Improved Heavy Dark Matter Annihilation Search from Dwarf Galaxies with HAWC

TL;DR

This work leverages the HAWC gamma-ray observatory to perform an indirect search for heavy WIMP dark matter via gamma rays from dwarf spheroidal galaxies. By implementing improved event reconstruction (Pass 5), neural-network energy estimation, and ML-driven cuts, and by expanding the dwarf sample and annihilation channels, the analysis achieves enhanced sensitivity up to $m_\chi \sim 10^4$ TeV. The signal model combines particle-physics spectra (HDMS with EW corrections) and astrophysical $J$-factors drawn from two catalogs (LS and GS), with a likelihood framework that stacks 11 SM channels across four decades of mass; no DM signal is detected, and stringent $\langle \sigma v \rangle$ upper limits are reported, notably constraining $\chi\chi \to b\bar{b}$ and $\tau^+\tau^-$ in the multi-TeV to PeV range. Overall, the results demonstrate HAWC's competitive reach into the PeV DM regime and underscore the importance of robust $J$-factor priors and advanced data-reduction techniques for indirect DM searches in dSphs.

Abstract

Understanding dark matter's elusive nature is crucial for the framework of particle physics and expanding the Standard Model. This analysis utilizes the High Altitude Water Cherenkov (HAWC) gamma ray Observatory to indirectly search for dark matter (DM) by studying gamma ray emission from dwarf spheroidal galaxies (dSphs). Selected for their high ratio of dark matter to baryonic matter, dSphs are useful for this type of search owing to the low background emission. In comparison to previous HAWC studies, we significantly improve our sensitivity to DM from dSphs due to improvements to our event reconstruction and reduced hadronic contamination. We expanded the number of dSphs studied, DM annihilation channels into the Standard Model (SM), and the amount of data collected on each previously studied dSph. We searched for DM signals in each dSph using the latest version of the algorithms used to reconstruct data from the primary detector of the HAWC instrument. We report that we do not detect evidence of DM from dSphs, so we place upper limits for the velocity-weighted DM annihilation cross-section ($\langleσv \rangle$) on the order of $10^{-23}~\text{cm}^3\text{s}^{-1}$ for a DM mass range of $1-10^4$ TeV.

Figures (12)

• Figure 1: Spectral hypotheses from PPPC and HDMS for DM annihilation: $\chi\chi \rightarrow W^-W^+$. The x-axis is the $\text{Log}_{10}$ of X, where X is the ratio of gamma ray energy to DM mass. The y-axis is the unitless, energy-weighted gamma ray counts. Solid lines are spectral models with EW corrections from the PPPC. Dash-dot lines are spectral models from HDMS. Red lines are models for $M_\chi = 1$ TeV, the lowest in our studied range. Blue lines represent models for $M_\chi = 100$ TeV, which are well above PYTHIA's limits, where corrections are necessary Rodd:HDM_spec.
• Figure 2: Photon spectra for $\chi\chi \rightarrow \gamma\gamma$ (left), $\chi\chi \rightarrow WW$ (middle), and $\chi\chi \rightarrow Z^0Z^0$ (right) after Gaussian convolution of line features. Both the 1 TeV and 10 PeV spectra have $\delta$-line features at $E_\gamma = m_\chi$. Redder lines are annihilation spectra with lower DM mass. Bluer lines are spectra from larger DM mass. All spectral models are sourced from the HDMSpectra (HDMS) models Rodd:HDM_spec. Axes are drawn according to the energy sensitivity of HAWC.
• Figure 3: Brazil bands on $\langle \sigma \mathit{v} \rangle$ versus $m_\chi$ for $\chi\chi \rightarrow b\overline{b}$, $t\overline{t}$, $u\overline{u}$, $d\overline{d}$, $W^-W^+$, $\nu_e\overline{\nu}_e$, $e^-e^+$, $\mu^-\mu^+$, $\tau^-\tau^+$, $\gamma\gamma$ and $Z^0Z^0$. Limits are with $\mathcal{LS}$$J$-factors DM_Strigari20. The solid line represents the observed combined limit. The dashed line represents the expected annihilation cross-section from HAWC background. The green and yellow bands show 68% and 95% containment respectively.
• Figure 4: Same as \ref{['fig:LSmtd_bb_1of2']} but with $\mathcal{GS}$$J$-factors Ando_2020.
• Figure 5: HAWC upper limits at 95% confidence level on $\langle \sigma \mathit{v} \rangle$ versus $m_\chi$ for $\chi\chi \rightarrow b\overline{b}$, $t\overline{t}$, $u\overline{u}$, $d\overline{d}$, $W^-W^+$, $\nu_e\overline{\nu}_e$, $e^-e^+$, $\mu^-\mu^+$, $\tau^-\tau^+$, $\gamma\gamma$ and $Z^0Z^0$. Limits are with $\mathcal{LS}$$J$-factors DM_Strigari20. The solid line represents the observed combined limit. Dashed lines represent limits from individual dSphs.