# ATLAS

### 2013 Nobel Prize in Physics and the characterization of the Higgs boson

High Energy Frontier
2013 Nobel Prize in Physics and the characterization of the Higgs boson: The 2013 Nobel Prize in Physics was awarded jointly to François Englert and Peter Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider”. The 2012 discovery paper constituted the “Observation of a new particle in the search of the Standard Model Higgs boson”. The scientific justification from the Nobel Prize Committee cited in addition two ATLAS papers in Physics Letters B  (2013). The first and second papers showed the consistency with the spin-0 and even parity characteristics of the discovered particle as well as couplings to bosons that were as expected for a Higgs boson.

Evidence for the spin-0 nature of the Higgs boson using ATLAS data

ATLAS Collaboration, Physics Letters B, 726, 120 (2013)

Measurement of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC

ATLAS Collaboration, Physics Letters B, 726, 88 (2013)

### Constraints on new phenomena via Higgs boson couplings and invisible decays

High Energy Frontier Neutrinos and Dark Matter
Constraints on new phenomena via Higgs boson couplings and invisible decays: A crucial question in particle physics is whether the Higgs boson discovered in 2012 is truly the fundamental scalar predicted by the Standard Model (SM). Strong theoretical arguments suggest that the SM is only an approximation to a more fundamental theory such as supersymmetry or composite Higgs models, which predict modified properties of the Higgs with respect to SM expectations. As published in the Journal of High Energy Physics (2015), the results of several analyses of production and decay rates of the Higgs boson in different channels were combined to determine how the couplings scale with mass and hence put constraints on various extensions of the SM. Vector boson processes and associated WH/ZH production set an upper limit on the Higgs boson decay branching ratio to invisible particles, such as dark matter, of 25%.

Constraints on new phenomena via Higgs boson couplings and invisible decays with the ATLAS detector

The ATLAS collaboration et al., Journal of High Energy Physics, 206 (2015)

### Exclusion of Obvious and Accessible Supersymmetry

High Energy Frontier
Exclusion of Obvious and Accessible Supersymmetry: The key feature of a proton collider is copious pair production of strongly interacting particles. The ATLAS collaboration scoured the entire dataset collected in the 8 TeV Large Hadron Collider run for an excess of events containing only particle jets, with an imbalance of transverse momentum. This would be the signature of strongly-produced supersymmetric particles decaying to Standard Model particles and the stable lightest supersymmetric particle, a weakly interacting particle detectable only by the hole it would leave, and thus an excellent candidate for dark matter. As published in the Journal of High Energy Physics (2015), no excess was found, and lower limits on masses of a large number of supersymmetric particles were obtained in a wide variety of benchmark and simplified models. These were in excess of one TeV for most strongly produced particles.

Summary of the searches for squarks and gluinos using $$\sqrt{s}=8$$ TeV pp collisions with the ATLAS experiment at the LHC

The ATLAS Collaboration et al., Journal of High Energy Physics, 54 (2015)

### Observation of Higgs boson production in association with a top quark pair

High Energy Frontier
Observation of Higgs boson production in association with a top quark pair: A probe of fundamental interest to further explore the nature of the Higgs boson is to measure its interaction with the top-quark, the most massive particle in the Standard Model. Indirect measurements of this interaction were previously made assuming no contribution from unknown particles. A more direct test of this coupling can be performed through the direct production of the Higgs boson in association with a top-quark pair, ttH. Measuring this process is challenging, because it is extremely rare: only one percent of Higgs bosons are expected to be produced this way. As submit to Physics Letters B (2018), using advanced analysis techniques, several independent searches for ttH production have been performed and combined, yielding the first observation of ttH production with a significance of 6.3 standard deviations relative to the background-only hypothesis.

Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector

The ATLAS Collaboration et al., Physics Letters B, 784, 173 (2018)

### Elusive dark matter and other exotic phenomena

High Energy Frontier Neutrinos and Dark Matter
Elusive dark matter and other exotic phenomena: A number of astrophysical measurements point to the existence of a new form of matter. For instance, the rotational speed of stars and observation of gravitational lensing effects strongly indicate the presence of so-called dark matter, in addition to our ordinary matter, that would compose a large fraction of our universe. Dark matter particles can be directly produced at the Large Hadron Collider, and one striking event signature would be the presence of an energetic jet of ordinary particles (called a monojet) and large missing energy due to dark matter particles escaping the ATLAS detector. As reported in the Journal of High Energy Physics (2018), a monojet final state constitutes a distinctive signature of beyond-Standard Model physics, and is also used to search for extra spatial dimensions and supersymmetry. Constraints have been set on various models.

Search for dark matter and other new phenomena in events with an energetic jet and large missing transverse momentum using the ATLAS detector

The ATLAS Collaboration et al., Journal of High Energy Physics, 126 (2018)

### Combination of searches for heavy resonances decaying into bosonic and leptonic final states

High Energy Frontier
Combination of searches for heavy resonances decaying into bosonic and leptonic final states: A generic prediction of many extensions of the Standard Model (SM) is the existence of heavy bosons decaying into pairs SM gauge bosons, as well as WH, ZH, or a pair of fermions. Specific searches for diboson resonances in several decay channels were combined to set constraints, using simple benchmark models, on the existence of a heavy hypothetical scalar, vector, or tensor particle. Analyses of leptonic final states  and were further combined with the diboson searches. Limit contours were obtained on the couplings of a heavy vector triplet (HVT) to quarks, leptons and the Higgs boson. The data exclude an HVT boson with mass below 5.5 (4.5) TeV in a weakly-coupled (strongly-coupled) scenario. Limits are also set on a Kaluza-Klein graviton.