HREPHAH 0H3HKA, 2004, moM 67, № 11, c. 1967-1972
NEUTRINO PHYSICS
FINAL RESULTS ON THE SEARCH FOR v^ — ve OSCILLATIONS
IN THE NOMAD EXPERIMENT
© 2004 B. A. Popov* (for the NOMAD Collaboration)
Dzhelepov Laboratory of Nuclear Problems, Joint Institue for Nucelar Research, Dubna, Russia
Received January 20, 2004
The results of the search for v^ — ve oscillations in the NOMAD experiment at CERN are presented. The experiment looked for the appearance of ve in a predominantly v^ wide-band neutrino beam at the CERN SPS. No evidence for oscillations was found. The 90% confidence limits obtained are Am2 < 0.4 eV2 for maximal mixing and sin2(20) < 1.4 x 10~3 for large Am2. This result excludes the LSND allowed region of oscillation parameters with Am2 > 10 eV2.
1. INTRODUCTION
The NOMAD experiment was designed to search for vT appearance from neutrino oscillations [1] in the CERN wide-band neutrino beam produced by the 450-GeV proton synchrotron (SPS). The detector was optimized to identify efficiently electrons from t- — e-vevT decays and therefore could also be used to look for ve appearance in a predominantly vn beam by detecting their charged current (CC) interactions veN — e-X. The main motivation for this search was the evidence for vp — ve and vp — ve oscillations found by the LSND experiment [2]. For vp — ve oscillations with Am2 > 10 eV2 and with the probability of 2.6 x 10-3 observed by LSND, a signal should be seen in the NOMAD data. The sensitivity of the NOMAD experiment to lower values of Am2 is limited by its L/Ev ratio of ^0.025 km/GeV, where L is the average source to detector distance and Ev is the average neutrino energy.
Preliminary results of the search for — ve oscillations in NOMAD were presented earlier [3]. In this paper the final results of a "blind" analysis [4] are reported.
2. NOMAD DETECTOR AND NEUTRINO BEAM
A detailed description of the NOMAD detector and its performance is given in [5]. The detector consisted of a large dipole magnet delivering a field of 0.4 T and housing several subdetectors, starting with an active target composed of 132 planes of
E-mail: popov@nusun.jinr.dubna.su
large 3 x 3-m drift chambers (DC) [6]. The walls of the chambers provided a low average density (0.1 g/cm3) target with a mass of 2.7 t. The density of the chambers was low enough to allow an accurate measurement of the momenta of the charged particles produced in the neutrino interactions. The chambers were followed by nine transition radiation detector (TRD) modules [7] each consisting of a polypropylene radiator and a plane of straw tubes operated with an 80%-xenon and 20%-methane gas mixture. An electromagnetic calorimeter (ECAL) [8] consisting of 875 lead glass blocks provided a measurement of the energies of electrons and photons with a resolution of a(E)/E = 3.2%/^£(GeV) + 1%. The ECAL was preceded by a lead-proportional tube preshower for better photon localization. A hadron calorimeter (HCAL) was located just beyond the magnet coil and was followed by two muon stations consisting of large-area DC, the first station located after 8, and the second one after 13 interaction lengths of iron.
The CERN West Area Neutrino Facility (WANF) neutrino beam [9] was produced by impinging 450-GeV protons extracted from the SPS onto a target consisting of beryllium rods adding up to a total thickness of 110 cm. The secondary particles emerging from the target were focused into a near parallel beam by two magnetic lenses (the horn and the reflector) providing toroidal magnetic fields. When running in neutrino mode, positively charged particles were focused. The focused particles then traversed a 290-m long decay tunnel followed by an iron and earth shield. Neutrinos originating from the decay of these particles traveled on average a distance of 625 m before reaching the NOMAD detector.
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Fig. 1. Composition of the v^ and ve energy spectra at NOMAD, within a transverse fiducial area of 260 x 260 cm, as predicted by the NOMAD simulation of the neutrino beam line.
Since the oscillation search implies a direct comparison between the measured and expected ratios of the number of ve CC to v^ CC interactions, an accurate prediction of the neutrino fluxes and spectra is crucial. They are computed with a detailed Monte Carlo simulation of the neutrino beam, referred to as NUBEAM and thoroughly described in [10]. This is implemented in three steps. First, the yields of the secondary particles from the interactions of 450-GeV protons with the Be target are calculated with the 2000 version of FLUKA [11], a generator of hadronic interactions. These yields are then modified in order to agree with all measurements currently available in the relevant energy and angular range, namely the
Average energies and relative abundances of the fluxes and charged current events of the four principal neutrino flavors at NOMAD, within a transverse fiducial area of 260 x 260 cm
Flavor Flux CC interactions
<£„>, Gev Rel. abund. (E), GeV Rel. abund.
