model from the outset, and is simple enough to be constrained by X-ray surface brightness and low-resolution SZ data. This also avoids ad-hoc parametrization of the electron density and temperature and therefore removes the possibility of introducing unphysical solutions for the total mass. The following section describes in detail the physical basis and parametrization of this model.

An Entropy-based model {#section:entropy_model}
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### The Dark Mass Halo

We construct a parametric model that is physically consistent and is therefore a reasonable description of the ICM on the angular scales to which CBI2 is sensitive. A suitable parametrization of the dark matter content is the hierarchical clustering NFW profile [@Navarro:1995], where the dark matter (DM) density as a function of radius is given by $$\rho_\rmn{DM}(r)= {\delta_\rmn{c}\rho_\rmn{crit}
    \over(r/r_\rmn{s})(1 + r/r_\rmn{s})^2},$$ where $\rho_{\rmn{crit}}$ is the universal critical density for closure (at the redshift of the cluster), $r_{\rmn{s}}$ is the scale radius and $\delta_{\rmn{c}}$ is the characteristic density contrast. The DM virial radius $r_\rmn{DM}$ is defined in this model as the radius of a sphere at which the mean interior DM density is equal to $200\rho_{\rmn{crit}}$. Note that the true virial radius of the cluster is slightly larger ($\sim 10\,\rmn{per cent}$) since one must 