probability of W −-boson decay. We show that the laser-assisted decay rate is equal to the laser-free one only when the famous Kroll-Watson sum rule is fulfilled. The notable effect of the laser on the leptonic decay rate was reasonably interpreted by the well-known quantum Zeno effect or by the opening of channels other than leptonic ones to decay. This work will pave the way for an upcoming one to study the hadronic decay of the W −-boson and then explore the laser effect on its lifetime and branching ratios.

Influence of intense laser fields on measurable quantities in [formula omitted]-boson decay

In principle, this paper suggests powerful laser technology as a promising instrument that can be experimentally useful to control the lifetime and branching ratio for an unstable particle decay. In a recent paper [ arXiv:2101.00224], we calculated theoretically the W−-boson leptonic decay (W−→ℓ−ν̄ℓ) in the presence of a circularly polarized laser and we showed that the laser significantly contributed to the diminution of the leptonic decay rate. In this paper, as a continuation of the previous one, we mainly deal with the theoretical calculation of the W−-boson hadronic decay (W−→qq̄′) and we combine the analytical results obtained in both papers to examine the effect of an intense laser, in terms of its field strength and frequency, on the three measurable quantities in W−-boson decay (total decay rate, lifetime and branching ratios). It was found that the laser has notably contributed to the reduction of the total decay rate leading to a longer lifetime. Most importantly, the two branching ratios (one for leptons and the other for hadrons) are affected (increased or decreased) by the presence of a strong external electromagnetic field. Combined together, these two complementary works may provide an in-depth and comprehensive study that would be useful for any experimental investigation in the future.

First lattice calculation of radiative leptonic decay rates of pseudoscalar mesons

We present a nonperturbative lattice calculation of the form factors which contribute to the amplitudes for the radiative decays $P\ensuremath{\rightarrow}\ensuremath{\ell}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}\ensuremath{\gamma}$, where $P$ is a pseudoscalar meson and $\ensuremath{\ell}$ is a charged lepton. Together with the nonperturbative determination of the corrections to the processes $P\ensuremath{\rightarrow}\ensuremath{\ell}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ due to the exchange of a virtual photon, this allows accurate predictions at $O({\ensuremath{\alpha}}_{\mathrm{em}})$ to be made for leptonic decay rates for pseudoscalar mesons ranging from the pion to the ${D}_{s}$ meson. We are able to separate unambiguously and nonpertubatively the pointlike contribution, from the structure-dependent, infrared-safe, terms in the amplitude. The fully nonperturbative $O(a)$ improved calculation of the inclusive leptonic decay rates will lead to the determination of the corresponding Cabibbo-Kobayashi-Maskawa matrix elements also at $O({\ensuremath{\alpha}}_{\mathrm{em}})$. Prospects for a precise evaluation of leptonic decay rates with emission of a hard photon are also very interesting, especially for the decays of heavy $D$ and $B$ mesons for which currently only model-dependent predictions are available to compare with existing experimental data.

Light-meson leptonic decay rates in lattice QCD+QED

The leading electromagnetic (e.m.) and strong isospin-breaking corrections to the $\pi^+ \to \mu^+ \nu[\gamma]$ and $K^+ \to \mu^+ \nu[\gamma]$ leptonic decay rates are evaluated for the first time on the lattice. The results are obtained using gauge ensembles produced by the European Twisted Mass Collaboration with $N_f = 2 + 1 + 1$ dynamical quarks. The relative leading-order e.m.~and strong isospin-breaking corrections to the decay rates are 1.53(19)\% for $\pi_{\mu 2}$ decays and 0.24(10)\% for $K_{\mu 2}$ decays. Using the experimental values of the $\pi_{\mu 2}$ and $K_{\mu 2}$ decay rates and updated lattice QCD results for the pion and kaon decay constants in isosymmetric QCD, we find that the Cabibbo-Kobayashi-Maskawa matrix element $ | V_{us}| = 0.22538(46)$, reducing by a factor of about $1.8$ the corresponding uncertainty in the Particle Data Group review. Our calculation of $|V_{us}|$ allows also an accurate determination of the first-row CKM unitarity relation $| V_{ud}|^2 + | V_{us}|^2 + | V_{ub}|^2 = 0.99988(46)$. Theoretical developments in this paper include a detailed discussion of how QCD can be defined in the full QCD+QED theory and an improved renormalisation procedure in which the bare lattice operators are renormalised non-perturbatively into the (modified) Regularization Independent Momentum subtraction scheme and subsequently matched perturbatively at $O(\alpha_{em}\alpha_s(M_W))$ into the W-regularisation scheme appropriate for these calculations.

