Observation of tau neutrino interactions

The DONUT experiment has analyzed 203 neutrino interactions recorded in nuclear emulsion targets. A decay search has found evidence of four tau neutrino interactions with an estimated background of 0.34 events. This number is consistent with the Standard Model expectation.     

A new way of detecting tau neutrinos

In this letter, a new kind of neutrino data analysis is proposed, which considers the special kinematic properties of charged current tau neutrino interactions which stimulate neutral currents due to hadronic decay of the tau lepton. It is shown that the differential cross section, with respect to a suitable scaling variable, of “true” neutral currents differs significantly from tau induced events. This analysis is shown to have good sensitivity for the presence of ν_gt interactions.     

Neutrino masses from three-particle decays

The purely leptonic decays of the tau and the radiative decay of the pion provide determinations of the tau neutrino and muon neutrino masses, respectively. The shift of the energy at which the tau decay spectrum attains its maximum and the forward-backward ratio are both large enough to determine tau neutrino masses of about 100 MeV. The photon endpoint energy and partially integrated differential decay rate in pion decay are sensitive to a neutrino mass as small as 100 keV. Thus, the present bounds on neutrino masses can be significantly improved.     

Probing a supersymmetric model for neutrino masses at ultrahigh energy neutrino telescopes

A bilinear R-parity breaking SUSY model for neutrino mass and mixing predicts the lightest superparticle to decay mainly into a pair of tau leptons or b quarks along with a neutrino for relatively light SUSY spectra. This leads to a distinctive triple bang signature of SUSY events at ultrahigh energy neutrino telescopes like IceCube or Antares. While the expected signal size is only marginal at IceCube, it will be promising for a future multi-km³ size neutrino telescope.     

Fully confined events indicative of proton decay in the Kolar Gold Fields detector

Three events have been recorded in the Kolar Gold Field experiment with tracks fully confined to the detector volume. It is shown that their characteristics are in conformity with the decay of bound nucleons and that the background due to neutrino interactions is extremely small. Based on these as well as the events reported earlier, we tentatively suggest a mean lifetime of about 7 × 10³⁰ yr for nucleons bound in iron nuclei.     

Evaluation of a test for the tau neutrino in the zero-muon channel

Extending previous work, we assess the feasibility of using the visible energy distribution for zero-muon events in a beam dump experiment as a possible means of observing the tau neutrino. The method would require an extremely precise knowledge of various neutrino fluxes and corresponding background reactions, and hence is not as reliable as the muon-trigger test proposed earlier.     

Measurement of atmospheric neutrino flux consistent with tau neutrino appearance

A search for the appearance of tau neutrinos from nu(mu) <--> nu(tau) oscillations in the atmospheric neutrinos has been performed using 1489.2 days of atmospheric neutrino data from the Super-Kamiokande-I experiment. A best fit tau neutrino appearance signal of 138+/-48(stat)-32(+15)(syst) events is obtained with an expectation of 78+/-26(syst). The hypothesis of no tau neutrino appearance is disfavored by 2.4 sigma.     

Search for charged Higgs bosons in e+e− collisions at s=189 GeV

A search for pair-produced charged Higgs bosons is performed with the L3 detector at LEP using data collected at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.4 pb⁻¹. Higgs decays into a charm and a strange quark or into a tau lepton and its associated neutrino are considered. The observed events are consistent with the expectations from Standard Model background processes. A lower limit of 65.5 GeV on the charged Higgs mass is derived at 95% confidence level, independent of the decay branching ratio Br(H^±→τν).     

Neutrino backgrounds in the Kolar Gold Field nucleon decay experiment

The neutrino events recorded in the Kolar Gold Field Nucleon Decay detector are analysed here. It is shown that there is good agreement between the observations and the estimates based on the intensities of atmospheric neutrinos and interaction cross-sections of neutrinos available from accelerator experiments. In the context of the search for proton decay, we show that the low energy (<2 GeV) neutrino events, which would provide the main background, are suppressed at thekgf site since it is situated near geomagnetic equator, where the geomagnetic cut-off rigidities are high. A comparison of the predicted characteristics ofv-induced events with thekgf observations shows that, within the statistical accuracy of the present data, the signal due to nucleon decay stands out distinctly within thev-induced background.     

Atmospheric neutrinos and discovery of neutrino oscillations

Neutrino oscillation was discovered through studies of neutrinos produced by cosmic-ray interactions in the atmosphere. These neutrinos are called atmospheric neutrinos. They are produced as decay products in hadronic showers resulting from collisions of cosmic rays with nuclei in the atmosphere. Electron-neutrinos and muon-neutrinos are produced mainly by the decay chain of charged pions to muons to electrons. Atmospheric neutrino experiments observed zenith-angle and energy dependent deficit of muon-neutrino events. Neutrino oscillations between muon-neutrinos and tau-neutrinos explain these data well. Neutrino oscillations imply that neutrinos have small but non-zero masses. The small neutrino masses have profound implications to our understanding of elementary particle physics and the Universe. This article discusses the experimental discovery of neutrino oscillations.     

