Arcadi Santamaria

Present and future bounds on non-standard neutrino interactions

We consider Non-Standard neutrino Interactions (NSI), described by four-fermion operators of the form (αγνβ)(γf), where f is an electron or first generation quark. We assume these operators are generated at dimension ≥ 8, so the related vertices involving charged leptons, obtained by an SU(2) transformation νδ→eδ, do not appear at tree level. These related vertices necessarily arise at one loop, via W exchange. We catalogue current constraints from sin 2θW measurements in neutrino scattering, from atmospheric neutrino observations, from LEP, and from bounds on the related charged lepton operators. We estimate future bounds from comparing KamLAND and solar neutrino data, and from measuring sin 2θW at the near detector of a neutrino factory. Operators constructed with νμ and νe should not confuse the determination of oscillation parameters at a νfactory, because the processes we consider are more sensitive than oscillations at the far detector. For operators involving ντ, we estimate similar sensitivities at the near and far detector.

Lepton flavour non-conservation at high energies in a superstring inspired standard model

Abstract We analyze some interesting phenomenological consequences of a class of models which extend the standard model by adding new neutral fermions. Such might arise in some models inspired on the heterotic superstring which have naturally light neutrinos. In these models it is possible to have sizable flavour changing decays of the Z to charged lepton pairs which might be detectable at SLC and/or LEP.

Gamma (Z ---> B anti-B): A Signature of Hard Mass Terms for a Heavy Top

Abstract We calculate analytically the weak radiative corrections to the weak neutral current gauge boson-bottom fermion vertex, keeping the mass mt of the internal fermion line for the relevant diagrams. We find, to order α, a hard mass-term dependence m t 2 M W 2 of the amplitude, for large mt values. Its origin comes from the unphysical charged Higgs coupling to fermions in the renormalizable gauge or, equivalently, from the longitudinal charged gauge boson couplings. The diagonal Z0 decay width to b-quarks decreases, due to these weak radiative corrections, by 0.6%–2.5% when the top mass mt varies from 45 to 200 GeV.

Spontaneous R parity violation in supersymmetry: A model for solar neutrino oscillations

Abstract Spontaneous breaking of R parity in the standard supergravity model implies the existence of a majoron which is strongly constrained by astrophysics. This is suggested as the origin of the small parameter required for the resonant amplification of neutrino oscillations to take place in the sun. To a good approximation only one neutrino type (most likely the tau neutrino) acquires a mass. Therefore the form of the leptonic charged current is very much restricted: it is fully described by two mixing angles and there is no CP violation. As a result, matter can only amplify oscillations in one channel which, for small mixings, is v l v τ . Agreement with the chlorine data on the one hand and the nonobservation of charged supersymmetric states in e + e - collisions on the other, tightly restricts the parameters of the model in a way that will be directly tested by collider experiments.

Prospects for the Zee-Babu Model at the CERN LHC and low energy experiments

We analyze the viability of the Zee-Babu model as an explanation of observed neutrino masses and mixings and the possibility that the model is confirmed or discarded in experiments planned for the very close future. The allowed parameter space is studied analytically by using some approximations and partial data. Then, a complete scanning of all parameters and constraints is performed numerically by using Monte Carlo methods. The cleanest signal of the model will be the detection of the doubly charged scalar at the LHC and its correlation with measurements of the branching ratio of {mu}{yields}e{gamma} at the MEG experiment. In addition, the model offers interesting predictions for {tau}{sup -}{yields}{mu}{sup +}{mu}{sup -}{mu}{sup -} experiments, lepton-hadron universality tests, the {theta}{sub 13} mixing in neutrino oscillations and the {sub ee} parameter of neutrinoless double beta decay.

Top quark mass from radiative corrections to the Z→bb¯ decay

The one-loop correction to the Zb b ¯ vertex presents a quadratic dependence on the top quark mass, which has its origin in the spontaneous symmetry breaking mechanism of the Standard Model. We study the possibility of fixing the top mass by comparing LEP measurements with theoretical predictions. Using the MS ¯ renormalization scheme, we calculate the top mass dependence of the Zb b ¯ vertex. For all Z-widths, we give simple approximate formulae which work at the 0.05% level. It is found that if some branching ratios involving the b-quark are measured at the 0.5% level the top mass will be determined with an error of 30 GeV using only LEP1 experiments.

Do the Quark Masses Run? Extracting m ¯ b ( m Z ) from CERN LEP Data

We present the first results of next-to-leading order QCD corrections to three-jet heavy quark production at the CERN e{sup +}e{sup {minus}} collider LEP including mass effects. Among other applications, this calculation can be used to extract the bottom-quark mass from LEP data and, therefore, to test the running of masses as predicted by QCD. {copyright} {ital 1997} {ital The American Physical Society}

Universal extra dimensions and Z → b b ¯

We study, at the one loop level, the dominant contributions from a single universal extra dimension to the process

αs(mZ) from τ decays with matching conditions at three loops

\stackrel{\ensuremath{\rightarrow}}{Z}b\overline{b}.

Effective Lagrangian approach to neutrinoless double beta decay and neutrino masses

By resorting to the gaugeless limit of the theory we explain why the result is expected to display a strong dependence on the mass of the top quark, not identified in the early literature. A detailed calculation corroborates this expectation, giving rise to a lower bound for the compactification scale which is comparable to that obtained from the