Double parton distributions: An introduction

In factorization theorems for double parton scattering (DPS) the non-perturbative parts of the cross section are encoded in universal functions, double parton distributions (DPDs) in the case of collinear factorization and double transverse momentum dependent parton distributions (dTMDs) for transverse momentum dependent (TMD) factorization.These distributions contain a wealth of information about the structure of the proton, in particular about correlations between two partons inside a proton with respect to their spin, colour, and spatial separation.In this talk we will give an introduction to these distributions, their spin and colour structure, and their renormalisation and rapidity scale dependence.Since presently DPDs and dTMDs are largely unconstrained experimentally, DPS cross section calculations are at present performed using model DPDs. We will briefly review one of the most used approximations, highlighting its shortcomings, and show how improved models can be constructed using DPD number and momentum sum rules