The S- and P-Wave Low-Lying Baryons in the Chiral Quark Model

Abstract

The 1S, 2S, 1P and 2P states of light baryons are investigated within the chiral quark model, paying particular attention to the well-known order reverse problem of 1P and 2S states. Besides a nonperturbative linearscreened confining interaction and a perturbative one-gluon exchange between quarks, we incorporate the Goldstone-boson exchanges taking into account not only the full octet of pseudoscalar mesons but also the scalar one. The scalar meson exchange potential simulates the higher order multi-pion exchange terms that appear in the chiral Lagrangian and its omission has been already admitted as a deficiency of the original model in describing, for instance, the ρ − ω splitting. The numerical approach to the three-body bound state problem is the so-called Gaußian expansion method, which is able to get a precision as good as Faddeev calculations. With a set of parameters fixed to different hadron and hadron-hadron observables, we find that the chiral potential could play an important role towards the issue on the mass order reverse problem. We extend our calculation to the qqQ and qQQ sectors (with q representing a light u-, d-, or s-quark and Q denoting the charm quark or the bottom one) in which many new states have been recently observed. Some tentative assignments are done attending to the agreement between theoretical and experimental masses; however, we admit that other sources of information are needed in order to make strong claims about the nature of these states.

Possible interpretation of the newly observed $$\Omega _b$$ states

“…It should be pointed out that although some people obtain the theoretical masses of the 2S states in the b family are about 6.3-6.4 GeV [15,17] and the possibility as the new observed b states cannot be excluded absolutely, however, Table 6 Partial widths (MeV) of strong decays for the λ-mode D-wave b baryons, whose masses (MeV) are taken the quark model predictions of Ref. [13] Decay mode the mass splitting between the 1P and 2S is about 100-150 MeV.…”

“…From the table, the four peaks observed by the LHCb Collaboration [10] are in the predicted mass region of the 1P wave excitations [11][12][13][14][16][17][18]. In addition, the possibility as 2S excitations cannot be excluded absolutely based simply on the predicted masses [15,17]. There are also some discussions of the strong decay properties of the excited − b states.…”

“…Before the LHCb's measurement [10], there exist many theoretical calculations of the mass spectrum of the excited b states with various quark models [11][12][13][14][15][16][17][18], Regge phenomenology [19], QCD two-point sum rule method [20,21], molecular model [22], etc. We collect the theoretical predictions in Table 1.…”

First Observation of Excited Ωb− States

“…Recently, the LHCb experiment observed five narrow states, assumed to be excited Ω 0 c baryons, which decay into Ξ þ c K − [7]. These states have been analyzed from the perspective of constituent quark models and lattice QCD [17][18][19][20][21][22][23][24][25][26][27][28][29][30][30][31][32][33], quark-diquark models [34][35][36][37][38][39][40][41][42][43][44], as well as molecular models [45][46][47][48][49][50] and pentaquark states [51][52][53]. Several of the models that seek to describe these peaks also make predictions for Ξ 0 b K − resonances.…”

Role of chiral symmetry in the nucleon excitation spectrum

“…It is clear that some controversy persists regarding how the Roper resonance in the continuum manifests on the lattice [5][6][7][8][9][10][11][12][13]. The χQCD collaboration advocate that their result is directly associated with the use of overlap fermions and stress the importance of implementing exact chiral symmetry when investigating the nucleon spectrum.…”