High Energy Collisions Of Lead Ions Produce Quark-Gluon Plasma

  • Physicists collide lead ions to create a Quark-Gluon Plasma at the Large Hadron Collider. 
  • The collision creates high-energy partons, which further split into two or more partons.
  • It gives new insights of color coherence and other properties of interactions of partons in the Quark-Gluon Plasma. 

A team of researchers collides lead ions to produce a quark-gluon plasma (QGP). The high energy collisions generate a unique structure of jets that could provide new methods to study QGP. The collisions also create high-energy gluons and quarks, collectively known as partons, which further produces hadron jets, like kaons and pions.

What’s Quark-Gluon Plasma?

QDP is a state of matter in quantum chromodynamics that exists at ultrahigh density and temperature. It is thought to consist of strong-interacting asymptotically-free quarks and gluons – state of matter that filled the universe after big bang. These quarks and gluons are confined by color confinement inside hadrons or atomic nuclei.

It’s an analogy with traditional plasma where electrons and nuclei, pinched inside atoms by electrostatic forces, can move freely.

How Did They Do It?

Now a research done at the Large Hadron Collider suggests that structure of jet formed by collisions of lead ions could reveal some of the effects of QGP on the partons. These collisions create high-energy partons, which further split into two or more partons.

This is quite similar to high-energy photons’ collisions that create electron-positron pairs. Since one cannot observe the splitting process directly, physicists tried to demonstrate that jet measurements might indirectly yield data on the splitting.

They extracted data of proton-proton and lead-lead collision experiments from 2015. For both experiments, they regenerated the structure of particles’s “subjets” within the jets. Using this, they deduced a factor responsible for sharing momentum among the partons associated with splitting process.

Reference: Phys. Rev. Lett | doi:10.1103/PhysRevLett.120.142302

The mismatch between both cases (parton splitting factors of proton and lead) proved that the factor relies on the partons-propagating medium. And because quark-gluon plasma is thought to be produced by only collisions of lead-lead, this factor (that depends on the medium) might also reveal effects of quark-gluon plasma on partons.

Technical Details

Quark-Gluon Plasma created in high energy collisionLarge Hadron Collider/CMS

In this study, physicists have presented the first calculation of the splitting function of lead-lead and proton-proton collisions at a center of mass energy of 5.02 TeV (tera electron volts) per nucleon pair.

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They used HERWIG++ and PYTHIA even generator to recreate the measured splitting function in peripheral lead-lead and proton-proton collision, at 15 percent level. A steeper zg distribution is seen in central lead-lead collisions, showing that the splitting process of parton is enhanced by the hot medium generated in heavy ion collisions. This gives new insights of color coherence and other properties of interactions of partons in the QGP.

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