Level 1 - Absolute Beginner

Just after the universe began, everything was very hot. It was so hot that matter was a thick soup. Scientists call this soup quark gluon plasma.

Scientists make tiny drops of this soup in a big machine in Switzerland. The machine is called the Large Hadron Collider. It crashes heavy atoms together.

In the new study, scientists looked at 13 billion crashes. Only about 2,000 of them were useful. In those, a small particle helped them find the answer.

They saw waves in the soup, like a boat makes waves in water. This shows the early universe acted like a liquid.

universe

Everything that exists, including all stars and space.

hot

Having a high temperature.

soup

A thick liquid food; here used to describe hot early matter.

machine

A device built to do work.

crash

To hit something hard.

particle

A very small piece of matter.

wave

A moving line on the surface of water.

liquid

Something that flows, like water.

Level 2 - Elementary

For the first few microseconds after the Big Bang, the universe was too hot for ordinary matter to exist. Instead there was quark gluon plasma, a soup of the smallest building blocks of matter at temperatures of several trillion degrees. Physicists often call it the first liquid in the universe.

Scientists can recreate tiny drops of it by smashing lead ions together at the Large Hadron Collider at CERN in Switzerland. The drops last less than a quadrillionth of a second, so measuring them is extremely hard.

The CMS collaboration wanted to see what happens when one fast quark travels through the plasma. The problem was that quarks are usually made in pairs, and the trail of one got mixed up with the trail of the other. The solution was clever. In about 2,000 collisions out of roughly 13 billion, a particle called a Z boson was produced. A Z boson passes through the plasma almost without touching it, so it works as a marker. Anything disturbed on the opposite side must come from the quark.

The team, led by physicists at MIT and published in the journal Physics Letters B, found splashes and swirling patterns exactly where the quark had passed. Yen-Jie Lee of MIT said the plasma is dense enough to slow a quark and leave marks like a liquid does.

microsecond

One millionth of a second.

quark

One of the tiny particles that make up protons and neutrons.

ion

An atom that has lost or gained electrons, so it carries a charge.

collision

An event in which two things crash into each other.

collaboration

A large team of scientists working together on one experiment.

marker

Something used to show where or what to look at.

swirl

A circular, twisting movement.

dense

Having a lot of material packed into a small space.

Level 3 - Intermediate

Physicists have long described quark gluon plasma as a near perfect liquid, a fluid so free of internal friction that nothing else in nature comes close. Proving that it responds collectively, the way water responds to a passing boat, has been much harder. A result published in Physics Letters B by the CMS collaboration at CERN now supplies the clearest evidence yet, and it rests on an unusually elegant piece of experimental reasoning.

The plasma itself exists only in the most extreme conditions. It filled the universe for the first few microseconds after the Big Bang at temperatures of several trillion degrees Celsius, and the droplets recreated by colliding lead ions at the Large Hadron Collider survive for less than a quadrillionth of a second. Within that instant, a quark produced in the collision tears through the medium, and theory says it should leave a wake behind it.

Earlier attempts to find that wake were defeated by geometry. Quarks are normally created in back to back pairs, so the disturbance caused by one overlapped the disturbance caused by the other, and neither could be isolated. The CMS team turned instead to the Z boson, a heavy particle that barely interacts with the plasma at all. Sifting roughly 13 billion collisions, they identified about 2,000 in which a Z boson was produced, and because the boson escapes almost untouched, any energy pattern on the opposite side can be attributed with confidence to the quark travelling the other way. Yi Chen's group at Vanderbilt University helped develop the technique alongside the MIT led team.

What the detectors recorded were splashes and swirling structures positioned exactly where the hybrid model built by the MIT theorist Krishna Rajagopal had predicted a wake should appear. Rajagopal, who was not involved in the measurement, noted that many physicists had long argued such wakes must exist. Yen-Jie Lee of MIT, who led the analysis, described the medium as dense enough to slow the quark and throw up patterns of exactly that kind, and called the plasma a primordial soup. The next step is quantitative: measuring how large the wakes are, how fast they travel, how far they reach and how quickly they fade.

fluid

A substance that flows, either liquid or gas.

friction

Resistance that slows motion between surfaces or layers.

collectively

As a whole, rather than as separate parts.

medium

The material through which something travels.

wake

The trail of disturbance left behind a moving object.

back to back

Pointing in opposite directions from the same origin.

sift

To examine a large quantity carefully to find something.

quantitative

Based on measured amounts rather than general description.

Level 4 - Advanced

The phrase near perfect liquid has attached itself to quark gluon plasma for two decades, and it has always carried an unpaid debt. A liquid is defined not by its temperature but by its collective response: push it, and the disturbance propagates. Demonstrating that the plasma does this when a single energetic parton traverses it, rather than merely losing energy to it, has resisted experimentalists since the first heavy ion runs. A measurement published by the CMS collaboration in Physics Letters B, volume 874, article 140120, closes much of that gap, and it does so through a piece of experimental design rather than brute statistics.

The medium under study is as transient as it is extreme. Quark gluon plasma filled the cosmos for the opening microseconds after the Big Bang at temperatures of several trillion degrees Celsius, and the droplets reconstituted in lead ion collisions at the Large Hadron Collider endure for under a quadrillionth of a second. Theory, specifically the hybrid strong interaction model developed by the MIT theorist Krishna Rajagopal, holds that a fast quark crossing such a medium must deposit momentum into it and leave a wake, a Mach like structure whose shape encodes the fluid's transport properties.

The obstacle was never sensitivity but confounding. Hard scattering produces quarks in approximately back to back pairs, so each wake sat in the acceptance region of the other, and no amount of additional luminosity could separate the two contributions. The CMS analysis replaced one of the partons with a colourless tag. A Z boson, carrying no colour charge, traverses the plasma essentially without interacting, which means that in events containing a Z the azimuthal region opposite the boson can be attributed entirely to the recoiling quark. Of some 13 billion lead ion collisions, roughly 2,000 yielded a Z, a severe reduction in sample size traded for an unambiguous geometry, with Yi Chen's group at Vanderbilt University contributing to the method alongside the MIT led analysis team.

The correlations of Z bosons with hadrons revealed precisely the splash and vortical structure Rajagopal's model anticipated, which is the strongest statement yet that the plasma reacts as a continuous fluid rather than as a collection of independent scatterers. Rajagopal, who was not an author, observed that the existence of such wakes had long been argued on theoretical grounds; Yen-Jie Lee of MIT, who led the work, emphasised that the medium is dense enough to decelerate a quark and generate liquid like structures, and used the phrase primordial soup to describe it. The programme now becomes metrological. Extracting the amplitude, propagation speed, spatial extent and decay time of the wake would place direct constraints on the plasma's viscosity and sound speed, and the collaboration intends to apply the Z tagging technique to larger datasets to do exactly that.

parton

A quark or gluon treated as a constituent of a proton or neutron.

propagate