arXiv subjects

Enrico Nardi

Publications and source records attributed to Enrico Nardi.

At least 91 records · Page 5Linked to original sources

arXiv2002

On the neutrino vector and axial vector charge radius

A Majorana neutrino is characterized by just one flavor diagonal electromagnetic form factor: the anapole moment, that in the static limit corresponds to the axial vector charge radius . As is the case for the vector charge radius of a Dirac neutrino, proving that this is a well defined physical quantity is non trivial. I will first describe briefly the origin of the long standing controversy about the physical or non physical nature of the neutrino charge radius. Then I will argue that, in contrast to Dirac neutrino electromagnetic form factors, for Majorana neutrinos cosmological and astrophysical arguments do not provide useful informations on , and therefore this quantity has to be studied by means of terrestrial experiment. Finally, I will discuss the constraints that can be derived on for the tau neutrino from a comprehensive analysis of the data on single photon production off Z-resonance, and I will conclude with a few comments on $ν_μ$ scattering data from the NuTeV, E734, CCFR and CHARM-II collaborations and on the limits implied for for the muon neutrino.

hep-ph↗

arXiv2002

Bounds on the tau and muon neutrino vector and axial vector charge radius

A Majorana neutrino is characterized by just one flavor diagonal electromagnetic form factor: the anapole moment, that in the static limit corresponds to the axial vector charge radius . Experimental information on this quantity is scarce, especially in the case of the tau neutrino. We present a comprehensive analysis of the available data on the single photon production process $e^+ e^- -> ν\barνγ$ off Z-resonance, and we discuss the constraints that these measurements can set on for the tau neutrino. We also derive limits for the Dirac case, when the presence of a vector charge radius is allowed. Finally, we comment on additional experimental data on $ν_μ$ scattering from the NuTeV, E734, CCFR and CHARM-II collaborations, and estimate the limits implied for and for the muon neutrino.

hep-ph↗

arXiv2002

Pulsar acceleration by asymmetric emission of sterile neutrinos

A convincing explanation for the observed pulsar large peculiar velocities is still missing. We argue that any viable particle physics solution would most likely involve the resonant production of a non-interacting neutrino $ν_s$ of mass $m_{ν_s}\sim 20$--50 keV. We propose a model where anisotropic magnetic field configurations strongly bias the resonant spin flavour precession of tau antineutrinos into $ν_s$. For internal magnetic fields $B_{int} \gsim 10^{15}$ G a $\barν_τ$-$ν_s$ transition magnetic moment of the order of $10^{-12}$ Bohr magnetons is required. The asymmetric emission of $ν_s$ from the core can produce sizeable natal kicks and account for recoil velocities of several hundred kilometers per second.

astro-ph↗

arXiv2000

Horizontal $U(1)_H$ symmetry: a non-anomalous model

Spontaneously broken Abelian gauge symmetries can explain the fermion mass hierarchies of the minimal supersymmetric standard model. In most cases it is assumed that the $U(1)_H$ symmetry is anomalous. However, non-anomalous models are also viable and yield an interesting phenomenology. Cancellation of the gauge anomalies implies the following results: unification of leptons and down-type quarks Yukawa couplings is allowed at most for two generations. The $μ$ term is necessarily somewhat below the supersymmetry breaking scale. The superpotential has accidental $B$ and $L$ symmetries, and R-parity is automatically conserved in the supersymmetric limit. Anomaly canncellation also implies that the determinant of the quark mass matrix must vanish, wich is possible only if $m_{up}=0$. This solves the strong CP problem and provides an unambiguous low energy test of the model.

hep-ph↗

arXiv2000

Non anomalous U(1)_H gauge model of flavor

A non anomalous horizontal $U(1)_H$ gauge symmetry can be responsible for the fermion mass hierarchies of the minimal supersymmetric standard model. Imposing the consistency conditions for the absence of gauge anomalies yields the following results: i) unification of leptons and down-type quarks Yukawa couplings is allowed at most for two generations. ii) The $μ$ term is necessarily somewhat below the supersymmetry breaking scale. iii) The determinant of the quark mass matrix vanishes, and there is no strong $CP$ problem. iv) The superpotential has accidental $B$ and $L$ symmetries. The prediction $m_{\rm up}=0$ allows for an unambiguous test of the model at low energy.

hep-ph↗

arXiv1999

Neutrino propagation in matter with general interactions

We present a general analysis of the effective potential for neutrino propagation in matter, assuming a generic set of Lorentz invariant non-derivative interactions. We find that in addition to the known vector and axial vector terms, in a polarized medium also tensor interactions can play an important role. We compute the effective potential arising from a tensor interaction. We show that the components of the tensor potential transverse to the direction of the neutrino propagation can induce a neutrino spin-flip, similar to the one induced by a transverse magnetic field.

