Physics Assistant Professor Manuel Franco Sevilla Answers Questions About Flavor Physics
The College of Computer, Mathematical, and Natural Sciences hosted a Reddit Ask-Me-Anything spotlighting experimental particle physics research.
University of Maryland Physics Assistant Professor Manuel Franco Sevilla participated in an Ask-Me-Anything (AMA) user-led discussion on Reddit to answer questions about “flavor” physics on July 22, 2025.
Franco Sevilla works at the LHCb experiment, one of the four detectors at the Large Hadron Collider (LHC) located at CERN, the European Organization for Nuclear Research in Geneva, Switzerland. LHCb is a leader in “flavor” physics—the study of the various types of quarks and leptons, the basic components of matter.
This Reddit AMA has been edited for length and clarity.
In layman's terms, can you explain why this type of research is important and relevant to the average person?
This is a very important question! However, it is difficult to give a simple answer because particle physics is “blue skies research,” that is, inherently open-ended. I can share a few reasons why I think it is good for societies to spend some resources on this kind of research.
Though it is impossible to be sure beforehand, simply understanding our world better sometimes leads to developments that end up improving our lives. Think of how all the quantum physicists of the early 20th century only cared about understanding the strange behavior at small scales, and ended up making possible semiconductors, lasers, and many other gadgets that we use today.
There is also the potential of indirect benefits. A couple of examples from particle physics are how the development of particle beams for colliders helped develop new therapies for cancer, or how the need to organize the enormous amounts of data coming from the LHC led to the development of the World Wide Web and the HTTP protocol.
This work can inspire students to develop STEM skills that benefit society. And curiosity is important! Understanding what the universe is made of, where it comes from, and where it is going is inherently interesting for many humans.
All this to say—there should always be a continuing debate on what level of resources blue skies research deserves, and how to best allocate them.
What's your opinion of the biggest unsolved mystery in your field and within all of physics?
It's not very original, but I’d say that in particle physics, the biggest unsolved mystery is the nature of dark matter. We really know that it is around us, and we know there is a lot of it, but we have no clue what it is made of.
The biggest mystery in all of physics could again be dark matter or the interpretation of quantum mechanics. What do those probabilities really mean?
If we could zoom infinitely into an atom, would we eventually find a truly fundamental particle, or does matter go on forever with smaller and smaller pieces?
Our current understanding is that there are a few truly fundamental particles, which are the quarks, leptons, and force carriers. Everything else is made up of smaller pieces.
Inside an atom, you would see the electrons, protons, and possibly neutrons. The electrons are fundamental, so it doesn't matter how much you zoom in because they have no size. In quantum field theory, we understand them as a point source of the field. The protons and neutrons do have a size, because they are made up of quarks and gluons. These quarks and gluons are moving around within a characteristic distance that gives protons and neutrons their size. But then, if you keep zooming, quarks and gluons have no size/width, because they are also fundamental. But you would continuously see new quarks/anti-quarks and gluons popping out of the vacuum.
If all of this sounds a bit messy, that's because it is. The structure of the Standard Model is incredibly simple and elegant, with just a few fermions and forces explaining the majority of the universe, but once you put a number of them together, things get very complex very quickly, giving us the awesome richness we are surrounded by.
Are the three generations of fundamental particles physically meaningful, or are they simply a classification based on mass?
The quarks within a generation are definitely related to each other. For instance, the third generation and super heavy top quark has enough rest mass (energy) to decay into any of the three -⅔ charged quarks via a W boson. Which one does it decay the most? Its third-generation buddy, the bottom quark. This is a pattern that occurs in all 3 generations, and is clearly visible in the CKM matrix (which tells you the transition probabilities) being almost diagonal.
There is no known connection between the quark and lepton generations, though, but seeing these particles neatly arranged in 3 columns is really suggestive. Thus, a lot of effort is being put into seeing if this is random or there is a deeper reason (like in grand unified theories). I’d definitely love to know!
Is the recently observed CP-symmetry breaking in baryons possibly an explanation for the matter-antimatter imbalance, or is it "just" a difference between matter and antimatter, unrelated to their imbalance?
I have to say, in my opinion, that result was painted in the media as bigger than it really is. It is a hard and interesting measurement, and the first time we observe CP violation in baryons, which is no small feat. But the level of CPv can be perfectly accommodated within the Standard Model, so it is way below what is needed to explain the matter-antimatter imbalance.
