Colour effective particles and confinement

Here I present a brief review of papers where the idea is pushed forward that colour confinement is realized by singular interaction at large distances between colour effective particles (constituent quarks, diquarks, massive effective gluons).Comment: Talk at Workshop ''Hadron Structure and QCD'' Gatchina, Russia, July 5 - July 9, 201

Systematics of q anti-q states in the (n,M^2) and (J,M^2) planes

In the mass region up to M < 2400 MeV we systematise mesons on the plots (n,M^2) and (J,M^2), thus setting their classification in terms of n^{2S+1}L_J q anti-q states. The trajectories on the (n,M^2)-plots are drawn for the following (IJ^{PC})-states: a_0(10^{++}), a_1(11^{++}), a_2(12^{++}), a_3(13^{++}), a_4(14^{++}), pi(10^{-+}), pi_2(12^{-+}), eta(00^{-+}), eta_2(02^{-+})$, rho(11^{--}), f_0(00^{++}), f_2(02^{++}). All trajectories are linear, with nearly the same slopes. At the (J,M^2)-plot we set out meson states for leading and daughter trajectories: for pi, rho, a_1, a_2 and P'.Comment: 6 pages, LaTeX, 16 EPS figures, epsfig.st

Partial decay widths of baryons in the spin-momentum operator expansion method

The cross sections for photo- and pion-induced production of baryon resonances and their partial decay widths to the two--body and multi--body final states are calculated in the framework of the operator expansion method. The approach is fully relativistic invariant, and it allows us to perform combined analyses of different reactions imposing directly the analyticity and unitarity constraints. All formulae are given explicitly in the form used by the Crystal Barrel collaboration in the partial wave analysis.Comment: 15 pages, 6 figure

Resonance f0(1500)f_0(1500): Is it a scalar glueball ?

The ratios of partial widths for the decay of a glueball into two pseudoscalar mesons are calculated under the assumption that the production of light quark pairs ($u\bar u,d\bar d,s\bar s$) in soft gluon-\-induced reactions goes on within universal symmetry breaking. Parameter of the violation of flavour symmetry is fixed by the central hadron production data in high energy hadron collisions and/or by the ratios of radiative decay amplitudes $J/\Psi \to \gamma\eta/\gamma\eta'$ and $J/\Psi \to \gamma \phi \phi/\gamma \omega \omega$. The ratios of coupling constants $glueball\to\pi\pi,K\bar K, \eta\eta,\eta\eta'$ which are calculated with this parameter coincide reasonably with those of $f_0(1500)$, supporting an idea about glueball nature of $f_0(1500)$.Comment: LaTex, 8 pages, 2 .eps figures in one uuencoded file, uses epsfi

Radiative decays of quarkonium states, momentum operator expansion and nilpotent operators

We present the method of calculation of radiative decays of composite quark-antiquark systems with different J^{PC}: (Q\bar Q)_{in} -> gamma (Q\bar Q)_{out}. The method is relativistic invariant, it is based on the double dispersion relation integrals over the masses of composite mesons, it can be used for the high spin particles and provides us with the gauge invariant transition amplitudes. We apply this method to the case when the photon is emitted by a constituent in the intermediate state (additive quark model). We perform the momentum operator expansion of the spin amplitudes for the decay processes. The problem of nilpotent spin operators is discussed.Comment: 21 pages, 1 figur

Mechanisms of the f0(980)f_0(980) production in the reaction π−p→π0π0n\pi^-p\to\pi^0 \pi^0n

The model of the pure one-pion exchange mechanism, which gives a good description of the GAMS results on the alteration of the $S$-wave $\pi^0\pi^0$ mass spectrum in the $f_0(980)$ region in the reaction $\pi^-p\to\pi^0\pi^0n$ with increasing $-t$, is compared with the recent detailed data on the $m$ and $t$ distributions of the $\pi^-p\to\pi^0\pi^0n$ events obtained by the BNL-E852 Collaboration. It is shown that the predictions of this model are not confirmed by the BNL data. Therefore the observed phenomenon should be explained by the different exchange mechanism. It is most likely to be the $a_1$ exchange mechanism.Comment: 11 pages including 3 figure

