Understanding the nature and identity of dark matter is a key goal in the physics community. In the case that TeV-scale dark matter particles decay or annihilate into standard model particles, very-high-energy (VHE) gamma rays (greater than 100 GeV) will be present in the final state. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an imaging atmospheric Cherenkov telescope array that can indirectly detect VHE gamma rays in an energy range of 100 GeV to > 30 TeV. Dwarf spheroidal galaxies (dSphs) are ideal candidates in the search for dark matter due to their high dark matter content, high mass-to-light ratios, and their low gamma-ray fluxes from astrophysical processes. This study uses a legacy data set of 638 hours collected on 17 dSphs, built over 11 years with an observing strategy optimized according to the dark matter content of the targets. The study addresses a broad dark matter particle mass range, extending from 200 GeV to 30 PeV. In the absence of a detection, we set the upper limits on the dark matter velocity-weighted annihilation cross section.
Figure 1: Profile likelihood for a joint-fit analysis with 17 dSphs for a dark matter particle mass of 1 TeV (left) and the VERITAS 95% upper limit on the dark matter cross section (right), considering the τ+τ− annihilation channel. The solid black line shows the joint-fit analysis result, and the dashed gray lines are from the individual dSph analyses.
Figure 2: Velocity-weighted annihilation cross section upper limits produced from 17 dSphs observations, by annihilation channel. The blue (green hatched) shaded uncertainty band depicts the 68% (95%) containment obtained from 300 realizations of viable dark matter density profiles. The blue solid curve denotes the median of the band. The orange, black, and red lines represent the upper limits derived from individual observations of Segue 1, Ursa Major II, and Ursa Minor, respectively. These limits result from a reference J-profile for each dSph (Table I). The red dotted-dashed line is the expected velocity-weighted annihilation cross section for a thermal-relic dark matter scenario.
Figure 3: Velocity-weighted annihilation cross section upper limit curves produced from VERITAS observations by channel compared with their null-hypothesis bands (H0; background-only hypothesis). We present upper limits derived from the jointfit dSph observations as Fig 2 (blue) and upper limits with the Poisson background fluctuation (orange). The orange (red hatched) uncertainty band depicts the 68% (95%) containment obtained from 300 realizations of random fluctuations of the background. The red dotted-dashed line is the expected velocity-weighted annihilation cross section for a thermal-relic dark matter scenario.
Figure 4: Left panel: VERITAS 95% confidence upper limits curves on the dark matter velocity-weighted annihilation cross section compared with benchmark theoretical models. The black solid line shows the partial-wave unitarity bound for a pointlike dark matter particle with angular momentum J = 0, while the dashed black lines show the unitarity bounds for composite dark matter particles of different radii. Right panel: VERITAS 95% confidence upper limits curves on the dark matter particle radius (units of femtometers) as a function of mass, for the nine annihilation channels considered. The dashed black line refers the proton charge radius as a reference [49]. The shaded areas denote exclusion regions.
Figure 5: Comparison of 95% confidence upper limit curves of the dark matter cross section with various conditions. The blue and orange lines result from the 126-hour Segue 1 observation with the J-profile adopted from [26] and [52], respectively. The blue (green hatched) shaded uncertainty band depicts the 68% (95%) containment obtained from 300 realizations of viable dark matter density profiles. The black dashed line denotes the previous VERITAS publication [11], which is from the 92-hour Segue 1 observation.
Figure 6: VERITAS upper limit curves obtained from 17 dSphs compared with other published upper limit curves. All curves show 95% confidence upper limits on the dark matter velocity-weighted annihilation cross section for the b-bar-b (left) and τ+τ− (right) annihilation channels. This work is represented with the solid black line. Other results are adapted from [2] (Fermi-LAT; blue dashed line), [9] (MAGIC; orange dashed line), [7] (H.E.S.S.; green dashed line), and [14] (HAWC; red dashed line). The red dotted-dashed line is the expected velocity-weighted annihilation cross section for a thermal-relic dark matter scenario.
Figure 7: VERITAS upper limit curves for three deep-exposure dSphs in two annihilation channels. The results for Segue 1, Ursa Major II, and Ursa Minor are displayed in the left, middle, and right panels, respectively. The results from τ+τ− and b-bar-b channels are depicted in blue and orange, respectively. A solid curve is produced from each dSph observation, while shaded and hatched uncertainty bands depict the 68% and 95% containment intervals obtained from 300 realizations of random fluctuations of the background. The red dotted-dashed line is the expected velocity-weighted annihilation cross section for a thermal-relic dark matter scenario.
