I’m really not one for New Year’s resolutions, but one that I ought to make is to do more writing for the US LHC blog. Fortunately, this is the right year to be making that resolution, as we will have quite a lot to say in 2015 — the year that the Large Hadron Collider returns! After two years of maintenance and improvements, everyone is raring to go for the restart of the machine this coming March. There is still a lot to do to get ready. But once we get going, we will be entering a dramatic period for particle physics — one that could make the discovery of the Higgs seem humdrum.
The most important physics consideration for the new run is the increase of the proton collision energy from 8 TeV to 13 TeV. Remember that the original design energy of the LHC is 14 TeV — 8 TeV was just an opening step. As we near the 14 TeV point, we will be able to do the physics that the LHC was meant to do all along. And it is important to remember that we have no feasible plan to build an accelerator that can reach a higher energy on any near time horizon. While we will continue to learn more as we record more and more data, through pursuits like precision measurements of the properties of the Higgs boson, it is increases in energy that open the door to the discovery of heavy particles, and there is no major energy increase coming any time soon. If there is any major breakthrough to be made in the next decade, it will probably come within the first few years of it, as we get our first look at 13 TeV proton collisions.
How much is our reach for new physics extended with the increase in energy? One interesting way to look at it is through a tool called Collider Reach that was devised by theorists Gavin Salam and Andreas Weiler. (My apologies to them if I make any errors in my description of their work.) This tool makes a rough estimate of the mass scale of new physics that we could have access to at a new LHC energy given previous studies at an old LHC energy, based on our understanding of how the momentum distributions of the quarks and gluons inside the proton evolve to the new beam energy. There are many assumptions made for this estimate — in particular, that the old data analysis will work just as well under new conditions. This might not be the case, as the LHC will be running not just at a higher energy, but also a higher collision rate (luminosity), which will make the collisions more complicated and harder to interpret. But the tool at least gives us an estimate of the improved reach for new physics.
During the 2008 LHC run at 8 TeV, each experiment collected about 20 fb-1 of proton collision data. In the upcoming “Run 2” of the LHC at 13 TeV, which starts this year and is expected to run through the middle of 2018, we expect to record about 100 fb-1 of data, a factor of five increase. (This is still a fairly rough estimate of the future total dataset size.) Imagine that in 2008, you were looking for a particular model of physics that predicted a new particle, and you found that if that particle actually existed, it would have to have a mass of at least 3 TeV — a mass 24 times that of the Higgs boson. How far in mass reach could your same analysis go with the Run 2 data? The Collider Reach tool tells you:
Using the horizontal axis to find the 3 TeV point, we then look at the height of the green curve to tell us what to expect in Run 2. That’s a bit more than 5 TeV — a 70% increase in the mass scale that your data analysis would have sensitivity to.
But you are impatient — how well could we do in 2015, the first year of the run? We hope to get about 10 fb-1 this year. Here’s the revised plot:
The reach of the analysis is about 4 TeV. That is, with only 10% of the data, you get 50% of the increase in sensitivity that you would hope to achieve in the entire run. So this first year counts! One year from now, we will know a lot about what physics we have an opportunity to look at in the next few years — and if nature is kind to us, it will be something new and unexpected.
So what might this new physics be? What are the challenges that we face in getting there? How are physicists preparing to meet them? You’ll be hearing a lot more about this in the year to come — and if I can keep to my New Year’s resolution, some of it you’ll hear from me.