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Haystacks and Needles

Measuring the number of proton collisions in ATLAS and probing them for the production of new exotic particles

Time: Thu 2022-06-02 13.00

Location: FD5, Roslagstullsbacken 21, Stockholm

Language: English

Subject area: Physics, Atomic, Subatomic and Astrophysics

Doctoral student: Giulia Ripellino , Partikel- och astropartikelfysik, Particle and Astroparticle Physics

Opponent: Assistant Professor Gabriella Pasztor, ELTE Institute of Physics, Hungary

Supervisor: Associate Professor Jonas Strandberg,

QC 220510


This thesis describes my work on the ATLAS luminosity measurement and on two searches for physics beyond the Standard Model, performed using √s = 13 TeV proton-proton (pp) collision data recorded by the ATLAS detector at CERN during Run 2 of the LHC in the years 2015-2018

ATLAS employs several detectors and methods to measure the relative luminosity and the absolute scale is determined using van der Meer scans during dedicated low-luminosity running periods in each year. The thesis presents the details of the track-counting method which is used to extrapolate the calibration to the high-luminosity physics data-taking regime. After data-quality selections, the full Run 2 pp data sample corresponds to an integrated luminosity of 139 fb-1, with an uncertainty of 1.7%.

The first analysis presented is a search for Supersymmetry in final states with two same-flavour opposite-charge leptons, jets, and large missing transverse momentum. The search uses 36.1 fb-1 of pp collision data collected in 2015 and 2016. Signal scenarios involving pair-production of squarks or gluinos decaying into final states with two leptons and the lightest supersymmetric particle are targeted by the analysis. The main Standard Model backgrounds are estimated with data-driven methods and the observed data is found to be consistent with the expectation. Results are interpreted in simplified Supersymmetry models and exclude gluinos and squarks with masses up to 1.85 TeV and 1.3 TeV at 95% confidence level, respectively.

The second analysis is a search for long-lived particles decaying into hadrons. The analysis selects events that contain multiple energetic jets and a displaced vertex and uses the full Run 2 pp collision dataset. Background estimates for Standard Model processes and instrumental effects are extracted from data. The observed yields are one event in the first signal region where 0.49±0.28 are expected and zero events in the second signal region where 0.7±0.4 are expected. The results are used to set limits at 95% confidence level on Supersymmetry scenarios with long-lived electroweakinos that decay via a small R-parity-violating coupling into triplets of quarks. Electroweakly produced neutralinos with masses up to 1.3 TeV  are excluded for lifetimes between 0.01 ns and 10 ns.