Synopsis: A Precise Probe of the Quark-Gluon Plasma

Properties of the quark-gluon plasma can be inferred from measurements of jets and Z bosons simultaneously produced in the ion collisions that create the plasma.
Synopsis figure
CMS Collaboration/CERN

Immediately after the big bang, the Universe was filled with a hot, dense soup of quarks and gluons. Scientists can recreate this quark-gluon plasma (QGP) in particle accelerators by smashing heavy ions together. The collisions also produce partons (quark and gluons) that subsequently produce hadron jets. The energy lost by the partons as they travel through the QGP can be determined from measurements of the jets, revealing properties of the QGP. Now, the CMS Collaboration at the Large Hadron Collider has measured a jet produced in coincidence with so-called Z bosons—the carriers of the weak interaction. These combined Z-boson–jet measurements may characterize the QGP more precisely than conventional measurements probing the jets only.

Z bosons interact with the QGP much more weakly than the partons do, so they don’t lose energy as they travel through it. Thus the bosons’ energy is very close to the initial energy of the partons emerging from the ion collisions. And the difference between the boson and parton energies can reveal the QGP’s properties.

The team carried out two sets of experiments—collisions between heavier lead ions and collisions between lighter protons. They then compared the energies of the Z bosons—observed by detecting electrons or muons from the bosons’ decays—to those of the coincident jets. They found that the ratio of parton energy over Z boson energy in lead collisions is smaller than in proton collisions. The finding is not unexpected: lead collisions create a large QGP for the partons to travel through and interact with. However, the measurements more precisely characterize the energy lost by partons. Such information can be used to discriminate between models with different QGP parameters.

This research is published in Physical Review Letters.

–Katherine Wright

Katherine Wright is a Contributing Editor for Physics.


Features

More Features »

Announcements

More Announcements »

Subject Areas

Particles and Fields

Previous Synopsis

Quantum Information

Plug-and-Play Photon Source

Read More »

Next Synopsis

Related Articles

Synopsis: An Expanding Universe in the Lab
Atomic and Molecular Physics

Synopsis: An Expanding Universe in the Lab

The rapid expansion of a Bose-Einstein condensate can mimic the expansion of the Universe. Read More »

Viewpoint: Homing in on Axions?
Particles and Fields

Viewpoint: Homing in on Axions?

A search for dark matter axions with unprecedented sensitivity tests a previously inaccessible parameter range for these hypothetical particles. Read More »

Focus: Neutrinos with a Single Energy
Particles and Fields

Focus: Neutrinos with a Single Energy

Neutrinos in a beam have a wide range of energies, but a new trick allowed researchers to isolate fixed-energy neutrinos, which can improve the precision in future experiments. Read More »

More Articles