Searches for New Physics

* Dark Energy

Dark Energy is the apparent accelerated expansion of the universe and  constitutes one of the biggest mysteries in modern physics. A plethora of models have been suggested to explain DE, ranging from new fundamental fields to modifications of General Relativity. The elucidation of the nature of  DE is a very active field of research in cosmology and particle physics with many experiments ongoing or under construction (see 1201.2434 and 1309.5380 for a review).

It has been suggested (astro-ph/0612452, astro-ph/0702615) that cosmological observations cannot distinguish between theories with new fundamental fields and modified gravity. Input from particle physics experiments is therefore necessary in order to understand the microscopic nature of DE.

I have set up the first collider search for DE in the ATLAS experiment, based on the model proposed in 1604.04299; the results were presented in ATL-PHYS-PUB-2018-008 and 1903.01400. I also serve as theory contact for the DE searches in ATLAS.

* Dark Matter search in Higgs final states

While there is ample evidence from astronomical observations for the existence of non-luminous matter in the Universe its microscopic nature still remains a mystery. Several experiments aiming to shed light on the nature of dark matter are under way. The LHC plays a key role in this programme. An attractive possibility that has been explored is that dark matter might interact with the Standard Model particles only via the exchange of Higgs bosons (usually involving an extended Higgs sector).

Since 2018 I have been leading the search for Dark Matter at the ATLAS experiment using final states with a Higgs boson decaying to b-quarks.

* Extended Higgs sector

One of the main questions after the discovery of a Higgs boson is whether this particle is part of an extended scalar sector, which could contain additional Higgs bosons. These arise in many extensions of the Standard Model, such as theories with Supersymmetry, axion models and baryogenesis models.

A large part of my research has focused on the search for a CP-odd Higgs boson (A) with the Run-2 LHC data.

I contributed to the first ATLAS results for the A→Zh search at 13 TeV, which was published as a conference note (ATLAS-CONF-2016-015). As one of the main analysers of the channel with 2 leptons and 2 b-jet I was involved in the development of the analysis software as well as the analysis optimisation.

I was appointed analysis contact of the following iteration of the A→Zh search with 36/fb of 13 TeV data, which was published in JHEP 03 (2018) 174. For this analysis, I developed a new method for combining two ways of reconstructing the Higgs boson candidate (resolved & merged), which boosted the analysis sensitivity. I also worked on the analysis framework and limit setting.

* Heavy Vector Bosons

The Standard Model predicts that the interactions of the Higgs boson with the other fundamental particles would drive its mass to be very large, in disagreement to what has been observed. This is known as the naturalness problem. Various models have been proposed to solve the problem. Among the most popular ones are models with dynamical symmetry breaking which involve a new stong interaction. These models generically predict the existence of heavy vector bosons, that have not been observed so far. A simplified phenomenological description that encapsulates the characteristics of these models has been proposed in 1402.4431 (Heavy Vector Triplet model); this predicts the existence of a W' and a Z' boson with masses in the TeV range.

I have been involved in the searches for W' and Z' bosons decaying to a W or Z boson and a Higgs boson with the Run-2 ATLAS data.

For the first result with 3.2/fb of data (PLB 765(2017) 32-52), I validated the 2-lepton analysis selection using an independent analysis framework.

I made significant contributions to the second iteration of the analysis, which was joint with the A→Zh search described above, where I served as analysis contact.  I have also contributed to the combination of the VV and Vh channels (1808.02380).