Rare Meson Decays Research Articles

Decay Bc+→D(s)(*)+ℓ+ℓ− within covariant confined quark model

We study the rare semileptonic decays of Bc mesons within the effective field theoretical framework of covariant confined quark model. The transition form factors corresponding to Bc+→D(*)+ and Bc+→Ds(*)+ are computed in the entire q2 range. Using form factors, we compute the branching fractions and compare them with the available theoretical results. We also compute various physical observables such as forward-backward asymmetry, longitudinal and transverse polarizations, as well as clean angular observables. Published by the American Physical Society 2024

Limits on ALP-neutrino couplings from loop-level processes

We obtain new bounds on effective axionlike particle (ALP)-neutrino interactions from their loop-level impact on the ALP couplings to electroweak gauge bosons γZ, ZZ and W+W−. Both types of interactions are connected via the chiral anomaly, which allows to overcome the fermion mass suppression of tree-level analyses. ALP couplings to charged leptons are considered simultaneously with ALP-neutrino ones, as required by gauge invariance. Complete one-loop computations are presented together with relevant kinematic limits. We leverage data obtained from rare meson decay measurements alongside observations from collider experiments, among others. The novel constraints are particularly competitive for ALP masses ≳100 MeV, evidencing the powerful potential of future ALP searches at colliders which target its couplings to electroweak gauge bosons. Published by the American Physical Society 2024

Decay process within the QCDSR approach* *Supported in part by the National Natural Science Foundation of China (12265010, 12265009) and the Project of Guizhou Provincial Department of Science and Technology (ZK[2021]024, ZK[2023]142)

In this paper, we investigate the charmed meson rare decay process using an approach based on QCD sum rules. First, the pion twist-2, 3 distribution amplitude (DA) moments and are calculated up to the tenth and fourth orders, respectively, in the QCD sum rules according to the background field theory. After constructing the light-cone harmonic oscillator model for the pion twist-2, 3 DAs, we obtain their behaviors by matching the calculated ξ-moments. Then, the transition form factors (TFFs) are calculated using an approach based on QCD light-cone sum rules. The vector form factor at the large recoil region is . Using the rapidly converging simplified series expansion of , we present the TFFs and corresponding angular coefficients in the whole squared momentum transfer physical region. Based on non-standard neutrino interactions, the decay can be related to the decay indirectly. Thus, we first describe the semileptonic decay process , differential decay widths, and branching fraction with . The differential/total predictions for forward-backward asymmetry, -differential flat terms, and lepton polarization asymmetry are also reported. The prediction for the branching fraction is .

B meson rare decays in the TNMSSM* *Supported by the National Natural Science Foundation of China (12075074, 12235008, 11535002, 11705045), the Natural Science Foundation for Distinguished Young Scholars of Hebei Province, China (A2022201017), the Natural Science Foundation of Guangxi Autonomous Region (2022GXNSFDA035068), and the Youth Top-Notch Talent Support Program of the Hebei Province, China.

We investigate the two loop electroweak corrections to B meson rare decays and in the minimal supersymmetry standard model (MSSM) extension with two triplets and one singlet (TNMSSM). The new particle contents and interactions in the TNMSSM can affect the theoretical predictions of the branching ratios and , and the corrections from two loop diagrams to the process can reach around . Considering the latest experimental measurements, the numerical results of and in the TNMSSM are presented and analyzed. The findings indicate that the results in the TNMSSM can fit the updated experimental data well, and the new parameters , clearly affect the theoretical predictions of and .

