Physicists get first glimpse of the zeptoverse from LHC experiment
Data from the Large Hadron Collider has revealed evidence of a long-sought particle process that serves as a guide into the realm of the smallest scales ever probed
Welcome to the zeptoverse, the world of the unprecedentedly small. A new analysis of data from the Large Hadron Collider has uncovered an extraordinarily rare particle decay that opens the door to this realm and its secrets.
The standard model of particle physics tabulates all known particles and how they interact, but it cannot explain dark matter and dark energy, which fill most of our universe. Consequently, researchers test its predictions in particle collider experiments, searching for signs of new particles and forces.
Now, Paula Collins and her colleagues at the Large Hadron Collider beauty (LHCb) experiment at the CERN particle physics laboratory near Geneva in Switzerland have found the first evidence of a rare decay involving the “beauty quark” particle, which opens a whole new world for these searches.
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“People have been searching for it for more than 40 years,” she says. “Everyone’s just over the moon.”
Beauty particles are often produced in the LHC’s particle smashes, but the researchers focused on a specific process where a beauty quark gets annihilated by an antiquark – its antimatter cousin – and their annihilation produces another particle and antiparticle pair, the electron-like muon and antimuon, which can be detected. This process is exceedingly rare and has eluded experiments for decades.
“These very, very rare decays, they’re almost impossible according to the current laws of physics,” says team member Harry Cliff at the University of Cambridge. “This happens only for 1 in 10 billion beauty particles.”
The finding is still preliminary, with relatively large uncertainties and a significance level of 3.2 sigma, while the gold standard for a discovery is 5 sigma. This means the chance that the signal in the experiment occurred randomly instead of being meaningful is about 0.3 per cent.
Cliff says future experiments ought to lower this uncertainty, but the data already reveals that we have reached the zeptoverse, a world at scales as small as sextillionths of a metre – the smallest ever explored. Undiscovered denizens of this world could influence the beauty particle decay process and because the process is so rare, researchers would immediately spot that change, says Collins. In this way, measuring this process is like surveying the zeptoverse.
“It was really a surprise that the team would be able to see this very rare event,” says Jure Zupan at the University of Cincinnati in Ohio. He says this was made possible by technical upgrades that the LHCb team has made in recent years. “It’s like a very powerful microscope where now you’re able to see much shorter distances because you don’t have a fog in front of your eyes,” says Zupan. “It’s absolutely exciting,” says Zoltan Ligeti at the Lawrence Berkeley National Laboratory in California.
This process is not only ultra-rare, but mathematical theories predict it very well, so investigating it makes for a particularly important test of the standard model, says Cristina Lazzeroni at the University of Birmingham in the UK.
Although the new measurement is consistent with the standard model, Ligeti says this doesn’t mean that all possible new particles have been ruled out. In his view, the new measurement implies that the effects of such particles must be small but not necessarily negligible and LHCb could quantify this further going forward. “What they measured is very interesting, but I think even more interesting is the promise [of future experiments],” says Zupan.
Cliff, however, is worried about the future of the team’s research because last year the UK government withdrew funding from a planned LHCb upgrade. “We are now really scrambling to figure out will we be able to continue doing this sort of research,” he says.
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