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Baryon resonances in large Nc QCD ; Stancu, Floarea in Reviews of Modern Physics (2015), 87 We review the current status and present open challenges of large $N_c$ QCD baryon spectroscopy. After introducing the $1/N_c$ expansion method we first shortly revisit the latest achievements for the ... [more ▼] We review the current status and present open challenges of large $N_c$ QCD baryon spectroscopy. After introducing the $1/N_c$ expansion method we first shortly revisit the latest achievements for the ground state properties. Next we discuss the applicability of this method to excited states, presenting two different approaches with their advantages and disadvantages. Selected results for the spectrum and strong and electromagnetic decays are described. We also present further developments for the applicability of the method to excited states, based on the qualitative compatibility between the quark excitation picture and the meson-nucleon scattering picture. We show that a quantitative comparison between results obtained from the mass formula of the $1/N_c$ expansion method and quark models brings convincing support to quark models and shortly discuss the implications of different large $N_c$ limits. We stress that the SU(6) spin-flavor structure of large $N_c$ baryon allows a convenient classification of highly excited resonances into SU(3) multiplets and predicts mass ranges for the missing partners. [less ▲] Detailed reference viewed: 15 (2 ULg)Large Nc baryons and Regge trajectories ; Stancu, Floarea in Bled Workshops in Physics (2013, November), 14(1), 37-42 The mixed symmetric positive and negative parity baryons are described in a similar way in the $1/N_c$ expansion method of QCD by using a procedure where the permutation symmetry is incorporated exactly ... [more ▼] The mixed symmetric positive and negative parity baryons are described in a similar way in the $1/N_c$ expansion method of QCD by using a procedure where the permutation symmetry is incorporated exactly. This allows to express the mass formula in terms of a small number of linearly independent operators. We show that the leading term follows a different Regge trajectory from that found for symmetric states, when plotted as a function of the band number $N$. [less ▲] Detailed reference viewed: 30 (5 ULg)[70,l+] baryons in large Nc QCD revisited: The effect on Regge trajectories ; Stancu, Floarea in Physical Review. D : Particles and Fields (2013), 87(April), 076012 A simple procedure within the 1/N_c expansion method where all the N_c quarks are treated on the same footing has been found successful in describing mixed symmetric negative parity baryon states ... [more ▼] A simple procedure within the 1/N_c expansion method where all the N_c quarks are treated on the same footing has been found successful in describing mixed symmetric negative parity baryon states belonging to the [{\bf 70},\ell^-] multiplets of the N = 1 and 3 bands. Presently it is applied to mixed symmetric positive parity [{\bf 70},0^+] and [{\bf 70},2^+] multiplets of the N = 2 band. We search for the most dominant terms in the mass formula. The results are compared to those obtained in the procedure where the system is separated into a core and an excited quark. We find that both the spin and isospin operators of the entire system of N_c quarks play dominant roles in describing the data, like for negative parity states. As a by-product we present the contribution of the leading spin-isospin singlet term as a function of the band number, which hints at distinct Regge trajectories for the symmetric and mixed symmetric states. [less ▲] Detailed reference viewed: 16 (4 ULg)Highly excited states of baryons in large Nc QCD ; Stancu, Floarea in Bled Workshops in Physics (2012, November), 13(1), 47-53 Detailed reference viewed: 19 (11 ULg)Negative parity baryons in the 1/Nc expansion: the three towers of states revisited. ; Stancu, Floarea in Physical Review. D : Particles and Fields (2012), D86 We discuss the compatibility between the quark-shell picture and the meson-nucleon scattering picture in large $N_c$ QCD for mixed symmetric $\ell$ = 1 states previously analyzed by using a simple ... [more ▼] We discuss the compatibility between the quark-shell picture and the meson-nucleon scattering picture in large $N_c$ QCD for mixed symmetric $\ell$ = 1 states previously analyzed by using a simple Hamiltonian including operators up to order $\mathcal{O}(N^0_c)$ defined in the standard ground state symmetric core + excited quark procedure. Here we introduce a Hamiltonian of order $\mathcal{O}(N^0_c)$ defined in a new approach where the separation of the system into two parts is not required. Three degenerate sets of states (towers) with the same quantum numbers as in the scattering picture and in the standard procedure are