Over 30 Nobel Laureates will debate this year in Lindau, Germany with about 400 young scientists from nearly 80 countries. The 66th Lindau Nobel Laureate Meeting from June 26th to July 1st is dedicated to the field of physics with a special focus on particle physics including neutrino physics. Prof. André Rubbia (ETH Zurich) is one of the leading neutrino experts in Switzerland. In the interview the 50-year-old researcher gives a glance at the hot topics of current research in worldwide neutrino physics and highlights the contribution of Swiss particle physicists.
Prof. Rubbia, the main speakers at the upcoming Lindau Nobel Laureate Meeting are both the Japanese researcher Takaaki Kajita and the Canadian astrophysicist Arthur McDonald, each of them winner of the Physics Nobel Prize 2015. The scientists got the Nobel Prize thanks to their discovery of the neutrino transformation and mass. What did they exactly discover?
We know three neutrino families, also called flavors: the electron neutrino, the muon neutrino and the tau neutrino. What Takaaki Kajita and Arthur McDonald discovered in different experiments is a new phenomenon called the neutrino flavor oscillation. That means the flavor of a neutrino can change ('oscillate') when it travels from the source to the detector. Kajita was studying the muon neutrinos with a very large neutrino observatory in Japan, called Super-Kamiokande. When he and his team have measured the arriving muon neutrinos in the detector they realized that there were less muon neutrinos than expected. What happened is a transformation of muon neutrinos to tau neutrinos as they crossed the earth. Arthur McDonald performed a similar experiment with the Sudbury Neutrino Observatory (SNO), a deep underground facility in Canada. In the SNO he studied the neutrino produced by the Sun, which is primarily the electron neutrino. McDonald and his team demonstrated in the experiment that the electron neutrino had changed to different flavors while traveling from Sun to Earth.
The flavor oscillation is a property we don't know from any another elementary particle?
Exactly, this kind of phenomenon has never been observed directly before. As a consequence of this oscillation we know that neutrinos have a (very small) mass. This is a spectacular discovery, since so far, according to the Standard Model of Particle Physics, neutrinos were assumed to be massless.
The neutrino was first postulated by Wolfgang Pauli in 1930, at that time working as a theoretical physicist at ETH Zurich. The three flavors of the neutrino then have been confirmed by experiments in 1956 (electron neutrino), 1962 (muon neutrino) and in the year 2000 (tau neutrino). Mission completed, a non-expert would think. But today the neutrino is a hot topic of particle physics again. Why?
The discovery of the neutrino mass by Kajita and McDonald actually has reopened the puzzle of the neutrino. The reason is the following: A particle like the electron has a spin. It “rotates” around itself, either right-handed or left-handed. Until the late nineteen nineties we thought that the neutrino has only one type of spin (left-handed). This was consistent with the assumption that the neutrino has no mass. Now if the neutrino has a mass it means that in principle there could also be what we call a right-handed neutrino. This neutrino would be a very special type of neutrino because in the Standard Model this right-handed neutrino would have no interaction with known matter and would be completely invisible. This is why we call it a sterile neutrino.
A rather weird idea!
Yes indeed, but the impact on the cosmology could be very large. Because this type of neutrinos could have been formed in the initial stage of the universe directly after the Big Bang. So it is one of the big questions today in particle physics whether the sterile neutrino exists and what has been their impact on the evolution of the universe. The discovery of this fourth neutrino would open a window to new particles of which we have absolutely no idea of existence up to now.
Is there a chance to find this particle within the next years?
Yes, absolutely. There are several types of experiments being currently built to prove the existence of a sterile neutrino.
If neutrinos oscillate, do their antimatter particles (the anti-neutrinos) oscillate as well?
That's exactly what we are trying to test in what we call a long-baseline experiment like DUNE, the Deep Underground Neutrino Experiment we are currently constructing at the Fermilab near Chicago. We are furthermore trying to test if the flavor oscillation is the same for neutrinos and anti-neutrinos. By studying this topic we look for an answer why there was more matter than anti matter produced throughout the Big Bang. The excess of matter became eventually the fundamental constituent of all the galaxies in our universe, all the stars and all the planets including the Earth and even ourselves.
What is the contribution of Swiss particle physics to worldwide neutrino research?
Today the University of Geneva (Prof. Alain Blondel), the University of Bern (Prof. Antonio Ereditato) and the ETH Zurich (with myself) are involved in the main neutrino experiments. One of them is the T2K (acronym for “Tokai to Kamioka”) experiment located in Japan. That experiment is sending a neutrino beam from the east coast (Tokai) to the west coast (Kamioka) of Japan. There we are studying the neutrino flavor oscillations and we are exploring the differences of oscillations between neutrinos and anti-neutrinos. Swiss researchers have contributed to this experiment with the construction of what is called the Near Detector between 2006 and 2011. Today, we are especially doing the physics exploitation by collecting data and studying the neutrino interactions.
You have visited this experiment last week and you spent the week before in the US, right?
Yes, at the Fermilab near Chicago. I'm actually one of the two spokespersons of the above mentioned DUNE experiment, together with Mark Thomson of the University of Cambridge. It is actually the first time that the US will host a very large neutrino project notably managed by two Europeans! We are building this project based on the CERN model with participation of many countries. This is a challenge, because USA is traditionally not so used to being internationalized.
And Switzerland sits in the front row.
Indeed, and another input of Swiss particle physicists is the development of a neutrino detector technology called Liquid Argon Time Projection Chamber. DUNE will be the first very large international project going to use our tool to detect neutrinos.
Let me finally come back to the Lindau Nobel Laureate Meeting. One of the Nobel Laureates joining the meeting is Carlo Rubbia, your father. Carlo Rubbia got the Nobel Prize in 1984 for discovering the W and the Z bosons at CERN. What makes the difference between an outstanding researcher as you are and a Nobel Prize winner as your father is? Is it more than a little bit of good luck?
That's a very good question. The Nobel Prize of Carlo Rubbia together with Simon Van der Meer was very much deserved, as well is the Nobel Prize of Takaaki Kajita and Arthur McDonald. And well, for the future we have to see. (He laughs.) We cannot be soothsayer, but for sure I think that the discovery of the sterile neutrino or the matter anti-matter asymmetry of the neutrino oscillation would be at the level of deserving a Nobel Prize. But this is not my main motivation. My main motivation is science.
Interview: Dr Benedikt Vogel