Symmetry-Protected $\alpha$-Attractor Hybrid Inflation in Supergravity and Constraints from ACT DR6 and DESI DR2

Abstract

We present a symmetry-protected supergravity effective realization of hybrid $\alpha$-attractor inflation with a sequestered St\"uckelberg uplift. The model contains four chiral multiplets, an inflaton modulus $T$, a stabilizer field $S$, and a pair of oppositely charged waterfall multiplets $(\Psi,\bar\Psi)$, together with a hidden St\"uckelberg $U(1)_D$ sector that generates an approximately constant positive contribution to the scalar potential. Along the real inflationary trajectory, the F-term potential realizes the E-model plateau \[ U(\phi)=V_0\left(1-e^{-\sqrt{2/(3\alpha)}\,\phi}\right)^2 , \] while the sequestered uplift shifts the total energy density without directly modifying the tree-level inflaton slope. We derive the background evolution, the analytic $N$--$\phi$ relation, the waterfall stability condition, and the leading predictions for the scalar spectral index and tensor-to-scalar ratio. The model retains the characteristic red-tilted $\alpha$-attractor behavior, with $n_s\simeq1-2/N_*$ and tensor amplitudes controlled primarily by the curvature parameter $\alpha$, while the uplift and the hybrid end point generate only subleading corrections at fixed $N_*$. We also discuss the assumptions required for an effectively single-field trajectory, including the treatment of the light angular component of $T$, and examine the sensitivity of the sequestered uplift to Planck-suppressed cross-couplings. The resulting construction provides a controlled supergravity embedding of hybrid $\alpha$-attractor inflation compatible with current CMB constraints, including Planck, ACT DR6, and DESI DR2, while remaining testable by future $B$-mode searches such as LiteBIRD and CMB-S4.