Spectroscopy of charmed baryons
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EMENTARY PARTICLES AND FIELDS Theory
Spectroscopy of Charmed Baryons E. I. Solovieva* National Research Nuclear University MEPhI, Kashirskoe sh. 31, Moscow, 115409 Russia Received February 20, 2015
Abstract—A present-day classification of charmed baryons is presented, a quark model for ground states is briefly described, and the energy levels of excited states are analyzed. In addition, a survey of experimentally observed states of charmed baryons is given. DOI: 10.1134/S1063778815090124
The spectroscopy of charmed baryons is beautiful and intricate. The presence of three quarks gives rise to numerous degrees of freedom, and this leads to a much greater number of states than in the region of charmed mesons. At the same time, a large mass difference between the charmed quark and light quarks offers a natural means for classifying and analyzing these states, which is Heavy Quark Effective Theory (HQET). The spectrum of known states featuring one charmed quark can be partitioned into three broad regions: that of ground states, which confirm the constituent quark model; that of low-lying excited states, which admit a good description within the HQET framework; and that of higher excitations, for which the situation is less clear.
2. EXCITED STATES
1. QUARK MODEL FOR GROUND STATES In the constituent quark model [1], baryons consisting of u, d, s, and c quarks can be systematized into SU (4) multiplets in accordance with the symmetry of the flavor, spin, and spatial wave functions. All states in each individual SU (4) multiplet have the same total angular momentum J and parity P , but they may have different quark flavors. One can treat C = 1 baryons as those that consist of a heavy c quark and a light diquark whose quantum numbers are j p , where j is the total angular momentum of the diquark and p is its ordinary spatial parity. Assuming isospin symmetry and denoting u and d by q, we can distinguish four possible compositions of the diquark: (i) qq of isospin 0 (the flavor wave function is antisymmetric), (ii) qq of isospin 1 (the flavor wave function is symmetric), (iii) sq of isospin 1/2, and (iv) ss of isospin 0 (the flavor wave function is symmetric). They correspond to the Λc , Σc , Ξc , and Ωc states. *
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A combination of a diquark with a charmed baryon yields possible states of charmed baryons. All J P = 1/2+ states belong to the same multiplet as the proton, while the J P = 3/2+ states are members of the same multiplet as Δ and Ω. There is yet another isospin doublet of states, Ξc of spin–parity J P = 1/2+ ; one denotes it by Ξc . It should be noted that, in many cases, the total angular momentum and parity of a state are assigned on the basis of quark-model predictions rather than on the basis of measurements. At the present time, an experimental determination of J P is one of the priority tasks in charmed-baryon spectroscopy.
Baryons may be assigned orbital (l) or radial (k) excitations. Since this is a three-body system, there are two degrees of freedom in each case (one denotes them by ρ an
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