Evidence of particles oscillating between matter and antimatter

UK physicists have successfully demonstrated that a subatomic particle can be transformed into an antimatter particle and back again for the first time ever.

The researchers from the University of Oxford have recorded the first evidence of charm mesons converting into their antimatter counterpart before returning to their original form, employing the use of CERN’s LHCb experiment to conduct their investigation.

The scientific community has understood for over a decade that charm mesons – subatomic particles comprised of a quark and an antiquark – travel in a phenomenon called mixing, where they are a mixture of their particle and antiparticle states, with this discovery solidifying that they can fluctuate between the two.

This novel information can aid scientists in solving some of the most complex physics conundrums that deviate from the Standard Model, such as whether these transitions are a result of an unknown particle not predicted by the guiding theory.

In a state known as quantum superposition, a charm meson can be itself and an antiparticle simultaneously, forming two particles, each with its own mass, with one being heavier than the other; superposition allows the charm meson to oscillate between the two states.

The researchers measured the difference in mass between the two particles through the utilisation of data attained during the second run of the Large Hadron Collider, which was 0.00000000000000000000000000000000000001 grams or in scientific notation 1×10-38g – a result that crosses the ‘five sigma’ level of statistical significance that is required to claim a discovery in particle physics.

Due to the clinical precision of the measurement, the researchers used a University of Warwick developed technique to ensure that their analysis method was as precise.

In the Standard Model, there are only four types of particles that can transform into an antiparticle; the mixing phenomenon has been observed in strange mesons in the 1960s, beauty mesons in the 1980s, with oscillations between the two states only observed in the strange-beauty meson in 2006.

Guy Wilkinson, a professor at the University of Oxford, whose group contributed to the analysis, said: “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.  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.”

Professor Tim Gershon at the University of Warwick, the developer of the analytical technique used to make the measurement, said: “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. 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.”

This revolutionary discovery opens up an array of avenues for physics exploration, potentially explaining the enigmatic occurrence of matter-antimatter asymmetry. A key question the team want to investigate is whether the rate of particle-antiparticle transitions is equal to that of antiparticle-particle transitions, and if they are dictated by unknown particles that the Standard Model does not predict.

Dr Mark Williams at the University of Edinburgh, 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.”

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