Chiral multiplets of excited mesons

It is shown that experimental meson states with spins J=0,1,2,3 in the energy range 1.9 - 2.4 GeV obtained in recent partial wave analysis of proton-antiproton annihilation at LEAR remarkably confirm all predictions of chiral symmetry restoration. Classification of excited $\bar q q$ mesons according to the representations of chiral $U(2)_L \times U(2)_R$ group is performed. There are two important predictions of chiral symmetry restoration in highly excited mesons: (i) physical states must fill out approximately degenerate parity-chiral multiplets; (ii) some of the physical states with the given $I,J^{PC}$ are members of one parity-chiral multiplet, while the other states with the same $I,J^{PC}$ are members of the other parity-chiral multiplet. For example, while some of the excited $\rho(1,1^{--})$ states are systematically degenerate with $a_1(1,1^{++})$ states forming (0,1)+(1,0) chiral multiplets, the other excited $\rho(1,1^{--})$ states are degenerate with $h_1(0,1^{+-})$ states ((1/2,1/2) chiral multiplets). Hence, one of the predictions of chiral symmetry restoration is that the combined amount of $a_1(1,1^{++})$ and $h_1(0,1^{+-})$ states must coincide with the amount of $\rho(1,1^{--})$ states in the chirally restored regime. It is shown that the same rule applies (and experimentally confirmed) to many other meson states.Comment: 14 pages, discussion and conclusion section is largely extende

Chiral symmetry restoration in hadron spectra

The evidence and the theoretical justification of chiral symmetry restoration in high-lying hadrons is presented.Comment: Invited talk given at Int. School on Nuclear Physics "Quarks in Hadrons and Nuclei", September 2002, Erice/Sicily/Italy; to appear in Progr. Part. Nucl. Phys., vol. 50; 10 page

Quark-gluonium content of the scalar-isoscalar states f_0(980), f_0(1300), f_0(1500), f_0(1750), f_0(1420 ^{+150}_{- 70}) from hadronic decays

On the basis of the decay couplings f_0 -> \pi\pi, K\bar K, \eta\eta, \eta\eta', which had been found before, in the study of analytical (IJ^{PC}=00^{++})-amplitude in the mass range 450-1900 MeV, we analyse the quark-gluonium content of resonances f_0(980), f_0(1300), f_0(1500), f_0(1750) and the broad state f_0(1420 ^{+ 150}_{-70}). The K-matrix technique used in the analysis makes it possible to evaluate the quark-gluonium content both for the states with switched-off decay channels (bare states, f^{bare}_0) and the real resonances. We observe significant change of the quark-gluonium composition in the evolution from bare states to real resonances, that is due to the mixing of states in the transitions f_0(m_1)-> real mesons-> f_0(m_2) responsible for the decay processes as well. For the f_0(980), the analysis confirmed the dominance of q\bar q component thus proving the n\bar n/s\bar s composition found in the study of the radiative decays. For the mesons f_0(1300), f_0(1500) and f_0(1750), the hadronic decays do not allow one to determine uniquely the n\bar n, s\bar s and gluonium components providing relative pecentage only. The analysis shows that the broad state f_0(1420 ^{+ 150}_{-70}) can mix with the flavour singlet q\bar q component only, that is consistent with gluonium origin of the broad resonance.Comment: 20 pages, LaTeX, 10 PostScript figures, epsfig.st

Parity doubling of highly excited mesons

Glozman has proposed that highly excited mesons and baryons fall into parity doublets, and that the f4(2050) on the leading Regge trajectory should have a nearly degenerate J^{PC} = 4^{-+} partner. A re-analysis of Crystal Barrel data does not support this idea. A likely explanation is that centrifugal barriers on the leading trajectory allow formation of the L=J-1 states, but are too strong to allow L=J states. Two new polarisation experiments have the potential for major progress in meson spectroscopy.Comment: 7 pages, 2 figures, 1 table. Further experimental detail added and additional algebra. Conclusions unchanged. To be published in Physics Letters