On July 4, 2012 – ten years ago – physicists at the CERN research center near Geneva announced a groundbreaking discovery: through collisions at the Large Hadron Collider (LHC) particle accelerator, they detected the long-sought Higgs boson – a particle that gives everything its mass. The existence of this elementary particle and its fields confirmed a theory put forward by several physicists some 50 years earlier – and which answered some fundamental questions in particle physics. Physicists have learned a lot about Higgs and its interactions over the past decade, but some fundamental questions about the particle and its effects remain unanswered.
Without mass, the universe would be a completely different place, and our world probably wouldn’t exist at all. Only mass and the related interactions of elementary particles, such as quarks and electrons, make matter possible. But where do these elementary particles get their mass from? The Standard Model of particle physics – the basis of our physical worldview – did not provide an answer for a long time. There was also an open question why the weak nuclear force carriers, the W and Z bosons, have a mass unlike all other carriers of the fundamental force. It wasn’t until the early 1960s that many theoretical physicists found a possible solution to these questions, including Robert Brout and Francois Englert in Belgium and Peter Higgs in the UK. Independently of one another, they concluded 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 the particles that make up matter and with the W and Z bosons, thus giving them their mass.
In a well-known analogy, British physicist David Miller compares this Brout-Englert-Higgs mechanism to a cocktail. If an important person enters the room, other guests quickly gather around them. A star can hardly move forward for the sake of all people – just like a large mass molecule that can only be accelerated with a lot of energy. According to the theory, if this Higgs field exists, it should also be manifested by a particle, the Higgs boson. “What better way to reconcile the standard model with the measurement data than this? If there is no Higgs boson, then the whole theory is pointless, ”said Peter Higgs in 2004. However, the search for this particle proved to be long and difficult – partly because the physicists did not know what energies they were looking for. should be looking for the Higgs boson.
A milestone in physics …
On July 4, 2012, the time has finally come: scientists from the CERN research center announced the long-awaited result: the unequivocal Higgs boson signal was detected independently of each other on two large detectors of the LHC particle accelerator, ATLAS and CMS. According to the data, it had a mass of 125 gigaelectronvolts. This fit perfectly into the mass range where previous research had suggested the Higgs boson. This can be demonstrated by a hump on the decay product curve generated by photon pairs or Z bosons released during Higgs decay. Both results achieved more than five standard deviations and thus fulfilled the requirement for an official particle discovery. “The discovery of the Higgs boson was a milestone in particle physics,” says Fabiola Gianotti, CERN CEO. “This marked the end of a decade of research and the beginning of a new era in research on this particular particle.” In 2013, Francois Englert and Peter Higgs received the Nobel Prize in Physics for being the other two representatives of theoretical theory. physicists who discovered the Higgs mechanism and predicted the Higgs boson.
In the meantime, ten years have passed as scientists, mainly from the LHC, continued to study the behavior and properties of the Higgs boson. One of the key questions was whether this newly discovered particle also interacts with other particles in the theoretically predicted manner. When demonstrating Higgs, it has already been established from the decays that there is such an interaction with other interaction particles such as photons and the W and Z bosons. In 2016, the so-called Yukawa interaction, the Higgs coupling with particles that make up matter such as quarks and leptons. Another prediction came true: according to the theory, the coupling of the Higgs boson with very heavy elementary particles, such as the quark, should be the strongest – thus they gain their great mass. In 2018, physicists at CERN demonstrated this strong coupling with the upper quark. “This discovery is a milestone in the study of the Higgs boson,” said ATLAS collaboration spokesman Karl Jakobs. “We have now observed all the couplings of the Higgs boson with the third generation heavy quarks and leptons, and all the important generation types 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 Gavin Salam of the University of Oxford and colleagues in a commentary on Nature. For example, it has hardly been investigated if and how the Higgs boson combines with lighter elementary particles and so-called particles. second generation fermions, including the muon, the heavier “brother” of the electron. There are also some unanswered questions about the Higgs boson itself. So it is not clear if this particle can interact with each other. If this so-called triple coupling does exist, its frequency and the energies at which it occurs could shed light on whether the Higgs field is as predicted by the Standard Model or if there is room for “new physics” in the form of undetected particles or forces. It is also unclear whether the Higgs boson is actually an undivided, real elementary particle, or whether it consists of other, as yet unknown particles.
Many of the unanswered questions about the Higgs boson are also closely related to some of the greatest puzzles in physics – dark matter, the domination of matter over antimatter, or the question of whether there was a phase of extremely violent expansion shortly after the Big Bang, so-called space inflation existed. “Even if the standard model has passed all experimental tests so far, such fundamental questions remain unanswered,” explain Salam and his colleagues. “The Higgs boson is to varying degrees related to potential solutions to these mysteries.” Physicists hope that the LHC’s third term at CERN, which begins on July 5, 2022, will provide at least some answers to these questions. Because their even more energetic collisions and even more optimized sensitivity of the detectors also offer new possibilities for studying Higgs and its decays. “We will measure the strength of the interaction of the Higgs boson with matter and force particles with unprecedented precision, and continue our search for the decay of the Higgs boson into dark matter particles and other variants of the Higgs boson,” says Andreas Hoecker, spokesperson for ATLAS collaboration.
Source: CERN, Gavin Salam (University of Oxford, UK) et al., Nature, doi: 10.1038 / s41586-022-04899-4