Today marks ten years since the historic discovery of the Higgs boson was announced. dr. Martin Bauer and Dr. Stephen Jones of Durham University look at what this has meant for particle physics.
Ten years ago, scientists announced the discovery of the Higgs boson, which helps explain why elementary particles (nature’s smallest building blocks) have mass.
For particle physicists, this was the end of a decades-long and immensely difficult journey – and arguably the most important achievement in the history of the field. But this end also marked the beginning of a new era of experimental physics.
Over the past decade, measurements of the properties of the Higgs boson have expanded the predictions of the standard model of particle physics (our best theory for particles). But it has also raised questions about the limitations of this model, such as whether there is a more fundamental theory of nature.
physicist Peter Higgs predicted the Higgs boson in a series of papers between 1964 and 1966, as an inevitable consequence of the mechanism responsible for giving elementary particles mass.
This theory suggests that particle masses result from elementary particles interacting with a field called the Higgs field. And according to the same model, such a field should also give rise to a Higgs boson — meaning that if the Higgs boson wasn’t there, it would end up falsifying the whole theory.
But it soon became clear that discovering this particle would be a challenge. When three theoretical physicists calculated the properties of a Higgs boson, they ended with an apology†
“We apologize to experimenters because they have no idea what the mass of the Higgs boson is … and because we are not sure about the couplings with other particles … For these reasons we do not want any major experimental searches for the Higgs boson. particle encourage.”
It wasn’t until 1989 that the first experiment with a serious chance of discovering the Higgs boson began its quest. The idea was to break together particles of such high energy that a Higgs boson could be created in a 27 km tunnel at CERN in Geneva, Switzerland – the largest electron positron (a positron is almost identical to an electron but has an opposite charge) collider ever built.
It ran for 11 years, but the maximum energy turned out to be only 5 pc too low to produce the Higgs boson.
Meanwhile, the most ambitious American collider in history, the Tevatron, had started taking data at Fermilab, close to Chicago. The Tevatron collided protons (which together with neutrons make up the atomic nucleus) and antiprotons (almost identical to protons but with opposite charges) with an energy five times higher than what was achieved in Geneva – certainly enough to create the Higgs.
But proton-antiproton collisions produce a lot of debris, making it much more difficult to extract the signal from the data. In 2011, the Tevatron stopped working – the Higgs boson again escaped detection.
In 2010, the Large Hadron Collider (LHC) began to collide with protons with seven times more energy than the Tevatron. Finally, by July 4, 2012, two independent experiments at CERN had each collected enough data to explain the discovery of the Higgs boson. In the following year, Higgs and his collaborator François Englert won the Nobel Prize “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of masses of subatomic particles”.
This sells it almost too short. Without the Higgs boson, the entire theoretical framework that describes particle physics on the smallest scale falls apart. Elemental particles would be massless, there would be no atoms, no humans, no solar systems and no structure in the universe.
Trouble on the horizon
Still, the discovery has raised new, fundamental questions. Experiments at CERN have continued to investigate the Higgs boson.
Its properties determine not only the masses of elementary particles, but also how stable they are. As it stands, the results indicate that: our universe is not in a perfectly stable state† Instead, similar to ice at its melting point, the universe could suddenly undergo a rapid “phase transition.” But instead of going from a solid to a liquid, like ice turning into water, this would involve a crucial change in the masses and natural laws of the universe.
The fact that the universe nevertheless appears stable suggests that something may be missing from the calculations – something we haven’t discovered yet.
After a three-year hiatus for maintenance and upgrades, the collisions at the LHC are now about to resume at an unprecedented energy, nearly double that used to detect the Higgs boson. This could help find missing particles that move our universe away from the apparent interface between being stable and rapidly undergoing a phase transition.
The experiment can also help answer other questions. Could the unique properties of the Higgs boson make it a portal to discovering dark matter, the invisible substance that makes up most of the matter in the universe? Dark matter does not charge. And the Higgs boson has a unique way of interacting with uncharged matter.
The same unique properties have led physicists to question whether the Higgs boson might not be a fundamental particle after all. Could there be a new, unknown force beyond the other forces of nature – gravity, electromagnetism, and the weak and strong nuclear forces? Perhaps a force that binds hitherto unknown particles into a composite object we call the Higgs boson?
Such theories can help to solve the controversial results of recent measurements that suggest that some particles don’t behave exactly as the Standard Model suggests they should. Studying the Higgs boson is thus vital to determine whether there is more physics to discover than the Standard Model.
Ultimately, the LHC will run into the same problem as the Tevatron. Proton collisions are messy and the energy of his collisions will only reach so far. Even though we have the full arsenal of modern particle physics – including advanced detectors, advanced detection methods and machine learning – at our disposal, there is a limit to what the LHC can achieve.
A future high-energy collider specifically designed to produce Higgs bosons would allow us to accurately measure its key properties, including how the Higgs boson interacts with other Higgs bosons. This, in turn, would determine how the Higgs boson interacts with its own field.
Studying this interaction could therefore help us investigate the underlying process that gives particle masses. Any disagreement between the theoretical forecast and a future measurement would be a crystal clear sign that we need to invent brand new physics.
These measurements will have a profound impact well beyond collision physics, guiding or limiting our understanding of the origin of dark matter, the birth of our universe and perhaps its ultimate fate.
dr. Martin Bauer is an associate professor of physics at the University of Durhamwhile Dr. Stephen Jones is an assistant professor of physics at the university.
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