hep-ph↗

arXiv1998

Unconventional superstring derived E$_{\bf 6}$ models and neutrino phenomenology

Conventional superstring derived E$_6$ models can accommodate small neutrino masses if a discrete symmetry is imposed which forbids tree level Dirac neutrino masses but allows for radiative mass generation. Since the only possible symmetries of this kind are known to be generation dependent, we explore the possibility that the three sets of light states in each generation do not have the same assignments with respect to the 27 of $E_6$, leading to non universal gauge interactions under the additional $U(1)'$ factors for the known fermions. We argue that models realising such a scenario are viable, with their structure being constrained mainly by the requirement of the absence of flavor changing neutral currents in the Higgs sector. Moreover, in contrast to the standard case, rank 6 models are not disfavoured with respect to rank 5. By requiring the number of light neutral states to be minimal, these models have an almost unique pattern of neutrino masses and mixings. We construct a model based on the unconventional assignment scenario in which (with a natural choice of the parameters) $m_{\nut}\sim O(10)$eV is generated at one loop, $m_{\num}$ is generated at two loops and lies in a range interesting for the solar neutrino problem, and $\nue$ remains massless. In addition, since baryon and lepton number are conserved, there is no proton decay in the model. To illustrate the non-standard phenomenology implied by our scheme we also discuss a second scenario in which an attempt for solving the solar neutrino puzzle with matter enhanced oscillations and practically massless neutrinos can be formulated, and in which peculiar effects for the $\num$ --> $\nut$ conversion of the upward-going atmospheric neutrinos could arise as well.

hep-ph↗

arXiv1998

Signals of Unconventional E$_6$ Models at $e^+e^-$ Colliders

Generation dependent discrete symmetries often appear in models derived from superstring theories. In particular, in the framework of E$_6$ models the presence of such symmetries is required in order to allow for the radiative generation of naturally small neutrino masses. Recently it was shown that by imposing suitable generation dependent discrete symmetries, a class of models can be consistently constructed in which the three sets of known fermions in each generation do not have the same assignments with respect to the {\bf 27} representation of E$_6$. In this scenario, the different embedding in the gauge group of the three generations implies in particular that the known charged leptons couple in a non--universal way to the new neutral gauge bosons $(Z_β)$ present in these models. We exploit this fact to study the signature of this class of models at present and future $e^+e^-$ colliders. We show that some signals of deviation from lepton universality as well as some other discrepancies with the standard model predictions which have been observed at the TRISTAN collider in the production rate of $μ$ and $τ$, can be accounted for if the $Z_β$ mass is not much heavier than 300 GeV. We also study the discovery limits for lepton universality violation of this type at LEP-2 and at the 500 GeV $e^+e^-$ Next Linear Collider (NLC). We show that models predicting unconventional assignments for the leptons will give an unmistakable signature, when the $Z_β$ mass is as heavy as $\sim 800$ GeV (LEP-2) and $\sim 2$ TeV (NLC).

hep-ph↗

arXiv1998

$B\toτμ(X)$ decays in SUSY models without R-parity

Being strictly forbidden in the standard model, experimental detection of the lepton flavor violating decays $B(\bar B)\toτ^+μ^-$ and $b(\bar b)\to Xτ^+μ^-$ would constitute an unmistakable indication of new physics. We study these decays in supersymmetric models without R-parity and without lepton number. In order to derive order of magnitude predictions for the branching ratios, we assume a horizontal U(1) symmetry with horizontal charges chosen to explain the magnitude of fermion masses and quark mixing angles. We find that the branching ratios for decays with a $τμ$ pair in the final state are not particularly suppressed with respect to the lepton flavor conserving channels. In general in these models ${\rm B}[b\toμ^+μ^-(X)]\lsim {\rm B}[b(\bar b)\toτ^+μ^-(X)] \lsim {\rm B}[b\toτ^+τ^-(X)]$. While in some cases the rates for final states $τ^+τ^-$ can be up to one order of magnitude larger than the lepton flavor violating channel, due to better efficiencies for muon detection and to the absence of standard model contributions, decays into $τμ$ final states appear to be better suited to reveal this kind of new physics.

