LHC's D-meson study wraps up antimatter 'flip' story

LHCb experiment The LHCb experiment has an established record studying these matter-antimatter "flips"

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Researchers at the Large Hadron Collider have witnessed particles called D-mesons flipping from matter into antimatter and back.

Antimatter is just like normal matter, but with opposite electric charge.

Such "oscillations" are well known among three other particle types, but this is the first time D-mesons have been seen doing it in a single study.

The team behind the collider's LHCb detector have put their results on the Arxiv repository.

The manuscript will be published in Physical Review Letters.

In the complicated zoo of subatomic physics, particles routinely decay into other particles, or spontaneously change from a matter type to their antimatter counterparts.

This "oscillation" forms an important part of the theory that attempts to tame the zoo - the Standard Model.

Mesons are part of a large family of particles made up of the fundamental particles known as quarks.

Start Quote

This is a nice moment, it's a sort of completeness”

End Quote Chris Parkes University of Manchester

The protons and neutrons at the centres of the atoms of matter we know well are each made up of three such quarks.

Mesons, on the other hand, are made of just two - specifically one quark and one antimatter quark.

Theory holds that four members of the meson family can undergo the matter-antimatter oscillation - the matter and antimatter quarks both flip to their opposites.

Three particle types - K-mesons and two types of B-mesons had been caught in the act before.

LHCb has already been intimately involved in refining those prior measurements; in March 2012, the team confirmed earlier oscillation observations of a meson called Bs, and published the result in Physics Letters B.

On Tuesday, the team published results that set a new record of precision on the oscillations of the B0 meson, in the same journal.

Statistics of a 'discovery'

Swiss franc coin
  • Particle physics has an accepted definition for a "discovery": a five-sigma level of certainty
  • The number of standard deviations, or sigmas, is a measure of how unlikely it is that an experimental result is simply down to chance, in the absence of a real effect
  • Similarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a "loaded" coin
  • The "three sigma" level represents about the same likelihood of tossing nine heads in a row
  • Five sigma, on the other hand, would correspond to tossing more than 21 in a row
  • Unlikely results are more probable when several experiments are carried out at once - equivalent to several people flipping coins at the same time
  • With independent confirmation by other experiments, five-sigma findings become accepted discoveries

The new paper represents the last of the four mesons that had never been seen flipping from matter to antimatter and back in a single measurement.

Other experiments had seen some evidence of the D-meson's flipping, but this is the first that crosses the "five sigma" level of statistical significance that particle physicists use to denote an official discovery.

"This is a nice moment, it's a sort of completeness," said Chris Parkes of the University of Manchester, spokesman for UK participation in the LHCb experiment.

"There are four systems in nature that oscillate, and this is the last one where a single-channel measurement has crossed the five-sigma threshold - we know now about mixing in all four of these systems," he told BBC News.

But what remains open is the question of why the Universe we see is made overwhelmingly of matter rather than antimatter - they should both have been created in equal measure during the Big Bang.

Mesons measured by LHCb have already hinted at an answer, and Prof Parkes says that is the next target.

"The LHC has had a tremendous first three years of operation, and now as we enter the first shutdown... we look forward to probing in great detail the [D meson] system using the data we've got so far and data to come."

The Standard Model and the Higgs boson

Standard model

The Standard Model is the simplest set of ingredients - elementary particles - needed to make up the world we see in the heavens and in the laboratory

Quarks combine together to make, for example, the proton and neutron - which make up the nuclei of atoms today - though more exotic combinations were around in the Universe's early days

Leptons come in charged and uncharged versions. Electrons - the most familiar charged lepton - together with quarks make up all the matter we can see; the uncharged leptons are neutrinos, which rarely interact with matter

The "force carriers" are particles whose movements are observed as familiar forces such as those behind electricity and light (electromagnetism) and radioactive decay (the weak nuclear force)

The Higgs boson came about because although the Standard Model holds together neatly, nothing requires the particles to have mass; for a fuller theory, the Higgs - or something else - must fill in that gap

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