Implementation of the SuSAv2-meson exchange current 1p1h and 2p2h models in GENIE and analysis of nuclear effects in T2K measurements

We first present the implementation and validation of the SuSAv2-MEC 1p1h and 2p2h models in the GENIE neutrino-nucleus interaction event generator and a comparison of the subsequent predictions to measurements of lepton and hadron kinematics from the T2K experiment. These predictions are also compared to those of other available models in GENIE. We further compare semi-inclusive predictions of the implemented 1p1h model to those of the microscopic model on which SuSAv2 is based—relativistic mean field—to begin to test the validity of widely used “factorization” assumptions employed by generators to predict hadron kinematics from inclusive input models. The results highlight that a more precise treatment of hadron kinematics in generators is essential in order to attain the few-% level uncertainty on neutrino interactions necessary for the next generation of accelerator-based long-baseline neutrino oscillation experiments.

Figure 1

Total 2p2h cross section for muon-neutrino and antineutrino interactions on carbon for the empirical model available in GENIE, the SuSAv2 2p2h model discussed in this manuscript and the Valencia model as implemented in GENIE [5, 44].

Figure 2

2p2h cross sections (in units of ${\mathrm{cm}}^2$) in bins of energy ($q_0$) and momentum ($q_3$) transfer for muon-neutrino interactions on carbon for different models. The top left is the GENIE empirical model, the top right is the implemented SuSAv2 2p2h prediction and the lower plot is the GENIE implementation of the Valencia model (where the 1.2 GeV cutoff in the model discussed in the text is clear).

Figure 3

A comparison of 2p2h double-differential cross sections in muon momentum for two different angular slices for muon-neutrino interactions on carbon for different models, split by the contribution from the different initial state correlated pairs: neutron-neutron ($nn$) or neutron-proton ($np$). The left plots are from the GENIE empirical model, the center are from the implemented SuSAv2 prediction and the right plots are from the GENIE implementation of the Valencia model.

Figure 4

Comparison of muon-neutrino single differential 1p1h cross sections on carbon at T2K kinematics as a function of the muon kinematics as both an inclusive (top panels) and a semi-semi-inclusive cross section (bottom panels), the latter meaning that a restriction that there are no protons with momenta above 500 MeV is applied. In the inclusive case the RMF, SuSAv2 model and SuSAv2-GENIE implementation are compared. In the semi-semi-inclusive case the RMF prediction is compared to those of GENIE using the implemented SuSAv2 model and variations of the factorization approach. These variations are split depending on whether an FSI cascade was applied and whether the nuclear removal energy is fixed or kinematic dependent (as described in Sec. 2). The full set of angular slices is shown in Appendix pp3.

Figure 5

Comparisons of data and model predictions for differential $\mathrm{CC}0\pi$ muon-neutrino cross sections on carbon in the T2K neutrino beam as a function of the muon kinematics when there are no protons with momenta above 500 MeV (making the model predictions semi-semi-inclusive). Two 1p1h predictions are shown (one from RMF, the other from SuSAv2 implemented in GENIE), in addition to the SuSAv2 2p2h and pion absorption contributions from GENIE. The (unstacked) contribution from 2p2h and pion absorption are shown, as are the total contributions when using each of the two 1p1h models. Goodness of fit is calculated to be ${\chi }_{\mathrm{RMF}}^2=171.87$ (59 bins) and ${\chi }_{\mathrm{SuSA}}^2=168.92$ (60 bins), where the latter includes a single extra bin from $-1.0$ to $-0.3\cos \theta$ (not shown). The data points are taken from [59].

Figure 6

The differential $\mathrm{CC}0\pi$ muon-neutrino cross sections on carbon in the T2K neutrino beam as a function of the muon kinematics when there is at least one proton with momentum above 500 MeV in the final state as measured by T2K and calculated using the SuSAv2 1p1h and 2p2h models implemented in GENIE alongside the pion absorption contribution. The (unstacked) contribution from each interaction mode 2p2h is shown. The goodness of fit is calculated to be ${\chi }_{1p}^2=100.72$ (33 bins). The combined goodness of fit for the SuSAv2 prediction here and in Fig. 5 is ${\chi }_{\text{combined}}^2=490.98$ (93 bins). The data points are taken from [59].

