arXiv subjects

Xing-Bo Yuan

Publications and source records attributed to Xing-Bo Yuan.

At least 19 recordsLinked to original sources

arXiv2026

Low-energy Muon-Nucleon scattering experiment: LUNE (White Paper)

The HIAF will provide high-intensity, high-quality muon beams with momenta from 0.5 to 7.5 GeV/c. This energy range is uniquely suited for precision muon scattering, bridging the gap between low-energy electron facilities and future high-energy lepton-ion colliders. In particular, HIAF will enable precision measurements with both positive and negative muon beams over a broad kinematic range, complementing existing electron-scattering facilities such as JLab, EicC and EIC. Based on HIAF muon source, the LUNE Collaboration has been established to address several fundamental questions in nuclear and particle physics, including the proton charge radius puzzle, nucleon electromagnetic structure, and the dynamics of quantum electrodynamics and hadronic interactions. The program proceeds in two phases, from elastic scattering to nucleon structure and beyond-Standard-Model searches. The experiment is expected to determine the proton charge radius with a precision of approximately 1.0\% using elastic muon-proton scattering. It will also perform systematic measurements of the proton electromagnetic form factors with both $\mu^+$ and $\mu^-$ beams, enabling precise studies of two-photon exchange effects and stringent tests of quantum electrodynamics. Beyond elastic scattering, LUNE will investigate TMD, gravitational form factors, and nuclear charge radii, providing new insights into the 3D structure of nucleons and nuclei. The experiment will further address important topics including Coulomb-distortion corrections, nuclear medium effects, and possible signatures of physics beyond the Standard Model. This white paper presents the scientific motivation, detector concept, expected performance, and long-term strategy of LUNE.

hep-ex↗

arXiv2026

Probing TeV-Scale Inverse-Seesaw Leptogenesis and Majorana Dark Matter in $U(1)_{B-L}$ Models at Multi-TeV Muon Colliders

We investigate a predictive and testable framework in which dark matter (DM), heavy-neutrino dynamics, and the BAU originate from correlated interactions within a local $U(1)_{B-L}$ extension of the SM. Unlike conventional $B-L$ constructions based on the type-I seesaw, we employ an inverse-seesaw mechanism realized through a sterile fermion $S_{1}$ and a complex scalar field $ϕ$, whose vacuum expectation value simultaneously generates the masses of the heavy neutrinos $N_{1,2}$ and the Majorana DM fermion $χ$ via Yukawa couplings. The small lepton-number-violating parameter induced by a higher-dimensional operator leads to naturally light active neutrinos together with TeV-scale heavy neutrinos and sizable active-sterile mixing, yielding distinctive collider signatures unavailable in minimal $B-L$ models. The relic abundance of $χ$ is governed by annihilation channels mediated by the same scalar and gauge interactions, producing a direct and model-specific correlation between successful leptogenesis and the observed DM relic density. A combined parameter-space analysis incorporating neutrino oscillation data, lepton-flavor-violating processes, direct-detection limits, and collider bounds on $N_{1,2}$ and $Z^\prime$ reveals a narrow yet robust region consistent with all these constraints. Representative benchmark points in this region are examined at a future multi-TeV muon collider. Heavy-neutrino production through electroweak processes yields striking signatures in the dilepton plus missing energy ($2\ell + E\!\!\!/_T$) and single-lepton plus di-jet plus missing energy ($1\ell + 2j + E\!\!\!/_T$) final states. These channels demonstrate that next-generation muon colliders offer a powerful and complementary probe of the inverse-seesaw origin of neutrino masses, the DM relic density, and the TeV-scale leptogenesis within such an extended $U(1)_{B-L}$ framework.

