Showing papers on "Lepton published in 1998"

Baryogenesis via neutrino oscillations

Abstract: We propose a new mechanism of leptogenesis in which the asymmetries in lepton numbers are produced through the $\mathrm{CP}$-violating oscillations of ``sterile'' (electroweak singlet) neutrinos. The asymmetry is communicated from singlet neutrinos to ordinary leptons through their Yukawa couplings. The lepton asymmetry is then reprocessed into baryon asymmetry by electroweak sphalerons. We show that the observed value of baryon asymmetry can be generated in this way, and the masses of ordinary neutrinos induced by the seesaw mechanism are in the astrophysically and cosmologically interesting range. Except for singlet neutrinos, no physics beyond the standard model is required.

Neutrino masses and leptogenesis with heavy Higgs triplets

Abstract: A simple and economical extension of the minimal standard electroweak gauge model (without right-handed neutrinos) by the addition of two heavy Higgs scalar triplets would have two significant advantages. Naturally small Majorana neutrino masses would become possible, as well as leptogenesis in the early universe which gets converted at the electroweak phase transition into the present observed baryon asymmetry.

Phenomenology of Massive Vectorlike Doublet Leptons

Abstract: Massive vectorlike electroweak doublets are generic in many extensions of the standard model. Even though one member of the doublet is necessarily electrically charged these particles are not easily detected in collider experiments. The neutral and charged states within the doublet are split by electroweak symmetry breaking. In the absence of mixing with other states, the radiatively generated splitting is in the range 200{endash}350 MeV for m{approx_gt} (1) /(2) thinspm{sub Z} . The charged state decays to the neutral one with an O()>cm decay length, predominantly by emission of a soft charged pion. The best possibility to detect these massive charged particles is to trigger on hard initial state radiation and search for two associated soft charged pions with displaced vertices. The mass reach for this process at LEPII is limited by luminosity rather than kinematics. {copyright} {ital 1998} {ital The American Physical Society}

Numerical simulations of string networks in the Abelian-Higgs model

Abstract: We present the results of a field theory simulation of networks of strings in the Abelian-Higgs model. From a random initial configuration the resulting vortex tangle approaches a self-similar regime in which the length density of lines of zeros of reduces as t-2. The network loses energy directly into scalar and gauge radiations supporting a recent claim that particle production, not gravitational radiation, is the dominant energy loss mechanism for cosmic strings. This means that cosmic strings in grand unified theories are severely constrained by high energy cosmic ray fluxes: Either they are ruled out, or an implausibly small fraction of their energy ends up in quarks and leptons.

Minimality Condition and Atmospheric Neutrino Oscillations

Abstract: A structure is proposed for the mass matrices of the quarks and leptons that arises in a natural way from the assumption that the breaking of $\mathrm{SO}(10)$ gauge symmetry is achieved by the smallest possible set of vacuum expectation values. This structure explains well many features of the observed spectrum of quarks and leptons. It reproduces the Georgi-Jarlskog mass relations and leads to a charm quark mass in reasonable agreement with data. It also predicts a large mixing angle between ${\ensuremath{ u}}_{\ensuremath{\mu}}$ and ${\ensuremath{ u}}_{\ensuremath{\tau}}$, as suggested by atmospheric neutrino data. The mixing angles of the electron neutrino are predicted to be small.

Large Leptonic Flavor Mixing and the Mass Spectrum of Leptons

Abstract: Implications of a simple model for the mass generation of leptons are studied, in particular for the upcoming long-baseline neutrino experiments. The flavor mixing angles are large (nearly maximal). The probability for the long-baseline u_\mu \leftrightarrow u_e oscillation is predicted to be about 1%.

Search for anomalous production of di-lepton events with missing transverse momentum in e+e- collisions at √S = 161 and 172 GeV

Abstract: Events containing two charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb\(^{-1}\) at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising from \(\mathrm{W}^+\mathrm{W}^-\) production with each W decaying l eptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented.

