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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Pich, A., & Rodriguez-Sanchez, A. (2016). Updated determination of alpha(s)(m(tau)(2)) from tau decays. Mod. Phys. Lett. A, 31(30), 1630032–15pp.
Abstract: Using the most recent release of the ALEPH tau decay data, we present a very detailed phenomenological update of the alpha(s)(m(tau)(2)) determination. We have exploited the sensitivity to the strong coupling in many different ways, exploring several complementary methodologies. All determinations turn out to be in excellent agreement, allowing us to extract a very reliable value of the strong coupling. We find alpha((nf =3))(s)(m(tau)(2)) = 0.328 +/- 0.012 which implies alpha((nf=5))(s)(M-Z(2)) = 0.1197 +/- 0.0014. We critically revise previous work, and point out the problems flawing some recent analyses which claim slightly smaller values.
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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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Pich, A. (2014). Precision tau physics. Prog. Part. Nucl. Phys., 75, 41–85.
Abstract: Precise measurements of the lepton properties provide stringent tests of the Standard Model and accurate determinations of its parameters. We overview the present status of tau physics, highlighting the most recent developments, and discuss the prospects for future improvements. The leptonic decays of the tau lepton probe the structure of the weak currents and the universality of their couplings to the W boson. The universality of the leptonic Z couplings has also been tested through Z -> l(+)l(-) decays. The hadronic tau decay modes constitute an ideal tool for studying low-energy effects of the strong interaction in very clean conditions. Accurate determinations of the QCD coupling and the Cabibbo mixing V-us have been obtained with tau data. The large mass of the tau opens the possibility to study many kinematically-allowed exclusive decay modes and extract relevant dynamical information. Violations of flavour and CP conservation laws can also be searched for with tau decays. Related subjects such as μdecays, the electron and muon anomalous magnetic moments, neutrino mixing and B-meson decays into tau leptons are briefly covered. Being one the fermions most strongly coupled to the scalar sector, the tau lepton is playing now a very important role at the LHC as a tool to test the Higgs properties and search for new physics at higher scales.
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Li, X. Q., Lu, J., & Pich, A. (2014). Bs,d(0) -> l(+)l(-) decays in the aligned two-Higgs-doublet model. J. High Energy Phys., 06(6), 022–39pp.
Abstract: The rare decays B-s,d(0) -> l(+)l(-) are analyzed within the general framework of the aligned two-Higgs doublet model. We present a complete one-loop calculation of the relevant short-distance 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.
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