hep-ph↗

arXiv2026

$\bar{B}_{s,d}^{0} \to J/ψμ^{+}μ^{-}$ Decays in QCD Factorization

Motivated by the first LHCb searches for the rare $\bar{B}_{s,d}^{0} \to J/ψμ^{+}μ^{-}$ decays, we perform a detailed study of these processes within the QCD factorization formalism. Since the transverse size of the $J/ψ$ meson is small in the heavy quark mass limit, this formalism is generally expected to hold for these decays. We include both the leading- and next-to-leading-order QCD corrections to the hard-scattering kernels, which are convoluted with the light-cone distribution amplitudes (LCDAs) of the initial- and final-state hadrons. It is numerically found that, depending on the model parameters for the leading-twist $B$-meson LCDA, the maximum branching ratios of $\bar{B}_{s}^{0}\to J/ψμ^{+}μ^{-}$ and $\bar{B}_{d}^{0} \to J/ψμ^{+}μ^{-}$, integrated over the dimuon invariant mass squared $q^2$ from $1\,\mathrm{GeV}^2$ to $(m_{B_{s,d}}-m_{J/ψ})^2$, can reach up to $2.21\times10^{-9}$ and $7.69\times10^{-11}$ at the leading order in $α_s$, respectively. After incorporating the non-factorizable one-loop vertex corrections, these branching ratios are further reduced by about one order of magnitude, with $\mathcal{B}(\bar{B}_{s}^{0} \to J/ψμ^{+}μ^{-})|_{q^2 \geq 1\,\mathrm{GeV}^2}=2.88\times10^{-10}$ and $\mathcal{B}(\bar{B}_{d}^{0} \to J/ψμ^{+}μ^{-})|_{q^2 \geq 1\,\mathrm{GeV}^2}=1.07\times10^{-11}$. In addition, we have presented the dimuon invariant mass distributions of the individual and total helicity amplitudes squared, as well as the differential and integrated longitudinal polarization fractions of the $J/ψ$ meson, which could be probed by the future LHCb and Belle II experiments with more accumulated data.

hep-ph↗

arXiv2026

An End-to-end Architecture for Collider Physics and Beyond

We present, to our knowledge, the first language-driven agent system capable of executing end-to-end collider phenomenology tasks, instantiated within a decoupled, domain-agnostic architecture for autonomous High-Energy Physics phenomenology. Guided only by natural-language prompts supplemented with standard physics notation, ColliderAgent carries out workflows from a theoretical Lagrangian to final phenomenological outputs without relying on package-specific code. In this framework, a hierarchical multi-agent reasoning layer is coupled to Magnus, a unified execution backend for phenomenological calculations and simulation toolchains. We validate the system on representative literature reproductions spanning leptoquark and axion-like-particle scenarios, higher-dimensional effective operators, parton-level and detector-level analyses, and large-scale parameter scans leading to exclusion limits. These results point to a route toward more automated, scalable, and reproducible research in collider physics, cosmology, and physics more broadly.

hep-ph↗

arXiv2025

Phenomenological anatomy of top-quark FCNCs induced by a light scalar singlet

Scalar singlets under the Standard Model gauge group appear naturally in many well-motivated New Physics scenarios, such as the composite Higgs models. Unlike the Higgs boson in the Standard Model, they can induce large flavour-changing neutral currents (FCNCs) in the top sector. We investigate systematically the effects of a light scalar singlet $S$ with top-quark FCNC couplings, by including the low-energy constraints from the $B_s \to μ^+ μ^-$ decay, the muon anomalous magnetic moment $(g-2)_μ$ and the neutron Electric Dipole Moment (EDM). We also perform a detailed Monte-Carlo simulation of the channel $pp \to t S +j$ with $S \to μ^+ μ^-$ and $S \to b \bar b$, and investigate the LHC sensitivity to the $tcS$ couplings. It is found that the scalar singlet $S$ can induce scalar-type contributions to the $B_s \to μ^+ μ^-$ decay, which do not suffer from the helicity suppression and contain a large CKM factor $V_{cs}^*V_{tb}$. As a result, constraints on the $tcS$ couplings from the measured branching ratio $\mathcal{B}(B_s \to μ^+ μ^-)$ are quite stringent, being even stronger than the expected LHC sensitivity in some parameter spaces. Besides the CP-conserving $tcS$ couplings, we have also considered the case of CP-violating $tcS$ couplings, with $y_{R,L}^{ct}=|y_{R,L}^{ct}|e^{iθ_{R,L}}$. It is found that the CP observables $\mathcal{A}_{ΔΓ_s}^{μμ}$ and $\mathcal{S}_{μμ}$ of the $B_s \to μ^+ μ^-$ decay are sensitive to the phase $θ_R$, while the neutron EDM can provide bounds on the phase difference $θ_L-θ_R$. Therefore, they are complementary to each other in probing the CP phases of the $tcS$ couplings.

