The Sgoldstino-Photon Portal of Superlight Gravitinos: Updated Stellar Bounds and Cosmological Outlook

We update the astrophysical and cosmological constraints on superlight gravitinos accompanied by light scalar (S) and pseudoscalar (P) sgoldstino partners through the two-photon portal. In the Brignole–Feruglio–Zwirner (BFZ) matching g = √2 M_γ / F = √(2/3) M_γ / (M_P m3/2), horizontal-branch (HB) globular-cluster cooling gives m3/2 ≳ 0.51–0.71 keV for a fiducial photino mass M_γ = 100 GeV in a single-species treatment, potentially reaching 0.72–1.0 keV under a conditional two-emissivity sharpening. A long-lived optical-mass (~4 eV) relic contributes a spectral-edge estimate m3/2 ≳ 0.97 f_φ1/2 keV via extragalactic background light arguments. The corresponding SUSY-breaking scale is √F ≳ 1.5–2 × 10^6 GeV. Cosmological dark-radiation benchmarks for the sgoldstino species are presented as conditional illustrations and explicitly not as predictions, because the actual decoupling of the sgoldstino sector is set by Primakoff-type processes through coloured and electroweak superpartners rather than by the pair channel alone, and specifying it requires the Lagrangian above the electroweak scale. This Letter complements the author's earlier studies Mori (1996) and Mori & Natale (2000) with modern inputs.