# Synopsis: Four-Quark State Confirmed

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

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

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