Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation $\ensuremath{\gamma}$ rays

Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation $γ$ rays

K. Abe et al. (T2K Collaboration)

Phys. Rev. D 90, 072012 – Published 31 October 2014

Abstract

We report the first measurement of the neutrino-oxygen neutral-current quasielastic (NCQE) cross section. It is obtained by observing nuclear deexcitation $γ$ rays which follow neutrino-oxygen interactions at the Super-Kamiokande water Cherenkov detector. We use T2K data corresponding to $3.01 \times 1 0^{20}$ protons on target. By selecting only events during the T2K beam window and with well-reconstructed vertices in the fiducial volume, the large background rate from natural radioactivity is dramatically reduced. We observe 43 events in the 4–30 MeV reconstructed energy window, compared with an expectation of 51.0, which includes an estimated 16.2 background events. The background is primarily nonquasielastic neutral-current interactions and has only 1.2 events from natural radioactivity. The flux-averaged NCQE cross section we measure is $1.55 \times 1 0^{- 38} {cm}^{2}$ with a 68% confidence interval of $(1.22, 2.20) \times 1 0^{- 38} {cm}^{2}$ at a median neutrino energy of 630 MeV, compared with the theoretical prediction of $2.01 \times 1 0^{- 38} {cm}^{2}$ .

Received 13 March 2014

DOI:https://doi.org/10.1103/PhysRevD.90.072012

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Vol. 90, Iss. 7 — 1 October 2014

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Figure 1
Distribution of the reconstruction quality parameter $Q_{rec}$ after the beam-unrelated selection cuts (timing, fiducial volume, $Q_{rec}$ ) have been applied. The inset shows the distribution before the energy-dependent cut on $Q_{rec}$ but including the timing and fiducial volume cuts. The T2K run 1–3 data are represented by points with statistical error bars, and the expectation is represented by stacked histograms showing the NCQE signal and the NC non-QE, CC, and beam-unrelated background components.
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Figure 2
The Cherenkov angle distribution in data and MC expectation after the beam-unrelated selections and the preactivity cut. The expectation has a three-peak structure corresponding to low-momentum muons around 28°, single $γ$ rays around 42°, and multiple $γ$ rays around 90°. A selection cut is applied at 34° to remove the muon events, but no attempt is made to separate single- and multiple- $γ$ events.
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Figure 3
$Δ T_{0}$ distribution of the data sample after all selection cuts except for beam timing, compared to the bunch center positions determined from high energy T2K neutrino events, indicated by eight dashed vertical lines. The on-timing and off-timing events are shown in solid and hashed, respectively.
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Figure 4
Vertex distribution of the final data sample in $Y$ versus $X$ after all selections have been applied. The solid and dotted lines indicate the boundaries of the inner detector and fiducial volume, respectively. The neutrino beam direction is indicated by the arrow.
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Figure 5
Comparison of the reconstructed energy spectrum between the selected data sample and the expectation after all selection cuts have been applied. The CC component is actually about twice as large as the beam-unrelated background, but it is less apparent since it is spread across all energies.
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Figure 6
The T2K measurement of the flux-averaged NCQE cross section, represented by a black point, compared with the calculated cross section from Ref. [8]. The dashed line shows the cross section versus neutrino energy, and the solid horizontal line shows the flux-averaged cross section. The vertical error bar on the data represents the 68% confidence interval on the measured cross section, while the horizontal error bar is placed at the central value from our data and represents 68% of the flux at each side of the median energy. The solid gray histogram shows the unoscillated T2K neutrino flux.
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Physical Review D

covering particles, fields, gravitation, and cosmology

Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation $γ$ rays

K. Abe et al. (T2K Collaboration)

Phys. Rev. D 90, 072012 – Published 31 October 2014

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Physical Review D

covering particles, fields, gravitation, and cosmology

Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation γ rays

K. Abe et al. (T2K Collaboration)

Phys. Rev. D 90, 072012 – Published 31 October 2014

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Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation $γ$ rays