Neutrino mass from laboratory: contribution of double beta decay to the neutrino mass matrix

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with ν oscillation experiments. The most sensitive experiment - since eight years the HEIDELBERG-MOSCOW experiment in Gran-Sasso - already now, with the experimental limit of m_ν < 0.26 eV practically excludes degenerate ν mass scenarios allowing neutrinos as hot dark matter in the universe for the smallangle MSW solution of the solar neutrino problem. It probes cosmological models including hot dark matter already now on the level of future satellite experiments MAP and PLANCK. It further probes many topics of beyond SM physics at the TeV scale. Future experiments should give access to the multi-TeV range and complement on many ways the search for new physics at future colliders like LHC and NLC. For neutrino physics some of them (GENIUS) will allow to test almost all neutrino mass scenarios allowed by the present neutrino oscillation experiments.     

Atmospheric neutrinos in the Soudan-2 and MACRO experiments

The results of Soudan-2 and MACRO experiments are summarized. Both experiments observe atmospheric neutrino anomalies in agreement with ν_μ → ν_τ oscillations with maximum mixing. The ν_μ → ν_s oscillations are disfavoured by the MACRO experiment at 98% C.L.     

Bounds on sterile neutrino mixing for cosmologically interesting mass range

This talk summarizes our recent work which studied the impact of resonant ν_e → ν_s and (ν_s is a sterile neutrino) conversions on supernova physics, under the assumption that the mass of the sterile state is in the few eV -cosmologically significant range.     

Final results from the ντ appearance search in the NOMAD experiment

An updated analysis of the full NOMAD data corresponding to 1.35 × 10⁶ charged current interactions has been performed to search for neutrino oscillations through ν_τ appearance. This document updates the recently published results on the ν_μ → ν_τ and ν_μ → ν_e oscillations search in NOMAD [1] with a unified analysis of the hadronic channels.     

Future atmospheric neutrino data from Super-Kamiokande

Expected sensitivity of future atmospheric neutrino data from Super-Kamiokande on neutrino oscillation physics is discussed. We expect that the accuracy of the sin ²θ₂₃ measurement will be improved with (exposure time) . By analyzing high energy fully contained events, it could be possible to statistically demonstrate the existence of charged current ν_τ interactions at the 3 standard deviation level with a few more years of data. Subdominant ν_μ → ν_e oscillations could be observed if θ₁₃ is near the present limit. However, significantly more data will be required to observe a 3 standard deviation effect.     

KARMEN: Neutrino oscillation limits and new results with the upgrade

The neutrino experiment KARMEN is situated at the beam stop neutrino source ISIS which provides ν_μ's, ν_e's and from the π⁺−μ⁺-decay at rest. The oscillation channels ν_{μ → νe} and are investigated with a 56 t liquid scintillation calorimeter. No evidence for oscillations could be found with KARMEN, resulting in 90% CL exclusion limits of sin²(2Θ) < 8.5 · 10⁻³ ( ) and sin²(2Θ) < 4.0 · 10⁻² (ν_μ → ν_e) for Δm² > 100 eV². In 1996, the experiment has been upgraded by an additional veto counting system with a total coverage of 300 m². The new system allows the identification of cosmic muons in the vicinity of the detector. Vetoing these muons suppresses energetic neutrons from deep inelastic scattering of muons as well as from μ-capture by a factor of 40. Up to 1996, these neutrons represented the main background for oscillation search. The experimental sensitivity for will be significantly enhanced towards sin²(2Θ) 1.0 · 10⁻³ after a further measuring period of 2–3 years.     

Large leptonic flavour mixing and the mass spectrum of leptons

Implications of a simple model for the mass generation of leptons are studied, in particular for the upcoming long-baseline neutrino experiments. The flavour mixing angles are large (nearly maximal). The probability for the long-baseline ν_μ↔ν_e oscillation is predicted to be about 1%.     

