Nikhef Colloquium

It has now been over 50 years since the inception of the quark model. The original papers by Gell-Mann and Zweig included the description of the now well known three-quark baryons and quark-antiquark mesons. They also included the possibility of "exotic" hadrons, such as mesons containing two quarks and two antiquarks (tetraquarks), or four quarks and an antiquark (pentaquarks). There is no clear reason why such exotic combinations of quarks should not exist. Indeed, in recent years strong tetraquark candidates have been discovered. However, until recently the observation of any lasting pentaquark candidates had eluded all searches. Using the LHCb Run 1 dataset, two $J/\psi p$ resonances consistent with pentaquark states have been observed in $\Delta b \to J/\psi pK^-$ decays and recently confirmed in $\Delta b0 \to J/\psi p\pi^-$ decays. We will briefly introduce the LHCb experiment, present the status of searches for pentaquarks and describe the amplitude analysis that led to the observation of the new states. Their nature is still disputed. Possible future studies that could determine their nature will be presented