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The long road to high-intensity LHC collisions

Integrated luminosity for LHC experiments through July 14, 2010.

Integrated luminosity delivered to the LHC experiments through July 14, 2010. Image copyright CERN.

It's getting more intense under France and Switzerland. Over the last few months the Large Hadron Collider has been quietly increasing the power of its two counter-rotating beams of protons, and serving up more and more particle collisions to the experiments dotted around the 27-kilometer ring.

An LHC beam is not a continuous stream of particles, but is made up of a number of packets of particles, called "bunches." Years from now, when the LHC beams are at full power, each beam will be composed of 2,808 bunches of more than 100 billion protons each. But when the LHC collided its very first protons in March, each beam contained only 4 bunches, each with less than 10 billion protons each. Much of the accelerator team's work over the last two months has focused on preparing the LHC, its protection systems, and its five pre-accelerators to handle beams with more higher-intensity bunches of protons.

The first major step in the march to greater intensity was to increase the number of protons in each bunch. By the end of May, bunches at "nominal" intensity--approximately 100 billion protons each--had been injected into the LHC and accelerated to 3.5 TeV.

Compared to the speed at which the number of protons in each bunch was increased to maximum, the road to 2,808-bunch beams will take much, much longer. To date, the beams have contained at most 13 bunches, with at most 9 bunches colliding in the LHC's experiments. An increase from 4 bunches to 13 over three and half months might seem extremely slow. But it's par for the course in particle physics, as scientists learn how to operate and understand the new accelerator, increasing the intensity, and thus the power, of the beam very slowly to ensure the safety of the LHC machinery and particle detectors.

While the LHC accelerator team has been working to increase beam intensity and provide more collisions, the physicists on the LHC experiments have been striving to record as many of the collisions as possible.  Physicists are also closely tracking a quantity called the integrated luminosity, a measure of the number of collisions that have taken place at the center of their detectors. The more collisions the experiments collect, the  better their ability to measure the properties of fundamental particles and forces.

As of this morning, the integrated luminosity recorded by the experiments is just above 250 inverse nanobarns, most of which was collected over the past week. This amount of data should have provided the ATLAS and CMS experiments with a few top quarks, as well as enough W and Z bosons for LHC scientists to start making their first measurements of the two particles. All the latest results from the LHC--and many other experiments from all fields of particle physics--will  be presented at next week's International Conference on High Energy Physics in Paris.

More details about the LHC's current status can be found in this week's CERN Bulletin, or by tuning into the LHC status updates presented at CERN approximately every two weeks.