MESA+ University of Twente
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Jeroen Hegeman (promotion date: 16 January 2009)

Measurement of the top quark pair production cross section in proton-antiproton collisions at √s = 1.96 TeV, Hadronic top decays with the D0 detector

Promotion date: 16. January 2009

Promotor: Prof. dr. ing. Bob van Eijk


Its large mass makes the top quark a key player in the realm of the postulated Higgs boson. Precision measurements of its particle mass, and the W boson, can provide indirect constraints on the Higgs boson mass.

The analysis described in this thesis sets out to measure the top-antitop quark production cross section at the mentioned centre-of-mass energy in the fully hadronic decay channel. Data are taken with the D0 detector between July 2002 and February 2006. A neural network is used to identify jets from b-quarks and a likelihood ratio method is used to separate signal from background.

It doesn’t sound like an ordinary MESA+ thesis project ...

No, it’s a rather special project. MESA+ and Twente University participate in subatomic research. The Low Temperature Division is directly involved in designing and constructing superconducting magnets for accelerators and experiments. The High Energy Physics chair participates in several collider experiments, like ATLAS at CERN and D0 in the US.

Being a member of the Nikhef institute as well, I went to the Fermi National Lab near Chicago to participate in the experiments first-hand. It is the second largest high energy collider, only surpassed by the new Large Hadron Collider at CERN in Switzerland.

Were you able to use the measurement results from these experiments directly?

Every analysis effort is a huge project on his own. Some 650 people, from all over the world, are working to keep the detector going and to prepare the data for analysis. As a member of this collaboration you also spend time on “service tasks”: you take shifts in the control room and help with tasks like calibration and the day-to-day running of the experiment.

This kind of team performance differs fundamentally from the relatively small research groups you find at MESA+ or Amolf in Amsterdam. Here, everybody is responsible for the whole process. In return, in DØ, every student is allowed to choose their own measurement and “decay channel”.

What did you achieve in your thesis project?

My measurements of top-quark pair production are in perfect agreement with the standard model predictions. In recent years we achieved a milestone in detector calibration, with a precision of approximately 2.5% for jets (collimated bunches of particles all detected close together in the detector) over a wide kinematic range. This, together with our newly-developed background model, makes our measurement the most precise estimate of the cross section in this particular decay channel.

What are your future plans?

After a vacation, I am going to work at the Large Hadron Collider at CERN. I've received a two-year fellowship contract. The planning is that the LHC will commence operation this summer. Hopefully it will be able to “catch” the Higgs boson, and perhaps the first hints of supersymmetry will be revealed.

I hope to fine-tune the model we developed in the US. I am a real academic type of researcher, always passionate about the puzzles involved. It's a great feeling being an intimate member of such a collaboration at the fore-front of scientific research. With the start of the LHC presenting the next big step in fundamental matter research these are exciting times to be involved in experimental particle physics.

What does, in your opinion, make MESA+ a unique kind of institute?

The spinoff companies surrounding academic research are really special. MESA+ should keep paying attention to close collaboration with companies. The beauty of fundamental research is that not only does it lead to knowledge and advanced education, but along the way one also often discovers many interesting and useful things, that might go unnoticed in a purely market-driven environment.