arXiv subjects

Xin-Qiang Li

Publications and source records attributed to Xin-Qiang Li.

At least 37 records · Page 2Linked to original sources

arXiv2024

$CP$ asymmetries in $τ\to K_Sπν_τ$ decays

We present here the CP asymmetries in the decay rate and angular distributions of $τ\to K_Sπν_τ$ decays in the Standard Model (SM) and beyond (BSM). The CP asymmetries in the SM are induced by the CP violation in $K^0-\bar{K}^0$ mixing. To investigate the BSM CP-violating (CPV) effects, a model-independent analysis is performed by using the low-energy effective field theory (LEFT) framework at $μ=2$~GeV. If one further assumes the BSM physics to stem from above the electroweak scale, the LEFT shall then be matched onto the SM effective field theory (SMEFT), the operators of which contributing to $τ\to K_Sπν_τ$ decays will also contribute to the neutron electric dipole moment (EDM) and $D^0-\bar{D}^0$ mixing. The stringent bounds from the latter suggest that no remarkable CPV effects can be observed in either the decay rate or the angular distributions. The prospects for future measurements of these observables are also mentioned.

hep-ph↗

arXiv2024

Study of the $B^-\to K^-ηη_c$ decay due to the $D\bar{D}$ bound state

We study the $B^- \to K^- ηη_c$ decay by taking into account the $S$-wave contributions from the pseudoscalar meson-pseudoscalar meson interactions within the unitary coupled-channel approach, where the $D\bar{D}$ bound state is dynamically generated. In addition, the contribution from the intermediate resonance $K_0^*(1430)^-$, with $K_0^*(1430)^- \to K^-η$, is also considered. Our results show that there is a clear peak around $3720$~MeV in the $ηη_c$ invariant mass distribution, which could be associated with the $D \bar{D}$ bound state. The future precise measurements of the $B^- \to K^- ηη_c$ process at the Belle II and LHCb experiments could be, therefore, used to check the existence of the $D \bar{D}$ bound state, and to deepen our understanding of the hadron-hadron interactions.

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

Probing new physics with polarized $τ$ and $Λ_c$ in quasielastic $ν_τ\!+\!n\!\to\! τ^-\!+\!Λ_c$ scattering process

The absence of semitauonic decays of charmed hadrons makes the decay processes mediated by the quark-level $c\to d τ^+ ν_τ$ transition inadequate for probing a generic new physics (NP) with all kinds of Dirac structures. To fill in this gap, we consider in this paper the quasielastic neutrino scattering process $ν_τ+n\to τ^-+Λ_c$, and propose searching for NP through the polarizations of the $τ$ lepton and the $Λ_c$ baryon. In the framework of a general low-energy effective Lagrangian, we perform a comprehensive analysis of the (differential) cross sections and polarization vectors of the process both within the Standard Model and in various NP scenarios, and scrutinize possible NP signals. We also explore the influence on our findings due to the uncertainties and the different parametrizations of the $Λ_c \to N$ transition form factors, and show that they have become one of the major challenges to further constrain possible NP through the quasielastic scattering process.

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

$B_c^- \to J/ψ(\to μ^+ μ^-)τ^- (\to π^- ν_τ, ρ^- ν_τ, \ell^-\barν_\ellν_τ)\barν_τ$ decays with visible final-state kinematics

The semitauonic $B_c^- \to J/ψτ^-\barν_τ$ decay is optimal to scrutinize possible new physics effects in $b \to c τ^- \barν_τ$ transitions as indicated by the current data on $R(D^{(*)})$ anomalies. In this work, we study the $B_c^- \to J/ψτ^-\barν_τ$ decay with both polarized $τ$ and $J/ψ$. Their subsequent decays, with $J/ψ\to μ^+ μ^-$ as well as $τ^- \to π^- ν_τ$, $τ^- \to ρ^- ν_τ$ and $τ^- \to \ell^-\barν_\ellν_τ$, are exploited to extract the energy and angular distributions of the charged final-state particles in the processes. Starting with the most general effective Hamiltonian relevant for the $b \to c τ^- \barν_τ$ transitions, including all possible Lorentz structures of the dimension-six operators with both left- and right-handed neutrinos, we first derive the five-fold differential decay rate in terms of the visible final-state kinematics. From this distribution, we then construct in total 34 normalized observables, among which nine refer to the CP-violating triple product asymmetries that vanish within the Standard Model. We also construct five new observables based on the combinations of these normalized observables that can only be attributed to the right-handed neutrinos. On the other hand, considering the low statistics of the fully differential distribution, we introduce some integrated observables with only one kinematic variable left, which are more promising to be measured due to the largely increased statistics. The sensitivities of all these observables to the different new physics scenarios are investigated in detail. Finally, assuming an ideal circumstance, we give an estimate of the statistical uncertainties of the nine CP-conserving observables at LHCb and found that $τ^-\to π^-ν_τ$ has the highest analyzing power among the three $τ$ decay channels.

