last week we celebrated 1 inverse femtobarn (1/fb) of integrated luminosity delivered to ATLAS and CMS. Of that data ATLAS has recorded about 95% and CMS about 92% so with a little more added ATLAS have now recorded over 1/fb.
The milestones have been celebrated with a CERN press release
Last week the LHC Control group held an Open meeting to report on progress of the beams and experiments. Slides and videos are available for some of the talks including the Machine Status Report by Steve Myers who revealed that during the last Machine Development period the bunch intensity was tested up to 195 billion protons, going well beyond the 170 billion ultimate intensity limit. The intensity currently in use is about 120 billion, but there is hope that this may be increased later in the year.
Although the LHC has delivered 1/fb in record time as a result of its better than expected early performance, there is some frustration that technical problems are holding it back from achieving even better results. A string of difficulties has been making it hard to get the beams circulating while other glitches cause the beams to be dumped early. The time in stable beams has been about 36% since they started running with 1092 bunches and it should be possible to do better than that.
Unidentified Falling Objects
In his talk Myres gave some more information about UFOs. These are mysterious rapid beam loss events thought to be caused by particles falling into the beam path. They can trigger the protection mechanisms to dump the beams. Studies have shown that they most often occur at the injection points and almost always shortly after injection causing problems before they get to stable beams. Surprisingly their frequency is not increasing with further intensity advances. They were 110 of these UFO events last year and already 5000 this year, but only the strongest cases can trigger a beam dump.
An extensive report on UFOs can be found here
RF Power Couplers
Another series of problems concerns the RF components. The couplers can take 200 kW of power and currently are being loaded up to 190 kW. This figure increases with beam intensity. If one of the ceramic couplers breaks it would put the LHC out of action for five to six weeks. These and other concerns have been preventing them from raising the bunch number to the next step of 1236 bunches. There is also a special report on the RF power issues and how they have been addressed. With the situation coming back under control it is hoped that the next luminosity step can still be taken this weekend.
In order to decide how to proceed for the rest of the year there will be a “mini-Chamonix” meeting on the 15th July. There We may hear more about addressing these and other problems as well as prospects for any further luminosity increases e.g. by raising bunch intensity.
Status Reports of the Experiments
At the LPCC meeting there were also reports from the individual LHC experiments. CMS has produced 80 papers using LHC data while ATLAS has about 190 and there are also good initial results from LHCb and ALICE. With the luminosity increasing at faster than expected rates there has been more pileup of events in the detector than anticipated. ATLAS reports an average of 6 events for each bunch crossing. There is significant impact on the calorimeter reconstruction resulting in increased systematic uncertainties in the analysis. Low transverse momentum jet events are the worst affected, but it is a small price to pay for so much extra data. Pileup will get worse if the bunch intensity is raised further.
Summary of Physics Results
Most of the physics results published so far have used just the 40/pb of data collected in 2010 with just a handful using up to 240/pb . A selective summary of results from ATLAS is shown on this slide (click to see full-sized). Within a few weeks we will have many more results including some using the 1/fb now collected. the EPS-HEP conference at the end of July is the next major opportunity for physics presentations.