Spherical collapse and halo mass function in the symmetron model
  We study the gravitational clustering of spherically symmetric overdensities
and the statistics of the resulting dark matter halos in the "symmetron model",
in which a new long range force is mediated by a $Z_2$ symmetric scalar field.
Depending on the initial radius of the overdensity, we identify two distinct
regimes: for small initial radii the symmetron mediated force affects the
spherical collapse at all redshifts; for initial radii larger than some
critical size this force vanishes before collapse because of the symmetron
screening mechanism. In both cases overdensities collapse earlier than in the
$\Lambda$CDM and statistically tend to form more massive dark matter halos.
Regarding the halo-mass function of these objects, we observe order one
departures from standard $\Lambda$CDM predictions. The formalism developed here
can be easily applied to other models where fifth-forces participate to the
dynamics of the gravitational collapse.
