LHC's exotic particle discovery may shed light on nucleus mystery

The world's largest physics experiment, the Large Hadron Collider (LHC) has detected a group of five sub-atomic particles whose existence has long been predicted by theory but had never before been observed. The particles, which are composed of unusual forms of the even smaller particles that make up most of matter, are hoped to provide insight into the mysterious "strong nuclear force", one of the fundamental forces of nature, which holds together the particles that make up the dense nuclei of atoms.

The new particles are all different types of particle known as the Omega-C baryon, an exotic cousin of the more familiar proton and neutron. These particles are made from quarks which have a group of quantum properties characterised by the names "up" and "down". The quarks making up the Omega-C baryon are heavier and much rarer, and have quantum properties characterised by the names "strange" and “charm”. These strange names known even more confusingly as "flavours", are used by physicists because the quantum properties do not have any similarity to properties found in the macroscopic world.

The Omega-C baryon was first discovered in 1964, after its existence was predicted by American physicist Murray Gell-Man in 1961. This particle, the first subatomic particle detected to not contain "up" or "down" quarks, consisted of three "strange" quarks. But because multiple quark flavours exist, it follows from theory that other sorts of Omega-C baryon must exist as well.

The discovery, described in this paper, was made in the LHCb experiment, the only one of the four detectors around the ring of the LHC that does not consist of a barrel-shaped array completely surrounding the point where the proton-proton collisions studied by the LHC are made to occur. Instead, LHCb effectively "sprays" the many different types of particle that result from collisions onto a wall of detectors. The b in its name stands for beauty, referring to a flavour of another type of quark. One of the goals of LHCb is to determine why there is much more matter than antimatter in the universe; it is also looking at a theory called quantum chromodynamics, which is concerned with how quarks stick together to form these other particles.

Like many discoveries at the LHC, the Omega-C baryon discovery was made by observing the particles it produces as it decays; exotic particles such as this tends to have very short lifetimes and cannot be detected directly. In this case, the parent particle was known as Omega-c-zero and consists of two "strange" and one ”charm" quark. The five Omega-C baryons detected are excited, or higher-energy, states, with masses ranging from 3000 to 3119MeV. By contrast, protons and neutrons have masses of 938MeV, and electrons of 0.511MeV, when at rest.

The discovery was made thanks to a recent upgrade of the LHC to run at higher energies, a corresponding upgrade to the LHCb detectors, and the huge amount of data that was generated by the LHC in its initial runs at lower energy that led to the discovery of the Higgs boson (which is responsible for the interactions that give matter mass). Dr Greig Cowan, of the University of Edinburgh, UK, who works on the LHCb experiment at Cern's LHC, said: "This is a striking discovery that will shed light on how quarks bind together. It may have implications not only to better understand protons and neutrons, but also more exotic multi-quark states, such as pentaquarks and tetraquarks."

Of the four fundamental forces known in the universe, the strong nuclear force that bind quarks together, and also binds protons to neutrons, is still the most mysterious. Like all discoveries in fundamental physics, it is very difficult to identify practical applications. However, other discoveries about the nature of the atomic nucleus are crucial to the development of magnetic resonance imaging, which has led to untold advances in medicine. Fundamental physics discoveries also often have applications in electronics.