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?"
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The work from LHCb for this project has built on the work that the Belle Collaboration did in 2008. At that time Belle reported evidence for an exotic structure, the Z(4430)-, that did not fit into the normal classification scheme. This state has 1 unit of electric charge and must contain a minimum of four quarks. Given this is a smoking gun for an exotic meson it has always been deemed important to confirm the Belle observations and understand whether the Z(4430)- is a real particle. Detailed studies show that the LHCb data can only be explained by the inclusion of the Z(4430)- and this state shows behaviour that is characteristic of a resonance ('phase motion across the peak').
The next steps are to search for other signs of this particle in other decays of B hadrons so that we can further study its properties. This may give signs of this same particle, allowing complementary ways to understand the nature of this state. This information will be essential to help understand what the nature of this state really is.
The theories give different predictions for rates of decays into different channels. They also have different predictions for sets of additional molecules or tetraquarks. This opens an exciting (but challenging) field that LHCb will be pursuing intensively in the coming years.
The result was presented for the first time on April 8th 2014 at the SM@LHC conference in Madrid, Spain.
LHCb is an experiment set up to explore what happened after the Big Bang that allowed matter to survive and build the Universe we inhabit today. Located in a vast underground cavern, 100 metres beneath the French countryside, LHCb is one of four large experiments based at the CERN laboratory near Geneva, Switzerland. The experiment, which involves about 700 scientists from 52 institutions around the world, has recorded the particles produced by the first circulating LHC proton beam on September 10th, 2008, and will run for at least 10 years.
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