Higher-order condensate corrections to \u03d2 masses, leptonic decay rates and sum rules

With the recent completion of NNNLO results, the perturbative description of the ϒ system has reached a very high level of sophistication. We consider the non-perturbative corrections as an expansion in terms of local condensates, following the approach pioneered by Voloshin and Leutwyler. The leading order corrections up to dimension eight and the potential NLO corrections at dimension four are computed and given in analytical form. We then study the convergence of the expansion for the masses, the leptonic decay rates and the non-relativistic moments of the ϒ system. We demonstrate that the condensate corrections to the ϒ(1S) mass exhibit a region with good convergence, which allows us to extract {overline{m}}_bleft({overline{m}}_bright)=4214pm 37 {left(mathrm{pert}.right)}_{-22}^{+20}left(mathrm{n}mathrm{o}mathrm{n}-mathrm{pert}.right) MeV, and show that non-perturbative contributions to the moments with n ≈ 10 are negligible.

First Lattice Calculation of the QED Corrections to Leptonic Decay Rates.

The leading-order electromagnetic and strong isospin-breaking corrections to the ratio of K_{μ2} and π_{μ2} decay rates are evaluated for the first time on the lattice, following a method recently proposed. The lattice results are obtained using the gauge ensembles produced by the European Twisted Mass Collaboration with N_{f}=2+1+1 dynamical quarks. Systematic effects are evaluated and the impact of the quenched QED approximation is estimated. Our result for the correction to the tree-level K_{μ2}/π_{μ2} decay ratio is -1.22(16)%, to be compared to the estimate of -1.12(21)% based on chiral perturbation theory and adopted by the Particle Data Group.

Electromagnetic Corrections to Hadronic Decays from Lattice QCD

A new method, recently introduced for the lattice calculation of electromagnetic and isospin corrections to weak decays [1], is discussed. Using this method, for the first time, the electromagnetic effects in the leptonic decay rates and have been evaluated in lattice QCD. Preliminary results for the electromagnetic corrections to charged (neutral) pion and kaon masses and leptonic decay rates are presented.

Towards a unified explanation ofRD(*),RKand(g−2)μanomalies in a left-right model with leptoquarks

We present a unified explanation for the $B$-decay anomalies in $R_{D^{(\ast)}}$ and $R_{K}$ together with the anomalous muon magnetic moment, consistent with the constraints from the current measurements of leptonic decay rates and $D^0-\bar{D}^0$, $B_{s}^0-\bar{B}_{s}^0$ mixings, within the framework of a minimal left-right symmetric gauge theory motivated by one of the low-energy subgroups of $E_{6}$ naturally accommodating leptoquarks.

Two-loop QCD corrections to [formula omitted] meson leptonic decays

The two-loop quantum chromodynamics (QCD) radiative corrections to the Bc meson leptonic decay rate are calculated in the framework of the non-relativistic QCD (NRQCD) factorization formalism. Two types of master integrals appearing in the calculation are obtained analytically for the first time. We calculate the short-distance coefficient of the leading matrix element to order αs2 by matching the full perturbative QCD calculation results to the corresponding NRQCD results. The result presented in this work helps the evaluation of both the Bc leptonic decay constant and the Cabibbo–Kobayashi–Maskawa (CKM) matrix element |Vcb| to the full next-to-next-to-leading-order (NNLO) degree of accuracy.

Precision tests of the Standard Model with Kaon decays at CERN

Recent results and prospects for precision tests of the Standard Model in kaon decay-in-flight experiments at CERN are presented. A measurement of the ratio of leptonic decay rates of the charged kaon at the level of 0.4% precision constrains the parameter space of new physics models with extended Higgs sector, a fourth generation of quarks and leptons or sterile neutrinos. Searches for heavy neutrino mass states and the dark photon in the ∼ 100 MeV/c2 mass range based on samples collected in 2003-2007 are in progress and prospects will be discussed. The NA62 experiment, starting in 2014, will search for a range of lepton number and lepton flavour violating decays of the charged kaon and the neutral pion at improved sensitivities down to ∼ 10-12, which will probe new physics scenarios involving heavy Majorana neutrinos or R-parity violating SUSY.

