Leptonic Decay Research Articles

(Semi)leptonic kaon decays and neutral kaon mixing from lattice QCD

We review recent progress on leptonic and semileptonic kaon decays and neutral kaon mixing from lattice QCD.

Covariant flavour effects in semileptonic K and D decay

We present constraints on the left-handed dimension-6 interactions that lead to semileptonic and leptonic decays of K, D mesons. We employ the flavour covariant description of the effective couplings, identify universal CP phases of New Physics and derive constraints from decay rates and CP-odd quantities. As a result, we can predict the maximal effects of such flavoured NP in D decays from stringent K decay constraints and vice-versa.

Weak nonleptonic decays of vector B -mesons

We study the radiative and weak nonleptonic decays of vector B-mesons within the covariant confined quark model (CCQM) developed in our previous papers. First, we calculate the matrix elements and decays widths of the radiative decays $B^\ast\to B\gamma$. The obtained results are compared with those obtained in other approaches. Then we consider the nonleptonic decays $B^\ast\to D^\ast V$ which proceed via tree-level quark diagrams. It is shown that the analytical expressions for the amplitudes correspond to the factorization approach. In the framework of our model we calculate the leptonic decay constants and the form factors of the $B^\ast\to D^\ast(V)$ transitions in the entire physical region of the momentum transfer squared. Finally, we calculate the two-body decay widths and compare our results with other models.

Flavor-violating charged lepton decays in the little Randall-Sundrum model

The little Randall-Sundrum (little RS) model receives significantly stronger constraints from the flavor observables in comparison to the Randall-Sundrum (RS) model. In this paper, we analyze the effect of the electroweak sector in little RS on flavor-changing decays of charged leptons. We compare the predictions of the model with the current limits on the flavor-violating branching ratios of $\ensuremath{\mu}\ensuremath{\rightarrow}eee$, $\ensuremath{\tau}\ensuremath{\rightarrow}eee$, $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\mu}\ensuremath{\mu}$, $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}ee$, $\ensuremath{\tau}\ensuremath{\rightarrow}e\ensuremath{\mu}\ensuremath{\mu}$, $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$, and $\ensuremath{\mu}Ti\ensuremath{\rightarrow}eTi$, and we show that the dominant constraint arises from the $\ensuremath{\mu}Ti\ensuremath{\rightarrow}eTi$ process which strongly limits the KK-1 gauge boson mass (${M}_{KK}$) to be $\ensuremath{\gtrsim}30.7\text{ }\text{ }\mathrm{TeV}$. We then derive and show that generalizing the electroweak gauge sector to include the brane localized kinetic term relaxes this constraint to $\ensuremath{\gtrsim}12\text{ }\text{ }\mathrm{TeV}$. Toward the conclusion, we comment on the possibility that the large flavor-violating currents can be mitigated by relaxing the assumption regarding the unnatural thinness and rigidity of the UV brane and discuss the possibility of suppression of these currents in the presence of fat fluctuating branes.

ALP Production in Weak Mesonic Decays

AbstractAxion–Like–Particles are among the most economical and well motivated extensions of the Standard Model. In this work ALP production from hadronic and leptonic meson decays are studied. The hadronization part of these decay amplitudes have been obtained using Brodsky–Lepage method or LQCD, at needs. In particular, the general expressions for ALP emission in mesonic s– and t–channel tree–level processes are thoroughly discussed, for pseudoscalar and vector mesons. Accordingly, the calculation of the decay amplitudes for and are presented. Finally, bounds on the (low–energy effective Lagrangian) ALP–fermion couplings are derived, from present and future flavor experiments.

Constraints on light leptophilic dark matter mediators from decay experiments

We study the influence of leptophilic dark matter interactions on decays of muons and ground state mesons in existing experiments. We consider a secluded dark sector exclusively interacting with leptons via either a (leptophilic) scalar or vector mediator. These interactions will therefore influence leptonic decays and deform the energy spectra. We first study the Michel decay of muons, mu ^+rightarrow e^+nu _e bar{nu }_mu , which allow us to constrain the parameter space reasonably well. Secondly, the rare pi ^pm , K^pm , D^pm and D_s^pm decays to enu will be considered. Scalar mediators would remove the Standard Model helicity suppression, so that strong constraints can be derived. The resulting bounds on the couplings of the light mediators to electrons and muons still turn out to be somewhat weaker than those from searches at low-energy e^+e^- colliders and the magnetic moment of the muon, respectively. Finally, we show that kaon and pion decays basically exclude a “Co-SIMP” scenario where a scalar dark matter particle has a dimension-5 coupling to electrons.

