With detectors positioned at distances of 147 and 220 m from the CMS interaction point and others inside CMS, the TOTal Elastic and Diffractive Cross Section Measurement (TOTEM) experiment will measure the total interaction cross-section of protons at the LHC.
The data collected by the experiment will help to improve knowledge of the internal structure of the proton and the principles that determine the shape and form of protons as a function of their energy. Furthermore, TOTEM will allow precise measurements of the LHC luminosity and individual cross-sections used by the other LHC experiments. Specific to the TOTEM experiment are the “Roman pots”. Veritable marvels of technology, these cylindrical vessels can be moved to within 1 mm of the beam centre. They contain detectors that will measure very forward protons, only a few microradians away from the beams, which arise from elastic scattering and diffractive processes.
Inelastic interactions between protons will be studied by gas electron multiplier (GEM) detectors installed in “telescopes”, placed in the forward region of the CMS detector, where the charged-particle densities are estimated to be in the region of 106 cm–2s–1. Each of the telescopes contains 20 half-moon detectors arranged in 10 planes, with an inner radius matching the beam pipe. TOTEM will exploit the full decoupling of the charge-amplification and charge-collection regions, which allows freedom in the optimization of the readout structure, a unique property of GEM detectors.
The closer that the Roman pot detectors can get to the path of the beam, the more precise the results. For the LHC, the Roman pots will collect data from a distance of 800 μm from the beam. Several improvements in TOTEM’s detectors will provide an unprecedented level of precision: the thin stainless-steel windows of less than 150 μm in thickness; the flatness of the windows (less than 30 μm); and the precision of the motor mechanism that moves the pots towards the beam. The pots used in the TOTEM experiment are manufactured by VakuumPraha in Prague, according to specification drawings produced at CERN.
In the final configuration, eight Roman pots will be placed in pairs at four locations at Point 5 on the LHC. There are two stations at each end of the CMS detector, positioned at distances of 147 m and 220 m from the collision point (interaction point 5). Although TOTEM and CMS are scientifically independent experiments, the Roman-pot technique will complement the results obtained by the CMS detector and by the other LHC experiments overall. The ATLAS experiment will also be using a pair of Roman pots based on the design developed by TOTEM, with slight adaptations to suit its own specific needs.
TOTEM has now installed all the Roman pots and has equipped a few of them with detectors. This will allow them to test the movement of the Roman pots with respect to the beams at the LHC start-up and to take some first data. Some detectors were also installed within CMS. After having gained experience this year, the remaining detectors will be installed during the winter shut-down to make the experiment fully operational for next year’s runs.
• Based on an article in CERN Bulletin 2008 issue 37–38.
LHCf looks forward to high energies
Positioned 140 m from the ATLAS interaction point, the LHCf experiment will attempt to improve the models that describe the disintegration of ultra-high-energy cosmic rays as they enter the atmosphere. This will allow their energies to be determined more accurately and their composition to be analysed with greater precision. This information will help support the hypotheses on the mysterious origins of cosmic rays.
The LHCf detectors are placed along the beam pipe just beyond the experiment cavern, at the point where the pipe splits into two. This location allows them to detect the neutral particles (or their decay products) that are emitted in the forward region and are not bent off course by the magnetic fields of ATLAS and the LHC magnets.
While the old generation of accelerators allowed researchers to verify the cosmic-ray disintegration models up to energies in the region of 1015 eV – LHCf will test them at energies of up to 1019 eV. Even if this year’s data is generated by lower-energy collisions, it will still be important as it will lie in the top-most region of data collected from previous experiments.
• Based on an article in CERN Bulletin 2008 issue 37–38.
