The objective of elementary particle physics is to understand what our world is made of at the elementary, most fundamental level. The standard model (SM) of elementary particle physics has served us tremendously well in explaining particle physics phenomena over the last decades. Despite its enormous success, it has substantial shortcomings, as some truly fundamental questions remain unanswered, e.g.,

  1. the SM does not provide any particle candidate(s) for Dark Matter, which makes up about 20% of our universe,
  2. the SM does not explain why the mass of the Higgs boson is so small compared to the range of potentially possible values,
  3. our Universe appears to be only metastable with the current measured value of the top quark mass (mt), and assuming that no new particles exist.
Currently, our group is working to address the first two questions by scrutinising proton-proton collisions with the ATLAS detector at the Large Hadron Collider (LHC) of CERN, which is discussed in detail below.
Previously, our group worked on addressing the last question through precise measurements of mt at ATLAS. We contributed to measurements of mt in the pole mass scheme from σ(tt) and σ(tt+1 jet) at ATLAS and performed, together with our DØ colleagues, the Tevatron's most precise single measurement of mt. All three results were the most precise single measurements of their kind in the world when they came out.

The ATLAS experiment

The main focus of our research is to use the ATLAS experiment to shed light on the first two questions outlined above, i.e. to search for potential Dark Matter candidate(s) and for new particles coupling to the SM Higgs boson. Both questions can be addressed through searches for anomalous production of Higgs bosons and/or massive gauge bosons, W± and Z, in proton-proton collisions at the LHC using the ATLAS detector:

  • we search for Dark Matter (DM) produced in association with a Higgs boson, which results in a striking signature: an energetic Higgs boson recoiling against missing transverse momentum;
  • Similarly, we search for DM produced in association with W/Z bosons;
  • In addition, we are involved in a systematic study and commissioning of a new benchmark model for New Physics searches, which results in prominent Higgs + DM or W/Z + DM signatures;
  • We also search for new particles like for instance the Kaluza-Klein graviton in Randal-Sundrum type of models, which resonantly decays to a pair of Higgs bosons, through the Feynman diagram displayed on the right.
Times are exciting for searches for New Physics beyond the SM like the ones above because of the dramatic increase in the centre-of-mass energy to √13 TeV of the LHC, and because of the large datasets avialable for data analysis (35 fb-1 of integrated luminosity in 2015+2016 alone!).

The striking signature of a new resonance decaying to a HH, WW, or ZZ pair with an invariant mass of 1 TeV and above is characterised by two SM bosons recoiling against each other in a back-to-back topology, each of them with a large transverse momentum in excess of 0.5 TeV. We focus on hadronic decay channels H→bb, Z→qq, and W→q'q to maximise the statistical sensitivity in our searches, as they yield the highest branching ratio. In the extreme kinematic regime with boson momenta in excess of 0.5 TeV, the H→bb, Z→qq, and W→q'q decay products become highly collimated and are merged into one single jet with typically two distinct subjets, as shown in a representative a G→WW candidate event display on the right. Such high momenta are also important in the Higgs + DM or W/Z + DM searches.

It is not always as easy to identify the subjets from the H→bb, Z→qq, and W→q'q decays as the event display on the right suggests. Therefore, to improve the sensitivity of searches for HH, WW, or ZZ pair-production at the energy frontier in Run II of the LHC, we are closely involved in improving the identification of jets from H→bb, Z→qq, and W→q'q decays. For instance, we co-developed a promising novel jet mass observable mTAS, which improves over the standard ATLAS jet mass reconstruction algorithms through considering the information from the tracker and from the calorimeter at the same time.

Our Level-1 Calorimeter trigger contribution

On the hardware side, we are working on the operation and calibration of the ATLAS Level-1 Calorimeter Trigger (L1Calo), which accounts for a major fraction of the ATLAS trigger menu. Our main focus is the operation of the L1Calo trigger through on-call and data taking shifts, and the experimental approaches to cope with high centre-of-mass energies and high instantaneous luminosities in Run II of the LHC. Here, we are collaborating closely with our colleagues from the ATLAS particle physics group of the Kirchhoff-Institut für Physik.

Our group

Our KIP ATLAS particle physics sub-group is quite an international bunch and we come from different academic systems with their own strengths. This creates an exciting and stimulating research atmosphere. Currently, we are two Master students and four Ph.D. students.

The DØ experiment

Besides its current ATLAS activities, in the past our group was involved in precision measurements of the mass of the top quark with the DØ detector in proton-antiproton collisions at a centre-of-mass energy just short of 2 TeV, at the Tevatron Collider of Fermilab. Incidentally, the Tevatron's most precise single measurement of the top quark mass was performed by our group in collaboration with other DØ institutions. Our measurement has been highlighted as a "Featured in Physics" Synopsis (2014), and as a "News and Views" article in the Nature magazine (Nature, 514, 174 (2014)). For more details on the measurement, please refer to the videocast of this measurement in the CERN-EP seminar.

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