
Abbas, G., Celis, A., Li, X. Q., Lu, J., & Pich, A. (2015). Flavourchanging top decays in the aligned twoHiggsdoublet model. J. High Energy Phys., 06(6), 005–26pp.
Abstract: We perform a complete oneloop computation of the twobody flavourchanging top decays t > ch and t > cV (V = gamma, Z), within the aligned twoHiggsdoublet model. We evaluate the impact of the model parameters on the associated branching ratios, taking into account constraints from flavour data and measurements of the Higgs properties. Assuming that the 125 GeV Higgs corresponds to the lightest CPeven scalar of the CPconserving aligned twoHiggsdoublet model, we find that the rates for such flavourchanging top decays lie below the expected sensitivity of the future highluminosity phase of the LHC. Measurements of the Higgs signal strength in the diphoton channel are found to play an important role in limiting the size of the t > ch decay rate when the charged scalar of the model is light.



Celis, A., Jung, M., Li, X. Q., & Pich, A. (2017). Scalar contributions to b > c(u) tau nu transitions. Phys. Lett. B, 771, 168–179.
Abstract: We perform a comprehensive analysis of scalar contributions in b > c tau nu transitions including the latest measurements of R(D(*)), the q(2) differential distributions in B > D(*) tau nu the tau polarization asymmetry for B > D*tau nu, and the bound derived from the total width of the Bc meson. We find that scalar contributions with the simultaneous presence of both left and righthanded couplings to quarks can explain the available data, specifically R(D(*)) together with the measured differential distributions. However, the constraints from the total Bc width present a slight tension with the current data on B > D*tau nu in this scenario, preferring smaller values for R(D*). We discuss possibilities to disentangle scalar new physics from other newphysics scenarios like the presence of only a lefthanded vector current, via additional observables in B > D(*)tau nu decays or additional decay modes like the baryonic Lambda(b) > Lambda(c)tau nu and the inclusive B > Xc tau nu decays. We also analyze scalar contributions in b > u tau nu transitions, including the latest measurements of B > tau nu providing predictions for Lambda(b) > p tau nu and B > pi tau nu decays. The potential complementarity between the b > u and b > c sectors is finally investigated once assumptions about the flavour structure of the underlying theory are made.



Celis, A., Jung, M., Li, X. Q., & Pich, A. (2013). Sensitivity to charged scalars in B > D(*)tau nu(tau) and B > tau nu(tau) decays. J. High Energy Phys., 01(1), 054–27pp.
Abstract: We analyze the recent experimental evidence for an excess of taulepton production in several exclusive semileptonic Bmeson decays in the context of twoHiggsdoublet models. These decay modes are sensitive to the exchange of charged scalars and constrain strongly their Yukawa interactions. While the usual TypeII scenario cannot accommodate the recent BaBar data, this is possible within more general models in which the chargedscalar couplings to uptype quarks are not as suppressed. Both the B > D(*)tau nu(tau) and the B > tau nu(tau) data can be fitted within the framework of the Aligned TwoHiggsDoublet Model, but the resulting parameter ranges are in conflict with the constraints from leptonic charm decays. This could indicate a departure from the family universality of the Yukawa couplings, beyond their characteristic fermion mass dependence. We discuss several new observables that are sensitive to a hypothetical chargedscalar contribution, demonstrating that they are well suited to distinguish between different scenarios of new physics in the scalar sector, and also between this group and models with different Dirac structures; their experimental study would therefore shed light on the relevance of scalar exchanges in semileptonic b > c tau()(nu) over bar (tau) transitions.



Chang, Q., Li, X. Q., & Yang, Y. D. (2011). The effects of a family nonuniversal Z ' boson on B > pi pi decays. Int. J. Mod. Phys. A, 26(78), 1273–1294.
Abstract: Motivated by the measured large branching ratio of (B) over bar (0) > pi(0)pi(0) (the socalled pi pi puzzle), we investigate the effects of a family nonuniversal Z' model on the treedominated B > pi pi decays. We find that the Z' coupling parameter zeta(LR)(d) similar to 0.05 with a nontrivial new weak phase phi(L)(d) similar to 50 degrees, which is relevant to the Z' contributions to the QCD penguin sector Delta C5, is needed to reconcile the observed discrepancy. Combined with the recent fitting results from B > pi K, pi K* and rho K decays, the Z' parameter spaces are severely reduced but still not excluded entirely, implying that both the “pi pi” and “pi K” puzzles could be accommodated simultaneously within such a family nonuniversal Z' model.



Chen, M. C., Li, X. Q., Liu, X. G., Medina, O., & Ratz, M. (2024). Modular invariant holomorphic observables. Phys. Lett. B, 852, 138600–13pp.
Abstract: In modular invariant models of flavor, observables must be modular invariant. The observables discussed so far in the literature are functions of the modulus tau and its conjugate, (tau) over bar. We point out that certain combinations of observables depend only on tau , i.e. are meromorphic, and in some cases even holomorphic functions of tau. These functions, which we dub “invariants” in this Letter, are highly constrained, renormalization group invariant, and allow us to derive many of the models' features without the need for extensive parameter scans. We illustrate the robustness of these invariants in two existing models in the literature based on modular symmetries, Gamma(3) and Gamma(5). We find that, in some cases, the invariants give rise to robust relations among physical observables that are independent of tau. Furthermore, there are instances where additional symmetries exist among the invariants. These symmetries are relevant phenomenologically and may provide a dynamical way to realize symmetries of mass matrices.



