What can(not) be measured with ton-scale dark matter direct detection
  experiments
  Direct searches for dark matter have prompted in recent years a great deal of
excitement within the astroparticle physics community, but the compatibility
between signal claims and null results of different experiments is far from
being a settled issue. In this context, we study here the prospects for
constraining the dark matter parameter space with the next generation of
ton-scale detectors. Using realistic experimental capabilities for a wide range
of targets (including fluorine, sodium, argon, germanium, iodine and xenon),
the role of target complementarity is analysed in detail while including the
impact of astrophysical uncertainties in a self-consistent manner. We show
explicitly that a multi-target signal in future direct detection facilities can
determine the sign of the ratio of scalar couplings $f_n/f_p$, but not its
scale. This implies that the scalar-proton cross-section is left essentially
unconstrained if the assumption $f_p\sim f_n$ is relaxed. Instead, we find that
both the axial-proton cross-section and the ratio of axial couplings $a_n/a_p$
can be measured with fair accuracy if multi-ton instruments using sodium and
iodine will eventually come online. Moreover, it turns out that future direct
detection data can easily discriminate between elastic and inelastic
scatterings. Finally, we argue that, with weak assumptions regarding the WIMP
couplings and the astrophysics, only the dark matter mass and the inelastic
parameter (i.e. mass splitting) may be inferred from the recoil spectra --
specifically, we anticipate an accuracy of tens of GeV (tens of keV) in the
measurement of the dark matter mass (inelastic parameter).
