The LHCf Si tracking system: Implementation and performances

The LHCf experiment is formed by two sampling EM calorimeters to study energy and transverse momenta of photons and π⁰ emitted from pp collisions at LHC (14 TeV center-of-mass energy) along both directions in the very-forward region; this measurement will give a precise calibration for hadron interaction models describing air showers initiated by very high energy cosmic-rays. The Si tracking system of LHCf is composed of 4 pairs of single-sided microstrip sensors (160 μm read-out pitch), integrated within one of the two calorimeters, interleaved with calorimeter layers (W absorber and scintillator planes); its purpose is to determine the shower transverse profile with uncertainty of few tens microns. The 3072 microstrips are read-out through highdynamic- range (400 MIP nominal) PACE3 chips, originally designed for the CMS Si Preshower detector; PACE3 output signals are digitized and processed by a set of fast dedicated ADC-FPGA electronics boards and transmitted to the LHCf storage system via optic fiber channels. The performances of the Si tracking system were measured at the CERN SPS H4 beam line in 2007, using 50-200 GeV electrons, 150-350 GeV protons and 100-150 GeV muons; the results of this beam test confirmed the spatial resolution expected from design and its energy dependence: for incident electrons in the energy range of interest (above 100 GeV) the resolution is better than 100 micron and improves with increasing energy.