arXiv subjects

J. Bernabeu

Publications and source records attributed to J. Bernabeu.

82 records · Page 5Linked to original sources

arXiv1996

Constraints on the Wr mass and CP-violation in left-right models

We update the constraints on the right-handed $W_R$ gauge boson mass, mixing angle $ζ$ with the left-handed $W_L$ gauge boson, and other parameters in general left-right symmetric models with different mechanisms of CP violation. Constraints mostly independent of any assumption on the quark sector are obtained from a re-analysis of muon decay data. Estimates of the left-right hadronic matrix elements in the neutral kaon system and their uncertainties are revised using large $N_c$ and Chiral Perturbation Theory arguments. With explicitly given assumptions on the long-distance $(ΔS=1)^2$ contributions to the $K_L-K_S$ mass difference, lower bounds on $M_{W_R}$ are obtained. With the same assumptions, one also gets strong upper bounds from the CP-violating parameter $ε_K$, for most of the parameter space of left-right models where the right-handed third family does not contribute in CP-violating quantities.

hep-ph↗

arXiv1996

Left-handed neutrino disappearance probe of neutrino mass and character

We explore the sensitivity to a non vanishing neutrino mass offered by dynamical observables, i.e., branching ratios and polarizations. The longitudinal polarization in the C.M. frame decreases by a 4% for $D^+ \rightarrow τ^+ ν_τ$ and $m_{ν_τ}=24$ MeV. Taking advantage of the fact that the polarization is a Lorentz variant quantity, we study the polarization effects in a boosted frame. By means of a neutrino beam, produced by a high velocity boosted parent able to flip the neutrino helicity, we find that an enhanced left-handed neutrino deficit, induced by a Wigner rotation, appears.

hep-ph↗

arXiv1996

Spontaneous CP violation in the left-right model and the kaon system

A left-right model with spontaneous $CP$ breakdown, consistent with the particle physics phenomenology, is presented. Constraints on free parameters of the model: mass of the new right handed gauge boson $M_2$ and ratio $r$ of the two vacuum expectation values of the bidoublet, are found from the measurement of $ε$ in the kaon system. For most of the parameter space, $M_2$ is restricted to be below 10 TeV. Higher masses can be achieved only by fine tuning of Kobayashi-Maskawa matrix elements, quark masses, $r$ and the phase $α$ which is the unique source of $CP$-violation in the model. Large number of combinations of signs of quark masses, which are observables of the model, are found to be not allowed since they contradict with data. The range of $ε'/ε$ the model predicts is around $10^{-4}$ in magnitude.

hep-ph↗

arXiv1996

Analytic O(alpha_s) Results for Bottom and Top Quark Production in e+ e- Collisions

We present a new derivation of the O(alpha_s) angular distribution of the outgoing $q$-quark in the production process $e^+ e^- \toγ,Z\to q\,\bar{q}(g)$. In our calculation, we express the three-particle phase-space integration of the gluon-bremsstrahlung process in terms of a general set of analytic integral solutions. A consistent treatment of the QCD one-loop corrections to the axial-vector current deserves special attention. This is relevant in the derivation of the forward-backward asymmetry predicted by the standard model. Finally, we provide the full analytical solutions for the differential rates in closed form and conclude with numerical estimates for bottom and top quark production.

hep-ph↗

arXiv1996

$γ$ - Z interferometry at a $Φ$-factory

We analyze the possibilities that the proposed $Φ$-factories offer to measure $γ-Z$ interference. In the unpolarized beam case, we study different signatures in the $ρπ$ channel, taking advantage of the presence of the near-by $a_1$ resonance. We build a C-odd forward-backward asymmetry, estimated to be around $10^{-5}$, and (P-even, T-even) and (P-odd, T-odd) alignments of the $ρ$, to be seen from the angular distribution of its $ππ$ decay products. With polarized electrons a left-right asymmetry around $2\times 10^{-4}$ is present in all channels. At leading order this asymmetry is independent of hadronic matrix elements and is sensitive to the $Z^0-s\bar{s}$ vector coupling. In the $ρπ$ channel, a combined left-right forward-backward asymmetry is considered.

