TOTEM's potential resides in making some unique observations. In addition to the precise measurement of the proton-proton interaction cross-section, TOTEM's physics programme will focus on the in-depth study of the proton's structure by looking at elastic scattering over a large range of momentum transfer. Many details of the processes that are closely linked to proton structure and low-energy QCD remain poorly understood, so TOTEM will investigate a comprehensive menu of diffractive processes - the latter partly in co-operation with the CMS experiment, which is located at the same interaction point (IP5) on the LHC.
Early measurements at CERN's Intersecting Storage Rings (ISR) revealed that the proton-proton interaction probability increases with collider energy. However, the nature of the correct growth with energy remains a delicate and unresolved issue. A precise measurement of the total cross-section at the world's highest-energy collider should discriminate between the different theoretical models that describe the energy dependence. The value of the total cross-section at LHC energies is also important for the interpretation of cosmic-ray air showers. All of the LHC experiments will use TOTEM's measurement to calibrate their luminosity monitors, in order to calculate the probability of measuring rare events.