Fresh Direct Evidence for Tiny Drops of Quark-Gluon Plasma

Particles of light from collisions of deuterons with gold ions provide direct evidence that energetic jets get stuck.

A photon (γ) and a jet of other particles emerges from a proton-proton (p-p) collision (left). But in a deuteron (d)-gold nucleus (Au) collision (right), the jet loses energy while the photon is not affected. Neutral pions (π0) are proxies for the jet. — Image courtesy of Valerie Lentz, Brookhaven National Laboratory

The Science

A new analysis of data from collisions of small nuclei made of just one proton and one neutron—known as deuterons—with much larger gold nuclei provides fresh evidence that these small collisions generate drops of quark-gluon plasma (QGP). The QGP is a soup of visible matter’s most fundamental building blocks. Using photons, which are not affected by the QGP, as a reference, the data show that energetic jets of particles from these collisions get “stuck,” or quenched, in the most central deuteron-gold collisions. The evidence adds to early observations of particle flow patterns that suggested these collisions could create a QGP.

The Impact

Scientists believe that a soupy mix of free quarks and gluons, the building blocks of protons and neutrons, permeated the early universe. Powerful particle colliders can recreate this QGP by smashing large nuclei together at nearly the speed of light. The energetic collisions melt the protons and neutrons, setting free the quarks and gluons that make up those particles. Particle flow patterns from collisions of small nuclei with large ones have suggested that even these small collisions can create a QGP. This was surprising because scientists thought small projectiles wouldn’t carry enough energy to create a QGP. The new evidence of jet quenching in deuteron-gold collisions supports the case that small drops of a QGP do form.

Summary

Collisions of gold nuclei with other gold nuclei at the Relativistic Heavy Ion Collider (RHIC), a Department of Energy Office of Science nuclear physics user facility at Brookhaven National Laboratory, routinely create the QGP. Analyses of particle flow patterns from collisions of smaller nuclei with gold suggested these small collisions might also generate QGP. This new analysis looked for “jet quenching,” a reduction in the number of energetic jets that is another key signature of QGP, in data from deuteron-gold collisions collected by RHIC’s former PHENIX detector. Jet quenching occurs when a jet-forming quark or gluon that is kicked free during a collision loses energy as it interacts with the free quarks and gluons in the QGP.

To search for quenching, the PHENIX scientists tracked “direct” photons, high energy particles of light that can be created in the collision right along with the kicked-free quarks and gluons. The photons do not interact strongly with quarks and gluons, so they don’t lose energy in QGP. But they do indicate how central, or head-on, the collisions are and how many energetic jets to expect. Here jets are characterized by their highest energy component, in this case neutral pions. The data show that the most central deuteron-gold collisions produced fewer energetic jets than expected—a clear sign of jet energy loss in tiny drops of QGP.

Contact

Yasuyuki Akiba
Brookhaven National Laboratory
[email protected]

Funding

This research was funded by the Department of Energy Office of Science, Nuclear Physics program, the National Science Foundation, and a range of U.S. and international universities and organizations (see the related scientific paper for a full list).