Hopefully, as we keep measuring CP-violating processes, we encounter something that explains that imbalance, but no dice so far.
As a layman who likes to read about the sciences, what exactly makes this specific field so interesting?
The Standard Model (SM) is an amazing theory with incredible predictive power. But it is old and has its limitations, so we particle physicists want to find something new. Any effect that breaks (violates) the SM would be super exciting, and lepton flavor universality violation (LFUV) is one of the ways you can break the SM.
What makes LFUV really interesting today is that a number of measurements, coming from independent experiments, all hint that LFUV may, in fact, be occurring. If confirmed, this would be monumental news! But we are still far from confirming these anomalies. We need more precision, and ideally more independent ways of looking at it to be really, really, really sure that LFUV is in fact occurring. The SM is so awesome that we require a very high bar to set it aside.
If you are interested, I wrote this little piece explaining all of this in a bit more detail in layman's terms.
Why are we unable to quantize gravity / find a graviton? Do you think there is a higher probability that we find a graviton particle or that gravity isn’t able to be quantized?
There are theories out there, like string theory, that successfully quantize gravity, but the question is how to find the one that truly describes the universe we live in, and how to prove it. Gravity is 36 orders of magnitude weaker than electromagnetism (that is, 10^36!), so if the graviton exists, it is going to be incredibly hard to find.
Given how successful we’ve been at quantizing the other 3 forces, I’d give a somewhat higher probability to the graviton existing than not existing, but I wouldn’t be surprised either way. But these prior probabilities are not very meaningful; at the end of the day, we scientists just need to keep coming up with ideas and measurements to determine what is real and what is not. And not be biased along the way!
In your opinion, what’s the most promising approach for finding (or killing) new physics relevant to the matter–antimatter asymmetry?
That's a very hard question. I don't think there is a clear path, so it's important not to put all our eggs in one basket. I think the current mix of particle physics measurements that aims to measure CP violation in the quark and lepton sectors (including neutrinos), as well as other measurements that may look at first order unrelated to CP violation, is the way to go. You just don't know where the solution is going to be ultimately found!
If given near-limitless funding, what experiment/idea related to your field would you pursue?
I don't have a super creative answer for your second question. I am very excited about the continuation of our current work at the high-luminosity LHC, which, for instance, should produce enough data to establish whether lepton flavor universality violation is real or not. But beyond this, I would love to have a super high-energy muon collider to see if we are finally able to produce new, cool, exotic particles. Who wouldn't?!
How important is quantum coherency in your line of work? How do you achieve it to observe specific behaviour of particles?
This is a big issue for quantum experiments that deal with low-energy matter, such as quantum computers. In high-energy physics, we collide particles in high-vacuum environments, and then the heavy particles decay before you get to meet them. (The very long-lived B meson only lasts 10^-12 seconds.) So we, for instance, generate coherent pairs of B and Bbar mesons, and they remain coherent without any issue until they decay. This is exploited in some measurements, where we tag the flavor of one of the B mesons by reconstructing the other B meson. (We know that if one is a Bbar meson, since they were generated coherently, the other one must have been a B meson.)
I have no idea what work is like for a physicist. Could you do a rough percent breakdown of what you spend your time doing at work? How much time (%) do you spend coding, math, meetings, etc?
There are many kinds of physicists, but I can give you a little insight into what it is like being an experimental particle physicist.
Our work is divided into two main tasks: data analysis and detector development (hardware). There’s quite a bit of freedom in how much time you spend on each. (There are some people who do 100% of either.) As for me, I’ve ended up at something like 50-50 (though some years are 100% data analysis and others 100% hardware).
When you do data analysis, you do some reading to learn about the latest techniques and physics, but spend most of the time processing data and writing code. You typically need some pretty high-level mathematical and statistical methods, together with a good physics understanding of what may be going on. Data analyses can be done by single people or groups (the Higgs discovery, for instance, involved hundreds of people).
For detector development, it varies significantly because each technology is different. But in general, they all have the active sensors (for instance, silicon sensors to detect charged particles or scintillators to measure the energy of particles), the electronics read-out, and the mechanical support structures. These are really complex (and fun!) projects involving many physicists and engineers. They can be exhausting because the deadlines are very tight and inflexible, but since they are quite social, they can be exhilarating. I had the time of my life when I was at CERN in the last half of 2022, coordinating the assembly and installation of the Upstream Tracker detector that I mentioned in the initial post!
