First roman pot tested by TOTEM

TOTEM, one of the smaller experiments of the LHC, successfully tested its first 'roman pot' detectors on 3 November. A total of eight will be installed in the LHC near the CMS cavern.

There is a small tribe in the land of CERN. Among its artefacts you may find colourfully painted rocks, a totem made of cardboard boxes, and a few roman pots. Known by the name of TOTEM, or 'TOTal, Elastic and diffractive cross-section Measurement' (not a tribe motto), they are a relatively small collaborative group in comparison to the main LHC experiments, with approximately 50 'tribe members'.

Unlike the four larger experiments that will analyse new particles produced as a result of the collisions, TOTEM will investigate the ones that almost missed each other. When two beams of protons travelling in opposite directions reach the collision point, not all of them will encounter another proton head on. Some will make slight contact, while others may not meet at all. For those that just brush pass another proton, they will be deflected from the direction of the beam by a minute amount - these are the 'forward particles' TOTEM will investigate, using special detectors called 'roman pots'.

The name originates from its invention in the early 1970s by the CERN Rome group, who used them to study the physics at the ISR (Intersecting Storage Ring), the world's first high-energy proton-proton collider. The small detectors are contained in cylindrical vessels, or 'pots'. 'Roman pots are considered a CERN speciality. Since their invention, they have been used in labs all around the world,' says Marco Oriunno, project engineer of TOTEM. Forward particles are used to measure the total collision rate, or cross-section, inside a collider. The closer the roman pot detectors can get to the path of the beam, the more precise the results. Several improvements will provide an unprecedented level of precision. As Marco continues, 'We've updated the original design with some new features. These are the very thin stainless steel windows of less than 150 microns in thickness, the flatness of the windows (less than 30 microns), and the precision of the motor mechanism that moves the pots towards the beam. A prototype was piloted at the SPS in 2004.' For the LHC, the roman pots will collect data from a distance of 800 microns from the beam. These technological achievements required multidisciplinary expertise from various departments during the design phase: experimental physics at the Physics Department (PH), vacuum physics at the Accelerator and Technology Department (AT), mechanical engineering at the Technical Support Department (TS), and motors, controls and radio frequency studies at the Accelerators and Beams Department (AB).

The roman pots used in the TOTEM experiments are manufactured by 'VakuumPraha' in Prague, according to specification drawings produced by Jean-Michel Lacroix at CERN. The Academy of Sciences of the Czech Republic is one of the TOTEM collaborators; it contributes to the physics and funds the production of the detectors. The other project collaborators include CERN, three universities in Italy, two in the USA, and one university each from Finland, Poland, Estonia and the UK.

In the final configuration, eight roman pots will be placed in pairs at four locations around Point 5 (Cessy) in the LHC. There are two stations at each end of the CMS detector, positioned at distances of 147m and 220m 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 needs.

The first roman pot tested by TOTEM on 3 November passed with flying colours. A further seven will be transported to CERN by the Christmas holiday for assembly, followed by laboratory tests. Seven years ago, before civil engineering work began on the CMS cavern, antique Roman pots from the third century AD were among the artefacts excavated at around the same location as the experiment. When the detectors are installed underground in spring 2007, the tribe of TOTEM may open a new chapter in local history - the roman pots which gave us a glimpse of the beginning of civilisation may now show us the birth of the Universe.