All the world is built out of 17 known elementary particles. Carlo Rubbia led the team that discovered two of them. In 1984, Rubbia shared the Nobel Prize in Physics with Simon van der Meer for their “decisive contributions” to the experiment that, the year before, had turned up the W and Z bosons. These particles convey one of the four fundamental forces, called the weak force, which causes radioactive decay.
Rubbia ran the experiment, called Underground Area 1 (or UA1), a bold and ambitious project at CERN laboratory near Geneva that sought traces of W and Z bosons in the chaos of high-energy particle collisions. Hundreds of billions of protons and antiprotons were accelerated close to the speed of light and then smashed together. At the time, antiprotons — which have a habit of rapidly destroying themselves when they come into contact with matter — had never been produced in abundance. Some of Rubbia’s peers favored alternative collider and detector designs, believing that antimatter was too volatile to be controlled in this way.
“We had zillions of different ideas. There was a lot of competition, but you cannot do two things at the same time,” Rubbia said. In the end, UA1 prevailed, and delivered.
More than three decades later, particle physics once again finds itself at a crossroads. A decision looms about which big particle-collider experiment to build next — if indeed one is built at all. While CERN’s Large Hadron Collider (LHC) has performed flawlessly, its collisions have yielded no signs of new particles beyond the expected 17, whose properties and interactions are described by the Standard Model of particle physics. This model makes incredibly accurate predictions about those particles’ behavior, yet it’s also understood to be an incomplete description of our world. It fails to include the gravitational force or dark matter — the mysterious substance that astronomers consider to be about five times more abundant than normal matter — or account for the universe’s matter-antimatter imbalance. Moreover, many theorists feel uneasy about the Standard Model’s inability to explain its own basic truths, such as why there are three families of quarks and leptons, and what determines the particles’ masses.
Rubbia, who at 85 remains at the forefront of the field, isn’t fazed by the absence of “new physics” in the LHC data. He urges his peers to press on in search of more and better data and to trust that answers will come. The Higgs boson — the 17th piece in the Standard Model puzzle — materialized at the LHC in 2012, and now Rubbia wants to explore its characteristics in depth with a state-of-the-art “Higgs factory.”
How best to do this is still up for debate, with competing designs ranging from a circular electron-positron collider 100 kilometers in circumference to a plasma wakefield accelerator, a tabletop experiment in which electrons “surf” on a wave of rapidly accelerating plasma. To Rubbia, the choice is clear: An innovative muon collider, he says, could produce thousands of Higgs bosons in clean conditions at a fraction of the time and cost of other experiments. Muons are simple like electrons but far heavier and thus capable of higher-energy collisions. Critics say such a machine is still far beyond our current technical abilities. But while it may be a technological moonshot, a muon collider offers the prospect of a precision instrument that could also potentially turn up evidence of new particles beyond the Standard Model’s.
Rubbia has spent most of his long career at CERN, including a five-year stint as director-general beginning in 1989. He has also taken a leadership role at Gran Sasso National Laboratory in his native Italy, which is looking for signs of the decay of the proton. (If seen, this would also offer clues about physics beyond the Standard Model.) An engineer and constant inventor, Rubbia has spent part of the last three decades pursuing radically novel energy sources — such as a nuclear power reactor that is driven by a particle accelerator.
Quanta caught up with Rubbia last month at the 69th Lindau Nobel Laureate Meeting in Germany. There, he addressed hundreds of young scientists from around the world, making the case for a muon collider as the best bet for learning more about the universe’s fundamental building blocks. A sharply dressed man with piercing blue eyes, he spoke with zeal, both onstage and off. The interview has been condensed and edited for clarity.
You discovered the W and the Z bosons. Why was this an important discovery?
Ha! I’ve never heard such a question! Particle accelerators are an essential part of the scientific program, which is fundamentally curiosity driven. And the discovery of the W and the Z bosons was one conclusion in the very long history of particle physics. There are particles of matter, like quarks and leptons, and those were reasonably well settled experimentally, but the question of the forces — that is, the particles which mediate the interactions between particles of matter — was something yet to be understood.
