Flavor Changing Neutral Current Processes Research Articles

The quark flavor-violating ALPs in light of B mesons and hadron colliders

The axion-like particle (ALP) may induce flavor-changing neutral currents (FCNCs) when the fermions’ Peccei-Quinn charges are not generation universal. The search for flavor-violating ALP couplings with a bottom quark so far focused on FCNC processes of B mesons at low energies. The recent measurements of B → K + X rare decays place stringent bounds on the quark flavor violations of a light ALP in different decay modes. In this work we propose a novel direct search for bottom flavor-violating interaction of a heavy ALP at the LHC and its upgrades, namely QCD production of an ALP associated with one b jet and one light jet p p → b j a. We consider the decay of the ALP to photons, muons and invisible ALP decays. The Boosted Decision Tree (BDT) algorithm is used to analyze the events and we train the BDT classifier by feeding in the kinematic observables of signal and backgrounds. Finally, we show the complementarity between the search prospects of hadron colliders and the low-energy B meson constraints from B meson mixing and B meson decays to a light ALP.

Search for a massless particle beyond the Standard Model in the Σ+ → p + invisible decay

A massless particle beyond the Standard Model is searched for in the two-body decay Σ+→p+invisible using (1.0087±0.0044)×1010J/ψ events collected at a center-of-mass energy of s=3.097GeV with the BESIII detector at the BEPCII collider. No significant signal is observed, and the upper limit on the branching fraction B(Σ+→p+invisible) is determined to be 3.2×10−5 at the 90% confidence level. This is the first search for a flavor-changing neutral current process with missing energy in hyperon decays which plays an important role in constraining new physics models.

A New Look at b → s Observables in 331 Models

Flavour changing neutral current (FCNC) processes are described by loop diagrams in the Standard Model (SM), while in 331 models, based on the gauge group SU(3)C×SU(3)L×U(1)X, they are dominated by tree-level exchanges of a new heavy neutral gauge boson Z′. By exploiting this feature, observables related to FCNC decays of K, Bd and Bs mesons can be considered in several variants of 331 models. The variants are distinguished by the value of a parameter β that plays a key role in this framework. Imposing constraints on the ΔF=2 observables, we select possible ranges for the mass of the Z′ boson in correspondence to the values β=±k/3, with k=1,2. The results are used to determine the impact of 331 models on b→s processes and on the correlations among them, in the light of new experimental data recently released.

On the role of LHC and HL-LHC in constraining flavor changing neutral currents

The Standard Model has no Flavor-Changing Neutral Current (FCNC) processes at the tree level. Therefore, processes featuring FCNC in new physics are tightly constrained by data. Typically, the lower bounds on the scale of new physics obtained from K−K¯ or B−B¯ mixing lie well above 10 TeV, surpassing the reach of current and future colliders. In this paper, we demonstrate, using a specific Z′ model that features flavor-changing interactions, that such limits can be severely weakened by specific choices of the quark mixing matrices with no prejudice while maintaining the CKM matrix in agreement with the data. We highlight the valuable role of the often-overlooked D0 mixing in deriving robust FCNC limits and show that the LHC and HL-LHC are promising probes for flavor-changing interactions mediated by a Z′ boson.

Exploring freeze-out and freeze-in dark matter via effective Froggatt-Nielsen theory

Motivated by the dynamical reasons for the hierarchical structure of the Yukawa sector of the Standard Model (SM), we consider an extension of the SM with a complex scalar field, known as `flavon', based on the Froggatt-Nielsen mechanism. In an effective theory approach, the SM fermion masses and mixing patterns are generated in orders of the parameter related to the vacuum expectation value of the flavon field and the cut-off of the effective theory. By introducing right-handed neutrinos, we study the viability of the lightest right-handed neutrino as a dark matter candidate, where the same flavon field acts as a mediator between the dark and the SM sectors. We find that dark matter genesis is achieved both through freeze-out and freeze-in mechanisms encompassing the 𝒪(GeV) – 𝒪(TeV) mass range of the mediator and the dark matter particle. In addition to tree-level spin-dependent cross section, the model gives rise to tree- and loop-level contributions to spin-independent scattering cross section at the direct detection experiments such as XENON and LUX-ZEPLIN which can be probed in their future upgrades. By choosing suitable Froggatt-Nielsen charges for the fermions, we also generate the mass spectrum of the SM neutrinos via the Type-I seesaw mechanism. Flavor-changing neutral current processes, such as radiative lepton decay, meson mixing, and top-quark decay remain the most constraining channels and provide testability for this minimal setup that addresses several major shortcomings of the SM.

