Study of electroweak penguin B decays at Belle II experiment

The electroweak b → sll (l = e, µ) transition is a flavor-changing neutral current process that mediates through a one-loop penguin diagram. The decay is considered to be a good probe for the New Physics as particles predicted in the beyond Standard Model theories can enter into the loop. The exclusive decay B → K (*) l + l − was first observed by the Belle experiment and it provides many observables such as the branching fraction, CP asymmetry, forward-backward asymmetry, and other angular observables. Recently, the LHCb experiment has reported some clue of a lepton flavor universality violation from the branching fraction ratio of the B → Kµ + µ − and B → Ke + e − decays. In this presentation, we report the status of the B → Kl + l − decay analysis at the Belle II experiment which started the data taking in 2019. We also, present an activity at the Belle II Chulalongkorn University group where we study the B → KJ/ψ decay which has the same topology as the B → Kl + l − .

Top and beauty synergies in SMEFT-fits at present and future colliders

We perform global fits within Standard Model Effective Field Theory (SMEFT) combining top-quark pair production processes and decay with b → s flavor changing neutral current transitions and Z → $$ b\overline{b} $$ b b ¯ in three stages: using existing data from the LHC and B-factories, using projections for the HL-LHC and Belle II, and studying the additional new physics impact from a future lepton collider. The latter is ideally suited to directly probe ℓ+ℓ− → $$ t\overline{t} $$ t t ¯ transitions. We observe powerful synergies in combining both top and beauty observables as flat directions are removed and more operators can be probed. We find that a future lepton collider significantly enhances this interplay and qualitatively improves global SMEFT fits.

Lepton Flavor Violating $h\to\tau\mu$ decay induced by leptoquarks

The flavor changing neutral current decay $h\to\tau\mu$ is studied in a renormalizable scalar leptoquark model with no proton decay. Analytical expressions for the one-loop level contributions of a scalar leptoquark to the decay width of the process $h\to\tau\mu$ are presented. We find a viable model parameter space via the current constraints on the muon ($g-2$), the decay $\tau\to\mu\gamma$, the LHC Higgs boson data and the direct leptoquark searches at the LHC. Then, we evaluate branching ratio of the decay $h\to\tau\mu$induced by leptoquarks; we find that it is of the order of $10^{-8}-10^{-7}$ ($10^{-8}$; $10^{-9}-10^{-8}$)for a scalar leptoquark mass equal to $m_{\Omega_{5/3}}=1$ TeV ($m_{\Omega_{5/3}}=2$; $m_{\Omega_{5/3}}=10$ TeV). Finally, we evaluate the number of events produced at future hadron colliders from both the $h\to\tau\mu$ decay and the potential Standard Model background finding a potentialevidence for the $h\to\tau\mu$ decay.

Structure of flavor changing Goldstone boson interactions

General flavor changing Goldstone boson (GB) interactions with fermions from a spontaneous global U(1)G symmetry breaking are discussed. This GB may be the Axion, solving the strong QCD CP problem, if there is a QCD anomaly for the assignments of quarks U(1)G charge. Or it may be the Majoron, producing seesaw Majorana neutrino masses by lepton number violation, if the symmetry breaking scale is much higher than the electroweak scale. It may also, in principle, play the roles of Axion and Majoron simultaneously as far as providing solution for the strong CP problem and generating a small Majorana neutrino masses are concerned. Great attentions have been focused on flavor conserving GB interactions. Recently flavor changing Axion and Majoron models have been studied in the hope to find new physics from rare decays in the intensity frontier. In this work, we will provide a systematic model building aspect study for flavor changing neutral current (FCNC) GB interactions in the fermion sectors, or separately in the quark, charged lepton and neutrino sectors and will identify in detail the sources of FCNC interactions in a class of beyond standard model with a spontaneous global U(1)G symmetry breaking. We also provide a general proof of the equivalence of using physical GB components and GB broken generators for calculating GB couplings to two gluons and two photons, and discuss some issues related to spontaneous CP violation models. Besides, we will also provide some details for obtaining FCNC GB interactions in several popular models, such as the Type-I, -II, -III seesaw and Left-Right symmetric models, and point out some special features in these models.