The Contribution of Charged Bosons with Right-Handed Neutrinos to the Muon g − 2 Anomaly in the Twin Higgs Models

We examine the charged boson and right-handed neutrino contribution to the muon g−2 anomaly in twin Higgs models with joint constraints of Higgs global fit data, precision electroweak data, leptonic flavor-changing decay μ→eγ, and the mass requirement of heavy-gauge bosons. It comes with the conclusion that some parameters, such as the coupling of charged Higgs to the lepton yμ, the top Yukawa yt, and heavy-gauge boson coupling to the lepton Vμ are constrained roughly in the range of 0.12≲yμ≲0.4, 0.4≲yt≲0.9, and 0.47≲Vμ≲1, respectively.

Measurements of the associated production of a W boson and a charm quark in proton–proton collisions at sqrt{s}=8,text {TeV}

Measurements of the associated production of a W boson and a charm ({text {c}}) quark in proton–proton collisions at a centre-of-mass energy of 8,text {TeV} are reported. The analysis uses a data sample corresponding to a total integrated luminosity of 19.7,text {fb}^{-1} collected by the CMS detector at the LHC. The W bosons are identified through their leptonic decays to an electron or a muon, and a neutrino. Charm quark jets are selected using distinctive signatures of charm hadron decays. The product of the cross section and branching fraction sigma (text {p}text {p}rightarrow text {W}+ {text {c}}+ text {X}) {mathcal {B}}(text {W}rightarrow ell upnu ), where ell = text {e} or upmu , and the cross section ratio sigma (text {p}text {p}rightarrow {{text {W}}^{+} + bar{{text {c}}} + text {X}}) / sigma (text {p}text {p}rightarrow {{text {W}}^{-} + {text {c}}+ text {X}}) are measured in a fiducial volume and differentially as functions of the pseudorapidity and of the transverse momentum of the lepton from the W boson decay. The results are compared with theoretical predictions. The impact of these measurements on the determination of the strange quark distribution is assessed.

Studying Same-Sign Top Pair Production through Top-Higgs FCNC Interactions at the HL-LHC

We investigate the potential of the HL-LHC for discovering new physics effects via the same-sign top pair signatures. We focus on the semileptonic (electron and muon) decay of the top quarks and study the reach for a simplified model approach where top quark flavor changing could occur through a neutral scalar exchange. A relatively smaller background contribution and clean signature are the advantages of the leptonic decay mode of the same-sign W bosons in the same-sign production processes of top quark pairs. Assuming the FCNC between top quark, up-type quark, and scalar boson from the new physics interactions, the branchings could be excluded of the order O 1 0 − 4 . We use angular observables of the same-sign lepton pairs and the top quark kinematics in the process which provide the possibility of separation of new physics signal from the SM backgrounds using machine-learning techniques. We find that the same-sign top quark pair production is quite capable of testing the top-Higgs FCNCs at the HL-LHC.

Michel parameters in the presence of massive Dirac and Majorana neutrinos

We analyse the effects of Dirac and Majorana neutrinos on leptonic decays of muon and tau leptons using the most general four-lepton effective interaction Hamiltonian of dimension six. We calculate the specific energy and angular distribution of the final charged lepton, including the polarizations of the initial and final charged leptons. We discuss the new generalized Michel parameters and focus on the effects of the heavy neutrino masses that would lead to sizable contributions on scenarios where the new sterile neutrinos have non-negligible mixing. Specifically, the most promising scenario is found for the case of τ decay with one heavy final-state neutrino with a mass around 102 − 103 MeV, where the linear term suppression could be of order 10−4, low enough to be measured in current and forthcoming experiments.

Resolving charged hadrons in QED — gauge invariant interpolating operators

Standard interpolating operators for charged mesons, e.g. JB = overline{b} iγ5u for B−, are not gauge invariant in QED and therefore problematic for perturbative methods. We propose a gauge invariant interpolating operator by adding an auxiliary charged scalar ΦB, {mathcal{J}}_B^{(0)} = JB ΦB, which reproduces all the universal soft and collinear logs. The modified LSZ-factor is shown to be infrared finite which is a necessary condition for validating the approach. At mathcal{O} (α), this is equivalent to a specific Dirac dressing of charged operators. A generalisation thereof, using iterated integrals, establishes the equivalence to all orders and provides a transparent alternative viewpoint. The method is discussed by the example of the leptonic decay B−→ ℓ− overline{nu} for which a numerical study is to follow. The formalism itself is valid for any spin, flavour and set of final states (e.g. B−→ π0ℓ− overline{nu} ).

