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

Abstract

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 theNOMAD neutrino detector.\ The new particles, if they exist, could be produced through the Primakoff effect in interactions of high energy photons, generated by the 450 $GeV$ protons in the CERN SPS neutrino target, with virtual photons from the WANF horn magnetic field.\ The particles would penetrate the downstream shieldingand would be observed in the NOMAD neutrino detector through their re-conversion into real high energy photons byinteracting with the virtual photons from the magnetic field of the NOMAD dipole magnet.\ From the analysis of the data collected during the 1996 run with $1.08\times10^{19}$ protons on target, 312 candidate events with energy between 5 and 140 $GeV$ were found.\ This number is in general agreement with the expectation of 272$\pm$18 background events from standard neutrino processes.\ A 90 $\% ~CL$ upper limit on the $a\gamma\gamma$-coupling $g_{a\gamma\gamma}$ 1.5$\times10^{-4}~GeV^{ -1}$ for $a$ masses up to 40 $eV$ is obtained.