by Andrew Grant,
From the January-February special issue of Discover Magazine; published online January 9, 2012
Particle physicists entered 2011 with high hopes. The Large Hadron Collider (LHC), a 17-mile tunnel straddling the border of France and Switzerland, was smashing protons together at unprecedented energies. By June the LHC had exceeded its full-year target of 70 million million collisions. But by the end of the year, many physicists were starting to sweat. The LHC delivered a flood of data, but none of it seemed to yield the discoveries everyone was hoping for. There were no signs of dark matter, no hints of extra dimensions, and not a whisper from the Higgs boson, the long-sought particle that is an essential component of the leading model of quantum physics.
One anxious physicist is Joseph Lykken, a theorist at the Fermi National Accelerator Laboratory in Illinois. He does not run the experiments—“they won’t allow me to touch anything, because I’d probably break it,” he says. His job is to predict what the LHC will find and then try to explain the results, whether they confirm his models or force him to throw up his hands and start over. Lykken talked to discover about the ups and downs of the past year and what to expect—as far as he can anticipate—in 2012.
Many people were expecting big news from the LHC in 2011. Why didn’t that happen?
There was one moment when we could have had a major discovery. It was early in the summer, when we first started to get proton collisions at a really high rate and could have seen something immediately. We didn’t, so there was some disappointment that there were no immediate signs of the Higgs boson or the other things we’re looking for, like dark matter. On the other hand, if you had made people bet their own money on whether we were going to see those kinds of things right away, I think most people would have bet that it would take longer. By the start of 2012 we should have five times as much data as we had at the beginning of the summer, so we may look again and see either a signal of a Higgs boson or maybe something more exotic that we hadn’t expected. People are excited, because they can smell that discoveries are probably close.
So how close are we to finding the Higgs boson?
We’re going to deliver on the Higgs boson, one way or the other. The lhc was advertised as a machine that would tell us definitely whether there is a Higgs boson and if so, what kind of particle it is. I think that’s going to turn out to be true. From this summer’s data, we’ve been able to eliminate many possibilities for the Higgs boson’s mass. By next summer I think we’re either going to see it or we’re going to have to revise our predictions for how it behaves. My gut feeling is that within the next year, we will discover a Higgs boson with a mass on the lower side of our expectations.
What if you’re wrong and the Higgs doesn’t show up?
That’s a possibility we talk about in hushed tones. In that case it’s going to be a real interesting crisis for theoretical physicists like me, because we’ve been saying that there has to be something like this in order to explain how the other particles behave and why they have mass. If that story is wrong, then we’re really back at the drawing board.
Does that possibility excite you or frustrate you?
I think I’d feel a little bit insecure. But what I think will be exciting is if we find something else that isn’t the Higgs boson that may lead us in a completely new direction. It would be great to find an extra dimension of space that could answer all the questions that the Higgs was supposed to answer. That’s the sort of drastic turn that would excite everybody, and it may happen in the near future.
Before it shut down in September, the Tevatron accelerator at Fermilab returned data suggesting a new particle or force. Could that be the drastic turn you’re looking for?
Back in April some data from the Tevatron included a strange bump that shouldn’t have been there if our models are correct. It was a strong enough signal to be very intriguing, but we can’t claim that we’ve found something new. The full data set hasn’t been analyzed. By next summer we should get the final word from the Tevatron data, and then the lhc should be able to tell us pretty definitively whether this bump is real. If we confirm it, it’s a big deal. All the explanations I’ve heard for that bump involve new forces of nature and other exotic effects.
What else is on the agenda for the LHC in 2012?
Next year is also exciting because of the machine itself. Right now the lhc is running at only half the energy it was designed for. By the end of 2012, we’re going to shut the machine down and try to upgrade the energy to maximum capacity. That means a much higher rate of collisions and a lot more data. It’s exciting, but it’s also challenging: The data get messier and messier when there are so many collisions. It becomes more difficult to find the needle in the haystack, because now it’s a bigger haystack and it’s got more junk in it.
What question do you most want the LHC to answer?
The most compelling problem for me is dark matter. As a particle physicist, my job is to figure out what things are made out of. Well, most of the universe (about 85 percent of all matter) is made out of dark matter, and I have no idea what that is. We have experiments in underground laboratories trying to directly detect dark matter particles coming in from space, and so far they have all come up empty. I don’t even know for sure that dark matter particles are particles in the ordinary sense of the word. So dark matter is the big mystery that you need the lhc to solve. If we can slam together particles of ordinary matter and create dark matter, then we can understand dark matter’s connection to ordinary matter and figure out why most of the universe is made of it.
Do you expect to find dark matter anytime soon?
It’s a long-term goal, but the lhc is a 20-year program. Of course you would like to see everything the first summer, but it’s not realistic to think it’s going to work like that. We ran the Tevatron for 25 years and we were still seeing interesting anomalies in the final year. So if you want to do lhc physics, you have to be in there for the long haul. We may have exciting discoveries next year and maybe not. We don’t know. This is discovery science: If we knew what we were looking for, it wouldn’t be that interesting.