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10 years since the Higgs boson was discovered

The historic discovery of the Higgs boson at the Large Hadron Collider (LHC) exactly ten years ago and the progress made since then has enabled the scientific community to make huge strides in our understanding of the universe

4 July 2022

Exactly ten years ago, on 4 July 2012, the ATLAS and CMS collaborations at CERN's Large Hadron Collider (LHC) announced the discovery of a new particle with characteristics consistent with those of the Higgs boson. The discovery was a milestone in the history of science and attracted worldwide attention. A year later, François Englert and Peter Higgs were honoured with the Nobel Prize in Physics, because already in the 1960s, together with the late Robert Brout, they predicted the existence of a new fundamental field, known as the Higgs field, which fills the universe, manifests itself as the Higgs boson and gives mass to elementary particles. 

"The discovery of the Higgs boson was a monumental milestone in particle physics. It marked the end of a decades-long research path and the beginning of a new era of studies focused on this very special particle," explains Fabiola Gianotti, Director General of CERN and spokesperson for the ATLAS experiment at the time of the discovery. "I remember the day of the announcement with emotion, a day of immense joy for the world particle physics community and for all the people who worked tirelessly for decades to make this discovery possible.

What has it been like so far?
The new particle discovered by the international ATLAS and CMS collaborations in 2012 looked a lot like the Higgs boson predicted by the Standard Model. But was it really that long-sought-after particle? As soon as the discovery was made, ATLAS and CMS began to investigate in detail whether the properties of the particle they had discovered actually matched the characteristics predicted by the Standard Model. 
Using data collected during the decay of this new particle into two photons, the experiments have shown that this new particle has no intrinsic angular momentum, or quantum spin, which matches the predictions of the Higgs boson from the Standard Model. In contrast, all other known elementary particles do have spin.

Moreover, by observing that Higgs bosons are produced and decay into pairs of W or Z bosons, ATLAS and CMS confirmed that the latter acquire their mass through their interactions with the Higgs field, as predicted by the Standard Model. 

The experiments have also shown that the top quark, the bottom quark and the tau lepton, which are the heaviest fermions, obtain their mass through their interactions with the Higgs field, again as predicted by the Standard Model. These observations also confirmed the existence of an interaction, or force, called the Yukawa interaction, which is part of the Standard Model, but is different from all the other forces in the model: it is mediated by the Higgs boson and its strength is not quantized, i.e. it is not given by multiples of a certain unit.

ATLAS and CMS measured the mass of the Higgs boson to be 125 billion electron volts (125 GeV), with an impressive precision of almost one per thousand. Knowing this value is important, because together with the mass of the heaviest known elementary particle, the top quark, and other parameters, the mass of the Higgs boson can help us determine the stability of the vacuum in the universe.

Front row, fourth person from the left: Teresa Rodrigo, at the time IFCA researcher and chair of the CMS Collaboration Board. / CPAN

There is still a lot of research ahead
What still needs to be learned about the Higgs field and the Higgs boson ten years on? A lot. Does the Higgs field also give mass to lighter fermions or could there be another mechanism at play? Is the Higgs boson an elementary or composite particle? Can it interact with dark matter and reveal the nature of this mysterious form of matter? What generates the mass of the Higgs boson? Does it have 'twins' or 'relatives'? 

Finding the answers to these and other questions will not only contribute to our understanding of the universe on its smallest scales, but may also help us unlock some of the biggest mysteries about the universe as a whole, such as how it came to be the way it is and what its ultimate fate might be. "The Higgs boson itself may point to new phenomena, including some that could be responsible for dark matter in the universe," says CMS spokesperson Luca Malgeri. "ATLAS and CMS are conducting many searches to probe all kinds of unexpected processes involving the Higgs boson."

While answers to some of these questions could be provided by data collected during the LHC's next imminent Run 3 or in later periods of accelerator operation, answers to other puzzles are believed to be beyond the reach of the LHC, requiring a future "Higgs factory". For this reason, CERN and its international partners are investigating the technical and financial feasibility of a much larger and more powerful machine, the Future Circular Collider (FCC). "High-energy colliders remain the most powerful microscope we have to explore nature at the smallest scales and discover the fundamental laws that govern the universe," says Gian Giudice, head of CERN's Theory department. 

Spanish perspective on the discovery of the Higgs boson
The Spanish research community played, and continues to play, a very important role in CERN's ATLAS and CMS collaborations, the experiments that announced the Higgs boson sighting on 4 July 2012. 

Since the start-up of the ATLAS detector, researchers from the Institute of High Energy Physics (IFAE), the Institute of Corpuscular Physics (IFIC), the Institute of Microelectronics of Barcelona (IMB-CNM) and the Autonomous University of Madrid (UAM) have been participating in it. On the other hand, the national presence also stands out in the CMS programme since the beginning of this experiment. Groups from the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Instituto de Física de Cantabria (IFCA, CSIC-UC), Universidad Autónoma de Madrid (UAM) and Universidad de Oviedo (UO) stand out. More recently, researchers from the Instituto Tecnológico de Aragón (ITAINNOVA) and the Centro Nacional de Microelectrónica (CNM) have joined the CMS collaboration.

