The pentaquark is proposed to be made from four quarks and one antiquark, which is an allowed combination by the theory of quantum chromodynamics. However, there was no convincing experimental evidence before 2002, despite many searches in particle physics experiments. In 2002, an experimental group in Japan presented results showing tentative evidence for the a pentaquark called the Θ~+, consisting of four light quarks and a strange antiquark. In the next few years, up to a dozen experimental groups published supporting evidence for the Θ~+, followed by a number of experiments with no evidence (and where the Θ~+ was expected to be seen). In later years, most of the experiments supporting the Θ~+ were repeated, but could not reproduce the evidence with better statistics. Today, there is little belief that the Θ~+ is real, and it remains a mystery how so many experiments could have claimed statistically-significant evidence for the pentaquark.
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