hep-ph↗

arXiv1998

Searching for New Physics in Rare $B \to τ$ Decays

The rare decays {$B^- \to τ\barν$}, \mbox{$B \to τ^+ τ^-$}, {$b \to X ν\barν$} and \mbox{$b \to X τ^+ τ^-$} all contain third generation leptons in the final state, and hence are sensitive to new physics that couples more strongly to the third family. We present model independent expressions for these decays that can be useful to study several types of new physics effects. We concentrate on supersymmetric models without R-parity and without lepton number. We also assume a horizontal U(1) symmetry with fermion horizontal charges chosen to explain the magnitude of fermion masses and quark mixing angles. This allows us to estimate the order of magnitude of the new effects, and to derive numerical predictions for the various decay rates and for the forward-backward asymmetry and the $τ$ polarization components measurable in \mbox{$b \to X τ^+ τ^-$}. In some cases the branching ratios are enhanced by more than one order of magnitude, rendering foreseeable their detection at upcoming B-factories. We also discuss how a measurement of asymmetries in \mbox{$b \to X τ^+ τ^-$} can be crucial in distinguishing between different sources of new physics.

hep-ph↗

arXiv1997

Renormalization Group Induced Neutrino Mass in Supersymmetry without R-parity

We study supersymmetric models without R parity and with universal soft supersymmetry breaking terms. We show that as a result of the renormalization group flow of the parameters, a misalignment between the directions in field space of the down-type Higgs vacuum expectation value $v_d$ and of the $μ$ term is always generated. This misalignment induces a mixing between the neutrinos and the neutralinos, resulting in one massive neutrino. By means of a simple approximate analytical expression, we study the dependence on the different parameters that contribute to the misalignment and to $m_ν$. In large part of the parameter space this effect dominates over the standard one-loop contributions to $m_ν$; we estimate 1 MeV $\lsim m_ν\lsim 1 GeV$. Laboratory, cosmological and astrophysical constraints imply $m_ν\lsim 100 eV$. To be phenomenologically viable, these models must be supplemented with some additional mechanism to ensure approximate alignment and to suppress $m_ν$.

hep-ph↗

arXiv1996

$B \toτ^+ τ^- (X)$ decays: first constraints and phenomenological implications

The study of $B \to τ^+ τ^- (X)$ decays can provide us with a better understanding of the third generation, and can be a useful probe of physics beyond the standard model. We present a model independent analysis of these decays. We classify new physics that can largely enhance the decay rates and we discuss the constraints implied by other processes. Experimentally, flavor changing neutral current $B$ decays into final state $τ$'s are still unconstrained. Searches for $B$ decays with large missing energy at LEP provide the first limits. We estimate that existing data already imply bounds on the $B_d \to τ^+ τ^-$, $B_s \to τ^+ τ^-$, and $B \to X τ^+ τ^-$ decay rates at the few percent level. Although these bounds are over four orders of magnitude above the standard model predictions, they provide the first constraints on some leptoquarks, and on some R-parity violating couplings.

hep-ph↗

arXiv1996

First limit on inclusive $B \to X_s ν\barν$ decay and constraints on new physics

The inclusive $B \to X_s ν\barν$ decay rate, on which no experimental bound exists to date, can be constrained by searching for large missing energy events in $B$ decays. Carefully examining the experimental and theoretical aspects of such an analysis, we argue that the published ALEPH limit on $\BR(B \to τ\barν)$ implies, conservatively, the bound $\BR(B \to X_s ν\barν)<3.9\times10^{-4}$, which is less than one order of magnitude above the standard model prediction. The LEP collaborations could significantly improve this bound by a dedicated experimental analysis. We study the constraints this new limit imposes on various extensions of the standard model. We derive new bounds on the couplings of third generation fermions in models with leptoquarks, and in supersymmetric models without R-parity. We also constrain models where new gauge bosons are coupled dominantly to the third generation, such as TopColor models and models based on horizontal gauge symmetries. For models which predict an enhanced effective $bsZ$ vertex, the constraint from $B \to X_s ν\barν$ is competitive with the limits from inclusive and exclusive $B \to X_s \ell^+ \ell^-$ decays.

hep-ph↗

arXiv1996

Neutrino Masses and Mixing in Supersymmetric Models without $R$ Parity

We study neutrino masses and mixing in Supersymmetric Models without $R$ parity and with generic soft Supersymmetry breaking terms. Neutrinos acquire mass from various sources: tree level neutrino--neutralino mixing, loop effects and non--renormalizable operators. Abelian horizontal symmetries (invoked to explain the smallness and hierarchy in quark parameters) replace $R$ parity in suppressing neutrino masses. We find lower bounds on the mixing angles: $\sinθ_{ij} \gsim m(\ell_i^-)/m(\ell_j^-)$ ($i<j$) and unusual order of magnitude predictions for neutrino mass ratios: $m(ν_e)/m(ν_μ)\sim\sin^2θ_{12}$; $m(ν_i)/m(ν_τ)\sim 10^{-7} \sin^2θ_{i3}$ ($i=1,2$). Bounds from laboratory experiments exclude $m_{ν_τ} \gsim 3\ MeV$ and the cosmological constraint that excludes $m_{ν_τ} \gsim 100\ eV$ is not evaded. Neither the solar nor the atmospheric neutrino problems are likely to be solved by $ν_μ-ν_e$ oscillations. These conclusions can be evaded if holomorphy plays an important role in the lepton Yukawa couplings.