Carlos Lacasta, researcher at IFIC, and Celso Martínez, researcher at IFCA, are currently the Spanish representatives of ATLAS and CMS, respectively. This position is designated as National Contact Physicist and the person in charge acts as the contact between the collaboration (ATLAS or CMS, in this case) and Spain. In the following lines, Lacasta and Martínez tell CPAN what the discovery of the Higgs boson meant and how this milestone was experienced in Spain.

The discovery of the Higgs boson: a perennial news story
"The discovery of the Higgs boson is news that will be kept alive for a long time: it will always be remembered that in July 2012 CERN showed us that a 125 GeV particle, very similar to the Higgs boson, had appeared in two different experiments, ATLAS and CMS, and there were even two of the three people who had thought of this boson almost 50 years earlier, Englert and Higgs," explains Celso Martínez. "For everyone it was very important, but for Teresa Rodrigo it was something very special, as she was the spokesperson for the CMS Collaboration Board," he adds, remembering his late colleague and IFCA researcher.

"The discovery of the Higgs boson is and has been very important for several reasons. The first and fundamental one is that it is a type of particle that has never been seen in a detector until now. In addition, we think it is a fundamental particle, i.e. it is not made up of other particles. We have discovered something that, although it had been "announced" for a long time, is really new and we need to focus on finding out the properties of this new particle," says Carlos Lacasta. And he continues: "The second reason is its own history. It was predicted back in 1964 to solve a problem that theoretical models had in calculating the numerical values of the observables that we could measure in experiments. Introducing the Higgs boson allowed those calculations to be made. Now, it had to be found and, with the knowledge and technology that we had at the time, it was already clear that this was not going to be an easy task".

Not an easy road
"After having searched - unsuccessfully - for the Higgs boson at the Large Electron Positron Collider (LEP), the predecessor of the LHC, we knew that something had to turn up at the LHC. And so it did: at the end of 2011, we saw that there was 'something' to pull on in the 125 GeV region. The meetings in which we shared our Higgs boson-related analyses became more and more frequent, until in the last few months they became daily. In the end we knew that the Higgs boson was there.... there was no other solution," explains Martínez.

Lacasta stresses the technological development needed to find this elusive boson: "The search for the Higgs boson was by no means easy. In fact, it took 50 years to find it. The Higgs is a particle predicted by a theoretical model that is unable to predict what mass the particle has, and so we had to start looking for it without knowing what energy the accelerators had to have to produce it. To give you an idea of the technological challenges that have had to be faced, I will tell you that of the 50 years it has taken to "find the Higgs boson", 30 have been devoted to the design and construction of the accelerator (the LHC) and the detectors (ATLAS and CMS) that first spotted it".

A singular and inexplicable emotion
"During the discovery there was a lot of emotion, the result of a titanic effort at international level not only to develop the technologies that made it possible, but also to 'manage' the international collaborations themselves, whose number of participants has grown steeply in recent decades. From Spain it was also very special. The Spanish research community had played very important roles in some of the aspects of the design, construction and operation of the detectors and in the generation of algorithms to search for and find among the millions of events produced those in which there could be a Higgs boson," says Lacasta.

"The meetings, analyses, counter-analyses and the actual Higgs boson candidates, which were checked one by one, kept us very busy in 2011 and 2012, but we were very excited. Discovering a particle is something that rarely happens in the life of an experimental particle physicist," says Martínez. Lacasta shares his excitement: "You can't always say that you have contributed to the discovery of a new particle, so you can imagine the emotion of the moment". 

IFCA and the Higgs
The IFCA's Particle and Instrumentation Group has been working for years and has been part of the team that discovered the Higgs boson in one of the two W boson decay channels. "Today we continue to study the properties of this boson, many of which still remain to be discovered, and for this we will use the data we collect in the new phase of LHC operation, which starts tomorrow, at an energy never before reached by any other experiment, and increasing the total amount of data we can use for physics analysis," explains Alicia Calderón, IFCA researcher and lecturer at the University of Cantabria. "We expect to produce several million Higgs bosons in the next 3-4 years," she adds. 

This will make it possible, on the one hand, to test the physics of the Higgs boson at a new energy, and on the other, to continue studying its properties with much greater precision. In addition, the IFCA will continue to understand the Higgs boson a little more, to resolve questions such as whether it is a fundamental boson or could have an internal structure, whether it is the only Higgs boson in existence, or whether it could be a portal connecting ordinary matter with dark matter. 

After the start of the LHC's third run, Run3, it will be the turn of the High Luminosity LHC (HL-LHC), which is expected to start in 2029 with a higher data throughput than Run3. "This will involve a much larger amount of data, which will allow us to measure new properties of the Higgs boson given the large amount of data," explains Calderón.

He adds that "to carry out the HL-LHC it is necessary to develop new technologies that can operate in very high radiation environments and that have a rapid response to the passage of particles, and also to develop new detectors that will improve the measurements in terms of precision". 

On the other hand, the CSIC researcher and member of the Particle and Instrumentation Group of the IFCA, Iván Vila, is currently working with his team on a new use of physics applied to medicine and health together with the Marqués de Valdecilla University Hospital, proton therapy. "Of course there is a technological transfer of all these new technological developments, and IFCA is involved in new technologies and their application in other scientific fields and in society, one of them, currently, is medical physics and the collaboration with Valdecilla in proton therapy as a treatment to cure cancer, this is what concerns us," says Vila. 

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