Towards understanding spectral degeneracies in nonstrange hadrons. Part I: Mesons as hadron strings vs. phenomenology

The spectroscopic regularities of modern empirical data on the nonstrange 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 perform a thorough phenomenological check of a recently proposed formula $M^2\sim L+n$, which explains the X-symmetry. A semiclassical hadron string picture is considered as a natural dynamical hypothesis leading to this spectrum. A quasiclassical correction is discussed. Using the obtained results it is argued that the spectrum of highly excited mesons should systematically reveal not only the chiral symmetry restoration in the sense of parity doubling, but also chiral-singlet states lying on the leading Regge trajectories.