Constraint on the Fine Structure Constant at z > 5 from Stacked JWST/NIRSpec Spectra.

We present a measurement of the fine structure constant α at z > 5 using stacked spectra in the rest frame of 212 JADES DR3 galaxies observed with JWST/NIRSpec G395M/G395H. We combine the Hβ λ4862.68 ˚A and [OIII] λ5008.24 ˚A emission lines to constrain a possible relative variation ∆α/α. From a bootstrap analysis with 100 iterations we obtain ∆α/α = +3.02±5.73 ppm (statistical), consistent with no variation at 0.53σ. The corresponding 2σ upper limit is |∆α/α| < 1.15 × 10−5 . The dominant systematic is the per-object dispersion (272 ppm) and the zSpec bias (−0.000626), which we correct using a multiline redshift (Hβ, Hα, [OIII], [NII]). This is, to our knowledge, the first upper limit on ∆α/α at z > 5 using emission lines.