Measurement of Dijet Cross Sections in Deep Inelastic ep Scattering at HERA

Thorben Theedt
Nov 2009

Thesis Type:

Dijet cross sections have been measured in deep inelastic neutral current electron-proton scattering at HERA. Cross sections have been measured differentially as functions of the photon virtuality, $Q^2$, the scaling variable, Bjorken x, the mean transverse jet energy, $\bar{E}_T$, the invariant dijet mass, $M_{jj}$, the difference in jet pseudorapidity, $\eta'=|\eta^{\mbox{jet1}}-\eta^{\mbox{jet2}}|$ and the momentum fraction, ξ. Cross sections as function of ξ have also been measured in different regions of the photon virtuality. The analysed data were recorded at a centre-of-mass energy of 318 GeV with the ZEUS detector in the years 1998, 1999, and 2000 and correspond to an integrated luminosity of $81.74\mbox{ pb}^{-1}$. The phase space of the analysis is defined by $125 < Q^2 < 5000\mbox{ GeV^2}$ and $|\cos\gamma_h| < 0.65$, where $\gamma_h$ is the polar angle of the hadronic final state. Jets are reconstructed with the inclusive $k_T$ cluster algorithm in the Breit reference frame. Dijet events are selected by applying an asymmetric cut on the transverse jet energy according to $E_{T,Breit}^{jet1(2)} > 12 (8)\mbox{ GeV}$, where the indices correspond to the two highest energetic jets in the pseudorapidity range $-2 < \eta_{Breit} < 1.5$. The measured cross sections are compared to QCD calculations in next-to-leading order (NLO) as implemented in the DISENT program. For the comparison the data and the theoretical predictions are corrected to hadron level. The applied correction procedures are based on Monte Carlo models. The data are (for all measured variables) in good agreement with the theoretical predictions. Dijet cross sections allow, as compared to inclusive-jet measurements, the reconstruction of additional kinematic observables like the invartiant dijet mass, $M_{jj}$, or the momentum fraction, ξ. The presented dijet analysis thus represents a very detailed test of perturbative QCD. Moreover, the cross sections measured in different regions of $Q^2$ are shown to be sensitive to the gluon density of the proton. They might thus be well suited to provide additional constraints on the gluon PDF when included in (NLO) QCD fits.

In the years 2000 and 2001 HERA was upgraded from HERA I to HERA II. The shutdown allowed an upgrade of the detector in order to adapt it to the high-luminosity environment and the physics program of HERA II. The detector upgrade comprised a new luminosity (monitoring) system which was designed to cope with the increased luminosity and the high rate of synchrotron radiation. The new luminosity system comprised an electromagnetic calorimeter located at about 6m from the interaction point in the rear region of the ZEUS detector. For the 6m (electron) tagger a trigger was designed and tested, a calibration procedure was established and the radiation damage of the scintillating fibres was monitored via a series of Cobalt scans. In 2007 a detailed study of multiplicities and energy flows based on NC low-$Q^2$ DIS jet data showed that the energy flow in the upgraded ZEUS detector was not described by the HERA II Monte Carlo predictions. The results triggered the so called single pion study, in which single pion events were used to investigate the response of the simulated detector. The study helped to improve the HERA II detector simulation and revealed imperfections of a (at ZEUS commonly used) reconstruction algorithm.

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