obtained. Thus the compatibility is equally achieved. The eigenvalues of the presently chosen Hamiltonian also have simple analytic expressions, depending linearly on the three dynamical coefficients entering the Hamiltonian. This reinforces the validity of the new approach which had already successfully described excited negative parity baryons in a large energy range. [less ▲] Detailed reference viewed: 18 (13 ULg)Excited baryons in the 1/Nc expansion. ; Stancu, Floarea in AIP Conference Proceedings (2012), 1432 We briefly describe the 1/Nc expansion method for studying baryon masses. Two approaches of the large Nc excited baryons have been proposed so far. The first one, based on the Hartree picture, treats the ... [more ▼] We briefly describe the 1/Nc expansion method for studying baryon masses. Two approaches of the large Nc excited baryons have been proposed so far. The first one, based on the Hartree picture, treats the baryon as a ground state core and an excited quark and the second one, suggested recently, considers the baryon globally, without decoupling the system. The masses of excited states of mixed orbital symmetry of nonstrange and strange baryons belonging to the lowest [70, −] multiplet are calculated in the 1/Nc expansion to order 1/Nc with the new method which allows to considerably reduce the number of linearly independent operators entering the mass formula. The status of the resonance Λ(1405) is discussed. [less ▲] Detailed reference viewed: 22 (7 ULg)SU(6) [70,1-] baryon multiplet in the 1/N_c expansion ; Stancu, Floarea in Physical Review. D : Particles and Fields (2011), 83 The masses of excited states of mixed orbital symmetry of nonstrange and strange baryons belonging to the lowest $[{\bf 70},1^-]$ multiplet are calculated in the $1/N_c$ expansion to order $1/N_c$ with a ... [more ▼] The masses of excited states of mixed orbital symmetry of nonstrange and strange baryons belonging to the lowest $[{\bf 70},1^-]$ multiplet are calculated in the $1/N_c$ expansion to order $1/N_c$ with a new method which allows to considerably reduce the number of linearly independent operators entering the mass formula. This study represents an extension to SU(6) of our work on nonstrange baryons, the framework of which was SU(4). The conclusion regarding the role of the flavor operator, neglected in previous SU(6) studies, is reinforced. Namely, both the flavor and spin operators contribute dominantly to the flavor-spin breaking. [less ▲] Detailed reference viewed: 12 (3 ULg)Negative parity baryons in the 1/N_c expansion : the quark excitation versus the meson-nucleon resonance picture ; Stancu, Floarea in Physical Review. D : Particles and Fields (2011), 84 In order to better understand the fundamental issue regarding the compatibility between the quark-shell picture and that of resonances in meson-nucleon scattering in large $N_c$ QCD we extend the work of ... [more ▼] In order to better understand the fundamental issue regarding the compatibility between the quark-shell picture and that of resonances in meson-nucleon scattering in large $N_c$ QCD we extend the work of Cohen and Lebed on mixed symmetric $\ell $ = 1 baryons to analyze excited states with $\ell $ = 3. We give an explicit proof on the degeneracy of mass eigenvalues of a simple Hamiltonian including operators up to order $\mathcal{O}(N^0_c)$ \emph{i. e.} neglecting $1/N_c$ corrections in the quark-shell picture in the large $N_c$ limit. We obtain three degenerate multiplets formed of $\ell = 3$ states, as in the case of $\ell $ = 1 baryons. The compatibility between this picture and that of resonances in meson-nucleon scattering is discussed in the light of the present results. [less ▲] Detailed reference viewed: 12 (2 ULg)Lowest negative parity baryons in the 1/Nc expansion ; Stancu, Floarea in Acta Physica Polonica. B, Proceedings Supplement (2010), 3 We review a recently proposed approach to study the lowest negative parity baryons within the $1/N_c$ expansion. The method is based on the derivation of the matrix elements of SU(2N$_f$) generators for ... [more ▼] We review a recently proposed approach to study the lowest negative parity baryons within the $1/N_c$ expansion. The method is based on the derivation of the matrix elements of SU(2N$_f$) generators for mixed symmetric $[N_c-1,1]$ flavor-spin states. Presently it is applied to the N = 1 band and a comparison is made with a former method based on the decoupling of the system into a symmetric core of $N_c-1$ quarks and an excited one. We prove that the decoupling is not necessary and moreover, it misses some important physical consequences. [less ▲] Detailed reference viewed: 17 (7 ULg)Group theoretical study of nonstrange and strange mixed symmetric baryon states [Nc-1,1] in the 1/Nc expansion ; Stancu, Floarea in Physical Review. D : Particles and Fields (2010), 81 Using