by Katie Yurkewicz, Fermilab, US LHC Communications and Harvey Newman, Caltech, US LUO Executive Committee Chair
(Originally published in January 2010 DPF newsletter)
The Large Hadron Collider at CERN in Geneva, Switzerland is now the world’s highest-energy particle accelerator, and is on its way to becoming the most powerful. Following more than 15 years of construction, and one year of repairs following a meltdown of a busbar between two superconducting magnets in September 2008, beams of protons once again circulated in the LHC on November 20. After rapidly capturing and circulating the beams, and completing the first series of measurements of the machine optics in a time shorter than expected, the LHC’s first-ever proton collisions at the injection energy of 450 GeV per beam (900 GeV in the center of mass) were recorded by the ALICE, ATLAS, CMS and LHCb detectors on November 23. Less than one week later, after preparing the “ramp” sequence to accelerate the beams while controlling the machine tune, the LHC set a world record for beam energy by accelerating protons to an energy of 1.18 TeV, surpassing the previous record of 0.98 TeV that had been held since 2001 by Fermilab. Collisions at 2.36 TeV center-of-mass energy soon followed, with the first physics runs taking place shortly after midnight on December 14. The last two weeks of the LHC’s 2009 run, which ended December 16, were particularly exciting. The accelerator met all of its operational goals in storing, measuring, accelerating, and squeezing the beams, and the experiments demonstrated their ability to produce rapid results.
The first runs at 900 GeV center-of-mass energy provided large data samples that allowed the experiments to improve on their calibration, alignment and timing, and test their reconstruction and analysis chains. Clean peaks soon appeared that showed light mesons such as pizeros, etas and lambdas decaying to photons or charged particles. Together with distributions of the charge multiplicity per event and jets and their spread,
these results signaled a good understanding of the detectors and their responses and provided an excellent beginning to the LHC physics program. The first physics runs at 2.36 TeV center-of-mass energy resulted in record data flows across the Atlantic and to several sites in Europe. Results at both collision energies were
reported by all LHC experiments at the CERN Council Meeting on December 18.
These first low-energy collisions are milestones on the way to the ultimate goal: billions of high-energy collisions in the center of the four main LHC detectors. Teams at CERN are now preparing the accelerator for collisions at 3.5 TeV per beam in the first quarter of 2010.
Scientists from U.S. universities and national laboratories celebrated along with their peers around the world as each milestone was passed, and eagerly await the next steps. In all, 10,000 people from 60 countries have helped conceive, design and build the LHC accelerator, its experiments, and the Worldwide LHC Computing Grid, including more than 1,700 scientists, engineers, students and technicians from 97 institutions in 32 U.S. states and Puerto Rico. U.S. participation is funded by the Department of Energy’s Office of Science and the National Science Foundation.
American participation is vital to the two biggest LHC experiments, ATLAS and CMS. U.S. scientists have contributed to the construction of almost every sub-system of the ATLAS and CMS detectors as well as their software, computing and networking systems, continue to play key roles in the operation of the detectors,
and are gearing up toward intensive work on all aspects of data analysis, from precision top quark and other measurements of Standard Model physics in a new energy range, to Higgs and supersymmetry searches, to more exotic new physics scenarios such as gravitons signaling the existence of extra dimensions, or low mass strings. In the ALICE experiment, American participation has ramped up over the last several years with the contribution of the detector’s electromagnetic calorimeter. U.S. scientists also contribute to the LHCb and TOTEM experiments.
On the accelerator front, scientists from American institutions have contributed critical components to the construction of the LHC, including the final-focus magnet systems that guide beams into collision in the heart of the experiments and superconducting beam separation dipole magnets.
Collisions at 3.5 TeV per beam are the initial goal for the LHC’s 2010 run, but more may be in store. Once a significant data sample has been collected by the LHC experiments and the operations team has gained experience in running the machine at that energy, beam energies may be increased to a maximum of 5 TeV per beam. At the end of 2010, the LHC will be run with lead ions for the first time. After that, the LHC will shut down and work will begin on moving the machine towards the design goal of 7 TeV per beam.
The speed at which the LHC was recommissioned and the beams were brought into collision, and the degree to which the beam optics are understood, speak both to the precision of the collider’s magnet lattice and construction accuracy, and the excellence of the teams who built and are operating the accelerator. The same can be said of the LHC experiments and the teams who built and operate the detectors, as well as their trigger and data acquisition, software and computing systems, who were ready and waiting to acquire, reconstruct, record and immediately analyze the first samples of events. Even these first steps have been an exciting experience for the thousands of physicists, students and engineers at sites throughout the world participating in the LHC program, as they embark on the long awaited physics program in a new range of collision energies.