Soft photon problem in leptonic B-decays

We point out at the peculiarity of B→μν_μ decay, namely the enhancement of the soft photon events which originate from the structure dependent part of the B→μν_μγ amplitude. This may be a dominant source of systematic uncertainty and compromise the projected experimental uncertainty on Γ(B→μν_μ). We show that the effect of these soft photons can be controlled if the experimental cut on identification of soft photons is lowered and especially if the better resolution in identifying the momentum of muon emerging from B→μν_μ is made. A lattice QCD computation of the relevant form factors would be highly helpful for a better numerical control over the structure dependent soft photon emission.     

Effects of superstrings in collisions

Superstring theory in d = 10 dimensions after Calabi—Yau compactification yields a minimum low-energy gauge group SU(3)_C × SU(2)_L × U(1)_Y × U(1)_E. The low-energy theory includes particles with the quantum numbers of 27 representations of E₆, each of which contains an extra neutrino ν^c conventionally called a “right-handed neutrino”. The contributions of ν and ν^c to through Z⁰ and Z_E mixing is calculated. Small contributions are found of the new right-handed neutrino and of the superstring boson Z_E to σ(e⁺e⁻ → γ + nothing).     

Liquid argon neutrino detectors

The Liquid Argon imaging technique, as proposed for the ICARUS detector, offers the possibility to perform complementary and simultaneous measurements of neutrinos, as those of CERN to Gran Sasso beam (CNGS) and those from cosmic ray events. For the currently allowed values of the Super—Kamiokande results, the combination of both CNGS and atmospheric data will provide a precise determination of the oscillation parameters. Since one can observe and unambiguously identify ν_e, ν_μ and ν_τ components, this technology allows to explore the full (3 x 3) mixing matrix. The same class of detector can be proposed for high precision measurements at a neutrino factory.     

Search for anomalous production of single photons at and 136 GeV

This letter reports the results of the measurement of single photon production in the reaction e⁺e⁻ → γ+ invisible particles at centre-of-mass energies and 136 GeV and an integrated luminosity of 5.83 pb⁻¹, collected with the DELPHI detector at LEP. The signal is compatible with the prediction of the Standard Model for the process , and the number of neutrino families has been determined to be N_ν = 3.1 ± 0.6. Limits have been derived on anomalous neutral gauge boson couplings and on compositeness in the framework of a specific model.     

Limits on the neutrino magnetic moment from solar neutrino experiments

If neutrinos have non-vanishing mass and non-vanishing magnetic moments, then electron neutrinos emitted in nuclear reactions in the solar interior may undergo flavour oscillations, spin precession or resonant spin-flavour precession. Assuming equal values for the magnetic moments of all neutrino flavours and using the data from Homestake and SuperKamiokande we obtain an upper limit on the neutrino magnetic moment and find μ_νe ≤ (2.2 − 2.3) × 10⁻¹⁰μ_B, within four standard solar models. We also point out that this limit may be further reduced if the detector threshold energy for the ν_e,χe⁻ scattering is decreased.     

Radiative neutrino decays in very strong magnetic fields

The radiative decay ν_H → ν_L + γ of massive neutrinos is analyzed in the framework of the standard model with lepton mixing for very strong magnetic fields B B_cr = m²_e/e 4.14 × 10¹³ G. The analysis is based on the approximate decay amplitude obtained by Gvozdev et al. Numerical results as well as analytical approximations for the decay rate are obtained for energies of the initial neutrino below and above the electron-positron pair creation threshold 2m_e.     

Single and multi-photon events with missing energy in eecollisions at = 189 GeV

Single and multi-photon events with missing energy are analysed using data collected with the L3 detector at LEP at a centre-of-mass energy of 189 GeV, for a total of 176 pb of integrated luminosity. The cross section of the process e⁺e⁻ → γ(γ) is measured and the number of light neutrino flavours is determined to be N_ν=3.011±0.077 including lower energy data. Upper limits on cross sections of supersymmetric processes are set and interpretations in supersymmetric models provide improved limits on the masses of the lightest neutralino and the gravitino. Graviton-photon production in low scale gravity models with extra dimensions is searched for and limits on the energy scale of the model are set exceeding 1 TeV for two extra dimensions.     

