Definition of "pentaquark"
pentaquark
noun
plural pentaquarks
(physics) Any of a class of subatomic particles (previously hypothetical, since detected, subject to confirmation) consisting of a group of five quarks (compared to three quarks in normal baryons and two in mesons), or more specifically four quarks and one antiquark (symbol Θ).
Quotations
Evidence for and against the heavier pentaquark states, the Ξ {\displaystyle \Xi } (1862) and the Θ c 0 {\displaystyle \Theta _{c}^{0}} (3100) observed at CERN and at HERA, respectively, are also discussed. I conclude that the evidence against the latter two heavier pentaquark baryons is rapidly increasing making their existence highly questionable.
2006, Volker D. Burkert, “Have Pentaquark States Been Seen?”, in Richard Brenner, Carlos P. De Los Heros, Johan Rathsman, editors, Lepton and Photon Interactions at High Energies: Proceedings of the XXII International Symposium, Sweden 30 June – 5 July 2005, River Edge, N.J., Singapore: World Scientific Publishing, abstract, page 185
A more recent famous example of false discoveries is the finding of a new particle called the pentaquark in experiments performed at accelerator laboratories in the mid-2000s. The pentaquark supposedly consists of four quarks and an antiquark bound together—a much different particle than the three quarks making up proton and neutron. Viewing the plots that were published in the literature between 2003 and 2005, we see a significant excess of events for the pentaquark, reaching almost 5 sigma—the gold-plated standard for the confirmation of new particles. Yet, further data and more detailed analysis again made the particle disappear.
2014, John W. Moffat, “Data that Go Bump in the Night”, in Cracking the Particle Code of the Universe: The Hunt for the Higgs Boson, page 113
It's a particle so elusive that even the world's largest physics experiment could only discover it by accident. The pentaquark has at last been found. […] "Pentaquarks have been reported several times in the past," says [Patrick] Koppenburg, "but they all appeared to be fakes." So when the team at the LHCb experiment detected what looked like the signature of a pentaquark, they were determined to take their time with the analysis. Now they are pretty certain they have it. The signal has a "10 sigma" stamp of accuracy, meaning that there is only a 1 in 1022 chance of it being a statistical fluctuation.
2015 July 15, “Pentaquark discovery at LHC shows long-sought new form of matter ”, in New Scientist, number 3030