Suat Baris Tuncay

Abstract:

This paper presents differential cross-section measurements of events containing a charged lepton, missing transverse momentum, two b-jets, and two light jets, consistent with the W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ final state. The analysis is based on 140 fb−1 of proton-proton collision data at s=13$$ \sqrt{s}=13 $$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider (2015–2018). Differential cross-sections are unfolded at particle level for a range of kinematic observables and in three kinematic regions. Both regions and observables are optimised for sensitivity to the interference between W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ produced with singly and doubly resonant top quarks. Results are compared with various theoretical predictions and deviations are observed in regions sensitive to the interference between singly and doubly resonant top-quark production. The unfolded results provide new information about the modelling of interference and off-shell effects of the W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ final state, improving the understanding of top-quark production mechanisms and providing new inputs for future Monte Carlo generator development.

Abstract:

A search for charged lepton flavour violation in τ → 3μ decays is performed in pp collisions at a centre-of-mass energy of 13 TeV using ATLAS data collected between 2016 and 2018, corresponding to an integrated luminosity of 137 fb − 1 . The search is optimised for the electroweak W → τν production channel, due to its higher trigger efficiency, but also considers potential signal events from heavy-flavour production. Data are collected using two-muon and three-muon triggers and a multivariate analysis is used to separate the signal from the background. An unbinned likelihood fit is then performed to the resulting three-muon invariant mass spectrum and the data are found to be compatible with the background-only hypothesis. The observed (expected) limit on the branching ratio B(τ → 3μ) is found to be 8.7 × 10 − 8 ( 7.5 × 10 − 8 ) at 90% CL.

Abstract:

A search is presented for the electroweak production of a top-quark pair in association with a $W$ boson and at least one additional jet known as the $t\overline{t}W{j}_{\mathrm{EW}}$ process. This process has embedded within it a $tW$ -scattering vertex, which is probed directly for the first time. The collision data were collected with the ATLAS detector during Run 2 of the LHC and correspond to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ at $\sqrt{s}=13\mathrm{TeV}$ . The search uses same-charge pairs of electrons and muons together with jets, of which at least one is $b$ -tagged. The properties of the most forward jet relative to the rest of the event are used to discriminate the electroweak $t\overline{t}W{j}_{\mathrm{EW}}$ production process from its strong $t\overline{t}W$ production counterpart. A measured (expected) 95% CL upper limit on the cross section is set at ${\sigma }_{t\overline{t}W{j}_{\mathrm{EW}}}<251\mathrm{fb}$ (230 fb), to be compared with the expected Standard Model (SM) cross section of 47.7 fb. Limits are set on the SM effective field theory (EFT) operators $O_{Ht}$ and $O_{HQ}^{(1)}$ , which modify the electroweak couplings of the top quark through contributions to the $tW$ -scattering vertex. The interpretation acts as a case study to emphasize the importance of energy-dependent sensitivity, multiprocess, and multioperator EFT contributions.

Abstract:

After ten years of operation at the LHC, the planar pixel sensors of the innermost barrel layer of the ATLAS Pixel detector have accumulated an average bulk damage fluence in excess of 2×1015 1 MeV-neutrons equivalent/cm2. The macroscopic effects of this radiation are an increase of the sensor leakage current, a loss of charge collection efficiency and an increase of the depletion voltage. Using regular bias voltage scans performed at the beginning and end of each data taking campaign the evolution of the pixel cluster charge and bulk depletion is studied as a function of particle fluence. Results are interpreted with the modelling provided by standalone TCAD and ATLAS Monte Carlo simulation including radiation damage effects. The dependence of the collected charge and the depletion voltage with integrated luminosity are studied through the full period of operation.

Abstract:

A model-independent search for low-mass resonances decaying into pairs of oppositely charged muons is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb−1, recorded by the ATLAS detector at the Large Hadron Collider between 2015 and 2018. The search targets hypothetical dimuon resonances in the invariant mass range from 35 GeV to 75 GeV. The modelling of this mass region is particularly challenging for conventional analytic background parameterisations. To address this, a Gaussian process regression technique is used to model the background. The dimuon mass spectrum is analysed for potential signals, and no statistically significant excess is observed. Upper limits at the 95% confidence level are set on the fiducial production cross-section of new resonances decaying promptly into muons, ranging from 20 fb to 110 fb, depending on the resonance mass. These results are further interpreted in the context of dark-photon and dark-matter-mediator models, leading to new constraints on their parameter spaces.