TOTEM: Thousands of interesting events

TOTEM is the LHC experiment dedicated to the measurement of the proton total cross-section. This first proton run produced a wealth of data that is allowing the collaboration to probe the proton as never before.


The TOTEM science programme concentrates on the fundamental properties of the proton by studying how it interacts with other protons. “We are there to measure the total cross-section of the proton, which describes the likelihood that some kind of interaction will occur between the protons”, explains Karsten Eggert, TOTEM spokesperson. “In order to do so, we have to understand all the individual processes and separately measure the different cross-sections”.

The interaction between protons can result in a modification of the individual kinetic energy (inelastic scattering) or of just the direction of propagation (elastic scattering). Besides the elastic scattering, TOTEM also studies more complex interaction processes, such as the so-called double pomeron exchange, in which both scattering protons lose part of their momentum. “During this first run we collected hundreds of thousands of elastic scattering events and were able to confirm that the diffractive pattern observed by previous experiments at much lower energies persists at high energies”, says Karsten Eggert.

Studying the spectra when the protons drastically change their direction of propagation allows scientists to look inside the proton without breaking it apart. “With this technique we can infer what the distribution of quarks and gluons might be inside the proton”, continues Karsten Eggert. “There are several theoretical models that predict how the proton behaves internally and our spectra will allow us to compare the different models with real data by the end of this year”.

So far, the LHC has been running with very focussed beams, which is not ideal for TOTEM. Next year, the machine will do special runs for TOTEM in which the beams will be brought onto more parallel orbits in the vicinities of the IP5 crossing point. Under these conditions, very tiny fractions of the momenta transferred from one proton to another can be observed. “This will allow us to really study all the different processes involved when two protons interact. We will then be able to measure the total cross-section”, says Karsten Eggert.

During the Christmas technical stop, the TOTEM collaboration will install new detectors (T1 chambers) inside CMS, on both sides of the interaction point, after which the experiment will be ready to go at full speed in the 2011 run.

by CERN Bulletin