Once again, Maggie McKee in New Scientist:
An oversized subatomic particle that has challenged models of quantum physics since its reported discovery in 2003 does not exist after all, a new study suggests. But in trying to track the particle down, scientists say they have stumbled on important new insights about the forces that bind the building blocks of matter.
The short-lived particle – called a pentaquark – was thought to consist of five subatomic particles called quarks. Quarks normally only associate in groups of two – producing short-lived mesons, or three – producing the protons and neutrons that make up the bulk of normal matter.
More here.