Research Experience

The search for the Higgs boson is one of the most interesting topics in high-energy physics today. The Tevatron has a good chance to find evidence for the Higgs before the LHC. I am analyzing the ZH->nunu bb channel which is one of the most sensitive channels accessible at the Tevatron. It is also one of the most challenging ones. We have to rely on an excellent b-jet identification and on the correct missing energy measurement. In order to maximize the sensitivity, dedicated triggers have been developed. We will use sophisticated multivariate techniques to extract a signal or set limits on the Higgs mass. This analysis is very sensitive to the presence of events affected by detector or software problems. Understanding and fixing these problems benefits the whole collaboration. Beside the analysis work, I made significant contributions to the common analysis tools by improving and verifying their correct behavior.

I represent the Higgs group at the D0 Trigger Board: the committee with responsibility for determining the trigger strategy and approving modifications to the D0 trigger-list. The trigger design in the high-luminosity environment of the Tevatron is very challenging. Therefore, clever approaches need to be found to maintain a competitive physics program while not overloading the online system and the reconstruction resources.

I lead the group responsible for the maintenance and operation of Trigger the Central Track Trigger (CTT) at D0. The CTT finds tracks on the first trigger level using the Central Fiber Tracker (CFT) which consists of 8 concentric double layers of scintillating fibers. Combinatorial logic implemented in FPGAs is used to identify track candidates by comparing fiber hits to predefined hit patterns. The CTT provides tracks, pre-shower, and occupancy information to the first level trigger framework and to other first and second level trigger systems. The CTT plays an essential role in D0 trigger strategies. Therefore, a high reliability of this trigger is mandatory and has been achieved thanks to continuing improvements to the installed hardware and firmware.

My task as CTT group convener includes the oversight and coordination of the daily operation, of development of online and offline monitoring tools, of the data quality assessment of the CTT system, and of plans and studies for further improvements of the trigger performance. I lead and coordinate the activities of students, postdocs, faculty, and engineers. I plan and, together with the D0 management, identify manpower needed to maintain the CTT as a component central to the success of D0. During my term as CTT group leader, we considerably improved our understanding of the trigger performance for Z->mu+ mu- events and of the homogeneity of trigger efficiency vs phi. This lead to improved trigger strategies which allows us to run most of the physics program up to the highest luminosities.

In addition, I train and assist the shifters monitoring the detector performance in real-time. Beside my leadership role, I was active in the actual implementation and improvement of software tools used to monitor the detector performance. This made the merge of the formerly 2 separate tracking shift positions into a single tracking shift possible, thereby reducing the manpower needed to operate the detector.

The first major task under my CTT group leadership was the successful upgrade of the track finding hardware. This upgrade be- came necessary as the original track finding hardware installed for the Tevatron run II limited the number of hit patterns to be considered. Thus, neighboring fibers had to be combined into one space point. This led to a substantial fake trigger rate at higher luminosities. Therefore, in spring 2006 the track-finding hardware was replaced to take advantage of the full granularity of the CFT. I lead the mechanical challenging installation of 40 new boards and rerouting of 600 cables in a limited space. I was also very hands-on in my direct involvement in testing the newly installed components and commissioning the new system.

Before taking over the group leadership, I studied the expected per- formance of the upgraded system in terms of efficiency and trigger rates using a parallel chain consisting of one-twentieth of the final system installed in the detector.

My primary interest is in radiative-penguin processes. I chose this topic as it has an excellent potential to find deviations from the Standard Model and to constrain new theories.

I picked the rare decay B –> gamma gamma as a first analysis topic. I developed the analysis in a common framework with other radiative penguin analyses. The main challenge in searching for these rare radiative decay modes is to attain sufficient background rejection. I used a 2-dimensional fitting technique for the background estimation and a simultaneous optimization of several selection variables in order to obtain the best upper limit, which will improve the current best upper limit set by BaBar by approximately a factor of 5. The results of this study are currently in internal review and will be published shortly. I contributed to code development and ntuple production for the measurement of branching fractions, CP and Isospin Asymmetries for the B –> K* gamma decay.

