Showing papers on "Lepton published in 1999"

New Limit for the Lepton-Family-Number Nonconserving Decay μ + -->e + γ

Abstract: An experiment has been performed to search for the muon- and electron-number non-conserving decay mu+ to e+_gamma. The upper limit for the branching ratio to be GAMMA(mu+ to e+_gamma)/GAMMA(mu+ to e+_nu_nubar) < 1.2e-11 with 90% confidence.

Angular distribution of neutron inverse beta decay, ν ¯ e + p → e + + n

Abstract: The reaction νe+p→e++n is very important for low-energy (Eν≲60 MeV) antineutrino experiments. In this paper we calculate the positron angular distribution, which at low energies is slightly backward. We show that weak magnetism and recoil corrections have a large effect on the angular distribution, making it isotropic at about 15 MeV and slightly forward at higher energies. We also show that the behavior of the cross section and the angular distribution can be well understood analytically for Eν≲60 MeV by calculating to O(1/M), where M is the nucleon mass. The correct angular distribution is useful for separating νe+p→e++n events from other reactions and detector backgrounds, as well as for possible localization of the source (e.g., a supernova) direction. We comment on how similar corrections appear for the lepton angular distributions in the deuteron breakup reactions νe+d→e++n+n and νe+d→e-+p+p. Finally, in the reaction νe+p→e++n, the angular distribution of the outgoing neutrons is strongly forward peaked, leading to a measurable separation in positron and neutron detection points, also potentially useful for rejecting backgrounds or locating the source direction.

Spin-polarized muons in condensed matter physics

Abstract: A positive muon is a spin-1/2 particle. Beams of muons with all their spins polarized can be prepared and subsequently implanted in various types of condensed matter. The subsequent precession and relaxation of their spins can then be used to investigate a variety of static and dynamic effects in a sample and hence to deduce properties concerning magnetism, superconductivity and molecular dynamics. Though strictly a lepton, and behaving essentially like a heavy electron, it is convenient to think of a muon as a light proton, and it is often found with a captured electron in a hydrogen-like atom known as muonium. This article outlines the principles of various experimental techniques which involve implanted muons and describes some recent applications. The use of muons in condensed matter physics has shed new light on subjects as diverse as passivation in semiconductors, frustrated spin systems, vortex lattice melting, and quantum diffusion of light particles.

Search for neutral heavy leptons in a high-energy neutrino beam

Abstract: A search for neutral heavy leptons (NHLs) has been performed using an instrumented decay channel at the NuTeV (E-815) experiment at Fermilab. The data were examined for NHLs decaying into muonic final states ({mu}{mu}{nu} , {mu}e{nu} , {mu}{pi} , and {mu}{rho} ); no evidence has been found for NHLs in the 0.25-2.0 GeV mass range. This analysis places limits on the mixing of NHLs with standard light neutrinos at a level up to an order of magnitude more restrictive than previous search limits in this mass range. (c) 1999 The American Physical Society.

Decay of pseudoscalars into lepton pairs and large N(c) QCD

Abstract: The counterterm combination that describes the decay of pseudoscalar mesons into charged lepton pairs at lowest order in chiral perturbation theory is considered within the framework of QCD in the limit of a large number of colours Nc. When further restricted to the lowest meson dominance approximation to large-Nc QCD, our results agree well with the available experimental data.

Gamma-ray and neutrino flares produced by protons accelerated on an accretion disc surface in active galactic nuclei

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

Cosmological implications of a relic neutrino asymmetry

Abstract: We consider some consequences of the presence of a cosmological lepton asymmetry in the form of neutrinos. A relic neutrino degeneracy enhances the contribution of massive neutrinos to the present energy density of the Universe, and modifies the power spectrum of radiation and matter. Comparing with current observations of cosmic microwave background anisotropies and large-scale structure, we derive some constraints on the neutrino degeneracy and on the spectral index in the case of a flat universe with a cosmological constant.

