So now we know that the LHC will be colliding beams at an energy of 7 TeV instead of 14 TeV, at least for a few months. Does this change anything from the point of view of the experiments? Yes!
We have been preparing for collisions at 14 TeV for over a decade, and in fact the final plans of 2,610 ATLAS physicists for what to do with the data just arrived here at my office (some of my work here at CERN has been a contribution to these studies). The 1,828 page, three volume set is a (very) comprehensive final evaluation of what we think we can do with the ATLAS detector and a plan for what physics topics we will study. This was all done with very sophisticated software simulations that took thousands of person-years to code and test. After all those years of preparation for collisions at 14 TeV, there are some changes that we have to make for working with real collision data at 7 TeV.
First, the laws of physics predict different rates of particle production at different energies. For example, our best tool for calibration is probably the particle called the Z boson. It is relatively easy to pick out from the rest of the data and it has been very well studied. So we can find it and measure its properties, then compare what we measure to the already known numbers, see if our detector is working as expected, and make any necessary changes to calibrate it. At 14 TeV, in 50/pb of data (perhaps a few months of data taking), we expected to have about 25,000 Z bosons that would decay into 2 electrons. In the same amount of data at 7 TeV, we expect about half the number of Z bosons. So it will take twice as long to accumulate enough data to achieve the same level of precision in our calibration.
There are also some differences in the behavior of particles produced at different collision energies. In higher energy collisions, particles tend to be produced with more kinetic energy (which means they start out moving faster) and this causes them to go flying off into different parts of our detector than particles produced in lower energy collisions. Also, having less energy means fewer particles overall are produced per collision. All of this is important because the simulations that I mentioned above, done at 14 TeV, led us to develop software reconstruction algorithms that would work at 14 TeV. So now we will have to produce new simulations corresponding to 7 TeV and re-do many studies.
Finally, the experimental signatures of new particles we may create, and of background processes that mimic these signatures that we have to know about to account for, will be different at 7 TeV and 14 TeV. The potential for discovery depends on the energy of collisions. At 7 TeV, we do have a chance at producing new particles that have never been seen by previous experiments, although not as good a chance as at 14 TeV. Again, we will have to re-do some studies to know quantitatively what our chances are, but preliminary studies show that we have a chance to find new particles even with just a few months of data at 7 TeV.
