How small can a liquid be?

Physicists have spent more than two decades studying one of the strangest forms of matter ever created in a laboratory: the quark-gluon plasma (QGP).

Quarks and gluons are the smallest pieces of matter. A QGP existed in the first few millionths of a second after the Big Bang, before the quarks bound together to form the first protons and neutrons.

Researchers routinely create small droplets of QGP by colliding heavy atomic nuclei at extremely high energies. These droplets are extraordinarily hot, reaching trillions of degrees centigrade. Their behaviour, however, has been more surprising: instead of acting like a gas, the QGP behaves like a fluid.

A new study in Physical Review Letters has now reported a new ‘boundary’ in this field. By smashing together oxygen nuclei at the Large Hadron Collider (LHC), researchers have found the smallest dollop of QGP — so far — that still behaves like a fluid.

Physicists have known for decades now that energising and colliding lead nuclei, which are much heavier than oxygen nuclei, produces QGP. The new study addresses a different question: how small can a droplet of matter be and still have fluid-like behaviour?

The LHC usually smashes together protons. The researchers picked oxygen ions because they occupy a middle ground between the tiny clumps of energy and matter created by smushed protons and the massive ones formed by lead ions. A middle ground that could reveal the tipping point where subatomic matter starts to resemble a fluid.

The research team analysed collisions between oxygen nuclei at the LHC, looking for fast-moving particles that emerged from the collision, and compared their numbers with what would be expected if no dense medium had formed.

If no dense medium forms, physicists can predict how many such particles should appear. But if a medium is present, some of the energetic quarks and gluons should lose energy before they escape, reducing the number of energetic particles eventually reaching the detector.

The researchers have reported signs of such a suppression. It was less pronounced in collisions involving the oxygen ions, but it was noticeable.

The finding suggests that oxygen-oxygen collisions can create a medium large enough to affect the motion of energetic quarks and gluons. The data “are also in better agreement with theoretical models that include quark–gluon energy loss than they are with models that omit it,” Physical Review Letters associate editor Nikhil Karthik wrote in Physics.

Properties like viscosity and flow are associated with fluids, and these fluids usually have an enormous number of particles. A droplet of water sliding down a wall has quadrillions of water molecules. A glob of honey big enough to show how slowly it slides has roughly a quintillion sugar molecules. Yet the QGP is much, much smaller — it has maybe thousands of quarks and gluons — but it is still a fluid.

According to the new finding, a strongly interacting medium can emerge even in collisions involving relatively light nuclei. At the same time, it makes the mystery of where exactly the transition between a fluid-like medium and a gas of largely independent particles emerges more intriguing.

mukunth.v@thehindu.co.in