On July 4, 2012 – ten years ago – physicists at the CERN research center near Geneva announced a groundbreaking discovery: through collisions in the particle accelerator Large Hadron Collider (LHC), they had detected the long-sought Higgs boson – the particle that gives everything its mass. The existence of this elementary particle and its field confirmed a theory that had been put forward by several physicists around 50 years earlier – and which answered some fundamental questions of particle physics. Physicists have learned a lot about the Higgs and its interactions over the past decade, but some fundamental questions about this particle and its effects still remain unanswered.
Without mass, the universe would be a completely different place and our world would probably not exist at all. Only the mass and the interactions of elementary particles such as quarks and electrons associated with it enable the formation of matter. But where do these elementary particles get their mass from? The Standard Model of particle physics – the basis of our physical world view – did not provide an answer for a long time. Also open was the question of why the carrier particles of the weak nuclear force, the W and Z bosons, have a mass unlike all other carriers of the basic forces. It was not until the early 1960s that a number of theoretical physicists came up with a possible solution to these questions, including Robert Brout and Francois Englert in Belgium and Peter Higgs in Great Britain. Independently of each other, they came to the conclusion that an invisible field pervading the entire universe could solve the problem. According to the theory, this scalar field, now known as the Higgs field, can interact with matter-forming particles and the W and Z bosons, thereby giving them their mass.
In a well-known analogy, the British physicist David Miller likens this Brout-Englert-Higgs mechanism to a cocktail party. If an important personality enters the room, a cluster of other guests quickly gathers around him. The celebrity can hardly move forward because of all the people – similar to a particle with a high mass that can only be accelerated with a lot of energy. According to the theory, if this Higgs field exists, then it should also be manifested by a particle, the Higgs boson. “What better way to reconcile the standard model with the measured data than this one? If there is no Higgs boson, then the whole theory makes no sense,” stated Peter Higgs in 2004. However, the search for this particle turned out to be lengthy and difficult – partly because physicists did not know at what energies they were looking for it should look for the Higgs boson.
A milestone in physics…
On July 4, 2012, the time had finally come: Scientists at the CERN research center announced the long-awaited result: the unambiguous signal of the Higgs boson had been detected independently of one another on the two large detectors of the particle accelerator LHC, ATLAS and CMS. According to the data, this had a mass of 125 gigaelectronvolts. This fitted perfectly with the mass range in which earlier investigations had suggested the Higgs boson. This could be demonstrated by a “hump” in the curve of the decay products, generated by the photon pairs or Z bosons released during the Higgs decay. Both results reached a significance of more than five standard deviations and thus fulfilled the requirement for the official discovery of a particle. “The discovery of the Higgs boson was a monumental milestone in particle physics,” says Fabiola Gianotti, Director General of CERN. “It marked the end of a decade-long search and the beginning of a new era in the study of this particular particle.” In 2013, Francois Englert and Peter Higgs received the Nobel Prize in Physics for being the two surviving representatives of the theoretical physicists who discovered the Higgs mechanism and the predicted the Higgs boson.
In the meantime, ten years have passed in which scientists, primarily at the LHC, have continued to research the behavior and properties of the Higgs boson. One of the crucial questions was whether this newly discovered particle also interacts with other particles in the way theoretically predicted. When demonstrating the Higgs, one had already established from the decays that there is such an interaction with other force particles such as photons and W and Z bosons. In 2016, the so-called Yukawa interaction, the coupling of the Higgs to matter-forming particles such as quarks and leptons, was also demonstrated for the first time. And another prediction came true: According to the theory, the coupling of the Higgs boson to very heavy elementary particles such as the top quark should be strongest – this is how they get their great mass. In 2018, the physicists at CERN demonstrated this strong coupling to the top quark. “This detection is a milestone in the research of the Higgs boson,” stated the spokesman for the ATLAS collaboration, Karl Jakobs. “We have now observed all couplings of the Higgs boson with the heavy quarks and leptons of the third generation and also all important types of generation of this particle.”
…and many unanswered questions
However, this does not mean that all questions about the Higgs boson have been answered. “In many ways, experimental exploration of the Higgs sector is still in its infancy,” said University of Oxford’s Gavin Salam and colleagues in a Nature commentary. For example, it has hardly been investigated whether and how the Higgs boson couples to lighter elementary particles and to particles of the so-called second generation of fermions, including the muon, the heavier “brother” of the electron. And there are also a few unanswered questions about the Higgs boson itself. So it is unclear whether this particle can interact with itself. If this so-called triple coupling exists, its frequency and the energies at which it occurs could shed light on whether the Higgs field follows Standard Model predictions or whether there is room for “new physics” in the form of undetected particles or forces . It is also unclear whether the Higgs boson is really an indivisible, genuine elementary particle or is composed of other, as yet unknown particles.
The many unanswered questions about the Higgs boson are also closely linked to some of the greatest mysteries in physics – dark matter, the dominance of matter over antimatter or the question of whether there was a phase of extremely rapid expansion shortly after the Big Bang, the so-called cosmic inflation existed. “Even if the standard model has so far passed all experimental tests, it leaves such fundamental questions unanswered,” explain Salam and his colleagues. “The Higgs boson is linked to varying degrees with potential solutions to these mysteries.” The physicists hope that the third term of the LHC at CERN, which begins on July 5, 2022, will provide at least some answers to these questions. Because their even more energetic collisions and the even further optimized sensitivity of the detectors also provide new possibilities for the investigation of the Higgs and its decays. “We will measure the strength of the interactions of the Higgs boson with matter and force particles with unprecedented precision and also continue the search for a decay of the Higgs boson into dark matter particles and for other variants of the Higgs boson,” says Andreas Hoecker, spokesman for the ATLAS collaboration.
Source: CERN, Gavin Salam (University of Oxford, UK) et al., Nature, doi: 10.1038/s41586-022-04899-4