Development of reconstruction algorithms for inelastic processes studies in the TOTEM experiment at LHC

The TOTEM experiment at the Large Hadron Collider (LHC) is designed and optimized to measure the total pp cross section at a center of mass energy of E = 14 TeV with a precision of about 1÷2 %, to study the nuclear elastic pp cross section over a wide range of the squared four-momentum transfer (10^{-3} GeV^2 < |t| < 10 GeV^2) and to perform a comprehensive physics program on diffractive dissociation processes, partially in cooperation with the CMS experiment. Based on the “luminosity independent method”, the evaluation of the total cross section with such a small error will in particular require simultaneous measurement of the pp elastic scattering cross section d\sigma/dt down to |t| ~10^{-3} GeV^2 (to be extrapolated to t = 0) as well as of the pp inelastic interaction rate, with a large coverage in the forward region. The TOTEM physics programme will be accomplished by using three different types of detectors: elastically scattered protons will be detected by Roman Pots detectors (based on silicon technology) placed about at 147 m and 220 m from the interaction point; inelastic processes will be detected by two tracking telescopes, T1 and T2 (based on gas detectors), embedded into the forward region of the CMS experiment. This thesis is structured in four chapters. Chapter 1 begins with an introduction to the LHC and to the beam dynamics. The TOTEM apparatus is then described with particular emphasis given to the T2 detector, on which the work reported in this thesis is related. This chapter also includes the description of the beam pipe around the interaction point, which is of interest for track reconstruction with TOTEM inelastic telescopes because of the possibility of multiple scattering and secondary particle production. In chapter 2 an overview on the TOTEM physics program is given and a description of some possible processes involving measurement with the T2 detector is reported. In chapter 3 the thesis work related to the development of algorithms for geometrical hit and track reconstruction in T2 is presented. The former reconstructs the geometrical position of the charged particle which ionize the active zone of the T2 triple-GEM chambers, starting from the digital signal of the activated read-out channels; the latter fits the reconstructed hits in order to obtain the particle track through the telescope. Moreover, in this chapter some studies for the optimization of the track selection criteria are reported, which in particular will be important for primary vertex reconstruction in inelastic events and hence for the measurement of the total cross section. In chapter 4 it is then investigated the possibility of hadronic jet reconstruction with the TOTEM inelastic telescopes, only relying on topological information (\eta,\phi) of charged particles. For this purpose, two innovative jet algorithms have been developed and tested at the particle level and then also applied at the reconstructed track level on Pythia Di-jet and Single Diffractive events, which allow jet reconstruction with a reasonable efficiency and relatively low fake-jet rate. These algorithms can in principle be combined with the energy information from the CMS CASTOR calorimeter for an optimal jet reconstruction in the very forward region, to be used in several analyses of interest.