[eng] The interest in two-photon interactions at the CERN Large Hadron Collider (LHC) is two-fold. First, as the cross-section for two-photon production of lepton pairs is precisely known, the measurement of dimuons rate is a strong candidate to calibrate the total integrated luminosity delivered by the LHC to the CMS experiment. This is made possible thanks to striking experimental signature of exclusive pair production and to unique kinematics properties of the muons -small accoplanarity and balance in transverse momentum-. The beam configuration leading to multiple interactions within the same
bunch-crossing, a new technique to impose exclusivity conditions within the tracking system only was built. The proof-of-principle of the method has been achieved with the observation of exclusive events from two-photon production and from photo-production of vector mesons, and the derivation of luminosity estimate for the 2010 data-taking period.
The second asset comes if the central measurement is done in coincidence with detection of outgoing scattered protons in very forward detectors. Simulations of the 'High Precision Spectrometer' acceptances for exclusive signals of 'new physics' pairs and for accidental coincidence
background showed that the installation of tagging stations at 240m and 420m of the interaction point of CMS, supplied with Gastof fast-timing detectors, is competitive to reconstruct the mass spectrum with a few GeV resolution, on an event-by-event basis.