Precise limits on cosmological variability of the fine-structure constant with zinc and chromium quasar absorption lines

The strongest transitions of Zn and Cr II are the most sensitive to relative variations in the fine-structure constant (Delta alpha/alpha) among the transitions commonly observed in quasar absorption spectra. They also lie within just 40 angstrom of each other (rest frame), so they are resistant to the main systematic error affecting most previous measurements of Delta alpha/alpha: long-range distortions of the wavelength calibration. While Zn and Cr II absorption is normally very weak in quasar spectra, we obtained high signal-to-noise, high-resolution echelle spectra from the Keck and Very Large Telescopes of nine rare systems where it is strong enough to constrain Delta alpha/alpha from these species alone. These provide 12 independent measurements (three quasars were observed with both telescopes) at redshifts 1.0-2.4, 11 of which pass stringent reliability criteria. These 11 are all consistent with Delta alpha/alpha = 0 within their individual uncertainties of 3.5-13 parts per million (ppm), with a weighted mean Delta alpha/alpha = 0.4 +/- 1.4(stat) +/- 0.9(sys) ppm (1 sigma statistical and systematic uncertainties), indicating no significant cosmological variations in alpha. This is the first statistical sample of absorbers that is resistant to long-range calibration distortions (at the <1 ppm level), with a precision comparable to previous large samples of similar to 150 (distortion-affected) absorbers. Our systematic error budget is instead dominated by much shorter range distortions repeated across echelle orders of individual spectra.