The Keller-Segel system has been widely validated as a model for cell movement under the combined effect of stochastic diffusion and a directed drift due to a chemoattractant emitted by the cells themselves (chemotaxis). In two dimensions, and in its simpler mathematical setting, it has been proved that the system admits a critical mass (below this mass there are global solutions, above the system blows-up). In higher dimensions this is not true and L d/2 is the critical norm but only less precise statements are available. Here, we study a variant for the diffusion law of the chemical potential in chemotaxis, based on a convolution operator or in other words on a fractional diffusion law. The aim of this paper is to enumerate the corresponding qualitative properties-global existence, blow up, stationary states-for the model with critical logarithmic kernel. In particular we show that this new system leads to a unified theory with critical mass in any dimension.