The Large Hadron Collider is almost done running for 2015. Proton collisions ended in early November, and now the machine is busy colliding lead nuclei. As we head towards the end-of-year holidays, and the annual CERN shutdown, everyone wants to know — what have we learned from the LHC this year, our first year of data-taking at 13 TeV, the highest collision energies we have ever achieved, and the highest we might hope to have for years to come?
We will get our first answers to this question at a CERN seminar scheduled for Tuesday, December 15, where ATLAS and CMS will be presenting physics results from this year’s run. The current situation is reminiscent of December 2011, when the experiments had recorded their first significant datasets from LHC Run 1, and we saw what turned out to be the first hints of the evidence for the Higgs boson that was discovered in 2012. The experiments showed a few early results from Run 2 during the summer, and some of those have already resulted in journal papers, but this will be our first chance to look at the broad physics program of the experiments. We shouldn’t have expectations that are too great, as only a small amount of data has been recorded so far, much less than we had in 2012. But what science might we hope to hear about next week?
Here is one thing to keep in mind — the change in collision energy affects particle production rates, but not the properties of the particles that are produced. Any measurement of particle production rates is inherently interesting at a new collision energy, as will be a measurement that has never been done before. Thus any measurement of a production rate that is possible with this amount of data would be a good candidate for presentation. (The production rates of top quarks at 13 TeV have already been measured by both CMS and ATLAS; maybe there will be additional measurements along these lines.)
We probably won’t hear anything new about the Higgs boson. While the Higgs production rates are larger than in the previous run, the amount of data recorded is still relatively small compared to the 2010-12 period. This year, the LHC has delivered about 4 fb-1 of data, which could be compared to the 5 fb-1 that was delivered in 2011. At that time there wasn’t enough data to say anything definitive about the Higgs boson, so it is hard to imagine that there will be much in the way of Higgs results from the new data (not even the production rate at 13 TeV), and certainly nothing that would tell us anything more about its properties than we already know from the full Run 1 dataset of 30 fb-1. We’ll all probably have to wait until sometime next year before we will know more about the Higgs boson, and if anything about it will disagree with what we expect from the standard model of particle physics.
If there is anything to hope for next week, it is some evidence for new, heavy particles. Because the collision energy has been increased from 8 TeV to 13 TeV, the ability to create a heavy particle of a given mass has increased too. A little fooling around with the “Collider Reach” tool (which I had discussed here) suggests that even as little data as we have in hand now can give us improved chances of observing such particles now compared to the chances in the entire Run 1 dataset as long as the particle masses are above about 3 TeV. Of course there are many theories that predict the existence of such particles, the most famous of which is supersymmetry. But so far there has been scant evidence of any new phenomena in previous datasets. If we were to get even a hint of something at a very high mass, it would definitely focus our scientific efforts for 2016, where we might get about ten times as much data as we did this year.
Will we get that hint, like we did with the Higgs boson four years ago? Tune in on December 15 to find out!
