TOTEM is an experiment dedicated to the measurement of total cross section, elastic scattering and diffractive processes at the LHC.

The total cross section will be measured using the luminosity independent method which based on the simultaneous detection of elastic scattering at low momentum transfer and of the inelastic interactions.

This method also provides an absolute calibration of the machine luminosity. The experimental apparatus consists of:
- Telescopes of "Roman pots" placed symmetrically on both sides of the intersection region. They will detect protons scattered at very small angles in elastic or quasi-elastic reactions.
- A forward inelastic detector covering about 4 pseudorapidity units in the forward cones (from eta=3 up to eta=7) with full azimuthal acceptance. This detector will measure the overall rate of inelastic reactions.

The Roman pots are special devices mounted on the vacuum chamber of the accelerator. In its retracted position the Roman pot leaves the full aperture of the vacuum chamber free for the beam as required at the injection.

Once the final energy is attained and the circulating beams are stable, the Roman pot is moved toward the machine axis by compressing the bellows until the inner edge of the detectors reaches a distance of the order of one millimeter from the beam. Particles emitted at very small angles can therefore be detected.