Benchmarking intranuclear cascade models for neutrino scattering with relativistic optical potentials

Abstract

Background: In neutrino oscillation experiments, the hadrons created in neutrino-nucleus collisions are becoming important observables. The description of final-state interactions (FSI) of hadrons with nuclei in the large phase space probed in these experiments poses a great challenge. In the analysis of neutrino experiments, which operate under semi-inclusive conditions, cascade models are commonly used for this task. The description of FSI under exclusive conditions on the other hand can be treated successfully by using relativistic optical potentials (ROP).Purpose: We formulate conditions under which the ROP approach and cascade model can be directly compared. Through this comparison the treatment of FSI in cascade models is studied and benchmarked.Method: We study single proton knockout with the T2K near-detector muon neutrino flux. We feed the NEUT cascade model with events distributed according to the cross section of a relativistic distorted-wave impulse approximation (RDWIA) calculation that uses the real part of an optical potential (rROP). We impose cuts on the missing energy of the resulting events to define a set of events which undergo only elastic FSI, these can be compared to RDWIA calculations with the full optical potential. Results:The NEUT cascade and ROP give similar cross sections for proton kinetic energies Tp > 150 MeV for carbon, oxygen and calcium nuclei. A necessary condition is that a realistic nuclear density is used to introduce events in the cascade. For Tp < 100 MeV the ROP and NEUT cross sections diverge strongly in shape and differences in magnitude are larger than 50 %. Single transverse variables allow to discriminate between different approaches to FSI, in particular the large δαT region is sensitive to the presence of non-elastic FSI. Due to experimental errors and a large non-QE contribution the comparison to T2K data does not give an unambiguous view of FSI. We discuss electron scattering data and provide results for kinematics covered in the e4ν analysis. We argue that with a simple cut in missing energy FSI can be studied with minimal confounding factors. Conclusions:The agreement of the ROP and NEUT cascade under T2K conditions lends confidence to these models as a tool in neutrino oscillation analyses for sufficiently large nucleon kinetic energies. Our results urge for caution when a cascade model is applied for small nucleon energies. The assessment of model assumptions relevant to this region are strongly encouraged. The approach presented in this paper provides novel constraints on cascade models from proton-nucleus scattering and can be easily applied to other neutrino event generators.

From inclusive to semi-inclusive one-nucleon knockout in neutrino event generators

“…In Ref. [20] the RDWIA with a real potential was used as input to the INC in the NEUT generator [6]. The results compare well with the T2K data of Ref.…”

“…The events are distributed according to the (Q 2 ) 2 -weighted cross section obtained in the RDWIA (for details see Refs. [3,20])…”

“…The sum is over the invariant masses of the residual system, given by the RMF model as in Ref. [20]; we only include scattering with protons. We impose cuts for the outgoing electron 40 • > θ e > 17 • and E e > 400 MeV, but include the full nucleon phase space.…”

“…Alternatively, the RDWIA events can be used directly in conjunction with a cascade model as done here and in Ref. [20]. This approach will be used in future work to study the influence of nucleon distortion on the semi-inclusive cross section, and to establish the applicability and limitations of the cascade models used in neutrino event generators.…”

Final state interactions in semi-inclusive neutrino-nucleus scattering: Applications to the T2K and MINERνA experiments

“…Moreover, this approach could give inconsistent results when the nuclear model used to generate the outgoing nucleon is different from the nuclear model used in the inclusive cross section, as is the case in the current SuSAv2 implementation in GENIE [29]. Furthermore, the results from this approach rely strongly on the semiclassical description of FSI commonly used in neutrino event generators [31,32], which have been shown to be unable to produce microscopic FSI predictions at low outgoing nucleon momenta [33].…”