January 19, 2021

"Doing physics today is like going down to the river and turning all the stones to see what appears" | Science

"Doing physics today is like going down to the river and turning all the stones to see what appears" | Science


The standard model of physics is a marvel of modern science. With 17 elementary particles, it describes the composition and behavior of the universe with a demonstrated reliability in the great physics experiments of the last decades. However, it is an incomplete theoretical framework: it does not include the force of gravity described by Einstein, it does not explain the composition of dark matter detected by cosmologists, and it did not predict that neutrinos – extremely light but non-disembodied particles – would have mass.

The scientist of the European Laboratory of Particle Physics (CERN) Marzio Nessi (1957, Muralto, Switzerland), who comes to Madrid to participate in the IV Cycle of Conferences organized by CERN and the BBVA Foundation, has been unraveling the mysteries of the standard model for years. Between 2000 and 2013 he was director of the Atlas experiment, the particle detector that found the Higgs boson between the debris of impacts between protons carried out by the Large Hadron Collider. Now, Nessi manages CERN's Neutrino Platform, which plays a fundamental role in neutrino research worldwide thanks to the construction of protoDUNE, two particle detectors of one thousand tons each. These instruments are miniature prototypes of four neutrino detectors that will be built in the US to find physics beyond the standard model and explain mysteries such as the asymmetry between matter and the antimatter of the universe.

Question. The Large Hadron Collider (LHC) was built to find the last piece of the standard model of particle physics, the Higgs boson. Now it takes years looking for physics beyond the standard model, but has not found anything. Is it time to consider new and radical approaches in the experimentation of particle physics?

Answer. Actually the LHC was built to find new physics. Complete the standard model [con el bosón de Higgs] it was only the first step, but in the long term it was always to find physics beyond. The main theory that studies is supersymmetry, but for now it is still a puzzle. We have looked for supersymmetry in all the possible configurations that occur to us, starting with the easy and then looking for more specific situations, but at the moment we have not found anything. However, there is still a lot of mathematical space to study: we are only at the beginning and we want to make analysis with more statistical power. In addition, we also look for more accurate measurements of the Higgs boson itself.

It is the dream of every physicist to describe all the forces with a single formula. In addition, the theory of supersymmetry also explains gravity, which is not included in the standard model. Solve many problems

P. What is the theory of supersymmetry?

R. In the standard model there are particles that describe the matter and particles that describe the forces. The particles of matter [fermiones] they have a concrete behavior, they can form atoms, and they have another characteristic, the quantum number – also called spin, something like the angular momentum of their rotation -, with a value of ½. The particles of forces have another behavior and their spin has a value of 1 or 0; They are called bosons. All the machinery of the standard particle model has been built around the concept of symmetry: if something works, its inverse should also work. Thus, if you invert the electric charge of the particles, everything should work the same. The last level of symmetry is the quantum number: we should be able to add ½ to the spin of the particles of matter and ½ to the spin of the force particles: the former would become bosons and the latter, in fermions.

If you add these particles [supersimétricas] to the calculations, many complicated mathematical elements are canceled that the physics is not able to explain, and everything becomes very clear. For example, by adding these particles all the forces of a given energy become equal. It is the dream of every physicist: to describe all the forces with a single formula. In addition, the theory of supersymmetry also explains gravity, which is not included in the standard model. Solves many problems, so for 20 or 30 years, it is the favorite theory.

P. Does not the desire to make the theory true with scientific objectivity interfere?

R. Maybe it's naive, or the fact that we find beauty in a symmetrical world. It may be a limitation, but it is the way our brain is constructed.

ProtoDUNE experiment
ProtoDUNE experiment

P. Physics experiments usually look for a very specific signal that predicts theoretical calculations. However, Atlas, the main experiment of the LHC that you directed for years, announced a change of strategy A few months ago: now analyzes data from all the accelerator collisions to look for any new pattern. How can you find something new without knowing what you are looking for?