group theory arguments we extend and complete our previous work by deriving all SU(6) exact wave functions associated with the spectrum of mixed symmetric baryon states [Nc-1,1] in the 1/Nc ... [more ▼] Using group theory arguments we extend and complete our previous work by deriving all SU(6) exact wave functions associated with the spectrum of mixed symmetric baryon states [Nc-1,1] in the 1/Nc expansion. The extension to SU(6) enables us to study the mass spectra of both strange and nonstrange baryons, while previous work was restricted to nonstrange baryons described by SU(4). The wave functions are specially written in a form to allow a comparison with the approximate, customarily used wave functions, where the system is separated into a ground state core and an excited quark. We show that the matrix elements of the flavor operator calculated with the exact wave functions acquire the same asymptotic form at large Nc, irrespective of the spin-flavor multiplet contained in [Nc-1,1], while with the approximate wave function one cannot obtain a similar behavior. The isoscalar factors of the permutation group of Nc particles derived here can be used in any problem where a given fermion system is described by the partition [Nc-1,1], and one fermion has to be separated from the rest. [less ▲] Detailed reference viewed: 22 (12 ULg)Mixed symmetric baryon multiplets in large $N_c$ QCD: two and three flavours ; Stancu, Floarea in Bled Workshops in Physics (2010), 11(1), 37-43 We propose a new method to study mixed symmetric multiplets of baryons in the context of the $1/N_c$ expansion approach. The simplicity of the method allows to better understand the role of various ... [more ▼] We propose a new method to study mixed symmetric multiplets of baryons in the context of the $1/N_c$ expansion approach. The simplicity of the method allows to better understand the role of various operators acting on spin and flavour degrees of freedom. The method is tested on two and three flavours. It is shown that the spin and flavour operators proportional to the quadratic invariants of SU$_S$(2) and SU$_F$(3) respectively are dominant in the mass formula. [less ▲] Detailed reference viewed: 15 (5 ULg)Comparative analysis of large Nc QCD and quark model approaches to baryons ; ; Stancu, Floarea et al in Acta Physica Polonica. B, Proceedings Supplement (2009), 2(2), 223-228 We show that a remarkable compatibility exists between the results of a potential model with constituent quarks and the 1/Nc expansion mass formula for strange and nonstrange baryon resonances. Such ... [more ▼] We show that a remarkable compatibility exists between the results of a potential model with constituent quarks and the 1/Nc expansion mass formula for strange and nonstrange baryon resonances. Such compatibility brings support to the basic assumptions of relativistic quark models and sheds light on the physical content of the model-independent large Nc mass formula. Good agreement between both approaches is also found for heavy baryons, made of one heavy and two light quarks, in the ground state band. [less ▲] Detailed reference viewed: 11 (3 ULg)Matrix elements of SU(6) generators for baryons with arbitrary $N_c$ quarks in mixed symmetric states $[N_c- 1,1]$ ; Stancu, Floarea in Nucl. Phys. (2009), A826 We derive general analytic formulae for the matrix elements of the SU(6) generators for mixed symmetric $[N_c-1,1]$ spin-flavor states with an arbitrary number $N_c$ of quarks. They are relevant for ... [more ▼] We derive general analytic formulae for the matrix elements of the SU(6) generators for mixed symmetric $[N_c-1,1]$ spin-flavor states with an arbitrary number $N_c$ of quarks. They are relevant for baryon spectroscopy in the $1/N_c$ expansion method applied to light baryons and can be used to study excited states. In this way previous work on non-strange baryons can be extended to both non-strange and strange baryons. [less ▲] Detailed reference viewed: 17 (1 ULg)Large N(c) QCD versus quark model ; ; Stancu, Floarea et al in Few-Body Systems (2008), 44 Detailed reference viewed: 19 (4 ULg)Group Theoretical Analysis of the Wave Function of the $[ 70,1^-]$ Nonstrange Baryons in the $1/N_c$ Expansion ; Stancu, Floarea in AIP Conference Proceedings (2008), 1038 Using standard group theoretical techniques we construct the exact wave function of the $[{\bf 70},1^-]$ multiplet in the orbital, spin and flavor space. This symmetric wave function is compared to that ... [more ▼] Using standard group theoretical techniques we construct the exact wave function of the $[{\bf 70},1^-]$ multiplet in the orbital, spin and flavor space. This symmetric wave function is compared to that customarily used in the $1/N_c$ expansion, which is asymmetric. The comparison is made by analyzing the matrix elements of various operators entering the mass formula. These matrix