hep-ph↗

arXiv2022

Linking the $R_{K^{(\ast)}}$ anomalies to the Hubble tension via a single right-handed neutrino

The updated measurements from the LHCb and SH0ES collaborations have respectively strengthened the deviations of the ratio $R_{K}$ in rare semi-leptonic $B$-meson decays and the present-day Hubble parameter $H_0$ in the Universe, implying tantalizing hints of new physics beyond the Standard Model. In this paper, we consider a simple flavor-specific two-Higgs-doublet model, where the long-standing $R_{K^{(*)}}$ anomalies can be addressed by a one-flavor right-handed neutrino. One of the intriguing predictions resulting from the parameter space for the $R_{K^{(*)}}$ resolution under flavor- and collider-physics constraints points toward a shift of the effective neutrino number, $ΔN_{\rm eff}=N_{\rm eff}-N_{\rm eff}^{\rm SM}$, as favored to ease the $H_0$ tension. Depending on whether the neutrino is of Dirac or of Majorana type, we show that the resulting shift is $ΔN_{\rm eff}\simeq 1.0$ for the former and $ΔN_{\rm eff}\simeq 0.5$ for the latter case, respectively. While the Dirac case is disfavored by the CMB polarization measurements, the Majorana solution is consistent with the recent studies via a combined data set from various sources. Consequently, such a simple flavor-specific two-Higgs-doublet model provides a link between the $R_{K^{(*)}}$ anomalies and the $H_0$ tension, which in turn can be readily verified or falsified by the upcoming measurements.

hep-ph↗

arXiv2022

Scotogenic Dirac neutrino mass models embedded with leptoquarks: one pathway to address the flavor anomalies and the neutrino masses together

If the leptoquarks proposed to account for the intriguing anomalies observed in the $B$-meson decays, $R_{D^{(\ast)}}$ and $R_{K^{(\ast)}}$, as well as in the anomalous magnetic moment of the muon, $(g-2)_μ$, can be embedded into the scotogenic Dirac neutrino model, all these flavor anomalies, together with the origin of neutrino masses and the nature of dark matter, would be potentially addressed in a unified picture. Among the minimal seesaw, one-loop, and two-loop realizations of the dimension-4 effective operator $\mathcal{L}_{4}$ for the Dirac neutrino masses, we show that plenty of diagrams associated with the two-loop realizations of $\mathcal{L}_{4}$ can support the coexistence of leptoquarks and dark matter candidates. After a simple match of these leptoquarks to those that can accommodate all the flavor anomalies, we establish the scotogenic Dirac neutrino models embedded with leptoquarks, which could address all the problems mentioned above.

hep-ph↗

arXiv2022

Rare top-quark decays $t \to cg(g)$ in the aligned two-Higgs-doublet model

We update the Standard Model (SM) predictions for the branching ratios of the rare top-quark decays $t \to cg(g)$, and evaluate the maximum rates that can be reached in the aligned as well as in the four conventional two-Higgs-doublet models (2HDMs) with $\mathcal{Z}_2$ symmetries. Taking into account the relevant constraints on the model parameters resulting from a global fit obtained at the $95.5\%$ confidence level, we find that the branching ratios of $t \to cg$ and $t \to cgg$ decays can reach up to $3.36\times 10^{-9}$ and $2.95\times 10^{-9}$ respectively, being therefore of the same order, in the aligned 2HDM (A2HDM). This is obviously different from the SM case, where the predicted branching ratio of the three-body decay $t \to cgg$ is about two orders of magnitude larger than that of the two-body decay $t \to cg$. On the other hand, compared with the SM predictions, no significant enhancements are observed in the four conventional 2HDMs with $\mathcal{Z}_2$ symmetries for the branching ratios of these two decays. Nevertheless, the predicted branching ratios of $t \to cg$ and $t \to cgg$ decays in the A2HDM are still out of the expected sensitivities of the future high-luminosity Large Hadron Collider and the Future Circular Collider in hadron-hadron mode.

