By Fernanda Morais
This semester, the Physics and Astronomy Department welcomed Visiting Assistant Professor Sean Foster, who is currently teaching Electromagnetic Theory I and Introductory Mechanics. He received his Ph.D. from Boston University in 2023, working on the Muon g-2 experiment, and did a postdoctorate at the University of Kentucky, working on both Muon g-2 and the PIONEER experiment. Before coming to Amherst, he taught at Colgate University.
This interview has been edited for length and clarity.
How did you become interested in physics? Why did you decide to pursue it?
I think the first time I got interested in physics was [when] my grandpa gave me a popular science book on modern physics topics. My dad’s a veterinarian and my mom [works] in microbiology, but I don’t think I really knew what physics was until I got this book. Then I started reading about h-bar and special relativity, and I was like, special relativity is a real thing, and time dilation actually happens. So I thought, “Oh, this is cool.” Then, when I started college, I wanted to take a physics course, and that course was quite interesting. It was not the usual starting physics course. It wasn’t mechanics, but more like a modern physics course. So [the] physics I was exposed to [through] popular science was what we learned in that physics class. Also, I had a really nice cohort of fellow first years who were interested in physics. So, the combination of the topic being interesting, and the fun group of people who were doing problem sets with me, made me keep doing this.
Could you talk about the focus of your research right now?
I’m in experimental particle physics, and for my Ph.D., I worked on an experiment called Muon g-2 (“muon g minus two”). That experiment studied a particle called the muon, which we often characterize as the heavy cousin to the electron. It has a lot of similar properties to the electron, but it’s more massive. The experiment was interested in using the muon as a way to probe our best model of nature, which is the Standard Model. We have various reasons to think that, while the Standard Model has done very well, it is not the end of the story. So, why the muon? You can measure the muon properties really precisely. Well, I guess you can also do that with the electron.
The physics I’m interested in is precision physics, where we measure some quantity of nature super precisely, while some theorists would calculate that same quantity super precisely. You would then put them head-to-head. If they agree, then that’s probably okay; you probably can’t say much [except] that the Standard Model is a fine theory. If they disagree, then you have some indication that there’s a gap in your Standard Model, and the fact that they disagreed might point you to something new.
The particular thing we were studying with g-2 was how the muon interacts magnetically. Because it has spin, if you put it in a magnetic field, it will precess. Then, you can measure the rate of that precession, and you measure that extremely precisely. We were measuring something like 100 parts per billion. A more meaningful analogy would be that at Fermilab, where this experiment was happening, [they have bison]. One hundred parts per billion is like measuring the weight of the bison to the precision of a sunflower seed.
That was actually the experiment I did in the past, and we did publish our final result this summer, which was super exciting. Now I’m working on another precision experiment, studying the pion. The pion decays, and this experiment is interested in looking at the decay modes of the pion. The Standard Model predicts what these decay modes should be, so if you measure them precisely and something breaks down, that might be where something new happens. This experiment is in the design stage, so we’re running simulations to figure out how we’re going to do it.
I know the timelines for experiments to get online in particle physics are extremely long. Do you have any idea about when the PIONEER experiment is going to happen?
This experiment is at a smaller scale, with less than 100 people, which correlates with the timeline for the experiment, but it’s a phased approach. Right now, even though we’re designing the experiment, we are also doing detector tests, building small components of the larger detector that we want to test, and [characterizing] the detector. We hope to do a first phase of a physics dataset, where we’re really going to do a measurement, by the end of the decade. That’s about a five-year horizon. And then the full plan for the experiment is about a decade horizon. For a grad student working on this as a project, it’s a good timeline, and since it’s small, you can really carve out your piece.
Why did you want to go into precision measurement?
When I first started in physics, I definitely had perfectionist tendencies. And going into experimental and then precision physics felt like a way to be actionable about [that] perfectionism.
I’m teaching 116 right now, and we do a lot with uncertainty in that course, which I think is great. It’s important to know that if I want to make a statement about measuring something, I can’t just give you the value — I need to give you an uncertainty on that value. And that’s what precision is all about. And I think it’s a way to harness the feeling of perfectionism, but in a useful manner. I think it’s useful to know whether [the uncertainties] are large or small when you’re asking a question.
Are you working with any students right now?
I’m not working with any Amherst students at the moment, but I did give a colloquium talk and got some interest. So I’m hoping to have some students. Probably not this semester, because I’m starting to teach and getting acclimated. But in the spring, I’m hoping to have some students work with me, which would be very exciting.
Have you mentored students in the past?