hep-ph↗

arXiv2024

Deciphering the Belle II data on $B\to K ν\barν$ decay in the (dark) SMEFT with minimal flavour violation

Recently, the Belle II collaboration announced the first measurement of $\mathcal B(B^+\to K^+ν\barν)$, which is found to be about $2.7σ$ higher than the SM prediction. We decipher the data with two new physics scenarios: the underlying $b\to s ν\barν$ transition is, besides the SM contribution, further affected by heavy new mediators that are much heavier than the EW scale, or amended by an additional decay channel with undetected light final states like dark matter or axion-like particles. These two scenarios can be most conveniently analyzed in the SMEFT and the dark SMEFT (DSMEFT) framework, respectively. We consider the flavour structures of the resulting effective operators to be either generic or satisfy the minimal flavour violation (MFV) hypothesis, both for the quark and lepton sectors. In the first scenario, once the MFV is assumed, only one SM-like low-energy effective operator induced by the SMEFT dim-6 operators can account for the Belle II excess, the parameter space of which is, however, excluded by the Belle upper bound on $\mathcal B(B^0\to K^{*0}ν\barν)$. In the second scenario, it is found that the Belle II excess can be accommodated by 22 of the DSMEFT operators involving one or two scalar, fermionic, or vector dark matters as well as ALPs. These operators also receive dominant constraints from the $B^0\to K^{*0}+$inv and $B_s\to$inv decays. In the MFV hypothesis, the number of viable operators is reduced to 14, and the $B^+\toπ^+ +$inv and $K^+\toπ^++$inv decays start to put further constraints. Within the parameter space allowed by all the current experimental data, the $q^2$ distributions (and $F_L$) of the $B\to K^{(*)}+$inv decays are studied for each viable operator. In addition, we, for the first time, calculate systematically the longitudinal polarization fraction $F_L$ of $K^*$ in the $B\to K^*+$inv decays within the DLEFT.

hep-ph↗

arXiv2023

Correlating the CDF $W$-mass shift with the muon $g-2$ and the $b \to s \ell^+ \ell^-$ transitions

Motivated by the latest CDF $W$-mass measurement as well as the muon $g-2$ anomaly and the discrepancies observed in $b \to s \ell^+ \ell^-$ transitions, we propose an extension of the Standard Model (SM) with the $SU(2)_L$-singlet vector-like fermion partners that are featured by additional $U(1)^\prime$ gauge symmetry. The fermion partners have the same SM quantum numbers as of the right-handed SM fermions, and can therefore mix with the latter after the electroweak and the $U(1)^\prime$ symmetry breaking. As a result, desirable loop-level corrections to the $(g-2)_μ$, the $W$-boson mass $m_W$ and the Wilson coefficient $C_9$ in $b \to s μ^+ μ^-$ transitions can be obtained. The final allowed parameter space is also consistent with the constraints from the $Z \to μ^+ μ^-$ decay, the neutrino trident production and the LHC direct searches for the vector-like quarks and leptons.

hep-ph↗

arXiv2023

Study of CP Violation in $D^{\pm}\rightarrow K^{\ast}(892)^{0} π^{\pm} + \bar{K}^{\ast }(892)^{0}π^{\pm}\rightarrow K_{S,L}^{0}π^0 π^{\pm}$ Decays