hep-ph↗

arXiv1996

Spontaneous breakdown of CP in left-right symmetric models

We show that it is possible to obtain spontaneous CP violation in the minimal $SU(2)_L \times SU(2)_R \times U(1)_{B-L}$, i.e. in a left right symmetric model containing a bidoublet and two triplets in the scalar sector. For this to be a natural scenario, the non-diagonal quartic couplings between the two scalar triplets and the bidoublet play a fundamental role. We analyze the corresponding Higgs spectrum, the suppression of FCNC's and the manifestation of the spontaneous CP phase in the electric dipole moment of the electron.

hep-ph↗

arXiv1995

Weak magnetic dipole moments in two-Higgs-doublet models

We investigate the effects of the new scalars in a two-Higgs-doublet model on the weak magnetic dipole moments of the fermions at the $Z$ peak. Proportionality of the Yukawa couplings to the fermion masses, and to $\tanβ$, makes such effects more important for the third family, and potentially relevant. For the $τ$ lepton, the new diagrams are suppressed by $v_τ= 2 \sin^2 θ_W - 1/2$, or by powers of $m_τ/M_Z$, but may still be comparable to the SM electroweak contributions. In contrast, we find that the new contributions for the bottom quark may be much larger than the SM electroweak contributions. These new effects may even compete with the gluonic contribution, if the extra scalars are light and $\tan β$ is large. We also comment on the problem of the gauge dependence of the vertex, arising when the $Z$ is off mass shell. We compute the contributions from the new scalars to the magnetic dipole moments for top-quark production at the NLC, and for bottom and $ τ$ production at LEP2. In the case of the top, we find that the SM electroweak and gluonic contributions to the $Z t {\bar t}$ vertex are comparable. The new contributions may be of the same order of magnitude as the standard-model ones, but not much larger.

hep-ph↗

arXiv1995

Neutrino pair synchrotron radiation from relativistic electrons in strong magnetic fields

The emissivity for the neutrino pair synchrotron radiation in strong magnetic fields has been calculated both analytically and numerically for high densities and moderate temperatures, as can be found in neutron stars. Under these conditions, the electrons are relativistic and degenerate. We give here our results in terms of an universal function of a single variable. For two different regimes of the electron gas we present a simplified calculation and compare our results to those of Kaminker et al. Agreement is found for the classical region, where many Landau levels contribute to the emissivity , but some differences arise in the quantum regime. One finds that the emissivity for neutrino pair synchrotron radiation is competitive, and can be dominant, with other neutrino processes for magnetic fields of the order $B \sim 10^{14} - 10^{15} G $.This indicates the relevance of this process for some astrophysical scenarios, such as neutron stars and supernovae.

astro-ph↗

arXiv1993

Universality-Breaking Effects in Leptonic Z Decays

We analyze the possibility of universality violation in diagonal leptonic decays of the $Z$ boson, in the context of interfamily "see-saw" models. In a minimal extension of the Standard Model with right-handed neutrino fields, we find that universality-breaking effects increase quadratically with the heavy Majorana neutrino mass and may be observed in the running $LEP$ experiments.

hep-ph↗

arXiv1993

$μ$-$e$ conversion in nuclei and Z$^\prime$ physics

Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank $> 4$) often predict also new charged fermions. A mixing of the known fermions with new states with {\it exotic} weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree level flavour changing neutral interactions mediated by $Z$ exchange, while if the mixing is only with new states with {\it ordinary} weak-isospin assignments, the flavour changing neutral currents are mainly due to the exchange of the lightest new neutral gauge boson $Z^\prime$. We show that the present experimental limits on $μ-e$ conversion in nuclei give a nuclear-model-independent bound on the $Z$-$e$-$μ$ vertex which is twice as strong as that obtained from $μ\to e e e$. In the case of E$_6$ models these limits provide quite stringent constraints on the $Z^\prime$ mass and on the $Z-Z^\prime$ mixing angle. We point out that the proposed experiments to search for $μ-e$ conversion in nuclei have good chances to find evidence of lepton flavour violation, either in the case that new exotic fermions are present at the electroweak scale, or if a new neutral gauge boson $Z^\prime$ of E$_6$ origin lighter than a few TeV exists.

hep-ph↗