hep-ph↗

arXiv2022

Searching and identifying leptoquarks through low-energy polarized scattering processes $e^-p\to e^-Λ_c$

We investigate the potential for searching and identifying the leptoquark (LQ) effects in the charm sector through the low-energy polarized scattering processes $\vec{e}^{\,-}p\to e^-Λ_c$, $e^-\vec{p}\to e^-Λ_c$, and $\vec{e}^{\,-}\vec{p}\to e^-Λ_c$. Considering only the longitudinally polarized processes, we show that the different LQ models can be disentangled from each other by measuring the four spin asymmetries, $A_{L}^e$, $A_{L}^p$, $A_{L3}^{ep}$, and $A_{L6}^{ep}$, constructed in terms of the polarized cross sections. Although it is challenging to accomplish the same goal with transversely polarized processes, we find that investing them in future experiments is especially beneficial, since they can directly probe into the imaginary part of the Wilson coefficients in the general low-energy effective Lagrangian. With our properly designed experimental setups, it is also demonstrated that promising event rates can be expected for all these processes and, even in the worst-case scenario -- no LQ signals are observed at all, they can still provide a competitive potential for constraining the new physics, compared with those from the conventional charmed-hadron weak decays and the high-$p_T$ dilepton invariant mass tails at high-energy colliders.

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

Effective neutrino number shift from keV-vacuum neutrinophilic 2HDM

If the Dirac neutrino masses are generated by a new scalar doublet with a vacuum at keV order, there would be rare hope to probe the framework in low-energy flavor physics, such as the lepton-flavor violating processes. However, the predicted neutrino Yukawa couplings being around the order of electronic Yukawa can realize a purely thermal Dirac leptogenesis, and thus render a more direct link between the high- and the low-energy CP violation. Despite its inert property in the low-energy flavor processes, the keV-vacuum induced Dirac neutrino model can generate a significant shift of the effective neutrino number, which can in turn be probed by the big-bang nucleosynthesis and cosmic microwave background epochs. We show that such a keV-vacuum induced Dirac neutrino model is already constrained by the current observations and will be probed with forecast sensitivity, serving therefore as a complementary avenue to test the Dirac leptogenesis.

hep-ph↗

arXiv2022

Prospects for discovering new physics in charm sector through low-energy scattering processes $e^-p\to e^- (μ^-)Λ_c$

We investigate the potential for discovering new physics through the low-energy scattering processes $e^-p\to e^-(μ^-)Λ_c$, which could be accessible in the forthcoming $ep$ scattering experiments once adapted to our proposed setups. In the framework of a general low-energy effective Lagrangian, we demonstrate that, compared with the conventional flavor-changing neutral-current weak decays of charmed hadrons and the dilepton productions at high-energy colliders, the low-energy scattering processes can provide more competitive potentials for hunting the underlying new physics. In some specific leptoquark models, we also show that promising event rates can be expected for both the scattering processes, and point out a possible way to distinguish the experimental signals due to the scalar leptoquarks from those of the vector ones.

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↗

arXiv2022

$CP$ asymmetry in the angular distributions of $τ\to K_Sπν_τ$ decays -- II: general effective field theory analysis

We proceed to study the $CP$ asymmetry in the angular distributions of $τ\to K_Sπν_τ$ decays within a general effective field theory framework including four-fermion operators up to dimension-six. It is found that, besides the commonly considered scalar-vector interference, the tensor-scalar interference can also produce a non-zero $CP$ asymmetry in the angular distributions. Bounds on the effective couplings of the non-standard scalar and tensor interactions are obtained under the combined constraints from the measured $CP$ asymmetries and the branching ratio of $τ^-\to K_Sπ^-ν_τ$ decay, with $\mathrm{Im}[\hatε_S]=-0.008\pm0.027$ and $\mathrm{Im}[\hatε_T]=0.03\pm0.12$, at the scale $μ_τ=2~\mathrm{GeV}$ in the $\mathrm{\overline{MS}}$ scheme. Using the best-fit values, we also find that the distributions of the $CP$ asymmetries can deviate significantly from the SM expectation in almost the whole $Kπ$ invariant-mass region. Nevertheless, the current bounds are still plagued by large experimental uncertainties, but will be improved with more precise measurements from Belle II as well as the proposed Tera-Z and STCF facilities. Assuming further that the non-standard scalar and tensor interactions originate from a weakly-coupled heavy new physics well above the electroweak scale, the $SU(2)_L$ invariance of the resulting SMEFT Lagrangian would indicate that very strong limits on $\mathrm{Im}[\hatε_S]$ and $\mathrm{Im}[\hatε_T]$ could also be obtained from the neutron electric dipole moment and the $D^0-\bar{D}^0$ mixing. With the bounds from these processes taken into account, it is then found that, unless there exist extraordinary cancellations between the new physics contributions, neither the scalar nor the tensor interaction can produce any significant effects on the $CP$ asymmetries in the processes considered.