Helicity Amplitudes for ${\cal O}(\alpha_s)$ Production of $W^+ W^-, W^\pm Z, Z Z, W^\pm \gamma,$ or $Z \gamma$ Pairs at Hadron Colliders

Abstract: We present the one-loop QCD corrections to the helicity amplitudes for the processes $q\qb \to W^+ W^-, Z Z, W^\pm Z, W^\pm \gamma$, or $Z \gamma$, including the subsequent decay of each massive vector boson into a pair of leptons. We also give the corresponding tree-level amplitudes with an additional gluon radiated off the quark line. Together, these amplitudes provide all the necessary input for the calculation of the next-to-leading order QCD corrections to the production of any electroweak vector boson pair at hadron colliders, including the full spin and decay angle correlations.

The Interplay between Proto-Neutron Star Convection and Neutrino Transport in Core-Collapse Supernovae

Abstract: We couple two-dimensional hydrodynamics to realistic one-dimensional multigroup flux-limited diffusion neutrino transport to investigate proto-neutron star convection in core-collapse supernovae, and more specifically, the interplay between its development and neutrino transport. Our initial conditions, time-dependent boundary conditions, and neutrino distributions for computing neutrino heating, cooling, and deleptonization rates are obtained from one-dimensional simulations that implement multigroup flux-limited diffusion and one-dimensional hydrodynamics. The development and evolution of proto-neutron star convection are investigated for both 15 and 25 M☉ models, representative of the two classes of stars with compact and extended iron cores, respectively. For both models, in the absence of neutrino transport, the angle-averaged radial and angular convection velocities in the initial Ledoux unstable region below the shock after bounce achieve their peak values in ~20 ms, after which they decrease as the convection in this region dissipates. The dissipation occurs as the gradients are smoothed out by convection. This initial proto-neutron star convection episode seeds additional convectively unstable regions farther out beneath the shock. The additional proto-neutron star convection is driven by successive negative entropy gradients that develop as the shock, in propagating out after core bounce, is successively strengthened and weakened by the oscillating inner core. The convection beneath the shock distorts its sphericity, but on the average the shock radius is not boosted significantly relative to its radius in our corresponding one-dimensional models. In the presence of neutrino transport, proto-neutron star convection velocities are too small relative to bulk inflow velocities to result in any significant convective transport of entropy and leptons. This is evident in our two-dimensional entropy snapshots, which in this case appear spherically symmetric. The peak angle-averaged radial and angular convection velocities are orders of magnitude smaller than they are in the corresponding "hydrodynamics-only" models. A simple analytical model supports our numerical results, indicating that the inclusion of neutrino transport reduces the entropy-driven (lepton-driven) convection growth rates and asymptotic velocities by a factor ~3 (50) at the neutrinosphere and a factor ~250 (1000) at ρ = 1012 g cm-3, for both our 15 and 25 M☉ models. Moreover, when transport is included, the initial postbounce entropy gradient is smoothed out by neutrino diffusion, whereas the initial lepton gradient is maintained by electron capture and neutrino escape near the neutrinosphere. Despite the maintenance of the lepton gradient, proto-neutron star convection does not develop over the 100 ms duration typical of all our simulations, except in the instance where "low-test" intial conditions are used, which are generated by core-collapse and bounce simulations that neglect neutrino-electron scattering and ion-ion screening corrections to neutrino-nucleus elastic scattering. Models favoring the development of proto-neutron star convection either by starting with more favorable, albeit artificial (low-test), initial conditions or by including transport corrections that were ignored in our "fiducial" models were considered. Our conclusions nonetheless remained the same. Evidence of proto-neutron star convection in our two-dimensional entropy snapshots was minimal, and, as in our fiducial models, the angle-averaged convective velocities when neutrino transport was included remained orders of magnitude smaller than their counterparts in the corresponding hydrodynamics-only models.

New Limits to the Infrared Background: Bounds on Radiative Neutrino Decay and on Contributions of Very Massive Objects to the Dark Matter Problem

Abstract: From considering the effect of γ-γ interactions on recently observed TeV gamma-ray spectra, improved limits are set to the density of extragalactic infrared photons which are robust and essentially model independent. The resulting limits are more than an order of magnitude more restrictive than direct observations in the 0.025–0.3 eV regime. These limits are used to improve constraints on radiative neutrino decay in the mass range above 0.05 eV and to rule out very massive objects as providing the dark matter needed to explain galaxy rotation curves. Lower bounds on the maximum distance which TeV gamma rays may probe are also derived.

