Non-linear Regge trajectories with AdS/QCD

In this work, we consider a non-quadratic dilaton $Φ(z)=(κ\,z)^{2-α}$ in the context of the static soft wall model to describe the mass spectrum of a wide range of vector mesons from the light up to the heavy sectors. The effect of this non-quadratic approach is translated into non-linear Regge trajectories with the generic form $M^2=a\,(n+b)^ν$. We apply this sort of fits for the isovector states of $ω$, $ϕ$, $J/ψ$ and $Υ$ mesons and compare with the corresponding holographic duals. We also extend these ideas to the heavy-light sector by using the isovector set of parameters to extrapolate the proper values of $κ$ and $α$ through the average constituent mass $\bar{m}$ for each mesonic specie considered. In the same direction, we address the description of possible non-$q\,\bar{q}$ candidates using $\bar{m}$ as a holographic threshold, associated with the structure of the exotic state, to define the values of $κ$ and $α$. We study the $π_1$ mesons in the light sector, and the $Z_c$, $Y$ and $Z_b$ mesons in the heavy sector as possible exotic vector states. Finally, the RMS error for describing these twenty-seven states with fifteen parameters (four values for $κ$ and $α$ respectively and seven values for $\bar{m}$) is $12.61\%$.

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