New light mediators and the neutrino fog: Implications from XENONnT nuclear recoil data

TL;DR

The study probes how new light mediators alter the neutrino fog in dark matter direct detection by analyzing XENONnT nuclear recoil data under two BSM scenarios: (i) DM–nucleon scattering mediated by a light boson with neutrinos scattering via CE NS, and (ii) DM–nucleon interactions via a contact-like SI/SD term while neutrinos acquire CE NS modifications from a light mediator. It derives 90% CL bounds on mediator couplings as functions of mediator mass, finding much stronger constraints when the mediator couples to DM than when it only modifies neutrino interactions. It then reevaluates the neutrino fog for both scenarios, showing significant morphology changes: light DM mediators can reduce the fog at low $m_ ext Chi$, while CE NS mediators can either enhance or suppress the fog depending on the mediator type and mass, with interference effects producing dips in the recoil spectrum. The results emphasize how mediator dynamics shape the irreducible background and suggest that future directional detectors may help overcome the neutrino fog barrier.

Abstract

Current ton-scale, xenon-based dark matter (DM) direct detection experiments have now reached the sensitivity required to observe solar neutrinos, marking the onset of the so-called neutrino fog. In this work, we explore how this fog is modified when either neutrinos or DM interact with nuclei through a new scalar, vector, or axial-vector interaction, considering both heavy and light mediators. Using the latest nuclear-recoil data from XENONnT, which show indications of coherent elastic neutrino-nucleus scattering from $^8$B solar neutrinos, we derive new strong bounds on light mediator couplings. We find that these limits are significantly more stringent when the mediator couples to DM, rather than when new physics affects only neutrino interactions. Building on these results, we recompute the expected neutrino fog and compare it with the corresponding constraints on spin-independent and spin-dependent DM-nucleon interactions. We show that the morphology of the neutrino fog can be markedly modified if either neutrinos or DM interact with nuclei through light mediators, even in light of these recent constraints.

Figures (7)

• Figure 1: Expected nuclear recoil spectra for XENONnT compared with data from XENON:2024ijkXENON:2024hup. Maroon histograms indicate the total background. Left panel: scenario (i), showing the SM CE NS contribution from $^8$B neutrinos (yellow), and together with benchmark DM–nucleus interactions mediated by light scalar (orange), vector (blue), or axial-vector (green) particles. For the BSM DM benchmarks, we fix $m_\chi = 100~\mathrm{GeV}$ and $m_X = 1~\mathrm{MeV}$. Right panel: scenario (ii), displaying the SM CE NS signal from $^8$B neutrinos (yellow), and benchmarks for CE NS in the presence of additional light scalar and vector mediators, together with a DM component corresponding to a point-like SI interaction with $m_\chi = 13~\mathrm{GeV}$ and $\left.\sigma_{\chi\mathfrak{n}}^{\mathrm{SI}}\right|_{\left|\mathbf{q}\right|^{2}\to 0} = 10^{-47}~\mathrm{cm^2}$. In all BSM CE NS cases, we fix $m_X = 1~\mathrm{MeV}$.
• Figure 2: Constraints at 90% CL in the new mediator coupling–mass plane, for scalar (left), vector (center), and axial-vector (right) DM-nucleon interactions. The solid, colored curves show the XENONnT limits obtained when the new mediator couples to DM, for $m_\chi = 4$ (yellow), 13 (blue), 100 (red), and 1000 GeV (green). The shaded gray regions show the corresponding bounds when new physics appears only in the CE NS sector (scenario (ii), from DeRomeri:2024iaw). See text for more details.
• Figure 3: 90% CL upper limit (orange shaded areas) on the SI DM–nucleon (upper row), SD DM-proton (middle row) and SD DM-neutron (bottom row) cross section derived from our analysis of XENONnT data. Left panels: standard definition of the neutrino fog with $^8$B flux normalization and uncertainty from SNO Baxter:2021pqoSNO:2011hxd. Right panels: neutrino fog with $^8$B flux normalization and uncertainty from the recent result of XENONnT XENON:2024ijk.
• Figure 4: 90% CL upper limit (orange shaded area) on the SI (upper panel), SD-proton (lower left panel), and SD-neutron (lower right panel) DM-nucleon cross section derived from our analysis of XENONnT data, under the assumption of DM interactions with a light mediator (scalar or vector for SI interactions, and axial-vector for the SD one). In all panels, the corresponding neutrino fog for scenario (i) is also shown.
• Figure 5: Left (right) panels: neutrino fog for CE NS interactions including a light scalar (top row), universal vector (middle row), or $B\!-\!L$ vector (bottom row) mediator, for SI DM-nucleon (SD DM-neutron) interactions. We fix the mediator masses to $m_{S} = m_{V} = m_{B\!-\!L} = 1~\mathrm{MeV}$ and the couplings to $g_S = 2\times10^{-6}$, $g_V = 3\times 10^{-6}$, and $g_{B\!-\!L} = 1\times10^{-5}$. The white dashed lines indicate the $n=2$ neutrino fog contours corresponding to the case in which neutrinos interact solely through the SM CE NS process.