QCD and heavy ion physics

The Quark-Gluon Plasma (QGP) is

• a low-viscosity fluid

• made of quarks and gluons

• made of hadrons

Answer explanation

The QGP is a hot and dense state of matter in which quarks and gluons, normally confined within hadrons (such as protons and neutrons), are able to move over larger distances. In such extreme conditions, quarks and gluons behave as an almost perfect fluid rather than as a gas of particles. Hadrons, on the other hand, are composite particles made of quarks and gluons that are bound together and do not exhibit the same fluid properties as the QGP.

The QGP only exists in human-made laboratories and is produced through ultra-relativistic heavy-ion collisions.

• True

• False

Answer explanation

Neutron stars also have the temperature and density conditions high enough for the production of a QGP in their core

In accelerators, the QGP can be detected directly since its lifetime is long enough to reach the detectors.

• True

• False

Answer explanation

The QGP's lifetime is on the order of yoctoseconds, which corresponds to a distance of around 10 femtometers (10 times the size of a proton); the detectors are positioned much further away (the innermost ATLAS detector layer sits at a radius of 33 mm).

One of the QGP's experimental signatures is the apparent suppression of jets.

• True

• False

Answer explanation

The jet-initiating parton propagates through the QGP and, via interactions with its constituents, thermalizes its energy with the medium. The resulting jet is then reconstructed with a smaller energy.

What happens when a photon propagates through the QGP?

• interacts with its constituents

• propagates almost obliviously to its existence