The NOMAD experiment is a short base-line search for ν µ → ν τ oscillations in the CERN neutrino beam. The ν τ '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 K 0 s 's, Λ 0 's and π 0 '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 π 0 → γ + 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 → π 0 conversion in the external Coulomb field of a nucleus. With 1.45×10 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(π 0 → γ + X) < (3.3 to 1.9)×10 −5 for X masses ranging from 0 to 120 MeV/c 2 is obtained.

NOMAD is a neutrino oscillation experiment designed to search for appearance in the CERN-SPS wide band beam. Signal detection relies on the identication of charged current i n teractions using kinematic criteria. The analysis of the 1995 data sample yields no oscillation signal. Combining all studied decay modes, a limit of sin 2 2 < 4:2 10 3 is obtained for large m 2 at the 90% condence level.

The NOMAD trigger system is described in the present paper. It is made up of a large area plastic scintillator veto system, two trigger scintillator planes inside a 0.4 T magnetic field and their associated trigger electronics. Special features of the system consist of the use of proximity mesh photomultipliers which allow the trigger scintillators to operate in the magnetic field, and the use of custom-built VME modules which perform the trigger logic decisions, the signal synchronisation and gate generation, event counting and livetime calculations. This paper also includes a description of each of the NOMAD triggers, with their calculated and measured rates, efficiencies and livetimes.

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 ν µ N andν µ N charged current events over the Q 2 range from 1 to 100 GeV 2 . 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.