CMS Forward Physics

The FSC system consists of three stations on each side of CMS; the inner stations, ST-1 (nearest to IP5) and ST-2, are each made of 2 counters, one above and the other below the beam vacuum pipe; the last ones, ST3 (farthest from IP5) are composed of 4 counters each. All counters are made of a square scintillator plate, with dimensions 25x25 cm2 and 1 cm thickness. The scintillators for the first two stations have a semi-elliptical cut-out on one edge, to fit closely around the beam pipe; the third stations are made of 4 counters each with a circular cut out in a corner. The FSC stations have a total of 16 scintillator plates (8 scintillator plates on each side of IP). Three type of tests were performed on the FSC counters before installing them in the LHC tunnel: However these counters do not see direct particles from IP5, but detect mainly showers from very forward particles interacting in the beam pipe and surrounding materials. Therefore the real - coverage of the FSC counters is not sharply defined, but it is smeared over a wider -range and has to be obtained from detailed simulations. As already remarked, the pseudorapidity  at very small values of  is a poor approximation to the true rapidity y.

The first CMS-TOTEM joint data taking with common triggers was performed in 2012, during a dedicated run with low pile-up and a non-standard beta= 90 m optics configuration, where beta is the amplitude function of the beam at the IP. The data were taken by each experiment with a bi-directional exchange of triggers; TOTEM RPs & T2 to CMS, and CMS jet & lepton triggers to TOTEM. The same events were saved by both experiments. The events reconstructed at CMS and at TOTEM are then synchronized offline by requiring the same LHC orbit and bunch numbers.