(S)neutrino properties in R-parity violating supersymmetry: I. CP-conserving Phenomena

Abstract: R-parity-violating supersymmetry (with a conserved baryon number B) provides a framework for particle physics with lepton number (L) violating interactions. We examine in detail the structure of the most general R-parity-violating (B-conserving) model of low-energy supersymmetry. We analyze the mixing of Higgs bosons with sleptons and the mixing of charginos and neutralinos with charged leptons and neutrinos, respectively. Implications for neutrino and sneutrino masses and mixing and CP-conserving sneutrino phenomena are considered. L-violating low-energy supersymmetry can be probed at future colliders by studying the phenomenology of sneutrinos. Sneutrino - antisneutrino mass splittings and lifetime differences can provide new opportunities to probe lepton number violation at colliders.

Lepton and quark mass matrices

Abstract: We propose a model that all quark and lepton mass matrices have the same zero texture. Namely, their (1,1), (1,3), and (3,1) components are zeros. The mass matrices are classified into two types. Type I is consistent with experimental data in the quark sector. For the lepton sector, if the seesaw mechanism is not used, type II allows a large ${\ensuremath{ u}}_{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{ u}}_{\ensuremath{\tau}}$ mixing angle. However, severe compatibility with all neutrino oscillation experiments forces us to use the seesaw mechanism. If we adopt the seesaw mechanism, it turns out that type I instead of type II can be consistent with experimental data in the lepton sector too.

Lepton flavor violation in the left-handed slepton production at future lepton colliders

Abstract: The Super-Kamiokande atmospheric neutrino data suggest the existence of a large lepton-flavor violating (LFV) interaction in the higher energy scale. If the minimal supersymmetric standard model is extended to have right-handed neutrinos, the left-handed sleptons in the second and third generations are expected to have LFV masses in the minimal supergravity scenario. In this article we study the LFV signals in the left-handed slepton production at ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ colliders and ${e}^{+}{e}^{\ensuremath{-}}$ linear colliders (LC's), ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{(e}^{+}{e}^{\ensuremath{-}})\ensuremath{\rightarrow}\ensuremath{\tau}\ensuremath{\mu}+4 \mathrm{jets}+E/$ and ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{(e}^{+}{e}^{\ensuremath{-}})\ensuremath{\rightarrow}\ensuremath{\tau}\ensuremath{\mu}l+2 \mathrm{jets}+E/$. The main background comes from decay of a tau lepton into a muon in the lepton-flavor conserving slepton pair production. They are significantly reduced by the energy and the impact parameter cuts for the muon. At ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ colliders (LC's) it may be possible to reach the mixing angle $\mathrm{sin}2{\ensuremath{\theta}}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{ u}}}\ensuremath{\gtrsim}0.1(0.5)$ and the mass difference $\ensuremath{\Delta}{m}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{ u}}}\ensuremath{\gtrsim}0.1(0.4)$ GeV for sneutrinos in the second and third generations at the statistical significance of $3\ensuremath{\sigma}$. Such a small mass difference and a mixing angle may be induced radiatively even if the Yukawa coupling constant for the tau neutrino is of the order of 0.1.

Lepton anomalous magnetic moments — a theory update

Abstract: Standard Model contributions to the electron, muon, and tau lepton anomalous magnetic moments, a 1 = (g l −2) 2 , are reviewed and upddated. The fine structure constant is obtained from the electron g e −2 and used to refine the QED contribution to the muon g μ −2 . Recent advances in electroweak and hadronic effects on g μ −2 are summarized. Examples of “New Physics” probed by the a μ Brookhaven experiment E821 are outlined. The prediction for a τ is also given.

Tau neutrino fluxes from atmospheric charm

Abstract: We present an evaluation of the atmospheric tau neutrino flux in the energy range between $10^2$ and $10^6$ GeV. The main source of tau neutrinos is from charmed particle production and decay. The $ u_\tau N\to \tau X$ event rate for a detector with a water equivalent volume of 1 km$^3$ is on the order of 60-100 events per year for $E_\tau>100$ GeV, reducing to 18 events above 1 TeV. Event rates for atmospheric muon neutrino oscillations to tau neutrinos are also evaluated.

Penetration of the Earth by Ultrahigh Energy Neutrinos Predicted by Low x QCD

Abstract: We calculate the cross sections for neutrino interactions with (isoscalar) nuclear targets in the energy domain all the way up to 10^12 GeV. Small x QCD effects are included by using a unified BFKL/DGLAP formalism which embodies non-leading log(1/x) contributions. The few free parameters which specify the input parton distributions are determined by fitting to HERA deep inelastic data. The attenuation of neutrinos transversing the Earth at different nadir angles is calculated for a variety of energy spectra for neutrinos originating from different sources (from Active Galactic Nuclei, Gamma ray bursts, top-down models), as well as for atmospheric neutrinos. For this purpose we solve the transport equation which includes regeneration due to neutral current neutrino interactions, besides attenuation.