Jung, M., Li, X. Q., & Pich, A. (2012). Exclusive radiative Bmeson decays within the aligned twoHiggsdoublet model. J. High Energy Phys., 10(10), 063–33pp.
Abstract: In the aligned twoHiggsdoublet model, the alignment of Yukawa matrices in flavour space guarantees the absence of treelevel flavourchanging neutral currents, while allowing at the same time for new sources of CP violation, implying potentially large effects in many lowenergy processes. In this work we study the constraints from exclusive radiative B > V gamma decays, where V denotes a light vector meson. The current experimental data on the CPaveraged branching ratios and the direct CP and isospin asymmetries are analyzed. It is found that, while the branching ratios and direct CP asymmetries do not constrain the parameter space much further compared to the inclusive B > Xs,Xd gamma decays, complementary constraints can be obtained from the isospin asymmetries Delta(K*gamma) and Delta(rho gamma). In addition, correlations between the various observables in exclusive B > V gamma and inclusive B > Xs,Xd gamma decays are investigated in detail, and predictions are made for several so far unmeasured observables.



Li, X. Q., Li, Y. M., Lu, G. R., & Su, F. (2012). Bs(0)(B)overbar(s)(0) mixing in a family nonuniversal Z ' model revisited. J. High Energy Phys., 05(5), 049–27pp.
Abstract: Motivated by the very recent measurements performed at the LHCb and the Tevatron of the Bs(0) – (B) over bar (0)(s) mixing, in this paper we revisit it in a family nonuniversal Z' model, to check if a simultaneous explanation for all the mixing observables, especially for the likesign dimuon charge asymmetry observed by the D0 collaboration, could be made in such a specific model. In the first scenario where the Z' boson contributes only to the offdiagonal element M12(s), it is found that, once the combined constraints from Delta Ms, phi(s) and Delta Gamma(s) are imposed, the model could not explain the measured flavourspecific CP asymmetry a(fs)(s), at least within its 1 sigma ranges. In the second scenario where the NP contributes also to the absorptive part Gamma(s)(12) via treelevel Z'induced b > c (c) over bars operators, we find that, with the constraints from Delta Ms, phi(s) and the indirect CP asymmetry in (B) over bar (d) > J/psi KS taken into account, the present measured 1 sigma experimental ranges for a(fs)(s) could not be reproduced too. Thus, such a specific Z' model with our specific assumptions could not simultaneously reconcile all the present data on Bs(0) – Bs(0) mixing. Future improved measurements from the LHCb and the proposed superB experiments, especially of the flavourspecific CP asymmetries, are expected to shed light on the issue.



Li, X. Q., Lu, J., & Pich, A. (2014). Bs,d(0) > l(+)l() decays in the aligned twoHiggsdoublet model. J. High Energy Phys., 06(6), 022–39pp.
Abstract: The rare decays Bs,d(0) > l(+)l() are analyzed within the general framework of the aligned twoHiggs doublet model. We present a complete oneloop calculation of the relevant shortdistance Wilson coefficients, giving a detailed technical summary of our results and comparing them with previous calculations performed in particular limits or approximations. We investigate the impact of various model parameters on the branching ratios and study the phenomenological constraints imposed by present data.



Li, X. Q., Su, F., & Yang, Y. D. (2011). Determination of the strong coupling gB*B pi from semileptonic B > pi l nu decay. Phys. Rev. D, 83(5), 054019–8pp.
Abstract: According to heavymeson chiral perturbation theory, the vector form factor f+(q(2)) of exclusive semileptonic decay B > pi l nu is closely related, at least in the softpion region ( i.e., q(2) similar to (m(B) – m(pi))(2)), to the strong coupling g(B*B pi) or the normalized coupling (g)overcap. Combining the precisely measured q2 spectrum of B > pi l nu decay by the BABAR and Belle collaborations with several parametrizations of the form factor f +(q(2)), we can extract these couplings from the residue of the form factor at the B* pole, which relies on an extrapolation of the form factor from the semileptonic region to the unphysical point q(2) = m(B*)(2). Comparing the extracted values with the other experimental and theoretical estimates, we can test these various formfactor parametrizations, which differ from each other by the amount of physical information embedded in. It is found that the extracted values based on the BecirevicKaidalov, BallZwicky and BourrelyCapriniLellouch parametrizations are consistent with each other and roughly in agreement with the other theoretical and lattice estimates, while the BoydGrinsteinLebed ansatz, featured by a spurious, unwanted pole at the threshold of the cut, gives a neatly larger value.



Li, X. Q., Yang, Y. D., & Yuan, X. B. (2012). Anomalous tqZ coupling effects in rare B and Kmeson decays. J. High Energy Phys., 03(3), 018–22pp.
Abstract: As a topfactory, the LHC is performing a direct study of topquark anomalous FCNC couplings, which are, however, correlated closely with the rare B and Kmeson decays. In this paper, we study the effects of anomalous tqZ (with q = u, c) couplings in the rare decays Bs,Bd > mu(+)mu(), B > Xs nu(nu) over bar, B > K(*)nu(nu) over bar, K+ > pi(+)nu(nu) over bar, and KL > pi(0)nu(nu) over bar. With the uptodate experimental bounds on the branching ratios of these channels, constraints on the lefthanded anomalous couplings Xct(L), and Xut(L) are derived, respectively. With these lowenergy constraints taken into account, we find that, for real couplings Xct(L) and Xut(L), the indirect upper bounds on B(t > qZ) are much lower than that from the D0 collaboration, but are still compatible with the 5 sigma discovery potential of ATLAS with an integrated luminosity of 10 fb(1). With refined measurements to be available at the LHCb, the future superB factories, the NA62 at CERN, and the KOTO at JPARC, closer correlations between the t > qZ and the rare B and Kmeson decays are expected in the near future, which will be helpful for the searches of thu e topquark FCNC decays at the LHC.

