Interview with Ivo de Medeiros Varzielas, an Expert in Flavour Symmetries

Ivo de Medeiros Varzielas is currently a Marie Curie fellow in the research group of Stephen King. His main research interest is the origin of fermion masses and his anwers should be invaluable for everyone intersted in doing beyond the standard model research. 

Physics Insider: Mr. de Medeiros Varzielas, what are you currently working on?

de Medeiros Varzielas: Our current understanding of particle physics is referred to as the Standard Model (because it has been extremely successful so far) and I work in different extensions of the Standard Model. The Standard Model is essentially a list of particles and a set of symmetries which are effectively the rules for how the particles interact. Extensions can be understood as adding more particles or symmetries.

Much of my work attempts to address the flavour problem of the Standard Model. One of the most promising approaches involves the use of family symmetries. The models I work on are often supersymmetric and with grand unified gauge groups. I also work on models with additional electroweak scalars.

Physics Insider: Why is this important?

de Medeiros Varzielas: In the Standard Model, the fundamental fermions (which have spin 1/2) appear in sets of 3 “generations”. The number of generations is unexplained.  Furthermore the coupling strengths between the fermions and the single Standard Model scalar (spin 0 boson) are left as free parameters that have to be measured (these couplings between fermions and a scalar are called Yukawa couplings). In the Standard Model, the Yukawa couplings are the origin of the fermion masses.

As an example, the electron (first generation) has two copies: the muon (second generation) and tau (third generation). These 3 leptons are identical apart from their mass that differs by orders of magnitude. This happens for the other fermions in the Standard Model, with the mass ratios between the third (heaviest) and first (lightest) generation being around 10^5.

The difference in masses is even more astonishing if one compares the tiny neutrino masses with the mass of the top quark (the third generation charge +2/3 quark, and the heaviest Standard Model fermion). Furthermore, the Yukawa couplings for the 3 generations are in general not aligned with the way the fermions couple with the electroweak interaction, meaning the generations mix (technically, the eigenstates of the Yukawa couplings are not the same as the eigenstates of the electroweak interaction). The observed 3 mixing angles for the quarks (1 small, 2 tiny) are very different from the 3 mixing angles for the leptons (1 nearly as large as it can be, 1 large and close to a peculiar value, and 1 small).

It is important to understand the features of the Standard Model related to the fermion masses and mixing, particularly as extensions addressing other issues of the Standard Model tend to make the flavour problem even worse (e.g. supersymmetric flavour problem).

If the generation of fermions transform into one another under a family symmetry it naturally explains how the 3 generations share the same properties (such as having the same electric charge). In that case, it is only because the 3 generations have different masses that the family symmetry is broken. This can occur similarly to how the electroweak symmetry of the Standard Model is broken, but in this case the details of how the family symmetry is broken provide an explanation for the mass ratios between generations and for the peculiar pattern of mixing angles. As a bonus, the family symmetry introduced to solve the flavour problem of the Standard Model will also typically solve the flavour problem of extensions meant to address other issues. Because of these reasons, family symmetries are a very natural and promising extension of the Standard Model.

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

de Medeiros Varzielas: I think at this date, most particle physicists would reply to this question by choosing the recent (2012) discovery of the scalar boson in the LHC. This discovery led to the award of the 2013 Nobel prize in Physics to theorists that proposed the underlying mechanism in the mid 1960s. The measured properties of the discovered scalar boson, such as its mass and how it couples to fermions, have far-reaching implications and are very important to distinguish between proposals beyond the Standard Model. Such a scalar boson was expected within the Standard Model since its inception in the late 1960s.

The top quark was officially discovered 20 years ago (1995) at the Tevatron, and it is also worth mentioning. The top was expected to be part of the Standard Model as early as 1973 and certainly more so when the other third generation fermions were discovered in the late 1970s (the tau lepton and the bottom quark).

Perhaps due to my focus on the flavour problem, I think my personal reply to this question is the discovery that neutrinos oscillate: in contrast with the other two discoveries mentioned above this was not expected within the Standard Model, where the neutrinos were expected to be massless and would not oscillate. Super Kamiokande provided clear evidence of atmospheric neutrino oscillation in 1998 and the Sudbury Neutrino Observatory finally confirmed in 2001 the existence of solar neutrino oscillations (an experimental programme that started with Homestake back in the 1960s). The observation of the reactor mixing angle by Daya Bay in 2012 is also a very important piece of the puzzle with respect to the flavour problem.

If (I hope “when”) the LHC experiments or any other experiments conclusively discovers something beyond the Standard Model, I will happily consider that a bigger discovery than neutrino oscillations!

Physics Insider: What was your biggest discovery?

de Medeiros Varzielas: As a theorist this is hard to answer, because even really clever ideas can be relatively worthless if nature simply doesn’t work that way.

An interesting concept I have been involved with is a concept referred to as geometrical CP violation, where the CP violation is to some extent predicted from the family symmetry employed – such as having the CP-violating phase predicted to be the angle of an equilateral triangle.

Another one I’m particularly proud of is a bit more technical to explain here. When I was a graduate student with my supervisor I developed a technique to combine grand unified models with family symmetries that I’m rather fond of. The specific papers I wrote as a graduate student have models that were ruled out by Daya Bay in 2012, but I already had written more recent papers using the same technique which remain viable – one of which was also with my supervisor and adds another interesting technique by involving scalar “mediators”.

