The proton as seen by TOTEM

TOTEM, one of the smaller experiments at the LHC, has recently recorded the first candidates of proton-proton elastic scattering at a collision energy of 7 TeV. Studying the elastic scattering between two protons is a powerful way of exploring the inner structure of the proton, one of the most common, yet still poorly understood, particles we observe in Nature.


One of the first elastic event candidates recorded by the TOTEM experiment. The proton tracks are reconstructed in the Roman Pots detectors 220m away from the intersection point IP5 (not to scale).

The elastic scattering between two colliding particles is a process in which the kinetic energy of the particles is the same before and after the interaction; only their direction of propagation is modified by the scattering. In more scientific terms, this means that particles transfer part of their momentum in the interaction but not their energy. By studying these kinds of processes, physicists can infer the inner structure of the interacting particles. One of the goals of the TOTEM experiment at the LHC is to use this technique to probe the proton.

“We are happy that we are seeing candidates for elastic and diffractive proton-proton scattering at the LHC for the first time” says Karsten Eggert, TOTEM Spokesperson. “Intensive studies of these phenomena started in the late 1970s at  CERN’s Intersecting Storage Rings (ISR) and continued at HERA and the Tevatron”. TOTEM’s physics programme looks at the elastic scattering of the two protons over a large range of momentum transfer. The higher the transferred momentum, the smaller the distance at which one proton probes the other.

The signals left on the TOTEM detector by a proton-proton interaction known as the “Double Pomeron Exchange”. This is the first time that such an interaction has been observed at a collision energy of 7 TeV.

To make these unique observations, TOTEM can count on the so-called Roman Pot detectors installed over a distance of 440 m on both sides of the CMS collision point (Point 5 of the accelerator ring). In the case of elastic scattering and many other diffractive events occurring when the protons collide in the LHC, particles need to be detected at tiny angles (less than 1 mrad) relative to the beam line. The Roman Pots have to be positioned  close to the beam line. “The positioning of these devices is a very delicate manoeuvre”, explains Mario Deile, TOTEM run co-ordinator. Working together with Ralph Assman’s collimation team, we have recently succeeded in moving our detectors extremely close to the beam and have been able to locate it with very high precision. We performed this exercise at 450 GeV per beam and will later do it at 3.5 TeV. At the end of the exercise, all the twelve Pots were aligned with high precision with respect to the beam centre and everything went very smoothly. This proves that we have acquired detailed information on our apparatus and the machine itself.” “We could see very clean tracks of particles scattering at very small angles and travelling along the beam line. This shows that all the detectors are working really well”, confirms Karsten Eggert.

Although, at first sight, the proton might seem to be one of the best known particles, its inner structure is far from being completely understood by scientists. This is the gap TOTEM is set to fill.

More information about the TOTEM experiment can be found here.
The TOTEM Collaboration will present its most recent results at the LHC Physics Day organised by the LPCC on 6 August.


by CERN Bulletin