24.3 1.0 47.5 1.0
17.2 0.068 42.0 0.024
Ve 36.4 0.010 58.2 0.015
Ve 27.6 0.0027 50.9 0.0015
SPY/NA56 [12] and NA20 [13] results. Finally, the propagation of the secondary particles is described by a simulation program based on GEANT3 [14].
The resulting energy spectra of v^ and ve, and of their components, are shown in Fig. 1. The v^ flux is predominantly due to decays of up to 60-GeV neutrino energy and to those of K + above this energy. The bulk of the ve flux comes from the decays of K+, with KL contributing at the level of about 18% and ^+ at the level of about 14%. The composition of the beam is shown in the table.
The neutrino fluxes generated by NUBEAM were used as an input to the NOMAD event generator to produce interactions of v^, ve, and ve. Deep-inelastic scattering events were simulated with a modified version of the LEPTO 6.1 event generator [15], with Q2 and W2 cutoffs removed. Quasielastic [16] and resonance production [17] events were generated as well. The GRV-HO parametrization [18] of the parton density functions and the nucleon Fermi motion distribution of [19], truncated at 1 GeV/c, were used along with JETSET 7.4 [20] to treat the fragmentation.
The secondary particles produced in these interactions were then propagated through a full GEANT3 [14] simulation of the NOMAD detector.
FINAL RESULTS ON THE SEARCH FOR vu — ve OSCILLATIONS
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3. DATA COLLECTION AND EVENT RECONSTRUCTION
The NOMAD experiment collected data from 1995 to 1998. Most of the running, a total exposure of 5.1 x 1019 protons on target (p.o.t.), was in neutrino 0.04 mode and yielded 1.3 x 106 CC interactions in the fiducial volume of the detector.
The trajectories of charged particles are recon- 0 structed from the hits in the DC and, from these trajectories, momenta are computed using the Kalman filter technique [21], which accounts for energy loss along the trajectory. As a first step the energy-loss model used is that for pions, resulting in a momentum estimate, pn, at the beginning of the track. Particles later identified as electrons or positrons are refitted [22] with an additional average energy loss due to bremsstrahlung, resulting in a new estimate, pe, of the momentum. Energy clusters in the ECAL not associated to charged particles are assumed to be due to photons.
Vertices are reconstructed from the trajectories of charged particles. The energy of the incident neutrino, Ev, is approximated by the total (visible) energy of an event computed from the sum of the energies of all observed primary particles and of photons.
Since the electron radiates bremsstrahlung photons in traversing the DC, in order to have an accurate measure of its energy, these photons must be identified and their energy added to the energy of the ECAL cluster at the end of the electron trajectory. Because of the curvature of the electron trajectory in the magnetic field these photons are located in a vertical band. The energy of photons in the ECAL and of photon conversions in the DC found in this region is included, resulting in a measure of the electron energy, Ebrem, with an average resolution of 2.1%.
Relative uncertainty
Ve/ V,
40
80
120 160 Energy, GeV
Fig. 2. Energy-dependent uncertainty in the prediction of
the Re/J, ratio.
The presence or absence of — ve oscillations is established by comparing the measured Rep with the one expected in the absence of oscillations. In order to avoid biases, we adopted a "blind analysis" strategy: the comparison of the measured to the predicted Rep is not made until the accuracy of the flux predictions and the robustness of the data analysis have been demonstrated and until all selection criteria are fixed. It should be noted that no-oscillation signal is expected to be measurable in ve since the intrinsic ratio of vp/ve in the beam is 4 times smaller than the intrinsic vp/ve ratio and the antineutrino statistics is limited.
4. PRINCIPLES OF OSCILLATION SEARCH
The vn ^ ve oscillation signal should manifest itself as an excess in the number of ve CC events over that expected for an intrinsic ve contamination in the beam (about 1% of vp). In order to reduce systematic uncertainties associated with absolute flux predictions and selection efficiencies, we study the ratio Rep of the number of ve CC to vp CC interactions. Due to different energy and radial distributions of incident electron and muon neutrinos, the contribution of the intrinsic ve component is smaller at low ve energies, Ev, where a low-Am2 oscillation signal is expected, and at small radial distances from the beam axis, r. Thus, the sensitivity of the search is increased by taking into account the dependence of Rep on Ev and r.
5. EVENT SELECTION
In order to calculate Rep, pure samples of ve CC and vp CC interactions are selected. A detailed description of the selection criteria is given in [4]. The initial data sample for ve CC interactions is compl
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