Physicists have long suspected that a difference in the properties of matter and antimatter is key to the early universe's survival. |
The first evidence of a difference between matter and antimatter was found in the 1960s in the decay of particles called neutral kaons. In 2001, accelerators in the United States and Japan found more evidence for a difference in particles called B mesons. In 2011 at CERN, Switzerland, evidence was found in a third system, D mesons, but there wasn't enough data to rule out a statistical fluke. New results from the Collider Detector at Fermilab near Chicago have confirmed CERN and bring the chances of a fluke down to about one in 10,000.
Such a matter-antimatter difference or "charge-parity (CP) violation" would have allowed normal matter to prevail over antimatter after the big bang. To witness CP violation, physicists study particles to see if there is any difference in the rate of decay between normal particles and their antiparticles. The accepted theory of elementary particles, the standard model, allows for a low level of CP violation—including that revealed in the discoveries of the 1960s and 2000s—but not enough to explain the prevalence of normal matter. So researchers have been trying to find cases in which CP violation is higher.
In November, the LHCb team (at CERN) reported that the decay rates differed by 0.8%, about eight times the amount the standard model is generally expected to allow. Unfortunately, the measurement was not very precise: The statistical significance was about 3 sigma, meaning there was about a 1% chance that it was a random blip in the data. The new data from Fermilab brings this down to about 0.01%, which is still not good enough for physicists who require 99.99994% (or "5 sigma") confidence.
Paul Harrison, an experimental particle physicist at the University of Warwick in the United Kingdom, says the 5-sigma standard is important because it helps avoid biases that arise in lopsided statistical distributions. But he thinks it is reassuring that the results come from two independent experiments. "I wouldn't be expecting a mistake in the experiments at this point," he says. "These guys are serious people. ... They've been at it a long time, and they know what they're doing."
To see whether the statistical significance can be improved toward 5 sigma, onlookers will have to wait until later this year, when the LHCb team examines the rest of its data. But even if the CP violation turns out to be real, there is the question of whether it is "new physics"—in other words, whether the observations can be explained without a change in the standard model.