Special note:

Please take note of the Dark Matter @ LHC workshop 2008 in Heidelberg #DMLHC2018, which we are organising! The conference will bring together experimental and theory communities interested in (collider-based, but also other) searches for dark matter, to discuss current hot topics and outline potential avenues into the future.


Despite its enormous success in the last decades, the standard model (SM) of elementary particle physics 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.
we are working to address the first two questions by scrutinising proton-proton collisions with the ATLAS detector at the Large Hadron Collider (LHC) of CERN -- see below!

we 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. Even today, there are only two single measurement which achieve a higher precision, using a much larger dataset at the LHC. All three mt results were the most precise single measurements of their kind in the world when they came out.

The ATLAS experiment

Our main focus is to search for potential Dark Matter candidate(s) and for new particles coupling to the SM Higgs boson. Both questions can be addressed with the ATLAS detector through searches for anomalous production of Higgs bosons and/or massive gauge bosons, W± and Z:

  • we search for Dark Matter produced in association with a Higgs boson, which results in a striking signature: an energetic Higgs boson recoiling against missing transverse momentum. Our latest result is published in Phys. Rev. Lett. and provides the world's strongest sensitivity. The next, more improved measurement is in preparation;
  • we search for Dark Matter produced in association with W/Z bosons and invisible Higgs boson decays. The signature is an energetic W/Z boson recoiling against missing transverse momentum. The publication is forthcoming;
  • We search for new mediators between the Dark Sector and the SM through their decays to b-quark pairs. This search is also sensitive to the SM production of boosted Higgs bosons which has not been experimentally observed yet! This effort is ongoing.
  • In addition, we are commissioning the next generation benchmark model for Dark Matter searches, which results in a diverse palette of experimental signatures including Higgs + Dark Matter and W/Z + Dark Matter. Our work will be published in the forthcoming White Paper from the LHC Dark Matter Working Group;
  • We contributed to the search for new particles decaying to a Higgs boson pair. The publication is forthcoming.

Times are exciting because we finally start to probe really interesting regions of phase space with "natural" coupling strengths of Dark Matter models, owing to the dramatic increase in the centre-of-mass energy to √13 TeV of the LHC, and to the large datasets becoming avialable for data analysis!

From an experimental perspective, we focus on highly energetic Higgs, W, and Z bosons with transverse momenta pT > 0.5 TeV because they provide an enhanced sensitivity to many New Physics scenarios. We analyse hadronic decay channels H→bb, Z→qq, and W→q'q that yield the highest branching ratio to maximise the sensitivity in our searches.

It is experimentally challenging to identify the subjets from the H→bb, Z→qq, and W→q'q decays. To fully exploit the physics potential of the LHC at 13 TeV, we are closely involved in improving the identification of jets from H→bb, Z→qq, and W→q'q decays. Recently, we co-developed a new jet mass observable mTA, which improves jet mass reconstruction algorithms through considering the information from the tracker and from the calorimeter at the same time. In addition, we proposed a new observable mTAS that follows a similar concept and is even more promising, and are in the process of commissioning it now.

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, creating an exciting and stimulating research atmosphere! Have a look at our group here.

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. We performed the Tevatron's most precise single measurement of the top quark mass in collaboration with other DØ institutions. Even today, there are only two single measurement which achieve a higher precision, using a much larger dataset at the LHC. 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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