  We present the FP420 R&D project, which has been studying the key aspects of
the development and installation of a silicon tracker and fast-timing detectors
in the LHC tunnel at 420 m from the interaction points of the ATLAS and CMS
experiments. These detectors would measure precisely very forward protons in
conjunction with the corresponding central detectors as a means to study
Standard Model (SM) physics, and to search for and characterise New Physics
signals. This report includes a detailed description of the physics case for
the detector and, in particular, for the measurement of Central Exclusive
Production, pp --> p + phi + p, in which the outgoing protons remain intact and
the central system phi may be a single particle such as a SM or MSSM Higgs
boson. Other physics topics discussed are gamma-gamma and gamma-p interactions,
and diffractive processes. The report includes a detailed study of the trigger
strategy, acceptance, reconstruction efficiencies, and expected yields for a
particular p p --> p H p measurement with Higgs boson decay in the b-bbar mode.
The document also describes the detector acceptance as given by the LHC beam
optics between the interaction points and the FP420 location, the machine
backgrounds, the new proposed connection cryostat and the moving ("Hamburg")
beam-pipe at 420 m, and the radio-frequency impact of the design on the LHC.
The last part of the document is devoted to a description of the 3D silicon
sensors and associated tracking performances, the design of two fast-timing
detectors capable of accurate vertex reconstruction for background rejection at
high-luminosities, and the detector alignment and calibration strategy.