Within the Standard Model, we investigate the CP violations and the $K_S^0-K_L^0$ asymmetries in $D^{\pm}\rightarrow K^{\ast}(892)^{0} π^{\pm} + \bar{K}^{\ast }(892)^{0}π^{\pm}\rightarrow K_{S,L}^{0}π^0 π^{\pm}$ decays basing on the factorization-assisted topological-amplitude (FAT) approach and the topological amplitude (TA) approach of Cheng and Chiang [Phys. Rev. D 104, 073003 (2021).]. We find that the CP violations in these decays $A_{CP}^{K_{S,L}^0}$ can exceed the order of $10^{-3}$ in the two approaches and consist of three parts: the indirect CP violations in $K^0 -\bar{K}^0$ mixing $A_{CP,K_{S,L}^0}^{mix}$, the direct CP violations in charm decays $A_{CP,K_{S,L}^0}^{dir}$ and the new CP violation effects $A_{CP,K_{S,L}^0}^{int}$, which are induced from the interference between two tree (Cabibbo-favored and doubly Cabibbo-suppressed) amplitudes with the neutral kaon mixing. The indirect CP violations in $K^0 -\bar{K}^0$ mixing play a dominant role in $A_{CP}^{K_{S,L}^0}$, the new CP violation effects have a non-negligible contribution to $A_{CP}^{K_{S,L}^0}$. We estimate the numerical results of the $K_S^0-K_L^0$ asymmetries $R_{K_{S}-K_{L}}^{D^{\pm}}$ and find that there exist a large difference between the numerical results in the FAT approach and that of the TA approach. We also find that if ones adopt the values of the decay time parameters $t_0 = 3.0 τ_S$ and $t_1 = 10.0 τ_S$, the new CP violation effect $A_{CP,K_{S}^0}^{int}$ would dominate the CP violation in $D^{\pm} \rightarrow K^{\ast}(892)^{0} π^{\pm} + \bar{K}^{\ast }(892)^{0}π^{\pm}\rightarrow K_{S}^0π^0 π^{\pm}$ decays and could be observed with $6.7\times 10^{6}$ and $6.5\times 10^{6}$ $D^{\pm}$ events-times-efficiency in the FAT approach and the TA approach, respectively. Our results could be tested by the LHCb, Belle II and BESIII experiments.

hep-ph↗

arXiv2022

Correlating the CDF $W$-boson mass shift with the $b \to s \ell^+ \ell^-$ anomalies

The recently updated measurement of the $W$-boson mass by the CDF collaboration exhibits a $7σ$ deviation from the SM expectation, which may imply a sign of new physics beyond the SM. The observed discrepancy could be explained by new fermions that carry the electroweak gauge charges and affect the vacuum polarization of gauge bosons. Notably, if the new fermions also have the same quantum numbers as of the SM quarks, they can mix with the latter and thus modify the penguin diagrams governing the $b \to s \ell^+ \ell^-$ transitions. Therefore, the $W$-boson mass shift could be related to the $b \to s \ell^+ \ell^-$ anomalies observed by the LHCb collaboration during the past few years. To investigate this possibility, we consider in this paper a model containing a vector-like top partner gauged under a new $U(1)^\prime$ symmetry. It is found that the latest CDF $m_W$ measurement and the $b \to s \ell^+ \ell^-$ anomalies can be simultaneously accommodated at $2σ$ level.

hep-ph↗

arXiv2022

Explaining the $b \to s \ell^+ \ell^-$ anomalies in $Z^\prime$ scenarios with top-FCNC couplings

Motivated by the recent anomalies in $b \to s \ell^+ \ell^-$ transitions, we explore a minimal $Z^\prime$ scenario, in which the $Z^\prime$ boson has a flavour-changing coupling to charm and top quarks and a flavour-conserving coupling to muons. It is found that such a $Z^\prime$ boson can explain the current $b \to s \ell^+ \ell^-$ anomalies, while satisfying other flavour and collider constraints simultaneously. The $Z^\prime$ boson can be as light as few hundreds GeV. In this case, the $t \to c μ^+ μ^-$ decay and the $tZ^\prime$ associated production at the LHC could provide sensitive probes of such a $Z^\prime$ boson. As a special feature, the $Z^\prime$ contributions to all rare $B$- and $K$-meson processes are controlled by one parameter. This results in interesting correlations among these processes, which could provide further insights into this scenario. In addition, an extended scenario, in which the $Z^\prime$ boson interacts with the $SU(2)_L$ fermion doublets with analogous couplings as in the minimal scenario, is also investigated.

