Jets

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Strong Coupling Constant alpha-s

    The value of the coupling constant alpha_s of the strong interactions was determined using 2-jet events in deep inelastic lepton-proton scattering. The production mechanism for such events is the photon-gluon-fusion process and the QCD-Compton process. Both processes contain gluon-quark vertices and are therefore sensitive to alpha_s. The quark and gluon distributions of the proton were taken from parameterizations based on structure function measurements. 

    Two methods to determine alpha_s were used: 
In one analysis the ratio of 2-jet to 1-jet events was measured and alpha_s was extracted as a function of the virtuality Q2 of the photon. The figure shows this measurement which confirms the running of the strong coupling constant that is predicted by perturbative QCD. 
    In the second analysis the 2-jet rate was measured as a function of the threshold parameter that indicates formation of two jets. This measurement gives the value alpha_s(M_Z^2)=0.118+-0.002(stat)+0.007-0.008(syst)+0.007-0.006(theory).

    Both measurements of alpha_s agree well with the values of other experiments.  

Gluon Distribution of the Proton

    In a direct extraction of the gluon distribution of the proton, 2-jet events from deep inelastic positron-proton scattering were analysed. Apart from the photon-gluon-fusion process, which allows the gluon to be determined, the QCD Compton process contributes to this 2-jet production. In the latter process the scattered quark from the proton radiates a gluon that causes the second jet. In order to determine the gluon distribution of the proton, the contribution of the QCD Compton events is subtracted from the jet production using a simultaneous fit with the proton structure function data at a fixed value of the scale Q2. The value of the strong coupling constant was set to the world average value and was allowed to vary within the corresponding errors. 
The figure shows the gluon distribution of the proton. The jet measurement complements the gluon extraction from the proton structure function data alone and provides a measurement up to gluon momenta of x = 0.01. 
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