The NOMAD Collaboration presents a study of opposite sign dimuon events in the framework of Leading Order QCD. A total of 2714 neutrino- and 115 antineutrino-induced opposite sign dimuon events with Eμ1,Eμ2>4.5 GeV, 151(GeV/c)2 are observed in the Front-Calorimeter of NOMAD during the 1995 and 1996 runs. The analysis yields a value for the charm quark mass of mc=1.3+0.3+0.3−0.3−0.3GeV/c2 and for the average semileptonic branching ratio of Bc=0.095+0.007+0.014−0.007−0.013. The ratio of the strange to non-strange sea in the nucleon is measured to be κ=0.48+0.09+0.17−0.07−0.12. The measured rate of charm-induced dimuon relative to single muon, as a function of neutrino energy, is consistent with the slow rescaling hypothesis of heavy quark production.

We carried out a model-independent search for light scalar or pseudoscalar particles a's (an example of which is the axion) that couple to two photons by using a photon-regeneration method at high energies allowing a substantial increase in the sensitivity to eV masses. The experimental set-up is based on elements of the CERN West Area Neutrino Facility (WANF) beam line and the NOMAD neutrino detector. From the analysis of the data collected during the 1996 run with 1.1 × 1019 protons on target, 312 condidate events with energy between 5 and 140 GeV were found. This number is in general agreement with the expectation of 272±18 background events from standard neutrino processes. A 90% C.L. upper limit on the aγγ-coupling gaγγ < 1.5 × 10−4 GeV−1 for a masses ranging up to 40 eV is obtained.

We report the first precision measurements of the scaled momentum, the charge multiplicity, and the thrust of hadronic jets in the Breit frame in Deep Inelastic Scattering nu(mu)N and <(nu)over bar(mu)N> charged cur rent events over the Q(2) range from 1 to 100 GeV2. The neutrino data, obtained in the NOMAD experiment at the CERN SPS, extend the Q(2)-evolution of these parameters by two orders of magnitude, and with commensurate precision, when compared to those reported by the ep and e(+)e(-) experiments. (C) 1999 Elsevier Science B.V. All rights reserved.

With additional data and improved algorithms, we have enhanced the sensitivity of our appearance search for nu(mu) --> nu(tau) oscillations in the NOMAD detector in the CERN-SPS wide-band neutrino beam. The search uses kinematic criteria to identify nu(tau) charged current interactions followed by decay of the tau(-) to one of several decay modes. Our "blind'' analyses of deep-inelastic scattering data taken in 1996 and 1997, combined with consistent reanalyses of previously reported 1995 data, yield no oscillation signal. For the two-family oscillation scenario, we present the contour outlining a 90% C.L. confidence region in the sin(2)2 theta(mu tau) - Delta m(2) plane. At large Delta m(2) the confidence region includes sin(2)2 theta(mu tau) < 1.2 X 10(-3) (a limit 3.5 times more stringent than in our previous publication), while at sin(2)2 theta(mu tau) = 1 the confidence region includes Delta m(2) < 1.2 eV(2)/c(4). (C) 1999 Published by Elsevier Science B.V. All rights reserved.

The NOMAD experiment includes a transition radiation detector that provides a 103 pion rejection factor, for a 90% electron identification efficiency. Such a rejection factor is required in the search for νμ→ντ oscillations in the τ electron decay channel and in the search for νμ→νe oscillations. Algorithms used for the electron–pion discrimination and results obtained on the data are described.

A search was made for a new light gauge boson X which might be produced in pi(0) -->, gamma + X decay from neutral pions generated by 450 GeV protons in the CERN SPS neutrino target. The X's would penetrate the downstream shielding and be observed in the NOMAD detector via the Primakoff effect, in the process of X --> pi(0) conversion in the external Coulomb field of a nucleus. With 1.45 x 10(18) protons on target, 20 candidate events with energy between 8 and 140 GeV were found from the analysis of neutrino data. This number is in agreement with the expectation of 18.1 +/- 2.8 background events from standard neutrino processes. A new 90% C.L. upper limit on the branching ratio Br(pi(0) --> Y + X)< (3.3 to 1.9)X 10(-5) for X masses ranging from 0 to 120 MeV/c(2) is obtained. (C) 1998 Elsevier Science B.V. All rights reserved.

