After nigh-on three decades, scientists at the CERN laboratory near Geneva are on the verge of completing the world’s most powerful particle collider. With all of the 1600 superconducting magnets that will be used to guide protons around the Large Hadron Collider (LHC) having been cooled to 1.9 K, the machine is finally ready to start circulating the first proton beams around its 27 km circumference underground ring.
At 9.30 a.m. local time on 10 September, a team of scientists and engineers will attempt to thread a single, low-intensity bunch of a few billion protons all the way round the €76.3 bn LHC. On the day itself, some 200 journalists from all over the world will be present at CERN to watch events unfold, with live footage of the control room set to be relayed into the lab’s science and innovation “globe”.
The beam will then be passed step by step through each of the LHC’s eight sectors until, by the end of the day, the beam should be fully circulating. After repeating the exercise for protons travelling in the opposite direction around the ring, which could take a further day or so, engineers will then tune the collider’s magnets so that the protons can circulate happily for periods of hours without veering off course.
At some point this autumn, strong focusing magnets will bring the two counter-rotating beams into collision at the LHC’s four interaction points, where the collider’s main experiments are located. If all goes to plan, the LHC will be providing proton–proton collisions at energies of 10 TeV by the end of the year, with a target of 14 TeV by spring next year. Then the real fun starts and the quest to peel back the next layer of nature can begin.
Start-up fever
With the world watching the switch-on — even Fermilab in the US will be having a pyjama party to celebrate, with director Pier Oddone allegedly planning to turn up in full nightwear — a lot is riding on a successful day’s work on 10 September. But two “injection tests” that took place last month bode well for the big start-up. These tests are designed to synchronize the LHC with the smaller accelerators that will feed it with protons. On 22 August scientists successfully injected the first protons in the counterclockwise direction of the LHC. This involved using a pulsed magnet to kick small bunches of protons out of the lab’s veteran Super Proton Synchrotron (SPS) and sending them down a 2.7 km-long transfer line towards the LHC. Indeed, scientists even managed to smash protons into a 28 tonne concrete collimator at the entrance to the LHC, which created muons that were detected by the LHCb experiment 200 m along the ring.
An earlier injection test on 7 August of the clockwise beam also proved successful — indeed, it turned out better than expected. After a few hours spent optimizing the injection process, engineers kicked one bunch of protons out of the transfer line into the LHC, where it travelled about 3 km through one of the LHC’s sectors before being stopped by a screen. “The passage of the beam first time caused some excitement in the control room, and the champagne was rolled out,” machine operator Roger Bailey told PhysicsWorld.
The test was then repeated several times to give the operations team lots of data to help make the start-up as smooth as possible. “The tests could not have gone better,” says CERN spokesperson James Gillies. “There are a lot of very happy people here.”
Towards 14 TeV
When it eventually reaches full steam, the LHC will produce the highest energy densities ever created in a lab. As such, it will be able to create fundamental particles that are too heavy to have been produced using existing machines such as the Tevatron at Fermilab. One of these could be the Higgs boson, which the LHC was built to find. If discovered, the Higgs would complete the Standard Model of particle physics by explaining how particles get their wildly different masses. The LHC may even see a “supersymmetric” world, where a new myriad of heavy particles mirror those of the Standard Model. Although based on much more speculative theories, the LHC may even find exotic entities such as mini black holes or evidence for additional dimensions of space–time.
But several steps need to be completed before the LHC can start hunting for new physics. Initially, the beams will have an energy of 450 GeV (the energy at which protons are injected from the SPS), producing 900 GeV collisions. But the target this year, possibly as early as a month after the first circulation, is to provide record-breaking 10 TeV collisions (5 TeV per beam) with 43 bunches each containing a few tens of billion protons. If all goes well, the first LHC data could be streaming out of the experiments just in time for the official LHC inauguration on 21 October, which will be attended by various heads of state.
The LHC will then be shut down for the winter, during which time the main bending magnets will be “trained” to handle beams at the full energy of 7 TeV (producing 14 TeV collisions) beginning in March or April 2009. When the machine is running at full capacity, nearly 3000 bunches each containing up to 100 billion protons will be hurtling around in each direction, producing half a billion collisions every second. However, it is likely to be at least a year before physicists amass enough data to understand their detectors well enough to be sure that what they are seeing is real.
