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Although CERN’s Large Hadron Collider has made a lasting impact on particle physics, it hasn’t yet open up a whole new frontier of particle physics like some scientists expected.
One scientist champions a theory that new physics could be hiding in what he calls the “zeptouniverse”—the realm of objects on the scale of the zeptometer (which is 1 quintillionth of a meter)—and that the best way to explore that universe is through observing kaon and B meson decays.
Future colliders will likely be able to directly observe the zeptouniverse, but for now, studying these decays could help us find new physics within the decade.
We humans have gotten pretty good at glimpsing the invisible. The Large Hadron Collider—our premier instrument for exploring the subatomic—can thoroughly explore the world of the attometer, which is (incredibly) just one-quintillionth of a meter. Famously, the LHC confirmed the existence of the Higgs boson in 2012, and physicists prepared for a rush of new particles to explain lingering mysteries of the universe like the existence of dark matter and matter-antimatter asymmetry.
But that explosion of discovery didn’t really materialize. That’s certainly not to say nothing has happened since then, but no major revelations on par with the Higgs have been discovered since. And now, a new article in New Scientist, written by particle physicist Harry Cliff who works on the LHCb experiment, details one theory as to why we haven’t found what we were expecting to find. At its most basic, many of these revelations could be hiding in what’s sometimes referred to as the “zeptouniverse,” which is a world that only exists at the 10-21-meter scale. The LHC can only analyze particles directly down to 50 zeptometers, but Cliff highlights a theory—largely championed by Technical University of Munich theoretical physicist Andrzej Buras—that these elusive particles could simply be beyond LHC’s detection capabilities.
Of course, a better detector could open up this frontier—CERN completed a feasibility study for the Future Circular Collider (FCC) just earlier this year. But Buras believes that we can explore this frontier of new physics indirectly without the need to wait the several decades required to finally probe this question (the FCC won’t perform high-energy physics until 2070). In 2020, Buras explored this question in an article for Physik Journal, writing:
Can we reach the Zeptouniverse, i.e., a resolution as high as 10–21m or energies as large as 200 TeV, by means of quark flavour physics and lepton flavour violating processes in this decade well before this will be possible by means of any collider built in this century?
In a paper uploaded to the preprint server arXiv last year, Buras identified seven possible targets for this investigation, which he dubbed the “magnificent seven,” according to New Scientist. All seven are extremely rare decays of particles containing strange and bottom quarks, which Cliff calls “echoes from the zeptouniverse.” Luckily for Buras, some experiments are already searching for these ultra-rare decays.
One example of such a decay starts with the B meson—a kind of composite particle made of different quarks, as Cliff explains. In 2023, the Belle II experiment in Japan captured this decay in action, producing another particle called a kaon (or K meson) and two neutrinos. However, because the experiment wasn’t set-up to directly detect neutrinos, information about them is limited.
This isn’t the only ultra-rare decay that’s been detected recently, either. In September of 2024, the NA62 experiment at CERN recorded the decay of a positively charged kaon into a pion and a matter-antimatter pair. It’s thought that less than one in 10 billion kaons should decay in this way. Because this interaction is sensitive to Standard Model deviations, it’s identified as one of the prime targets for finding new physics. Today, the KOTO experiment in Japan is searching for a second confirmation of this kaon decay.
“The search for new particles and forces beyond those of the Standard Model is strongly motivated by the need to explain dark matter, the huge range of particle masses from the tiny neutrino to the massive top quark, and the asymmetry between matter and antimatter that is responsible for our very existence,” Buras wrote last year in the trade magazine CERN Courier. “As direct searches at the LHC have not yet provided any clue as to what these new particles and forces might be, indirect searches are growing in importance.”
Scientists are only beginning to peer inside the unknown frontier of the zeptouniverse, and until next-generation colliders are up and running, these extremely rare decays are our only windows into that universe.
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