Smashing protons into lead ions creates quark-gluon plasma that behaves like liquid

Dec 06, 2013
Figure 1: Inside the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider. Credit: Michael Hoch/CERN

The Large Hadron Collider (LHC) at CERN (European Organization for Nuclear Research) in Switzerland is best known for its discovery of the Higgs boson, formed during collisions between bunches of protons travelling close to the speed of light. However, it has also been smashing protons into ions of lead to generate clouds of quarks and gluons—the fundamental particles inside the protons and neutrons of the atomic nucleus. 

Experiments at the LHC have recently shown that this is more liquid than physicists had expected. Adam Bzdak from the RIKEN BNL Research Center at Brookhaven National Laboratory in the United States and colleague Vladimir Skokov of Western Michigan University now offer an explanation for the effect.

When two lead ions collide, the quark–gluon plasma they create flows like a liquid. This hydrodynamic flow carries other created in the collision, like boats drifting along a fast-flowing river.

Swapping one of the colliding ions for a proton should have made this hot soup behave less like a liquid because there would be fewer particles involved. Therefore, it came as a surprise when scientists at the LHC found that ejecta from these collisions were also carried along on a wave of plasma.

Bzdak and Skokov calculated what should have happened to the pions, and kaons generated in the collisions had the quarks and gluons acted independently, without hydrodynamic interactions. They then compared their results from this 'wounded nucleon' model with the LHC's data for the collisions. They found that pions—the lightest particles—behaved very nearly as forecast by the model, whereas the heavier kaons and protons had more momentum than predicted. The more would receive a greater momentum boost from hydrodynamic effects, says Bzdak. "It's an indication of hydrodynamics."

Bzdak and Skokov's calculations could help to refine scientists' understanding of the quark–gluon plasma that filled the Universe during its first moments. Pinning down how quarks and gluons interact at different energies would also help to refine the quantum theories that describe their behavior.

Bzdak notes that there is an alternative explanation for the LHC's observations, called the color glass condensate model. The model predicts that at very high energies, protons become saturated with a seething mass of extra gluons, which explains the extra momentum gained by more massive particles spraying from the collision. The next challenge for physicists, says Bzdak, is to test other experimental predictions of the two models to work out which of them offers the best description of the quark–gluon plasma.

Explore further: First glimpse inside a macroscopic quantum state

More information: Bzdak, A. & Skokov, V. Average transverse momentum of hadrons in proton–nucleus collisions in the wounded nucleon model. Physics Letters B 726, 408–411 (2013).

add to favorites email to friend print save as pdf

Related Stories

Quark matter's connection with the Higgs

Aug 27, 2012

(—You may think you've heard everything you need to know about the origin of mass. After all, scientists colliding protons at the Large Hadron Collider (LHC) in Europe recently presented stunning ...

RHIC's perfect liquid a study in perfection

Jun 18, 2013

( —When heavy ions (the nuclei of heavy atoms such as gold and lead) collide at high energies at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) and Europe's Large Hadron Coll ...

World's smallest droplet

May 17, 2013

( —Physicists may have created the smallest drops of liquid ever made in the lab. That possibility has been raised by the results of a recent experiment conducted by Vanderbilt physicist Julia Velkovska and her ...

Recommended for you

First glimpse inside a macroscopic quantum state

40 minutes ago

In a recent study published in Physical Review Letters, the research group led by ICREA Prof at ICFO Morgan Mitchell has detected, for the first time, entanglement among individual photon pairs in a beam ...

Theory of the strong interaction verified

13 hours ago

The fact that the neutron is slightly more massive than the proton is the reason why atomic nuclei have exactly those properties that make our world and ultimately our existence possible. Eighty years after ...

Fluctuation X-ray scattering

17 hours ago

In biology, materials science and the energy sciences, structural information provides important insights into the understanding of matter. The link between a structure and its properties can suggest new ...

Understanding spectral properties of broadband biphotons

18 hours ago

Advances in quantum optical technologies require scientists to control and exploit the properties of so-called biphotons. Biphotons occur when two photons become 'quantum-entangled' - spatially separate entities ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Dec 06, 2013
I remember when we were seeing signs of something that *looked* like a QGP ish thing in similar experiments at BNL, but to my recollection it was a pretty controversial claim at the time. Is that more soundly resolved now? That the density of particles is sufficient even from a p-Pb collision to make a QGP?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.