Review of Flavor Anomalies

Lepton flavor universality exists in the Standard Model, and hence any observation of the violation of this universality will be a hint for new physics. Recent experimental searches for processes violating this symmetry have attracted much attention among theorists and experimentalists alike. In recent years, such hints have been observed in flavor changing neutral current weak processes such as b→sll and charged current weak processes such as b→clν processes by collider experiments like Belle, Belle II, BaBar, LHCb, ATLAS, and CMS collaborations, where b,s,c are the bottom, strange, and charm quarks, respectively, and l,ν stand for lepton and the corresponding lepton neutrino, respectively. This article is a review of some of the interesting anomalies observed in the B-sector and includes decays of Bs mesons.

The flavor of a light charged Higgs

The ATLAS Collaboration has recently reported a search for light-charged Higgs in t → H+b decay, with {H}^{+}to coverline{b} . An excess with a local significance of approximately 3σ is found at {m}_{H^{+}} ≈ 130 GeV, with a best-fit value of BR(t → H+b) × BR( {H}^{+}to coverline{b} ) = (1.6 ± 0.6) × 10−3. We study the implications of such a hypothetical signal in multi-Higgs doublet models. We take into account constraints from searches for other charged Higgs decays and from flavor-changing neutral current processes. Two Higgs doublet models with flavor structure dictated by natural flavor conservation (NFC), minimal flavor violation (MFV), or the Froggatt-Nielsen (FN) mechanism cannot account for such excess. A three-Higgs doublet model with NFC can account for the signal. The Yukawa couplings of the neutral pseudoscalar A in the down sector, {hat{Y}}_A^D , should be larger by a factor of 4 – 6 compared to the corresponding Yukawa couplings of the Higgs h, {hat{Y}}_h^D . We further present two minimal scenarios, one in which a single Yukawa coupling in the down sector, {left({hat{Y}}_A^Dright)}_{bb} , gives the only significant contribution, and one in which two Yukawa couplings in the up sector, {left({hat{Y}}_A^Uright)}_{tt} and {left({hat{Y}}_A^Uright)}_{tc} , give the only significant contributions, and we discuss possible tests of these scenarios.

Flavor changing neutral current processes and family discrimination in 3-3-1 models

In 3-3-1 models, anomaly cancellation requires that one of the three families of quarks transforms as triplet by , with the other two transforming necessarily as anti-triplet. This is an important feature of the model because, with it, we explain family replication. Thus it is mandatory to discriminate which of the families will transform as triplet by because the main consequence of anomaly cancellation in 3-3-1 models is the arising of processes violating flavor at the tree level by means of neutral currents mediated by gauge and scalar fields, and each case leads to different results. In this work, we consider the 3-3-1 model with right-handed neutrinos. Among the spectrum of 3-3-1 particles that contribute to the flavor changing neutral processes, there is a pseudoscalar that may be the lightest of the 3-3-1 particles, and this then should give the main contribution to such processes. We then calculate its contribution to the mixing transition and confront it with the current experimental results. We do this for the three cases in which one of the family of quarks transforms as a triplet by . According to our findings, each case leads to different constraints on the mass of the pseudoscalar, and the case in which the third family of quarks transforms as triplet seems to be the favoured one. We also obtain the most stringent bounds on the mass of the pseudoscalar of the model.

FCNC B and K meson decays with light bosonic Dark Matter

We consider decays of B and K mesons into a pseudo-scalar or vector meson plus missing energy. Within the SM, these modes originate from flavor changing neutral current (FCNC) processes with two neutrinos in the final state. In this paper we consider the experimental upper bounds on these modes and interpret the difference between these bounds and the SM prediction as a window into new light invisible particles. In particular we consider the case where some new symmetry requires the new particles to be produced in pairs. We first construct the general low energy effective Lagrangian coupling an FCNC with two dark sector particles of spin zero, one-half and one. We then present numerical estimates for the constraints that can be placed on these interactions, finding that an effective new physics scale from mathcal{O} (10)- mathcal{O} (1011) GeV can be probed, with the exact value strongly depending on the interaction structure as well as the mass of the invisible particle. For we incorporate into our constraints the effect of using only the signal regions of NA62, and for the q2-dependent efficiency of Belle II.