Zee Mass Matrix and Bi-Maximal Neutrino Mixing

Abstract: We investigate neutrino masses and mixings within the framework of the Zee mass matrix, with three lepton flavors. It is shown that the bi-maximal solution is the only possibility to reconcile atmospheric and solar neutrino data, within this ansatz. We obtain two almost degenerate neutrinos, which are mixtures of all three neutrino flavors, with heavy masses $\simeq \sqrt{\Delta m_{atm}^2}$. The predicted mass of the lightest neutrino, which should consist mostly of $ u_{\mu}$ and $ u_{\tau}$, is $\simeq \Delta m_{\odot}^2/(2\sqrt{\Delta m_{atm}^2})$.

Approximate treatment of lepton distortion in charged-current neutrino scattering from nuclei

Abstract: The partial-wave expansion used to treat the distortion of scattered electrons by the nuclear Coulomb field is simpler and considerably less time-consuming when applied to the production of muons and electrons by low- and intermediate-energy neutrinos. For angle-integrated cross sections, however, a modification of the {open_quotes}effective-momentum{close_quotes} approximation seems to work so well that for muons the full distorted-wave treatment is usually unnecessary, even at kinetic energies as low as 1 MeV and in nuclei as heavy as lead. The method does not work as well for electron production at low energies, but there a Fermi function often proves perfectly adequate. Scattering of electron neutrinos from muon decay on iodine and of atmospheric neutrinos on iron is discussed in light of these results. {copyright} {ital 1998} {ital The American Physical Society}

Supersymmetry reach of Fermilab Tevatron upgrades: The largetanβcase

Abstract: The Yukawa couplings of the tau lepton and the bottom quark become comparable to, or even exceed, electroweak gauge couplings for large values of the supersymmetry parameter tanthinsp{beta}. As a result, the lightest tau slepton {tilde {tau}}{sub 1} and bottom squark {tilde b}{sub 1} can be significantly lighter than corresponding sleptons and squarks of the first two generations. Gluino, chargino, and neutralino decays to third generation particles are significantly enhanced when tanthinsp{beta} is large. This affects projections for collider experiment reach for supersymmetric particles. In this paper, we evaluate the reach of the Fermilab Tevatron p{bar p} collider for supersymmetric signals in the framework of the minimal supergravity model. We find that the reach via signatures with multiple isolated leptons ({ital e} and {mu}) is considerably reduced. For very large tanthinsp{beta}, the greatest reach is attained in the multijet+E{sub T}{sup miss} signature. Some significant extra regions may be probed by requiring the presence of an identified {ital b} jet in jets+E{sub T}{sup miss} events, or by requiring one of the identified leptons in clean trilepton events to actually be a hadronic 1 or 3 charged prong tau. In an appendix, we present formulas for chargino, neutralino, and gluino three bodymore » decays which are valid at large tanthinsp{beta}. {copyright} {ital 1998} {ital The American Physical Society}« less

Long baseline neutrino oscillation experiments and CP violation in the lepton sector

Abstract: We discuss possibilities to investigate the effects of CP (and T) violation in the lepton sector in neutrino oscillation experiments. We consider the effects of CP violation in the framework of two schemes of mixing of four massive neutrinos that can accommodate the results of all neutrino oscillation experiments. Using the constraints on the mixing parameters that follow from the results of short-baseline neutrino oscillation experiments, we derive rather strong upper bounds on the effects of CP violation in nu_mu nu_e transitions in long-baseline neutrino oscillation experiments. We show that the effects of CP violation in nu_mu nu_tau transitions in long-baseline oscillation experiments can be as large as is allowed by the unitarity of the mixing matrix. The matter effects, which complicate the problem of searching for CP violation in long-baseline experiments, are discussed in detail. We consider the T-odd asymmetries whose measurement could allow to reveal T and CP violation in the lepton sector independently from matter effects.

