Synopsis

Four-Quark State Confirmed

The LHCb experiment provides conclusive evidence for the existence of the four-quark particle called Z (4430).

LHCb @ CERN 2013

LHCb @ CERN 2013 ×

First there were hints and hedges, but now there is a definitive observation of a four-quark particle called Z (4430). Studying B meson decays, the LHCb collaboration has collected a strong enough signal to declare in the latest Physical Review Letters that Z (4430) is a bona fide particle. It joins other exotic particles, such as Z c (3900), that defy the conventional wisdom that quarks only combine in pairs or triplets (see 17 June 2013 Viewpoint).

In 2008, the Belle experiment in Japan reported a peak in B meson decay data that suggested a negatively charged particle with mass 4 . 5 times that of a proton. The quark content of this Z (4430) particle posed a puzzle: its decay implied it contained a charm quark and anticharm, while its charge required two more quarks (a down and anti-up, for example)—giving a total of four. However, subsequent studies by SLAC’s BaBar experiment called into question the evidence for a new particle.

The LHCb experiment at CERN in Geneva, which is primarily set up to study bottom-quark physics in the LHC’s proton-proton collisions, has collected 25 , 000 relevant B 0 decays at energies of 7 and 8 tera-electron-volts. This sample is a factor of 10 larger than the data sets of Belle and BaBar. The analysis by the LHCb collaboration shows a highly significant signal (about 14 standard deviations above background) that removes any doubt that Z (4430) is a real particle. The team also confirms that the particle has a spin of 1 and a positive parity, which rules out the interpretation of the particle signatures as merely arising from a pair of (two-quark) D mesons. The only remaining explanation, according to the researchers, is that Z (4430) is a bound state of four quarks. – Michael Schirber