FCNC processes of charmed hadrons with invisible scalar

We study the invisible scalar of S in charmed meson and baryon decays with flavor-changing neutral currents (FCNCs) based on the model-independent effective Lagrangian between the quarks and invisible scalar. From the bounds of the coupling constants extracted from the recent BES III experiment on the decay of D0→π0ν¯ν, we predict that the decay branching ratios of D+→π+S, Ds+→K+S, Λc→pS, Ξc0(+)→Σ0(+)S, and Ξc0→ΛS can be as large as 10.6,2.53,2.39,0.963,5.77, and 2.92×10−4 with mS≈1.1GeV, respectively. Some of these decay modes are accessible to the ongoing experiments, such as BES III, LHCb and Belle II, as well as the future ones, such as FCC-ee.

Rare decays at LHCb

Flavour-changing neutral-current processes, such as b → sℓ + ℓ −, are forbidden at tree level in the Standard Model and hence might receive comparatively large corrections from new particles. This proceeding paper highlights recent measurements from LHCb on b → sℓ + ℓ − and purely-leptonic decays, including tests of lepton flavour universality and searches for lepton flavour violation.

Connecting b → sℓℓ with and

We discuss the consequences of deviations from the Standard Model observed in b → sμ + μ − transitions for flavour-changing neutral-current processes involving down-type quarks and neutrinos. We work within an effective field theory approach respecting the SM gauge symmetry, including right-handed currents, a flavour structure based on approximate U(2) symmetry, and assuming only SM-like light neutrinos. We discuss correlations among (hs = K, K*, Xs ), and branching ratios in the case of linear Minimal Flavour Violation and in a more general framework.

Probing the Anomalous tqgamma Couplings in Photon-Proton Collisions

The top quark flavor changing neutral current processes are extremely suppressed within the Standard Model. Nevertheless, they could be enhanced in a new physics model beyond the Standard Model. Investigating the top quark’s flavor changing neutral current interactions at colliders would be an important test in terms of new physics. In this work, we examine the potentials of the processes \({e^{-}} {p} {\rightarrow } {e^{-}} {\gamma } {p} {\rightarrow } {e^{-}} W^{+} b {\gamma }{\rightarrow } {e^{-}} {j} {\bar {j}} b {\gamma }\) and \({e^{-}} {p} {\rightarrow } {e^{-}} {\gamma } {p} {\rightarrow } {e^{-}} W^{+} b {\gamma }{\rightarrow } {e^{-}} \textit {l} {\nu }_{\textit {l}} b {\gamma }\) at the FCC-eh with \(\sqrt {s}=7.08\) and 10.0 TeV to study anomalous tqγ couplings via effective Lagrangians. We obtain 95% confidence level sensitivities on the anomalous coupling parameters at the two FCC-eh energies and various integrated luminosities. We find that our limits are at least two orders of magnitude better than the current LHC experimental results.

Searching for BSM physics in Yukawa couplings and flavour symmetries

In the framework of the Standard Model Effective Field Theory, we compare the lower bounds on the scale of new physics possibly contributing to the foverline{f}h effective couplings, obtained from the measurements of different observables, under the assumption that the Wilson coefficients of the relevant dim 6 operators respect certain flavour structure: either the Minimal Flavour Violation (MFV) ansatz or a flavour symmetry, often invoked to explain the observed pattern of fermion masses and mixings. We perform a global analysis of the bounds following from the limits on the diagonal couplings measured in the Higgs boson production and decays at the LHC experiments. Another set of bounds is obtained from the limits on non-diagonal couplings constrained by the variety of flavour changing neutral current (FCNC) and radiative decay processes. With the present precision of the LHC data, the FCNC data give stronger bounds on the scale of new physics than the collider data (obviously, for the MFV ansatz only collider data are relevant): once the Wilson coefficients respect some flavour structure, the obtained bounds are in the TeV range. In the quark case, these limits are compatible with a few percent deviations from the SM Yukawa couplings and only mildly more stringent than those obtained from the available collider data. For leptons, instead, the FCNC bounds are stronger and then a signal in the near future collider data would mean the violation of the flavour symmetry or indicate the presence of additional beyond the Standard Model contributions, affecting the flavour observables, that leads to cancellations.

