Ionization-based search for magnetic monopoles using the NOvA Far Detector

TL;DR

This work reports a dedicated search for highly ionizing magnetic monopoles in the cosmic-ray flux using the NOvA Far Detector, a 14 kt surface-based tracking calorimeter. The analysis combines an ionization-based online trigger with offline 3D track reconstruction to identify long, uniform, highly ionizing through-going tracks, supported by detailed energy-loss modeling for $g=g_D$ monopoles across a broad $\beta$ range. No monopole signal is observed, and a comprehensive set of 90% C.L. flux limits is derived across multiple speed $\beta$ and mass regimes, including $\phi_{90\%}<2\times10^{-16}\ \mathrm{cm^{-2}\,s^{-1}\,sr^{-1}}$ for heavy monopoles with $0.005<\beta<0.8$ and $>10^{13}$ GeV, and $\phi_{90\%}<8\times10^{-16}\ \mathrm{cm^{-2}\,s^{-1}\,sr^{-1}}$ for lighter monopoles with $>10^8$ GeV arriving from above. The NOvA FD’s large area and modest overburden enable sensitivity to lower masses and slower speeds than many prior searches, providing complementary constraints to earlier experiments and improving the monopole flux bounds in several regions of parameter space.

Abstract

We report a search for highly-ionizing magnetic monopoles in the cosmic-ray flux using a 2,713-day dataset collected during 2015--2025 with the NOvA Far Detector, a 14-kiloton segmented detector located on the Earth's surface in Minnesota, United States. The search is sensitive to monopoles across a wide range of speeds, $7 \times 10^{-4} < β< 0.995$, and is sensitive to masses as low as $2 \times 10^5~\mathrm{GeV}$ for the fastest monopoles. No signal was observed. With the detector's large surface area and minimal overburden, we achieve the strongest flux limits reported to date in several regions of speed and mass. For heavy monopoles with masses above $10^{13}~\mathrm{GeV}$ that are able to reach the detector from above or -- crossing the Earth -- from below, we find a flux limit $φ_{90\%} < 2 \times 10^{-16}\, \mathrm{ cm^{-2} s^{-1} sr^{-1}}$ (90\% C.L.) for monopoles with $0.005 < β< 0.8$. Across the same range of speeds, we report a limit ${φ_{90\%}} < 8 \times 10^{-16}\, \mathrm{ cm^{-2} s^{-1} sr^{-1}}$ for light monopoles with masses above $10^8~\mathrm{GeV}$ that can reach the detector from above.

Figures (9)

• Figure 1: Schematic diagram of the NOvA detectors. For each detector, the coordinate system origin is set at the center of the front face. The z-axis points forward along the length of the detector, the y-axis points vertically upward, the x-axis points horizontally along the width of the detector, and together they form an orthogonal right-handed system. The inset illustrates the internal structure of the detectors, where cells are arranged in alternating vertical and horizontal orientations. This geometry enables precise 3D reconstruction of particle trajectories and energy deposition.
• Figure 2: Solid angle coverage as a function of monopole speed ($\beta$) and mass (top), and as a function of monopole mass for selected values of monopole speed, $\beta = 10^{-3}, 10^{-2}, 10^{-1}, 5 \times 10^{-1}, 8 \times 10^{-1}$ and $9 \times 10^{-1}$ (bottom).
• Figure 3: Trigger and overall efficiency as a function of monopole speed ($\beta$).
• Figure 4: Distributions of off-track width (top) and width difference (bottom) for data and simulated signal sample. In each plot, the simulated event distribution is area-normalized to the data. The top (bottom) distribution corresponds to events passing the trigger, and selection requirements 1--4 and 6 (1--5). The dashed vertical line with an arrow marks the cut value on the variable itself.
• Figure 5: Distributions of mean ADC for data and simulated signal events, shown for events with $\beta_{\text{reco}} < 0.03$ (top) and $\beta_{\text{reco}} \geq 0.03$ (bottom). In each plot, the simulated event distribution is area-normalized to the data for visual comparison. In each case, trigger and selection requirements 1--6 have been applied. These plots are used to define the signal region (note suppressed zero on horizontal axis). Most of the background events are rejected by the selection criteria 5 and 6 due to irregular or clustered cosmic activity.