Subatomic particle seen changing to antiparticle and back for the first time

An extraordinarily precise measurement made by UK researchers using the Large Hadron Collider beauty (LHCb) experiment at CERN has provided the first evidence that charm mesons can change into their antiparticle and back again.

For more than 10 years, scientists knew that charm (D⁰, ‘D-zero’) mesons, subatomic particles that contain a quark and an antiquark, can travel as a mixture of their particle and antiparticle states, a phenomenon called mixing. However, this new result shows for the first time that they can oscillate between the two states.

In the strange world of quantum physics, the D⁰ meson can be itself and its antiparticle at once. This state, known as quantum superposition, results in two particles each with their own mass – a heavier and lighter version of the D⁰ particle. This superposition allows the D⁰ to oscillate into its antiparticle and back again.

The research, published today in Physical Review Letters, received funding from the Science and Technology Facilities Council (STFC), and involved colleagues from the Universities of Edinburgh, Oxford and Warwick.

Dr Mark Williams at the School of Physics and Astronomy’s Particle Physics Experiment research group, who convened the LHCb Charm Physics Group within which the research was performed, said:

Tiny measurements like this can tell you big things about the Universe that you didn’t expect.

Prof Franz Muheim from the School of Physics and Astronomy’s Particle Physics Experiment research group, who is chair of the LHCb Editorial Board and was internal reviewer of the paper said:

This observation of the mixing between neutral charmed meson is a further milestone in trying to elucidate the difference between the matter-antimatter asymmetry in the Universe.

Using data collected during the second run of the Large Hadron Collider, the researchers measured a difference in mass between the two particles of 0.00000000000000000000000000000000000001 grams – or in scientific notation 1x10^-38g. This small mass difference leads to a very slow rate of oscillation: the time taken for one transition from particle to antiparticle and back is over 1000 times longer than the typical particle lifetime, making this a very rare process. It took all of the data collected by the LHCb experiment from 2015-2018 to make this discovery, along with a novel method to reduce experimental uncertainties.

There are only four types of particle in the Standard Model, the theory that explains particle physics, that can turn into their antiparticle. The mixing phenomenon was first observed in Strange (K⁰) mesons in the 1960s and in beauty (B) mesons in the 1980s. Until now, the only other particle that has been seen to oscillate is the strange-beauty (B_S) meson, a measurement made in 2006.

This discovery of charm meson oscillation opens up a new and exciting phase of physics exploration; researchers now want to understand the oscillation process itself, potentially a major step forward in solving the mystery of matter-antimatter asymmetry. A key area to explore is whether the rate of particle-antiparticle transitions is the same as that of antiparticle-particle transitions, and specifically whether the transitions are influenced/caused by unknown particles not predicted by the Standard Model. 

The result, 1x10^-38g, crosses the ‘five sigma’ level of statistical significance that is required to claim a discovery in particle physics.