The TOTEM experiment is designed to explore the proton interactions when they survive intact after the collisions. With this goal, special detectors have been placed far from the interaction points to detect a small angular deflection, due to elastic and inelastic interactions. This is why this physics is known as the 'forward' physics that is inaccessible by the other LHC experiments. TOTEM has made significant contributions to the field of particle physics, leaving a lasting legacy in our understanding of proton-proton interactions at high energies

As CERN's 'longest' experiment, TOTEM detectors were spread across almost half a kilometre around the CMS interaction point. TOTEM experiment has the equipment either in the CMS cavern and in the LHC tunnel, including four particle Telescopes in the forward region of the CMS as well 26 Roman Pots Detectors in the tunnel.

The TOTEM T2 detector, pictured during the last run in summer 2023

The 'telescopes' – T1 and T2 – were cathode-strip chambers and Gas Electron Multipliers (GEM), upgraded for run 3 with two scintillator telescopes (see picture above),  to track the particles emerging from collisions at the CMS interaction point. Meanwhile, 'Roman Pots', named for their shape and first use by physicists from Rome in the 1970s, were performing measurements of scattered protons. 

TOTEM data have been used to measure precisely the proton-proton cross-section at different energies, from 900 GeV up to 13,6 TeV, either total, elastic and inelastic, giving also evidence of the existence of a gluon compound, called Odderon. The existence of such type of gluon compound is hidden in the QCD theory but was never seen before.

With the forward telescopes, TOTEM measured the track density at small angles, giving a calibration for all the ultra-high energy cosmic rays experiments. The proton cross-section at 900 GeV has not yet been published, but the analysis is ongoing. The data taken in a special run in 2018 together with CMS are now under study for the search of the so-called Glueballs. This type of particle is foreseen by the QCD and is made exclusively from gluons, as suggested by its name. These elusive particles, in fact can be produced easily in the gluon fusion interaction, typical of diffractive physics. The protons momenta are directly correlated to the central CMS decay products, making the presence of these particles evident. 

TOTEM has also advanced the use of Roman Pot detectors, which are specialized devices that can detect protons scattered at very small angles relative to the beam line. In this collaboration with CMS, TOTEM with its Roman Pots has taken data also in high luminosity runs with the project CT-PPS (CMS TOTEM Precision Proton Spectrometer). Several analyses have been published for the search of Beyond Standard Model (BSM) physics. In particular, the interactions seen in these events are coming from photon fusion, a channel never studied before at the LHC. The searches cover the study of the anomalous quartic coupling production of W-W, Z-Z and γ-γ. Another search focuses on dark matter production together with a Z or γ particles using the missing mass method. In fact, the balancing of the scattered protons kinematics and the decay products seen in the CMS central detectors can isolate an eventual invisible particle. Other analyses are still underway using this method, searching for invisible particles in association with a Higgs, and for exclusive γ-γ and γ-Z production.

The CT-PPS pixel detector, a collaborative innovation by TOTEM and CMS, captures ultra-precise data during high-luminosity LHC runs, advancing searches for Beyond Standard Model physics.

The TOTEM Roman Pots, precision instruments for detecting scattered protons, installed deep within the LHC tunnel, have paved the way for groundbreaking discoveries in forward physics.