Пентаquark: Искусственно созданная субатомная частица

Human made subatomic particle

A pentaquark is a human made subatomic particle, consisting of four quarks and one antiquark bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them. As quarks have a baryon number of , and antiquarks of , the pentaquark would have a total baryon number of 1, and thus would be a baryon. Further, because it has five quarks instead of the usual three found in regular baryons ( "triquarks"), it is classified as an exotic baryon. The name pentaquark was coined by Claude Gignoux et al. (1987) and Harry J. Lipkin in 1987; however, the possibility of five quark particles was identified as early as 1964 when Murray Gell Mann first postulated the existence of quarks. Although predicted for decades, pentaquarks proved surprisingly difficult to discover and some physicists were beginning to suspect that an unknown law of nature prevented their production. The first claim of pentaquark discovery was recorded at LEPS in Japan in 2003, and several experiments in the mid 2000s also reported discoveries of other pentaquark states. However, other researchers were not able to replicate the LEPS results, and the other pentaquark discoveries were not accepted because of poor data and statistical analysis. On 13 July 2015, the LHCb collaboration at CERN reported results consistent with pentaquark states in the decay of bottom Lambda baryons On 26 March 2019, the LHCb collaboration announced the discovery of a new pentaquark that had not been previously observed. On 5 July 2022, the LHCb collaboration announced the discovery of the pentaquark. Outside of particle research laboratories, pentaquarks might be produced naturally in the processes that result in the formation of neutron stars.

A quark is a type of elementary particle that has mass, electric charge, and colour charge, as well as an additional property called flavour, which describes what type of quark it is (up, down, strange, charm, top, or bottom). Due to an effect known as colour confinement, quarks are never seen on their own. Instead, they form composite particles known as hadrons so that their colour charges cancel out. Hadrons made of one quark and one antiquark are known as mesons, while those made of three quarks are known as baryons. These 'regular' hadrons are well documented and characterized; however, there is nothing in theory to prevent quarks from forming 'exotic' hadrons such as tetraquarks with two quarks and two antiquarks, or pentaquarks with four quarks and one antiquark. The binding mechanism for pentaquarks is not yet clear. They may consist of five quarks tightly bound together, but it is also possible that they are more loosely bound and consist of a three quark baryon and a two quark meson interacting relatively weakly with each other via pion exchange (the same force that binds atomic nuclei) in a "meson baryon molecule".

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 1530 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 1522 and 1555, 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).

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 (J/ψ), 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.

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.

The discovery of pentaquarks will allow physicists to study the strong force in greater detail and aid understanding of quantum chromodynamics. In addition, current theories suggest that some very large stars produce pentaquarks as they collapse. The study of pentaquarks might help shed light on the physics of neutron stars.