Detection of astrophysical tau neutrino candidates in IceCube

High-energy tau neutrinos are rarely produced in atmospheric cosmic-ray showers or at cosmic particle accelerators, but are expected to emerge during neutrino propagation over cosmic distances due to flavor mixing. When high energy tau neutrinos interact inside the IceCube detector, two spatially separated energy depositions may be resolved, the first from the charged current interaction and the second from the tau lepton decay. We report a novel analysis of 7.5 years of IceCube data that identifies two candidate tau neutrinos among the 60 “High-Energy Starting Events” (HESE) collected during that period. The HESE sample offers high purity, all-sky sensitivity, and distinct observational signatures for each neutrino flavor, enabling a new measurement of the flavor composition. The measured astrophysical neutrino flavor composition is consistent with expectations, and an astrophysical tau neutrino flux is indicated at 2.8sigma significance.

Search for type-III seesaw heavy leptons in leptonic final states in pp collisions at sqrt{s} = 13~text {TeV} with the ATLAS detector

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139,text {fb}^{-1} of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.

Kℓ3 form factors at the physical point: Toward the continuum limit

We present updated results for the form factors of the kaon semileptonic $(K_{\ell 3})$ decay process calculated with $N_f = 2 + 1$ nonperturbatively $O(a)$-improved Wilson quark action and Iwasaki gauge action at the physical point on large volumes of more than (10 fm)$^4$. In addition to our previous calculation at the lattice spacing $a = 0.085$ fm, we perform a calculation at the second lattice spacing of $0.063$ fm. Using the results for the form factors extracted from 3-point functions with the local and also conserved vector currents at the two lattice spacings, continuum extrapolation and interpolation of the momentum transfer are carried out simultaneously to obtain the value of the form factor $f_+(0)$ at the zero momentum transfer in the continuum limit. After investigation of stability of $f_+(0)$ against several fit forms and different data, we obtain $f_+(0) = 0.9615(10)(^{+47}_{\ -3})(5)$, where the first, second, and third errors are statistical, systematic errors from choice of the fit forms and isospin breaking effect, respectively. Combining our value of $f_+(0)$ and experimental input of the $K_{\ell 3}$ decay, one of the Cabibbo-Kobayashi-Maskawa matrix elements $|V_{us}|$ is determined as $|V_{us}| = 0.2252(^{\ +5}_{-12})$, whose error contains the experimental one as well as that in the lattice calculation. This value is reasonably consistent with the ones determined from recent lattice QCD results of $f_+(0)$ and also the one determined through the kaon leptonic decay process. We observe some tension between our value and $|V_{us}|$ evaluated from the unitarity of the CKM matrix with $|V_{ud}|$, while it depends on the size of the error of $|V_{ud}|$. It is also found that $|V_{us}|$ determined with our phase space integrals through six $K_{\ell 3}$ decay processes is consistent with the above one using $f_+(0)$.

(Semi-) leptonic D(s) decays at BESIII

BESIII has collected 2.93fb−1 and 6.32fb−1 of e+e− collision data samples at the center-of-mass energies 3.773 and 4.178–4.226 GeV, respectively. We report recent measurements of the (semi-) leptonic decays Ds→ℓ+νℓ (ℓ=μ,τ) and D(s)→Xℓ+νℓ [X=η,K1(1270) or denotes generic decays, and ℓ=e,μ]. The decay constant fDs+, the semileptonic form factor f+η(0) and the CKM matrix elements |Vcd(s)| are determined precisely. These results are important to verify the theoretical calculations of fDs+ and f+η(0), and the unitarity of the CKM matrix. Precision tests of lepton-flavor universality with (semi-) leptonic D(s) decays are also performed.

Peculiarities of pancreatic dysfunction in patients with chronic obstructive pulmonary disease

We consider the production of a Standard Model Higgs boson decaying to bottom quarks in association with a vector boson W±/Z in hadron collisions. We present a fully exclusive calculation of QCD radiative corrections both for the production cross section and for the Higgs boson decay rate up to next-to-next-to-leading order (NNLO) accuracy. Our calculation also includes the leptonic decay of the vector boson with finite-width effects and spin correlations. We consider typical kinematical cuts applied in the experimental analyses at the Large Hadron Collider (LHC) and we find that the full NNLO QCD corrections significantly decrease the accepted cross section and have a substantial impact on the shape of distributions. We point out that these additional effects are essential to obtain precise theoretical predictions to be compared with the LHC data.