Light dilaton in rare meson decays and extraction of its CP property

The dilaton ϕ is a pseudo-Nambu-Goldstone boson associated with the spontaneous breaking of scale invariance in a nearly conformal theory, and couples to the trace of the stress-energy tensor. We analyze experimental constraints on a light dilaton with mass in the MeV–GeV range from rare meson decays. New model-independent inclusive bounds for the b → sϕ transition largely exclude the parameter space of a light dilaton that could explain the muon g − 2 anomaly. Despite similarities between a dilaton and a Higgs-portal scalar, the dilaton-photon coupling is enhanced compared to the Higgs-portal scalar due to contributions from loops of the conformal sector. Consequently, the shortened lifetime of the dilaton relaxes bounds from K → π + invisible searches at the NA62 experiment and constraints from the Big Bang Nucleosynthesis. We utilize this fact to search for the dilaton signature at a lepton collider such as the ongoing Belle II experiment. Further, we demonstrate how to extract the CP property of the dilaton using the variation of the differential cross-section of e+e− → e+e−ϕ with the azimuthal angle between the outgoing leptons.

New dark matter candidate in the B-L\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B-L$$\\end{document} symmetric SSM

The B-L symmetric SUSY model (B-LSSM) introduces a new Z′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z'$$\\end{document} gauge boson and two singlet scalars, which indicates there are more neutralinos in the B-LSSM comparing with the minimal SUSY extension of standard model (MSSM). These new neutralinos provide more dark matter (DM) candidates in the B-LSSM, we find that the super partner of the new scalar singlet can serve as the lightest supersymmetric particle (LSP) and be considered as a DM candidate. The properties of this new DM candidate are analyzed in this work. Taking into account the experimental constraints on Higgs boson mass, B meson rare decays and muon anomalous magnetic dipole moment, the numerical results are obtained and presented. The properties of new DM candidate in the B-LSSM satisfy the observed relic density Ωh2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Omega h^2$$\\end{document} and comply with the latest upper bounds on σSIχ10-n\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _\ extrm{SI}^{\\chi _1^0-n}$$\\end{document}, and its mass is expected to greater than 80GeV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$80\\;\ extrm{GeV}$$\\end{document} approximately.

New constraint on dark photon at T2K off-axis near detector

The T2K experiment is one of the most powerful long-baseline experiments to investigate neutrino oscillations. The off-axis near detector called ND280 is installed 280 m downstream from the neutrino production target to measure the neutrino energy spectrum. In this paper, we study the capability of the ND280 detector to search for the dark photon produced through the meson rare decay and proton bremsstrahlung processes at the proton beam dump. We find that the ten-year operation of T2K with the ND280 detector excludes the unexplored parameter region for the dark photon mass and kinetic mixing. We also show that a broader parameter region can be searched by the ND280 in the future T2K operation for dark photon as well as U(1)B−L gauge boson.

Lepton flavor violating decays of vector mesons in the MRSSM* *Supported by the Natural Science Foundation of Hebei Province (A2022104001, A2022201017), the Shanxi Scholarship Council of China (2021-31), the youth top-notch talent support program of the Hebei Province, and the Foundation of Baoding University (2018Z01)

In this study, we analyze the rare decays of the neutral vector mesons and in the scenario of the minimal R-symmetric supersymmetric standard model using the effective Lagrangian method. The predicted branching ratios are dominated by the mass insertion parameters , i.e., the off-diagonal inputs, and the contributions of different parts are comparable. Taking into account the experimental constraints on the mass insertion parameters, the predicted branching ratios for the most promising processes are ten orders of magnitude smaller than the present experimental bounds.

Searches for heavy QCD axions via dimuon final states

Heavy QCD axions are well-motivated extensions of the QCD axion that address the quality problem while still solving the strong CP problem. Owing to the gluon coupling, critical for solving the strong CP problem, these axions can be produced in significant numbers in beam dump and collider environments for axion decay constants as large as PeV, relevant for addressing the axion quality problem. In addition, if these axions have leptonic couplings, they can give rise to long-lived decay into lepton pairs, in particular, dominantly into muons above the dimuon threshold and below the GeV scale in a broad class of axion models. Considering existing constraints, primarily from rare meson decays, we demonstrate that current and future neutrino facilities and long-lived particle searches have the potential to probe significant parts of the heavy QCD axion parameter space via dimuon final states.

