Showing papers on "Lepton published in 2004"

Measurement of the total active B-8 solar neutrino flux at the Sudbury Neutrino Observatory with enhanced neutral current sensitivity

Abstract: The Sudbury Neutrino Observatory has precisely determined the total active (nu(x)) B-8 solar neutrino flux without assumptions about the energy dependence of the nu(e) survival probability. The measurements were made with dissolved NaCl in heavy water to enhance the sensitivity and signature for neutral-current interactions. The flux is found to be 5.21+/-0.27(stat)+/-0.38(syst)x10(6) cm(-2) s(-1), in agreement with previous measurements and standard solar models. A global analysis of these and other solar and reactor neutrino results yields Deltam(2)=7.1(-0.6)(+1.2)x10(-5) eV(2) and theta= 32.5(-2.3)(+2.4) degrees. Maximal mixing is rejected at the equivalent of 5.4 standard deviations.

Neutrino mass models

Abstract: This is a review article about neutrino mass models, particularly see-saw models involving three active neutrinos that are capable of describing both the atmospheric neutrino oscillation data and the large mixing angle (LMA) MSW solar solution, which is now uniquely specified by recent data. We briefly review the current experimental status, show how to parametrize and construct the neutrino mixing matrix, and present the leading order neutrino Majorana mass matrices. We then introduce the see-saw mechanism and discuss a natural application of it to current data using the sequential dominance mechanism, which we compare with an early proposal for obtaining LMAs. We show how both the Standard Model and the Minimal Supersymmetric Standard Model may be extended to incorporate the see-saw mechanism and show how the latter case leads to the expectation of lepton flavour violation. The see-saw mechanism motivates models with additional symmetries such as unification and family symmetry models, and we tabulate some possible models before focusing on two particular examples based on SO(10) grand unification and either U(1) or SU(3) family symmetry as specific examples. This review contains extensive appendices that include techniques for analytically diagonalizing different types of mass matrices involving two LMAs and one small mixing angle, to leading order in the small mixing angle.

Muon Track Reconstruction and Data Selection Techniques in AMANDA

Abstract: The Antarctic Muon And Neutrino Detector Array (AMANDA) is a high-energy neutrino telescope operating at the geographic South Pole. It is a lattice of photo-multiplier tubes buried deep in the polar ice between 1500 and 2000 m. The primary goal of this detector is to discover astrophysical sources of high-energy neutrinos. A high-energy muon neutrino coming through the earth from the Northern Hemisphere can be identified by the secondary muon moving upward through the detector. The muon tracks are reconstructed with a maximum likelihood method. It models the arrival times and amplitudes of Cherenkov photons registered by the photo-multipliers. This paper describes the different methods of reconstruction, which have been successfully implemented within AMANDA. Strategies for optimizing the reconstruction performance and rejecting background are presented. For a typical analysis procedure the direction of tracks are reconstructed with about 2° accuracy. © 2004 Elsevier B.V. All rights reserved.

Flavor mixing and CP violation of massive neutrinos

Abstract: We present an overview of recent progress in the phenomenological study of neutrino masses, lepton flavor mixing and CP violation. We concentrate on the model-independent properties of massive neutrinos, both in vacuum and in matter. Current experimental constraints on the neutrino mass spectrum and the lepton flavor mixing parameters are summarized. The Dirac- and Majorana-like phases of CP violation, which are associated respectively with the long-baseline neutrino oscillations and the neutrinoless double beta decay, are discussed in detail. The seesaw mechanism, the leptogenesis scenario and the strategies to construct lepton mass matrices are briefly described. The features of flavor mixing between one sterile neutrino and three active neutrinos are also explored.