Gamma-ray and neutrino flares produced by protons accelerated on an accretion disk surface in AGN

Abstract: We discuss the consequences of almost rectilinear acceleration of protons to extremely high energies in a reconnection region on the surface of an accretion disk which surrounds a central black hole in an active galaxy. The protons produce $\gamma$-rays and neutrinos in interactions with the disk radiation as considered in several previous papers. However, in this model the secondary $\gamma$-rays can initiate cascades in the magnetic and radiation fields above the disk. We compute the spectra of $\gamma$-rays and neutrinos emerging from regions close to the disk surface. Depending on the parameters of the reconnection regions, this model predicts the appearance of $\gamma$-ray and neutrino flares if protons takes most of the energy from the reconnection region. In contrast, if leptons take most of the energy, they produce pure $\gamma$-ray flares. The $\gamma$-ray spectrum expected in the case of hadronic cascading is compared with the spectrum observed during the flare in June 1991 from 3C 279. The neutrino flares which should accompany these gamma-ray flares may be detected by future large scale neutrino telescopes sensitive at $\sim 10^{5}$ TeV.

Search for SUSY in (Leptons +) Jets + E_T^miss final states

Abstract: We study the observability of the squarks and gluinos in CMS at LHC. Classical E_T^miss + jets final state as well as a number of additional multilepton signatures (0 leptons, 1 lepton, 2 leptons of the same sign, 2 leptons of the opposite sign and 3 leptons) are investigated . The detection of these sparticles relies on the observation of an excess of events over Standard Model background expectations. The study is made in the framework of a minimal SU(5) mSUGRA model as a function of m_0, m_1/2 for 4 sets of model parameters : tan(beta) = 2 or 35 and sign(mu) = +/- 1 and for fixed value of A_0 = 0. The CMS detector response is modelled using CMSJET 4.51 fast MC code (non-GEANT). The results obtained are presented as 5 sigma detection contours in the m_0, m_1/2 planes and with optimized selection cuts in various regions of the parameter space. The result of these investigations is that with integrated luminosity L=10^5 pb^-1 the squark and gluino mass reach is about 2.5 TeV and covers most of the interesting parts of parameter space according to neutralino relic density expectations. The influence of signal and background cross-section uncertainties on the reach contours is estimated. The effect of pile-up on signal and background is also discussed. This effect is found to be insignificant for E_T^miss and single lepton signatures, whilst only a minor deterioration is seen for multilepton final states.

Explicit Quark-hadron Duality in Heavy-light Meson Weak Decays in the 't Hooft Model

Abstract: We compute the nonleptonic weak decay width of a heavy-light meson in 1+1 spacetime dimensions with a large number of QCD colors (the {close_quote}t Hooft model) as a function of the heavy quark mass. In this limit, QCD is exactly soluble, and decay modes are dominated by two-particle final states. We compare the results to the tree-level partonic decay width of the heavy quark in order to test quark-hadron duality in this universe. We find that this duality is surprisingly well satisfied in the heavy quark limit, in that the difference between the sum of exclusive partial widths and the tree-level partonic width approaches a constant as M{r_arrow}{infinity}, and the deviation is well-fit by a small 1/M correction. We comment on the meaning of this conclusion and its implications for the use of quark-hadron duality in hadronic physics. {copyright} {ital 1997} {ital The American Physical Society}

An upper limit on the tau neutrino mass from three- and five-prong tau decays

Abstract: A bound on the tau neutrino mass is established using the data collected from 1991 to 1995 at \(\sqrt{s} \simeq m_{Z}\) with the ALEPH detector. Two separate limits are derived by fitting the distribution of visible energy vs invariant mass in \(\tau^{-} \rightarrow 2\pi^{-} \pi^{+} u_{\tau}\) and \(\tau^{-} \rightarrow 3\pi^{-} 2\pi^{+} (\pi^{0}) u_{\tau}\) decays. The two results are combined to obtain a 95% confidence level upper limit of 18.2 MeV/c\(^2\) on the mass of the tau neutrino.