Discoveries of the pentaquark states

Abstract

Protons and neutrons are not fundamental particles, instead are formed by quarks. Thousands of multi-quark particles were discovered in cosmic rays or particle accelerators. However, within the past decades after the quark model was proposed by Gell-Mann and Zweig, only two categories of strongly interacting particles were known: Baryons (such as protons and neutrons) that are made from three quarks, and mesons that are made from one quark and one antiquark. But what other configurations of quarks and antiquarks can form a particle? This question is theoretically difficult to answer. Yet new particles with different combinations of quarks keep popping up in experiments. In 2015, the best evidence for the existence of an exotic type of particle called as pentaquark, a never-before-seen formation of five quarks, was discovered by physicists working on the Large Hadron Collider beauty (LHCb) experiment at European Organization for Nuclear Research (CERN). The detection of this particle is a major breakthrough in learning more about the nature of the strong forces that bound quarks together. In a sample of about 26000 Λ b 0 → J/ψK − p decays collected by the LHCb detector, a significant enhancement structure was observed in the J/ψp mass distribution. The pentaquark signal populated well separatedly from those contributions of Λ * → K − p on the “Dalitz” plot using the K–p and J/ψp invariant mass-squared as independent variables. The amplitude analysis revealed two pentaquark states were needed to describe dataset. They were formally named as Pc(4450)+ and Pc(4380)+ with statistical significances greater than 9 standard deviations: One has a mass of (4449.8 ±1.7±2.5) MeV and a width of (39 ±5±19) MeV, while the other is wider, with a mass of (4380 ±8±29) MeV and a width of (205 ±18±86) MeV. Both pentaquarks that LHCb physicists observed decaying strongly into a J/ψ particle and a proton, must have a minimal quark content of c c ¯ uud, and thus are charmonium pentaquarks. Recently the analysis has been reoptimized based on the data collected to date. A sample of 2.5×105 Λ b 0 → J/ψK − p decays was selected, nine times more than those used in the previous analysis. A new pentaquark state Pc(4312) was discovered with a mass near 4312 MeV. The Pc(4450) state was found to be the superposition of two nearby states at 4440 and 4457 MeV respectively. In an attempt to interpret the discoveries by the LHCb experiment, the pentaquark was conjectured as a tightly bound system of all five quarks, or a weakly bound “molecular” system of a baryon and a meson. Accurate description of a multi-quark system using the theory of the strong force and quantum chromodynamics (QCD) is an extremely difficult problem. In fact, there are many combinations how five quarks are bound within the pentaquark. The five quarks could be tightly bound within a single structure, for example. Another possibility is that a quark and an antiquark are bound together to form a meson and the remaining three quarks form a baryon. The meson and baryon could then be bound to each other to create a structure resembling a subatomic molecule. Exploring the structure of pentaquarks as one of novel frontiers in high energy physics paves a new era to study the nature of strong interactions, which is an important stepping-stone in our good understanding of the strong force, the least well known of the four forces in nature.