Phenomenological tests of supersymmetric A 4 family symmetry model of neutrino mass

Abstract: Recently Babu, Ma and Valle proposed a model of quark and lepton mixing based on $A_4$ symmetry. Within this model the lepton and slepton mixings are intimately related. We perform a numerical study in order to derive the slepton masses and mixings in agreement with present data from neutrino physics. We show that, starting from three-fold degeneracy of the neutrino masses at a high energy scale, a viable low energy neutrino mass matrix can indeed be obtained in agreement with constraints on lepton flavour violating $\mu$ and $\tau$ decays. The resulting slepton spectrum must necessarily include at least one mass below 200 GeV which can be produced at the LHC. The predictions for the absolute Majorana neutrino mass scale $m_0 \geq 0.3$ eV ensure that the model will be tested by future cosmological tests and $\beta\beta_{0 u}$ searches. Rates for lepton flavour violating processes $\ell_j \to \ell_i + \gamma$ in the range of sensitivity of current experiments are typical in the model, with BR$(\mu \to e \gamma) \gsim 10^{-15}$ and the lower bound BR$(\tau \to \mu \gamma) > 10^{-9}$. To first approximation, the model leads to maximal leptonic CP violation in neutrino oscillations.

Prospective analysis of spin- and CP-sensitive variables in H rightarrow ZZ rightarrow l + 1 l - 1 l + 2 l - 2 at the LHC

Abstract: A possibility to prove spin and CP-eigenvalue of a Standard Model (SM) Higgs boson is presented. We exploit angular correlations in the subsequent decay H \(\rightarrow\) ZZ \(\rightarrow\) 4l (muons or electrons) for Higgs masses above 200 GeV. We compare the angular distributions of the leptons originating from the SM Higgs with those resulting from decays of hypothetical particles with differing quantum numbers. We restrict our analysis to the use of the Atlas-detector which is one of two multi-purpose detectors at the upcoming 14 TeV proton-proton-collider (LHC) at CERN. By applying a fast simulation of the Atlas detector it can be shown that these correlations will be measured sufficiently well that consistency with the spin-CP hypothesis 0 + of the Standard Model can be verified and the 0- and \(1^\pm\) can be ruled out with an integrated luminosity of 100 fb-1.

Study of tau-pair production in photon-photon collisions at LEP and limits on the anomalous electromagnetic moments of the tau lepton

Abstract: Tau-pair production in the process e(+)e(-) --> e(+)e(-) tau(+)tau(-) was studied using data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000. The corresponding integrated luminosity is 650 pb(-1). The values of the cross-section obtained are found to be in agreement with QED predictions. Limits on the anomalous magnetic and electric dipole moments of the tau lepton are deduced.

Universal texture of quark and lepton mass matrices with an extended flavor 2 3 symmetry

Abstract: Against the conventional picture that the mass matrix forms in the quark sectors will take somewhat different structures from those in the lepton sectors, on the basis of the idea that all the mass matrices of quarks and leptons have the same texture, a universal texture of quark and lepton mass matrices is proposed by assuming a discrete symmetry ${Z}_{3}$ and an extended flavor $2\ensuremath{\leftrightarrow}3$ symmetry. The texture is described by three parameters (including the phase parameter). The neutrino masses and mixings are investigated according to this ansatz.

Fermion masses and mixing in extended technicolor models

Abstract: We study fermion masses and mixing angles, including the generation of a seesaw mechanism for the neutrinos, in extended technicolor (ETC) theories. We formulate an approach to these problems that relies on assigning right-handed $Q=-1/3$ quarks and charged leptons to ETC representations that are conjugates of those of the corresponding left-handed fermions. This leads to a natural suppression of these masses relative to the $Q=2/3$ quarks, as well as the generation of quark mixing angles, both long-standing challenges for ETC theories. Standard-model-singlet neutrinos are assigned to ETC representations that provide a similar suppression of neutrino Dirac masses, as well as the possibility of a realistic seesaw mechanism with no mass scale above the highest ETC scale of roughly $10^3$ TeV. A simple model based on the ETC group SU(5) is constructed and analyzed. This model leads to non-trivial, but not realistic mixing angles in the quark and lepton sectors. It can also produce sufficiently light neutrinos, although not simultaneously with a realistic quark spectrum. We discuss several aspects of the phenomenology of this class of models.

High Scale Mixing Unification and Large Neutrino Mixing Angles

Abstract: Starting with the hypothesis that quark and lepton mixings are identical at or near the grand unified theory scale, we show that the large solar and atmospheric neutrino mixing angles together with the small reactor angle U e3 can be understood purely as a result of renormalization group evolution provided the three neutrinos are quasidegenerate and have the same CP parity. The mechanism is found to work if the common Majorana mass for the neutrinos is larger than 0.1 eV, which falls right in the range reported recently and also in the range which will be probed in planned experiments.

