Measurement of dijet production in neutral current deep inelastic scattering at high Q**2 and determination of alpha_s

Dijet production has been studied in neutral current deep inelastic e + p scattering for 470 < Q 2 < 20000 GeV 2 with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb − 1 . Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q 2 allows both a precise determination of α s ( M Z ) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the parton distribution functions of the proton. The value of α s ( M Z ), as determined from the QCD fit, is α s ( M Z ) = 0 . 1166 ± 0 . 0019 (stat . ) − (th ).