Counter-clockwise beam test produces historic particle collisions.
Champagne corks popped at the Large Hadron Collider (LHC) this weekend after one of the facility's four giant particle detectors tasted its first authentic data. Crammed into a stuffy control room on the afternoon of Friday 22 August, physicists tracked the debris produced by protons that had struck a block of concrete during a test of the €3 billion (£2.1 billion) collider's beam-injection system.
Some 15 years in construction, the LHC is based at the European particle facility CERN near Geneva, Switzerland, and is due to fully switch on its proton beams on 10 September. But the LHC's particle detectors have been recording hits from cosmic rays for several months — and Friday's test now marks the first time particle tracks have been reconstructed from a man-made event generated by the collider. "It's amazing to have seen the first LHC tracks," Themis Bowcock of University of Liverpool, UK, who led the team, told Nature. "It's quite overwhelming actually."
The first useful physics data is expected to come in October, when the two counter-rotating beams of protons racing through the LHC's 27-kilometre-long tunnels are made to collide, packing sufficient energy into a small enough space to produce fundamental particles from thin air. Full high-energy collisions at a combined energy of 14 trillion electron volts will begin next spring, exceeding the energies accessible to the current world record holder — the Tevatron at Fermilab in Batavia, Illinois — by a factor of seven. The LHC's high-energy collisions will allow physicists to search for new particles such as the fabled Higgs boson, which is thought to be responsible for conferring the property of mass on other particles.
The purpose of this weekend's injection test was to make sure protons are magnetically kicked out of the smaller Super Proton Synchrotron (SPS) — the last link in a chain of other CERN accelerators that whip protons up to faster speeds — at the precise moment the LHC is ready to accept them. For this transfer process to happen smoothly, magnetic pulses in the accelerator chain must be synchronized to within a fraction of a microsecond.
But researchers on the LHCb experiment, which will study B mesons to look for tiny differences in the behaviour of matter and antimatter, spotted an opportunity to put part of their experiment through a test of its own. After a week of around-the-clock preparation and a tense afternoon spent waiting for the green light from the CERN Control Centre, their efforts paid off.
At 17:20 on Friday, physicists in the control centre fired a few billion protons from the SPS down a 2.7-kilometre transfer line into a 28-tonne concrete collimator positioned at the entrance to the LHC, some 200 metres upstream from the LHCb detector. Muons produced in the pileup sailed straight through the block and travelled along the LHC beam pipe to generate electrical hits in successive silicon discs of the Vertex Locator (VELO), an instrument that will track particles produced within a few millimetres of the proton-proton collisions. Greeted by claps and cheers, the team then used software to display the paths of about half a dozen muons on a laptop screen within minutes of the event.
According to an excited Chris Parkes of Universityof Glasgow, UK, who heads the UK VELO team, the test went much better than anyone had expected. "This represents a tremendous amount of work by a big team over 10 years," he said on Friday.
Shoot it up
Meanwhile, in CERN's sleek blue control centre, a 15-minute drive away along the Swiss-French border, the champagne (and beer) was still on ice. Having successfully flicked protons from the SPS to the LHC's back door, the next step for the operations group was to remove the collimator and shoot a bunch of protons into the counter-clockwise beam pipe of the LHC. When the same feat was attempted in the other direction on 8 August, the first protons to enter the LHC had travelled the expected 3 kilometres to the end of Sector 12 of the ring on the first attempt.
After overcoming a few software and timing problems, the team matched the feat in the opposite direction just before 21:00 on Friday. "We opened the tap and it went straight through," said Paul Collier of CERN's operations group. "The quality of the machine alignment is superb." Some scientists on the team had expected they would need to adjust a few corrector magnets to steer the protons around.
Tests continued over the weekend, allowing more detailed measurements of the beam and providing further tracks in LHCb. The successful trials mean that an eighth of the LHC has now been tested with a beam of protons, and CERN officials can be quietly confident that things will go smoothly on start-up in September.
"These guys make it look easy," CERN's director of accelerators and beams, Steve Myers, told Nature from the control room floor. "It took longer to write the documentation telling us how to do it than it did to shoot the beam through."
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Chalmers, M. Double first for Large Hadron Collider. Nature (2008). https://doi.org/10.1038/news.2008.1061