  We analyze the problem of correlating pp interaction data from the central
detectors with a subevent measured in an independent system of leading proton
detectors using FP420 as an example.
  FP420 is an R&D project conducted by a collaboration formed by members of
ATLAS and CMS to investigate the possibility of detecting new physics in the
central exclusive channel, PP -> P + X + P,where the central system X may be a
single particle, for example a Standard Model Higgs boson. With standard LHC
optics, the protons emerge from the beam at a distance of 420m from the
Interaction Point, for M_X ~ 120 GeV. The mass of the central system can be
measured from the outgoing protons alone, with a resolution of order 2 GeV
irrespective of the decay products of the central system. In addition, to a
very good approximation, only central systems with 0^++ quantum numbers can be
produced, meaning that observation of a SM or MSSM Higgs Boson in this channel
would lead to a direct determination of the quantum numbers.
