Interview with Werner Rodejohann, an Expert in Neutrino Physics

Werner Rodejohann is the group leader of the MANITOP group at the Max-Planck Institute in Heidelberg. MANITOP stands for Massive Neutrinos: Investigating their Theoretical Origin and Phenomenology and the group’s goal is “to shed light on the theoretical origin of (Majorana) neutrino masses and to explore the phenomenological consequences of the model predictions and of possible mechanisms giving rise to neutrino mass”. Here he discusses, among others, his biggest discovery and shares invaluable tips for students who are interested in neutrino physics.

Physics Insider: Mr. Rodejohann, what are you currently working on?

Rodejohann: Currently we are working on a project dealing with the neutrino telescope IceCube, located at the south pole. Cosmic rays in the form of ultra-high energetic neutrinos have been detected there for the first time a few years ago. We are trying to find out how the presence of new particles changes the event rates. Other projects investigate how particular mechanisms to explain neutrino mass influence the Higgs boson, and the „translation” of a rare nuclear process related to neutrinos into a LHC cross section. Then we have some ideas on dark matter that we pursue. The cool thing about neutrinos is that they are connected to various other fields, which leads to broad and colorful projects.

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

Rodejohann: The finding that neutrinos have mass of course! In the Standard Model of particle physics (which was completed after 100+ years of research, and recently found its last ingredient, the Higgs boson), neutrinos are massless. We know however that they must have mass, since they oscillate. What that is?  Just as quarks and charged leptons, neutrinos come in 3 different „flavors”, called electron-, muon- and tau-neutrinos. Suppose you produce an electron-neutrino and let it travel a certain distance L. Experimentally you want to check whether it is still an electron neutrino. You will find that the original electron-neutrino has some probability to be a muon-neutrino! The probability goes with the sine of L, hence the name neutrino oscillations. Physically it is a quantum mechanical interference effect over macroscopic distances, and it has been confirmed e.g. on Earth with distances of close to $1000$ km! The point is that it only works if neutrinos have mass, in contradiction to what the Standard Model says. Experiments like SuperKamiokande in 1998 and SNO in 2002 have shown that neutrinos oscillate, and have been rewarded with Nobel prizes.
We know of several shortcomings of the Standard Model, such as dark matter, dark energy, the baryon asymmetry of the Universe, but massive neutrinos is the only physics beyond the Standard Model that we can test and work with in the lab. That’s what makes this topic so important.

Beyond the fact that neutrinos have mass, there are several puzzling aspects. The mass turns out to be very tiny. Moreover, the probability of flavor change is quite high, while the analogous thing for quarks is quite small. Apart from explaining the origin of neutrino mass, we need to address these issues. Usually those explanations link neutrinos with other fields such as dark matter, cosmology, collider physics, etc.

Physics Insider: What was your biggest discovery?

Rodejohann: I was involved in some of the early analyses that link neutrino parameters with the baryon asymmetry of the Universe: a rather embarrassing prediction of the Standard Model is that we should not exist, because equal amounts of matter and antimatter should have been produced in the early Universe, and should have annihilated completely, leaving nothing but photons. Since we apparently exist there is thus a need for an asymmetry between matter and antimatter, the so-called baryon asymmetry.

Turns out that some mechanisms for neutrino mass can also generate the baryon asymmetry as an unexpected bonus. We tried to see how the measurable parameters of neutrinos are connected to the baryon asymmetry within such mechanisms.
I am also pretty happy with my work on neutrinoless double beta decay, including a looooong review article. This process is the only possibility to find out if neutrinos are their own antiparticles (like the photon) and its observation would tell us a lot about neutrinos and other physics beyond the Standard Model. Many fascinating experiments around the world are looking for this decay, and many theorists are predicting its half-life, including me and my co-authors.

Another paper I am proud of is about the absolute lower limit on a certain parameter that was unknown at the time, only upper limits existed. Assuming this parameter was originally zero, we found a situation where the usual small corrections do not give an effect. So we had to evaluate the even smaller next-leading corrections ($2$ loop corrections, for the experts) and thus found the lower limit on this parameter, which numerically turns out to be about $10^{-14}$. I was pretty proud of this result, and a few months later this parameter was found to be $0.1$ That’s life 🙂

Physics Insider: What is your advice to a student who wants to make a career in your field? Which books do you recommend to a student who wants to start doing research in your field?

Rodejohann: Apart from being smarter than the others I would advise students to identify the field they really love, and not blindly follow a current „hot” topic. After finding a field close to your heart, inform yourself which groups are good in that field and try to go there. Too often I have seen smart students with good prospects blindly follow hot topics, or go to places just because they like the country or the town. They will never produce really good work and their career is over before it even started.

Regarding literature, I would suggest the very detailed book by Giunti & Kim: Fundamentals of Neutrino Physics and Astrophysics.

Physics Insider: What defines a good student from an advisor’s perspective?

Rodejohann: Be smart. Be efficient and/or innovative. Don’t get frustrated quickly if projects don’t go as planned. Have your own view on things, be independent. No need to have the best grades of the year, being good in exams is helpful but no guarantee to be a good student. Perfectly knowing the technical details is good, but having „street smartness” in physics is even better: have a gut feeling on which things work and how.

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

Rodejohann: I’d prefer us humans to find out the secrets of nature by ourselves. In addition, a fairy does not seem to be a trustworthy source of information.

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

Rodejohann: The biggest question in our field is: what is the origin of neutrino mass? We have invented many mechanisms that can generate neutrino masses, but do not know the one realized by nature. We may find this out next Tuesday or in 50 years, depending on the model and its details. That’s research, you never know. One can make educated guesses on those things, based on experience and other results, but at the end our ideas might be totally wrong.

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

Rodejohann: drink less, study more.

Physics Insider: Which books did influence you the most?

Rodejohann:A Song of Ice and Fire“, of course 🙂 In physics, I have to mention Halzen & Martin: Quarks and Leptons, as well as Bjorken & Drell: Relativistic Quantum Mechanics. Excellent books, never outdated. I’d also like to mention Particles and Nuclei: An Introduction to the Physical Concepts, by Povh et al. I learned particle and nuclear physics in my 5th semester from this book, and was a few years ago asked by the authors to join the author list to add new chapters and help updating it. I encourage everybody to buy a few copies.