Shifting the neutrino fog: studying the Isospin-violating Dark Matter case

TL;DR

This work analyzes how isospin-violating dark matter (IVDM) alters the neutrino fog in direct-detection experiments, moving beyond the conventional isospin-conserving assumption. By employing a profile-likelihood framework, the authors quantify a dynamic discovery limit that accounts for neutrino backgrounds and DM model parameters, and they illustrate two representative IVDM realizations: the Scotogenic model and an effective $Z$ portal DM. The results show that the neutrino fog can be significantly lowered or raised depending on the $f_n/f_p$ ratio, with Xenon and Germanium targets exhibiting distinct sensitivities. The findings emphasize that the neutrino background is model-dependent and that a multi-target strategy is essential to disentangle DM signals from neutrino backgrounds and to constrain isospin-violating couplings. This work provides a path toward mitigating the neutrino fog by exploiting model-specific interference patterns in target materials.

Abstract

First observation of solar neutrinos through coherent elastic neutrino-nucleus scattering by dark matter (DM) direct detection (DD) experiments makes the study of the neutrino fog of the most relevance. This irreducible neutrino background depends on the target material as well as other experimental parameters. Recently, it has also been remarked the dependence of the neutrino fog on the DM models under consideration. In this work, we study the case of Isospin-violating dark matter (IVDM) models, discussing specific examples of DM models and making a detailed analysis of the implications of IVDM on the neutrino fog. We also explore the conditions under which this background can be mitigated for specific DM models.

Figures (6)

• Figure 1: Left: Neutrino energy spectra for Solar, Atmospheric (Atm), and the DSNB Baxter_2021. Right: Xenon recoil spectra showing the irreducible neutrino backgrounds (coloured) compared to potential WIMP signals (black dotted). The recoil spectra of a WIMP with mass $6$ GeV and cross section $\sigma_{\chi-n} = 5\times10^{-45}$ cm$^2$ is shown to overlap with $^8$B neutrinos, while a $m_\chi = 100$ GeV and $\sigma_{\chi-n} = 2.5\times10^{-49}$ cm$^2$ WIMP overlaps with atmospheric neutrinos.
• Figure 2: Feynman diagrams for the SI inelastic scattering process of DM ($N_1$) off a nucleus at the lowest loop level in the Scotogenic model. This process is mediated via a photon and has been calculated in Schmidt:2012yg.
• Figure 3: Feynman diagram for the DM ($\chi$) -quark ($q$) elastic scattering in the effective $Z$-portal DM model.
• Figure 4: The discovery limit for a Xenon-based detector plotted in the WIMP mass ($m_\chi$) versus WIMP-nucleon SI cross-section ($\sigma_{n-\chi}$) plane for different values of the ratio $f_n/f_p$. The standard Isospin-conserving scenario, $f_n/f_p=1$, is indicated by the thick black line within the peach-coloured region.
• Figure 5: Evolution of the neutrino fog in the WIMP-nucleon SI cross-section, $\sigma_{n-\chi}$, versus WIMP mass, $m_\chi$, plane for a Xenon-based detector. The solid curves represent discovery limits for experimental exposures ranging from $10^{-2}$ ton-year (light pink) to $10^7$ ton-year (dark maroon). The panels compare two Isospin-violating scenarios: the Z-portal DM model with $f_n / f_p=-22.18$ (left) and the Scotogenic model with $f_n/ f_p=0$ (right). For reference, the blue dash-dotted line indicates the projected limit for the standard Isospin-conserving case ($f_{n}/f_{p}=1$) at one ton-year exposure.