(In)Visible signatures of the minimal dark abelian gauge sector

In this paper we study the present and future sensitivities of the rare meson decay facilities KOTO, LHCb and Belle II to a light dark sector of the minimal dark abelian gauge symmetry where a dark Higgs S and a dark photon ZD have masses ≲ 10 GeV. We have explored the interesting scenario where S can only decay to a pair of ZD’s and so contribute to visible or invisible signatures, depending on the life-time of the latter. Our computations show that these accelerator experiments can access the dark Higgs (mass and scalar mixing) and the dark photon (mass and kinetic mixing) parameters in a complementary way. We have also discussed how the CMS measurement of the SM Higgs total decay width and their limit on the Higgs invisible branching ratio can be used to extend the experimental reach to dark photon masses up to ~ 10 GeV, providing at the same time sensitivity to the gauge coupling associated with the broken dark abelian symmetry.

Flavor probes of axion-like particles

Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model (SM). We investigate the phenomenology of an ALP with flavor-changing couplings, and present a comprehensive analysis of quark and lepton flavor-changing observables within a general ALP effective field theory. Observables studied include rare meson decays, flavor oscillations of neutral mesons, rare lepton decays, and dipole moments. We derive bounds on the general ALP couplings as a function of its mass, consistently taking into account the ALP lifetime and branching ratios. We further calculate quark flavor-changing effects that are unavoidably induced by running and matching between the new physics scale and the scale of the measurements. This allows us to derive bounds on benchmark ALP models in which only a single (flavorless or flavor-universal) ALP coupling to SM particles is present at the new physics scale, and in this context we highlight the complementarity and competitiveness of flavor bounds with constraints derived from collider, beam dump and astrophysical measurements. We find that searches for ALPs produced in meson decays provide some of the strongest constraints in the MeV-GeV mass range, even for the most flavorless of ALP models. Likewise, we discuss the interplay of flavor-conserving and flavor-violating couplings of the ALP to leptons, finding that constraints from lepton flavor-violating observables generally depend strongly on both. Additionally, we analyze whether an ALP can provide an explanation for various experimental anomalies including those observed in rare B-meson decays, measurements at the ATOMKI and KTeV experiments, and in the anomalous magnetic moments of the muon and the electron.

Probing new physics effects in Λ b → Λ(→pπ −)ℓ + ℓ − decay via model-independent approach

In the last few decades, the FCNC transitions played a pivotal role in the ongoing efforts to test the Standard Model (SM) parameters. These transitions are also in the limelight to constrain the various New Physics (NP) scenarios. Regarding this, both the rare meson and baryon decays are pertinent as they occur through FCNC transitions and contain information about the heavy to light quark transition sector, where the SM has not yet examined totally. Therefore, it is interesting to analyze a phenomenological rich four-body decay channel . Here, we incorporate the NP through a model-independent approach, which modifies both the SM Wilson coefficients (WCs), and introduces the new vector, axial-vector, scalar, pseudo-scalar, and tensor operators. To check the SM and to explore the possibilities of the NP, several physical observables such as the branching ratio , forward-backward asymmetries , and , longitudinal (transverse) polarization fractions , asymmetry parameters , the angular moments, and some other angular observables, extracted from certain foldings, have been examined to compare our results with LHCb. To see which new coupling among VA, SP, and T can accommodate the available experimental data, we have examined the influence of these couplings separately to the values of these observables. For the angular moments, most of the LHCb observations lie close to the SM, and the current constraints on new VA couplings do not change the SM results significantly. For angular coefficients, , and the agreement with the experimental measurement gets improved if we invoke the tensor couplings along with SM WCs. We hope that when experimental measurements become more accurate, together with B meson decays, the will help us to put constraints on the parametric space of these WCs.

