Synopsis: Four-Quark State Confirmed

The LHCb experiment provides conclusive evidence for the existence of the four-quark particle called Z(4430).
Synopsis figure
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 Zc(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 B0 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


Announcements

More Announcements »

Subject Areas

Particles and Fields

Previous Synopsis

Atomic and Molecular Physics

Crisis Averted for the Bose Glass

Read More »

Next Synopsis

Biological Physics

Modeling Biodiversity

Read More »

Related Articles

Synopsis: Pentaquark Discovery Confirmed
Particles and Fields

Synopsis: Pentaquark Discovery Confirmed

New results from the LHCb experiment confirm the 2015 discovery that quarks can combine into groups of five. Read More »

Synopsis: Searching for Majorana Neutrinos
Particles and Fields

Synopsis: Searching for Majorana Neutrinos

The KamLAND-Zen collaboration has run the most sensitive search to date for a radioactive decay that could reveal whether neutrinos are Majorana fermions. Read More »

Viewpoint: Hunting the Sterile Neutrino
Particles and Fields

Viewpoint: Hunting the Sterile Neutrino

A search for sterile neutrinos with the IceCube detector has found no evidence for the hypothetical particles, significantly narrowing the range of masses that a new kind of neutrino could possibly have. Read More »

More Articles