Yukawaon model in the quark sector and nearly tribimaximal neutrino mixing

For the purpose of deriving the observed nearly tribimaximal neutrino mixing, a possible yukawaon model in the quark sector is investigated. Five observable quantities (2 up-quark mass ratios and 3 neutrino mixing parameters sin²2θ_atm, tan²θ_solar and |U₁₃|) are excellently fitted by two parameters (one in the up-quark sector and another one in the right-handed Majorana neutrino sector).     

Further evidence for neutrino oscillations from LSND: the νμ → νe decay-in-flight channel

A search for ν_μ → ν_e oscillations has been conducted at the Los Alamos Meson Physics Facility (LAMPF) using ν_μ from π⁺ decay in flight. An excess in the number of beam-related events from the ν_e C → e⁻ X inclusive reaction is observed. The excess is too large to be explained by normal ν_e contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of (2.6 ± 1.3 ± 0.5) × 10⁻³ is consistent with the previously reported oscillation evidence from LSND.     

ATLAS discovery potential for a heavy charged Higgs boson

The sensitivity of the ATLAS detector to the discovery of a heavy charged Higgs boson is presented. Assuming a heavy SUSY spectrum, the most promising channels above the top quark mass are H^± → tb and H^± → τ^±ν_τ which provide coverage in the low and high tanβ regions up to ∼ 600 GeV. The achievable precisions on the charged Higgs mass and tanβ determination are also discussed. The H^± → W^± h⁰ channel, though restricted to a small MSSM parameter space, shows a viable signal in NMSSM where the parameter space is less constrained. The observation of the channel H⁻ → τ⁻_L ν_τ + c.c. may constitute a distinctive evidence for models with singlet neutrinos in large extra dimensions.     

Theoretical aspects of neutrino mass and lepton flavour violation

We consider lepton flavour violation (LFV) in the charged lepton sector both from the bottom-up effective Lagrangian approach and from the top-down approach via various case studies that have been analysed. The implications for LFV studies at the LHC is briefly discussed. Finally the nature of LFV in the neutrino sector is considered, paying particular regard to the implications of the recent measurements of ?? ₁₃.     

Neutrino mixing and masses in a left–right model with mirror fermions

In the framework of a left–right model containing mirror fermions with gauge group SU(3) _C ⊗SU(2) _L ⊗SU(2) _R ⊗U(1) _Y′, we estimate the neutrino masses, which are found to be consistent with their experimental bounds and hierarchy. We evaluate the decay rates of the Lepton Flavor Violation (LFV) processes μ→eγ, τ→μγ and τ→eγ. We obtain upper limits for the flavor-changing branching ratios in agreement with their present experimental bounds. We also estimate the decay rates of heavy Majorana neutrinos in the channels N→W ^± l ^∓, N→Zν _l and N→Hν _l , which are roughly equal for large values of the heavy neutrino mass. Starting from the most general Majorana neutrino mass matrix, the smallness of active neutrino masses turns out from the interplay of the hierarchy of the involved scales and the double application of seesaw mechanism. An appropriate parameterization on the structure of the neutrino mass matrix imposing a symmetric mixing of electron neutrino with muon and tau neutrinos leads to tri-bimaximal mixing matrix for light neutrinos.     

中微子非标准相互作用对大亚湾实验的影响

大亚湾中微子实验的目标是测量中微子混合矩阵中的最小混合角θ13.如果考虑中微子的非标准相互作用(NSIs),中微子的振荡概率公式要做相应的改写,其效应将和θ13纠缠在一起,从而降低了实验对θ13的敏感度(sensitivity).讨论了在NSIs存在的情况下大亚湾实验对θ13的敏感度,发现这个实验不可能同时测量出NSIs和θ13的值.由于当θ13=0时反应堆产生的反中微子将没有振荡现象(NSIs的效应也将消失),如果大亚湾实验测量到了中微子振荡效应,那将表明θ13≠0;但是,由于非零的θ13的效应和NSIs的效应有可能抵消而导致中微子没有振荡.如果大亚湾实验没有测量到中微子振荡,不能排除非零的θ13.