hep-ph↗

arXiv2021

Study of CP violation and CPT violation in $K^{\ast}(892)\rightarrow K_{S,L}^{0}π$ decays at BESIII

The decays $K^{\ast}(892)\rightarrow K_{S,L}^{0}π$ can be used to study CP violation and CPT violation. The $K^{\ast}(892)$ (hereinafter referred to as $K^{\ast}$) meson can be produced via $J/ψ$ decays at BESIII. In this paper, we study CP violation and $K_S^0-K_L^0$ asymmetry in the $J/ψ$ decays involving the $K^{\ast}$ meson in the final states and obtain the following results \begin{align} {\mathcal A}_{CP}\left(J/ψ\rightarrow K^{\ast}f_{J/ψ}\rightarrow K^0_{S}πf_{J/ψ}\right)=\left(3.64\pm0.04\right)\times 10^{-3},\nonumber\\ {\mathcal A}_{CP}\left(J/ψ\rightarrow K^{\ast}f_{J/ψ}\rightarrow K^0_{L}πf_{J/ψ}\right)=\left(-3.32\pm0.04\right)\times 10^{-3},\nonumber \end{align} and \begin{align} &R\left(J/ψ\rightarrow K^{\ast}f_{J/ψ}\rightarrow K^0_{S,L}πf_{J/ψ} \right)=\left(3.51\pm0.03\right)\times 10^{-3},\nonumber\\ &R\left(J/ψ\rightarrow \bar{K}^{\ast} \bar{f}_{J/ψ}\rightarrow K^0_{S,L}π\bar{f}_{J/ψ} \right)=\left(-3.45\pm0.03\right)\times 10^{-3}.\nonumber \end{align} Basing on two cases: the samples of $10^{10}$ and $10^{12}$ $J/ψ$ events, we calculate the expected numbers of the observed signal events on the CP violation and the $K_S^0-K_L^0$ asymmetry in the $J/ψ$ decays with $K^{\ast}$ meson in the final states, we find that the BESIII experiment may be able to unambiguously observe CP violation and $K_S^0-K_L^0$ asymmetry for each of these two cases. We study the possibility to constraint the CPT violation parameter $z$ and discuss the sensitivity for the measurement of $z$ in $J/ψ$ decays with $K^{\ast}$ meson in the final states at BESIII. The sensitivity of the measurement of $z$ depend on the measured precision of the parameters $m_L-m_S$, $Γ_L$, $Γ_S$, $p$ and $q$ and the consistence between the values of $t_0$ and $t_1$ and the event selection criteria in experiment.

hep-ph↗

arXiv2021

Effect of $K^0-\bar{K}^0$ Mixing on CP and CPT Violations in $B_{c}^{\pm}\rightarrow B^{\pm} K_{S,L}^{0}$ Decays

The large data sample of the $B_c$ meson collected at the LHC experiment and the HL-LHC experiment provides us the opportunity to study the $B_c$ decays and the related physics. In this paper, we investigate the effect of $K^0-\bar{K}^0$ mixing on the the branching ratios, CP violations and CPT violations in the $B_{c}^{\pm}\rightarrow B^{\pm} K_{S,L}^{0}$ decays. We find that some of the $B_c^{\pm}\rightarrow B^{\pm} K_{S,L}^0\rightarrow f_{B^{\pm}} f_{K_{S,L}^0}$ decay chains have large branching ratios, whose maximum value can exceed the order of $10^{-6}$, the minimum number of $B_c^\pm$ events times efficiency for observing the decays at three standard deviations (3$σ$) level is about $ 10^6$. We study the CP asymmetries in the $B_c^{\pm}\rightarrow B^{\pm} K_{S,L}^0$ decays and find that the CP asymmetries can exceed the order of $10^{-3}$, which are dominated by $K^0-\bar{K}^0$ mixing. We give the most promising processes to observe the CP violations and the ranges of the numbers of $B_c^\pm$ events-times-efficiency needed to observe the CP asymmetries at a significance of 3$σ$ in these decays. We investigate the possibility to constraint the CPT violation parameter $z$ in the $B_c^{\pm}\rightarrow B^{\pm} K_{S,L}^0\rightarrow f_{B^{\pm}} f_{K_{S,L}^0}$ decays and give the most promising processes to constraint the parameter $z$.