Now, the W and Z were postulated and discussed by many people, but the experimental realization required very high energies — for the time, at least. Don’t forget these fundamental choices are coming from nature, not from individuals. Theorists can do what they like, but nature is the one deciding in the end.
So how did you create such high energies?
We first of all had to learn how to construct a colliding beam machine instead of having a single accelerator. And so we modified an existing circular accelerator at CERN so that particles and antiparticles could be injected.
The question of accumulating antiprotons was a serious problem because antiprotons were barely discovered at Berkeley some years before — and they only made a handful of particles. Here we needed to make a hundred billion particles every morning. Not only that, but we had to cool them by an enormous factor so they could fit inside the circular accelerator. I have to acknowledge the enormous help and support of Léon Van Hove, John Adams and others; without them it would’ve been impossible.
After a few years, we eventually entered the energy domains of the W and Z, and indeed they were there! However the proton-antiproton collisions were very complicated collisions; a lot of other interactions were happening, so we went further. We transformed the CERN facilities from a proton-antiproton collider into an electron-positron collider, and we built a new ring: the 27-kilometer Large Electron-Positron Collider experiment. This produced tens of thousands of W and Z bosons, all in perfect, clean conditions. It led to more Nobel Prizes and completed the story of the W and the Z. Now of course, there’s one more piece missing, which is the Higgs boson.
But haven’t we already found the Higgs?
Yes, that was six years ago. The question now is: How can we produce Higgs bosons abundantly, in clean conditions? And that is requiring novelty.
The Higgs is the first and only scalar particle — meaning it has only size and no direction — that we have amongst the basic forces of nature. Every particle has a different story, and therefore this has to be studied and understood on its own. Unlike the other forces, the Higgs field has no preferred direction and looks the same when you reflect it in a mirror. Understanding it is at least as important as the observation of the W and the Z, and this will conclude the story of the elementary particles in the Standard Model.
Not everyone agrees that time and resources should be focused on a “Higgs factory”; they say pushing to the next energy frontier to search for new particles should be a priority.
You could build a circular machine three times the size of the Large Hadron Collider to collide electrons and positrons; you could upgrade the LHC, or even build a next-generation linear accelerator. Probing higher energies offers the hope of new physics — it could be supersymmetry, it could be something else, I don’t know what. But before exploring higher energies, it makes sense to me to build a muon collider, and to clarify the question of the Higgs first. Here we already have a particle that we want to explore. We may even find signs of new physics by studying the Higgs very precisely. For that we don’t need to go to a 100-kilometer-around tunnel. Think about how many days it takes to walk 100 kilometers! And it all has to be extremely functional, every single piece has to work — it’s a miracle if people succeed in making it work.
Remember, in the 1950s, Enrico Fermi said that by the year 2000, the accelerator ring would have the circumference of the Earth. It’s an absurd statement, of course, but there’s a point: Do we direct resources for the realization of mastodonic, gigantic devices, which might be achieved but will take 20 to 30 years? It took us 10 billion euros and 20 years to discover the Higgs particle. And so if you want to go further, it’s going to be more costly and more complicated.
There’s no doubt that there’s a solution: creating muon pairs. A muon experiment is a small ring, which is one-hundredth the size of the LHC. It can be done in existing accelerators: Both CERN and the European Spallation Source can produce enough protons to make a sufficient number of muons.
You make it sound easy! Isn’t it still very difficult to create a narrow beam of muons —that is, to “cool” them — so that they can be used in an existing particle collider?
Yes, there are huge challenges, but in my view there is nothing major which represents substantial risk. We have proposed a so-called initial cooling experiment which is done on a very small scale, in which we’ll start to build all the basic ideas. It requires a lot of tests and verification of the behavior of muon cooling, but it can be done in a few years.
Then to go from that to a big machine is something which can be done with conventional technologies. And it can be done with relatively small cost — by smart people of course — in a reasonably short time. There’s a lot of work to do, but what’s wrong with doing new work to improve things?