I joined the data processing team when the centralized skim production needed to be ramped up within a new computing model. The skimming splits the reconstructed data into 123 different physics-streams making the handling of the huge data sample of 250fb-1 collected by BaBar and the associated Monte-Carlo samples accessible for the analysis groups. I debugged the skimming tools and developed scripts to automate the skimming and to monitor thousands of skimming jobs at SLAC, which use up to 1200 CPUs and 10TB of disk space. Various problems needed to be identified and solved on a stringent time scale imposed by ICHEP'04. I successfully achieved this task in close collaboration with members in the areas of data quality, reconstruction, computing and physics coordination. This enabled many exciting results like for example the discovery of direct CP violation in B° –> K+ pi-. I was appointed skimming manager for the upcoming round of skimming, which is a prerequisite for many interesting results for LeptonPhoton'05.

I used the opportunity of taking shifts to learn as much as possible about the detector operation. The experience I gained from increasingly challenging shifts was rewarded by becoming expert shift leader supervising the detector operation during critical periods of data taking.

I was interested in learning more about the beams of the PEP-II accelerator. Therefore, I started a project to investigate beam-induced heat in components of the asymmetric electron-positron storage ring PEP-II. Steadily increasing beam currents cause components to fail due to temperature-related problems. Accessing existing temperature data from the beam control system, I tried to predict likely candidates for failures. Unfortunately, the available data is influenced by many other factors, which are only fragmentary documented. Therefore, it was impossible to make any sensible prediction within the scope of this project.

I designed and commissioned a new prototype SubFarm Input (SFI) application within a common application framework shared with all other DataCollection (DC) applications in the ATLAS experiment at CERN.

DataCollection is a subsystem responsible for the movement of event data from the Readout Subsystem (ROS) to the high level triggers. I took responsibility for the SFI, a key component in the event building system. It receives event fragments from ROS via a switched network, assembles the event fragments into a complete event, and makes the event available to the last stage of the online event selection. The main challenge I had to cope with is the high event fragment rate of 4kHz originating from about 200 network connections and handling 40MB/s input and 40MB/s output rate. Therefore, the design must meet high performance requirements and maximize the efficiency of the memory management while being as fail-save as possible. I planned and coordinated the vital and successful integration and testing of various DataCollection components and applications. It required integrating the configuration, control, and monitoring of data-flow components and applications with one another, and with the infrastructure and services provided by the ATLAS Online Software. I developed an automatic set-up procedure for the database describing the system enabling the deployment on up to 220 computing nodes.

For my Ph.D. thesis I worked for the ATLAS High Level Triggers, where I focused on the performance of the Event Filter, the last stage of the online event selection. I studied the vital electron trigger, where a very high background rejection is mandatory to enable the search of physics beyond the standard model. I used the Higgs –> 4e and Z –> 2e channels for benchmarking. This work yielded firmer estimates of the electron trigger rate and the efficiency of all three trigger levels. In addition, I investigated where the execution times of offline algorithms could be reduced in order to meet the stringent time constraints for the online event selection. These measurements contributed to a better understanding of the performance and size of the trigger system and were presented in the Trigger/DAQ/DCS Technical Proposal published in 2000.

For my Master thesis I worked on the optical readout of the SemiConductor Tracker (SCT) of the ATLAS detector. I took a leading role in building a machine to automatically scan up to 144 light emitting diodes (LEDs) or vertical cavity surface emitting laser diodes (VCSELs). We used this machine to investigate the annealing behavior of 250 LEDs from two different manufactures and about 200 VCSELs after proton and/or neutron irradiation with doses comparable or above those expected during 10 years of LHC running. We performed an accelerated life time test of these devices, too. I wrote a large part of the object oriented software (C++) to steer the scanning machine and to execute the measurements. I made most of the measurements of the irradiated devices and was heavily involved in the data analysis. The scanning machine was subsequently used for testing irradiated PIN diodes by a group at the University of Birmingham, U.K. I instructed them how to use the hard- and software.

I participated in the NA52 experiment at the CERN SPS. This experiment was looking for long-lived strange quark matter in Pb+Pb collisions at 158 A GeV/c. It also measured particle and anti-particle yields in heavy ion collisions. I contributed to the data acquisition and was involved during the data-taking periods in monitoring the experiment as well as setting it up for several data-taking and test-beam runs. I participated in many discussions about heavy-ion physics and analysis issues.

I spent three months as summer student in the low energy research section of the Paul Scherrer Institute, Villigen, Switzerland. The Philips cyclotron was built 1973 as a proton source. Later it had been used as heavy ion accelerator, too. This damaged the original electro-static deflection mirror of the injection system. I implemented a numerical simulation of the reflector using successive overrelaxation for the field and a Runge-Kutta algorithm for the tracking. I estimated the influence of the observed damage of the mirror onto the ion tracks. I proposed an improved mirror geometry which was later successfully implemented.