Finding BSM Needles in Electromagnetic Haystacks at DUNE

Abstract

In this work, motivated by several beyond the Standard Model signal topologies, we perform detailed background mitigation analyses for the DUNE near detector. Specifically, we investigate $e^+ e^-$, $e^- γ$, $γ$, and $γγ$ final states that may arise from long-lived particles, including light mediators, dark matter, heavy neutral leptons, and axion-like particles (ALPs), decaying or scattering inside the liquid argon detector. To this end, we employ both photophilic and leptophilic ALPs as phenomenological benchmarks. The aforementioned final states leave a hard electromagnetic signature with no hadronic activity above the detector energy thresholds. Nevertheless, such signatures are not immune to backgrounds from neutrino scattering in the detector, which are in the focus of our study. In order to model realistic experimental analyses, we take into account particle misidentification rates, cross-contamination effects, and detector responses. We calculate confidence limit projections for DUNE, thereby presenting realistic capabilities for constraining or discovering new physics manifested through electromagnetic showers.

Figures (13)

• Figure 1: Top row: The $g_{a\gamma}$-driven (a) ALP production from Primakoff scattering and detection (b,c) due to inverse Primakoff and di-photon decays. Middle row: $g_{ae}$-driven ALP production from Compton scattering, associated production from electron-positron annihilation, bremsstrahlung, and resonant annihilation. Bottom row: $g_{ae}$-driven detection through decays to an electron-positron pair, inverse Compton scattering, and electron-positron pair production through atomic scattering.
• Figure 2: Schematic for the treatment of ALP flux, shown for a photon-produced ALP. ALP propagation to the detector is shown; the acceptance to the detector is decided by integrating the angular spectrum over the detector solid angle. Here, the angles $\theta_a^\prime, \phi_a^\prime$ are with respect to the parent photon direction, which can be translated to the laboratory coordinates via eq. \ref{['eq:angle']}.
• Figure 3: One-dimensional flux histograms: Photon, positron, and electron fluxes in the DUNE graphite target from GEANT4 simulation based on a $10^5$ POT sample, binned in steps of 20 MeV over the $e^\pm /\gamma$ energies (left) and their angles with respect to the beam axis binned in steps of 1 mrad (right) at production. In the left panel, we also show the energy distributions of the forward component of the fluxes after a 5 mrad angular cut (dotted histograms). The shallow "knee" over 0.1-0.2 rad region in the electron angular spectrum in the right panel can be interpreted as a result of Compton scattering by high-energy cascade electrons and $\gamma$-rays.
• Figure 5: Di-photon final state: we show the total energy spectrum (upper right), opening angle (bottom left), and invariant mass (bottom right) distributions for the neutrino-induced backgrounds and the ALP signal. The ALP mass and coupling benchmark values are chosen to yield statistically significant event rates in the limit of smaller $g_{a\gamma}$ couplings (cyan and pink histograms) or larger coupling (dark blue and red histograms). The neutrino-induced backgrounds include the rates from $e^+$ and $e^-$ mis-ID that contaminate the $2\gamma$ signal as $(1e^+)^* \gamma$ or $(1e^-)^* \gamma$.
• Figure 6: Single photon final state: Top right: the ALP energy spectrum from collinear $a \to \gamma \gamma$ decays are shown against the 1$\gamma$ final state backgrounds as well as 1$e^-$ and 1$e^+$ final states with an 18% mis-ID rate applied. Background labels denoted with $(1\gamma)_c$ stem from photons from true $2\gamma$ events in which one of the photons escaped containment in the fiducial detector volume. Bottom left: the ALP signal and background angular spectra for the single shower angle $\theta_\gamma$ taken with respect to the beam axis. Bottom right: the same as top right but after the $\theta_\gamma < 1^\circ$ angular cut. Note that the $1e^+$ contributions from the $\bar{\nu}_\mu$, $\nu_\mu$, and $\nu_e$ fluxes (purple shades) are zero.