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 − .

The charm of 331

We perform a detailed analysis of flavour changing neutral current processes in the charm sector in the context of 331 models. As pointed out recently, in the case of Z′ contributions in these models there are no new free parameters beyond those already present in the Bd,s and K meson systems analyzed in the past. As a result, definite ranges for new Physics (NP) effects in various charm observables could be obtained. While generally NP effects turn out to be small, in a number of observables they are much larger than the tiny effects predicted within the Standard Model. In particular we find that the branching ratio of the mode D0→ μ+μ−, despite remaining tiny, can be enhanced by 6 orders of magnitude with respect to the SM. We work out correlations between this mode and rare Bd,s and K decays. We also discuss neutral charm meson oscillations and CP violation in the charm system. In particular, we point out that 331 models provide new weak phases that are a necessary condition to have non-vanishing CP asymmetries. In the case of ∆ACP, the difference between the CP asymmetries in D0→ K+K− and D0→ π+π−, we find that agreement with experiment can be obtained provided that two conditions are verified: the phases in the ranges predicted in 331 models and large hadronic matrix elements.

In BLMSSM * *Supported by National Natural Science Foundation of China (11805140, 11347185, 11905002), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (2017113), Natural Science Foundation of Shanxi Province (201801D221021, 201801D221031), Natural Science Foundation of Hebei Province (A2020201002), the China Scholarship Council (201906935031)

Applying the effective Lagrangian method, we study the flavor changing neutral current process within the minimal supersymmetric extension of the Standard Model, where baryon and lepton numbers are local gauge symmetries. Constraints on the parameters are investigated numerically with the experimental data for the branching ratio of . Additionally, we present the corrections to direct CP-violation in and time-dependent CP-asymmetry in . With appropriate assumptions on the parameters, we find the direct CP-violation is very small, while one-loop contributions to can be significant.

Pair invariant mass spectrum and polarization asymmetry in the event generation of gamma-ray conversions

We examine the dielectron invariant mass spectrum and the variation of the polarization asymmetry as a function of that mass in the event generator of the five-dimensional Bethe–Heitler differential cross section of the conversion of linearly polarized photons that we have implemented recently as a Geant4 Physics Model. We compare the results obtained with simulated samples to analytical expressions.Studies of b→sγ decays at LHC and Belle II experiments could provide hints of physics beyond the standard model in flavor-changing neutral current processes; issues have been reported in the simulation of the properties of the background induced by genuine γ-ray conversions in their detectors. Our results demonstrate that using the five-dimensional model would help solve these issues.

Spin- 1/2 invisible particles in heavy meson decays

The flavor-changing neutral current decay processes of the $B$ and $B_c$ mesons with the final states involving spin-$1/2$ particles are investigated. By considering the background of the Standard Model where $\nu\bar\nu$ contributes the missing energy and the experimental upper bounds for the branching fractions, we get the constraints of the coupling constants of the quark-antiquark and the assumed invisible particles $\chi\bar\chi$. The constraints of the coupling constants are then used to study the similar processes of the $B_c$ meson. At some specific region of $m_\chi$, the upper limit of BR($B_c\to D_{(s)}\chi\bar\chi$) is of the order of $10^{-6}$, while for BR($B_c\to D^\ast_{(s)}\chi\bar\chi$), it is $10^{-5}$. The possibility of distinguishing $\chi$ to be a Majorana or Dirac fermion by the differential branching fractions is also discussed.

Eligibility of EFT Approach to Search for tqg FCNC Phenomenon

The Effective Field Theory (EFT) approach is widely used in the search for possible deviations from the predictions of the Standard Model. Such an approximation of possible BSM physics is valid up to a certain levels of energy scale and accuracy. In this article, we investigate potential limitation of the EFT approach related to unitarity to describe possible contributions of flavor changing neutral currents (FCNC) involving the top quark. The numerical and analytical calculations of the FCNC processes used in the EFT approach demonstrate the constant asymptotic behavior of the total cross section with increasing energy. It is shown that the EFT approach for studying the possible contribution of FCNC does not violate the restrictions following from perturbative unitarity, the asymptotic behavior of the cross section does not exceed the Froissart bound, and the approach itself can be used to set the corresponding experimental limits for FCNC couplings or Wilson coefficients at present and future colliders.