DarkBit: A GAMBIT module for computing dark matter observables and
  likelihoods
  We introduce DarkBit, an advanced software code for computing dark matter
constraints on various extensions to the Standard Model of particle physics,
comprising both new native code and interfaces to external packages. This
release includes a dedicated signal yield calculator for gamma-ray
observations, which significantly extends current tools by implementing a
cascade decay Monte Carlo, as well as a dedicated likelihood calculator for
current and future experiments (gamlike). This provides a general solution for
studying complex particle physics models that predict dark matter annihilation
to a multitude of final states. We also supply a direct detection package that
models a large range of direct detection experiments (DDcalc), and provides the
corresponding likelihoods for arbitrary combinations of spin-independent and
spin-dependent scattering processes. Finally, we provide custom relic density
routines along with interfaces to DarkSUSY, micrOMEGAs, and the neutrino
telescope likelihood package nuLike. DarkBit is written in the framework of the
Global And Modular Beyond the Standard Model Inference Tool (GAMBIT), providing
seamless integration into a comprehensive statistical fitting framework that
allows users to explore new models with both particle and astrophysics
constraints, and a consistent treatment of systematic uncertainties. In this
paper we describe its main functionality, provide a guide to getting started
quickly, and show illustrative examples for results obtained with DarkBit (both
as a standalone tool and as a GAMBIT module). This includes a quantitative
comparison between two of the main dark matter codes (DarkSUSY and micrOMEGAs),
and application of DarkBit's advanced direct and indirect detection routines to
a simple effective dark matter model.
