A Global Analysis of Dark Matter Signals from 27 Dwarf Spheroidal
  Galaxies using 11 Years of Fermi-LAT Observations
  We search for a dark matter signal in 11 years of Fermi-LAT gamma-ray data
from 27 Milky Way dwarf spheroidal galaxies with spectroscopically measured
$J$-factors. Our analysis includes uncertainties in $J$-factors and background
normalisations and compares results from a Bayesian and a frequentist
perspective. We revisit the dwarf spheroidal galaxy Reticulum II, confirming
that the purported gamma-ray excess seen in Pass 7 data is much weaker in Pass
8, independently of the statistical approach adopted. We introduce for the
first time posterior predictive distributions to quantify the probability of a
dark matter detection from another dwarf galaxy given a tentative excess. A
global analysis including all 27 dwarfs shows no indication for a signal in
nine annihilation channels. We present stringent new Bayesian and frequentist
upper limits on the dark matter cross section as a function of dark matter
mass. The best-fit dark matter parameters associated with the Galactic Centre
excess are excluded by at least 95% confidence level/posterior probability in
the frequentist/Bayesian framework in all cases. However, from a Bayesian model
comparison perspective, dark matter annihilation within the dwarfs is not
strongly disfavoured compared to a background-only model. These results
constitute the highest exposure analysis on the most complete sample of dwarfs
to date. Posterior samples and likelihood maps from this study are publicly
available.
