Equal and Opposite

By Neil Savage

For many scientists, July 4, 2012, was not just another day. It was the day the world stopped for a moment and listened as two sets of researchers—working on two different experiments that posed the same question—announced the discovery of what appeared to be the Higgs boson. The Higgs is a subatomic particle that researchers believe provides evidence of the Higgs field, which pervades the universe and gives other particles their mass. Without it, the quarks and electrons that make up matter would never collect into atoms or galaxies. It was the piece of evidence that scientists needed to prove the dominant theory of the creation of the universe, which, until last summer, was just a theory.

Among the Boston University scientists anticipating the announcement were Tulika Bose and Kevin Black, assistant professors in the Department of Physics, who have been working on the two major experiments at the Large Hadron Collider (LHC) in Switzerland. In addition to holding prominent roles in these high-profile experiments, Bose and Black also happen to be husband and wife.

The two met while conducting graduate work at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. Now, they are in a competition of sorts: Bose works on the compact muon solenoid (CMS), one of the two main detection systems at the LHC, while Black works on the other, known as A Toroidal LHC Apparatus (ATLAS).

What is the LHC designed to look for and how does it work?

TULIKA BOSE: We use the Standard Model, a theory describing our knowledge of particle physics that was constructed back in the 1950s and 1960s as a result of experimental as well as theoretical work. A big leader in this was Sheldon Glashow. So we had these particles called quarks and leptons and these forces—the weak force, the electric force, the magnetic force, and the strong force. Then we found that you could actually treat this electromagnetic force and the weak force as the manifestation of a single force, the electroweak force.

What we came up with was that there were force carriers, the W and Z bosons, which carry the electroweak force. And the carrier for the electromagnetic force is the photon. The predictions told us that these particles should be massless for the theory to be true. However, what we found—by work that Glashow did as well as when we experimentally discovered W and Z bosons—were the W and Z are quite massive in comparison to the photon.

“Since this was first proposed in the ’60s we have been searching for the Higgs boson, because it will tell us the Higgs mechanism is correct and allow us to determine why particles acquire mass.”

Peter Higgs and others tried to explain why you expect massless bosons, but end up with actually massive W and Z bosons. This came to be called the Higgs mechanism. To know the Higgs mechanism is correct, you have to find the particle associated with it, the Higgs particle. Since this was first proposed in the ’60s we have been searching for the Higgs boson, because it will tell us the Higgs mechanism is correct and allow us to determine why particles acquire mass.

KEVIN BLACK: You have two counter-circulating beams of high-energy protons. They collide and produce a large number of particles that spray outwards, and they’re your only handle on what happened in that collision. The tracking detector sits in a magnetic field and has many, many different sensors, and it tries to see where the particles go. By measuring where each of the particles interacted and in seeing how much it bent the magnetic field, you can get the measurement of its momentum. From the detector hits we try to reconstruct tracks and different types of particles that came out of that collision so that we can know something about what happened in detail. Basically, you’re trying to connect the dots and measure the properties of the collision from those hits.

There are a couple of different particles that don’t actually get absorbed. One that we’re interested in is called a muon, so there’s a special system just to detect the muons in the very outermost portion of the detector. That’s the portion that I work on, and I’ve worked on the reconstruction algorithms of those particles.

How do your two experiments differ?

BLACK: They’re very, very similar. What they differ in are the details of how the detector is optimized. CMS has somewhat better inner detector-tracking. They have a larger tracking volume, a higher magnetic field. The ATLAS detector is somewhat better at measuring high-energy jets of particles. And there are different particles that are produced, so one detector can be really good at one thing and the other one could be really good at another thing.

“The two experiments … came up with results that were about the same…. Nothing gives people more confidence than that. It was really amazing. None of us expected that they would be so close.”

BOSE: The main reason places like the LHC and the Fermilab Tevatron have multiple experiments is because they anticipate a much stronger result when you have two completely different experiments—both using different technology and independent data sets—arriving at the same conclusion.

When the announcement came for the actual discovery—this was the July 4, 2012—the two spokespeople went up right after one another and showed their results and the two results were almost exactly the same. The CMS could have found a Higgs boson with a mass and ATLAS could have found a Higgs boson with a completely different mass and then all bets would be off. But the two experiments went up without any prior discussion and came up with results that were about the same in terms of sensitivity and pointing to the same particle. Nothing gives people more confidence than that. It was really amazing. None of us expected that they would be so close.

What’s it like to be married to someone who’s working on a project so similar to your own?

BOSE: We tend to avoid discussions of work, especially results. It’s not just that we are on two experiments and we’re doing similar physics, we’re actually on competing experiments. And, in a way, both of us would like our experiments to be the first out there. So we might discuss other things related to the experiment, but not details. This sort of keeps things simple.

“People never believe us when we say we don’t talk about the details.”

BLACK: People never believe us when we say we don’t talk about the details, but it’s actually not hard. We just don’t force each other into situations where we have to ask those questions. I think it’s a good way to be able to interact. It’s often good to get other people’s perspectives when they are working on similar things, but not necessarily working so close to you that they know exactly what you’re doing. If you only talk to the same person on a project, you can sort of get boxed into a particular idea. So it’s great, if you have some idea about something and haven’t fleshed it out in detail, to talk to somebody you trust and say, “What do you think about this idea?”

Now that the LHC has found the Higgs boson, what’s left to be done?

BOSE: The Higgs really helps us to explain this question of how particles acquire mass, but it does not explain a number of other problems. Why is it that some particles have much higher masses than others? We don’t understand this. Then there is this big question of matter. At the Big Bang, when the universe was created, we had equal amounts of matter and antimatter. Now everything we see around us, ourselves included, is composed of matter particles. So where is all the antimatter? There is also the issue of gravity. We know gravity exists, but the Standard Model cannot include gravity in its current incarnation, or how gravity compares with the other forces. All of this means that the Standard Model is good, but it’s good only up to a certain approximation.

BLACK: It’s this bizarre situation in which we now have this very, very good agreement with data in the realm of particle physics. But in fact if you look at the energy and matter content of the universe, the Standard Model only describes about 5 percent of that. So 95 percent of that we have no idea about. There’s a big open question about what this stuff is. One thing we’re curious about is dark matter, which is some sort of particle that’s interacting gravitationally, and then there’s dark energy, which is some sort of force that is pushing the universe apart. We really have almost no knowledge about it, but we know it’s incredibly important to the large-scale universe.

What is BU’s role at the LHC?

BLACK: If you combine the ATLAS and CMS experiments we have six faculty members, four research scientists, four postdocs, and nine graduate students working at the LHC. And because these experiments are very large, it’s very helpful to get a critical mass of people working on them. Having a significant group of people who can work on a similar project really helps, and in particular, having people around you to bounce ideas off of or ask for help on a project is very useful. It does tend to attract students.

BOSE: BU is one of very few universities that’s on both ATLAS and CMS. So when students come to BU, they have this wide choice of which experiment to work on. Then within these groups, because we have major responsibilities in multiple projects, they can choose what interests them, and I think that’s a very attractive thing for them to have.

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