Little Story of Infinity : General Alcazar's hat and a Nobel Prize?!?

Could there be a connection between General Alcazar's hat and a Nobel Prize?!?

Although as surprising as it may seem, the answer is: YES! The Mexican hat worn by the general represents the shape of the potential, one of the properties of quantum fields describing the infinitely small, associated with one of the most famous particles in the world: the Brout-Englert-Higgs (BEH) boson, names after the researchers who proposed its existence.

Let's start below the Alps…

Yes, you read that right! To observe this famous boson, physicists, engineers, technicians,... from all over the world work at CERN, the European Organization for Nuclear Research, which is a gigantic center for research in particle physics. CERN notably houses the Large Hadron Collider (LHC), a circular particle accelerator, with a 27 kilometers circumference, buried under the Alps between Switzerland and France. It is the largest and most powerful accelerator built to date. Equipped with thousands of superconducting magnets, it increases the energy of particles, such as protons, each time they rotate around the accelerator, and thus reach billions of electron-volts!

Let's come back to our boson

The BEH boson and its unique potential explain how elementary particles, such as the electron for example, acquire mass. The solution to this fundamental question was provided almost 60 years ago by several teams of theoretical researchers independently, but it took decades for it to be confirmed experimentally. The announcement of this major scientific discovery was made at CERN 10 years ago today, on July 4, 2012.

Researchers from the Belgian universities of Antwerp, Brussels, Ghent, and Louvain-la-Neuve are studying particle physics and are part of the CMS experiment which, among other things, made this grandiose discovery possible.

Let's meet Barbara Clerbaux, professor at the ULB, who has been involved for nearly 20 years in particle physics.

The discovery of this boson, independently predicted by Professors Brout, Englert and Higgs, celebrates its tenth anniversary. We imagine that the announcement of such a discovery is being prepared for many months. What is your best memory of this period?

Professor Barbara Clerbaux: The discovery was announced during a seminar on July 4, 2012 at CERN. During this seminar, the two independent experiments, CMS and ATLAS, announced that they had seen a signal compatible with the production of the famous Brout-Englert-Higgs boson, for the same mass value at around 125 GeV, i.e. 125 times the mass of the proton. For the record, during the famous seminar at CERN, the researchers of the CMS experiment did not know the details of the results of the ATLAS experiment and vice versa! It was therefore particularly fascinating to live this moment of sharing the results and to see the coherence of the signal.

Another very intense and unforgettable moment was the announcement of the Nobel Prize to Peter Higgs and Francois Englert in 2013. I had the chance to be president of the physics department at ULB during this very special moment, we therefore celebrated this prize within the Faculty of Sciences and the university. These were very moving moments. It should be noted that Francois Englert is an extraordinary personality: brilliant, committed (if not rebellious), and endearing. What also struck me a lot is how everyone in Belgium, and not only the scientific community, was touched by this announcement and felt proud of this very first Nobel Prize in Physics for Belgium. It is necessary to underline the effort made by the journalists and by the population to try to explain/understand what a scalar boson really is... mission (almost) impossible if one does not have notions of quantum mechanics and of special relativity!

What did this discovery represent for you and your team, as well as for your fellow theorists at ULB?

This is of course a source of great pride for CMS experimentalists in Belgium. It is the result of many years of work for all the Belgian teams involved in this extraordinary experience. We have been part of the experiment since its very beginning: we even participated in the construction of part of the detector, namely the tracker, an important one because it detects the passage of charged particles.

From the perspective of our fellow theorists, this was also a major event. It should be understood that Robert Brout and François Englert proposed the famous symmetry breaking mechanism and therefore the existence of a scalar boson - the famous BEH boson - in 1964! Their scientific article appeared at the same time (in fact a little before) as that of Peter Higgs. It will therefore have been necessary to wait 48 years before being able to discover this famous particle!

Pictures of the original papers from 1964. © Prof. Jean-Marie Frère

This delay between the prediction and the observation of the BEH boson is explained by the fact that it is massive and that its production rate is low, meaning it is a rare process! This particle is unstable and to detect it we must analyze the products of its decay, which can take place in a series of different channels. It's a bit like trying to reconstruct the shape of a vase from its pieces after breaking it.

For our ULB teams, it was a bit of a loop that was taking shape: prediction and observation, almost 50 years apart.

In ten years, how has this discovery evolved? Was it the beginning of a new era or rather the answer to a long quest?

