CERN scientists at the world’s largest particle accelerator have made a unique antimatter breakthrough, which they believe could crack open the mystery behind the Big Bang itself.
The discovery, carried out at the Large Hadron Collider (LHC) near Geneva in Switzerland, pointed towards a heavier, short-lived cousin of protons and neutrons called the beauty-lambda baryon (Λb).
Composed of an up quark, a down quark, and a beauty quark, this composite subatomic particle decays at a different rate than its antimatter counterpart, a difference caused by a phenomenon known as the charge-parity (CP) violation, as recently observed.
Describing how particles with opposite charges don’t behave identically, the charge-parity (CP) violation is of key importance in cosmological theories that seek to explain the observed dominance of matter over antimatter in the universe.
The find offers new insight into why matter fits the Standard Model’s patterns and why it may have triumphed over antimatter nearly 14 billion years ago.
Study insights
The violation has sparked decades of investigation among physicists, ever since it was first spotted in the 1960s in mesons, particles made of a quark and an antiquark. Meanwhile, baryons, which are made of three quarks, were also expected to exhibit this phenomenon, but experiments like LHCb had only found subtle hints of the violation in them until now.
“The reason why it took longer to observe CP violation in baryons than in mesons is down to the size of the effect and the available data,” LHCb spokesperson Vincenzo Vagnoni explains, adding that it took more than 80,000 baryon decays to spot the matter-antimatter asymmetry for the first time.
“We needed a machine like the LHC capable of producing a large enough number of beauty baryons and their antimatter counterparts, and we needed an experiment at that machine capable of pinpointing their decay products,” he says.
Particles and their antimatter partners usually have the same mass but opposite charges, however, when they decay, like during radioactive processes, the same violation causes a crack in this expected symmetry, causing them to behave differently.
This effect shows up as a difference in how quickly particles and their antimatter twins decay into lighter particles, a process physicists can track using advanced detectors and analysis tools.
In their latest work, scientists analyzed data gathered by the LHCb detector during the collider’s first two runs, spanning 2009 to 2018. They looked for instances where the Λb particle decayed into a proton, a kaon, and two oppositely charged pions, and did the same for its antimatter counterpart, the anti-Λb. Ultimately, they counted the number of decays for each particle and calculated the difference between them.
First-of-its-kind detection
Results showed a 2.45 percent difference in decay rates between Λb and anti-Λb particles, enough to confirm CP violation with a 5.2 sigma level of confidence, the gold standard for a scientific discovery.
While CP violation in baryons has long been expected, the complex predictions of the Standard Model remain too imprecise to allow a thorough comparison with the LHCb measurement.
Oddly enough, the amount of CP violation it predicts is far too small to account for the matter–antimatter asymmetry observed in the universe. According to the researchers, this points to possible new sources of CP violation beyond the Standard Model.
“The more systems in which we observe CP violations and the more precise the measurements are, the more opportunities we have to test the Standard Model and to look for physics beyond it,” Vagnoni says. “The first ever observation of CP violation in a baryon decay paves the way for further theoretical and experimental investigations of the nature of CP violation, potentially offering new constraints for physics beyond the Standard Model.”
Meanwhile, Joachim Mnich, PhD, CERN director for research and computing, congratulated the LHCb collaboration on the result. “It again underlines the scientific potential of the LHC and its experiments, offering a new tool with which to explore the matter-antimatter asymmetry in the Universe,” Mnich concludes in a press release.