Some aspects of thermal leptogenesis

Abstract: Properties of neutrinos may be the origin of the matter–antimatter asymmetry of the universe. In the see–saw model for neutrino masses, this leads to important constraints on the properties of light and heavy neutrinos. In particular, an upper bound on the light neutrino masses of 0.1 eV can be derived. We review the present status of thermal leptogenesis with emphasis on the theoretical uncertainties and discuss some implications for lepton and quark mass hierarchies, CP violation and dark matter. We also comment on the 'leptogenesis conspiracy', the remarkable fact that neutrino masses may lie in the range where leptogenesis works best.

Lepton mixing angle $\theta_{13} = 0$ with a horizontal symmetry $D_4$

Abstract: We discuss a model for the lepton sector based on the seesaw mechanism and on a D4 family symmetry. The model predicts the mixing angle �13 to vanish. The solar mixing angle �12 is free—it will in general be large if one does not invoke finetuning. The model has an enlarged scalar sector with three Higgs doublets, together with (�)

Two-phase emission detector for measuring coherent neutrino-nucleus scattering

Abstract: Coherent scattering is a flavor-blind, high-rate, as yet undetected neutrino interaction predicted by the Standard Model. We propose to use a compact (kg-scale), two-phase (liquid-gas) argon ionization detector to measure coherent neutrino scattering off nuclei. In our approach, neutrino-induced nuclear recoils in the liquid produce a weak ionization signal, which is transported into a gas under the influence of an electric field, amplified via electroluminescence, and detected by phototubes or avalanche diodes. This paper describes the features of the detector, and estimates signal and background rates for a reactor neutrino source. Relatively compact detectors of this type, capable of detecting coherent scattering, offer a new approach to flavor-blind detection of man-made and astronomical neutrinos, and may allow development of compact neutrino detectors capable of nonintrusive real-time monitoring of fissile material in reactors.

Massive neutrinos and flavour violation

Abstract: In spite of the large lepton flavour violation (LFV) observed in neutrino oscillations, within the Standard Model, we do not expect any visible LFV in the charged lepton sector (? ? e, ?, ? ? ?, ?, etc). On the contrary, the presence of new physics close to the electroweak scale can enhance the amplitudes of these processes. We discuss this in general and focus on a particularly interesting case: the marriage of low-energy supersymmetry (SUSY) and seesaw mechanism for neutrino masses (SUSY seesaw). Several ideas presented in this context are reviewed both in the bottom-up and top-down approaches. We show that there exist attractive models where the rate for LFV processes can attain values to be probed in pre-LHC experiments.

Supersymmetric origin of neutrino mass

Abstract: Supersymmetry with breaking of R-parity provides an attractive way to generate neutrino masses and lepton mixing angles in accordance with the present neutrino data. We review the main theoretical features of the bilinear R-parity breaking (BRpV) model, and stress that it is the simplest extension of the minimal supersymmetric standard model (MSSM), which includes lepton number violation. We describe how it leads to a successful phenomenological model with hierarchical neutrino masses. In contrast with see-saw models, the BRpV model can be probed at future collider experiments, such as the Large Hadron Collider or the Next Linear Collider, since the decay pattern of the lightest supersymmetric particle provides a direct connection with the lepton mixing angles determined by neutrino experiments.

TeV scale resonant leptogenesis from supersymmetry breaking

Abstract: We propose a model of TeV-scale resonant leptogenesis based upon recent models of the generation of light neutrino masses from supersymmetry-breaking effects with TeV-scale right-handed (rhd) neutrinos, Ni. The model leads to large cosmological lepton asymmetries via the resonant behaviour of the one-loop self-energy contribution to Ni decay. Our model addresses the primary problems of previous phenomenological studies of low-energy leptogenesis: a rational for TeV-scale rhd neutrinos with small Yukawa couplings so that the out-of equilibrium condition for Ni decay is satisfied; the origin of the tiny, but non-zero mass splitting required between at least two Ni masses; and the necessary non-trivial breaking of flavour symmetries in the rhd neutrino sector. The low mass-scale of the rhd neutrinos and their superpartners, and the TeV-scale A-terms automatically contained within the model offer opportunities for partial direct experimental tests of this leptogenesis mechanism at future colliders.

