Possible evidence for a variable fine-structure constant from QSO absorption lines: motivations, analysis and results

An experimental search for variation in the fundamental coupling constants is strongly motivated by modem high-energy physics theories. Comparison of quasar (QSO) absorption-line spectra with laboratory spectra provides a sen sitive probe for variability of the finestructure constant, alpha, over cos mological time-scales. We have previously developed and applied a new metho d providing an order-of-magnitude gain in precision over previous optical a strophysical constraints. Here we extend that work by including new quasar spectra of damped Lyman-alpha absorption systems. We also reanalyse our pre vious lower-redshift data and confirm our initial results. The constraints on alpha come from simultaneous fitting of absorption lines of subsets of t he following species: Mg I, Mg ii, Al ii, Al iii, Si ii, Cr ii, Fe ii, Ni I I and Zn II. We present a detailed description of our methods and results b ased on an analysis of 49 quasar absorption systems (towards 28 QSOs) cover ing the redshift range 0.5 < z < 3.5. There is statistical evidence for a s maller alpha at earlier epochs: Delta alpha/alpha = (-0.72 +/- 0.18) x 10(- 5). The new and original samples are independent but separately yield consi stent and significant non-zero values of Delta alpha/alpha. We summarize th e results of a thorough investigation of systematic effects published in a companion paper. The value we quote above is the raw value, not corrected f or any of these systematic effects. The only significant systematic effects so far identified, if removed from our data, would lead to a more signific ant deviation of Delta alpha/alpha from zero.