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Pich, A., & Rodriguez-Sanchez, A. (2016). Determination of the QCD coupling from ALEPH tau decay data. Phys. Rev. D, 94(3), 034027–26pp.
Abstract: We present a comprehensive study of the determination of the strong coupling from tau decay, using the most recent release of the experimental ALEPH data. We critically review all theoretical strategies used in previous works and put forward various novel approaches which allow one to study complementary aspects of the problem. We investigate the advantages and disadvantages of the different methods, trying to uncover their potential hidden weaknesses and test the stability of the obtained results under slight variations of the assumed inputs. We perform several determinations, using different methodologies, and find a very consistent set of results. All determinations are in excellent agreement, and allow us to extract a very reliable value for alpha(s)(m(tau)(2)). The main uncertainty originates in the pure perturbative error from unknown higher orders. Taking into account the systematic differences between the results obtained with the contour-improved perturbation theory and fixed-order perturbation theory prescriptions, we find alpha((nf=3))(s) (m(tau)(2)) = 0.328 +/- 0.013 which implies alpha((nf=5))(s) (M-Z(2)) = 0.1197 +/- 0.0015.
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Mandal, R., & Pich, A. (2019). Constraints on scalar leptoquarks from lepton and kaon physics. J. High Energy Phys., 12(12), 089–40pp.
Abstract: We present a comprehensive analysis of low-energy signals of hypothetical scalar leptoquark interactions in lepton and kaon transitions. We derive the most general effective four-fermion Lagrangian induced by tree-level scalar leptoquark exchange and identify the Wilson coefficients predicted by the five possible types of scalar leptoquarks. The current constraints on the leptoquark Yukawa couplings arising from lepton and kaon processes are worked out, including also loop-induced transitions with only leptons (or quarks) as external states. In the presence of scalar leptoquark interactions, we also derive the differential distributions for flavour-changing neutral-current transitions in semileptonic kaon modes, including all known effects within the Standard Model. Their interference with the new physics contributions could play a significant role in future improvements of those constraints that are currently hampered by poorly-determined non-perturbative parameters.
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Murgui, C., Peñuelas, A., Jung, M., & Pich, A. (2019). Global fit to b -> c tau nu transitions. J. High Energy Phys., 09(9), 103–45pp.
Abstract: We perform a general model-independent analysis of b -> c tau(nu) over bar (tau) transitions, including measurements of R-D, R-D*, their q(2) differential distributions, the recently measured longitudinal D* polarization F-L(D)*, and constraints from the B-c -> tau(nu) over bar (tau) lifetime, each of which has significant impact on the fit. A global fit to a general set of Wilson coefficients of an effective low-energy Hamiltonian is presented, the solutions of which are interpreted in terms of hypothetical new-physics mediators. From the obtained results we predict selected b -> c tau(nu) over bar (tau) observables, such as the baryonic transition Lambda(b) -> Lambda(c)tau(nu) over bar (tau), the ratio R-J/psi, the forward-backward asymmetries A(FB)(D()*()), the tau polarization asymmetries P-tau(D()*()), and the longitudinal D* polarization fraction F-L(D)*. The latter shows presently a slight tension with any new-physics model, such that an improved measurement could have an important impact. We also discuss the potential change due the very recently announced preliminary R-D(*) measurement by the Belle collaboration.
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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 B-c meson. We find that scalar contributions with the simultaneous presence of both left- and right-handed couplings to quarks can explain the available data, specifically R(D-(*)) together with the measured differential distributions. However, the constraints from the total B-c 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 new-physics scenarios like the presence of only a left-handed 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 -> X-c 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.
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Abbas, G., Celis, A., Li, X. Q., Lu, J., & Pich, A. (2015). Flavour-changing top decays in the aligned two-Higgs-doublet model. J. High Energy Phys., 06(6), 005–26pp.
Abstract: We perform a complete one-loop computation of the two-body flavour-changing top decays t --> ch and t --> cV (V = gamma, Z), within the aligned two-Higgs-doublet 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 CP-even scalar of the CP-conserving aligned two-Higgs-doublet model, we find that the rates for such flavour-changing top decays lie below the expected sensitivity of the future high-luminosity phase of the LHC. Measurements of the Higgs signal strength in the di-photon 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.
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