I have worked with undergrads and other grad students. In my last role as a postdoc, I was mentoring a graduate student who was taking over some of the work I was doing, and he was doing a similar analysis as me, but on a new data set. I was also mentoring an undergrad who was doing some simulation projects for the g-2 experiment, and that was really fun.
I’d like to bring Amherst students into [these experiments] so they aren’t only working with me, but with a whole collaboration [that’s composed] of people all over the world, at different stages. There are undergraduates working at different institutions on the experiment, and lots of grad students working on the experiment as their thesis, and then there are postdocs, professors, and people supporting the experiment: administrators, safety people, engineers. There are lots of interesting people that you can learn from by being part of the collaboration. We have something like two collaboration meetings a year, where we’ll travel to one of the collaborating institutions and have a meeting. And those are really fun, because then you can see all the people you’ve been having Zoom meetings with weekly.
What are you teaching here, and how are your first few weeks of teaching?
I’m teaching PHYS-347, the upper-division electricity and magnetism course. I’m also co-teaching PHYS-116, the introductory mechanics course [geared towards] non-majors, with Professor Friedman and Professor Modir. Professor Modir is also a new visitor, and it’s been really nice that both of us have been starting this semester. We’re doing the labs and discussion for that course and using the existing structure for the course while learning together. We worked through the labs over the summer before the semester, so it’s really nice to work with her and get acclimated to the labs.
Things have been going good the first week or so. At first, you don’t really know your schedule and the cadence of things yet. So it was a lot of trying to figure out [when] I have the time to prepare for class and things like that. I think that subsided a little bit. I’m still getting used to the structure of the schedule that teaching a course enforces.
A lot of students who are new to physics feel like it’s really daunting. How do you navigate this as their instructor?
The 116 labs are an investigative science learning environment, which I think puts this feeling at the forefront for students. We understand that physics can have this reputation of being difficult, and we want to recognize that, but also lean into it a bit. Those labs are structured to get students to explore some physical phenomena related to mechanics. They’re going to be able to design an experiment, do some hypothesis testing, and make observations about what happens when you do an experiment. They’re going to do all the things that a scientist does. And part of doing those things, and part of being a scientist, is that things will not always work, and I think that the “not always working” part can relate to the feeling of “not getting this.” But [we built] that into [the course] — the mindset where you’ve tried something, and then you reflect on what didn’t go right, and that’s okay.
Do you have any advice for students who want to pursue physics as a career? Or specifically particle physics?
There were always people involved when I [was getting] interested in physics. My grandpa first gave me that book, and then in college, I had a good cohort of students. I think it’s really important for physics to be a group activity… In grad school I lost some of that group, but then, I got attached to these collaborations … Finding and being around a group of people that are somehow aligned with your interests has helped career-wise. I feel like those have driven my decisions [about] where I want to spend my time. I don’t know if that’s a good way to make your career happen or not. But if you’re working with people [who also ask] interesting questions like that, it helps a lot. That is connected to particle physics. The nice thing about particle physics is that it is very collaborative. I like that a lot.
Switching gears a little bit, but what do you like to do in your free time?
I enjoy cooking and baking. I haven’t done a lot this semester because of teaching, but I do enjoy baking. I was trying to think, what’s an interesting baked item I can share? Last year, for the holidays, my wife and I, and also some of her family, made a fruitcake. It’s half cake batter and half dried fruit. You bake it, and then over the course of a few months, you feed it. Yeah, the word is feed. You pour some sort of alcohol on it, and then it ages. You do this every week for a few months, and then at the end of it, you get this fruitcake. So it’s true labor. Every Saturday morning, we would take it out, open the foil up, and then we would feed it a couple of ounces of Brandy.
Do you keep it in the fridge?
No, because you’re basically preserving it with alcohol. And then, after the two months, you coat it in marzipan, and then you ice it. And then at that point, apparently, it is good to go. We got this recipe from this book, Dessert Person, by Claire Saffitz. So if you’re interested, or if anyone is interested, you can check out that book. But I will say there’s a warning. It’s a holiday, December time, cake, but the recipe tells you that if it’s December, and you want to make this, you’re too late. That was definitely one of the more interesting projects I’ve done. Outside of baking, I like walking my dog around campus. She’s a little bit of a monster, but she is friendly.
What’s your favorite thing to bake?
Well, I don’t know if that was my favorite thing to bake. That was more of a project. I do like to make breakfast items like scones and biscuits, because they’re kind of quick, and then you can enjoy them immediately. In a way, it’s the opposite extreme. I like the long projects, but I also want to balance it out.
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