Physics Letters B 762 (2016) 124–130
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Emission of neutron–proton and proton–proton pairs in neutrino
scattering
I. Ruiz Simo
a
, J.E. Amaro
a,∗
, M.B. Barbaro
b,c
, A. De Pace
c
, J.A. Caballero
d
, G.D. Megias
d
,
T.W. Donnelly
e
a
Departamento de Física Atómica, Molecular y Nuclear, and Instituto de Física Teórica y Computacional Carlos I, Universidad de Granada, Granada 18071, Spain
b
Dipartimento di Fisica, Università di Torino, Via P. Giuria 1, 10125 Torino, Italy
c
INFN, Sezione di Torino, Via P. Giuria 1, 10125 Torino, Italy
d
Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, Apdo. 1065, 41080 Sevilla, Spain
e
Center for Theoretical Physics, Laboratory for Nuclear Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
29 July 2016
Received
in revised form 12 September
2016
Accepted
13 September 2016
Available
online 16 September 2016
Editor: W.
Haxton
We use a recently developed model of relativistic meson-exchange currents to compute the neutron–
proton
and proton–proton yields in (ν
μ
, μ
−
) scattering from
12
C in the 2p–2h channel. We compute the
response functions and cross sections with the relativistic Fermi gas model for different kinematics from
intermediate to high momentum transfers. We find a large contribution of neutron–proton configurations
in the initial state, as compared to proton–proton pairs. In the case of charge-changing neutrino scattering
the 2p–2h cross section of proton–proton emission (i.e., np in the initial state) is much larger than for
neutron–proton emission (i.e., two neutrons in the initial state) by a (ω, q)-dependent factor. The different
emission probabilities of distinct species of nucleon pairs are produced in our model only by meson-
exchange
currents, mainly by the isobar current. We also analyze other effects including exchange
contributions and the effect of the axial and vector currents.
© 2016 The Authors. Publishe d by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
The identification of nuclear effects in neutrino scattering is
essential for modern neutrino oscillation experiments [1–7]. In
particular the sensitivity of the neutrino energy reconstruction to
multi-nucleon events has been stressed in recent data analyses [8].
In the MINERvA neutrino experiment an enhanced population of
multi-proton states has been observed between the quasielastic
and peaks. On the other hand, observation of events with a pair
of energetic protons at the interaction vertex accompanying the
muon in
40
Ar(ν
μ
, μ
−
) reaction has been reported in the ArgoNeuT
experiment [9]. From these events several back-to-back nucleon
configurations have been identified and associated with nuclear
mechanisms involving short-range correlated (SRC) neutron–proton
(np) pairs in the nucleus [10]. However in [11] these “hammer
events” have been modeled by a simple pion production and reab-
sorption
model without nucleon–nucleon correlations, suggesting
that the distribution of pp pairs in the final state is less sensi-
*
Corresponding author.
E-mail
address: amaro@ugr.es (J.E. Amaro).
tive to details of the initial pair configuration. In the opinion of
the authors of [11], the events cannot teach us anything significant
about SRC. The NUWRO event generator supports that the excess
of back-to-back events in ArgoNeuT has a kinematic origin and is
not directly related to SRC [12].
SRC
with back-to-back configurations have been also identified
in two-nucleon knock-out electron scattering experiments on
12
C
for high momentum transfer and missing momentum [13,14]. In
this case one expects an excess of np pairs over pp pairs [15,
16].
The experiment reported a number of np pairs 18 times
larger than their pp counterparts. The analysis of these experi-
ments
is compatible with theoretical single-nucleon and nucleon
pair momentum distributions in variational Monte Carlo calcula-
tions,
where the importance of the tensor forces in the ground-
state
correlations of nuclei has been emphasized [17,18]. While the
kinematics of the experiments have been selected to minimize the
contribution from other mechanisms that can induce two-particle
emission, such as meson-exchange currents (MEC) and isobar exci-
tations
[13], the contribution of MEC cannot be ruled out a priori
[19].
In
this work we investigate the relative effect of MEC on the
separate pp and np emission channels in the inclusive 2p–2h neu-
http://dx.doi.org/10.1016/j.physletb.2016.09.021
0370-2693/
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.
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