The spectroscopic regularities of modern empirical data on the non-strange mesons up to 2.4 GeV can be summarized as a systematic clustering of states near certain values of energy. It is getting evident that some unknown X-symmetry triggers the phenomenon. We review the experimental status of this symmetry and recent theoretical attempts put forward for explanation of broad degeneracy.

We investigate the possible corrections to the linear Regge trajectories for the lightquark meson sector by matching two-point correlators of quark currents to the Operator Product Expansion. We find that the allowed modifications to the linear behavior must decrease rapidly with the principal quantum number. After fitting the lightest states in each channel and certain low-energy constants the whole spectrum for meson masses and residues is obtained in a satisfactory agreement with phenomenology. The perturbative corrections to our results are discussed.Keywords: QCD; sum rules; large-Nc.The observed masses squared of mesons with given quantum numbers form linear trajectories 1,2 depending on the number of radial excitation n. This is a strong indication that QCD admits an effective string description, as this type of spectrum is characteristic e.g. of the bosonic string. In the bosonic string model the slope of all trajectories must be equal since this quantity is proportional to the string tension depending on gluedynamics only. However, there exist sizeable deviations from the string picture. In the present analysis we examine possible corrections to the linear trajectories in the vector (V), axial-vector (A), scalar (S), and pseudoscalar (P) channels 3 . Our method is based on the consideration of the two-point correlators of V,A,S,P quark currents in the large-N c limit of QCD 4 . On the one hand, by virtue of confinement they are saturated by an infinite set of narrow meson resonances, that is, they can be represented by the sum of related meson poles in Euclidean space:expressing the quark-hadron duality 5 . Here J ≡ S, P, V, A; Γ = i, γ 5 , γ µ , γ µ γ 5 . Further we denote F S,P ≡ G S,P m S,P . On the other hand, their high-energy asymptotics 1

Abstract. Modern spectroscopic data on light non-strange meson spectrum is analyzed. It is argued that the observed regularities of experimental spectrum for highly excited states favour a partial restoration of all approximate classical symmetries of QCD Lagrangian (conformal, chiral and axial) broken by the quantum corrections. The rate of restoration of classical symmetries is estimated. The dependence of the resonance widths from the corresponding masses is systematically checked. On average, it turns out to be universal for the high excitations as predicted by the effective string description.

The global features of spectrum of highly excited light nonstrange mesons can be well understood within both chiral symmetry restoration scenario combined with the relation M 2 ∼ J + n and within a nonrelativistic description based on the relation M 2 ∼ L + n. The predictions of these two alternative classifications for missing states are different and only future experiments can distinguish between the two. We elaborate and compare systematically the predictions of both schemes, which may serve as a suggestion for future experiments devoted to the search for missing states.