hep-ph↗

arXiv2019

$b \to cτ\barν$ decays in supersymmetry with $R$-parity violation

In the past few years, several hints of lepton flavour universality (LFU) violation have emerged in the $b \to c τ\barν$ and $b \to s \ell^+ \ell^-$ data. Quite recently, the Belle Collaboration has reported the first measurement of the $D^*$ longitudinal polarization fraction in the $B \to D^* τ\barν$ decay. Motivated by this intriguing result, together with the recent measurements of $R_{J/ψ}$ and $τ$ polarization, we study $b \to c τ\barν$ decays in the Supersymmetry (SUSY) with $R$-parity violation (RPV). We consider $B \to D^{(*)} τ\barν$, $B_c \to η_c τ\barν$, $B_c \to J/ψτ\barν$ and $Λ_b \to Λ_c τ\barν$ modes and focus on the branching ratios, the LFU ratios, the forward-backward asymmetries, polarizations of daughter hadrons and $τ$ lepton. It is found that the RPV SUSY can explain the $R_{D^{(*)}}$ anomalies at $2σ$ level, after taking into account various flavour constraints. In the allowed parameter space, the differential branching fractions and LFU ratios are largely enhanced by the SUSY effects, especially in the large dilepton invariant mass region. In addition, a lower bound $\mathcal B(B^+ \to K^+ ν\barν) > 7.37 \times 10^{-6}$ is obtained. These observables could provide testable signatures at the High-Luminosity LHC and SuperKEKB, and correlate with direct searches for SUSY.

hep-ph↗

arXiv2019

Phenomenology of $b\to cτ\barν$ decays in a scalar leptoquark model

During the past few years, hints of Lepton Flavour Universality (LFU) violation have been observed in $b \to c τ\barν$ and $b \to s \ell^+ \ell^-$ transitions. Recently, the $D^*$ and $τ$ polarization fractions $P_L^{D^*}$ and $P_L^τ$ in $B \to D^* τ\barν$ decay have also been measured by the Belle collaboration. Motivated by these intriguing results, we revisit the $R_{D^{(*)}}$ and $R_{K^{(*)}}$ anomalies in a scalar leptoquark (LQ) model, in which two scalar LQs, one being $SU(2)_L$ singlet and the other $SU(2)_L$ triplet, are introduced simultaneously. We consider five $b \to c τ\barν$ mediated decays, $B \to D^{(*)}τ\barν$, $B_c \to η_c τ\barν$, $B_c \to J/ψτ\barν$, and $Λ_b \to Λ_c τ\barν$, and focus on the LQ effects on the $q^2$ distributions of the branching fractions, the LFU ratios, and the various angular observables in these decays. Under the combined constraints by the available data on $R_{D^{(*)}}$, $R_{J/ψ}$, $P_L^τ(D^*)$, and $P_L^{D^*}$, we perform scans for the LQ couplings and make predictions for a number of observables. It is found numerically that both the differential branching fractions and the LFU ratios are largely enhanced by the LQ effects, with the latter being expected to provide testable signatures at the SuperKEKB and High-Luminosity LHC experiments.

hep-ph↗

arXiv2018

Flavor Violating Higgs Couplings in Minimal Flavor Violation

Motivated by the rencent LHC data on the lepton-flavor violating (LFV) decays $h\to \ell_1 \ell_2$ and $B_{s,d}\to \ell_1 \ell_2$, we study the Higgs-mediated flavor-changing neutral current (FCNC) interactions in the effective field theory (EFT) approach without and with the minimal flavor violation (MFV) hypothesis, and concentrate on the later. After considering the $B$ and $K$ physics data, the various LFV processes, and the LHC Higgs data, severe constraints on the Higgs FCNC couplings are derived, which are dominated by the LHC Higgs data, the $B_s - \bar B_s$ mixing, and the $μ\to e γ$ decay. In the general case and the MFV framework, allowed ranges of various observables are obtained, such as $\mathcal B (B_s\to \ell_1 \ell_2)$, $\mathcal B (h\to \ell_1 \ell_2)$, $\mathcal B (h\to q_1 q_2)$, and the branching ratio of $μ\to e$ conversion in Al. Future prospects of searching for the Higgs FCNC interactions at the low-energy experiments and the LHC are discussed.

