After a two week break from physics runs, the Large Hadron Collider is once again clocking up proton-proton collisions at a steady rate. The break included a few days of machine development time, a planned technical stop for routine maintenance, Van der Mere scans to study the beam size and distribution and finally alignments for the TOTEM and ALFA experiments.
Now they are back to the process of building up the luminosity. The current fill has 768 bunches matching previous runs except that they are now injecting 108 bunches at a time compared to the previous scheme using 72 bunch injections. This is important because the next step up to 912 bunches will require the 108 injections and that is now expected for the next run later today.
The 912 bunch step is likely to take the luminosity through another milestone by peaking at more than 1000/μb/s. This is the official target for 2011 luminosity but it will be exceeded further as they step up through 1056, 1200 and then 1380 bunches in the next couple of weeks. These last three steps will require 144 bunch injections taking the injection systems to new limits. In the last few days they already injected 1308 bunches during a scrubbing run, but without ramping to full energy for collisions.
(Update 23-May-2011: A record luminosity of 1100/μb/s was reached on the third fill with 912 bunches per ring this morning.)
With the current plans it should be possible to reach luminosity of at least 1500/μb/s for the long run from June to October during which there will be 124 days allocated to proton-proton physics runs. So how much further will it be possible to increase the luminosity and when? As part of the machine development time they tried out some processes that could potentially increase luminosity further. Possibilities for higher luminosity without updating the hardware are the following
- Increase of bunch intensity
- Improved emittance
- Better Squeeze
- More bunches
Increase of bunch intensity: The proton bunches currently circulating start out with 115 billion protons in each. This is the “nominal” bunch intensity that was originally planned for the collider with its present hardware. However, it is possible to inject an “ultimate” intensity of 170 billion protons into each bunch from the SPS. The luminosity increases in proportion to the bunch intensity in both beams so the potential luminosity increase is a factor of (170/115)2 = 2.2. It had been anticipated that the beams would become too unstable at these levels but already last year they found that reaching nominal intensity was easier than expected. During the latest machine development time they injected a few ultimate intensity bunches to see how they perform. Once again they found that there was no problem with the head-on beam-beam tune shifts that they had expected to be a limiting factor, so the way is open to a further increase of luminosity.
The beam operators are being publicly cautious about when this could happen. Paul Collier who heads the team said in a message on the LHCPortal forum that “The studies with very intense bunches are for the future – no plans to push higher bunch intensities into operation for the moment.”
Improved emittance: The emittance is related the size of the beam Transversely emittance should be 3.75μm but with better than expected performance it has been typically 2μm to 3μm. During the machine development tests they were able to reduce this to 1.5μm. If they can do this for injections of full trains it represents a further increase of luminosity.
Despite the cautious words there are signs that increased intensity and improved emittance could be introduced into the physics runs this year taking luminosities up to “a few” times the 1000/μb target figure. Whether this happens and to what level will depend on how smooth operations go during the first few weeks of the long run with 1380 bunches.
Better Squeeze: The squeeze is a process of focusing the beams into a smaller cross section as they pass the collision points inside the detectors. The current squeeze is represented by a figure of β* = 1.5m but a tighter squeeze may be possible. The nominal squeeze is 0.55m but this will not be possible until the full 7TeV beam energy is available after the long shutdown. After that further improvements are still possible. As Paul Collier explained, the minimum β* is limited by the aperture of the triplets and also by the chromatic aberrations introduced by the very tight squeeze. Another test during the machine development period looked at a new method of ATS injection which is a novel and compicated scheme to bypass the strength limit of the existing lattice sextupoles and allow a much smaller β*. Values of around 0.15m should be possible with existing hardware.
These developments are definitely not for this year. Moving to a tighter squeeze by any means will require a long process of resetting collimators and building up luminosity again in steps. Too much time would be lost to make this worthwhile for current runs but in 2012 a smaller β* might be used.
More bunches: The last route to increased luminosity is more bunches. To exceed 1380 bunches per beam they must use a lower bunch separation time of 25ns compared to the 50ns spacing currently in use. Since they travel at virtually the speed of light this smaller spacing places the bunches just 7.5m apart. Such a change might require a larger crossing angle to avoid parasitic collisions as bunches come close 7.5m away from the desired interaction points. This would again mean resetting the collimators. Furthermore, it is not known if the vacuum and cryogenics can yet cope with the higher intensities that twice as many bunches would imply, especially if ultimate luminosity bunches are being used as well. So realistically this is not likely to happen this year either, however, we can expect some tests of 25ns spacing this year during further scheduled machine development times and a switch to 25ns for 2012 is a possibility if the tests go well.
Altogether this could take luminosities up to a remarkable 10000/μb/s next year, if everything works out. 5000/μb/s is probably a more realistic target. Further down the line there will be upgrades allowing for yet higher intensities and a better squeeze. Peak luminosities of 50000/μb/s are anticipated after a few years. At some point the pile-up from collisions coming too frequently to be separated will be a problem and the luminosities will have to be limited. Pile-up is already expected to be a challenge for ATLAS and CMS if futher luminosity increases are introduced this year. Even then it is worth pushing the peak luminosity higher because it will make it possible to sustain the maximum desired luminosity during much of the length of a long run. This is already being done for LHCb and ALICE.
With so much data the potential for new discoveries and precision measurements over the coming years is huge. In the shorter timespan of the next two or three months we can already anticipate some great results. There are two conferences this summer that are especially worth following. The Physics at LHC 2011 conference starting on June 6th will be an opportunity to show results using at least 200/pb of data collected up to now. That is significantly more than the 40/pb already analysed in detail. On July 21st another meeting for the International Europhysics Conference on High Energy Physics could already be presenting first results from as much as 2000/pb worth of collisions. However, if there are any really important discoveries forthcoming there will be a longer process of analysis, checking and consultation before going public. Rumors can of course be based on what they don’t show straight away.