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Brookhaven boffins boggle at baryons

Oi, stranger! Freeze ME out, will you?

By Richard Chirgwin, 25 Aug 2014

It's a long way from being a “discovery”, but it's getting physicists excited: the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab is accumulating evidence that could point to so-called “strange” Baryons.

Predicted by theory but never observed in the wild, the particles we're talking about are so named because they include at least one strange quark. As Brookhaven's release notes, the oddities can't be observed directly, but their effects can be inferred from the behaviour of matter around them.

“These heavy strange baryons, containing at least one strange quark, still cannot be observed directly, but instead make their presence known by lowering the temperature at which other strange baryons 'freeze out' from the quark-gluon plasma (QGP)” created in the collider, the statement says.

Baryonic matter is the stuff of the everyday: three-quark hadrons that form the basis of most of the matter in the universe. The same theory that describes the formation of mundane matter also predicts that short-lived baryons containing strange quarks can also exist.

In explaining the “freezing out” effect, the lab likens it to how the presence of salt changes the freezing temperature of water: “These 'invisible' hadrons are like salt molecules floating around in the hot gas of hadrons, making other particles freeze out at a lower temperature than they would if the 'salt' wasn't there,” said theoretical physicist Swagato Mukherjee.

So by observing the formation of omega baryons, cascade baryons, and lambda baryons in the RHIC – and particularly the temperatures at which they're observed – the BNL team is inferring the existence of the strange baryons.

Quantum chronodynamics provides the equations describing the interactions between quarks and gluons in a four-dimensional matrix (three spatial and one time dimension), and running the calculations through its supercomputer, the laboratory came up with a prediction of the behaviour of the strange baryons.

That prediction was then tested against the data observed coming from the collider, the laboratory says, and the outcome is that the findings “are helping physicists quantitatively plot the points on the phase diagram that maps out the different phases of nuclear matter, including hadrons and quark-gluon plasma, and the transitions between them under various conditions of temperature and density”.

The work has been published in Physical Review Letters, here. ®

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