In February, a team of researchers at the DZero collaboration — one of the two experiments conducted at the Fermilab’s Tevatron Collider before it was shut down in 2011 — announced the discovery of a new particle. The particle, named X(5568), was believed to be a part an exotic family called “tetraquarks.”
However, new research has cast doubts over the discovery. Scientists at the European Organization for Nuclear Research (CERN) in Geneva announced in a research note Monday that they had failed to detect the particle in the debris of proton-proton collisions at the Large Hadron Collider (LHC).
Quarks, the fundamental particles that make up protons and neutrons, come in six different “flavors” — up, down, strange, top, bottom and charm. Each quark has an antimatter equivalent known as antiquark. Quarks usually come in packages of two or three. Both protons and neutrons — contained within the nucleus of an atom — are made up of three quarks bound together.
However, the 1964 model that first proposed the existence of quarks also postulates the existence of tetraquarks, composed of four quarks, and pentaquarks, composed of five quarks. Last August, scientists at CERN’s Large Hadron Collider announced the discovery of a pentaquark, in which quarks were arranged in a pattern never before seen in over five decades of experimental searches. Prior to that, in 2014, experiments at LHC had also provided strong evidence of the existence of a tetraquark.
However, while researchers at the DZero collaboration identified the tetraquark with a statistical significance of over five sigma — which is the benchmark for claiming a discovery in particle physics — scientists at the LHCb experiment found no such evidence. So, while the DZero collaboration spotted an excess 133 pairs of BS mesons and pi mesons — produced when X(5568) decays — the CERN scientists spotted none.
“The claimed X(5568) state, if confirmed, would differ from any of the previous observations, as it must have constituent quarks with four different flavours [bottom, strange, up and down]. As such, it would be unique among observed exotic hadrons in having its mass dominated by one constituent quark,” the researchers wrote in the note. “[However] no significant excess is found and thus the existence of the X(5568) state is not confirmed.”