Detection of back-to-back proton pairs in charged-current neutrino interactions with the ArgoNeuT detector in the NuMI low energy beam line

Abstract

Short range nucleon-nucleon correlations in nuclei (NN SRC) carry important information on nuclear structure and dynamics. NN SRC have been extensively probed through two-nucleon knockout reactions in both pion and electron scattering experiments. We report here on the detection of two-nucleon knock-out events from neutrino interactions and discuss their topological features as possibly involving NN SRC content in the target argon nuclei. The ArgoNeuT detector in the Main Injector neutrino beam at Fermilab has recorded a sample of 30 fully reconstructed charged current events where the leading muon is accompanied by a pair of protons at the interaction vertex, 19 of which have both protons above the Fermi momentum of the Ar nucleus. Out of these 19 events, four are found with the two protons in a strictly back-to-back high momenta configuration directly observed in the final state and can be associated to nucleon Resonance pionless mechanisms involving a pre-existing short range correlated np pair in the nucleus. Another fraction (four events) of the remaining 15 events have a reconstructed back-to-back configuration of a np pair in the initial state, a signature compatible with one-body Quasi Elastic interaction on a neutron in a SRC pair. The detection of these two subsamples of the collected (µ − + 2p) events suggests that mechanisms directly involving nucleon-nucleon SRC pairs in the nucleus are active and can be efficiently explored in neutrino-argon interactions with the LAr TPC technology. Introduction -The systematic study of the impact of nuclear effects on the determination of neutrino cross sections in the "few-GeV region" and on neutrino oscillation parameters, has developed into a very active field of theoretical and experimental research over the last decade [1]. Effects of long-range two-nucleon processes in the nuclear target are widely recognized as a source of apparent cross section enhancement in the neutrino Charged Current Quasi-Elastic (CC QE) response [2], sufficient to provide a possible solution to the excess of neutrino event rate in the energy range around ∼1 GeV observed by MiniBooNE [3]. Since emitted nucleons in Cherenkov detectors are typically below detection threshold, the socalled CCQE-like sample can in fact find contributions from both single nucleon knock-out events from a genuine CC QE reaction and from two-nucleon emission events produced for example by two-body meson-exchange currents (MEC).

Nuclear transparency in Monte Carlo neutrino event generators

“…Another strategy to learn about two-body current contribution is to also use information from the final state protons. ArgoNeuT experiment reported results on the distribution of relative proton angles in a two-proton sample of events [21,22]. MINERvA experiment used protons in a study of CC0π events on carbon, iron and lead targets with Q 2 (four-momentum transfer) derived from the leading proton only, as opposed to the common approach of using the muon information [23].…”

Final-state interactions in two-nucleon knockout reactions

“…This is a remarkable result that could help to quantify the effect of two-nucleon knockout contributions to quasi-elastic neutrino-nucleus and anti-neutrino nucleus responses [34]. computer Infrastructure) and services used in this work were provided by Ghent University, the Hercules Foundation uation with the nuclear medium is calculated with the aid of a classical survival probability P ( r ).…”

Characterization of nuclear effects in muon-neutrino scattering on hydrocarbon with a measurement of final-state kinematics and correlations in charged-current pionless interactions at T2K

“…This process includes not only other reactions such as pion production, in which the pion is absorbed inside the nucleus, but also 2p2h excitations involving two-nucleon knockout. CC0π (sometimes called "CCQE-like") interactions have been extensively measured [19,[35][36][37][38][39][40][41][42][43][44][45], yet the unambiguous identification of various nuclear effects has proved difficult. This is primarily because the often measured single-particle final-state kinematics, such as momentum and angular distributions, are determined by both the intrinsic dynamics of Eq.…”