nternational teams of physicists working with the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory (Long Island, New York) believe they have created the state of matter that existed milliseconds after the Big Bang. Four papers this period (#1, #2, #3, and #5) showcase results in which RHIC has fleetingly, but repeatedly, created the temperatures and matter densities that prevailed when the universe was 1 µs old. This primordial matter has startled researchers with its unexpected properties, behaving as a liquid with low viscosity.

Inside the 4-km circumference of the RHIC particle accelerator two beams of gold ions travelling at 99.995% of the velocity of light collide head on, releasing an enormous amount of energy in a tiny volume. RHIC, which began operations in 2000, is designed to defeat the strong force that binds the nucleus, and so reveal the quark-gluon matter imprisoned in the nucleus.

Quantum chromodynamics predicts that during the first 10^-4 s of the existence of the universe, its material content consisted of a plasma of quarks, together with gluons that bind them. RHIC is designed to create the extreme matter that once filled the extremely hot universe before there were protons and neutrons. In 1999, physicists at CERN (Geneva, Switzerland) used a heavy-ion collider to get the first glimpse of quark-gluon plasma, thus demonstrating that this phase transition of deconfined quarks is accessible to experimental investigation.

RHIC delivers a higher energy than CERN could achieve, and therefore a higher temperature in the center of mass reference frame. It probes more closely the properties of quark-gluon plasma. Fleetingly the collision temperature is about 150,000 times greater than that of the solar interior. In 380 pages the four Hot Papers describe the results obtained in the first three years of RHIC by nearly 1,000 scientists and engineers, arranged in four teams, each with a dedicated detector.

The PHENIX detector (#1) records many different particles emerging from collisions, including photons. The latter are important because photons alone emerge unchanged from the maelstrom of a RHIC collision. Escaping photons carry vital information about the temperature of the collision. Paper #2 is from the STAR Collaboration, which specializes in tracking the thousands of particles produced by each ion collision. STAR is a huge detector with a mass of 1,200 tons specifically designed to look for signs that quark-gluon plasma has been released.

The team operating the PHOBOS experiment (#3) operate on the premise that unusual and unpredictable events may occur. Although interesting collisions may be very rare, they could lead to new physics. For each collision PHOBOS obtains the global picture of the event. In the BRAHMS experiment (#5) the focus is on the charged hadrons flung out of the collision. A central collision results in about 7,000 particles, some 4,600 of which are charged (pions, kaons, protons, and their antiparticles). The charged particle parameters provide the metrics for determining the energy density in the very early stages of the collision process.

Exceptional demands are placed on the instrumentation at RHIC, operating on distance scales of a few proton radii and a time scale of 10^-23 s. By drawing together results from overlapping investigations, our four Hot Papers reach the conclusion that the gold-gold collisions have created an exotic new form of matter that is far more remarkable than predicted.

Theorists had confidently expected that the quark-gluon soup would be plasma-like, meaning that it would be an ensemble of free flying quarks and gluons. Not so, according to the nuclear fireballs unleased at RHIC, where the results took an unexpected turn. Instead of random explosive debris, the Brookhaven experimenters find evidence for collective behavior: the hot matter behaves as a perfect liquid with extremely low viscosity. In fact it is the most nearly perfect liquid ever observed. What this means is that quark-quark interactions are stronger than expected.