hep-ph↗

arXiv2021

Baryogenesis from Hierarchical Dirac Neutrinos

We present here a novel and testable mechanism for leptogenesis, which is characterized by a purely thermal generation of lepton asymmetry via scalar decay. Guided by the thermal mass matrix diagonalization in the finite-temperature regime, we propose a scenario in which the baryon asymmetry is formulated in terms of the masses and mixing from hierarchical Dirac neutrinos, as well as a vacuum scale accountable for the sub-eV neutrino masses. This allows a natural and direct link between the high-scale \textit{CP} asymmetry and the low-energy Dirac \textit{CP}-violating phase without involved model structures, and thus circumvents the haunting problem encountered in general leptogenesis scenarios. The mechanism can also be applied as a prototype for solving the baryon asymmetry problem to a broad class of well-motivated model buildings.

hep-ph↗

arXiv2021

Simple estimate of BBN sensitivity to light freeze-in dark matter

We provide a simple analysis of the big-bang nucleosynthesis (BBN) sensitivity to the light dark matter (DM) generated by the thermal freeze-in mechanism. It is shown that the ratio of the effective neutrino number shift $ΔN_ν$ over the DM relic density $ω\equiv Ωh^2$, denoted by $R_χ\equivΔN_ν/ω$, cancels the decaying particle mass and the feeble coupling, rendering therefore a simple visualization of $ΔN_ν$ at the BBN epoch in terms of the DM mass. This property drives one to conclude that the shift with a sensitivity of $ΔN_ν\simeq \mathcal{O}(0.1)$ cannot originate from a single warm DM under the Lyman-$α$ forest constraints. For the cold-plus-warm DM scenarios where the Lyman-$α$ constraints are diluted, the ratio $R_χ$ can be potentially used to test the thermal freeze-in mechanism in generating a small warm component of DM and a possible excess at the level of $ΔN_ν\simeq \mathcal{O}(0.01)$.

hep-ph↗

arXiv2021

Probing new physics in class-I $B$-meson decays into heavy-light final states

With updated experimental data and improved theoretical calculations, several significant deviations are being observed between the Standard Model predictions and the experimental measurements of the branching ratios of $\bar{B}_{(s)}^0\to D_{(s)}^{(*)+} L^-$ decays, where $L$ is a light meson from the set $\{π,ρ,K^{(\ast)}\}$. Especially for the two channels $\bar{B}^0\to D^{+}K^-$ and $\bar{B}_{s}^0\to D_{s}^{+}π^-$, both of which are free of the weak annihilation contribution, the deviations observed can even reach 4-5$σ$. Here we exploit possible new-physics effects in these class-I non-leptonic $B$-meson decays within the framework of QCD factorization. Firstly, we perform a model-independent analysis of the effects from twenty linearly independent four-quark operators that can contribute, either directly or through operator mixing, to the quark-level $b\to c\bar{u} d(s)$ transitions. It is found that, under the combined constraints from the current experimental data, the deviations observed could be well explained at the $1σ$ level by the new-physics four-quark operators with $γ^μ(1-γ_5)\otimesγ_μ (1-γ_5)$ structure, and also at the $2σ$ level by the operators with $(1+γ_5)\otimes(1-γ_5)$ and $(1+γ_5)\otimes(1+γ_5)$ structures. However, the new-physics four-quark operators with other Dirac structures fail to provide a consistent interpretation, even at the $2σ$ level. Then, as two specific examples of model-dependent considerations, we discuss the case where the new-physics four-quark operators are generated by either a colorless charged gauge boson or a colorless charged scalar, with their masses fixed both at the $1$~TeV. Constraints on the effective coefficients describing the couplings of these mediators to the relevant quarks are obtained by fitting to the current experimental data.

hep-ph↗