From Colour--Casimir Recoupling to Near-Threshold Scattering: Algebraic Diagnostics and a Pion-Mass Test for the $T_{cc}^{+}$ Channel

Abstract

The near-threshold $T_{cc}^{+}$ is a clean testing ground for how constituent-level colour structure connects to hadronic scattering. We report three results and state what each does and does not establish. (i)~\emph{Colour algebra.} In the two-dimensional colour-singlet basis of $cc\bar u\bar d$, a general pairwise colour operator has direct $DD^*$ matrix element $V_{MM}=-\tfrac43(v_{13}+v_{24})$, containing only the two intra-meson coefficients, while meson--hidden-colour mixing is $V_{MH}^{\rm off}=\tfrac{\sqrt2}{3}Δ_v$ with $Δ_v=v_{14}+v_{23}-v_{12}-v_{34}$; uniform edges give no mixing. The projection is exact within this basis and is checked numerically on 10\,000 random edge assignments. A same-Hamiltonian decomposition shows that it re-expresses the central colour operator ($116.46$ MeV off-diagonal) rather than adding an independent dynamical input. (ii)~\emph{Negative dynamical results.} For Coulomb-type Gaussian edges the hidden-colour diagonal lies below the meson diagonal, so the attractive second-order shift $-2Δ_v^2/(9Δ_H)$ does not apply. In a Gaussian constituent Hamiltonian calibrated to $M_D$ and $M_{D^*}$, the lowest compact level lies $148.6$ MeV above its model $DD^*$ threshold (iii)~\emph{Pion-mass test.} Couplings fixed at the $m_π\simeq280$ MeV lattice pole ($-5.2$ MeV) are carried to $m_π\simeq391$ MeV by changing only the reduced mass and compact-sector gap, both refitted to the lattice $D$ and $D^*$ masses. A one-channel separable Lippmann--Schwinger benchmark ($Λ=650$ MeV, $μ=967.7$ MeV, $C_G\simeq-36.15~\mathrm{GeV}^{-2}$) is calibrated to the $5.2$ MeV pole and serves only as a regulator-sensitivity study. We do not reproduce the complex lattice pole or perform a full coupled-channel calculation.