J. Bernabeu

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.

Non-decoupling of heavy neutrinos and lepton flavour violation

We consider a class of models predicting new heavy neutral fermionic states, whose mixing with the light neutrinos can be naturally significant and produce observable effects below the threshold for their production. We update the indirect limits on the flavour non-diagonal mixing parameters that can be derived from unitarity, and show that significant rates are in general expected for one-loop-induced rare processes due to the exchange of virtual heavy neutrinos, involving the violation of the muon and electron lepton numbers. In particular, the amplitudes for μ-e conversion in nuclei and for μ → ee+e− show a non-decoupling quadratic dependence on the heavy neutrino mass M, while μ → eγ is almost independent of the heavy scale above the electroweak scale. These three processes are then used to set stringent constraints on the flavour-violating mixing angles. In all the cases considered, we point out explicitly that the non-decoupling behaviour is strictly related to the spontaneous breaking of the SU(2) symmetry.

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.

The Physics Programme Of The MoEDAL Experiment At The LHC

The MoEDAL experiment at Point 8 of the LHC ring is the seventh and newest LHC experiment. It is dedicated to the search for highly-ionizing particle avatars of physics beyond the Standard Model, extending significantly the discovery horizon of the LHC. A MoEDAL discovery would have revolutionary implications for our fundamental understanding of the Microcosm. MoEDAL is an unconventional and largely passive LHC detector comprised of the largest array of Nuclear Track Detector stacks ever deployed at an accelerator, surrounding the intersection region at Point 8 on the LHC ring. Another novel feature is the use of paramagnetic trapping volumes to capture both electrically and magnetically charged highly-ionizing particles predicted in new physics scenarios. It includes an array of TimePix pixel devices for monitoring highly-ionizing particle backgrounds. The main passive elements of the MoEDAL detector do not require a trigger system, electronic readout, or online computerized data acquisition. The aim of this paper is to give an overview of the MoEDAL physics reach, which is largely complementary to the programs of the large multipurpose LHC detectors ATLAS and CMS.

Monochromatic neutrino beams

In the last few years spectacular results have been achieved with the demonstration of non vanishing neutrino masses and flavour mixing. The ultimate goal is the understanding of the origin of these properties from new physics. In this road, the last unknown mixing [Ue3] must be determined. If it is proved to be non-zero, the possibility is open for Charge Conjugation-Parity (CP) violation in the lepton sector. This will require precision experiments with a very intense neutrino source. Here a novel method to create a monochromatic neutrino beam, an old dream for neutrino physics, is proposed based on the recent discovery of nuclei that decay fast through electron capture. Such nuclei will generate a monochromatic directional neutrino beam when decaying at high energy in a storage ring with long straight sections. We also show that the capacity of such a facility to discover new physics is impressive, so that fine tuning of the boosted neutrino energy allows precision measurements of the oscillation parameters even for a [Ue3] mixing as small as 1 degree. We can thus open a window to the discovery of CP violation in neutrino oscillations.In the last few years spectacular results have been achieved with the demonstration of non vanishing neutrino masses and flavour mixing. The ultimate goal is the understanding of the origin of these properties from new physics. In this road, the last unknown mixing [Ue3] must be determined. If it is proved to be non-zero, the possibility is open for Charge Conjugation-Parity (CP) violation in the lepton sector. This will require precision experiments with a very intense neutrino source. Here a novel method to create a monochromatic neutrino beam, an old dream for neutrino physics, is proposed based on the recent discovery of nuclei that decay fast through electron capture. Such nuclei will generate a monochromatic directional neutrino beam when decaying at high energy in a storage ring with long straight sections. We also show that the capacity of such a facility to discover new physics is impressive, so that fine tuning of the boosted neutrino energy allows precision measurements of the oscillation parameters even for a [Ue3] mixing as small as 1 degree. We can thus open a window to the discovery of CP violation in neutrino oscillations.

Studying indirect violation of CP, T and CPT in a B-factory

In this work we analyze the observable asymmetries one can build from entangled B-meson states, in order to extract information on the parameters ε and δ which govern indirect violation of discrete symmetries. The traditionally proposed observables, based on flavour tags, are not helpful for the study of the Bd-system, where the tiny value of ∆Γ clears up such asymmetry effects. Our study makes instead use of CP tags in order to build new asymmetries where the different parameters can be separated out. For this separation, it is decisive to achieve a good time resolution in the measurement of entangled state decays. Nevertheless, even with no temporal information, as would be the case in a symmetric factory, it is still possible to extract some information on the symmetries of the system. We discuss both non-genuine and genuine observables, depending on whether absorptive parts can mimic or not asymmetry effects. 1 banuls@ific.uv.es 2 Jose.Bernabeu@uv.esAbstract In this work we analyze the observable asymmetries one can build from entangled B -meson states, in order to extract information on the parameters e and δ which govern indirect violation of discrete symmetries. The traditionally proposed observables, based on flavour tags, are not helpful for the study of the B d -system, where the tiny value of ΔΓ clears up such asymmetry effects. Our study makes instead use of CP tags in order to build new asymmetries where the different parameters can be separated out. For this separation, it is decisive to achieve a good time resolution in the measurement of entangled state decays. Nevertheless, even with no temporal information, as would be the case in a symmetric factory, it is still possible to extract some information on the symmetries of the system. We discuss both non-genuine and genuine observables, depending on whether absorptive parts can mimic or not asymmetry effects.

Tau spin correlations at the Z peak. Aplanarities of the decay products

Abstract The production of τ pairs at the Z peak leads to interesting dynamical information contained in their spin density matrix. Besides the longitudinal polarization of each τ lepton, there are two independent spin-spin correlations associated with the transverse (within the production plane) and normal (to the production plane) polarization components. The transverse-normal spin correlation is both a parity-odd and time reversal-odd observable. We analyze the correlated angular distribution of the decay products as polarization analyzers of the τ + τ − system. We find two azimuthal asymmetries of the decay products and the incoming beam, which are able to disentangle these spin correlation observables without reconstructing the τ direction.

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.

Intrinsic CPT violation and decoherence for entangled neutral mesons

Abstract We present a combined treatment of quantum-gravity-induced decoherence and intrinsic CPT violation in entangled neutral-kaon states. Our analysis takes into consideration two types of effects: first, those associated with the loss of particle–antiparticle identity, as a result of the ill-defined nature of the CPT operator, and second, effects due to the nonunitary evolution of the kaons in the space–time foam. By studying a variety of ϕ-factory observables, involving identical as well as general final states, we derive analytical expressions, to leading order in the associated CPT violating parameters, for double-decay rates and their time-integrated counterparts. Our analysis shows that the various types of the aforementioned effects may be disentangled through judicious combinations of appropriate observables in a ϕ factory.

Charge radius of the neutrino

Using the pinch technique we construct at one-loop order a neutrino charge radius, which is finite, depends neither on the gauge-fixing parameter nor on the gauge-fixing scheme employed, and is process-independent. This definition stems solely from an effective proper photon-neutrino one-loop vertex, with no reference to box or self-energy contributions. The role of the