Study of the charge-exchange reactions\pi ^ - p \to (\pi ^0 ,\eta ,\eta \prime )n at 63 GeV

Muon electron pairs were detected in an Al multiplate spark chamber, exposed to a neutrino beam from the CERN PS. The leptons were not accompanied by other particles, except occasionally by protons. The background came mainly from muon associated π⁰ production, with one decay gamma lost. It was determined empirically, together with the small contribution from υ _e reactions. For electron energies above 2 GeV the background is 5.7±1.5 events, whereas 18 (μe)-candidates have been observed. Hence the effect is established, with a rate of about 10^?4 as compared to the muonic reactions above 3 GeV. Charm creation as the origin of this (μe)-production process is excluded; heavy neutral lepton production does not fit the kinematics observed. Instead the events are compatible with the two-body decay of an object with variable invariant mass of order 1 GeV, possibly resulting from axion interactions.     

Tau neutrino mass from semileptonic decays

The semileptonic three particle decays of the tau provide determinations of the tau neutrino mass. The shift of the maximal energy of the observable final state particles islinear in the neutrino mass. The endpoint energy of the pion and a partially integrated decay rate in τ → πωv _τ and τ → πρV _τ are sensitive to a neutrino mass smaller than 100 MeV. Thus, the present bound on \(m_{v_\tau } \) can significantly be improved.     

Decay of high-energy astrophysical neutrinos

Existing limits on the nonradiative decay of one neutrino to another plus a massless particle (e.g., a singlet Majoron) are very weak. The best limits on the lifetime to mass ratio come from solar neutrino observations and are tau/m greater, similar 10(-4) s/eV for the relevant mass eigenstate(s). For lifetimes even several orders of magnitude longer, high-energy neutrinos from distant astrophysical sources would decay. This would strongly alter the flavor ratios from the phi(nu(e)):phi(nu(mu)):phi(nu(tau))=1:1:1 expected from oscillations alone and should be readily visible in the near future in detectors such as IceCube.     

Precision measurements and tau neutrino physics in a future accelerator neutrino experiment

We investigate prospects of building a future accelerator-based neutrino oscillation experiment in China, including site selection, beam optimization and tau neutrino physics aspects. CP violation, non-unitary mixing and non-standard neutrino interactions are discussed. We simulate neutrino beam setups based on muon and beta decay techniques and compare Chinese laboratory sites by their expected sensitivities. A case study on the Super Proton–Proton Collider and the China JinPing Laboratory is also presented. It is shown that the muon-decay-based beam setup can measure the Dirac CP phase by about 14.2° precision at 1σ CL, whereas non-unitarity can be probed down to ∣α_ij∣ ≲ 0.37 (i ≠ j = 1, 2, 3) and non-standard interactions to $| {\epsilon }_{{\ell }{\ell }^{\prime} }^{m}| \lesssim $ 0.11 (${\ell }\ne {\ell }^{\prime} =e$, μ, τ) at 90% CL, respectively.     

Do high energy neutrino beams contain new kinds of neutrinos from charged heavy lepton decays?

The possibility is considered that new kinds of neutrinos produced promptly in hadronic interactions as the decay products of charged heavy leptons may contaminate some high-energy neutrino beams. With a contamination at the level of a fraction of a per cent they could contribute observably to high-energy dilepton events.     

Weak and electromagnetic nuclear decay signatures for neutrino reactions in Super-Kamiokande

We suggest the study of events in the Super-Kamiokande neutrino data due to charged- and neutral-current neutrino reactions followed by weak and/or electromagnetic decays of struck nuclei and fragments thereof. This study could improve the prospects of obtaining evidence for tau production from nu(mu)-->nu(tau) oscillations and could augment the data sample used to disfavor nu(mu)-->nu(sterile) oscillations.     

Strange particle production and charmed particle search in the gargamelle neutrino experiment

Data is given for single and multiple strange particle production in neutrino reactions in both charged and neutral current channels. A total of 15000 neutrino and antineutrino events has been examined for possible evidence of semi-leptonic decay of a charmed particle. One candidate has been observed. Upper limits on charmed particle production cross-sections are given as a function of the neutrino energy and the invariant mass of the final state hadrons.     

Search for lepton-flavor violation in the decay tau- --> l- l+ l-

A search for the lepton-flavor-violating decay of the tau into three charged leptons has been performed using 91.5 fb(-1) of data collected at an e(+)e(-)center-of-mass energy around 10.58 GeV with the BABAR detector at the SLAC storage ring PEP-II. In all six decay modes considered, the numbers of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range (1-3)x10(-7) at 90% confidence level.     

Improved limits on the lepton-flavor violating decays tau{-}-->l{-}l{+}l{-}

A search for the neutrinoless, lepton-flavor violating decay of the tau lepton into three charged leptons has been performed using 376 fb{-1} of data collected at an e{+}e{-} center-of-mass energy around 10.58 GeV with the BABAR detector at the SLAC PEP-II storage rings. In all six decay modes considered, the numbers of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range (4-8)x10{-8} at 90% confidence level.     

Search for the Decay B+-->K+ tau-/+ mu+/-

We present a search for the lepton flavor violating decay B+-->K+ tau-/+ mu+/- using 383 x 10;{6} BB[over ] events collected by the BABAR experiment. The branching fraction for this decay can be substantially enhanced in new physics models. The kinematics of the tau from the signal B decay are inferred from the K+, mu, and other B in the event, which is fully reconstructed in one of a variety of hadronic decay modes, allowing the signal B candidate to be fully reconstructed. We observe no excess of events over the expected background and set a limit of B(B+-->K+ tau mu)<7.7 x 10(-5) at 90% confidence level, where the branching fraction is for the sum of the K+ tau- mu+ and K+ tau+mu- final states. We use this result to improve a model-independent bound on the energy scale of flavor-changing new physics.