Yesterday the LHC passed another milstone when they ramped a trillion protons per beam up to the current running energy of 3.5 TeV. The number of bunches in use is now 12 per beam with a filling scheme giving 8 collisions per turn to each experiment.
The first run at this new intensity was dumped before stable beams could be declared for physics. The reason given was problems with orbit feedback. A second run this morning has been much more successful with stable beams declared and a new record for peak luminosity of about 1.5 x 1030 cm-2s-1. That’s 1.5 inverse micro barns per second. This follows a healthy run the previous night with 9 bunches that provided about 25 inverse nanobarns of integrated luminosity to add to the totals of around 100 inverse nanobarns so far.
It has not been all plain sailing. An uncomfortable proportion of attempts to reach stable beams have failed for a variety of reasons that caused the beams to be dumped prematurely. Typical sources of problems include glitches in the electricity network, problems with hardware and software components or just human errors. Even more worrying is the heavy losses from the beams once they are circulating. This is partly due to the electromagnetic interactions between the protons when the bunches collide and this has been mitigated by reducing the intensity of each bunch to about 90 billion protons rather than the 115 billion nominal value. It also seems likely that The Hump continues to dog operations causing the beams to disperse vertically and hit the collimators.
Despite these problems progress continues to just about keep up with the ambitious plans laid out in June. Although the multitude of problems can be frustrating, the rate of progress overall is very good compared with accelerators of the past. The collider beam teams are continually learning how to avoid failures and improve stability.
The next step is to use multi-bunch injection to push towards even higher intensities. They have already carried out tests with four bunches being fed from the SPS into the LHC ring in one go. Now they plan to use this technique on the next physics runs. The aim is to get intensities as high as possible by the end of July then run without further changes during August when some of the technical experts will be on leave.
The focus of excitement is now turning towards the ICHEP conference starting in one weeks time. Tommaso Dorigo has been mischievously pumping up the tension with rumours of an observation of a low mass Higgs at the Tevatron. Lobos Motl has prepared an excellent discussion of what that would mean for physics if it is true. Let’s hope his effort is not wasted! At ICHEP we might also expect further clarification of recent reports on neutrino physics, and perhaps some other new surprises.
Update: They followed the first 12 bunch run with another that is still ongoing after 12 hours with luminosity still above 1 inverse microbarn per second. Combining the three runs of the last three days provides about 100 inverse nanobarns of integrated luminosity. That doubles the total to date. After so many failed runs over the previous couple of weeks the LHC is suddenly looking on great form. The target for the end of 2011 is 100 inverse picobarns. If they can increase the luminosity by another factor of 10 to 20 that should be easily achievable.