Candidate collision event in the CMS detector. Image copyright CERN.

The first protons collided in the Large Hadron Collider today at CERN outside Geneva, Switzerland. The four largest detectors at the LHC all recorded candidate collision events. Scientists at CERN, throughout the United States, and around the world celebrated the news.

"This is a very exciting moment after so many years of preparation,” said Andrew Lankford from the University of California, Irvine, deputy spokesperson for the ATLAS experiment. Beams were first tuned to produce collisions in the ATLAS detector, which recorded its first candidate for collisions at 2:22 p.m. local time. "The real accomplishment belongs to the accelerator scientists for bringing the beams into collision so quickly after they were first circulated,” he added.

These first collisions are another milestone on the way to the ultimate goal: high-energy collisions of protons in the center of the LHC experiments. They follow a weekend of rapid progress for the LHC. After more than one year of repairs, on Friday evening, November 20, beams were once again circulating in the collider. Over the weekend, the LHC team carefully studied the beams one at a time. Today at approximately 1:30 local time, two beams circulated at the same time for the first time in the LHC. As the two circulating beams passed through each other, protons from each beam hit one another, and the resulting spray of particles registered in the ALICE, ATLAS, CMS, and LHCb detectors.

Celebrations on Friday, November 20, when the first beams of 2009 successfully circulated in the LHC. Image copyright CERN.

“It’s a great achievement to have come this far in so short a time,” said CERN Director General Rolf Heuer in a statement issued by the laboratory. “But we need to keep a sense of perspective – there’s still much to do before we can start the LHC physics programme.”

These particular collisions happened against the odds. When the LHC is fully operational, each beam will consist of almost 3000 bunches of more than one hundred billion protons each. Despite the enormous number of protons, each bunch will still contain mostly empty space, and operators will "squeeze" them to increase the chances of two protons colliding. Today, during the testing phase of the accelerator, each beam only contained one bunch of several billion protons, and the beams were not squeezed. Thus the chance of two protons colliding as the bunches passed through each other was very small, and resulted in relatively few recorded collisions in each experiment.

"This is another great technical achievement for the LHC accelerator team and allows the collaborations on the LHC experiments to make further progress in preparing for first high-energy collision data," said Bob Cousins from the University of California, Los Angeles, deputy spokesperson for the CMS experiment. "We are getting a chance to test drive our detectors with real collision data."

More than 1700 scientists, engineers, students, and technicians from 97 US universities and national laboratories have helped design and build the LHC accelerator and its four massive particle detectors, known by their acronyms: ALICE, ATLAS, CMS and LHCb. They are joined by an estimated 8500 colleagues from 59 countries around the world. US participation from institutions in 32 states and Puerto Rico is supported by the Department of Energy’s Office of Science and the National Science Foundation.

“Everyone’s very excited,” said Tom LeCompte from the Department of Energy's Argonne National Laboratory, the physics coordinator for the ATLAS experiment. “We will use these very first collisions to determine if our detector is ‘in time,’ by which I mean that when a collision occurs, every part of the detector sees it happening at exactly the same time.”

Precise timing is critical for these huge detectors, where millions of separate detector elements, some separated from each other by tens of meters, must be synchronized to within one billionth of a second. The first collisions will also be used by scientists to calibrate and test many other parts of the complex detectors.

The first two protons collided at the relatively low energies with which they were injected into the LHC, 450 GeV each. Over the next few months, LHC scientists will raise the beam energy, aiming for collisions at the world-record energy of 3.5 TeV per beam in early 2010. With these high-energy collisions, the teams on the LHC experiments will embark on their quest to solve some of the mysteries of the universe.

American scientists have contributed critical components to the construction of the LHC accelerator and experiments, continue to play key roles in the operation of the detectors, and will be vital to the success of the experiments in their search for new phenomena such as the Higgs boson and the particles that make up dark matter.

For more images of candidate collision events, check out the CERN press release, the page of ATLAS public event displays, the ALICE Web site, and the CMS e-commentary.