Abstract
We present a study of exclusive neutral pion production in neutrino–nucleus Neutral Current interactions using data from the NOMAD experiment at the CERN SPS. The data correspond to 1.44×106 muon-neutrino Charged Current interactions in the energy range 2.5⩽Eν⩽300 GeV. Neutrino events with only one visible π0 in the final state are expected to result from two Neutral Current processes: coherent π0 production, ν+A→ν+A+π0 and single π0 production in neutrino–nucleon scattering. The signature of coherent π0 production is an emergent π0 almost collinear with the incident neutrino while π0's produced in neutrino–nucleon deep inelastic scattering have larger transverse momenta. In this analysis all relevant backgrounds to the coherent π0 production signal are measured using data themselves. Having determined the backgrounds, and using the Rein–Sehgal model for the coherent π0 production to compute the detection efficiency, we obtain 4630±522(stat)±426(syst) corrected coherent-π0 events with Eπ0⩾0.5 GeV. We measure σ(νA→νAπ0)=[72.6±8.1(stat)±6.9(syst)]×10−40 cm2/nucleus. This is the most precise measurement of the coherent π0 production to date.
Highlights
The Neutrino Oscillation MAgnetic Detector (NOMAD) experiment at CERN used a neutrino beam [12] produced by the 450 GeV protons from the Super Proton Synchrotron (SPS) incident on a beryllium target and producing secondary π±, K±, and KL0 mesons
Downstream of the magnet was a hadron calorimeter, followed by two muonstations each comprising large area drift chambers and separated by an iron filter placed at 8- and 13-λ’s downstream of the electromagnetic calorimeter (ECAL), that provided a clean identification of the muons
The photons will either both convert in the drift chambers (DC) target, or one of the photons will convert in the tracker and the other will be measured in the electromagnetic calorimeter (ECAL), or both photons will be measured in the ECAL
Summary
Precise measurement of π0 production when a neutrino scatters coherently off a target nucleus, ν + A → ν + A + π0, depicted in Figure 1, is challenging: the cross-section (σ) of coherent-π0 (Cohπ0 ) is 0.003 of the inclusive neutrino charged current (CC) interactions at Eν ≃ 25 GeV [1]; the single π0 is notoriously refractory to accurate identification in neutrino detectors. The past cross-section measurements of Cohπ0 have been poor, with a precision no better than ≃ 30% [2,3,4,5,6]; recently the MiniBOONE experiment has reported the fraction of Cohπ0 in all exclusive NC π0 production [7] This challenge is the primary motivation for the present analysis. The contribution of the isovector current to the Cohπ0 process is minimal where Z0 can be viewed as a ρ meson which produces a π0 exchanging an isoscalar ω with A This minimal contribution of the isovector current to the Cohπ0 arises from two reasons: (a) the cross section of the isovector ρ-A interaction is zero in the forward direction, a direction preferred by the nuclear form factor; and (b) the vector component has a contribution proportional to (1 − 2 sin θW ) reducing the isovector contribution further, the net reduction with respect to the axial part being a factor of 3.5.
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