Interview with Steve F. King, an Expert in Flavour Models

One of the most exciting questions left unanswered by the standard model is: why are there three almost identical copies of all elementary particles? For example, why does the muon, a heavier copy of the electron, exist? “Who ordered that?

Steve F. King is a professor at the School of Physics and Astronomy at the University of Southampton and one of the leading experts when it comes to solutions of the flavour problem.

We talk about his biggest discovery, his current work and he shares lots of advice for students who want to help solving the flavour puzzle.

So without further ado, meet Mr. King.

Physics Insider: What are you currently working on?

King: I am working on various theories beyond the Standard Model which involve discrete flavour symmetries. These theories range from multi-Higgs models, to supersymmetric grand unified models and string models based on F-theory and M-theory. I am also interested in the resulting LHC signals, including flavour-changing at LHCb, as well as the cosmological implications of such theories.

Do not feel that you have to follow the latest fashionable trend like a shoal of fish. Instead, decide what is interesting for you and pursue it, in collaboration with like-minded others.

Physics Insider: Why is this important?

King: The goal is to find a theory beyond the Standard Model which addresses the outstanding questions such as the flavour problem: the origin of three families of quarks and leptons (including neutrinos) with their observed pattern of masses, mixing and CP violating parameters.

Physics Insider: What was the biggest advance/discovery in your field in the last 20 years?

King: Undoubtedly the discovery of neutrino mass and mixing, following the breakthrough experiment by Super Kamiokande in 1998. This is the first and so far only particle physics evidence for physics beyond the Standard Model.

Physics Insider: What was your biggest discovery?

King: The sequential dominance (SD) mechanism, which provides a mechanism for understanding in a simple way how the seesaw mechanism can give rise to large mixing angles. The idea is that one right-handed neutrino gives the dominant contribution to the heaviest (atmospheric) neutrino mass, while a second subdominant right-handed neutrino is responsible for the next heaviest (solar) neutrino mass. There may be a third right-handed neutrino responsible for the lightest physical neutrino mass, but this is approximately decoupled from the seesaw mechanism.

The main prediction of the SD scheme is the normal neutrino mass hierarchy m1<< m2 < m3 which has yet to be verified experimentally.
The main success of the scheme so far is that it predicted the value of the reactor angle theta13 should be of the order of the ratio of the second and third physical neutrino masses theta13 ~ m2/m3 ~0.2 about a decade before it was measured.

The atmospheric and solar mixing angles are related to ratios of Yukawa couplings of the dominant and subdominant right-handed neutrinos to the left-handed lepton doublets. If these ratios take simple values, due to some discrete symmetry, then precise values of the angles are predicted. This type of model is called constrained sequential dominance (CSD).

Physics Insider: What is your advice to a student who wants to make a career in your field?

King: First of all study very hard at undergraduate level and get the best degree grade that you can. This will be the stepping stone to winning a PhD position at a good university. You (the student) should choose a successful researcher in the field of interest who can guide you towards becoming an experienced researcher.

Physics Insider: If some fairy would offer to answer you one question about nature; what would it be?

King: I would ask my fairy:

“What is the theory of flavour?”
i.e. the origin of the three families of quarks and leptons (including neutrinos) and the understanding of their masses, mixing angles and CP violating phases.
I would hope that the theory of flavour would involve understanding of the unification of forces, perhaps including supersymmetry, and shed light on the cosmological questions such as matter-antimatter asymmetry, dark matter and dark energy.

Physics Insider: How far do you think are we away from answering this question?

King: Somewhere between 5 and 50 years, depending in whether some theorist achieves a conceptual breakthough (5 years) or if not then we must wait for future experiments to “rub our noses in the answer” as Feynman once said. Unfortunately this may take some time (I am allowing 50 years but of course I would hope it would be sooner)!

Physics Insider: If you could give your 20 year old self one piece of advice, what would it be?

King: Have patience! Your physics career will last a long time, so do not expect to achieve everything all at once. Do not feel that you have to follow the latest fashionable trend like a shoal of fish. Instead, decide what is interesting for you and pursue it, in collaboration with like-minded others.

Physics Insider: What math is necessary to be able to work in your field?

King: Linear algebra and vector spaces on which quantum mechanics and quantum field theory are founded, as well as group theory, including both lie algebras (applied to gauge theories) and finite groups (applied to discrete symmetries such as CP symmetry).

Physics Insider: Which books do you recommend to someone who wants to do research in your field?

King: A good modern field theory text such as the one by Peskin and Schroeder and a good book on group theory such as the one by Georgi.

Physics Insider: Which books did influence you the most?

King: Georgi’s book on Lie Algebras in Particle Physics, and Bjorken and Drell’s field theory books, although there are now many more books on field theory to choose from.

Physics Insider: What was the best physics or math book you’ve ever read?

King: Shankar’s book on Quantum Mechanics.

Physics Insider: What do you wish you would’ve known earlier in your career/ when you started studying physics?

King: I do not wish I could have known anything earlier since without mistakes one learns nothing of lasting value. I think I am saying that experience is the best teacher and cannot be taught!

Physics Insider: Mr. King, we thank you for this interview.

To learn more about Mr. King’s work, you should have a look at his papers. For example, his neutrino mass review paper is highly readable even for non-experts. If you liked this interview, you should have a look at our interview with Michal Malinský, an expert in Grand Unified Theories.