Spot the baryon
A hint of matter and antimatter behaving differently to each other has been spotted in a new particle for the first time. If the find bears out, it could help explain the existence of all the matter in the universe, and why it was not snuffed out by antimatter long ago.
Physicists think that the big bang should have produced equal amounts of matter and antimatter. But these contrasting particles annihilate each other in a puff of energy whenever they meet, so they should have destroyed each other long ago.
The fact that there is enough matter in the universe today for us to exist and wonder why, means that some mechanism must have favoured matter over antimatter.
“Today we have this complete imbalance between matter and antimatter. We have no evidence of antimatter in the universe,” says Nicola Neri of the National Institute for Nuclear Physics in Milan, Italy. “This is one of the main questions we’d like to answer.”
One way the two could differ is by violating a rule about the way the laws of physics affect particles and antiparticles known as CP symmetry. Previously, experiments showed that CP symmetry is in fact violated in particles called mesons, which are made up of a quark and an antiquark. Those results garnered two Nobel prizes, one in 1980 and one in 2008.
But it wasn’t enough. “The sources we’ve found so far are not sufficient to explain this huge imbalance,” Neri says.
Keep calm and baryon
Now, Neri and his colleagues have checked another kind of particle: baryons, which are made of three quarks and no antiquarks. Neutrons and protons, the building blocks of matter, are baryons.
They watched for differences in the decay of baryons and their antimatter counterparts made of three antiquarks, and they were lucky. The particles decayed in a way that seems to violate CP symmetry.
“This is the first hint, a sign that something is going on there,” Neri says.
During the first run of the Large Hadron Collider at CERN near Geneva, between 2011 and 2012, the large international team used the LHCb experiment to watch the decay of heavy baryons called lambda-b particles, which are about six times heavier than a neutron.
They observed about 6500 instances of lambda-b particles decaying into a proton and three particles called pions, and about 1000 instances of a different decay that included particles called kaons as well. Theory suggested that there should be a lot of CP violation in these events, but because they needed the extreme energies of the LHC to be produced, they had never been seen before.
“It was not anticipated that we could have such a large signal yield,” Neri says. “That was a nice surprise.”
The kaon decay looked normal. But the pion version showed a deviation from the standard predictions to a statistical significance of 3.3 sigma, meaning random fluctuations would produce a similar signal less than once every 1000 times.
Not a chance
Particle physicists consider that level of significance evidence that something strange is happening, but it’s not quite enough to declare discovery. That will have to wait for 5-sigma, when the odds of random fluctuations producing a similar signal are less than one in a million.
But the LHC has been upgraded and collected more data since these measurements, and is still ramping up to its full potential. Neri expects to increase their data set by at least a factor of 10.
And even if the signal goes away with more data, it’s still useful to be able to compare CP violation in baryons and mesons, Neri says.
“We can do studies for the first time in baryon decays and make good comparisons with decays with similar quark transitions, and in that way get information on the underlying physics,” he says. “We are entering an era with LHCb where we can make precision measurements in CP violation in heavy baryons. You open a new series of measurements with this kind of result. That’s the excitement.”
“It’s an important observation,” says David MacFarlane at the SLAC National Accelerator Laboratory in Menlo Park, California, who was on the team that measured CP violation in mesons. “The more systems we see CP violations in, the more chance we have to understand whether the standard model is correct, or whether there are other sources.”
Journal reference: Nature Physics, DOI: 10.1038/nphys4021