Pentaquark

Evidence for and against the heavier pentaquark states, the #92;Xi(1862) and the #92;Theta⁰#95;c(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.

Source: wiktionary

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.

Source: wiktionary

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.

Source: wiktionary