Fires of creation probed by quarks

Scientists have confirmed that a powerful particle accelerator has recreated the intense conditions that existed just microseconds after the beginning of the universe. The experiments have also revealed a surprise about quarks, the fundamental building blocks of every atomic nucleus.

Quarks are normally held together by gluons, but immediately after the big bang these ingredients existed as a hot quark–gluon plasma (QGP). Understanding how this soup condenses into the discrete particles that make up ordinary matter can help to reveal how the subatomic world works.

To generate quarks of various flavors—known by names such as ‘charm’ or ‘strange’—the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) in Upton, US, smashes particles together at close to the speed of light.

Scientists working there conducted a series of experiments there in 2004/5 to create and study an unusual particle called J/ψ, made up of a charm quark paired with its opposite number, the anti-charm quark.

The scientists first smashed protons together, and used the PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) detector to spot thousands of J/ψ particle decays1 (Fig. 1).

When they switched protons for gold atoms, the heavier missiles created more intense explosions expected to generate a quark–gluon plasma2. They saw that, as expected, some of the J/ψ particles from the initial explosion were melting in this hot bath, lowering their overall numbers.

“This supports the theoretical prediction that J/ψ will melt in a QGP, and thus provides strong evidence for QGP formation at RHIC,” says Yasuyuki Akiba of RIKEN’s Nishina Center for Accelerator-Based Science, Wako, who is part of the PHENIX team.

Surprisingly, they also found that the central, hotter region of the collision actually hosted more J/ψ particles than the cooler outskirts. This suggests that charm and anti-charm quarks produced at the heart of the collision can recombine into J/ψ. “This is a very intriguing explanation but, at present, the data cannot rule out other possibilities,” adds Akiba.

The team have now just finished collecting a new set of data on gold–gold collisions which will allow them to measure J/ψ much more precisely. They hope they will be able to track the characteristic motion of J/ψ particles produced by recombination of quarks, dubbed ‘elliptic flow’, which would distinguish them from existing J/ψ that failed to melt in the QGP. This would allow them to calculate the balance between melting and recombination effects, revealing more about the primordial QGP.