Matter and anti-matter are always thought of as opposites. If they interact, they turn into pure energy. But there are cases, thanks to the peculiar laws of quantum mechanics, where particles and antiparticles are somewhat coexisting. Now, a new particle can be added to those cases.
Physicists report that the charm meson (subatomic particle that contains a quark and an antiquark, with one of them being of the charm type) D0 can be found in a superposition state of its particle – made of one charm quark and an up antiquark – and its antiparticle, which is made of one charm antiquark and an up quark. This is the fourth particle known to be able to do that after the strange-beauty mesons oscillations discovered decades ago. The findings, submitted for publication in Physical Review Letters and available as a preprint at arXiv, are yet to be peer-reviewed.
“Charm meson particles are produced in proton–proton collisions and they travel on average only a few millimetres before transforming, or decaying, into other particles,” Professor Tim Gershon at the University of Warwick, developer of the analytical technique used to make the measurement, said in a statement.
“By comparing the charm meson particles that decay after travelling a short distance with those that travel a little further, we have been able to measure the key quantity that controls the speed of the charm meson oscillation into anti-charm meson – the difference in mass between the heavier and lighter versions of charm meson.”
And this difference in mass between the matter and antimatter version of the charm meson is tiny. Putting it into a familiar unit of mass, we have a difference of 0.00000000000000000000000000000000000001 grams – or in scientific notation 1x10-38 grams (3.5x10-40 ounces). That’s like spotting a difference of ten grams between two supermassive black holes, like Sagittarius A*.
Achieving this precise measurement is possible by looking at a huge amount of particle decays. A total of 30.6 million decays of the charm meson were studied in the LHCb experiment, part of the Large Hadron Collider.
“What makes this discovery of oscillation in the charm meson particle so impressive is that, unlike the beauty mesons, the oscillation is very slow and therefore extremely difficult to measure within the time that it takes the meson to decay,” Professor Guy Wilkinson at University of Oxford, whose group contributed to the analysis, said:
“This result shows the oscillations are so slow that the vast majority of particles will decay before they have a chance to oscillate. However, we are able to confirm this as a discovery because LHCb has collected so much data.”
The observations provide more insights into a major debate in physics: the matter-antimatter asymmetry. According to the Standard Model of Particle Physics, matter and antimatter should have formed in equal numbers in the Big Bang and then eventually decayed together, leaving an empty universe.
Clearly, there was a little extra matter but it’s not exactly clear why. If the rate of oscillation from particle to antiparticle is different from the opposite direction, it could give us some clues. It could also be due to something beyond the standard model that might have caused this difference, and studying this particle further might tell us what.