Argonne National Laboratory

Tom LeCompte

By Louise LernerFebruary 5, 2012

Tom LeCompte served as the physics coordinator for the ATLAS (A Toroidal LHC ApparatuS) experiment, a 3,000-person collaboration at the Large Hadron Collider in Europe. This experiment studies the collisions of protons at the highest energies yet achieved.

What do you study?
I do particle physics. What that means is that I study the properties of matter, energy, space and time at very small scales. Much, much smaller than the atoms that we're made out of. To give you kind of an idea of the sorts of scales we're talking about, if an atom were the size of the Earth, the particles we're looking right now are at the scale of a pea. And I'm working at the Large Hadron Collider, which collides protons together with the highest energies we've ever managed, and ultimately the sensitivity will move from the size of a pea to the size of a single sprinkle on a Pop-Tart.

So that's really what we're doing: looking for new particles. Often these particles are predicted to explain something about the world we live in. For example, one you may have heard of is the Higgs boson.

How does the Higgs fit in the model?
What the Higgs does, in the model, is keep our sun burning for billions of years, rather than immediately exploding in a flash of energy, and it explains why there's only one kind of light. Without the Higgs, our theories would predict there'd be two kinds of light, two kinds of electricity, and two different kinds of magnetism, but obviously in the world we live in, there's only one kind of each.

The Higgs mechanism explains this by saying it works on only a very short range. We don't see it because this force can't even propagate outside of an atom, so it's as if it doesn't exist.

So the search for the Higgs is really one particle that's predicted to explain a phenomenon that's so common that we don't even think about it. We don't say, "Gee, it's unusual that the stars last for billions of years. It's unusual that there's only one kind of light and not two." But in fact it is kind of unusual when you start looking at the theory behind it. So then you need to ask yourself, Do I have an explanation, and if so, does it make a prediction that I can test?

What do you do at the Large Hadron Collider?
We're colliding protons together. One of the advantages is that it gets you down to these tiny scales that I was talking about, and also it produces much more energy for collisions to make these new particles.

And the instruments—I work on the ATLAS detector, part of which was built at Argonne—are far more sensitive than anything we've ever done before. We had 10 years to design them, that's one of the reasons the experiments look so good; the collaborations have had so much time to build the experiments to really do what they want; the other reason is that if you're going to put the sort of resources you have at LHC, that no single country could do on its own, it takes a global collaboration, you really want to make sure you get the best value out of that investment. You really don't want to miss anything.

That's part of the reason there are four different experiments at the LHC; besides overlapping strengths, if something is seen by one, it'll also be seen by another, possibly in a different way.

ATLAS is a general-purpose experiment, designed to do many things. We have 3,000 collaborators, all with their own scientific interests; we're doing hundreds of experiments simultaneously. So in some ways, we talk about the ATLAS experiment, but really it's a confederation of many different people with different technical skills and physics interests that come together and allow us to do many things at the same time.

I was the physics coordinator at the LHC. It was my job to ensure that we got maximum scientific output among this rather diverse and sometimes chaotic group. I mean, you don't want to have everyone marching in lockstep; while that's efficient, it's not very good scientifically—you want people feel free to say "I don't understand that, we need to think about this instead." You always want to be able to have that argument in science. It's kind of like being the conductor of an orchestra. You have to provide overall direction, but you've got really excellent musicians there: let them do what they do well, so that the whole thing works. It's not the guy flapping his arms around that's making the music.

This is the first time we've achieved such high-energy collisions; are there any challenges with working at the frontier?
Well, here's what I tell people science really is: you make a thousand plots. For 9,999 of them, you know the answer; for one, you don't. And when you get the right answer on those 9,999 plots, then when you look at the thousandth plot where you didn't know the answer, then you have some confidence that you're right.

But to make sure that you're not making some small but horrible mistake somewhere, you really need to check a lot of data. They may each have a low probability of being wrong, but if I've got 1,000 things that each have a 1 percent chance of being wrong, I have 10 things wrong right there.

You really have to be meticulous and slow. Especially what we're doing at the LHC, which is discovery physics. When you're doing something brand new, you need to make sure: did you really see a star over there or do you have a smudge on your lens? And even though you know for sure, you have to go and check.

That's why it takes some time before we get results to everyone—we are making sure it's all valid.

How long have you been on this project?
At some level for 10 years; maybe the last four have been full-time.

What are you hoping to find at the LHC?
I think what we're really hoping for is a surprise. I think what we're all really hoping for is a result that comes through and we all say, "Well, that's not at all what we expected."

First of all, you never learn anything from getting the right answer—you always learn by getting the wrong answer.

We know that the current model we have of the world is wrong. It makes nonsensical predictions in certain regions. For example, it predicts probabilities of something happening as greater than one. Which you might think doesn't sound so bad, but the probability of that not happening is also less than zero, and that's not possible.

So we know our theory has its problems; we know then, that at some very short scale it has to be replaced with some new theory that does better. We have our ideas of what that might look like; I'd certainly like to show that one of them is right; but I'd even more like to see that they're all wrong and that we find something even more interesting.

There's some precedent for that. Every time we've been able to go one more layer down or one more layer up, we've seen more things and discovered that the world is a more complicated and more beautiful place than we thought. Really we're kind of hoping for something that's a complete surprise.

What keeps you at it?
Well, I have the best job in the world, right? I get to come in and think about how the universe is put together. Yes, it takes me a long time to make the measurements that need to happen to show it, but what's nice about it is you're looking under a lot of rocks. You never know which is the one until you pick it up and see under it. But for that one brief moment, you're the only person in the world who really understands that.

Then of course you get to tell your friends.

Do you worry that we won't find the new big particle physics theory in your lifetime?
I'm certain that the LHC, even if it doesn't give up the right answer, it'll exclude a lot of wrong answers and provide strong hints. Whether we can see everything or not is another question.

I'm certain we'll get much, much closer in the next twenty years. There's no way out for Nature. We've got her closed in on all sides.

But beyond that, is the LHC going to be able to explain everything that's going on? Maybe we'll need to look more at astrophysics after that? We don't know yet.

What do you do in your spare time?
I'm a trumpet player. I play with a couple of rehearsal bands, and now again I get called to sub. I played professionally through grad school. I do mostly big band jazz. I like playing the fourth trumpet part. It's a guilty pleasure of mine - you never know exactly what you're going to get; the intervals are funky, maybe you're playing with the woodwinds or maybe with the brass.

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