Renormalization group improved pQCD prediction for Υ(1S) leptonic decay

Abstract The complete next-to-next-to-next-to-leading order short-distance and bound-state QCD corrections to Υ(1S) leptonic decay rate Γ(Υ(1S) → ℓ+ℓ−) has been finished by Beneke et al. [8]. Based on those improvements, we present a renormalization group (RG) improved pQCD prediction for Γ(Υ(1S) → ℓ+ℓ−) by applying the principle of maximum conformality (PMC). The PMC is based on RG-invariance and is designed to solve the pQCD renormalization scheme and scale ambiguities. After applying the PMC, all known-type of β-terms at all orders, which are controlled by the RG-equation, are resummed to determine optimal renormalization scale for its strong running coupling at each order. We then achieve a more convergent pQCD series, a scheme- independent and more accurate pQCD prediction for Υ(1S) leptonic decay, i.e. $$ \varGamma {\left.{}_{Y(1S)\to {\mathrm{e}}^{+}{\mathrm{e}}^{-}}\right|}_{\mathrm{PMC}}=1.27{0}_{-0.187}^{+0.137} $$ Γ Υ 1 S → e + e − P M C = 1.27 0 − 0.187 + 0.137 keV, where the uncertainty is the squared average of the mentioned pQCD errors. This RG-improved pQCD prediction agrees with the experimental measurement within errors.

Leptonic decay constantsfK,fD, andfDswithNf=2+1+1twisted-mass lattice QCD

We present a lattice QCD calculation of the pseudoscalar decay constants fK, fD and fDs performed using the gauge configurations produced by the European Twisted Mass Collaboration with Nf = 2 + 1 + 1 dynamical quarks, which include in the sea, besides two light mass degenerate quarks, also the strange and charm quarks with masses close to their values in the real world. The simulations are based on a unitary setup for the two light mass-degenerate quarks and on a mixed action approach for the strange and charm quarks. We use data simulated at three different values of the lattice spacing in the range 0.06 - 0.09 fm and at pion masses in the range 210 - 450 MeV. Our main results are: fK+ / fpi+ = 1.184 (16), fK+ = 154.4 (2.0) MeV, which incorporate the leading strong isospin breaking correction due to the up- and down-quark mass difference, and fK = 155.0 (1.9) MeV, fD = 207.4 (3.8) MeV, fDs = 247.2 (4.1) MeV, fDs / fD = 1.192 (22) and (fDs / fD) / (fK / fpi) = 1.003 (14) obtained in the isospin symmetric limit of QCD. Combined with the experimental measurements of the leptonic decay rates of kaon, pion, D- and Ds-mesons our results lead to the following determination of the CKM matrix elements: |Vus| = 0.2269 (29), |Vcd| = 0.2221 (67) and |Vcs| = 1.014 (24). Using the latest value of |Vud| from superallowed nuclear beta decays the unitarity of the first row of the CKM matrix is fulfilled at the permille level.

Rare and forbidden kaon decays at NA62

A precision lepton universality test by measurement of the helicity suppressed ratio of leptonic decay rates of the charged kaon with ∼ 150000 K ± → e ± ν decays collected by the NA62 experiment in 2007-08 is presented. The record accuracy of 0.4% constrains the parameter space of new physics models with extended Higgs sector, a fourth generation of quarks and leptons or sterile neutrinos.An improved upper limit on the rate of the lepton number violating decay K ± → π ∓ μ ± μ ± , which probes the resonant enhancement of the rate in the presence of heavy Majorana neutrinos in the ∼ 100 MeV range, is presented.The rare decays K + → π + νν are excellent processes to make tests of new physics at the highest scale complementary to LHC thanks to their theoretically cleaness. The NA62 experiment at CERN SPS aims to collect of the order of 100 events in two years of data taking, keeping the background at the level of 10%.