elements are calculated by the help of isoscalar factors of the permutation group, specially derived for this purpose as a function of $N_c$. We also compare two distinct methods used in the study of the $[{\bf 70},1^-]$ multiplet. In the first method the generators are divided into two parts, one part acting on a subsystem of $N_c-1$ quarks called core and another on the separated quark. In the second method the system is treated as a whole. We show that the latter is simpler and allows to clearly reveal the physically important operators in the mass formula. [less ▲] Detailed reference viewed: 8 (1 ULg)New look at the (70,1-) baryon multiplet in the 1/N(c) expansion ; Stancu, Floarea in Nucl. Phys. (2008), A811 So far, the masses of excited states of mixed orbital symmetry and in particular those of nonstrange $[{\bf 70},1^-]$ baryons derived in the $1/N_c$ expansion were based on the separation of a system of ... [more ▼] So far, the masses of excited states of mixed orbital symmetry and in particular those of nonstrange $[{\bf 70},1^-]$ baryons derived in the $1/N_c$ expansion were based on the separation of a system of $N_c$ quarks into a symmetric core and an excited quark. Here we avoid this separation and show that an advantage of this new approach is to substantially reduce the number of linearly independent operators entering the mass formula. A novelty is that the isospin-isospin term becomes as dominant in $\Delta$ as the spin-spin term in $N$ resonances. [less ▲] Detailed reference viewed: 7 (0 ULg)The wave function of $[70,1^-]$ baryons in the $1/N_c$ expansion ; Stancu, Floarea in Phys. Rev. (2008), D77 Much work has been devoted to the study of nonstrange baryons belonging to the $[\textbf{70},1^-]$ multiplet in the framework of the $1/N_c$ expansion. Using group theoretical arguments here we examine ... [more ▼] Much work has been devoted to the study of nonstrange baryons belonging to the $[\textbf{70},1^-]$ multiplet in the framework of the $1/N_c$ expansion. Using group theoretical arguments here we examine the relation between the exact wave function and the approximate one, customarily used in applications where the system is separated into a ground state core and an excited quark. We show that the exact and approximate wave functions globally give similar results for all of mass operators presented in this work. However we find that the inclusion of operators acting separately on the core and on the excited quark deteriorates the fit and leads to unsatisfactory values for the coefficients which encode the quark dynamics. Much better results are obtained when we include operators acting on the whole system, both for the exact and the approximate wave function. [less ▲] Detailed reference viewed: 11 (1 ULg)Baryonic mass formula in large N-c QCD versus quark model ; ; et al in Physical Review D (2007), 75(9), We establish a connection between the quark model results and the 1/N-c expansion mass formula used in the description of baryon resonances. We show that a remarkable compatibility exists between the two ... [more ▼] We establish a connection between the quark model results and the 1/N-c expansion mass formula used in the description of baryon resonances. We show that a remarkable compatibility exists between the two methods. [less ▲] Detailed reference viewed: 9 (1 ULg)Masses of [70, l+] baryons in large N-c QCD ; Stancu, Floarea in International Journal of Modern Physics A (2007), 22(2-3), 663-666 Previous work is extended from SU(2) to SU(3) and we present results for the mass spectrum of the [70,l(+)](l=0,2) nonstrange and strange baryons in the 1/N-c expansion. We show that the dominant term is ... [more ▼] Previous work is extended from SU(2) to SU(3) and we present results for the mass spectrum of the [70,l(+)](l=0,2) nonstrange and strange baryons in the 1/N-c expansion. We show that the dominant term is the spin-spin interaction and its contribution vanishes at large excitations. [less ▲] Detailed reference viewed: 15 (1 ULg)The $[ 70,1^-]$ baryon multiplet in the $1/N_c$ expansion revisited ; Stancu, Floarea in AIP Conference Proceedings (2007), 892 The mass splittings of the baryons belonging to the $[{\bf 70},1^-]$-plet are derived by using a simple group theoretical approach to the matrix elements of the mass formula. The basic conclusion is that ... [more ▼] The mass splittings of the baryons belonging to the $[{\bf 70},1^-]$-plet are derived by using a simple group theoretical approach to the matrix elements of the mass formula. The basic conclusion is that the first order correction to the baryon masses is of order $1/N_c$ instead of order $N^0_c$, as previously found. The conceptual difference between the ground state and the excited states is therefore removed. [less ▲] Detailed reference viewed: 14 (1 ULg) |
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