Steve Myers who is the director of the LHC beams has just delivered an optimistic update on the ongoing LHC commissioning process. This comes at a critical moment as they are about to move out of a prolonged commissioning phase with a restart of physics runs expected today.
Myers expressed some frustration over the time taken to do the setting up of the machine when parameters are changed. He said he felt like renaming the LHC as the large Loss-Map non-Collider because they have to keep doing runs to map where the protons are lost round the beam pipe.
However, that process is now finished ready for the next series of runs and there was good news about beam performance. The bunches can now be separated by just 150 ns which will allow up to 384 bunches in each beam. the crossing angle is set at 170 micro-radians and they found that they only had problems when this reduced to about 80 micro radians. This means they may have room to use a smaller bunch spacing later without changing the crossing angle.
Control of beta-beating in the beams is also better than expected. They use 100 quadrapole magnets to keep the bunches in good shape and this has been very effective. The beam aperture measured at 450 GeV is especially good and this may mean that they can use more squeeze to increase luminosity without adding more bunches.
In a review of the protection systems it was concluded that they are working very well. Myers suggested that this allows him to increase the number of bunches in the ring more quickly over the next few weeks. In a previous plan the number was to increase by 48 bunches each week. Now he is suggesting to do this twice a week. This will make up for time lost over the last few weeks. It might also allow for two weeks of running with 384 bunches, during which time they may even attempt to squeeze the beams further. Currently they use a beta* of 3.5m but if they can reduce it to 2m they can more easily reach the target of 1/fb desired for next year’s runs.
However, they must still be cautious and this aggressive schedule can only be realised if each run ticks all the boxes indicating that everything is under control. The number of bunches injected in each bunch train will start at 8, then be increased to 12 and then 24. Twelve bunches is already the safe intensity limit above which the collider can be damaged if they lose control. When they inject twice this limit at one go the risk increases.
During questions Myers also said that next year they may attempt to reduce the bunch spacing to 75ns or even 50ns. This would allow 2 or 3 times more bunches and the same factors for increase of luminosity. If Myers’ optimism proves to be justified they could even exceed the 1/fb target for 2011.
Following Myer’s update this moring there were presentations from CMS and ATLAS which present results using up to 3/pb of data for the first time. At recent conferences they have only shown up to about 1/pb. Some of the plots are really exciting and you can get the full set of slides at http://indico.cern.ch/conferenceDisplay.py?confId=105780 Here is just a selection from ATLAS
They see up to 10 jets and agreement with the monte carlo is considered surprisingly good.
I’ve added a zoom to this slide so we can peer into the noise🙂
During questions after his ATLAS talk, Thomas Le Compte was asked why he had not shown di-photon plots. He said he could not comment but that these results were of interest to lots of people …😀