CERN Accelerating science

Interview with Guido Tonelli

by Panos Charitos

Guido Tonelli, former spokesperson of the CMS experiment and one of the protagonists of the Higgs boson discovery, discusses the meaning of one of the major breakthroughs of the century, as well as the importance of pushing beyond the current limits of knowledge. He reflects on the present situation in particle physics and the significance of having a vision for the future.​


How do you see the present situation in particle physics following the recent findings of the LHC?

We are living a truly magic moment in physics. With the discovery of the Higgs boson we have closed a chapter that was opened approximately 50 years ago. We have found the last missing piece and the Standard Model of fundamental interactions is now complete. Still, while we celebrate another triumph of this successful theory, we are left with a long list of open questions, whose answer would probably require the emergence of new paradigms.


Why the discovery of the Higgs boson was so important? 

We have understood the specific mechanism chosen by nature to break the perfect symmetry between electromagnetic and weak interactions. We can now reconstruct the very special moment, in the early life of our Universe, 10-11 seconds after the Big-Bang, when a new scalar field occupied every corner of the immense object originated from a tiny fluctuation of the vacuum. The mechanism with which the Higgs boson forever separated electromagnetic and weak interactions and assigned a well-defined mass to elementary particles is now understood. It allows light quarks and gluons to form stable protons and attract electrons, therefore generating the first atoms. The current shape of everything, including us, was somehow determined in those moments. The evolution of matter eventually led to the formation of galaxies, stars and planets, including a very special planet located in a comfortable corner of our solar system, where several forms of life became possible.

Do you think that we have reasons to search beyond the Standard Model of particle physics? 

We are very proud of the success of the Standard Model, but we are also aware that it does not include particles or forces responsible for dark matter and dark energy. It does not explain the dynamics of inflation, neither is it able to consistently unify the major interactions. Last but not least, it does not include gravity. We already know that sooner or later we shall be forced to abandon the Standard Model as a general theory in favor of a new, more complete description of nature. The beauty of our job is that this could happen anytime. It could happen tomorrow, if the analysis of the current LHC data unveils new states of matter, but it could also take years of efforts or perhaps a completely new generation of accelerators.

How should one proceed in trying to address today's open questions in high-energy physics?

We have to follow two independent routes: direct searches and precision measurements. With the LHC running at 13TeV, we are going to explore a completely new region that could hide very heavy particles. So far, we have explored the region up to ~1TeV and within the next twenty years we could extend this reach by factors. Furthermore, in the high luminosity regime, the LHC will produce a huge amount of Higgs bosons that could be studied in great detail. For the moment, we have only measured the major characteristics of the newly discovered particle and the errors are still large. The Higgs boson is a unique object that couples to all particles, including, possibly, some of the very massive ones foreseen in supersymmetry or extra dimensions models. If they exist, and they will be too heavy to be produced directly, they can be "seen" indirectly, by measuring deviations from the expected properties of the new scalar boson. Statistics matter a lot, since we will need to produce tens of millions of Higgs bosons to improve precision and study very rare decays that could hide anomalies. It must be noted that the heavier the unknown particle interacting with the Higgs, the smaller the effect on its properties. This is why now is the time to start looking beyond the LHC.

FCC is the right project to coherently address these issues. I consider it a sort of dream machine. Imagine first the FCC-ee option, where we could perform the most precise measurements of all parameters of the Standard Model, stressing it to the extreme limit. Consider, then, the 100TeV pp collider. For the first time, we could study the Higgs away from the stable equilibrium in which it has been for 13.8 billion years. With the new machine, the electroweak phase transition could be studied in detail and we could measure the Higgs self-coupling, one of the most important parameters for checking the evolution of its potential. We would also have the opportunity to directly explore new states of matter, up to masses of tens of TeV. Nature might have surprises for us in this new energy regime, but this is not granted. It is certain, however, that after FCC our conception of matter will not be the same as today.

Is it timely to think ahead and design future colliders that could extend the present intensity and energy frontier? 

It is urgent to start thinking forward to the next generation of colliders. The very first discussions on the LHC started in the mid-1980s and the accelerator was completed in 2008. If we want a new machine at the end of the high luminosity phase of the LHC, the time to act is now. The CERN management has been very wise in launching the FCC study group. Since 2014, we have been attracting a growing interest in this initiative. Hundreds of scientists are making enormous progress, both in addressing the most important physics cases and in finding solutions to many technological challenges.

How would you describe the relation between scientific and technological developments?

Without fundamental physics, there can be no serious technological developments. Think for a moment of laser or microelectronics technology. Do you think they would have been possible without Maxwell's equations and quantum mechanics? When interviewed by journalists on this subject, I sometimes joke about the BRB, the Big Red Button. Let's suppose that we could have it somewhere, say at CERN. Its function would be to switch off all major physics discoveries of the last 150 years, erasing therefore their applications in modern society. Whenever politicians or the general public would start questioning the economic impact of investments in research, it would be enough to switch off the BRB for a few moments. I am sure that, within hours, the entire world would line up in front of CERN's doors offering money and asking physicists to continue their research. 

Guido Tonelli was one of the speakers of the public event "​Macchine per scoprire dal Bosone di Higgs alla Nuovo Fisica​" that was held in Rome during the 2016 FCC Week.


Do you think that recent data from the LHC and experiments in other fields, i.e. astrophysics, cosmology, could offer a new understanding of the fundamental theories of particle physics?

