As Steve mentioned last week, the new LHC schedule for this year and next was revealed last Monday. The most prominent features of the announcement were the anticipated start date (autumn) and duration (one year) of the run. However, there were two other important parameters that were also discussed at the Chamonix meeting last week. We learned the luminosity and beam energy that the LHC team would try to achieve during the run. Both of these choices have important implications for the physics we can do with the data from the run.
Luminosity is closely related to the rate of events we observe in a detector. The rate for a given process is partly determined by the luminosity and partly by the cross-section for the process: the larger the cross-section, the more common it is. Think of how much easier it is to hit a softball than a regular baseball — for the batter, the softball has a larger cross-section than the smaller baseball. Physicists can measure the cross-section of a given process, but we can’t change it. It’s fixed by nature and depends on the details of the interaction we wish to study. Only the luminosity, or number of collisions, is under our control. Since our analyses depend on gathering as large a sample as possible, we try to design an accelerator which delivers as high a luminosity as we can. It’s like giving the batter more pitches to get a hit.
We can raise the luminosity by improving four main factors:
- First, we can increase the number of particles in the accelerator. The LHC doesn’t contain a steady stream of protons. Instead, they are grouped into bunches of about 1010 to 1011 protons. The more particles in the bunch, the more chances there are for a collision to occur when the bunches pass through each other at the interaction point inside our detector.
- We can also increase the frequency with which the collisions occur. At the beginning, the LHC will have only a few bunches in each beam, and so the number of times the beams cross will be smaller. Eventually, there will be thousands of bunches inside the machine, and they will cross 40 million times per second.
- We can improve the quality of the bunches. There is an ideal path through the accelerator, and a high-quality bunch will follow this path as closely as possible. In order to obtain the best quality, we have to handle the bunches carefully as they make their way through the CERN accelerator complex into the LHC.
- Finally, we can focus the beam more tightly, especially as it approaches the interaction point where the particles collide. The smaller the size of the beam at the collision point, the higher the number of collisions.
Eventually, as the LHC runs more and more smoothly, the average number of collisions per bunch crossing will go up from less than one to about twenty. Though it can be more challenging to correctly reconstruct an event with many collisions, the additional chances to see very rare interactions are worth it.
The beam energy should also be as high as possible. Because of the conservation of energy, we can never produce particles with more energy than that of the incoming beams. However, since the LHC collides protons, the typical energy for the interactions is much less than the beam energy. This is because the proton is really a collection of more fundamental particles (quarks and gluons), each carrying only a fraction of the overall energy. By maximizing the overall energy of the proton beams, the typical energy of the quarks and gluons is higher, and we see proportionately more interesting collisions.
In an ideal world, we could set the energy and luminosity of the beams to whatever values were best for the particular physics study we wanted to do. In practice, the LHC accelerator team will try to optimize these factors against more pragmatic concerns about the schedule for the machine (e.g. the electricity to run the machine is more expensive in the winter, and time must be left for necessary safety checks and upgrades to the machine components) and the capabilities of the hardware (it’s safer to run at a lower energy at first, because the LHC magnets will need to be trained for a higher field before the design energy can be achieved.) It looks like the current schedule, which has the LHC machine operating at 10 TeV starting in the fall, is a good compromise between these factors and the experiments’ desire for as much good data as possible, as soon as possible.