Physics and Beam Monitoring with Forward Shower Counters (FSC) in CMS

We propose to add forward shower counters, FSC, to CMS along the beam pipes, with 59 m . z . 140 m. These will detect showers from very forward particles with 7 . η . 11 interacting in the beam pipe and surrounding material. They increase the total rapidity coverage of CMS to nearly∆Ω = 4π, thus detecting most of the inelastic cross section σinel, including low mass diffraction. They will help increase our understanding of all high cross section processes, which is important for understanding the “underlying event” backgrounds to most physics searches. To the extent that the luminosity is well known, they may (together with all of CMS) provide the best measurement of σinel at the LHC. They are most useful when the luminosity per bunch crossing is still low enough to provide single (no pileup) collisions. They will allow measurements of single diffraction: p+ p→ p⊕X (where ⊕ means a rapidity gap) for lower masses than otherwise possible, and double diffraction: p + p → X ⊕ X with a large central rapidity gap. They can also be used as rapidity gap detectors for double pomeron exchange and central exclusive processes. Studies of exclusive processes such as γγ → μ+μ− (for luminosity calibration and eventually momentum calibration of forward spectrometers) can be made more cleanly requiring gaps in the FSC counters. Models of forward particle production can be tested indirectly through simulations of hit patterns in the counters. This may reduce the uncertainty on very high energy (E ∼ 10 eV) cosmic ray shower parameters. For heavy ion collisions, the counters act as crude forward calorimeters detecting nuclear fragments (supplementing the ZDC), as well as enabling the study of coherent quasi-elastic scattering e.g. Pb + Pb→ Pb ⊕X⊕ Pb via two-photon interactions. The counters can also be used for real-time monitoring, and if desired for vetoing in the level 1 trigger,both incoming and outgoing beam halo-generated backgrounds (separated by timing) and beam conditions generally. These counters represent a significant enhancement of the beam monitoring, and will make an invaluable contribution to the understanding of the background environment and its topology. They can also provide an additional luminosity monitor, up to luminosities such that the number of interactions per bunch crossing is 〈nX〉 ∼ 5. This note discusses mainly the physics issues; more technical details will be presented in another note. Basically we propose a set of scintillation counters at several locations between 59 m and 140 m along the beam pipes (on both sides), and read out by DAQ electronics identical to that of the HF, with some inputs to the level 1 trigger. Bunch-by-bunch information on rates etc. will be provided for LHC operations. The cost is very modest, given the added value to many physics studies in CMS and to our knowledge of beam conditions generally. a) contact person b) Also at Iowa State University c) Some authors are not members of CMS, but have contributed to this note.