Never-before-seen type of ‘exotic’ particle discovered by Large Hadron Collider

1 Jul 2020

Illustration of a tetraquark composed of two charm quarks and two charm antiquarks. Image: Daniel Dominguez/LHCb/CERN

CERN has welcomed the ‘very exciting’ discovery of a new type of tetraquark particle that could be the first of a previously undiscovered class of particles.

CERN’s Large Hadron Collider beauty (LHCb) experiment – established to help explore what happened immediately after the Big Bang – has discovered a particle never seen before. In a paper published today (1 July) to the ArXiv preprint server, researchers said this could be the first of a previously undiscovered class of particles.

Its discovery could help physicists better understand the complex process of how quarks bind themselves together into composite particles, such as protons and neutrons found inside atom nuclei.

Typically, quarks combine in groups of threes or twos to form particles called hadrons. However, for decades it has been theorised that four-quark or five-quark hadrons – referred to as tetraquarks and pentaquarks – could exist.

“Particles made up of four quarks are already exotic, and the one we have just discovered is the first to be made up of four heavy quarks of the same type, specifically two charm quarks and two charm antiquarks,” said the outgoing spokesperson of the LHCb collaboration, Giovanni Passaleva.

“Up until now, the LHCb and other experiments had only observed tetraquarks with two heavy quarks at most and none with more than two quarks of the same type.”

Spotted in a ‘bump’

As they are made up of unusual combination of quarks, these particles are seen as ideal for studying one of the four known fundamental forces of nature, the strong interaction that binds protons, neutrons and the atomic nuclei that make up matter. By learning more about their strong interactions, it could help determine whether new, unexpected processes are a sign of new physics or just standard physics.

LHCb’s incoming spokesperson, Chris Parkes, added: “These exotic heavy particles provide extreme and yet theoretically fairly simple cases with which to test models that can then be used to explain the nature of ordinary matter particles, like protons or neutrons.

“It is therefore very exciting to see them appear in collisions at the LHC for the first time.”

The unusual tetraquark was discovered using the particle-hunting technique of looking for an excess of collision events, known as a ‘bump’, in datasets ranging from 2009 to 2013 and 2015 to 2018. This revealed a bump in the mass distribution of a pair of particles consisting of a charm quark and a charm antiquark.

However, it is not completely clear whether the new particle is a ‘true tetraquark’ – a system of four quarks tightly bound – or a pair of two-quark particles loosely bound in a molecule-like structure. CERN said that either way the discovery will help test models of quantum thermodynamics.

Colm Gorey was a senior journalist with Silicon Republic