Another view of the Gell-Mann-Okubo mass formula

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1 3 February 017 Another view of the Gell-Mann-Okubo mass formula Jean Pestieau 1 The Gell-Mann-Okubo mass formula for light hadrons assumes that mass effective operator is in the 8 representation of flavour SU(3). To be in very good agreement with experimental data, we have to assume that the mass effective operator is in part also in the 7 representation. From that, we derive improved mass formulas. Fifty five years ago, the mass formula of Gell-Mann-Okubo (GMO) for light hadrons (composed of valence quarks u, d, s) met a great success in the elucidation of their mass spectrum in terms of flavour SU(3) singlets, octets and decuplet. In this note, we propose to review GMO mass formula and to improve it. We start with GMO for the baryon octet 3, expressed as M N + M Ξ = 3M Λ + M Σ (1) with 4 M N [m(p) + m(n)]/ MeV M Ξ [m(ξ ) + m(ξ )]/ MeV M Λ m(λ) MeV (a) (b) (c) M Σ [m(σ ) + m((σ ) + m(σ +)]/ MeV (d) In fact, M N + M Ξ 3M Λ M Σ 5.8 MeV (3) GMO is satisfied up to 1 % because 5.8 MeV / M Ξ Centre for Cosmology, Particle Physics and Phenomenology (CP3), Université catholique de Louvain, chemin du Cyclotron, B-1348 Louvain-la-Neuve, Belgium. M. Gell-Mann, unpublished, 5 march 1961; S.Okubo, Progr. Theor. Phys. (Kyoto), 7, 949 (196); M. Gell-Mann and Y Ne eman, The Eightfold Way, W.A. Benjamin,Inc., New-York (1964). 3 We performed a similar analysis for the baryon decuplet. Here we have to take in consideration 8, 7 and 64 representations of flavour SU(3) as components of mass effective operator. 4 Hadron experimental values used in this note come from: C.Patrigani et al. (Particle Data Group), Chin. Phys. C, 40, (016),

2 Now, let us look GMO mass formula from an other point of view, expressing it as M N + M Ξ 4M Λ = M Σ M Λ (4) As indicated in eq. (3), this formula is approximated and is quite different of the expression 3M N + 3M Ξ 6M Λ = M Σ M Λ (5a) which is in excellent agreement with experimental data. Let us note that eq. (5a) is equivalent to M N + M Ξ 3M Λ M Σ = [M Σ M Λ ]/3 5.8 MeV (5b) This is the GMO improved mass formula for baryons. We are going to show from where comes eqs. (5). To derive GMO mass formula, we use the interaction picture of the perturbation theory at the first order, with state vectors (in the limit where flavour SU(3) is exact) sandwiching mass effective operator which breaks flavour SU(3) in the direction of the hypercharge axis Y. This mass effective operator has two components, one belonging to 8 representation and another one belonging to 7 representation of flavour SU(3). Mass of baryon B, belonging to light octet representation is expressed then as M B = M 0 Δ 8a Y/ + (Δ 8s + Δ 7 )[I(I + 1) Y²/4]/10 + Δ 7 Y²/4 (6) or, explicitly as M N = M 0 Δ 8a / + (Δ 8s + Δ 7 )/0 + Δ 7 /4 M Ξ = M 0 + Δ 8a / + (Δ 8s + Δ 7 )/0 + Δ 7 /4 M Λ = M 0 M Σ = M 0 + (Δ 8s + Δ 7 )/5 (6a) (6b) (6c) (6d) M 0 is the mass SU(3) symmetric part. Δ 8a, Δ 8s et Δ 7 are contributions to baryon octet masses corresponding to antisymmetric 8, symmetric 8 and 7 representations of flavour SU(3) group 5 respectively: Δ 8a = M Ξ M N MeV (7) Δ 8s = M N M Ξ M Λ + 6M Σ MeV (8) Δ 7 = M N + M Ξ 3M Λ M Σ 5.8 MeV (9) GMO mass formula [eqs. (1) or (4)] is obtained assuming octet dominance in the mass effective operator, i.e. assuming Δ 7 = 0 (10) 5 For definitions of Δ 8a, Δ 8s et Δ 7, see table 11 (p. 6) of W. Bieterholz et al.,

