Search for eV (pseudo) scalar penetrating particles in the SPS neutrino beam

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 substant ial increase in the sensitivity to eV masses. The experimental set-up is ba sed on elements of the CERN West Area Neutrino Facility (WANF) beam Line an d the NOMAD neutrino detector. The new particles, if they exist, could be p roduced through the Primakoff effect in interactions of high energy photons , generated by the 450 GeV protons in the CERN SPS neutrino target, with vi rtual photons from the WANF horn magnetic field. The particles would penetr ate the downstream shielding and would be observed in the NOMAD neutrino de tector through their re-conversion into real high energy photons by interac ting with the virtual photons from the magnetic field of the NOMAD dipole m agnet. From the analysis of the data collected during the 1996 run with 1.08 x 10( 19) protons on target, 312 candidate events with energy between 5 and 140 G eV were found. This number is in general agreement with the expectation of 272 +/- 18 background events from standard neutrino processes. A 90 % CL up per limit on the a gamma gamma-coupling g(a gamma gamma) < 1.5 x 10(-4) GeV -1 for a masses up to 40 eV is obtained. (C) 2000 Elsevier Science B.V. All rights reserved.