|Matter and anti-matter of hydrogen atom|
By Jon Cartwright, Science Now, February 29, 2012
The big bang created a lot of matter—along with the same amount of antimatter, which wiped out everything and brought the universe to an untimely end. That's what accepted theoretical physics tell us—though things clearly didn't work out that way. Now, results from a U.S. particle smasher are providing new evidence for a subtle difference in the properties of matter and antimatter that may explain how the early universe survived.
The first evidence of a difference between matter and antimatter was found in the 1960s in the decay of particles called neutral kaons, which led to the awarding of a Nobel Prize in physics. In 2001, accelerators in the United States and Japan found more evidence for a difference in particles called B mesons. Then last year at CERN's Large Hadron Collider (LHC) near Geneva, Switzerland, evidence was found in a third system, D mesons, but there wasn't enough data to rule out a statistical fluke. The new results—which come from the Collider Detector at Fermilab (CDF) experiment near Chicago—are still not conclusive evidence, but they bring the chances of a fluke down to about one in 10,000. "I'm sure in a few days everyone in the field will feel much more confident that this is actually real," says Giovanni Punzi, spokesperson of the CDF experiment.
Physicists have long suspected that a difference in the properties of matter and antimatter is key to the early universe's survival. Such a difference—technically known as charge-parity (CP) violation—would have allowed normal matter to prevail over antimatter so that normal matter could go on to form all of the stuff we see in the universe today.
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.
The LHCb detector at CERN, and CDF at Fermilab, are two such experiments. They trace the paths of D0 meson particles and their antiparticles. These can decay into pairs of either pions or kaons, and by tallying these decay products, the LHCb and CDF teams can calculate the difference in decay rates between the D0 particles and antiparticles.
In November, the LHCb team reported that the decay rates differed by 0.8%—some eight times the amount the standard model is generally expected to allow, and perhaps enough to help explain the origin of matter's prevalence over antimatter. Unfortunately, the measurement was not very precise: The statistical significance was about 3 sigma, meaning there was about one chance in a 100 that it was a random blip in the data.
The latest CDF results—announced earlier today at a meeting in La Thuile, Italy—drastically decreased the odds of a fluke. They point to CP violation at the level of 0.6%, with a statistical significance of 2.7 sigma. Combined with the previous LHCb results, the CDF results bring the significance to about 3.8 sigma—or about one chance in 10,000 that the CP violation is a random blip.
The results cannot be claimed as a bona fide discovery, which requires a statistical significance of 5 sigma—or the chance of it being random at less than one in a million. Still, particle physicists are excited. "We cannot yet say for sure it is CP violation," says Angelo Carbone, a member of the LHCb collaboration. "But it's close."
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 current standard model can explain it.
Particle theorist Sebastian Jaeger at the University of Sussex in the United Kingdom thinks the answer is uncertain because no one is sure how far the standard model can be pushed. "The main issue is that CP [violation] is difficult to quantify—it's rather challenging, from a theoretical point of view, to make a prediction for it. ... So even if the significance becomes 5 or 10 sigma, the standard model may still not be ruled out."