Neutral-current background induced by atmospheric neutrinos at large liquid-scintillator detectors. I. Model predictions

The experimental searches for diffuse supernova neutrino background and proton decay in next-generation large liquid-scintillator (LS) detectors are competitive with and complementary to those in the water-Cherenkov detectors. In this paper, we carry out a systematic study of the dominant background induced by atmospheric neutrinos via their neutral-current (NC) interactions with the ${}^{12}\mathrm{C}$ nuclei in the LS detectors. The atmospheric neutrino fluxes at the location of Jiangmen Underground Neutrino Observatory (JUNO) are used, as the JUNO detector is obviously a suitable representative for future LS detectors. Then, we implement the sophisticated generators genie and nuwro to simulate the neutrino interactions with the carbon nuclei, and the package talys to deal with the deexcitations of final-state nuclei. Finally, the event rates for the production of additional nucleons, $\gamma$’s, $\alpha$’s, pions, and kaons are obtained and categorized, and the systematic uncertainty of the NC background represented by a variety of data-driven nuclear models is estimated. The implications of the NC background from atmospheric neutrinos for the detection of diffuse supernova neutrino background and proton decay are also discussed.

Figure 1

The predicted fluxes ${\varphi }_{\nu }(E_{\nu })\times E_{\nu }^3[{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}{\mathrm{GeV}}^2]$ (upper panel) and the associated relative uncertainty (lower panel) ${\sigma }_{\mathrm{rel}}$ of atmospheric neutrinos for $\nu ={\nu }_{\mu },{\overline{\nu }}_{\mu },{\nu }_e,{\overline{\nu }}_e$ at the JUNO site, as calculated by the Honda group in Ref. [53].

Figure 2

The cross section $\sigma (E_{\nu })/E_{\nu }[{10}^{-38}{\mathrm{cm}}^2{\mathrm{GeV}}^{-1}]$ of the NC interactions between neutrinos and the ${}^{12}\mathrm{C}$ nucleus has been given in the left panel, while that for antineutrinos $\sigma (E_{\overline{\nu }})/E_{\overline{\nu }}[{10}^{-38}{\mathrm{cm}}^2{\mathrm{GeV}}^{-1}]$ is given in the right panel, where the results for all six typical models in Table 1 have been shown.

Figure 3

The distribution of the total event rate induced by atmospheric neutrino NC interactions with respect to the neutrino energy for each of six representative interaction models in Table 1. The atmospheric neutrino fluxes in Fig. 1 and the inclusive cross sections in Fig. 2 have been input in the calculations, and the ratio of the event rate in each model to that in Model-N1 has been shown in the lower panel.

Figure 4

The event rates for the NC interactions of atmospheric neutrinos (with energies ranging from 100 MeV to 10 GeV) with ${}^{12}\mathrm{C}$ nuclei in the exclusive channels, which have been categorized by the daughter residual nuclei and the associated superscript “$*$”, highlight possible excited states. In each channel, the event rates predicted by Model-G, Model-N1, and Model-N5 are shown as the solid filled histogram, the striped histogram with slashes, and the striped histogram with backslashes, respectively. For each exclusive channel in a specified model, the contributions from the processes with one or more extra nucleons are represented by the colored bars. The results for ${}^{12}\mathrm{C}$ and other isotopes with the atomic number $Z<3$ or the mass number $A<6$ have been given in the rightmost panel and labeled “others”.

Figure 5

The event rates for the NC interactions of atmospheric neutrinos (with energies ranging from 100 MeV to 10 GeV) with ${}^{12}\mathrm{C}$ nuclei in the exclusive channels, which have been categorized by the daughter residual nuclei in their ground states and the deexcitation of the excited residual nuclei has been simulated by using talys. The notations and the patterns of the histograms in each panel exactly follow those in Fig. 4.

Figure 6

The kinetic energy spectra of $p$ or $n$ from the NC interactions of atmospheric neutrinos with ${}^{12}\mathrm{C}$ from Model-G (red dashed curve). Model-$\mathrm{N}i$ for $i=1$, 2, 3, 4 (black dashed curve) and Model-N5 (blue dashed curve) have been presented in the upper panel, where those of $p$, $n$, and $\alpha$ from the deexcitation of final-state nuclei in the NC interactions have been denoted by blue, magenta, and orange solid curves, respectively. The gray band along the black dashed curve represents $1\sigma$ deviation from the average of the predictions from Model-$\mathrm{N}i$ for $i=1$, 2, 3, 4. The colored band along the solid curve stands for $1\sigma$ deviation from the average of all six models, for which the deexcitation processes are the same. In the lower panel, the energy spectrum of $\gamma$ from the deexcitation of final-state nuclei has been shown as gray histograms. The production rates are shown with respect to the kinetic energies of relevant particles, and the QEL with MEC effects dominates the production.

Figure 7

The event rates for the NC interactions of atmospheric neutrinos (with energies ranging from 100 MeV to 10 GeV) with ${}^{12}\mathrm{C}$ nuclei in the exclusive channels, which are categorized by the associated neutron multiplicities. The predictions from Model-G, Model-$\mathrm{N}i$ for $i=1$, 2, 3, 4 and Model-N5 are denoted as dots, squares, and triangles, respectively. Note that the squares with error bars stand for the mean value of the predictions from Model-$\mathrm{N}i$ (for $i=1$, 2, 3, 4) and $1\sigma$ deviation. In all the panels, the raw rates are extracted from those in Fig. 5, and the gray bands represent the $1\sigma$ uncertainties from all six models. In the rightmost panel of the second row for a single neutron, the total rate $(16.5\pm 2.8){\mathrm{kt}}^{-1}{\mathrm{yr}}^{-1}$ (black) and that $(3.1\pm 0.5){\mathrm{kt}}^{-1}{\mathrm{yr}}^{-1}$ for the DSNB-like events (blue) should be read off from the vertical axis with red ticks.

Figure 8

Neutron multiplicity distributions of the NC interactions of atmospheric neutrinos (with energies ranging from 100 MeV to 10 GeV) with ${}^{12}\mathrm{C}$ nuclei have been categorized by the multiplicities of the charged pions in the COH, RES, and DIS processes. The predictions simulated by using Model-G, Model-N1, and Model-N5 are denoted by dots, squares, and triangles, respectively.

Figure 9

The multiplicity distribution of charged kaons from the NC interactions of atmospheric neutrinos (with energies ranging from 100 MeV to 10 GeV) with ${}^{12}\mathrm{C}$ nuclei in the COH, RES, and DIS processes. Note that there are no NC events with more than three charged kaons in Model-G, Model-N1, and Model-N5.

Figure 10

The differential event rate for the production of ${\pi }^{\pm }$ and $K^{\pm }$ with respect to the energy transfer, where the results of any number of ${\pi }^{\pm }$ and $K^{\pm }$ are given in the left panel and those of the exclusive one single ${\pi }^{\pm }$ or $K^{\pm }$ are shown in the right panel. Note that only Model-G and Model-N1 have been implemented to illustrate the difference between genie and nuwro generators.