Electromagnetic flavor-changing lepton decays from Lorentz and CPT violation

Lorentz- and $CPT$-violating effects initiating two-body electromagnetic flavor-changing decays of charged leptons are studied in the framework of Lorentz-violating effective field theory. An analysis of data from experiments at the Paul Scherrer Institute and at the Stanford Linear Accelerator measuring the branching ratios of these decays provides 576 constraints on independent flavor-changing effects in the charged-lepton sector, consistent with no Lorentz and $CPT$ violation at the level of parts in ${10}^{\ensuremath{-}13}--{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$.

Leptonic cascade decays of a heavy Higgs boson through vectorlike leptons at the LHC

We demonstrate the potential of fully leptonic cascade decays of a heavy neutral Higgs boson through vectorlike leptons as a simultaneous probe for extended Higgs sectors and extra matter particles at the LHC. The processes we explore are unique in that their event topologies lead to di-boson-like leptonic final states with a lepton pair which does not reconstruct the mass of a gauge boson. By recasting existing 2ell +{E}_R^{textrm{miss}} and 3/4ℓ searches channels using run2 data from the LHC we obtain model independent bounds on the masses of heavy scalars and vectorlike leptons and use these results to explore future prospects at the HL-LHC. Our results can be directly applied to any kind of new physics scenarios sharing the final states and the event topology. For concreteness, we apply our results to a benchmark scenario: a two Higgs doublet model type-II augmented with vectorlike leptons. Remarkably, even with current data the sensitivity of our analysis shows a reach for masses of a heavy neutral Higgs and vectorlike leptons up to 2 TeV and 1.5 TeV, respectively. Even for low tan β ≳ 1, the analysis retains sensitivity to heavy Higgs masses slightly above 1 TeV. The future sensitivities at the HL-LHC extend the reach for heavy Higgses and new leptons to 2.7 TeV and 2 TeV, respectively.

Effect of large light-heavy neutrino mixing and natural type-II seesaw dominance to lepton flavor violation and neutrinoless double beta decay

We derive the lower bound on absolute scale of lightest neutrino mass for normal hierarchy and inverted hierarchy pattern of light neutrinos by studying the new physics contributions to charged lepton flavour violations in the framework of a TeV scale left–right symmetric model. In the model the fermion sector comprises of the usual quarks and leptons plus a fermion singlet per generation and the scalar sector consists of isospin doublets, triplets and a bidoublet. The framework allows large light-heavy neutrino mixing where the light neutrino mass formula is governed by natural type-II seesaw mechanism unlike the generic type-II seesaw dominance which assumes suppressed light-heavy neutrino mixing. We demonstrate how sizeable loop-induced contribution to light neutrino mass is kept under control such that light neutrino mass formula is dominantly explained by type-II seesaw mechanism. We examine the heavy neutrino contributions with large light-heavy neutrino mixing to charged lepton flavour violating processes like mu rightarrow e gamma , mu rightarrow 3 e and mu rightarrow e conversion inside a nuclei. We present a complementary study between neutrinoless double beta decay and charged lepton flavour violation taking into account single beta decay bound, double beta decay bound and cosmology bounds on neutrino mass sum.

Neutrinoless double beta decay and the muonium-to-antimuonium transition in models with a doubly charged scalar

The lepton number and flavor violations are important possible ingredients of the lepton physics. The neutrinoless double beta decay and the transition of the muonium into antimuonuim are related to those violations. The former can give us an essential part of fundamental physics, and there are plenty of experimental attempts to observe the process. The latter has also been one of the attractive phenomena, and the experimental bound will be updated in planned experiments at new high-intensity muon beamlines. In models with a doubly charged scalar, not only can those two processes be induced, but also the active neutrino masses can be induced radiatively. The flavor violating decays of the charged leptons constrain the flavor parameters of the models. We study how the muonium-to-antimuonium transition rate can be as large as the current experimental bound, and we insist that the updated bound of the transition rate will be useful to distinguish the models to generate the neutrinoless double beta decay via the doubly charged scalar. We also study a possible extension of the model to the left-right model which can induce the neutrinoless double beta decay.