Leptonic decay of the ϒ(1S) meson at third order in QCD.

We present the complete next-to-next-to-next-to-leading order short-distance and bound-state QCD correction to the leptonic decay rate Γ(ϒ(1S)→ℓ+ℓ-) of the lowest-lying spin-1 bottomonium state. The perturbative QCD prediction is compared to the measurement Γ(ϒ(1S)→e+e-)=1.340(18) keV.

VusfromπandKdecay constants in full lattice QCD with physicalu,d,s, andcquarks

We determine the decay constants of the pi and K mesons on gluon field configurations from the MILC collaboration including u, d, s and c quarks. We use three values of the lattice spacing and u/d quark masses going down to the physical value. We use the w_0 parameter to fix the relative lattice spacing and f_pi to fix the overall scale. This allows us to obtain a value for f{K^+}/f{pi^+} = 1.1916(21). Comparing to the ratio of experimental leptonic decay rates gives |Vus| = 0.22564(28){Br(K^+)}(20){EM}(40){latt}(5){Vud} and the test of unitarity of the first row of the Cabibbo-Kobayashi-Maskawa matrix: |Vud|^2+|Vus|^2+|Vub|^2 - 1 = 0.00009(51).

Contributions of theW-boson propagator to theμandτleptonic decay rates

We derive closed expressions and useful expansions for the contributions of the tree-level W-boson propagator to the the muon and tau leptonic decay rates. Calling M and m the masses of the initial and final charged leptons, our results in the limit m=0 are valid to all orders in M^2/M_W^2. In the terms of O(m_j^2/M_W^2) (m_j=M,m), our leading corrections, of O(M^2/M_W^2), agree with the canonical value (3/5) M^2/M_W^2, while the coefficient of our subleading contributions, of O(m^2/M_W^2), differs from that reported in the recent literature. A possible explanation of the discrepancy is presented. The numerical effect of the O(m_j^2/M_W^2) corrections is briefly discussed. A general expression, valid for arbitrary values of M_W, M and m in the range M_W>M>m, is given in the Appendix. The paper also contains a review of the traditional definition and evaluation of the Fermi constant.

Precision measurement of the ratio of the charged kaon leptonic decay rates

A precision measurement of the ratio RK of the rates of kaon leptonic decays K±→e±ν and K±→μ±ν with the full data sample collected by the NA62 experiment at CERN in 2007–2008 is reported. The result, obtained by analysing ∼150000 reconstructed K±→e±ν candidates with 11% background contamination, is RK=(2.488±0.010)×10−5, in agreement with the Standard Model expectation.

Hints of R-parity violation in B decays into τν

In this article we show that the recently observed enhanced semi-leptonic and leptonic decay rates of the B meson into \tau \nu modes can be explained within the frame work of R-parity violating (RPV) MSSM. In particular, RPV contributions involving the exchange of right-handed down-type squarks give a universal contribution to the B+ --> \tau \nu, B --> D \tau \nu and the B --> D* \tau \nu decays. We find that the masses and couplings that explain the enhanced B decay rates are phenomelogically viable and the squarks can possibly be observed at the LHC.

Kaon Physics at CERN – Recent Results from the NA48/2 and NA62 Experiments

This letter presents recent results from the kaon experiments NA48/2 and its successor NA62 at CERN. With samples of 2.5 × 106 K µ 3 ± and 4.0 × 106 K e 3 ± events collected by NA48/2 in 2004, precise measurements of the form factors were performed. The Chiral Perturbation Theory was tested through a new study of K ± → π± γγ decays based on data taken with minimal trigger conditions. Using the beam line and detector of NA48/2 in the years 2007-2008, NA62 performed a precison measurement of the helicitysuppressed ratio of the charged kaon leptonic decay rates K ± → e+ ν and K ± → μ + ν Using the full data set, which is about ten times larger than the previous world data sample, the total uncertainty on this ratio is only 0.4%, representing a precision test of μ – e lepton universality. Finally, the main stage of the NA62 experiment is presented. The planned measurement of the ultra rare decay K + → π+ at 10% precision will provide a powerful test of the Standard Model. The experimental challenges to reach this goal require new detectors with exceptional performances.