Research at DØ (2004 - 2008)

In the DØ experiment, I was a Ph.D. student and my thesis topic was a cross section measurement of top pair production in the lepton+jets channel using b-tagging. Studying properties of the top quark is interesting because it gives insights into BSM, electroweak (EWK) symmetry breaking and perturbative Quantum Chromo Dynamics (QCD). Since the top quark decays into a W boson and a b-quark with almost 100% branching fraction, b-jet identification gives us a power to discriminate top-anti-top signal and other backgrounds. The analysis presented in my thesis gave the best result at that time of the top pair production cross section with 1 fb^-1 using the so-called Neural Network (NN) b-tagging [1]. In addition, I developed a brand-new b-tagging algorithm using a soft lepton tagging (SLT) technique with a NN. The fake rate of this new algorithm was reduced by about factor of 10 compared to the SLT only without the NN and with no significant signal efficiency loss.

I made major contributions to hardware, most significantly, the luminosity monitor (LM) detector, which plays a crucial role in DØ physics analyses by providing the normalization for the luminosity. During Run IIa of the DØ experiment, the LM detector and the electronic readout system were upgraded in order to perform in the higher luminosity environment. I did not only participated in commissioning of the electronics of the LM readout system, but also developed a GUI based calibration system. I additionally was responsible for the calibration of the LM detector during the development [2].