Holly Pacey and the Half-Spin Particles: Searching for new physics with leptons at the ATLAS experiment

Whilst the Large Hadron Collider (LHC) has spent just over a decade pushing experimental boundaries in energy and intensity of proton-proton collisions, it has yet to discover new physics beyond the Standard Model. The Standard Model remains one of the most robustly tested theories of all time, despite evidence that it is incomplete, such as its inability to account for dark matter or the weakness of gravity. It is thus highly motivated that new particles should appear in proton-proton collisions at the ATLAS detector, around the TeV scale. This thesis contains three searches for such new particles which represent substantial improvements to constraints on possible new physics beyond the Standard Model. They demonstrate that data remains consistent with the Standard Model in collision events with two leptons in the final state. The first two searches seek electroweak supersymmetry, using 80.5 fb⁻¹ and the full 139.5 fb⁻¹ of the ATLAS Run 2 dataset, respectively. The first search placed the first ATLAS Run 2 limits on chargino pair production decaying via W-bosons, excluding chargino masses up to 410 GeV for a massless neutralino. This was surpassed by the second search, resulting in world-leading exclusion limits on this process, up to 420 GeV, as well as excluding direct slepton production with slepton masses up to 700 GeV and chargino pair production decaying via sleptons or sneutrinos with chargino masses up to 1 TeV. The third search uses an innovative technique to pursue signs of lepton charge-flavour asymmetry in the full Run 2 ATLAS dataset. Whilst it can be shown that the ratio of e⁺μ⁻ to e⁻μ⁺ events within Standard Model proton-proton collisions should not exceed one, this need not be the case for collision processes beyond the Standard Model. This search measures the ratio in data to be 0.988±0.005 and 0.987±0.005 in two regions of phase space (one with missing transverse momentum and one with a hard jet in the final state, respectively), as well as providing measurements binned in different variables. Furthermore, the search places the first direct exclusion limits at the LHC for single-production of a smuon within an R-parity violating supersymmetry model with the λ'₂₃₁ coupling switched on, excluding left-handed smuons up to 1.2 TeV for neutralino masses below the top quark mass and λ'₂₃₁ couplings up to one. It also interprets results in a scalar leptoquark model, placing the first LHC limits on a singly-produced leptoquark decaying to an eμ final state, excluding masses up to 2.2 TeV for leptoquark couplings λ(eu) and λ(µc) up to a value of one

The ATLAS inner detector trigger performance in pp collisions at 13 TeV during LHC Run 2

The design and performance of the inner detector trigger for the high level trigger of the ATLAS experiment at the Large Hadron Collider during the 2016-18 data taking period is discussed. In 2016, 2017, and 2018 the ATLAS detector recorded 35.6 fb$^{-1}$, 46.9 fb$^{-1}$, and 60.6 fb$^{-1}$ respectively of proton-proton collision data at a centre-of-mass energy of 13 TeV. In order to deal with the very high interaction multiplicities per bunch crossing expected with the 13 TeV collisions the inner detector trigger was redesigned during the long shutdown of the Large Hadron Collider from 2013 until 2015. An overview of these developments is provided and the performance of the tracking in the trigger for the muon, electron, tau and $b$-jet signatures is discussed. The high performance of the inner detector trigger with these extreme interaction multiplicities demonstrates how the inner detector tracking continues to lie at the heart of the trigger performance and is essential in enabling the ATLAS physics programme

Observation of electroweak production of two jets in association with an isolated photon and missing transverse momentum, and search for a Higgs boson decaying into invisible particles at 13 TeV with the ATLAS detector

