Exotic particle confirmed by CERN

Scientists at CERN’s Large Hadron Collider (LHC) have confirmed the existence of a new class of subatomic particles called exotic hadrons. The LHCb collaboration, one of the four large experiments at the LHC, found that the exotic object does not fit into the pattern of particles observed up until now.

Hadrons are subatomic particles that can take part in the force that binds protons inside the nuclei of atoms. The protons and neutrons that form atomic nuclei consist of particles, called quarks, bound together. A subset of hadrons, called mesons, is formed from quark-antiquark pairs, while the rest - baryons - are made up of three quarks.

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. Many searches for such exotic states have been performed, with one of the most recent experiments conducted by the Belle Collaboration in 2008. At that time Belle reported evidence for an exotic structure, the Z(4430)-, that did not fit into the normal classification scheme, but the observation was questioned.

The existence of the Z(4430)- particle has now been confirmed, with detailed studies showing that the LHCb data can only be explained by the inclusion of the Z(4430)-. This state shows behaviour that is characteristic of a resonance (‘phase motion across the peak’).

This image shows the so-called Argand diagram proving to the experts that the Z(4430)- structure seen in the data (black points) represents really the resonant particle production and decay, since it follows approximately a circular path (red circle).

“We have now undisputable proof that nature is actually able to form more complex, more sophisticated objects of quarks than what was initially proposed by Gell-Man in 1964,” said LHCb physicist Richard Jacobsson.

The team analysed 25,000 interesting decays of neutral B mesons selected from data from 180 trillion proton-proton collisions in the LHC. Jacobsson explained that the B meson decayed the three particles, and they analysed the outcome to see if an object would be formed in the intermediate stage in the decay process.

“What we have seen is that nature is actually forming an object of four quarks,” he said.

A view of the LHCb experiment at underground Point 8 on the LHC. The prominent tube is the LHC beam pipe, in which protons circulate at close to the speed of light. Image: Anna Pantelia/CERN.

The researchers’ next steps are to search for other signs of the particle in other decays of B hadrons so they can further study its properties. This may give signs of this same particle, allowing complementary ways to understand the nature of this state and what it really is.

“LHCb’s observation and measurement of the Z(4430)- is going to help us explore this feature of matter,” noted Professor Tara Shears, LHCb lead for the University of Liverpool. Jacobsson further noted that understanding the dynamics of the strong force could help in our comprehension of, for example, the compositeness of a neutron star.

“LHCb’s measurement also demonstrates the experiment’s versatility,” Professor Shears added. “Who would have thought that an experiment designed to investigate the strange features of antimatter could also help us understand QCD and matter better?”

The researchers’ results were presented on 8 April at the SM@LHC conference in Madrid, Spain. They can be viewed online at http://arxiv.org/abs/1404.1903