ATLAS Thesis Award goes to the Dark Side

Ruth Pöttgen is a postdoctoral researcher in the ATLAS group at Stockholm University. In 2015, she obtained her Ph.D. at the Johannes Gutenberg – University in Mainz, Germany, for her thesis on a “Search for Dark Matter in Events with a highly energetic jet and missing transverse momentum at a centre-of-mass energy of 8 TeV with the ATLAS Detector”. At the ATLAS collaboration meeting in February, Ruth was awarded one out of 4 ATLAS Thesis Awards for outstanding contributions to the ATLAS-Experiment in the context of a Ph.D. thesis; more than 100 theses were eligible.

Ruth Pottgen
Ruth Pottgen

During her Ph.D., Ruth was based at CERN within its doctoral student programme, funded by a Wolfgang-Gentner scholarship. She was active member of the CERN group responsible for the central trigger — a vital component of the ATLAS experiment — and contributed both to its day-to-day operation as well as its upgrade during the long LHC shutdown from 2013-2015.

In 2012, she joined one of her colleagues from the central trigger group in a search for weakly interacting massive particles (WIMPs) — a popular generic class of candidates for Dark Matter particles, for which also various searches are pursued within OKC.

While there is ample compelling evidence for the existence of Dark Matter at various cosmological scales, an unambiguous direct observation of Dark Matter in the laboratory is still pending. The Standard Modell (SM) of particle physics does not provide a viable candidate to account for all of the observed Dark Matter, which is estimated to amount to roughly 80% of the matter content of the universe. WIMPs could, in a natural way, account for the present day abundance of Dark Matter. Such particles feature in many theories for physics beyond the SM, e.g. the lightest supersymmetric particle in R-parity conserving super-symmetry models.

A number of experiments searches for evidence for WIMPs and can be grouped roughly into three different categories. Direct searches look for the nuclear recoil in a target material due to the scattering of WIMPs off nuclei, while indirect searches try to detect the products of WIMP annihilation into SM particles. In recent years, also the search for WIMP pair production at hadron colliders has gathered momentum. It is in many regards complementary to the other search approaches and has become the third pillar for the hunt for Dark Matter.

Like neutrinos, WIMPs do practically not interact with the detector material and are hence themselves ‘invisible’. If they are, however, produced together with some other, ideally highly energetic object, their presence can be inferred from the missing contribution to the momentum balance. One possibility is that the other object is a hadronic jet (a bundle of particles). Such events are commonly referred to as mono-jet events. The signal is then expected to manifest itself as an excess above the Standard Model prediction at large missing transverse energy. While the direct searches are essentially background-free experiments, one of the major challenges in the collider-based search is the careful estimation of irreducible backgrounds, dominated by the production of Z-bosons together with jets, where the former decay in neutrino-antineutrino pairs.

The preliminary ATLAS results based on half of the data set collected at a centre-of-mass energy of 8 TeV in 2012 had been presented by Ruth at the ISAPP summer school on Dark Matter, held in Stockholm and organised by OKC in 2013. The results were discussed with great interest and Ruth was awarded with one of the three poster awards.

For the analysis of the full 2012 proton-proton dataset (20 fb-1) in her thesis, Ruth optimised the event selection with respect to the sensitivity for a WIMP signal. The search was performed in eight signal regions of increasing missing transverse momentum and no significant excess was observed. Model-independent limits on the cross section for new physics as well as bounds on parameters of two different, but related models for Dark Matter were derived.

Together with Ruth, three members of the SU ATLAS group were and are still involved in the mono-jet analysis, but focussing on a different interpretation.

Ruth is currently based at CERN and coordinates the ATLAS search for first and second generation scalar leptoquarks, which is to be published for the Moriond conference in March. Afterwards, she’ll return to Stockholm and is eager to explore possibilities of a collaboration with Dark Matter experts at OKC.

The “Award ceremony” will take place on today, February 25, at CERN.

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