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Electron recombination in highly ionizing stopping protons and deuterons is studied in the ArgoNeuT detector. The data are well modeled by either a Birks model or a modified form of the Box model. The dependence of recombination on the track angle with respect to the electric field direction is much weaker than the predictions of the Jaffe columnar theory and by theoreticalcomputational simulations.

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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).

The ArgoNeuT collaboration presents measurements of inclusive muon neutrino and antineutrino charged current differential cross sections on argon in the Fermilab NuMI beam operating in the low energy antineutrino mode. The results are reported in terms of outgoing muon angle and momentum at a mean neutrino energy of 9.6 GeV (neutrinos) and 3.6 GeV (antineutrinos), in the range 0• < θµ < 36 • and 0 < pµ < 25 GeV/c, for both neutrinos and antineutrinos.The liquid argon time projection chamber (LArTPC) [1] is a detection technique that provides three-dimensional imaging of charged particle interactions with fine spatial resolution, ideal for the study of neutrino interactions [2-4] and searches for rare phenomena such as proton decay. In the U.S., ArgoNeuT (Argon Neutrino Test) [5] and MicroBooNE [6] represent an important development towards the realization of multikiloton-scale LArTPC detectors. With the recent decision for the future U.S. long-baseline neutrino detector to be a large LArTPC [7], it is crucial to understand both neutrino and antineutrino interactions on argon for future CP violation searches. Neutrino cross sections have been measured on a variety of targets [8][9][10][11]; however, measurements on liquid argon are virtually non-existent [3]. In this Letter, we present flux-averaged ν µ and ν µ charged current (CC) differential cross sections as measured with the ArgoNeuT LArTPC exposed to the NuMI beam [12] in a neutrino kinematic range relevant for current and future accelerator-based neutrino experiments. The integrated cross sections are also reported. This paper, representing the first measurement of ν µ -Ar cross sections, follows the analysis approach of our previous measurement [3], differing in its study of the NuMI beam operating in antineutrino mode in place of neutrino mode.The NuMI beam can be configured to produce a ν µ or ν µ enhanced beam by changing the polarity of the current in the magnetic focusing horns. The neutrinos in the antineutrino beam have a broad energy spectrum and originate from the decay of forward-going π + 's and K + 's that were not defocused by the horns. The antineutrinos from focused π − 's come in a narrowly distributed, lower energy spectrum. Neutrino interactions comprise almost 60% of all the neutrino/antineutrinoinduced events in the detector in antineutrino mode [13]. This fact enables the measurements of both ν µ and ν µ CC differential cross sections using this data sample.ArgoNeuT [5] is the first LArTPC to take data in a low energy neutrino beam. ArgoNeuT collected neutrino and antineutrino events in Fermilab's NuMI beamline at the MIarXiv:1404.4809v3 [hep-ex]