The Large Hadron Collider has settled into a steady period of running and is now delivering about one inverse femtobarn each week (actually it was about 0.98/fb in the last seven days including a single fill of over 200/pb yesterday.) There are 17 days until the cut-off for ICHEP (contribution list now filling out) , the biggest international particle physics conference of the HEP calendar that is being held in Melbourne from 4th July. If there are no snags they should still be able to reach the target of 5/fb delivered and the experiments will show new Higgs results with at least 4/fb at 8 TeV. That is about as potent as last years 5/fb at 7 TeV. If they get similar bumps at the same places as last year that should settle the existence of the Higgs at least unofficially, but unlucky statistical fluctuations could still leave the result hanging. Official discovery will probably have to wait a little longer. I think they will hold back from combining the new data with last years until both experiments can claim discovery. ATLAS and CMS have shown very similar sensitivity and resolution especially in the crucial diphoton channel despite completely different detector technologies. It would not be fair if one of them got the discovery first due to lucky statistics. They can conveniently time their combinations to avoid that hapening.
According to Paul Collier (commenting at LHCportal) they still have some scope to slowly increase the bunch intensity up to 160 billion protons per bunch for another 20% increase in luminosity. That is the limit for this year and realistically I think that 1/fb per week will be about the average amount delivered over the remaining 17 week of proton physics minus any time for recovery from technical stops, extra MD or special runs for TOTEM. They should comfortably reach their target of 15/fb total delivered to each of ATLAS and CMS.
Full Higgs combinations of ATLAS+CMS have not been seen since autumn last year when they had just 2/fb. Although they have not given any explanation for this there are several factors that come into play. The sheer complexity and quantity of all the data means that the big combinations require enormous amounts of computer resources (when done exactly). By time they could complete the calculation the experiments have usually added something new making the answer obsolete before it is ready. I imagine they can’t afford to waste their computer resources or manpower in that way with so many other things to be done. Another problem is that the two experiments have seen maximum peaks at slightly different Higgs masses. The favorite theory for this is that the energy calibration has worked out slightly differently through systematic errors in the two experiments. The consequence is that the combination only gives a small improvement in significance over the individual results. But I think the most important reason for not doing the full combination now is that they can reach discovery level this year with both experiments separately. It will make a more spectacular and convincing presentation of the discovery if they can do it that way rather than with a cross-experiment combination. I could be wrong but I think there is a good chance that they will do full combinations of ATLAS+CMS only during the long shutdown as part of a more detailed analysis to compare observations with the standard model predictions. Until then people will have to be content with unofficial approximate versions.
While you are waiting for ICHEP do not miss the meeting at Blois. The first session has some particularly interesting talks not least of which is a presentation by Nima Arkani-Hamed on why he thinks a 125 GeV Higgs smells like SUSY. It’s a 30 minute talk but there is already a YouTube version online where he crams it into 1 hour 42 minutes and still skips slides at the end. You are recommended to set aside the time to watch it.
Update 25-May-2012: They have now passed the milestone of delivering 1/fb in one week to ATLAS and CMS. Here is a table of the fills that did it with the latest still running.
|fill||start date time||dur||ATLAS /nb||CMS /nb|