The discovery of the Higgs boson celebrates its tenth anniversary
In 2012, researchers at the European Organisation for Nuclear Research (CERN) caused a sensation by announcing the discovery of the Higgs boson, the ultimate elementary particle. Ten years later, what are the consequences of this event for particle physics? Here are how physicists at Université Paris-Saclay respond.
“I think we have it.” At the CERN headquarters in Geneva on 4 July 2012, Rolf-Dieter Heuer (Managing Director of the international research centre) uttered these words in front of a huge audience to announce the discovery of a new subatomic particle, a few characteristics of which he knew at the time were very similar to the boson theorised by Peter Higgs in 1964. The British researcher, who was in Switzerland when the discovery of the boson which now bears his name was announced, later received the 2013 Nobel Prize in Physics for his work which led to the discovery of this new particle.
The announcement of this discovery caused an enormous stir in the scientific community as it validated the theory describing particles and matter. “It was the final piece in what is known as the Standard Model of particle physics,” explains Sébastien Descotes-Genon from the Irène-Joliot Curie – Physics of Two Infinities Laboratory (IJCLab – Univ. Paris-Saclay, Univ. Paris Cité, CNRS).
“If it weren’t for this field, everything would move at the speed of light”
The physicist from the IJCLab explains. “This model is the result of the insight, gained over nearly 50 years, into what matter is like at the smallest scales currently being probed. Today, we’re even able to answer the question ‘What is matter made of?’ using a very small number of particles which we call fermions.” Among these fermions, specialists differentiate between quarks, of which there are six, and leptons (which include electrons, for example), of which there are also six. Scientists also associate three fundamental interactions with these particles which are necessary for the construction of matter. These are electromagnetic interaction and two interactions on a subatomic scale – namely strong and weak interactions.
“Up until the 1990s, we made an inventory of all elementary particles, their interactions and behaviours, while incorporating them into a coherent framework. For this, we needed an unified description of electromagnetic and weak interactions. Much theoretical work was done in this respect,” continues Sébastien Descotes-Genon. “However, when we tried to find a cohesive description of all the electromagnetic and weak interactions, we had to add an ‘extra ingredient’ to the Standard Model...and this was the Higgs boson.” More precisely, it is the Higgs field, which is the key to the Standard Model and explains how the electromagnetic and weak interactions, although very different in appearance, are in fact two sides of a single interaction. Providing enough energy to a field creates an excitation which, like a wave on the sea, spreads and interacts with its environment, and which corresponds to a particle that we can observe. To confirm their theory of the Standard Model of particle physics, physicists spent years relentlessly exploring the Higgs field in search of a significant excitation which corresponded to the elusive boson.
The LHC or Large Hadron Collider at CERN was built underground between Switzerland and France in 2008. Today, it still remains the most powerful and largest particle accelerator in the world. From its beginning, it has carried the hope of the entire community that the Higgs boson will one day be observed.
“The LHC was built in part to observe phenomena at the energy scales at which we expected the Higgs boson, or something else that would link the weak and electromagnetic interactions, to be present. It was designed to collide particles at energies sufficient to excite the Higgs field, create the boson particle and allow the study of its decay,” explains Sébastien Descotes-Genon. In fact, it is not the Higgs boson itself which scientists have been observing since its discovery in 2012, but the particles into which it decays. “The announcement on 4 July 2012 referred to the observation on several decay channels of events with similar energies which were not simply background. We therefore knew, with a certain level of statistical confidence, that this corresponded to a specific physical phenomenon,” explains Sébastien Descotes-Genon.
“The Higgs field is everywhere. Unlike other fields, and this is what makes it special, it has the property of disturbing and ‘slowing down’ the propagation of other particles. If there were no such field, nothing would have mass and everything would go at the speed of light,” adds the physicist from the IJCLab.
The boson at Paris-Saclay
For ten years now, researchers throughout the world have been working to define the Higgs boson ever more precisely. The A Toroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid (CMS) collaborations have both been historically linked to the Higgs boson and its study. Moreover, it was scientists from these two experiments who announced the particle’s discovery in 2012. Since then, their goal has been to measure all possible properties of the Higgs boson. “To begin with, it was ‘basic’ things, such as its mass or spin (a quantum property which describes sensitivity to the electromagnetic field), as well as the frequency of the different modes of production of the Higgs boson and its decays,” outlines Sébastien Descotes-Genon.
With this in mind, several laboratories attached to Université Paris-Saclay have been involved in the international scientific effort to develop particle-acceleration systems. The sensors used by ATLAS were in part designed, tested and built at the IJCLab, while the Institute for Research on the Fundamental Laws of the Universe (Irfu – Univ. Paris-Saclay, CEA) helped to develop the sensors for ATLAS and CMS. The two laboratories are also involved in discussions on future particle accelerators.
“The CEA Paris-Saclay group plays an essential role in the calibration of photon energy by participating significantly in the design of the electromagnetic calorimeter, which detects electrons and photons and measures their energy,” explains Julie Malclès, who manages the CMS team at the Particle Physics Departement of Irfu. “The Higgs boson can notably be decayed into two photons. This diphoton channel is ideal to observe the boson, because it allows us to access to the four main production modes of the boson, with a good sensitivity. We have participated in many studies via this channel, such as the coupling between the boson and the top quark.” Apart from the formation and decay of the Higgs boson, Julie Malclès’ team are also wishing to improve the accuracy of the various characteristic measurements of the particle. “Our work aims at reducing the uncertainties on the measurements of the boson properties. For this, the improvement of the analysis methods, with for example strategies based on artificial intelligence, plays a very important role. The increased number of collisions studied is also crucial to reduce the statistical uncertainties. This will require a redesign of the collider, to collect more collisions, and also a rejuvenation of the detectors, with drastic improvements in performance.”
“Within the framework of a collaboration between the Institute of Theoretical Physics (IPhT – Univ. Paris-Saclay, CNRS, CEA) and the IJCLab, we’re also interested in the theoretical considerations which the results and observations generate, in particular to determine the space potential,” says Sébastien Descotes-Genon, before pondering. “What alternative assumptions to the Standard Model are still possible?”
The ATLAS Collaboration. A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery. Nature 607, 52–59 (2022).
The CMS Collaboration. A portrait of the Higgs boson by the CMS experiment ten years after the discovery. Nature 607, 60–68 (2022).