The experiments being prepared for CERN’s Large Hadron Collider (LHC) received their first beams in May and preliminary results are expected soon. No, the LHC, scheduled to begin operations in 2005, hasn’t been brought forward by five years, but with LHC-type test beams being delivered from the SPS synchrotron, the project to upgrade CERN’s existing accelerator complex to supply the LHC is well on course.
The upgrade was required because bunches of particles travelling around the LHC will be just 25 ns apart. This is a lot closer together than bunches in the Large Electron Positron (LEP) collider and has been chosen because the phenomena that LHC experiments will be looking for are extremely rare. A 25 ns bunch spacing will give the experiments as many collisions as possible without swamping them with data.
The Booster was the first of CERN’s synchrotrons to be upgraded. It had its radiofrequency system changed and its maximum energy was “boosted” from 1 to 1.4 GeV to provide the high beam brightness required for the LHC.
With a beam energy of 26 GeV, the veteran Proton Synchrotron (PS) is the best place to group particles into short intense bunches before sending them on to the larger accelerators. This is done using new radiofrequency cavities. A cavity operating at 40 MHz produces bunches spaced by 25 ns by applying a rapid change in voltage.
Bunch lengths are also measured in nanoseconds – the time it takes a bunch to pass a fixed point – and this procedure squeezes the bunches down to just 10 ns. If every note in Beethoven’s Fifth Symphony were that short, the music would be over in a fraction of a second. Nevertheless, it is still not short enough because the next accelerator in the CERN chain, the Super Proton Synchrotron (SPS), operates at 200 not 40 MHz. This means that SPS bunches must be 5 ns long at the most; 10 ns bunches from the PS would be too long.
To overcome this problem, more particle gymnastics are performed by the 40 MHz cavities working together with more new cavities operating at 80 MHz. This squeezes the bunches to less than 5 ns – short enough to transfer into the SPS.
The new 40 and 80 MHz cavities for the PS are one of the first joint ventures resulting from the Canadian contribution to the LHC. Essential elements of the cavities were built in collaboration with the Canadian TRIUMF laboratory. Their installation began in 1996 and the PS was ready to hand over its first LHC-type beams to the SPS in 1997. Two years of fine-tuning and optimization ensued to prepare realistic test beams for the LHC experiments.
The PS celebrated its 40th birthday in 1999 and, to mark the occasion, an optimized LHC-type beam was accelerated and handed over to the SPS on 27 October. That provided the cue for the SPS team to ready itself for the LHC. With modifications to the PS being tailored to fitting in with the existing configuration of the SPS, the modifications to the larger machine were less extensive.
By the end of the year an LHC-type beam had been steered around the SPS and extracted towards the tunnel that will eventually take beams into the LHC. By May this year the SPS was ready to provide its first LHC-type test beams for the experiments.
The goal of this year’s tests, carried out by the ATLAS, CMS and LHCb experiment collaborations, was to test electronics and detector prototypes in realistic LHC conditions – a milestone required by the LHC committee. The ALICE collaboration, whose lead-ion beams will come together at the relatively sedate rate of once every 125 ns, did not use the new test beam.
ATLAS tested its semiconductor (SCT) and pixel trackers, and its transition radiation tracker (TRT) systems. CMS tested its silicon tracker and its resistive plate chamber (RPC) muon trigger system. LHCb tested its fast (40 MHz) front-end electronics read-out system based on the response of its calorimeter.
All three experiments report encouraging preliminary results. The ATLAS SCT group, for example, used the tests to investigate the efficiency of its electronics at associating hits in the detector with the correct beam bunch, and to see how well the electronics could keep track of hits from several bunches in the electronics pipeline at the same time.
CMS took the opportunity to test prototype silicon tracker modules with the full electronics read-out chain. The results showed that the detector and electronics are up to the task of operating under the stringent timing constraints imposed by the LHC’s 40 MHz bunch-crossing frequency. The LHCb system also proved itself capable of distinguishing clearly between adjacent bunches.
More LHC-type test beam running is foreseen for the coming years, with the goal of testing final electronics and several sub-detector prototypes together.
