Jets at Low Q² at HERA and Radiation Damage Studies for Silicon Sensors for the XFEL

Hanno Perrey
Jun 2011

Thesis Type:

In the first part this thesis, jet cross-sections were measured for inclusive jet, inclusive dijet, and inclusive trijet production at photon virtualities in the range of $10 < Q^2 < 100\mbox{ GeV^2}$.
The data analyzed were recorded with the ZEUS detector in the years 2004-2007 corresponding to an integrated luminosity of $296\mbox{ pb^{-1}}$. Events in neutral current deep inelastic scattering were selected in the above stated $Q^2$ region for an inelasticity of $0.2 < y < 0.6$. The jets were reconstructed in the Breit frame, where the virtual boson and the proton collide head on. The jets were required to carry a transverse momentum in the Breit frame of $p_{T,B} > 8\mbox{ GeV}$ and to have a pseudorapidity in the laboratory frame in the range of $-1 < eta_{\mbox{lab}} < 2.5$. For the dijet and trijet samples, an additional requirement was imposed on the invariant dijet mass of $M_{jj} > 20\mbox{ GeV}$ to avoid phase space regions where the fixed order calculations are sensitive to infrared divergences. The presented analysis is the first jet analysis at such low values of $Q^2$ to exploit the full HERA-II ZEUS data set, and as such is performed at significant higher luminosities than previous publications.
Overall, the next-to-leading order (NLO) calculations correctly predict the measured cross-sections within the uncertainties in all studied quantities and over most of the investigated regions of phase space, except in the pseudorapidity region close to the proton beam direction ("forward" region) in inclusive jet production where the prediction is considerably below the data. The uncertainty of the NLO prediction, dominated by the uncertainty associated with the choice of the renormalization scale, is typically larger than the experimental uncertainty, which is for the most part dominated by the uncertainty of the jet energy scale.
The large theoretical uncertainties indicate the need for calculations including higher-orders. Such NNLO calculations will allow to fully exploit the sensitivity of the low $Q^2$ jet data in QCD PDF fits and in fits to extract values of $\alpha_s$.

In the second part of this thesis, a study of radiation damage of silicon sensors by 12 keV X-rays for doses up to 1 GGy is presented. For this study, an irradiation facility has been set up at HASYLAB at DESY. Test structures (gate-controlled diodes) have been irradiated and the properties of the $\mbox{Si-SiO_2}$ interface under high irradiation have been studied using current versus voltage (I/V), capacitance versus voltage (C/V), and thermally depolarization relaxation current (TDRC) measurements. In addition to a strong increase and subsequent decrease of the interface current and the flat-band voltage as function of dose, strong hysteresis effects have been found.
The data can be qualitatively described by a model which includes interface traps and fixed oxide charges. The model predictions were used in combination with in C/V and TDRC measurements to separately determine the different types of traps present at the $\mbox{Si-SiO_2}$ interface and the charges in the $\mbox{SiO_2}$.
The parameters extracted in this studies are to be implemented into simulations with the goal of reproducing the measurements and later use them for the design of radiation hard sensors for the AGIPD project.

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