Recently, the team of Prof. Geng Lisheng of the School of Reliability and Systems Engineering and the foreign collaborators have used the effective field theory description to study the three pentaquarklike states discovered by the LHCb Collaboration of the European Nuclear Center. They found that these particles may resemble the molecular Dbar(*) Sigma_c(*) and predicted four additional molecular pentaquarks, which presents ground-state mesons containing anti-charm quark and ground-state baryons containing charm quark. On June 21, the research was punished online titled “Emergence of a Complete Heavy-Quark Spin Symmetry Multiplet: Seven Molecular Pentaquarks in Light of the Latest LHCb Analysis” on Phys. Rev. Lett. (122, 242001 (2019)) and drew the attention of Science.
Exploring the basic structure of matter has always been an important frontier topic in physics. The study of spectroscopy plays a significant role in understanding the laws of physics in the microscopic world, and promotes the establishment of shell and collective models from quantum mechanics to nuclear physics. In 1961, Gell-Mann and Ne'eman proposed the octagonal method, which classified the known baryons and mesons into different SU (3)-flavor multiplets and predicted the existence and properties of Omega particles. In 1964, the discovery of Omega particle was recorded as a major breakthrough in particle physics, marking the establishment of SU (3) symmetry theory. In the subsequent simple quark model established by Gell-Mann and Zweig, baryons consist of three valence quarks and mesons consist of a pair of positive and negative quarks, but the basic theory of strong interaction QCD does not prohibit the existence of more complex hadronic structures, such as hybrid states, colloidal spheres, four-quark states, five-quark states, and molecular states.
Since 2003, a large number of new exotic hadronic states have been discovered in high-energy physics experiments, which could hardly be explained by the simple quark model. In 2015, the two pentaquarklike states, Pc(4380) and Pc(4450), were first detected by the LHCb Collaboration. In 2019, they found three narrow formants on the J/psi p invariant mass spectrum of Lambda_b, of which Pc(4312) is a new pentaquarklike state, and Pc(4450) discovered in 2015 splits into two states, Pc(4440) and Pc(4457).
These narrow pentaquarklike states are concentrated near the threshold of Dbar(*) Sigma_c(*). Based on model-independent analysis of the effective field theory, the Beihangteam points out that these pentaquarklike states are likely to be the molecular states of Dbar(*) Sigma_c(*). In its system, the heavy quark spin-flavor symmetry means that the leading order interaction of Dbar(*) Sigma_c(*) needs to be described by contact potentials with two low-energy effective constants. Using the newly observed Pc (4440) and PC (4457) of the LHCb Collaboration as input, the predicted mass of bound state of Dbar Sigma_c is close to that of Pc (4312), and the mass of bound state of Dbar Sigma_c(*) is close to that of the previously discovered Pc (4380). In addition, the effective field theory predicts the existence of three Dbar* Sigma_c* molecular states with spins of 1/2,3/2 and 5/2 respectively. The Beihang team believes that it is probably the first complete seven-state molecular. If other molecular pentaquarks of this multiplet are confirmed subsequently, it will be an important and successful application of heavy quark symmetry to explain exotic hadrons, and may open a new chapter in the study of heavy-flavor hadronic spectrum.
The mass and spin-parity of seven-state moleculars predicted by effective field theory
The paper is written by Liu Mingzhu, a doctoral student of Beihang, Pan Yawen, an undergraduate, Peng Fangzheng, a postgraduate, Mario Sanchez, a doctoral student of the University of Bordeaux, Prof. Geng Lisheng from Beihang, Prof. Manuel Pavon Valderrama and Prof. Atsushi Hosaka from Osaka University. The program is supported by the National Natural Science Foundation of China, the Outstanding Youth Scientist Team of Beihang University, Atsushi Hosaka and so on.
The article can be read at: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.242001.
The news report of Science on this research can be read at: https://www.sciencemag.org/news/2019/06/exotic-particles-called-pentaquarks-may-be-less-weird-previously-thought?from=singlemessage&isappinstalled=0.
Reported by Liu Rui
Reviewed by Wang Wenwen
Edited by Jia Aiping
Translated by Lu Baihui