Figure 7

The regularized T2K measurement of $\mathrm{CC}0\pi$ muon-neutrino cross sections on carbon at T2K kinematics as a function of the single transverse variables [67] compared to predictions from the GENIE-implemented SuSAv2 and Valencia $1\mathrm{p}1\mathrm{h}+2\mathrm{p}2\mathrm{h}$ models, each of which is added to GENIE’s pion absorption prediction. The (unstacked) contribution from 2p2h and pion absorption is shown, as are the total contributions when using each of the two 1p1h models. For $\delta p_T$ a shape only comparison is also shown. Goodness of fit are calculated as follows. For $\delta p_T$: ${\chi }_{\mathrm{SuSA}}^2=20.5$, ${\chi }_{\mathrm{Valencia}}^2=27.1$. For $\delta {\alpha }_T$: ${\chi }_{\mathrm{SuSA}}^2=45.3$, ${\chi }_{\mathrm{Valencia}}^2=31.4$. For $\delta {\varphi }_T$: ${\chi }_{\mathrm{SuSA}}^2=40.1$, ${\chi }_{\mathrm{Valencia}}^2=36.8$. The data points are taken from [59].

Figure 8

Comparison of the T2K $\mathrm{CC}0\pi$ measurement of the muon-neutrino cross section on carbon with the SuSAv2 model ($1\mathrm{p}1\mathrm{h}+2\mathrm{p}2\mathrm{h}$) and a pion-absorption contribution as implemented in GENIE. The (unstacked) contribution from each interaction mode is shown separately, as well the total prediction. Comparison between 1p1h and 2p2h GENIE implementation (histograms) and the microscopic calculations (smooth curves) is also shown for model implementation validation. The goodness of fit is ${\chi }^2=255.8$ (67 bins). The data points are taken from [71].

Figure 9

Total 2p2h cross section for muon-neutrino and antineutrino interactions on carbon for different implementations of SuSAv2. The param label specifies that the calculation is made using the SuSAv2 parametrization of the microscopic model (either in GENIE or using the original fortran code).

Figure 10

Comparison of the T2K $\mathrm{CC}0\pi$ measurement of muon-neutrino interactions on carbon with the SuSAv2 and Valencia models ($1\mathrm{p}1\mathrm{h}+2\mathrm{p}2\mathrm{h}$) each with an additional pion-absorption contribution as implemented in GENIE. The (unstacked) contribution from each interaction mode is shown separately, as well the total prediction. The top plots are the SuSAv2 predictions whilst the Valencia ones are below. The data points are taken from [71].

Figure 11

Comparison of the T2K $\mathrm{CC}0\pi$ measurement of muon-neutrino interactions on carbon where there are no protons above 500 MeV with the SuSAv2 and Valencia models ($1\mathrm{p}1\mathrm{h}+2\mathrm{p}2\mathrm{h}$) each with an additional pion-absorption contribution as implemented in GENIE. The (unstacked) contribution from each interaction mode is shown separately, as well the total prediction. The top plots are the SuSAv2 predictions whilst the Valencia ones are below. The data points are taken from [59].

Figure 12

An extended version of Fig. 4. A comparison of muon-neutrino single differential 1p1h cross sections on carbon at T2K kinematics as a function of the muon kinematics as both an inclusive and a semi-semi-inclusive cross section, the latter applying a restriction that there are no protons with momenta above 500 MeV. In the inclusive case the RMF, SuSAv2 model and SuSAv2-GENIE implementation are compared. In the semi-semi-inclusive case the RMF prediction is compared to those of GENIE using the implemented SuSAv2 model and the factorization approach. The latter is split depending on whether an FSI cascade was applied and whether the nuclear removal energy is fixed or kinematic dependent (as described in Sec. 2).