American Scientist

Sally C. Seidel is the faculty member of the University of New Mexico’s Collider Physics Group and a collaborator on the ATLAS experiment at the Large Hadron Collider housed at CERN, the European Organization for Nuclear Research, in Switzerland. The ATLAS experiment focuses on activity involving massive particles and was part of the effort to discover of the Higgs boson in 2012. Seidel’s research focuses on improving the understanding of heavy quark bound states, which in turn can help to elucidate the strong force, one of the four fundamental forces of nature. Her group also works on developing particle-tracking detectors. Seidel, one of Sigma Xi’s Distinguished Lecturers, spoke with editor-in-chief Fenella Saunders about her research.

How many particles do physicists think are out there?

If we are talking about the most fundamental particles, there are only about a dozen, the quarks and the leptons and so forth. Often particle physicists speak somewhat casually about particles, and we include what we would think of as bound states, combinations of quarks and so forth. In that case, there are hundreds of them.

What is the difference between fundamental particles and something like bound states, or particles that are made from other particles?

The fundamental particles, as far as we know, are typically pointlike objects. They might have mass, but sometimes they don’t have any physical dimension. The particles that are like protons are physically extended, and when we collide them in colliders like the Large Hadron Collider, we can determine that they actually have structure.

How can a fundamental particle have mass, but no volume? Can you explain how that works?

I cannot. It’s a mystery. That’s something we’re still working on.

Is there a defining line between a particle and a force? For instance, because there are certain particles that are responsible for forces, where does that defining line happen?

It is the case that some of the particles seem in nature to be constituents of physical bodies or things, like the proton, and some of them seem to have the role of transmitting forces. We are able to relate the photon, which is the particle of light, as also being the transmitter of the electromagnetic force. There are particles like that for the strong force, the weak force, and so forth.

What are the strong force and the weak force?

The strong force is the strongest of the fundamental forces, and it’s the one that binds the quarks into a nucleus. It’s the one that is able to overcome the repulsion that the quarks that have the same electromagnetic charge otherwise experience and which would otherwise cause the nucleus to disintegrate. The weak force is something entirely different. It is the force that allows quarks and leptons to change their identity, and it’s at the basis of radioactive decay.

What do you see as the purpose of a particle collider?

The purpose of a particle collider is to collect a lot of energy at a very small point, and we do that by colliding two particles that are carrying a lot of energy. Mostly these particles are carrying that energy in the form of their momentum, and some of it in the form of their mass. Typically the moment of collision creates a little microscopic fireball, which evolves according to the laws of nature. Then we look and see what nature produces when you give it that much energy. In that sense, we are replicating a tiny Big Bang sort of event.

What particles is your group studying?

Members of my group are part of the ATLAS Experiment at the Large Hadron Collider. There are four general-purpose experiments at the LHC, and ATLAS is one of them. Within the collaboration, there are people who are studying the Higgs boson, properties of the top quark, people who are searching for new exotic particles that have properties that might help us see some unifying principle in nature. My group is interested in searching for new particles, new principles, and new properties, and we, particularly historically, have used the heavier quarks as signatures of those. The bottom quark, in particular, is the second heaviest quark. We typically look for signatures that involve a bottom quark for a number of reasons—for example, because its coupling to the Higgs boson is the strongest. Typically, it’s one of the easier signatures to reconstruct.

What did you find in your study that combined two quarks?

The University of New Mexico group contributed an analysis published by the ATLAS experiment, that involved the discovery of the first excited state of the B_C meson, which is a bound state of a bottom and a charm quark.

This study described the excited state as being almost like one of the quarks was orbiting the other, like an electron in an atom. Is that accurate?

That’s a reasonable model. One could think of the bottom quark as being so heavy that the motion of the two particles relative to each other in the bound state might be approximately nonrelativistic.

If you’re talking about an excited state, is it analogous to the orbitals of an electron being moved up to an excited state?

Yes. Our initial motivating interest was to learn what bound states can be sustained in nature, which may give us an opportunity to understand better how the strong force works and what the binding principles of the strong force are. We’re experimentalists, so we made the measurement. We’re not theorists, but in principle theorists who build quantum chromodynamics models might use the information of the mass of this particle combined with other bound states to infer some features of the strong interaction.

Many scientists collaborate on work at the Large Hadron Collider. What are the advantages of working in large collaborations?

There are four collaborations on the Large Hadron Collider. The ATLAS collaboration, the one that I work on, has 3,000 people. The members of the collaboration form subcollaborations so that people who have related expertise—people who naturally will support, enhance, and stimulate the work of one another—work together. People who do, for example, particle tracking, as the University of New Mexico does, form smaller groups, and then within particle tracking there are people who are interested in smaller and smaller, more and more specific aspects of that topic, until in the end the number of people who are talking to each other about a particular question may be only five people or may be only 15 or 30 people, despite the fact that there are 3,000 of us in the entire collaboration.

Someone might wake up in the morning and connect to a meeting and talk to 20 people about a technology that’s in development. Then they might have a video conference with a different group of 15 or 20 people and talk about an analysis that’s searching for a new particle, and then maybe later in the day talk with yet some other small group, maybe 15 or so people about some other new development, maybe a new method for data storage, data reconstruction, electronics, something like that.

In the end, the fact that there are 3,000 people is barely visible to a typical collaborator. We have very small and tightly networked communities that we work within. That seems to have evolved naturally. That’s not imposed by the collaboration. It’s a natural way that scientists work. It’s good for the students, because they find themselves working in a community that is largely without any hierarchy of age or experience. There could be people who are graduate students who are communicating with people who are very experienced, distinguished members of the community, all talking about the same problem, same questions, and everyone growing together, moving forward together toward a solution.

What is it like to work with such an international team?

The collaboration draws upon expertise almost without consideration of national borders. A typical meeting that I might have with collaborators does not require me usually to travel any place other than as far as my laptop, where I can run some video conferencing software, and I may connect and meet with 20 people who are in 20 different locations, and I may not even know where they are, what their nationality is, or where they’re physically connecting from. We’re just working together on the science.

What advice would you give to students working on a PhD?

I think a student who has decided to attempt a PhD should select the single topic that they think is most important and most interesting to them. If you’re working every day on something that is the most important thing to you, it’s the easiest thing in the world to do. You don’t feel conflicted about it, and you feel encouraged by the worthwhileness of the topic.