While the large experiments at the LHC have been collecting the first inverse picobarn of integrated luminosity at 7 TeV in the centre of mass, the two smaller experiments installed at the collider have also passed milestones.
TOTEM, which stands for TOTal cross-section, Elastic scattering and diffraction dissociation Measurement at the LHC, is designed to measure elastic scattering over a range of momentum transfer, as well as a variety of diffractive processes. To make these observations, the experiment needs to detect particles at angles of less than 1 mrad relative to the beam line, so TOTEM includes detectors in Roman Pots at a distance of 220 m on either side of the CMS collision point (Point 5). The Roman Pots can move in close to the beam line, so the collaboration has to work closely with the LHC collimation experts. Now they have succeeded in moving the detectors close to the beam, locating it with very high precision, first at the beam energy at injection of 450 GeV, and then at the normal running energy of 3.5 TeV per beam. This led to TOTEM’s sighting of not only the first candidates for proton–proton elastic scattering at 7 TeV, but also the first candidate for the diffractive process of double-Pomeron-exchange – the first time that such an interaction has been seen at 7 TeV.
LHCf, which stands for “LHC forward”, meanwhile, has already completed its first run at the LHC. This experiment consists of two independent detectors located at 140 m either side of the ATLAS collision point (Point 1). It studies the secondary particles created during the head-on collisions in the LHC, the goal being to compare the various models that are used to estimate the primary energy of ultrahigh-energy cosmic rays from the showers of particles that the primaries create in the atmosphere. LHCf was designed to work with high-energy particles, but at a low luminosity, and the experiment has now collected sufficient data to complete the first phase of the research programme at 450 GeV and 3.5 TeV per beam. The results of the data analysis at 450 GeV will be available by the end of the year, while data at 3.5 TeV will be analysed in 2011. The UA7 experiment carried out at the SPS proton–antiproton collider in the 1980s has already provided information for collisions at beam energies of 450 GeV. LHCf will be the first to provide results at 3.5 TeV and beyond
The detectors used for this first run were removed during a short technical stop of the LHC at the end of July. These were mainly plastic scintillators. The collaboration will now work on replacing them with more radiation-resistant crystal scintillators, to be ready by 2013 when the LHC will run at 7 TeV per beam. The collaboration also plans to change the position of the silicon detectors to improve the performance of the experiment in measuring the energy of the interacting particles.
