Understanding the energy resolution of liquid argon neutrino detectors

Available estimates for the energy resolution of DUNE vary by as much as a factor of 4. To address this controversy, and to connect the resolution to the underlying physical processes, we build an independent simulation pipeline for neutrino events in liquid argon, combining the public tools genie and fluka. Using this pipeline, we first characterize the channels of nonhermeticity of DUNE, including subthreshold particles, charge recombination, and nuclear breakup. Particular attention is paid to the role of neutrons, which are responsible for a large fraction of missing energy in all channels. Next, we determine energy resolution, by quantifying event-to-event stochastic fluctuations in missing energy. This is done for several sets of assumptions about the reconstruction performance, including those available in the literature. The resulting migration matrices, connecting true and reconstructed neutrino energies, are presented. Finally, we quantify the impact of different improvements on the experimental performance. For example, we show that dropping particle identification information degrades the resolution by a factor of 2, while omitting charge deposits from deexcitation gammas worsens it by about 25%. In the future, this framework can be used to assess the impact of cross section uncertainties on the oscillation sensitivity.

Figure 1

A neutrino event at DUNE: a conceptual illustration.

Figure 2

An example simulated 4 GeV ${\nu }_{\mu }$ event using genie and fluka. The magenta energy deposits are caused by neutrons undergoing multiple scatterings; the orange color denotes energy originally carried by the prompt charged pion.

Figure 3

Hadronic energy distributions produced in the scattering on argon of 4 GeV neutrinos (blue) and antineutrinos (brown). The average hadronic energy is 1.6 GeV for $\nu$, and 1.0 GeV for $\overline{\nu }$.

Figure 4

Same as the blue histogram in Fig. 3, but broken down according to the physical processes involved (per genie default tune).

Figure 5

Hadronic composition of ten CC ${\nu }_{\mu }+{}^{40}\mathrm{Ar}$ scattering events. The neutrino energy is 4 GeV in each event.

Figure 6

Hadronic energy budget after primary neutrino interaction. A set of 10 000 4 GeV ${\nu }_{\mu }+{}^{40}\mathrm{Ar}$ scattering events has been averaged over. Shown are the fractions of the hadronic energy that go into prompt neutrons (n, pro), subthreshold particles according to Table 1 (th, pro) and the rest (ion).

Figure 7

Examples of 500 MeV neutron events. The neutrons are injected at (0, 0).

Figure 8

Energy distribution of the neutrons in 4 GeV neutrino interactions. The top histogram corresponds to prompt neutrons, which come from the nucleus struck by the neutrino; the bottom one corresponds to secondary neutrons, which are knocked out of other argon nuclei as the event develops in the detector.

Figure 9

Energy breakdown of neutrons at different energies. Shown are the energy fractions that go to ionization (solid blue) and to nuclear breakup (dashed orange). Also shown is the fraction of the ionization energy in particles above the CDR thresholds (dashed blue). The ionization charges are given before charge recombination.

Figure 10

The final energy budget of the first ten events from genie, depicted earlier in Fig. 5. The left and right panels show two fluka realizations. Here, “charge” reflects the total ionization produced, “rec” refers to charge lost to recombination, “nucl” is the energy that goes into the breakup of argon nuclei in the detector and “$\nu$” is the energy carried away in neutrinos from $\pi /\mu$ decays. The corresponding processes involving neutrons are labeled separately.

Figure 11

Histograms of the energy fraction that goes to ionization charge, for hadron- and electron-induced showers. Initial neutrinos have a flat spectrum in the [2, 4] GeV range.

Figure 12

Hadronic energy budget after fully propagating neutrino events (cf. Fig. 6). The averaging was performed over a set of 10 000 4 GeV ${\nu }_{\mu }$ CC interactions. Shown are the fractions of the hadronic energy that go into ionization charge (charge) above and below the CDR thresholds, that are lost to recombination (rec), lost to nuclear breakup (nucl) and, finally, that escape as decay neutrinos ($\nu$). As in Fig. 10, the corresponding processes for neutrons are shown separately ($n$).

Figure 13

The energy budget for ${\nu }_e+{}^{40}\mathrm{Ar}$ (left panel) and ${\overline{\nu }}_e+{}^{40}\mathrm{Ar}$ (right panel) scattering events as a function of the (anti)neutrino energy. Both the electromagnetic shower from the final-state electron and the hadronic system are included. The hadronic energy channels are the same is in Fig. 12. In contrast to Fig. 12, however, hit-finding thresholds have been applied here (see text).

Figure 14

Simulations of reconstructed neutrino energies for $E_{\nu }=3\mathrm{GeV}$ true energy in the CC ${\nu }_e+{}^{40}\mathrm{Ar}$ scattering process. The histograms correspond to three different sets of assumptions, as described in the text.

Figure 15

Same as Fig. 14, but for ${\overline{\nu }}_e+{}^{40}\mathrm{Ar}$ scattering.

Figure 16

Migration matrices for ${\nu }_e$ (top) and ${\overline{\nu }}_e$ (bottom) CC scattering in argon in the three scenarios considered in the text: CDR thresholds (left), total charge calorimetry (middle), and hit-finding thresholds with charge recombination corrections (right).

Figure 17

Energy resolution numbers for each of the three reconstruction scenarios considered in the text. Solid curves correspond to ${\nu }_e$ scattering, and dashed curves to ${\overline{\nu }}_e$.

Figure 18

Comparison of the histograms of the total charge created in 4 GeV interactions of ${\nu }_e$: our simulations in scenario 2 vs the findings of Ref. [12].

Figure 19

Histogram of reconstructed neutrino energies for $E_{\nu }=3\mathrm{GeV}$ true energy in the CC ${\nu }_e+{}^{40}\mathrm{Ar}$ scattering process in scenario 3, broken down based on event type (cf. green histogram in Fig. 14).

Figure 20

The average energy fractions in different prompt final-state hadrons, for ${\nu }_e+{}^{40}\mathrm{Ar}$ (left panel) and ${\overline{\nu }}_e+{}^{40}\mathrm{Ar}$ (right panel), as a function of the (anti)neutrino energy.