Down, at the bottom of the matter

If we imagine the universe as a clock – but it isn't: the comparison is far from adequate – we can think of particle physicists as curious people who take it apart to understand how it works, with what mechanisms, springs and cogs. Except that they don't have the instruction manual. In fact, it's up to them to write it. But why do they do it? What do they discover? And what is the use of what they discover?

We discuss this topic with Marco Zaro, professor of particle physics at the University of Milan, who will give a lecture at the Cantonal Library in Bellinzona on Wednesday 25 February at 6.30 p.m., organised in collaboration with the Istituto ricerche solari Aldo e Cele Daccò (IRSOL) and the Ticino Astronomical Society.

What was the exact moment or specific concept during your journey that made you realise that particle physics would be your path?

Rather than a specific moment, I would say that it was a combination of many factors. Even before university, I felt quite inclined towards mathematics and science, partly thanks to family «influences» – my father is a geologist and my mother taught mathematics and science – and above all thanks to the teachers I had, especially at secondary school. This guided my choice of university faculty. Specifically, I opted for physics because of the fascination I glimpsed while studying it at secondary school and because it used mathematics to explain reality. My university years confirmed this, once again thanks to some teachers who were able to keep my passion for the subject alive and help it grow, but also thanks to friends and fellow students who were invaluable companions on my journey. Since I was seen as the «best» student, I often answered various questions, which may have been perceived as trivial, but which each time helped me understand and discover aspects of the subjects I had perhaps overlooked. During my university studies, I focused on the theoretical study of physics, and I was fascinated by aspects of theoretical physics that could be verified or falsified experimentally. After taking a very exciting course on the subject and visiting CERN, I decided to pursue a career in (theoretical) particle physics. My degree thesis and subsequent PhD in Belgium cemented this decision.

Fifty years passed between the Higgs boson hypothesis and its discovery at CERN. How does a physicist cope with the challenge of working on a hypothesis that could take decades or even generations to be confirmed?

This fact has always struck me. There are many cases in which a hypothesis, perhaps dictated by «aesthetic» arguments – an extra particle is needed to make a theory more symmetrical – and therefore by categories that belong to our mind, has proved capable of explaining reality, which is other than ourselves. For the Higgs boson, it took 50 years between the hypothesis and the discovery. For neutrinos, whose existence was motivated by the need not to violate the principle of energy conservation, it took 25 years, and for gravitational waves, 100. In addition to the amazement when a hypothesis is then verified by reality, there is a great sense of humility, in the sense that one's hypothesis, however beautiful or exciting from a theoretical point of view, must always be subjected to the scrutiny of reality.

What is the anomaly or experimental data that most keeps particle physicists awake at night and could point the way to a new physics after the Higgs boson?

We certainly have many indications that the theory we use to describe how fundamental particles interact, the Standard Model, is currently incomplete. Before talking about its flaws, however, we must remember that it is an extremely successful theory, capable of explaining phenomena ranging from the anomalous magnetic moment of the electron, where experiment and theory agree to within less than one part in a trillion (!), to the phenomena we see at the Large Hadron Collider (LHC), which occur at energies millions of times higher. There are some phenomena, such as the fact that dark matter is needed to explain the evolution of the universe, or the fact that we cannot explain why we are all made of matter and see little antimatter in the universe, that the Standard Model cannot explain. Other phenomena, such as the fact that the masses of elementary particles are so different – between neutrinos, the lightest particles, and top quarks, the heaviest particles, there are about 15 orders of magnitude – are more problematic from an aesthetic point of view. The latter problem, being also related to Higgs boson physics and therefore to aspects that I deal with, is perhaps the one that intrigues me the most.

Since the announcement of the discovery of the Higgs boson 14 years ago, particle physics has not enjoyed the same level of media attention. Why is this? Has nothing else of such significance been discovered? Could we say that, since the LHC came into service, research in this field has stalled somewhat?

First of all, I would not use media attention as a yardstick for determining the relevance of a scientific field. The discovery of the Higgs boson was certainly one of those events that happen once in a century and represented a milestone for the entire scientific community, not just for particle physicists. Media attention merely crowned this success, which would have been exceptional even without it. This event could not have happened without the thousands of people who built and operated the LHC, together with the ATLAS and CMS experiments that analyse its data, and without the theoretical physicists who provided and continue to provide predictions to understand what this Higgs boson really is. We are talking about thousands of people who made this discovery possible with their contributions. After the discovery of the Higgs boson, first of all we have one more particle to study, whose characteristics are, on the one hand, very well predicted by the Standard Model and, on the other hand, can be linked to physics that we do not know beyond the Standard Model. For now, at the level of precision that the data we currently have allows us to achieve, the Standard Model explains these characteristics very well, and this is certainly a significant success for this beautiful theory. As we acquire more data, the level of precision will continue to improve, and this requires the ability to make increasingly refined theoretical predictions. Finally, for some more specific measurements, even more data is needed in order to draw meaningful conclusions. So I would certainly not say that research has come to a standstill.

The public often asks a legitimate question: what is the point? How do you respond to those who question the usefulness of fundamental research, particularly in high-energy physics?

There are two ways to answer this question. The first is that fundamental physics does have an impact because it requires the development of new technologies, and these technologies prove to be very important for everyone's life. For example, the need to share data from the first accelerators at CERN led to the creation of the World Wide Web. Or the construction of particle accelerators led to the development of increasingly advanced superconducting materials, which are now used in many other fields (magnetic levitation trains, high-field magnets for medical imaging, etc.), or to the fact that some forms of cancer are treated using particle accelerators (we have one in Italy in Pavia). However, it is certainly true that all these spin-offs were not planned, and one might wonder whether it would have been worth it even if they had not happened. One could dismiss the question by saying that the costs of all this are negligible. In 2025, CERN received around 1.5 billion Swiss francs from its member states for all its activities, and since the population of the European Union is 500 million, this amounts to a contribution of 3 Swiss francs per person per year. All this in the face of even possible repercussions. But that is not enough. This brings us to the second way of answering the question: human beings have always been curious about the nature of the phenomena they observe. Think of the observation of the sky or Democritus hypothesising about atoms. There has always been a desire for knowledge of reality that went beyond «What can I use what I know for?». The creation of technology does not distinguish human beings from other animals, but what characterises them is the search for meaning, for a reason. This desire to know is what drives fundamental research, just as the desire for beauty drives an artist to create their works. And in a sense, if we stopped following this desire and giving it space in its various forms (art, science, literature, etc.), there would be little left to distinguish us from animals.