Both! It is indeed the answer to a long quest that has reinforced our research since the 1960s and confirmed our understanding of the microscopic world. Thanks to this mechanism introduced by Brout and Englert, we can include the mass terms in our model and thus explain the mass of elementary particles. This was indeed fundamental: we know for example that an electron, which is an elementary particle, has a non-zero mass, but it remains to explain where it comes from! For 10 years now, CMS and ATLAS experimentalists have been studying this one-of-a-kind BEH particle. The CMS and ATLAS experiments reproduce the interactions present in the first moments of the Universe. From now on, we know with precision the mass of the boson, its modes of production, and its most frequent modes of decay. More than a hundred scientific articles have been published by CMS and ATLAS on the study of this scalar boson.

But somewhat paradoxically, this also places us in front of a great difficulty: our model, which describes all current experiences so well and precisely, cannot be complete. Indeed, it cannot explain or describe certain observations such as what is the nature of dark matter? Why is our world made of matter and not antimatter? Why some fundamental particles, such as neutrinos, have a very low but non-zero mass contrary to what our model suggests. These questions are fundamental and we have no answers today. We must therefore go beyond our current understanding, pursue a new quest, and we believe that the scalar boson plays a crucial role in this.

What is your research subject today, the question that interests you?

I have been working on the CMS experience for twenty years already. I worked on different subjects but always around the theme of the search for new physics beyond our current model. At the moment, I am looking for other scalar particles (other than the BEH boson), "exotic" scalar bosons in a way. Indeed, nothing imposes that the boson of BEH discovered is unique and the question is to know if this boson is unique or if there are other scalar bosons in nature. This information is crucial to move forward.

And in ten years, at the celebration of the 20th anniversary of this discovery, what more could we have learned?

Ten years from now, I hope, we will have been able to answer some of these fundamental questions. The CMS and ATLAS experiments will continue to take data between 2022 and 2025: this is the third run of the LHC which begins on July 5!

Then there will be a technical stop and a major update of the machine and the detectors: this is phase 2 of the LHC, it will allow the accumulation of a total of 10 times more data than in the previous phase. The entire CMS tracker will have to be replaced, and once again Belgium is making a major contribution to the construction of this instrument. LHC phase 2 will allow the properties of the BEH boson to be studied in much greater detail and perhaps discover signs of new physics.

But it is not only at CERN that this quest takes place, a whole series of experiments whether in particle physics, astrophysics or cosmology try to highlight this new physics, such as the search for dark matter, understanding the matter-antimatter asymmetry, the study of neutrinos, etc. A new discovery may therefore be imminent!

How about a Calculus boson?

No doubt that Tintin's favorite professor, Cuthbert Calculus, deserved to have a boson in his name! But why don't we talk about particles in Tintin's universe?

Professor Calculus is indeed a nuclear physicist, and in 1947, he even managed to send Tintin and his friends to the moon with an atomic rocket. But at that time, although the first elementary particles had been discovered, such as the electron and the muon (another elementary particle heavier than the electron), quantum theory was at its very beginning! No wonder the professor didn't take the leap from atomic to sub-atomic!

It's your turn!

Do you also want to try to discover the BEH boson at CERN? Download the free game Higgsy, which will allow you to analyze LHC events like CERN physicists!

Glossary:

Elementary particle: indivisible component of matter, fundamental building block of our universe
BEH boson: elementary particle predicted by physicists Robert Brout, François Englert, and Peter Higgs. This work earned the last two the Nobel Prize in Physics in 2013 (Robert Brout having unfortunately passed away before).
LHC (Large Hadron Collider): the most powerful particle accelerator built to date, which has made it possible in particular to highlight the existence of the BEH boson. The accelerated protons collide with each other at 4 specific locations around the accelerator. Four major experiments are installed at these points to observe the particles produced during proton collisions: CMS and ATLAS, but also LHCb and ALICE (read more).
CMS (Compact Muon Solenoid): experiment around the LHC at CERN (read more)
ATLAS: experiment also located around the LHC (read more). The two experiments work independently with slightly different designs in order to be able to provide validation of the results.

Want to know more?

About the author :

Gwenhaël W. De Wasseige is assistant professor at UCLouvain in astroparticle physics. Each month, Gwenhaël will tell us through her Little Story of Infinity the latest news about the universe and the domain of the infinitely small.