Given that no new particles — aside from the anticipated Higgs boson — showed up at the LHC, what are the chances that the next accelerator will uncover new physics?
Actually, there are other interesting experiments, not just accelerators. The neutrino experiments going on at the South Pole, for example, are becoming a new alternative to making bigger and more complex accelerator systems. And I think competition between the two is very productive; it will create the results of years to come.
I’m a bit concerned that the future of particle physics at CERN does not involve, so far, any new alternative after the termination of the LHC program. When I was responsible for the activities at CERN, whenever we had one machine, we had the next one coming. We need to have more courage, and collectively agree on alternatives.
How did growing up under fascism and living through the Second World War influence your life?
You don’t know what Europe was then. When I was 4 years old, I remember I had a radio that my father had constructed — radios then were very complicated systems with antennas and everything else. And we heard Hitler shouting on the radio. And then the war came: 88 million people were killed in a few years. We were all exposed to this terrible situation, a tremendous thing which left us completely at zero — and then we had to rebuild.
Yet you seem to take an optimistic attitude in all your work.
Oh yes, I’m very optimistic. You couldn’t afford not to be optimistic. Optimism was the most important thing after the vicissitudes of such a complicated history, and Europe has advanced tremendously since then. The integration of Europe through science is phenomenal. This is a very important success, passing from single countries — hating each other, fighting each other, at war with each other — to a situation where there is a complete consensus within Europe.
You spent a couple of years doing research in the United States before moving to CERN in 1960. What drew you back to Europe?
I wanted to work at CERN because I was not ready to totally give up my European nature and become an American citizen. A lot of European colleagues of mine moved to America and became lawful, perfectly acceptable American citizens. But I like to do things my own way, and so I wanted to come back, because I felt that Europe was a place where progress was possible. Indeed, particle science over the last few decades has been European.
Aside from particle physics, you’ve also been heavily involved in pursuing sustainable energy technologies. Why is that?
During my lifetime the population of the planet has gone up by a factor of three and a half, but the primary energy use has gone up by a factor of twelve — and there’s no way that the children who are born today will afford another twelve times on top of us. And so sustainable energy is a crucial problem, and I find it very exciting to see that there are new methods which can allow us to solve it.
What novel method would you put your money on?
We have renewable energies and fossil fuels, a little bit of nuclear too. Out of these, natural gas is extremely abundant: We have normal gas, but we also have shale gas, and we have clathrates — which maybe you don’t know about. They are found in the depths of the ocean, a combination of natural gas and water, and they are ten times more abundant than conventional natural gas. And so we have enough natural gas for thousands of years.
Now of course, natural gas produces carbon dioxide, and that’s the biggest worry that everyone has today. So how can you go ahead and produce these things without carbon dioxide emissions? Well, we’ve developed methods which allow us to prevent carbon dioxide emissions by taking the methane and, instead of producing carbon dioxide, producing black carbon and hydrogen. In principle this decomposition occurs at a very high temperature, about 2,000 degrees Celsius, which is impossible to use, but with our new methods with these new metals it can be done at 1,000 degrees.
You often seem to push for leap-change technologies.
Experimental physics is founded on curiosity-driven observation. You should never do what other people do. You have to do something unique because it allows you to make your own mistakes and to modify things — to change your mind 25 times before coming to the right solution. This is the kind of pragmatic way I have of proceeding in this field.
Here at Lindau you’re speaking to hundreds of young scientists about your work and your ideas for the future. What do you hope to pass on to them?
Young scientists have enough to do themselves without my opinion! We had the right to drive our own future, and they will have the right to master their own future. I can listen to them, but it’s not up to me to tell them what to do.
Given the current crisis in particle physics, and the huge hurdles humanity faces in creating a sustainable future, are you still an optimist?
I’m as optimistic today as I was in the past. The discussion is complicated, the choices are difficult, but so far over my long lifetime I’ve always seen the results to be positive. And so I’m sure a solution will also be found this time.