Gabriel A. González-Sprinberg

Model independent bounds on the tau lepton electromagnetic and weak magnetic moments

Using LEP1, SLD and LEP2 data, for tau lepton production, and data from p¯ p colliders and LEP2, for W decays into tau leptons, we set model independent limits on non-standard electromagnetic and weak magnetic moments of the tau lepton. The most general effective Lagrangian giving rise to tau moments is used without further assumptions. Precise bounds (2σ) on the non-standard model contributions to tau electromagnetic (−0.007 < a < 0.005), tau Z-magnetic (−0.0024 < aZ < 0.0025) and tau W-magnetic (−0.003 < κ W < 0.004) dipole moments are set from the analysis.

The tau weak-magnetic dipole moment

Abstract We calculate the prediction for the anomalous weak-magnetic form factor of the tau lepton at q 2 = M 2 within the Standard Model. With all particles on-shell, this is an electroweak gauge-invariant quantity. Its value is a τ w ( M Z 2 )= − (2.10+0.61 i ) × 10 −6 . We show that the transverse and normal components of the single-tau polarization of tau pairs produced at e + e − unpolarized collisions are sensitive to the real and absorptive parts of the anomalous weak-magnetic dipole moment of the tau. The sensitivity one can achieve at LEP in the measurement of this dipole moment is discussed.

Top quark tensor couplings

We compute the real and imaginary parts of the one-loop electroweak contributions to the left and right tensorial anomalous couplings of the tbW vertex in the Standard Model (SM). For both tensorial couplings we find that the real part of the electroweak SM correction is close to 10% of the leading contribution given by the QCD gluon exchange. We also find that the electroweak real and imaginary parts for the anomalous right coupling are almost of the same order of magnitude. The one loop SM prediction for the real part of the left coupling is close to the 3σ discovery limit derived from b → sγ. Besides, taking into account that the predictions of new physics interactions are also at the level of a few percents when compared with the one loop QCD gluon exchange, these electroweak corrections should be taken into account in order to disentangle new physics effects from the standard ones. These anomalous tensorial couplings of the top quark will be investigated at the LHC in the near future where sensitivity to these contributions may be achieved.

Top quark anomalous tensor couplings in the two-Higgs-doublet models

A bstractWe compute the one loop right and left anomalous tensor couplings (gR and gL, respectively) for the top quark, in the aligned two-Higgs-doublet model. They are the magnetic-like couplings in the most general parameterization of the tbW vertex. We find that the aligned two-Higgs doublet model, that includes as particular cases some of the most studied extensions of the Higgs sector, introduces new electroweak contributions and provides theoretical predictions that are very sensitive to both new scalar masses and the neutral scalar mixing angle. For a large area in the parameters space we obtain significant deviations in both the real and the imaginary parts of the couplings gR and gL, compared to the predictions given by the electroweak sector of the Standard Model. The most important ones are those involving the imaginary part of the left coupling gL and the real part of the right coupling gR. The real part of gL and the imaginary part of gR also show an important sensitivity to new physics scenarios. The model can also account for new CP violation effects via the introduction of complex alignment parameters that have important consequences on the values for the imaginary parts of the couplings. The top anomalous tensor couplings will be measured at the LHC and at future colliders providing a complementary insight on new physics, independent from the bounds in top decays coming from B physics and b → sγ.

Tau anomalous magnetic moment form factor at super B/flavor factories

The proposed high-luminosity B/flavor factories offer new opportunities for the improved determination of the fundamental physical parameters of standard heavy leptons. Compared to the electron or the muon case, the magnetic properties of the τ lepton are largely unexplored. We show that the electromagnetic properties of the τ , and in particular its magnetic form factor, may be measured competitively in these facilities, using unpolarized or polarized electron beams. Various observables of the τ ’s produced on top of

CP violation and electric-dipole-moment at low energy τ production with polarized electrons

The new proposals for high luminosity B/Flavor factories, near and on top of the � resonances, allow for a detailed investigation of CP-violation in the �-pair production. In particular, bounds on the tau electric dipole moment can be obtained from genuine CP-odd observables related to the �-pair production. We perform an independent analysis from low energy (10 GeV) data by means of linear spin observables. We show that for a longitudinally polarized electron beam a CP-odd asymmetry, associated to the normal polarization term, can be measured at these low energy facilities both at resonant and non resonant energies. In this way stringent and independent bounds to the tau electric dipole moment, which are orders of magnitude below other high or low energy bounds, can be obtained.

Tau spin correlations and the anomalous magnetic moment

We show that the precise determination of the Tau magnetic properties is possible in the next generation accelerators, specially at B/Flavour factories. We define spin correlation observables suitable to extract the real part of the magnetic form factor that, for the first time, will allow to test the standard model-QED predictions. In particular, the predicted QED-dependence with both the momentum transfer and the lepton mass can be precisely measured. Until now, the most stringent bounds on the τ magnetic moment aτ come from LEP data with strong assumptions on the physics involved on the observed process. In this paper, we find three different combinations of spin correlations of the outgoing Taus that disentangle the magnetic moment form factor of the Tau lepton in the electromagnetic vertex. These combinations of asymmetries also get rid off the contributions coming from Z-mediating amplitudes to the defined correlations. Using unpolarized electron beams and an integrated luminosity of 15 × 1018b−1, the sensitivity to the τ magnetic moment form factor is of the order 10−6. This sensitivity is two orders of magnitude better than the present existing high- or low-energy bounds on the magnetic moment and would allow its actual measurement with the precision of a few per cent.

CP violation and electric dipole moment at low energy τ-pair production

CP violation at low energy is investigated at the tau electromagnetic vertex. High statistics at B factories, and on top of the Upsilon resonances, allows a detailed investigation of CP-odd observables related to the tau-pair production. The contribution of the tau electric dipole moment is considered in detail. We perform an analysis independent from the high energy data by means of correlation and linear spin observables at low energy. We show that different CP-odd asymmetries, associated to the normal-transverse and normal-longitudinal correlation terms can be measured at low energy accelerators, both at resonant and non resonant energies. These observables allow to put stringent and independent bounds to the tau electric dipole moment that are competitive with other high or low energy results.

The top quark right coupling in the tbW-vertex

The most general parametrization of the tbW vertex includes a right coupling