Frixione isolation

Frixione isolation is a way of requiring prompt photons (i.e. not from fragmentation) in an infrared-safe way. Only such photons are accepted for which the total amount of hadronic (or partonic) transverse energy $E_{T}$ in an angular cone of radius $r$ is smaller than $E_{T}^{\max }(r)$, defined as

$E_{T}^{\max }(r) \equiv \epsilon_{\gamma} p_{T}^{\gamma}\left(\frac{1-\cos r}{1-\cos R}\right)^{n},$

where $p_{T}^{\gamma}$ is the photon transverse momentum. The isolation criterion $E_{T}<E_{T}^{\max }(r)$ has to be fulfilled for all cones with $r \leq R$. Example values are $\epsilon_{\gamma}=0.5$, $n=1$ and $R=0.4$.