TOTEM: The experiment to measure the total proton–proton cross section at LHC

TOTEM is an experiment dedicated to the measurement of the proton–proton total cross section (σ TOT ) at LHC, i.e. the probability that two protons interact at the center of mass energy of 14 TeV. In addition to that, it will also study the proton–proton elastic scattering and diffractive dissociation processes. TOTEM foresees specific measurements and experimental techniques which are very different from the other ‘general purpose’ experiments at LHC. A precise ‘luminosity independent’ measurement of σ TOT will be achievable in special beam optics runs by simultaneously measuring: 1) the elastic scattering rate at low transfer momentum, possibly as small as t = 10 GeV, and 2) the inelastic scattering rate with the largest possible coverage to reduce losses to few percents. The first goal requires detectors located into units mounted into the vacuum chamber of the accelerator, called Roman Pots (RPs), as the scattered protons are emitted at angles of the order of 10 μrad, therefore without leaving the beam– pipe. The latter requires the measurement of all the inelastically produced particles in the very forward direction with respect to the pp collision point; this can be achieved by using tracking detector telescopes with a complete azimuthal coverage around the beam–pipe.