Particle physics won’t die, even if the LHC doesn’t find new particles

An illustration of events recorded with the CMS detector in 2012 shows the expected characteristics of the decay of a Higgs boson (dashed yellow lines) into a pair of photons (green towers).Credit: Thomas McCauley/CMS Collaboration

Ten years ago, on July 4, 2012, scientists around the world were celebrating the new capital that researchers had found evidence supporting the existence of the Higgs boson. This fundamental particle, whose existence was predicted as a result of theories developed in the mid-1960s1,2, was discovered by teams working on the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN, the European laboratory for particle physics near Geneva in Switzerland. This discovery was a crowning achievement for the LHC and for the thousands of engineers, researchers, support staff and technicians who contributed to its realization.

And yet, as the LHC prepares to start the third of its planned five runs, some believe this could be the last gasp for particle physics – or, at least, for physics dealing with particle collisions. at high energy. Their reasons? The researchers hoped to improve the current theoretical description of fundamental particles and their interaction – the standard model of particle physics – which is considered incomplete. Many are disappointed that the LHC has yet to find hints of something at odds with the Standard Model that could represent a step towards a more complete theory.

The Standard Model is a set of theories developed between the 1950s and the 1970s. Despite its phenomenal accuracy, it does not incorporate gravity or dark matter, and it does not explain the abundance of matter over antimatter. in the Universe, nor certain aspects of neutrinos.

field of fear

In the search for answers, theoretical physicists regularly hypothesize the existence of new particles, some of which should be within the detection limits of existing colliders. But the LHC, the largest in the world, has not yet spotted any. Some fear that the field will be in trouble if the LHC cannot supply the goods. Critics argue that in the absence of clear clues, it is unreasonable to expect governments to potentially find billions of dollars for another large-scale collider and continue an indiscriminate hunt. Without another collider, the field itself could soon wither away.

These concerns are legitimate, but they assume that the sole purpose of the LHC is to chase particles. Spotting a new particle is by no means a trivial result. However, particle colliders such as the LHC are also essential for deepening our understanding of the behavior of known particles, including the Higgs boson itself. Physicist Peter Higgs is one of the best-known names in science, but researchers still know little about the particle that bears his name. And, collision after collision, the LHC continues to reveal new information about the Higgs boson.

In 2012, researchers at CERN estimated the mass of the newly detected particle. They cautiously labeled it a candidate for the Higgs boson, but realized that more evidence would be needed to establish with certainty that the candidate matched the predictions of the Standard Model.

Thirty times more data

According to the theory, particles have no mass; rather, their masses result from their continuous interaction with something called the Higgs field (or the Brout-Englert-Higgs field) that pervades the entire Universe. The Higgs boson is a wave in this area. The strength of any particle’s interactions with the Higgs field should be proportional to its mass, which means that heavier particles, such as the top quark, should interact more strongly than lighter particles.

Since 2012, the LHC has produced 30 times more data on particle collisions involving a Higgs boson. Building on these data, the ATLAS and CMS collaborations report results this week on Higgs interactions with heavy particles.3,4. The results match the pattern predicted by theorists more than 50 years ago.

To complete the picture, researchers also need to probe the interactions between the Higgs boson and lighter particles – those, like electrons, that form the building blocks of everyday matter. As Giulia Zanderighi of the Max Planck Institute for Physics in Munich, Germany, and her colleagues explain in Perspective, the LHC should allow scientists to achieve this goal, but a new generation of colliders will probably be needed to achieve the Work completed5. How best to study these interactions at the LHC and what other experiments might be needed are active areas of research.

The process matters

Yet deeper questions about the Higgs boson remain unanswered. Unlike all other known particles, its interactions in the Universe do not occur through any of the four known forces: the electromagnetic force, the weak and strong nuclear forces, and the gravitational force. This is remarkable, and physicists hope that this unique feature will allow the Higgs boson to shed light on some fundamental questions. For example, if the Higgs boson interacts with dark matter or other unknown particles, these interactions could leave an observable trace in the behavior of the boson. Both theoretical and experimental physicists explored these questions long before the 2012 discovery, so resolution may still be a long way off.

Particle physics isn’t done yet with what physicist and philosopher Thomas Kuhn called “normal science” in his book The structure of scientific revolutions (1962). Kuhn’s normal science describes the work of scientists who theorize, observe, and experiment within a given framework, such as the Standard Model. That’s not to say there couldn’t be what Kuhn called a “paradigm shift,” whereby a radical shift in perspective is needed to accommodate changing evidence. But as Rolf-Dieter Heuer, who was Director General of CERN at the time of the Higgs discovery, writes in Natural PhysicsLHC data is also needed in the search for physics beyond the Standard Model6. Identifying any anomaly requires a good understanding of what is expected.

To those who fear that particle physics is nearing its last breath, we urge you to let science take its course, prepare for surprises, and remember that it took more than four decades for one aspect of a theory is confirmed by experiment. In science, the process rarely makes headlines, but it matters just as much as the result.

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