Soon after the genesis in 1964 of the quark model, theorists began to ask if particles could be made from more than three quarks. Quantum chromodynamics, the theory of how quarks interact, does not rule out higher combinations. Theorists soon speculated on the existence of assemblies of four or more quarks, triggering a long history of searches for exotic resonances in baryon collisions, without making any progress. This situation might have changed in 1997, when Maxim Polyakov, Dmitri Diakonov, and Victor Petrov at the Petersburg Nuclear Physics Institute in Russia proposed the pentaquark, as a new bound state of matter. However, their musings met great skepticism, and could easily have been forgotten. Then, in 2000, Dmitri Diakonov and Takashi Nakano (Osaka University, Research Center for Nuclear Physics) struck up conversations at the lunch counter while attending a conference in Copenhagen. Diakonov thought that the experiments then being conducted by Nakano and his team could produce a five-quark particle. Crucially, Diakonov suggested a new method of data analysis that should, he thought, reveal the pentaquark’s presence.

For their quest, the Japanese team used the Spring-8 synchrotron near Kobe, the world’s largest third-generation synchrotron radiation facility. They directed a beam of 2.4 GeV gamma rays at a carbon target, and studied the missing-mass spectrum of the nuclear reaction in which a neutron absorbs a gamma ray and then decays to a K⁺, and K^- meson plus a neutron. The missing-mass spectrum has a sharp resonance at 1.54 GeV/c², which indicates the formation of a particle of that mass, about 1.5 times the proton mass. This can be attributed to a meson-baryon molecular resonance, or, much more exciting, an exotic five-quark baryon composed of two ups, one down, one strange, and one anti-down quark. The half-life of the pentaquark, essentially a fleeting fusion of a neutron and a K⁺ meson, is 10^-20 s.

For Science Watch, Nakano offers the following comments. "Our result is important because it indicates a new way to confine quarks in a particle. So far about 10 experimental groups have observed the peak, although there is still no observation which undeniably proves the existence of the pentaquark, which belongs to a rare species of the hadron family." He adds that in the early Big Bang universe free quarks existed, and the pentaquark "may give a clue to understand how quarks now form protons and neutrons and can never escape from them."

The early universe features in Hot Paper #10, which reports on polarization in the cosmic microwave background (CMB), as measured by the Degree Angular Scale Interferometer (DASI). This ground-based instrument is at the South Pole, where it profits from the very dry atmosphere. Paper #10 reports the first detection of polarization of the CMB, which University of Chicago astrophysicists first announced in September 2002. One of them, lead author John Kovac, tells Science Watch, "An initial detection of CMB polarization had long been sought; with its achievement cosmology passes a significant milestone." Many ongoing experiments are now taking up the challenge of precision polarization measurements, spurred on by the Chicago results. Looking to the future, Kovac adds, "A new generation of experiments will push into uncharted territory, where the polarization should contain the signatures of the physics of inflation and dark energy."