Lowering the scale of Pati-Salam breaking through seesaw mixing

We analyse the experimental limits on the breaking scale of Pati-Salam extensions of the Standard Model. These arise from the experimental limits on rare-meson decay processes mediated at tree-level by the vector leptoquark in the model. This leptoquark ordinarily couples to both left- and right-handed SM fermions and therefore the meson decays do not experience a helicity suppression. We find that the current limits vary from mathcal{O} (80–2500) TeV depending on the choice of matrix structure appearing in the relevant three-generational charged-current interactions. We extensively analyse scenarios where additional fermionic degrees of freedom are introduced, transforming as complete Pati-Salam multiplets. These can lower the scales of Pati-Salam breaking through mass-mixing within the charged-lepton and down-quark sectors, leading to a helicity suppression of the meson decay widths which constrain Pati-Salam breaking. We find four multiplets with varying degrees of viability for this purpose: an SU(2)L/R bidoublet, a pair of SU(4) decuplets and either an SU(2)L or SU(2)R triplet all of which contain heavy exotic versions of the SM charged leptons. We find that the Pati-Salam limits can be as low as mathcal{O} (5–150) TeV with the addition of these four multiplets. We also identify an interesting possible connection between the smallness of the neutrino masses and a helicity suppression of the Pati-Salam limits for three of the four multiplets.

Signals of the QCD axion with mass of 17 MeV/c2 : Nuclear transitions and light meson decays

The QCD axion remains experimentally viable in the mass range of O(10 MeV) if (i) it couples predominantly to the first generation of SM fermions; (ii) it decays to $e^+ e^-$ with a short lifetime $\tau_a\lesssim 10^{-13}\,$s; and (iii) it has suppressed isovector couplings, i.e., if it is piophobic. Remarkably, these are precisely the properties required to explain recently observed anomalies in nuclear de-excitations, to wit: the $e^+e^-$ emission spectra of isoscalar magnetic transitions of $^{8\!}$Be and $^{4\!}$He nuclei showed a "bump-like" feature peaked at $m_{e^+e^-}\sim 17$ MeV. In this article, we argue that on-shell emission of the QCD axion (with the aforementioned properties) provides an extremely well-motivated, compatible explanation for the observed excesses in these nuclear de-excitations. The absence of anomalous features in other measured transitions is also naturally explained: piophobic axion emission is strongly suppressed in isovector magnetic transitions, and forbidden in electric transitions. This QCD axion hypothesis is further corroborated by an independent observation: a $2-3\,\sigma$ deviation in the measurement of $\Gamma(\pi^0\to e^+e^-)$ from the Standard Model theoretical expectation. This article also includes detailed estimations of various axionic signatures in rare light meson decays, which take into account contributions from low-lying QCD resonance exchange, and, in the case of rare Kaon decays, the possible effective implementations of $\Delta S=1$ octet enhancement in chiral perturbation theory. These inherent uncertainties of the effective description of the strong interactions at low energies result in large variations in the predictions for hadronic signals of the QCD axion; in spite of this, the estimated ranges for rare meson decay rates obtained here can be probed in the near future in $\eta/\eta^\prime$ and Kaon factories.

Self-interacting neutrinos: Solution to Hubble tension versus experimental constraints

Exotic self-interactions among the Standard-Model neutrinos have been proposed as a potential reason behind the tension in the expansion rate, H0, of the universe inferred from different observations. We constrain this proposal using electroweak precision observables, rare meson decays, and neutrinoless double-\{beta} decay. In contrast to previous works, we emphasize the importance of carrying out this study in a framework with full Standard-Model gauge invariance. We implement this first by working with a relevant set of Standard- Model-Effective-Field-Theory operators and subsequently by considering a UV completion in the inverse See-Saw model. We find that the scenario in which all flavors of neutrinos self-interact universally is strongly constrained, disfavoring a potential solution to the H0 problem in this case. The scenario with self-interactions only among tau neutrinos is the least constrained and can potentially be consistent with a solution to the H0 problem.

