Anti-Electron Neutrinos at High-Energy Neutrino Experiments: Identification Strategies and Physics Potential

Abstract

Most existing and proposed high energy neutrino experiments have excellent muon charge identification capabilities, enabling the distinction of $ν_μ$ and $\bar ν_μ$ charged current interactions. In contrast, distinguishing electrons and positrons from $ν_e$ and $\bar ν_e$ interactions is typically impossible, as they quickly interact within the characteristically dense detector material and fail to reach the spectrometer. In this letter, we propose a compact and cost-effective plastic target, placed right before the spectrometer, to maximize the rate of electrons and positrons that reach the spectrometer before interacting. We demonstrate that, when installed at the FASER experiment, the Forward Physics Facility, or SHiP, this setup could enable the first separate measurement of $ν_e$ and $\barν_e$ cross sections at high energy. Additionally, this setup opens new opportunities to study forward particle production at collider neutrino experiments, such as constraining forward $Λ$ hyperon production, and, by reducing flux uncertainties, significantly improve limits on non-standard neutrino interactions in neutral currents.

Figures (7)

• Figure 1: Schematic of the setup, illustrating the position of the proposed plastic target and the path of the electrons and positrons through the experiment.
• Figure 2: The energy dependence of the detector performance metrics for a 20 cm detector. The result of the BDT analysis (black) is contrasted with the result of a simple cut and count analysis (blue).
• Figure 3: The events passing the BDT selection criteria and containing an electron of $E_{e} >100~\text{GeV}$, originating at a given depth in the detector, for neutrino energies of 357 GeV (brown), and 1129 GeV (teal). The solid (dashed) lines correspond to $\nu_e$CC ($\overline{\nu}_e$CC) events, and the numbers of events originating at a given depth are given on the left vertical axis. Detectors deeper than 50 cm add no significant contributions to the number of signal events with a high-energy electron. The black dashed line indicates the BDT charge identification success rate, with values given on the right vertical axis.
• Figure 4: Predicted precision of the neutrino-nucleon cross sections for $\nu_e$ and $\bar{\nu}_e$, normalized to the neutrino energy, using a plastic target at FASER, the FPF and SHiP. We also show previous measurements performed at Gargamelle Gargamelle:1977pmg, E53 Baltay:1988au, DONuT DONuT:2007bsg and FASER$\nu$FASER:2024ref. The dotted line shows the predicted cross section obtained using the Bodek-Yang model Bodek:2002vp as implemented in GENIE Andreopoulos:2009rqGENIE:2021npt. See text for details.
• Figure 5: Flux of $\nu_e$, $\bar{\nu}_e$, their sum, and their difference, originating from the unconstrained $K^0$, $\Lambda$ and charm hadrons passing through the plastic target at FASER and the FPF. The neutrino flux from $\Lambda$ decays is shown for comparison. The errorbars depict the expected precision of its measurement as inferred from the statistical uncertainties in the number of $\nu_e$ and $\bar{\nu}_e$ CC interaction events in the plastic target.