An experiment that ran for eight years at the world’s largest and most powerful particle accelerator has revealed a tantalizing gap in our understanding of the basic building blocks of matter. Though the results are still preliminary, they hint at the possible discovery of new and exotic physics in the near future.
Scientists at CERN’s Large Hadron Collider (LHC), the reigning mega-machine of world particle physics, have spent years smashing protons together at high speeds to observe the decay of so-called “beauty quarks,” which are tiny subatomic particles.
Known as the LHCb experiment (“b” stands for “beauty”), this vast international collaboration has repeatedly detected an unusual anomaly that clashes with the predictions of the Standard Model of particle physics, a highly successful and well-corroborated theory that explains many of the forces and particles observed in the universe.
Beauty quarks decay into smaller units called leptons, a category that includes the subatomic particles electrons and muons. Though muons are more massive than electrons, these two “flavors” of lepton are otherwise functionally the same, meaning that the decay of beauty quarks should produce both of them at similar abundances.
But here’s the catch: the LHCb collaboration keeps seeing evidence that more electrons are created from this decay process compared to muons, a “violation” that could “imply physics beyond the Standard Model,” according to a new study that will be published in Nature Physics and has been released on the arXiv preprint server.
“The result is not conclusive yet,” cautioned Chris Parkes, an experimental particle physicist at the University of Manchester and the deputy leader of the LHCb, in a call. “I guess you'd describe it, maybe, as an intriguing hint” that has made the team “cautiously excited.”
The LHCb team considers these findings to be tentative in part because it is incredibly rare to witness the decay of beauty quarks into electrons and muons. Though the experiment collided billions of protons together from 2010 to 2018, this specific decay process was only captured a few thousand times.
These ultra-scarce events produced an overall ratio of about 85 muons to every 100 electrons, indicating a slight but persistent preference for electrons in the aftermath.
Right now, the odds that these results represent a statistical fluke or an instrumental error are about one in 1,000. In physics jargon, this percentage is called a “three-sigma” level of standard deviation. However, Parkes and his colleagues won’t be truly confident in the existence of this anomaly until they attain a five-sigma level, in which the possibility of statistical error is downgraded to a one-in-a-million chance.
“It's the one-in-1,000 level where you start to scratch your head and say, ‘Wow, this is a bit intriguing,’” Parkes said. “Yes, it could still be a statistical fluctuation. It's not conclusive.”
That said, “the holy grail—the measure in physics of a gold-plated ‘Eureka moment’—is normally set at the five-sigma level,” he added. “That would be tremendous.”
In their quest to reach this level, the LHCb collaboration is currently installing a next-generation version of their experiment that will collect up to five to ten times more data than the first iteration. As a result, the team should be able to gather as much information within the next few years as they did from 2010 to 2018. The entire LHC is currently shut down for the upgrade, which is expected to be completed in 2022, so the experiment should resume next year.
Moreover, an experiment called Belle-II, operated at the SuperKEKB collider in Japan, will also examine the decay process of beauty quarks in the 2020s, which could help corroborate LHCb’s weird findings.
If the discrepancy persists in these advanced experiments, and that hallowed five-sigma level is attained, it would upend the concept of “lepton universality,” which is the notion that leptons should behave the same way in interactions that don’t have anything to do with their mass.
Lepton universality is not some debatable sideshow of the Standard Model. It’s a core pillar that would radically challenge our assumptions if toppled.
“It really is a very firm prediction of the theory,” Parkes said. “If it did turn out that we confirm the hints of today, it would be quite a big change in our understanding of how fundamental particles work.”
Some scientists have already proposed trippy new ideas that might explain the overabundance of electrons relative to muons. One example is the speculative existence of a particle called a leptoquark that might interact with electrons and muons differently, thereby allowing one flavor of lepton to outpace the other in the decay process.
When brainstorming these possibilities, it’s possible to “step into the realm of wild speculation and philosophy,” Parkes admitted. But he noted that while the Standard Model is incredibly successful and has withstood major challenges for decades, it has its limits.
Many phenomena in the universe—such as dark matter, the prevalence of matter over antimatter, or the mechanics of gravity on quantum scales—cannot be explained by the model. For this reason, it’s crucial to find the weaknesses in this battle-tested framework, because those “here be dragons” areas may be exactly where broader truths about our reality can be uprooted.
“Who knows where this road that we're starting on today could lead?” Parkes said. “First, we would need to confirm it, then we would look twice to look at what the explanations might be.”
“But of course, the dream would be that, maybe, it's the very first steps on the road to trying to answer some of these big questions that have been in physics for many years and for which we don't have explanations,” he concluded.