This paper presents the measurement of the electroweak production of two jets in association with a $Z\gamma$ pair with the $Z$ boson decaying into two neutrinos. It also presents the search for invisible or partially invisible decays of a Higgs boson with a mass of 125 GeV produced through vector-boson fusion with a photon in the final state. These results use data from LHC proton-proton collisions at $\sqrt{s}$ = 13 TeV collected with the ATLAS detector corresponding to an integrated luminosity of 139 fb$^{-1}$. The event signature, shared by all benchmark processes considered for measurements and searches, is characterized by a significant amount of unbalanced transverse momentum and a photon in the final state, in addition to a pair of forward jets. For electroweak production of $Z\gamma$ in association with two jets, the background-only hypothesis is rejected with an observed (expected) significance of 5.2 (5.1) standard deviations. The measured fiducial cross-section for this process is 1.31$\pm$0.29 fb. Observed (expected) upper limit of 0.37 (0.34) at 95% confidence level is set on the branching ratio of a 125 GeV Higgs boson to invisible particles, assuming the Standard Model production cross-section. The signature is also interpreted in the context of decays of a Higgs boson to a photon and a dark photon. An observed (expected) 95% CL upper limit on the branching ratio for this decay is set at 0.018 (0.017), assuming the 125 GeV Standard Model Higgs boson production cross-section

The ATLAS inner detector trigger performance in pp collisions at 13 TeV during LHC Run 2

The design and performance of the inner detector trigger for the high level trigger of the ATLAS experiment at the Large Hadron Collider during the 2016-18 data taking period is discussed. In 2016, 2017, and 2018 the ATLAS detector recorded 35.6 fb$^{-1}$, 46.9 fb$^{-1}$, and 60.6 fb$^{-1}$ respectively of proton-proton collision data at a centre-of-mass energy of 13 TeV. In order to deal with the very high interaction multiplicities per bunch crossing expected with the 13 TeV collisions the inner detector trigger was redesigned during the long shutdown of the Large Hadron Collider from 2013 until 2015. An overview of these developments is provided and the performance of the tracking in the trigger for the muon, electron, tau and $b$-jet signatures is discussed. The high performance of the inner detector trigger with these extreme interaction multiplicities demonstrates how the inner detector tracking continues to lie at the heart of the trigger performance and is essential in enabling the ATLAS physics programme

Evolution of Regional, Age and Gender Demographics in the ATLAS Collaboration

The ATLAS Collaboration consists of more than 5000 members, from over 100 different countries. Regional, age and gender demographics of the collaboration are presented, including the time evolution over the lifetime of the experiment. In particular, the relative fraction of women is discussed, including their share of contributions, recognition and positions of responsibility, including showing how these depend on other demographic measures

EW SUSY Production at the LHC

The direct production of electroweak SUSY particles, including sleptons, charginos, and neutralinos, is a particularly interesting area of search at the LHC. While the lightest neutralino is a well motivated and studied candidate for dark matter in models with R-parity conservation, the small production cross sections of electroweak production leads to difficult searches. This talk will highlight the most recent results of searches performed by the ATLAS and CMS experiments for supersymmetric particles produced via electroweak processes, including analyses with leptonic and hadronic final states

EW SUSY Production at the LHC

EW SUSY searches at the LHC covering all LHC experiments (Speaker from ATLAS

Missing Transverse Momentum Recontruction in ATLAS

Missing transverse momentum (MET) is a critical observable for physics searches in proton-proton collisions at the Large Hadron Collider. This talk describes these various novel approaches and their performance. ATLAS employs a suite of working points for missing transverse momentum (MET) reconstruction, and each is optimal for different event topologies. A new neural network can exploit various event properties to pick the optimal working point on an event-by-event basis and also combine complementary information from each of the working points. The resulting regressed "METNet" offers improved resolution and pileup resistance across a number of different topologies compared to the current MET working points. Additionally, image-based de-noising neural network techniques are studied; these also provide significant resolution improvements and pileup resistance

EW SUSY production at the LHC

The direct production of electroweak SUSY particles, including sleptons, charginos, and neutralinos, is a particularly interesting area of search at the LHC. While the lightest neutralino is a well motivated and studied candidate for dark matter in models with R-parity conservation, the small production cross sections of electroweak production leads to difficult searches. This talk will highlight the most recent results of searches performed by the ATLAS and CMS experiments for supersymmetric particles produced via electroweak processes, including analyses with leptonic and hadronic final states