Exploring charm decays with missing energy in leptoquark models

We investigate the possibility that scalar leptoquarks generate consequential effects on the flavor-changing neutral-current decays of charmed hadrons into final states with missing energy "Image missing" carried away by either standard model or sterile neutrinos. We focus on scenarios involving the R2, $$ {\tilde{R}}_2 $$ R ˜ 2 , and $$ {\overline{S}}_1 $$ S ¯ 1 leptoquarks and take into account various pertinent constraints, learning that meson-mixing ones and those inferred from collider searches can be of significance. We find in particular that the branching fractions of charmed meson decays D →"Image missing", M = π, ρ, and Ds→"Image missing" and singly charmed baryon decays $$ {\Lambda}_c^{+} $$ Λ c + →"Image missing" and Ξc→"Image missing" are presently allowed to attain the 10−7-10−6 levels if induced by R2 and that the impact of $$ {\tilde{R}}_2 $$ R ˜ 2 is comparatively much less. In contrast, the contributions of $$ {\overline{S}}_1 $$ S ¯ 1 , which couples to right-handed up-type quarks and the sterile neutrinos, could lead to branching fractions as high as order 10−3. This suggests that these charmed hadron decays might be within reach of the BESIII and Belle II experiments or future super charm-tau factories and could serve as potentially promising probes of leptoquark interactions with sterile neutrinos.

(g − 2)μ versus K → π + Emiss induced by the (B − L)23 boson

To address the long-standing (g − 2)μ anomaly via a light boson, in ref. [1] we proposed to extend the standard model (SM) by the local (B − L)23, under which only the second and third generations of fermions are charged. It predicts an invisible Z′ with mass $$ \mathcal{O}(100) $$ O 100 MeV, and moreover it has flavor-changing neutral current (FCNC) couplings to the up-type quarks at tree level. Such a Z′, via KL→ π0 + Z′(→$$ v\overline{v} $$ v v ¯ ) at loop level, may be a natural candidate to account for the recent KOTO anomaly. In this article, we investigate this possibility, to find that Z′ can readily do this job if it is no longer responsible for the (g − 2)μ anomaly. We further find that both anomalies can be explained with moderate tuning of the CP violation, but may contradict the B meson decays.

Lepton flavor violating decays in model

Motivated by the hints of lepton flavor violating (LFV) decays, we study Bd,s → μτ, Bs,d → τe, and τ– → e– + e+ + e– decays in [Formula: see text] model. These LFV decays present interesting patterns that may reveal the shape of new physics beyond the standard model. Considering the effect of both Z- and [Formula: see text]-mediated flavor-changing neutral current we calculate the branching ratios for these decays. We find the branching ratios [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text].

Probing the anomalous tqγ couplings through single top production at the future lepton–hadron colliders

The measurements of the top quark flavor changing neutral current interactions are one of the most important goals of the top quark physics program in the present and the future collider experiments. These measurements provide direct information on nonstandard interactions of the top quark. Within the framework of new physics beyond the Standard Model, these interactions can be defined by an effective Lagrangian. In this study, we have investigated the potential of the future [Formula: see text] colliders on the top quark flavor changing neutral current interactions through the subprocesses [Formula: see text], where [Formula: see text]. These subprocesses have been produced through the main reaction [Formula: see text] at the LHC-[Formula: see text], the FCC-[Formula: see text] and the SPPC-[Formula: see text]. For the main reaction, the total cross-sections have been calculated as a function of the anomalous [Formula: see text] couplings. In addition, sensitivities on [Formula: see text] at 95% Confidence Level have been calculated. We obtain that the best constraints on [Formula: see text] are at the order of [Formula: see text] which is four orders of magnitude better than the LHC’s experimental results.

Studying flavor-changing neutral tqZ couplings: Current constraints and future prospects

Possible nonstandard tuZ and tcZ interactions, which induce flavor-changing neutral-current decays of the top quark, are studied in the effective-Lagrangian framework. The corresponding Lagrangian consists of four kinds of nonstandard couplings coming from [Formula: see text] invariant dimension-6 effective operators. The four coupling constants in each interaction are treated as complex numbers independent of each other and constraints on them are derived by using the present experimental limits of the branching fractions for [Formula: see text] and [Formula: see text] processes. Future improvements of those constraints are also discussed as well as possibilities of measurements of these couplings at the High-Luminosity Large Hadron Collider.