Double Parton Scattering in pbar-p Collisions at root s = 1.8 TeV

In a paper submitted to Physical Review Letters, the CDF collaboration
announced the first clear observation of two independent collisions of
the constituents(i.e. partons, the generic name for quarks and gluons) 
of the proton within the same event, hence the name "Double Parton 
scattering."  The rate for double parton scattering depends on how the 
partons are distributed within the parton, particularly the correlation 
that relates a given parton to its neighbors. This correlation is hard to 
calculate in the present theoretical models, making the measurement 
particularly important.

Double parton scattering looks very much like two separate proton-
antiproton scatterings superimposed on top of each other. Among the
variety of scatterings recorded by CDF are events with two collimated
sprays of particles ("jets") and events with one jet and an energetic
photon. Using data recorded in 1992-93, CDF examined those events that
contain a photon and 3 jets and found that over half were made up of
independent photon +jet and jet-jet subsystems: double parton scatters.

The rate for these events is governed by two factors-- by the individual
rates for two parton-parton scatterings and by the spatial distribution
of the partons within the parton. The latter factor is the most 
interesting for understanding the details of proton structure. The
experimental challenge is to sort out the two effects. The breakthrough
for CDF came by measuring the rate for photon-jet and jet-jet
collisions in separate events and comparing with the rate for both collisions
to occur in the same event. When taking the ratio, the individual
rates cancel, leaving only the interesting part related to the spatial
distribution of the partons within the proton. The CDF results show
no evidence for local correlations ("clumping") among the partons that 
make up the proton.