# ATLAS at the LHC

## Introduction

During 2019, the first year of the second long shutdown of the LHC (LS2), the group has been actively involved in the analysis of the full LHC Run-2 dataset, and the maintenance and commissioning of detector elements under IFAE responsibility, in preparation for the start of the LHC Run 3 in 2021. The group has also maintained its visibility within the ATLAS Collaboration through a number of important management positions.

## The year 2019 was characterized by an intense analysis effort using the full Run-2 dataset

The year 2019 was characterized by an intense analysis effort using the full Run-2 dataset, which corresponds to $139 fb^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV. The IFAE team continued to play a leading role in several physics research lines, including Higgs boson studies, dark-matter searches, searches for Supersymmetry, and searches for new phenomena in top-quark final states. A summary of the main analysis activities is provided below.

## Higgs Boson Physics

During Run 2 the IFAE team is participating strongly in measurements of $t\bar tH$ production, and searches for additional Higgs bosons.

#### Probing top-Higgs interactions

Direct tests of top-Higgs interactions are extremely interesting, since the top quark is the Standard Model (SM) particle most strongly coupled to the Higgs sector. The most sensitive direct probe of the top-Higgs Yukawa coupling is provided by the measurement of the $t\bar tH$ cross section. During Run 1 and throughout Run 2 the IFAE team has actively participated in the $t\bar tH (\to b \bar b)$ search, resulting in several public results. The combination of several ATLAS searches using a partial Run-2 dataset recently led to the observation of ttH production [Phys. Lett. B 784 (2018) 173], a milestone result that represents the first step towards a precise measurement of the top-Higgs Yukawa coupling. Since 2018 the IFAE team (A. Juste and S. Kazakos) is also involved in the ttH search with $H \to WW^*/ZZ^*/\tau \bar\tau$ decays, giving multilepton final states. Using a partial Run-2 dataset, the IFAE team recently produced a preliminary result (ATLAS-CONF-2019-04) on the measurement of the $t\bar tH$ production cross section in multilepton final states (see Figure 1), with A. Juste acting as Corresponding Editor. This analysis also highlighted some issues with the modeling of the dominant $t\bar tW$ background, which would benefit from improved theoretical predictions. Motivated by these findings, the IFAE team has started participating in the measurement of differential cross sections for ttW production, using the full Run-2 dataset. Such measurement will be followed by the actual ttH analysis. Both results will be part of S. Kazakos’ PhD Thesis.

#### Searches for additional Higgs bosons

During 2019 the IFAE team (M. Bosman, J. Glatzer, Ll.M. Mir, I. Riu, and A. Salvador) continued its strong involvement in the search for a heavy charged Higgs boson $(H^+)$ decaying to a top quark and a bottom quark. IFAE is the only institute in charge of the full Run-2 analysis in the standard lepton-plus-jets final state. A sophisticated approach, which uses a parameterized neural network (see Figure 2), was developed to improve the search sensitivity. I. Riu is the ATLAS analysis contact for this search.

In addition, during 2019 the IFAE team has continued to develop a novel search for a light charged Higgs boson appearing in top-antitop-quark $(t\bar t)$ events, when one of the top quarks undergoes the decay $t \to H^+b$, followed by the decay of the $H^+$ boson into a bottom quark and a charm quark. Such scenario is predicted in several extensions of the Higgs sector, and is poorly tested experimentally. N. Orlando is the ATLAS analysis contact for this search, which capitalizes on the experience gained from a previous search also led by IFAE, this time for flavor-violating top quark decays $t \to H(\to b \bar b)c)$ (JHEP 05 (2019) 123). The results of this new search are expected during 2020.