The Large Hadron Collider today returned to physics runs. Over the last few days they had been filling the collider for scrubbing runs in an effort to remove the e-cloud problem. In fact they only did a few hours of scrubbing runs before surprisingly returning to physics with 50ns bunch spacing.
On the face of it this is great news because it suggests that they have concluded that the problems are not too severe and physics with 50ns spacing will be the norm for this year. However I have heard no official word so I can’t confirm that they wont change back to 75ns spacing.
With the 50ns spacing they will be able to get 1400 bunches into the collider. This is half the design limit of the machine which may only be achievable when they move to the higher energy of 14 TeV in 2014. During the scrubbing runs they were already able to circulate 1020 bunches, so perhaps it will not be long before they reach similar numbers with colliding beams. It is very impressive that they have reached this point so quickly.
For today they have been running with 228 bunches providing a luminosity of 250/ub/s just below the records of a few weeks ago. They need three runs and 20 hours of stable beams before they can go to the next step at about 300 bunches. Just one more runs of about 5 hours should do it. Today’s runs have already added about 10/pb to the total integrated luminosity delivered to each of ATLAS and CMS.
Update 16-Apr-2011: The next step is set as 336 bunches and still with 50ns spacing. Expected peak luminosity is about 360/ub/s which will be a new record, fingers crossed. Total luminosity delivered for 2011 has passed the 40/pb delivered in 2010.
Update 17-Apr-2011: After a few false starts a good long run with 336 bunches is now in progress. It started with a peak luminosity of 380/ub/s in ATLAS and 360/ub/s for CMS. After 9 hours the integrated luminosity has passed 10/pb, the highest yet for a single run. This may be a good moment to speculate about how much data they can collect before the “Physics at LHC” conference to be held in Italy from 6-11 June. The answer depends on many things but I don’t think 0.5/fb is overly optimistic at this stage assuming they proceed with continuous physics runs and increasing luminosity according to the plan which has six weeks of running before the conference. To give some meaning to that amount of data here is the expected Higgs limits for ATLAS.
0.5/fb would exclude the Higgs between 140GeV and 180GeV, unless it is there. Of course even if they do collect 0.5/fb before the conference they would have to do the analysis very quickly to be able to report it in time and I don’t expect that to be possible. Other results are more likely to be shown and the Higgs limit should just be taken as an indication of how much progress has been made when these landmarks are reached.