A more sensitive search for νμ→ντ oscillations in NOMAD

Abstract

With additional data and improved algorithms, we have enhanced the sensitivity of our appearance search for ν µ → ν τ oscillations in the NOMAD detector in the CERN-SPS wide-band neutrino beam. The search uses kinematic criteria to identify ν τ charged current interactions followed by decay of the τ − 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 2θ µτ − ∆m 2 plane. At large ∆m 2 , the confidence region includes sin 2 2θ µτ < 1.2 × 10 −3 (a limit 3.5 times more stringent than in our previous publication), while at sin 2 2θ µτ = 1, the confidence region includes ∆m 2 < 1.2 eV 2 /c 4 .

Updated results from the ντ appearance search in NOMAD

“…In order to reliably compute background rejections, based on the hadronic system, as large as 10 5 , the MC results are corrected using a data simulator based on the data themselves [1]. We perform this correction by using a sample of ν µ CC events from the data, removing the identified muon, and replacing it with a MC-generated lepton of appropriate momentum vector, where can be a ν, an e − or a τ − decay.…”

“…The enlarged data sample was supported by an increase in the number of the simulated Monte Carlo (MC) neutrino interactions with respect to Ref. [1], leading to a corresponding improvement in the kinematic selection of ν τ CC events and to a more accurate estimation of the different background sources. The analyses described in Ref.…”

“…Recent searches for ν µ → ν τ [1] and ν e → ν τ [2] oscillations, based on about 950 000 ν µ CC interactions in the detector fiducial volume, yielded no evidence for oscillations.…”

Statistical Pattern Recognition: Application to νμ→ντOscillation Searches Based on Kinematic Criteria

“…The kinematic approach was validated after several years of successful operation at the CERN WANF neutrino beam [17,18]. This shortbaseline experiment set the most competitive limit for ν µ → ν τ oscillations at high values of ∆m 2 [19].…”

“…Kinematic identification of the τ decay [18], which follows the ν τ CC interaction, requires excellent detector performance: good calorimetric features together with tracking and topology reconstruction capabilities. In order to separate ν τ events from the background, a basic criteria can be used: an unbalanced total transverse momentum due to neutrinos produced in the τ decay.…”