hep-ph↗

arXiv2018

$R_{K^{(*)}}$ and related $b\to s\ell\bar\ell$ anomalies in minimal flavor violation framework with $Z'$ boson

A recent LHCb measurement of the ratio $R_{K^*}$ of $B\to K^*μ\barμ$ to $B\to K^*e\bar e$ branching fractions has produced results in mild tension with the standard model (SM). This adds to the known anomalies also induced by the $b\to s\ell\bar\ell$ transitions, resulting in a confidence level now as high as 4$σ$. We analyze whether the parameter space preferred by all the $b\to s\ell\bar\ell$ anomalies is compatible with a heavy $Z'$ boson assumed to have nonuniversal couplings to SM fermions dictated by the principle of minimal flavor violation (MFV). We deal with the MFV couplings of the $Z'$ to leptons in the context of the type-I seesaw scenario for generating neutrino masses. The flavor-violating $Z'$ interactions are subject to stringent constraints from other processes, especially $B$-$\bar B$ mixing, charged lepton decays $\ell_i\to\ell_j\ell_k\bar\ell_l$ occurring at tree level, and the loop induced $μ\to eγ$. We perform scans for parameter regions allowed by various data and predict the ranges for a number of observables. Some of the predictions, such as the branching fractions of lepton-flavor violating $τ\to3μ$, $B\to K eμ$, $K_L\to eμ$, and $Z\to\ell\ell'$, are not far below their experimental bounds and therefore could be probed by searches in the near future. The viable parameter space depends strongly on the neutrino mass hierarchy, with a preference for the inverted one.

hep-ph↗

arXiv2018

Search for sterile neutrinos decaying into pions at the LHC

We study the possibility to observe sterile neutrinos with masses in the range between 5 GeV and 20 GeV at the LHC, using the exclusive semileptonic modes involving pions, namely W to lepton + N to n pions + lepton+lepton (n = 1, 2, 3). The two pion and three pion modes require extrapolations of form factors to large time-like $q^2$, which we do using vector dominance models as well as light front holographic QCD, with remarkable agreement. This mass region is difficult to explore with inclusive dilepton+dijet modes or trilepton modes and impossible to explore in rare meson decays. While particle identification is a real challenge in these modes, vertex displacement due to the long living neutrino in the above mass range can greatly help reduce backgrounds. Assuming a sample of $10^9$ W bosons at the end of the LHC Run 2, these modes could discover a sterile neutrino in the above mass range or improve the current bounds on the heavy-to-light lepton mixings by an order of magnitude, $U_{l N}^2 \sim 2 \times 10^{-6}$. Moreover, by studying the equal sign and opposite sign dileptons, the Majorana or Dirac character of the sterile neutrino may be revealed.

hep-ph↗

arXiv2017

Constraints and Implications on Higgs FCNC Couplings from Precision Measurement of $B_s \to μ^+μ^-$ Decay

We study constraints and implications of the recent LHCb measurement of ${\cal B}(B_s \to μ^+μ^-)$ for tree-level Higgs-mediated flavor-changing neutral current (FCNC) interactions. Combined with experimental data on $B_s$ mass difference $Δm_s$, the $h \to μτ$, and the $h \to τ^+τ^-$ decay branching ratios from the LHC, we find that the Higgs FCNC couplings are severely constrained. The allowed regions for $B_s \to μτ$, $ττ$ and $h \to sb$ decays are obtained. Current data allow large CP violation in the $h \to τ^+ τ^-$ decay. Consequences of the Cheng-Sher ansatz for the Higgs Yukawa couplings are discussed in some detail.

hep-ph↗