Exotic particle confirmed

April 10th 2014

Summary: A new measurement performed by the LHCb collaboration has confirmed the existence of the exotic object labelled the Z(4430)-. This 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

Image: Mass squared plotted for the particles into which the Z(4430)- decays. The blue histogram shows the Z(4430)- contribution which is required to explain the distribution seen in the data (black points)

(Credit LHCb)

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.

Dr. Greig Cowan, STFC Ernest Rutherford Fellow at the University of

Edinburgh and one of the lead analysts on this project says,

"This is a fantastic result from the LHCb collaboration. It confirms

previous signs of this exotic state and shows, for the first time, that it

has has the characteristic behaviour of a resonance. In addition we have

also been able to pin down the quantum numbers and properties of this

state with higher precision than previous experiments."

Speaking about what this means for particle physics research Professor

Tara Shears, LHCb lead for the University of Liverpool, said "We've always

taken the existence of two and three quark particle states for granted,

but there's no reason why more complicated versions shouldn't occur.

LHCb's observation and measurement of the Z(4430)- is going to help us

explore this feature of matter. LHCb's measurement also demonstrates the

experiment's versatility - who would have thought that an experiment

designed to investigate the strange features of antimatter could also help

us understand QCD and matter better?"


Dr Greig Cowan,  University of Edinburgh, Greig.Cowan ATNOSPAM

Thanks to Jake Gilmore, STFC Media Manager, for his help in preparing this press release


The UK participation in the international LHCb experiment is from eleven institutes.

University of Birmingham, University of Bristol, University of Cambridge, University of Edinburgh, University of Glasgow, Imperial College London, University of Liverpool, University of Manchester, University of Oxford, STFC Rutherford Appleton Laboratory, University of Warwick

UK participation in the experiment is funded by the Science and Technology Facillities Council (STFC), with contributions from the participating institutes, the Royal Society and European Union.