Atmospheric dark matter and XENON1T excess

Very recently, the Xenon1T collaboration has reported an intriguing electron recoil excess, which may imply for light dark matter. In order to interpret this anomaly, we propose the atmospheric dark matter (ADM) from the inelastic collision of cosmic rays (CRs) with the atmosphere. Due to the boost effect of high energy CRs, we show that the light ADM can be fast-moving and successfully fit the observed electron recoil spectrum through the ADM-electron scattering process. Meanwhile, our ADM predicts the scattering cross section $\sigma_e \sim {\cal O}(10^{-38}- 10^{-39}$) cm$^{2}$, and thus can evade other direct detection constraints. The search for light meson rare decays, such as $\eta \to \pi + \slashed E_T$, would provide a complementary probe of our ADM in the future.

Probing dark sectors with long-lived particles at Belle II

We propose a new search for light scalar singlets in rare meson decays. For tiny interactions, the scalar is long-lived at detector scales and decays into displaced pairs of leptons or light mesons. We show that Belle II has a remarkable potential to probe scalars in the GeV range with couplings as small as $10^{-5}$. The predicted sensitivity is higher than at the long-baseline experiments FASER and NA62. We also investigate signatures of invisibly decaying scalars in rare meson decays with missing energy.

A random clockwork of flavor

We propose a simple clockwork model of flavor which successfully generates the Standard Model flavor hierarchies from random order-one couplings. With very few parameters we achieve distributions of models in excellent agreement with observation. We explain in some detail the interpretation of our mechanism as random localization of zero modes in theory space. The scale of the vectorlike fermions is mostly constrained by lepton flavor violation with secondary constraints arising from rare meson decays.

High energy muons in extensive air showers

The production of very high energy muons inside an extensive air shower is observable at ν telescopes and sensitive to the composition of the primary cosmic ray. Here we discuss five different sources of these muons: pion and kaon decays; charmed hadron decays; rare decays of unflavored mesons; photon conversion into a muon pair; and photon conversion into a J/ψ vector meson decaying into muons. We solve the cascade equations for a 1010.5 GeV proton primary and find that unflavored mesons and gamma conversions are the two main sources of E≥ 108.5 GeV muons, while charm decays dominate at 105.5 GeV< E< 108.5 GeV. In inclined events one of these muons may deposite a large fraction of its energy near the surface, implying fluctuations in the longitudinal profile of the shower and in the muon to electron count at the ground level. In particular, we show that 1 out of 6 proton showers of 1010.5 GeV include an E>106 GeV deposition within 500 g/cm2, while only in 1 out of 330 showers it is above 107 GeV . We also show that the production of high energy muons is very different in proton, iron or photon showers (e.g., conversions γ→ μ+ μ− are the main source of E≥ 104 GeV muons in photon showers). Finally, we use Monte Carlo simulations to discuss the validity of our results.

The new physics reach of null tests with D rightarrow pi ell ell and D_s rightarrow K ell ell decays

|Delta c|=|Delta u|=1 processes are unique probes of flavor physics in the up-sector within and beyond the Standard Model (SM). SM tests with rare semileptonic charm meson decays are based on an approximate CP–symmetry, a superior GIM–mechanism, angular distributions, lepton-universality and lepton flavor conservation. We analyze the complete set of null test observables in D rightarrow pi ell ell ^{(prime )} and D_s rightarrow K ell ell ^{(prime )} decays, ell ^{(prime )}=e, mu , and find large room for new physics safely above the SM contribution. We identify signatures of supersymmetry, leptoquarks and anomaly-free U(1)^prime –models with generation-dependent charges, for which we provide explicit examples. Z^prime –effects in c rightarrow u ell ell ^{(prime )} transitions can be sizable if both left-handed and right-handed couplings to quarks are present.