R. It is true that we are living the physics of unknown 'strangers'. The Higgs boson was an unknown 'acquaintance': although not everyone was convinced that it was going to appear, we had a theoretical idea that guided us. Now a theory does not guide us and we do not know what we are going to find. It's like going down to the river and turning all the stones to see what appears. We have many minds, especially young minds, to make sure we analyze all the data we collect. Artificial intelligence is also used to look for patterns in the data. But there are problems: a couple of years ago, we found an intriguing signal that turned out to be a statistical fluctuation, but people got excited. Hundreds of theories appeared to try to explain the phenomenon.

P. There is a wild competition in science for being the first to publish the theory.

Neutrinos must have mass because they oscillate: they change flavor [clase de partícula] when moving. But what we see is not reality, it is what nature wants to teach us

R. Exactly, that is the difficulty. But it is part of normal life in any society. People want recognition for their work. Especially theoretical physicists, they have to demonstrate only with their brain, pencil and paper that they are right, and they have to fight to defend their point of view. That is why experimental physics is so important: it has to be the guide that says "this theory is, it is outside, it is also outside". In the LHC happens all the time: theories are destroyed one after another and people have to start over.

P. The discoveries that have been made about neutrinos recently, such as the fact that they have mass, they are already outside the predictions of the standard model. Does this have anything to do with supersymmetry?

R. We do not know. Neutrinos must have mass because they oscillate: they change flavor [clase de partícula] when moving. But what we see is not reality, it is what nature wants to teach us. Below is another level of reality, the mass eigenstates [atributos fundamentales de los neutrinos], we do not know. They are mixed according to energy, distance, factors like that, and what we see is a combination, but not the underlying motive. We know how neutrinos behave because we have ways to define the parameters that describe their behavior, but why they behave like that, what are those background characteristics … that's another story. We could be seeing only the surface of something much more complex.

ProtoDUNE experiment
ProtoDUNE experiment

P. You direct CERN's participation in the deep underground neutrino experiment (DUNE) that is being built in the United States. Will it teach us something about these basic and unknown attributes of neutrinos?

R. DUNE is an experiment to study the oscillation of neutrinos at very large scales, with a very powerful beam of neutrinos. We will observe the oscillations and describe them, studying all their parameters. Some of these parameters are known and others are unknown: for example, we could observe the phenomenon that explains the asymmetry between matter and the antimatter of the universe, or why we are all made of matter.

P. DUNE is a huge experiment in basic science, but also, can that knowledge have practical value?

Each protoDUNE tank weighs a thousand tons and contains liquid argon at 184 degrees below zero. It was a necessary step to demonstrate that it will work. ProtoDUNE works perfectly, but you will never see neutrinos.

R. It's like the Higgs boson. You can not tell anyone that the Higgs boson is going to be relevant in your life. It is pure knowledge, which means knowledge for the future. But there is also a need to develop new technologies that could find unexpected applications in the civilian world. DUNE has several scientific objectives: with its huge subterranean detectors of neutrinos, we can also see if the protons disintegrate, we can understand the explosion of a supernova because it emits neutrinos, and the oscillation experiment can tell us why there are three families [sabores] of particles. There may be a fourth type of neutrino and we would have to reevaluate our model of the whole environment. The question is how much value each one gives to that fundamental knowledge.

P. What is the goal of the newly built prototype at CERN, protoDUNE?

R. Since DUNE is such a large experiment – it will use four detectors of 20,000 tons each, buried a mile deep – we thought it would be stupid and risky to build something like this without taking some intermediate steps. With protoDUNE we have shown that we can build and operate detectors of this type. Each protoDUNE tank weighs a thousand tons and contains liquid argon at 184 degrees below zero. It was a necessary step to demonstrate that it will work. ProtoDUNE works perfectly, but you will never see neutrinos. In the spirit of globalization, we decided not to build our own neutrino experiment at CERN to concentrate all our efforts on the international project in the US. Now we are going to open a call for proposals: we want the prototypes to be used to make physics. They are huge and precious detectors, maybe someone has a good idea to use them in the search for dark matter or something like that.

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