Physicists have seen traces of quarks in the matter of the early universe
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- Physicists have seen traces of quarks in the matter of the early universe
In the first moments after the birth of the universe, there was an unusual state of matter in which quarks and gluons moved at temperatures of several trillion degrees. Today, physicists can recreate short-lived quark-gluon plasma in heavy ion collisions at the Large Hadron Collider.
Now, researchers have received direct evidence that quarks passing through it are capable of creating waves and vortex movements similar to the trace of a particle in a liquid. About how quarks leave traces in the matter of the early Universe — in the material of Izvestia.
The Quark trace: what was seen in the quark-gluon plasma
The study was published in the scientific journal Physics Letters B. Scientists studied how the quark-gluon plasma reacts to a fast-moving particle. As a result, they found characteristic changes in the energy distribution — bursts and vortex motion — in the region opposite to the direction of the detected Z boson.
The resulting picture corresponds to theoretical calculations, according to which plasma should react to the movement of a quark like a liquid.
Yenji Lee, Professor of Physics at the Massachusetts Institute of Technology
There has long been a debate in our field about whether plasma should react to quarks. Now we see that plasma is incredibly dense, so much so that it can slow down quarks and create splashes and vortices like a liquid. Thus, the quark-gluon plasma is indeed a primeval soup.
Liquid from the first moments: why plasma is compared to water
In the early universe, temperatures reached several trillion degrees. Quarks and gluons were in a state called a quark-gluon plasma. As the universe expanded and cooled, they combined into protons, neutrons, and other particles that make up the matter around us today.
Quark-gluon plasma is considered to be a state of matter that existed at the earliest moments in the history of the universe. It is formed even today in experiments during collisions of heavy ions, but it exists for an extremely short time.
At the Large Hadron Collider, physicists recreate small volumes of such plasma by colliding heavy ions at almost the speed of light. The resulting drops of quark-gluon plasma live for less than one quadrillionth of a second. Therefore, it is impossible to directly observe them — the properties of the plasma have to be restored by the distribution of particles that appeared after the collision.
One quark instead of two: how physicists were able to see the wave
Previously, scientists tried to detect traces of the movement of quarks by studying pairs of quarks and antiquarks. However, this approach created a problem: the particles move in opposite directions and can leave overlapping trails, which makes it difficult to determine the contribution of each quark.
Yenji Lee
When two quarks are formed, the problem is that when these two quarks move in opposite directions, one quark obscures the trace of the second quark. We have developed a new technique that allows us to observe the effect of a single quark on the quark-gluon plasma through another pair of particles.
The researchers used another method — the Z boson as a kind of label. At the same time, the Z boson almost does not interact with the surrounding plasma, so it can be used as a reference point to determine the direction of the quark's movement.
The Z boson is a neutral elementary particle associated with the weak interaction. In some collisions, a quark and a Z-boson arise with large momenta and fly away in approximately opposite directions.
This made it possible to look for changes in the plasma precisely on the side where the quark was supposed to move. The waves and other disturbances that occur there could be attributed to the impact of a single particle.
13 billion collisions — and about 2 thousand necessary events
The team analyzed data on about 13 billion heavy ion collisions at the Large Hadron Collider. Among them, the researchers identified about 2 thousand events in which the Z boson was formed.
For these events, scientists traced how energy was distributed in a short-lived quark-gluon plasma. In the direction opposite to the Z boson, a characteristic pattern with bursts and vortex motion has repeatedly appeared. Since the Z boson itself almost does not interact with the plasma, the observed perturbations were associated with the movement of the quark.
The results were consistent with the predictions of the hydrodynamic model.
Yenji Lee
We have received the first direct evidence that a quark does indeed carry more plasma with it during its movement. This will allow us to study the properties and behavior of this exotic liquid in unprecedented detail.
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