Scientists at CERN’s Large Hadron Collider (LHC) have confirmed the existence of a new class of subatomic particles, exotic hadrons.

A new measurement performed by the LHCb collaboration, one of the four large experiments at the LHC at CERN, has confirmed the existence of the exotic object labelled the Z(4430)-. This particle does not fit into the pattern of particles we have seen up to now. The LHCb result confirms an observation made by the Belle collaboration in 2008 that was later questioned and so resolves this previously unclear situation.

We and everything around us are made of atoms, and atoms are made in turn of smaller constituents. Atomic nuclei are orbited by electrons. The protons and neutrons that form atomic nuclei consist of three fundamental particles, called quarks, bound together. Other combinations of quarks can occur - particles containing two bound quarks (mesons) are also seen in nature. However, until now all particles containing quarks (hadrons) have conformed to one of these two types; quarks seem to like to come in twos or threes.

However, the underlying theory of quantum chromodynamics (QCD) that describes the behaviour of quarks allows for many different quark combinations, such as four quark states, to bind together into hadrons. Over the last forty years many searches for such exotic states have been performed but until now there was no conclusive proof of their existence. Several more mundane explanations for the Z(4430)- signal seen by Belle had been put forward, but the LHCb result establishes that for the first time we have seen the "smoking gun" signal for resonant behaviour of a particle that contains at least four quarks/antiquarks.

The "4430" refers to the (approximate) mass of this state, corresponding to roughly four times the mass of a proton.

Source: STFC website