Limits on dark matter WIMPs using upward-going muons in the MACRO detector

TL;DR

This study searches for non-baryonic dark matter in the form of WIMPs by detecting neutrino-induced upward-going muons from WIMP annihilation in the Sun and Earth with the MACRO detector. The analysis uses angular cones and time-of-flight discrimination to suppress atmospheric backgrounds and produces flux upper limits in multiple cones for both celestial bodies. The limits are interpreted within supersymmetric neutralino models (Bottino et al.), constraining parameter combinations and, in particular, excluding several DAMA/NaI–favored configurations; Earth limits are among the most stringent indirect bounds to date. The work demonstrates the power of indirect searches to probe WIMP scenarios complementary to direct detection.

Abstract

We perform an indirect search for Weakly Interacting Massive Particles (WIMPs) using the MACRO detector to look for neutrino-induced upward-going muons resulting from the annihilation of WIMPs trapped in the Sun and Earth. The search is conducted in various angular cones centered on the Sun and Earth to accommodate a range of WIMP masses. No significant excess over the background from atmospheric neutrinos is seen and limits are placed on the upward-going muon fluxes from Sun and Earth. These limits are used to constrain neutralino particle parameters from supersymmetric theory, including those suggested by recent results from DAMA/NaI.

Figures (7)

• Figure 1: (a) Nadir distribution of measured (black circles) and expected (solid line and dashed line) through-going upward muons. The expected distributions are multiplied by the ratio of the measured events over the expected ones outside the largest window ($30^{\circ}$). The normalization factors are 0.82 for the solid line (no oscillations) and 1.19 for the dashed line ($\nu_{\mu}-\nu_{\tau}$ oscillations for maximum mixing and $\Delta m^{2} = 0.0025$ eV$^{2}$). (b) Muon flux limits (90$\%$ c.l.) as a function of the angle from the vertical (the angle varies form $3^{\circ}$ to 30$^{\circ}$ in steps of $3^{\circ}$). In both plots, the dashed line is obtained in the hypothesis of $\nu_{\mu} \rightarrow \nu_{\tau}$ oscillations of the atmospheric neutrino background with maximum mixing and $\Delta m^{2} = 0.0025$ eV$^{2}$.
• Figure 2: (a) Distribution of measured (black circles) and expected (solid line) upward-going muons as a function of the cosine of the angle from the Sun direction. (b) Muon flux limits (90% c.l.) as a function of the search cone around the direction of the Sun.
• Figure 3: Nadir angle distribution of muons induced by neutralino annihilation inside the Earth for several neutralino masses. The angular ranges including 90$\%$ of the signal are indicated.
• Figure 4: Neutrino-muon angular separation distribution for neutrinos from $\tilde{\chi} - \tilde{\chi}$ annihilation in the Sun for several neutralino masses. The angular ranges with 90$\%$ of the signal are shown.
• Figure 5: Upward-going muon flux vs $m_{\chi}$ for $E_{\mu}^{\rm th} =$ 1 GeV from the Earth Bottino98. Each dot is obtained varying model parameters. In this plot values of $\mu > 0$ are considered. Similar results are obtained for $\mu <0$. Solid line: MACRO flux limit (90$\%$ c.l.). The solid line representing the flux limit for the no-oscillation hypothesis is indistinguishable in the log scale from the one for the $\nu_{\mu}-\nu_{\tau}$ oscillation hypothesis, but the expectations could be about two times lower. The open circles indicate the models excluded by direct measurements (particularly the DAMA/NaI experiment DAMA96) and assume a local dark matter density of 0.5 GeV cm$^{-3}$. See Fig. \ref{['fig7']} for the comparison for the same density between the MACRO flux limit and the allowed values of parameters based on recent DAMA/NaI results.