3 In contradistinction, if we wish to satisfy eqs. (5) a mass formula in strict agreement with experiments, we find from eqs. (6) that necessarily (Δ 8s + Δ 7 )/5 = 3 Δ 7 (11) and that Δ 7 = [ M Σ M Λ ] /3 5.8 MeV (1) It is obvious that eqs. (5) are equivalent to 3M N + 3M Ξ = 5M Λ + M Σ (13) to be compared to eq. (1). Eq. (1) is not an approximated result due to flavour SU(3) breaking but to the neglect of 7 representation in the mass effective operator. Eqs. (5) or (13) are in agreement with experiments at least at the same level of precision as the Coleman-Glashow mass formula. 6 m(n) m(p) + m(ξ ) m( (Ξ ) = m(σ ) m(σ + ) (14) Next, we extend our results to four light mesons octets with J PC = 0 +, 1, + + and 3, denominated J = 0, 1, and 3 respectively. We assume that mass formula for light mesons will be mass quadratic. For mesons, we have a mixing between singlet state under flavour SU(3) and the I = 0 member of the corresponding octet: η η (0 + ), ω φ (1 ), f f ' ( + + ), ω 3 φ 3 (3 ). For each octet, we have a mixing angle denoted θ J. The GMO mass formulas for light mesons 7 are expressed as 4M K 3M η8 M π = 0, θ (15a) 4M K* 3M ω8 M ρ = 0, θ (15b) 4M K* 3M f _8 M a =0, θ 3.1 (15c) 4M K*3 3M ω3_8 M ρ3 = 0, θ (15d) We propose an alternative to GMO mass formulas given in eqs. (15 a-d): 4M K 3M η8 M π = + [ M π M η8 ]/3, θ 0 18 (16a) 4M K* 3M ω8 M ρ = + [ M ρ M ω8 ]/3, θ (16b) 4M K* 3M f _8 M a = (M a M f _8 )/3, θ 35. (16c) 4M K*3 3M ω3_8 M ρ3 = (M ρ3 M ω3_8 )/3, θ (16d) with sin θ 0 = 1/( 5 + 1) and sin θ J = 1/ 3 when J = 1, or 3. sin θ J = 1/ 3 separates I = 0 meson states containing (ssbar) only and the ones containing (uubar + ddbar) only as valence quarks. 6 S.Coleman and S.L. Glashow, Phys.Rev. Lett., 6, 43 (1961). See also P.A. Carruthers, Introduction to unitary symmetry, Interscience Publishers (1966). 7 table 15..

4 sin θ 0 = 1/( 5 + 1) 8 is compatible with different η η mixing determinations : 14 > θ 0 >, taking into account axial strong anomaly. To derive GMO mass formulas [eqs. (15a-d)] and improved GMO mass formulas [eqs. 16a-d)], we proceed as for baryon octet. Masses of mesons of isospin I, belonging to light octet representation J, are expressed 9 as M² J,I = M² J,0 + (Δ' J,8 + Δ' J,7 )[I(I + 1) Y²/4]/10 + Δ' J,7 Y²/4 (17) or, explicitly as M² J,1/ = M² J,0 + (Δ' J,8 + Δ' J,7 )/0 + Δ' J,7 /4 (17a) M² J,0 = M² J,0 (17b) M² J,1 = M² J, 0 + (Δ' J,8 + Δ' J,7 )/5 (17c) M J,0 is the mass SU(3) symmetric part. Δ' J,8 et Δ' J,7 are contributions to meson octet (masses)² corresponding to 8 and 7 representations of flavour SU(3) group 10 respectively: Δ J,8 = 4M² J,1/ M² J,0 + 6M² J,1 (18) Δ J,7 = 4M² J,1/ 3M² J,0 M² J,1 (19) GMO mass formulas [eqs. (15a-d)] are obtained assuming octet dominance in the (mass)² effective operator, i.e. assuming Δ J,7 = 0 (0) Modified and improved GMO mass formulas [eqs. 16a-d] are obtained assuming (Δ J,8 + Δ J,7 )/5 = + 3 Δ J,7 (1) when J = 0 or 1 and (Δ J,8 + Δ J,7 )/5 = 3 Δ J,7 () when J = or 3 Eqs. (0), (1) and () imply that, to be in agreement with experimental data, (mass)² effective operator for light hadrons belonging to octet must be part not only of 8 representation but also of 7 representation. Furthermore, there is an intimate relation between contributions coming from 8 and 7 representations. 8 It is amusing to note that ( 5 + 1)/ is the golden ratio. 9 A meson of isospin I in an octet has one possible value for Y². I = ½, 0 and 1 imply Y² = 1, 0, 0 respectively. 10 For definitions of Δ J,8 et Δ J,7, see table 1 (p. 6) of W. Bieterholz et al.,

5 Appendix From eq. (16a), we get M η8 599 MeV M η8 = M η cos θ 0 + M η' sin θ 0 (θ 0 = 18 ) From eq. (16b), we get M ω8 947 MeV M ω8 = M ω sin θ 1 + M φ cos θ 1 (θ 1 = 35. ) From eq. (16c), we get M f_ MeV M f_8 = M f sin θ + M f cos θ (θ = 35. ) From eq. (16d), we get M ω3_ MeV M ω3_8 = M ω3 sin θ 3 + M φ3 cos θ 3 (θ 3 = 35. )