This talk presents the current status of the search for nu(mu) --> nu(tau) oscillations with the NOMAD detector. Significant progress has been made with respect to our published results. The analysis techniques have been improved and a larger data sample, corresponding to the 1995-1997 runs has been used. The search yields no evidence of the presence of oscillations. The updated limit on the neutrino oscillation mixing angle is sin(2) 2 theta(mu tau) < 1.2 x 10(-3), in the limit of large Delta m(2).

The NOMAD experiment is a short base-line search for νμ − ντ oscillations in the CERN neutrino beam. The ντ's are searched for through their charged current interactions followed by the observation of the resulting τ− through its electronic, muonic or hadronic decays. These decays are recognized using kinematical criteria necessitating the use of a light target which enables the reconstruction of individual particles produced in the neutrino interactions. This paper describes the various components of the NOMAD detector: the target and muon drift chambers, the electromagnetic and hadronic calorimeters, the preshower and transition radiation detectors and the veto and trigger scintillation counters. The beam and data acquisition system are also described. The quality of the reconstruction and individual particles is demonstrated through the ability of NOMAD to observe Ks0's, Λ0's and π0's. Finally, the observation of τ− through its electronic decay being one of the most promising channels in the search, the identification of electrons in NOMAD is discussed.

The NOMAD experiment includes a transition radiation detector that provides a 10 3 pion rejection factor, for a 90% electron identi-cation eeciency. Such a rejection factor is required in the search for ! oscillations in the electron decay channel and in the search for ! e oscillations. Algorithms used for the electron-pion discrimination and results obtained on the data are described.

A transition radiation detector to identify electrons at 90% efficiency with a rejection factor against pions of 103 on an area of 2.85 × 2.85 m2 has been constructed for the NOMAD experiment. Each of its 9 modules includes a 315 plastic foil radiator and a detector plane of 176 vertical straw tubes filled with a xenon-methane gas mixture. Details of the design, construction and operation of the detector are given.

The NOMAD experiment is a short base-line search for νμ → ντ oscillations in the CERN neutrino beam. The ντ ’s are searched for through their charged-current interactions followed by the observation of the resulting τ− through its electronic, muonic or hadronic decays. These decays are recognized using kinematical criteria necessitating the use of a light target which enables the reconstruction of individual particles produced in the neutrino interactions. This paper describes the various components of the NOMAD detector: the target and muon drift chambers, the electromagnetic and hadronic calorimeters, the preshower and transition radiation detectors, and the veto and trigger scintillation counters. The beam and data acquisition system are also described. The quality of the reconstruction of individual particles is demonstrated through the ability of NOMAD to observe Ks ’s, Λ 0’s and π0’s. Finally, the observation of τ− through its electronic decay being one of the most promising channels in the search, the identification of electrons in NOMAD is discussed.

A search was made for a new light gauge boson X which might be produced in p TMgqX decay from neutral pions generated by 450 GeV protons in the CERN SPS neutrino target. The X’s would penetrate the downstream shielding and be observed in the NOMAD detector via the Primakoff effect, in the process of XTMp 0 conversion in the external Coulomb field of a nucleus. With 1.45=10 protons on target, 20 candidate events with energy between 8 and 140 GeV were found from the analysis of neutrino data. This number is in agreement with the expectation of 18.1"2.8 background events from Ž 0 . Ž . y5 standard neutrino processes. A new 90 % C. L. upper limit on the branching ratio Br p TMgqX 3.3 to 1.9 =10 for X masses ranging from 0 to 120 MeVrc is obtained. q 1998 Elsevier Science B.V. All rights reserved.