Color Transparency Effects in Quasi-elastic Nuclear Reactions

The Lagrangians and Hamiltonians of classical field theory require compensating gauge fields in order to realize local internal gauge invariance with the standard transformation laws. This principle is developed in the extended covariant canonical transformation formalism, which also accommodates changes of spacetime coordinates. The construction yields locally U$(N)$-invariant matter--gauge-field actions and, in a separate application, diffeomorphism-covariant actions for ordinary vector fields and an independent affine connection. The latter application adopts a covariant-derivative coupling prescription whose antisymmetric vector-momentum sector contains a direct interaction with torsion. This interaction is not required by diffeomorphism covariance alone and does not apply to the internal gauge connection with its standard inhomogeneous transformation law. The compensating terms are distinguished from additional invariant interactions and from the gravitational dynamics, which must be specified independently.

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arXiv2026

Nucleon electromagnetic form-factors in a minimal Gari-Krümpelmann model including two-meson continua

The Gari--Krümpelmann (GK) model provided one of the first semiphenomenological descriptions of the electromagnetic nucleon form-factors over a wide range of momentum transfer. It combined vector-meson dominance at low and intermediate momentum transfer with perturbative-QCD constraints at large momentum transfer. The present work extends this framework by including the dispersive two-pion continuum explicitly in the isovector channel. The two-pion contribution is taken from modern dispersive analyses and embedded into the intrinsic high-$Q^2$ structure of the GK ansatz. The physical $ρ$ dynamics is then interpreted as part of the correlated two-pion spectral function. Similarly, we include in the isoscalar spectral function the correlated $K\bar K$ and $ρπ$ continua, where the former also includes the $ϕ$-meson pole. The direct coupling terms remain essential for the transition to the asymptotic regime. This gives a cleaner separation between long-range continuum dynamics, coherent vector-meson contributions, and direct quark-current dynamics. With just 7 parameters, we can describe the large body on form-factor data and differential cross sections from electron-proton scattering in the space-like region.

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arXiv2026

$^{86}$Ni: Possible doubly-magic nucleus with neutron halo

Self-consistent mean-field calculations with semi-realistic interactions have predicted that $N=58$ becomes a magic number near Ni, where the $n2s_{1/2}$ orbit is the highest occupied state. As the last neutrons occupy the shallowly bound $s$-orbit, $^{86}$Ni can be highly distinctive, being a doubly-magic nucleus and possessing a neutron halo simultaneously. With minimal ambiguity in the many-body wave function, it may provide a unique opportunity to investigate, \textit{e.g.}, reaction mechanisms in halo nuclei. As an example, we present differential cross sections for proton elastic scattering predicted via a folding potential without local approximations, and compare them with those obtained using an empirical potential.

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