Searches for Neutral Higgs Bosons in Extended Models

Abstract: Searches for neutral Higgs bosons produced at LEP in association with Z bosons, in pairs and in the Yukawa process are presented in this paper. Higgs boson decays into b quarks, tau leptons, or other Higgs bosons are considered, giving rise to four-b, four-b+jets, six-b and four-tau final states, as well as mixed modes with b quarks and tau leptons. The whole mass domain kinematically accessible at LEP in these topologies is searched. The analysed data set covers both the LEP1 and LEP2 energy ranges and exploits most of the luminosity recorded by the DELPHI experiment. No convincing evidence for a signal is found, and results are presented in the form of mass-dependent upper bounds on coupling factors (in units of model-independent reference cross-sections) for all processes, allowing interpretation of the data in a large class of models.

Momentum transfer in complex plasmas.

Abstract: Momentum transfer in complex plasmas (systems consisting of ions, electrons, neutrals, and charged macroscopic grains) is investigated assuming an interaction potential between the charged species of the screened Coulomb (Yukawa) type. Momentum transfer cross sections and rates are derived. Applications of the results are discussed; in particular, we classify the possible states of complex plasmas in terms of the momentum transfer due to grain-grain collisions and its competition with that due to interaction with the surrounding medium. The resulting phase diagrams are presented.

Theta-13 as a Probe of Mu-Tau symmetry for Leptons

Abstract: Many experiments are being planned to measure the neutrino mixing parameter θ13 using reactor as well as accelerator neutrino beams. In this note, the theoretical significance of a high precision measurement of this parameter is discussed. It is emphasized that it will provide crucial information about different ways to understand the origin of large atmospheric neutrino mixing and move us closer towards determining the neutrino mass matrix. For instance if exact μτ symmetry in the neutrino mass matrix is assumed to be the reason for maximal νμ−ντ mixing, one gets θ13 = 0. Whether θ13 (Δm 2 ⊙/Δm 2 A )1/2 or θ13 Δm 2 ⊙/Δm 2 A can provide information about the way the μτ symmetry breaking manifests in the case of normal hierarchy. We also discuss the same question for inverted hierarchy as well as possible gauge theories with this symmetry.

CMB signals of neutrino mass generation

Abstract: We propose signals in the cosmic microwave background (CMB) to probe the type and spectrum of neutrino masses. In theories that have spontaneous breaking of approximate lepton flavor symmetries at or below the weak scale, light pseudo-Goldstone bosons recouple to the cosmic neutrinos after nucleosynthesis and affect the acoustic oscillations of the electron-photon fluid during the eV era. Deviations from the Standard Model are predicted for both the total energy density in radiation during this epoch, $\ensuremath{\Delta}{N}_{\ensuremath{ u}}$, and for the multipole of the $n$'th CMB peak at large $n$, $\ensuremath{\Delta}{l}_{n}$. The latter signal is difficult to reproduce other than by scattering of the known neutrinos, and is therefore an ideal test of our class of theories. In many models, the large shift $\ensuremath{\Delta}{l}_{n}\ensuremath{\approx}8{n}_{S}$ depends on the number of neutrino species that scatter via the pseudo-Goldstone boson interaction. This interaction is proportional to the neutrino masses, so that the signal reflects the neutrino spectrum. The prediction for $\ensuremath{\Delta}{N}_{\ensuremath{ u}}$ is highly model dependent, but can be accurately computed within any given model. It is very sensitive to the number of pseudo-Goldstone bosons, and therefore to the underlying symmetries of the leptons, and is typically in the region of $0.03l\ensuremath{\Delta}{N}_{\ensuremath{ u}}l1$. This signal is significantly larger for Majorana neutrinos than for Dirac neutrinos, and, like the scattering signal, varies as the spectrum of neutrinos is changed from hierarchical to inverse hierarchical to degenerate.