Jigsaw puzzles at CERN – Happy Birthday, Higgs particle
Happy birthday, Higgs particle
Ten years after the discovery of the “God Particle” the new round of measurements begins at Cern. Will she bring the advanced new physics? Will a new window on the mysteries of the universe open soon?
Birthday presents for minors are often a bit overambitious, so many a book of world literature ends up on the shelf unread because the recipient refuses to pick up anything other than a mobile phone. In fact, the Higgs particle, which celebrated its tenth birthday last week, neither has a cell phone nor can it read. This means that the journal “Nature”, which published three overview articles on the elementary particle on the anniversary of its discovery, is probably on the safe side, you can’t do much wrong there.
In any case, ten years after the discovery is no worse time to look back – and forward. Especially since the discoverer of the Higgs particle, the Large Hadron Collider (LHC), which costs billions and is technically almost unrivalled, which is located at the European research center Cern near Geneva, has just started its third measurement period. Is Higgs – some exaggeratedly call it “God particle” – now a constant source of new, exciting insights, as portrayed by his fans? Or as the only major discovery of the LHC so far, just a bitter disappointment, as critics see it?
With the discovery of the Higgs particle, which was announced on July 4, 2012 to great international applause, the “Standard Model of particle physics” was complete. It was the last piece of the puzzle that was missing from the theory, and at the same time a central part of this theory. The Higgs field, which is inextricably linked to the particle of the same name, surrounds us all like an invisible ether. Only through the interaction with this field do the other elementary particles get their mass.
A nightmare seemed to come true
At the same time, however, the discovery of the Higgs seemed to confirm a nightmare. Because that Higgs would show up sooner or later was considered practically certain. Because the Standard Model was so well confirmed experimentally, it seemed extremely unlikely that something as central as the Higgs could simply be a mistake (it would have been all the more exciting if it had turned out that way).
Actually, one had hoped that the LHC would primarily provide indications of new physics that go beyond the Standard Model. Ultimately, this still leaves many questions unanswered: What is the dark matter and dark energy that fill the universe made of? Why are the masses of the elementary particles not completely different? Why is there so much more matter than antimatter? And how can one integrate gravity into particle physics? The Higgs particle, which only shows where the Standard Model is right, but does not provide the long-awaited indication of where the model is wrong: many physicists felt shivers down their spines in this standstill scenario.
But sometimes it just takes time. “I was a bit disappointed that we didn’t immediately find something completely new with the LHC,” says Sandra Kortner from the Max Planck Institute for Physics in Munich, who heads a project at the Atlas detector of the LHC. “But reason says: Maybe we just need more data. I’m optimistic, we still have a lot to do.”
It is also not the case that the work of the past ten years has been awarded. “The Higgs particle is unique,” says Kortner. Since it first appeared back then, its interactions have been measured from all directions. The physicists have learned a lot, and a lot is still open. For Kortner, this remains the most exciting thing for the third LHC measurement round, which begins on Tuesday after months of ramping up and adjusting the accelerator. “We will be able to observe further decays of the Higgs into lighter particles and also better understand how two Higgs particles interact with each other,” says Kortner.
“They are standing in front of a desert and they don’t know how big it is.”
Others are less optimistic about the chances of progress. “You’re standing in front of a desert and you don’t know how big it is,” University of Minnesota physicist Marvin Marshak told Science magazine. After all: In the next four years, the LHC will work with slightly higher energy than before, with 13.6 teraelectronvolts (TeV) it is now quite close to its performance limit of 14 TeV. After this round of measurements, another major upgrade to the so-called “High Luminosity LHC” is to follow, after which the number of particle collisions in particular is to be multiplied again.
For the third measurement period, however, the accelerators and above all the reliable detectors were optimized over a period of three years. When protons once again circulate in the LHC’s 27-kilometre ring tunnel at almost the speed of light, colliding and collapsing into countless debris in quantum mechanical processes, the detectors can now collect more data from these collisions and process them better than ever before.
“It would be the discovery of a new force in nature and could fundamentally change our understanding of the universe.”
This could lead to completely new discoveries. There were a number of anomalies in the data collected to date. For example, new physics seems to be revealed in the rare decays of so-called B mesons. B mesons are particles made up of two different quarks, the elementary building blocks of an atomic nucleus. “If this is confirmed, it would be the greatest discovery in particle physics in recent decades,” says Nico Serra from the University of Zurich, who is involved in measurements with the detector name LHCb. “It would be the discovery of a new force in nature and could fundamentally change our understanding of the universe.”
Perhaps these measurements of the B mesons, like many other measurements that indicate deviations from the Standard Model of particle physics, turn out to be coincidences. But it is quite possible that one or the other will be confirmed in tough, tiring detailed work. “I measured the mass of the W boson for ten years,” says Matthias Schott from the University of Mainz, who also does research at the LHC. “But you just have to keep at it. Measurements in the precision range are just as exciting and spectacular as results that are immediately apparent.»
Does the Higgs boson have siblings?
Not everything has been extracted from the data collected so far. “We now work a lot with deep neural networks, which can be used to identify irregularities in the data very well,” says Schott. The allegedly effective field theories are also a fundamentally new, still relatively young approach. Instead of testing each new, exotic theory individually, it looks at which ingredients of new theories the data could match. “That way we can see in which direction we need to continue our search,” says Schott.
There is also still much to be learned about the Higgs field itself, such as exactly how its potential energy behaves. There may also be siblings of the Higgs particle to be discovered, some theories predict several Higgses. “Whatever we discover in the coming centuries, we will learn,” writes a team led by Giulia Zanderighi in the birthday edition of “Nature”. Either by finding evidence for parts of the Standard Model that are still unclear. “Or by opening a window to new horizons and the mysteries of the universe.” It doesn’t work entirely without pathos.
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