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

de Medeiros Varzielas: “Don’t!” is a tempting reply here. An academic career in general can be rather unforgiving and I believe that a career in theoretical particle physics is particularly so. It is almost by definition fundamental research and it is not very surprising that applied research has comparatively more employment opportunities.

My main advice, independent of field, is to get in touch with people that are doing it at all levels of the career. If possible talk to the experts that have Senior positions or Junior positions. But it is important to talk to Postdocs with varying number of years since they graduated. Talk to graduate students as well.

Spending a few years as a graduate student in theoretical particle physics will of course give you a better idea. You can discuss career prospects with your supervisor, decide if you want to continue before you get your diploma, and apply to many Postdoc positions – which in this field typically have deadlines for application around a year before they actually start!

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

de Medeiros Varzielas: I would really like to know the answer to the flavour problem, but the hierarchy problems (scalar mass and Dark Energy / cosmological constant) are in my opinion less understood.

If I could just ask something along the lines of “What is the Theory of Everything?”, that should include the answers to all these problems, to combining gravity with particle physics, and also as bonus one gets to know what Dark Matter is. Maybe that would be considered cheating, but who knows what some fairy would think is fair?

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

de Medeiros Varzielas: This is impossible to know (for fundamental research at least). To know how long it would take to answer we would have to know what we need to do, and if we know what to do, to some extent we already have the answer (up to perhaps some calculation or measuring some free parameters). There is no way to extrapolate from whatever progress we arguably have made since the question was formulated, because without knowing the answer we can’t even quantify our progress.

I’m usually an optimistic person, but I don’t expect to be alive when it is answered (if it ever is). I hope to be wrong about this though! If you pin me down to a number, I’ll go with 99 years from now.

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

de Medeiros Varzielas: “Beware of time travel as the consequences of changing your own past are too unpredictable to be worth the risk”. I think I honestly would not do it even if I had the chance. In any case, I’m mostly happy with my choices so far and even the ones that I may think were suboptimal may have led to a greater good. I try not to have regrets as there is nothing I can do about what has already happened, and I try to avoid the “sunk cost fallacy” as much as I can. This isn’t to say I don’t believe in learning from my mistakes – although I prefer to learn in advance from someone else’s mistakes if possible!

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

de Medeiros Varzielas: I’m not sure if this is a complete answer but I’ll try. The usual lots of linear algebra and calculus, and a bit of complex analysis, formal logic and statistics. Specifically for what I do, group theory as well.

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

de Medeiros Varzielas:

For undergraduate Quantum Mechanics:

“Quantum Mechanics”, Claude Cohen-Tannoudji, Bernard Diu, Frank Laloe

For more advanced students:

“An Introduction to Quantum Field Theory”, Michael E. Peskin, Daniel V. Schroeder

“Gauge Theory of Elementary Particle Physics” Ta-Pei Cheng, Ling-Fong Li

For the following I’m a bit biased as I know the authors personally, regardless I think they are good recommendations:

“CP Violation”, Gustavo C. Branco, Luis Lavoura, Joao P. Silva

“Grand Unified Theories”, Graham G. Ross

“Group Theory: A Physicist’s Survey”, Pierre Ramond

“Journeys Beyond The Standard Model”, Pierre Ramond

Physics Insider: Which books did influence you the most?

de Medeiros Varzielas: In general I think the hilarious books of Douglas Adams and Terry Pratchett, sadly both died younger than the average life expectancy. In terms of scientific books this is a good place to mention “The cosmic onion” by Frank Close, which I read before my undergraduate degree, and I believe played a part in my interest in particle physics. I don’t doubt I would have gone to a Physics degree and I’m relatively certain I would end up doing theoretical particle physics even without having read it, but I can’t be absolutely certain!

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

de Medeiros Varzielas: A funny answer to give here would be the Particle Data Group book (although it is worth having a look, and it is freely available online).

It is hard to pick a best one. Maybe because I read this one as an undergraduate, the first that came to my mind in this question was “Quantum Mechanics” by Claude Cohen-Tannoudji, Bernard Diu, Frank Laloe. I think it is very well written and clear – even when I was reading the French version with the French reading skill I had kept from high-school (I got the English version later though).

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

de Medeiros Varzielas: Lots of tricky questions! This is to some extent a variation of the previous one involving time travel, so I’ll start by being consistent and repeat that I would not want to change the past.

I can add here that while I was a graduate student and shortly during my first postdoc I hadn’t realized it was so relevant to get papers published in peer-reviewed journals. I may have picked that up from my supervisor, I think it is fair to say he didn’t worry too much about it – although of course he was already a Professor at Oxford and in my (biased) opinion one of the top scientists in our field.

I think part of it was that I naively imagined that having the papers available in the arXiv, anyone can read them and decide how good the work is. Of course it doesn’t work that way for many reasons: those that work in the field don’t have enough time to read all the papers from someone to decide that, and those that are outside the field don’t have the required knowledge. Peer-review is not perfect but it provides a kind of quality check.

In any case, this is why I have an unpublished paper from 2006 (with my supervisor) and another from 2008: I didn’t even submit them for publication!

Would have helped me a bit.