Where the b's are

The last of the 42 modules for the LHCb Vertex Locator (VELO) arrived safely at the lab during the first week of March. VELO's high-precision silicon detector will rest a mere 5 millimetres from the beam line itself.

Assembly of one half of LHCb's VELO, containing 21 of the 42 modules.

LHCb's Vertex Locator (VELO) team had reason to celebrate this month with the arrival of the last of the 42 silicon detector modules at the beginning of March. Consisting of two rows of 21 double-sided half moon-shaped silicon detectors, each about eight centimetres in diameter, the VELO modules, designed and constructed at Liverpool University, will allow scientists to get a precise track of the particles spraying out of the forward regions of the detector. This is the area where scientists expect to find the greatest concentration of the b/anti-b pairs that will help them explain the imbalance between the amounts of matter and antimatter we observe in the universe.

'Everyone says their device is unique,' said LHCb-VELO Project Leader Paula Collins. 'This one truly is. This is probably one of the most difficult, complicated modules we have in the LHC.' Because of its proximity (5 millimetres) to the beam line, VELO must face the double responsibilities of acting as detector and beam pipe at the same time. To keep a wayward beam from drilling a hole through one of the modules, NIKHEF, Amsterdam, another major partner in the VELO, created a custom-made set of bellows that would allow both sides of the VELO to retract to the safer distance of three centimetres away from the beam line while the beam is being set up. Furthermore, to maintain the LHC vacuum at 10-8 millibar, a special corrugated foil separates the LHC beam line from the VELO detector vacuum, ensuring that VELO's presence in the beam pipe does not compromise other experiments.

For Collins, the arrival of the final VELO module is indeed a cause for celebration for the Liverpool team. It is the culmination of at least ten years of dedicated development-not to mention the successful end to the complicated process of transporting the modules from Liverpool to CERN. The modules are so sensitive to vibrations that even dropping a pin onto one could break or dislodge the delicate bond wires that connect the silicon strips to the electronic chips. 'We had a special security check-in procedure arranged with EasyJet so it wouldn't hurt the modules,' Collins said. When bomb scares postponed the plane scheme, the Liverpool team turned to cars; the first set were personally chauffeured to CERN by Themis Bowcock of Liverpool University. So far, not a single module has been damaged. 'If you drop a VELO module, you've dropped two per cent of the whole detector,' Collins said.

The arrival of the final module allows the VELO team to finish their assembly and prepare to install the detector in the pit, which they hope to do by summer. After that they can begin commissioning the experiment-but not before they have had a massive party. 'The end is clearly in sight now,' Collins said. 'We'll find out soon: is 42 really the answer we were looking for?'

Paula Collins, LHCb-VELO project leader, and Raphael Dumps, Project Mechanical Engineer, make adustments to one half of the VELO.

Each VELO module consists of two layers of the half-moon-shaped silicon detectors connected to readout electronics and a cooling system.