hep-ph↗

arXiv1995

Top-Charm flavor changing contributions to the effective $bsZ$ vertex

We analyze the effects of a tree level flavor changing $tcZ$ vertex induced by a mixing with new isosinglet $Q=2/3$ quarks, on the effective $bsZ$ vertex. We compute the contributions arising from the new electroweak penguin diagrams involving one insertion of the $tcZ$ vertex. We show that a generalized GIM mechanism ensures the cancellation of the mass independent terms as well as of the new divergences. Unexpectedly, the presence of a $tcZ$ coupling cannot enhance the rates for the $Z$ mediated flavor changing decays \bsll\ and \bsnn, implying that these processes cannot be used to set limits on the $tcZ$ coupling. The additional effects of the heavy isosinglets are compared with the well studied effects of new isodoublets appearing in multi-generational models.

hep-ph↗

arXiv1995

Supersymmetry without R-Parity and without Lepton Number

We investigate Supersymmetric models where neither R parity nor lepton number is imposed. Neutrino masses can be kept highly suppressed compared to the electroweak scale if the $μ$-terms in the superpotential are aligned with the SUSY-breaking bilinear $B$-terms. This situation arises naturally in the framework of horizontal symmetries. The same symmetries suppress the trilinear R parity violating terms in the superpotential to an acceptable level.

hep-ph↗

arXiv1995

Natural R Parity Conservation with Horizontal Symmetries. a Four Generation Model

In most supersymmetric models the stability of the proton is ensured by invoking R-parity. A necessary ingredient to enforce R-parity is the possibility of distinguishing the lepton superfields from the Higgs ones. This is generally achieved either by assuming different charges under some matter parity, or by assigning the superfields to different representations of a unified gauge group. We want to put forward the idea that the replica of the fermion generations, which constitute an intrinsic difference between the fermions and the Higgs superfields, can give a clue to understand R-parity as an accidental symmetry. More ambitiously, we suggest a possible relation between proton stability and the actual number of fermion generations. We carry out our investigation in the framework of non-Abelian horizontal gauge symmetries. We identify $SU(4)_H$ as the only acceptable horizontal gauge group whichcan naturally ensure the absence of R parity violating operators, without conflicting with other theoretical and phenomenological constraints. We analyze a version of the supersymmetric standard model equipped with a gauged horizontal $SU(4)_H$, in which R-parity is accidental. The model predicts four families of fermions, it allows for the dynamical generation of a realistic hierarchy of fermion masseswithout any ad hoc choice of small Yukawa couplings, it ensures in a natural way the heaviness of all the fourth family fermions (including the neutrino) and it predicts a {\it lower} limit for the $τ$-neutrino mass of a few eV. The scale of the breaking of the horizontal symmetry can be constrained rather precisely in a narrow window around $\sim 10^{11}$ GeV. Some interesting astrophysical and cosmological implications of the model are addressed as well.

hep-ph↗

arXiv1995

Realistic SUSY model with four fermion families, natural R parity and $ν_τ$ in the eV Range

We study an extension of the supersymmetric standard model with four families and gauged horizontal symmetry $SU(4)_H$, in which R parity automatically follows as a consequence of gauge invariance and of the field content of the model. Beyond ensuring R parity conservation, the horizontal symmetry allows for the dynamical generation of a hierarchical pattern of fermion masses, without the need of any ad hoc choice of small Yukawa couplings. The structure of the mass matrices implies that the fourth family does not mix with the other three, and that the $b',t',τ',ν'$ masses are all naturally in the 100 GeV range. The model has other interesting phenomenological implications. The scale of the horizontal symmetry breaking is constrained by cosmological and astrophysical arguments to be $\sim 10^{11}$ GeV. Then the late $b'$ decays could explain a magnitude of the isotropic cosmic gamma flux. In addition, a lower bound of a few eV is found for the mass of the $τ$-neutrino, which can thus provide a hot component for the cosmological dark matter. Due to R parity conservation, the lightest supersymmetric particle is stable and can provide the cold component. The $ν_e$ and $ν_μ$ masses can naturally be in the correct ranges for a solution of the solar or atmospheric neutrino problems via neutrino oscillations.

hep-ph↗