We are a lucky generation of scientists: in a very short period of time, we have witnessed two major breakthroughs in physics. They will probably influence our field for decades. The first was the discovery of the Higgs boson at the LHC in 2012. The second was the recent detection of gravitational waves by the LIGO interferometer. It looks like we now have two new powerful tools to better understand the fine structure of our Universe. The Higgs boson could be the portal to discovering new particles or new interactions at the microscopic scale, but it could also provide hints about cosmic inflation. It is well known that a scalar particle could produce the very special potential that could lead to inflation. The Higgs is the first fundamental scalar particle observed in nature so far. Did it play a role in the very early stage of the evolution of the Universe? To carefully answer this question, we would probably need better measurements of the Higgs and a new round of precision astrophysics measurements. The recently discovered gravitational waves could be a fantastic new tool to explore these very first instants of our Universe.

Could we be in the middle of a revolution in physics that calls for a change of scientific paradigm?

This is the secret hope of every scientist. I am particularly puzzled by gravity. It is frankly embarrassing that the most popular of the major interactions is still such a mysterious subject. The extreme weakness of gravity makes the effort to unify it with the other interactions very difficult, not to mention the long-lasting attempts of building a convincing quantum description of it. The in-depth understanding of gravity will likely be the target of the next generation of gravitational waves experiments. The challenge will be to improve, by orders of magnitude, the sensitivity of the current interferometers. High-energy physics at accelerators could also play a role in carefully testing the theoretical models based on additional spatial dimensions or in discovering new behaviors of gravity at a very small scale. I am sure that a breakthrough in understanding gravity would imply a change of paradigms in physics similar to the ones that marked the beginning of the 20-th century. 

​Why did you choose to study particle physics and what would be your advice to young scientists?

Curiosity and passion were my main driving forces. I studied in Pisa and I still remember the feeling of entering rooms on whose the walls hang original drawings of Galilei or the front page of Fermi's thesis. Each student in Pisa feels the strength of a powerful tradition that has continued through strong personalities, like Bruno Pontecorvo or Carlo Rubbia. I have been very lucky, when I was student, in having met some incredible professors. I remember, in particular, Adriano di Giacomo and Lorenzo Foà. They were excellent mentors, with the special gift of widening the horizons of each young student interacting with them. Furthermore, they both showed a huge confidence in young people. I entered the field, as many other colleagues, thanks to their encouragement. The advice I can give to the new generation of young scientists is very simple: follow your passion and try to make your dreams come true. I don't know if you will succeed, but I am sure that you'll never regret it.

In an age of turbulence like the present one, why do you think it is important to continue investing in fundamental research? 

Investments in fundamental research are the answer to the current economic crisis. In the new order that will emerge from this turbulent period, the hierarchy will be defined by the role of each country in producing knowledge and innovation. Countries taking the lead in these areas will guide the world for the whole century. If Europe wants to continue playing an important role in a planet that is becoming more and more competitive, it should secure its current leadership in high-energy physics. The right move would be to soon launch FCC as a new, global project. With new actors, like China and Japan, trying to take center stage, every delay or hesitation could be extremely dangerous.

Finally, I would like to ask you about your new book and what motivated you to write it? 

I have written the book The imperfect origin of everything mostly to tell our story to young people, with the secret hope that some of the readers will discover the same passion in them. It is the story of my generation, of people that were young scientists in their 30s when they started discussing the idea of building the LHC, the new, fantastic accelerator that would discover the Higgs boson. The book describes the many crises we have faced to build this marvelous machine and its challenging detectors, but also the deep emotions of the last months of this long-lasting hunt. The last part deals with the many open questions in physics and the new challenges facing our field today, including the FCC initiative. Quite surprisingly, the book has become a best seller in Italy. Actually, in the last two years, several physics books written for the general public have become very popular in our country. This has therefore generated serious attention by the media, which regularly cover all major events and report on the most significant scientific results. It is a sort of magic moment for physics that has already translated into a significant increase of physics students in Universities. Since I am always optimistic, I do hope that this trend will continue to grow and involve the government, yielding new resources to the field.


Guido Tonelli is professor of Physics at the University of Pisa, former spokesperson of the CMS experiment and the author of "LA NASCITA IMPERFETTA DELLE COSE – THE IMPERFECT BIRTH OF THINGS" (Rizzoli Publications).

The big rush to the God Particle and the new Physics that will change the world. The discovery of the Higgs boson as told by one of its protagonists. An accomplishment that will revolutionize the idea that we have of ourselves and of the world around us. “Everything is precarious. The human condition is as fragile as the gigantic structures of the universe, even those that seem immortal.” At that precise moment, a hundredth of a billionth of a second after the Big Bang, our destiny was decided. In a universe where matter and antimatter were equivalent, and that therefore could have gone back any moment to being pure energy, a slight preference of the Higgs boson for matter instead of antimatter can have been enough for the world as we know it to be produced. “Here is the flaw, the subtle imperfection that gave birth to everything. An anomaly that gives rise to a universe that can evolve for billions of years.” If everything comes from there, we need to understand in each detail that crucial moment, rebuild it frame by frame, in slow motion and from different angles. For this reason, at CERN in Geneva, LHC was created: the most powerful particle collider in the world, the place most similar to the first moment of life of the universe that man has been able to build. For this, for years, the world’s best physicists have been working day and night, at the four corners of the planet. This is how the “God particle” was captured. And this is why the studies continue, to better understand how all this was born and how our story will end: if in the cold and in the dark, or in a cosmic catastrophe that would give us the privilege of a much more spectacular exit. Guido Tonelli is one of the protagonists of this great adventure, one of the leaders of this army of visionaries. Here, with the attitude of an explorer, he tells what it means to push beyond the extreme limit of knowledge, what it means to make the discovery of the century on the day of one’s birthday, what it means to understand how it all began and will probably end.


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