  Double parton scattering (DPS) has become very relevant as a background to
interesting analyses performed by the experiments at the LHC. It encodes
knowledge of correlations between the proton constituents not accessible in
single parton scattering. Within perturbative QCD DPS is described in terms of
partonic subprocesses and double parton distributions (DPDs). There exists a
large number of different DPDs describing the different possible states of two
partons inside a proton. They include correlations between the two partons and
interferences between the two hard subprocesses. Taking the probability
interpretation of the DPDs as starting point, we derive limits on the
interference DPDs and thereby constrain the size of correlations between two
partons inside an unpolarized proton.
