Last night the first collisions of protons at the world-record energy of 2.36 TeV (1.18 TeV per beam) were recorded by the ATLAS experiment at CERN. The ATLAS team posted an image of one candidate collision event on its Web site.
In addition to setting world records--for proton beam energy on November 29, and last night for proton collision energy--the team operating the Large Hadron Collider at CERN, and the teams operating the LHC experiments, have been hard at work. Here are a few more highlights from the last week and a half.
One of the main objectives for the LHC team over the past week and a half has been preparing to circulate and collide beams at higher intensities. This involves increasing the number of bunches that make up each LHC beam as well as the number of protons in each bunch.
"Higher intensity means more luminosity, more luminosity means more events for the experiments, more events gives more physics," explained Fermilab's Jim Strait, a former project manager for the US contributions to the LHC accelerator who has been at CERN for the last year and a half assisting with LHC commissioning.
At full intensity, LHC beams will have 2808 bunches each. The very first collisions at the LHC, at an energy of 450 GeV per beam on November 23, took place between one-bunch beams. On December 4, the LHC injected and circulated the very first two-bunch beam at an energy of 450 GeV. Around midnight on December 6, the first four-bunch beams were circulated and steered to collide in the center of the experiments.
Another main objective has been to deliver collisions at an energy of 450 GeV per beam to the LHC experiments. Collisions at this low energy--the energy at which the beams are injected into the LHC--are crucial to the experiments for calibrating and testing their detectors. The first sustained period of multi-bunch collisions at 450 GeV per beam occurred on December 6, and the experiments have continued to collect collision data at injection energy over the past few days.
The teams on the LHC experiments have been busily analyzing the first experimental data. The first LHC paper using collision data was prepared by the ALICE collaboration and accepted by the European Physical Journal C on December 1.
Although the LHC has already reached several milestones, there is much work still to do. By the time CERN shuts down for its two-week winter break on December 18, CERN hopes to have achieved a sustained period of collisions at an energy of 2.36 TeV. The next step will be collisions at a total energy of 7 TeV in early 2010, at which point the teams on the experiments will have their first chance to search for new physics.
"Almost all the work is still ahead to commission the machine and learn how to operate the beams," said Strait. "They've only been at it two week. There's been fantastic progress, but it's still only two weeks."