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

December 6, 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: Quark matter's connection with the Higgs

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).

Related Stories

Quark matter's connection with the Higgs

August 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 evidence strongly ...

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 ...

RHIC's perfect liquid a study in perfection

June 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 Collider (LHC), ...

Recommended for you

Fusion reactors 'economically viable' say experts

October 2, 2015

Fusion reactors could become an economically viable means of generating electricity within a few decades, and policy makers should start planning to build them as a replacement for conventional nuclear power stations, according ...

Iron-gallium alloy shows promise as a power-generation device

September 29, 2015

An alloy first made nearly two decades ago by the U. S. Navy could provide an efficient new way to produce electricity. The material, dubbed Galfenol, consists of iron doped with the metal gallium. In new experiments, researchers ...

Invisibility cloak might enhance efficiency of solar cells

September 30, 2015

Success of the energy turnaround will depend decisively on the extended use of renewable energy sources. However, their efficiency partly is much smaller than that of conventional energy sources. The efficiency of commercially ...

Extending a battery's lifetime with heat

October 1, 2015

Don't go sticking your electronic devices in a toaster oven just yet, but for a longer-lasting battery, you might someday heat them up when not in use. Over time, the electrodes inside a rechargeable battery cell can grow ...

1 comment

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.