Mid-2000s The requirement to include an antiquark means that many classes of pentaquark are hard to identify experimentally – if the flavour of the antiquark matches the flavour of any other quark in the quintuplet, it will cancel out and the particle will resemble its three-quark hadron cousin. For this reason, early pentaquark searches looked for particles where the antiquark did not cancel. This coincided with a pentaquark state with a mass of predicted in 1997. The proposed state was composed of two
up quarks, two
down quarks, and one
strange antiquark (uudd). Following this announcement, nine other independent experiments reported seeing
narrow peaks from and , with masses between and , all above 4 σ. However this 'discovery' was later revealed to be due to flawed methodology (https://www.osti.gov/biblio/21513283-critical-view-claimed-theta-sup-pentaquark).
2015 LHCb results representing the decay of a lambda baryon into a kaon and a pentaquark . In July 2015, the
LHCb collaboration at
CERN identified pentaquarks in the channel, which represents the decay of the bottom lambda baryon into a
J/ψ meson , a
kaon and a
proton (p). The results showed that sometimes, instead of decaying via intermediate
lambda states, the decayed via intermediate pentaquark states. The two states, named and , had individual
statistical significances of 9 σ and 12 σ, respectively, and a combined significance of 15 σ – enough to claim a formal discovery. The analysis ruled out the possibility that the effect was caused by conventional particles. The search for pentaquarks was not an objective of the LHCb experiment (which is primarily designed to investigate
matter-antimatter asymmetry) and the apparent discovery of pentaquarks was described as an "accident" and "something we've stumbled across" by the Physics Coordinator for the experiment. and Hall C E2-16-007 experiments at
JLab. The major challenge in these studies is a heavy mass of the pentaquark, which will be produced at the tail of photon-proton spectrum in JLab kinematics. For this reason, the currently unknown branching fractions of pentaquark should be sufficiently large to allow pentaquark detection in JLab kinematics. The proposed
Electron Ion Collider which has higher energies is much better suited for this problem. An interesting channel to study pentaquarks in proton-nuclear collisions was suggested by Schmidt & Siddikov (2016). This process has a large cross-section due to lack of electroweak intermediaries and gives access to pentaquark wave function. In the fixed-target experiments pentaquarks will be produced with small rapidities in laboratory frame and will be easily detected. Besides, if there are neutral pentaquarks, as suggested in several models based on flavour symmetry, these might be also produced in this mechanism. This process might be studied at future high-luminosity experiments like After@LHC and NICA.
2019 LHCb results On 26 March 2019, the LHCb collaboration announced the discovery of a new pentaquark, based on observations that passed the 5-sigma threshold, using a dataset that was many times larger than the 2015 dataset. with a significance of 15-sigma. Designated PψsΛ(4338)0, its composition is described as udsc, representing the first confirmed pentaquark containing a strange quark. ==Applications==