Neutrino Masses in E_6 Unification

Abstract: We show that the neutrino masses with large mixing between u_\mu and u_\tau are naturally reproduced in a supersymmetric E_6 grand unification model with an anomalous U(1)_X symmetry. We propose a simple scenario which incorporates a novel mechanism called `E-twisting' by which all the characteristic features of the fermion mass matrices, not only the quark/lepton's Dirac masses but also the neutrino's Majorana masses, can be well reproduced despite all the members in 27 of each generation are assigned a common U(1)_X charge.

ICOOL: a simulation code for ionization cooling of muon beams

Abstract: Current ideas for designing a high luminosity muon collider require significant cooling of the phase space of the muon beams. The only known method that can cool the beams in a time comparable to the muon lifetime is ionization cooling. This method requires directing the particles in the beam at a large angle through a low Z absorber material in a strong focusing magnetic channel and then restoring the longitudinal momentum with an RF cavity. We have developed a new 3-D tracking code ICOOL for examining possible configurations for muon cooling. A cooling system is described in terms of a series of longitudinal regions with associated material and field properties. The tracking takes place in a coordinate system that follows a reference orbit through the system. The code takes into account decays and interactions of /spl sim/50-500 MeV/c muons in matter. Material geometry regions include cylinders and wedges. A number of analytic models are provided for describing the field configurations. Simple diagnostics are built into the code, including calculation of emittances and correlations, longitudinal traces, histograms and scatter plots. A number of auxiliary files can be generated for postprocessing analysis by the user.

The Small observed baryon asymmetry from a large lepton asymmetry

Abstract: Primordial Big-Bang Nucleosynthesis (BBN) tightly constrains the existence of any additional relativistic degrees of freedom at that epoch. However a large asymmetry in electron neutrino number shifts the chemical equilibrium between the neutron and proton at neutron freeze-out and allows such additional particle species. Moreover, the BBN itself may also prefer such an asymmetry to reconcile predicted element abundances and observations. However, such a large asymmetry appears to be in conflict with the observed small baryon asymmetry if they are in sphaleron mediated equilibrium. In this paper we point out the surprising fact that in the Standard Model, if the asymmetries in the electron number and the muon number are equal (and opposite) and of the size required to reconcile BBN theory with observations, a baryon asymmetry of the Universe of the correct magnitude and sign is automatically generated within a factor of two. This small remaining discrepancy is naturally remedied in the supersymmetric Standard Model.

Implication of Super-Kamiokande data on R-parity violation

Abstract: R-parity violating bilinear (soft) terms in the supersymmetric standard model would be the leading source for nonzero neutrino masses and mixing. We point out that the mixing between neutralinos (charginos) and neutrinos (charged leptons) driven by the bilinear terms take factorized forms, which may enable us to probe the neutrino mixing parameters in a collider. It is then shown that the Super-Kamiokande data on atmospheric neutrinos require all the baryon number violating couplings to be substantially suppressed: $\lambda''_{\rm any} <10^{-9}$.

Neutrino Masses in E6 Unification

Abstract: We show that neutrino masses with large mixing between νμ and ντ are naturally reproduced in a supersymmetric E6 grand unification model with an anomalous U(1)X symmetry. We propose a simple scenario which incorporates a novel mechanism called ‘E-twisting’ by which all the characteristic features of the fermion mass matrices, not only the quark/lepton Dirac masses but also the neutrino’s Majorana masses, can be reproduced despite the fact that all the members in 27 of each generation are assigned a common U(1)X charge.

Neutrino mixing and CP-violation

Abstract: The prospects of measuring the leptonic angles and CP-odd phases at a neutrino factory are discussed in two scenarios: 1) three active neutrinos as indicated by the present ensemble of atmospheric plus solar data; 2) three active plus one sterile neutrino when the LSND signal is also taken into account. For the latter we develop one and two mass dominance approximations. The appearance of wrong sign muons in long baseline experiments and tau leptons in short baseline ones provides the best tests of CP-violation in scenarios 1) and 2), respectively.