Jun. 28 (Beihang News) - Recently, the team of Professor Geng Lisheng of the School of Physics and Nuclear Energy Engineering and its foreign partners have used the effective field theory description to study the three pentaquark states discovered previously by the LHCb Collaboration of the European Organization for Nuclear Research. They find the molecular interpretation of these particles as Dbar(*) Sigma_c(*), and predict four additional molecular pentaquarks of Dbar(*) Sigma_c(*). The Dbar(*) and Sigma_c(*) here are the ground state meson containing anti-charm quarks and the ground state baryon containing charm quarks. On June 21, the research finding, titled “Emergence of a Complete Heavy-Quark Spin Symmetry Multiplet: Seven Molecular Pentaquarks in Light of the Latest LHCb Analysis”, was published online onPhys. Rev. Lett.122, 242001 (2019), and hasalso drawn the attention ofScience Magazine.
Exploring the basic structure of matter has always been an important frontier topic of physics. The research of spectroscopy has played a significant role in understanding the physical laws of microcosmos, and promoted the establishment of the shell model and collective movement model from the field of quantum mechanics to nuclear physics. In 1961, Gell-Mann and Ne'eman proposed the Eightfold Way, which classified the known baryons and mesons (collectively called hadrons) into different SU(3) multiplets and predicted the existence and properties of the Omega particle. In 1964, the discovery of Omega particle made history as a major breakthrough in particle physics, marking the establishment of the SU(3) symmetry theory. In the subsequent simple quark model established by Gell-Mann and Zweig, a baryon was composed of three valence quarks, and a meson was composed of one antiquark pair. However, QCD, the basic theory of strong interaction, did not confine the existence of more complex hadron structures, such as hybrid state, gluon, tetraquark state, pentaquark state, molecular state and so on.
Since 2003, a large number of new exotic hadron states have been discovered in high-energy physics experiments, which were difficult to interpret by simple quark models. Since the mass of these exotic states was mostly close to the mass threshold of two or more hadron states, the molecular state was therefore a very probable explanation, just as deuteron could be considered as a weakly bound molecular state composed of protons and neutrons.
In 2015, the LHCb Collaboration of the European Organization for Nuclear Research detected two pentaquark states, Pc(4380) and Pc(4450), for the first time. In 2019, based on higher statistic data, the LHCb Collaboration found three narrow formants on the J/psi p invariant mass spectrum of Lambda_b decay, of which Pc(4312) was a new pentaquark state. The Pc (4450) discovered in 2015 split into two narrow states, Pc (4440) and Pc (4457).
These narrow pentaquark states are close to the threshold of Dbar(*)Sigma_c(*). Based on the analysis of model-independent effective field theory, the Beihang team points out that these pentaquark states are likely to be the molecular states of Dbar(*)Sigma_c(*). In the Dbar(*)Sigma_c(*) system, heavy quark spin symmetry means that the leading order interaction of Dbar(*)Sigma_c(*) only needs to be described by contact potentials of two low-energy effective constants. By using the newly observed Pc(4440) and Pc(4457) by LHCb Collaboration as inputs, the predicted mass of the Dbar Sigma_c bound state is very close to Pc(4312), and the mass of the Dbar Sigma_c(*) bound state is very close to the previously discovered Pc(4380). In addition, the effective field theory also predicts the existence of three Dbar* Sigma_c* molecular states with spins of 1/2, 3/2, and 5/2, respectively (see Fig. 1). The Beihang team believes that this is very likely to be the first complete septuple molecular state. If other molecular pentaquarks of this multiplet are to be confirmed by subsequent experiments, it will be an important and successful application to explain exotic hadron state using heavy quark symmetry, and it is possible to uncover a new chapter in the study of heavy-flavor hadron spectrum.
Fig. 1: Mass and spin-parity of the septuple molecular state predicted based on effective field theory. Scenario A: The predicted spin of Pc(4457) and Pc(4440) is 3/2 and 1/2; Scenario B: The predicted spin of Pc(4457) and Pc(4440) is 1/2 and 3/2. Their parities are both negative.
The authors of the thesis are Liu Mingzhu, a doctoral student of Beihang University, Pan Yawen, an undergraduate, Peng Fangzheng, a postgraduate, Dr. Mario Sanchez of the University of Bordeaux, Professor Geng Lisheng and Professor Manuel Pavon Valderrama of Beihang University, and Professor Atsushi Hosaka of Osaka University. The work is funded by the Major Project Program of National Natural Science Foundation of China, the Outstanding Young Scientists Team of Beihang University, and the Ministry of Education, Science and Culture of Japan (Atsushi Hosaka), etc.
The research article is available at:https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.242001
Science Magazinenews report:
https://www.sciencemag.org/news/2019/06/exotic-particles-called-pentaquarks-may-be-less-weird-previously-thought?from=singlemessage&isappinstalled=0
(Reviesed by Wang Wenwen)
Edited by Jia Aiping
