Mu3e

Abstract:

MALTA2 is a depleted monolithic active pixel sensor (DMAPS) designed for tracking at high rates and typically low detection threshold of 200e⁻. Threshold calibration is crucial to understanding the charge collection in the pixel. A simple procedure is developed to calibrate the threshold to unit electrons making use of a dedicated charge injection circuit and an Fe-55 source with dominant charge deposition of 1611e⁻. The injection voltage corresponding to the signal under exposure of Fe-55 is determined and serves as the basis for charge calibration. The charge injection circuit incorporates a capacitance with design value of Cinj=230aF. Experimentally, the average capacitance value for non-irradiated samples is found to be Cinj,exp=255±35aF and varies between sensors by 14% in terms of standard deviation. The deviation from the design value as well as among sensors motivates the need for the presented calibration procedure, which is proposed to be performed for each MALTA2 sensor and reduces the uncertainty down to 3% depending on the sensor.

Abstract:

This Letter reports the observation of W + W − γ triboson production in 140 fb − 1 of data collected by the ATLAS detector from proton–proton collisions at a centre-of-mass energy of s = 13 TeV at the LHC. Events with an opposite-charge e μ pair, a high transverse-momentum photon, and significant missing transverse momentum are considered. The observed (expected) significance of the signal is 5.9 (6.0) standard deviations. The measured fiducial cross-section, defined for the W + W − γ → e ± μ ∓ ν ν ¯ γ final state is 6.2 ± 0.8 (stat.) ± 0.6 (sys.) fb, in good agreement with the Standard Model prediction of 6.1 − 0.7 + 1.0 fb. Constraints on the Wilson coefficients of 13 dimension-8 operators describing physics beyond the Standard Model through anomalous quartic gauge-boson couplings are derived using the effective.

Abstract:

This paper presents a search for physics beyond the Standard Model targeting a heavy resonance visible in the invariant mass of the lepton-jet system. The analysis focuses on final states with a high-energy lepton and jet, and is optimised for the resonant production of leptoquarks — a novel production mode mediated by the lepton content of the proton originating from quantum fluctuations. Four distinct and orthogonal final states are considered: e+light jet, μ+light jet, e+b-jet, and μ+b-jet, constituting the first search at the Large Hadron Collider for resonantly produced leptoquarks with couplings to electrons and muons. Events with an additional same-flavour lepton, as expected from higher-order diagrams in the signal process, are also included in each channel. The search uses proton-proton collision data from the full Run 2, corresponding to an integrated luminosity of 140 fb−1 at a centre-of-mass energy of s=13 TeV, and from a part of Run 3 (2022–2023), corresponding to 55 fb−1 at s=13.6 TeV. No significant excess over Standard Model predictions is observed. The results are interpreted as exclusion limits on scalar leptoquark S~1 production, substantially improving upon previous ATLAS constraints from leptoquark pair production for large coupling values. The excluded S~1 mass ranges depend on the coupling strength, reaching up to 3.4 TeV for quark-lepton couplings yde = 1.0, and up to 4.3 TeV, 3.1 TeV, and 2.8 TeV for ysμ, ybe, and ybμ couplings set to 3.5, respectively.

Abstract:

Measurements of tt¯ℓ+ℓ-$$t\bar{t}\ell ^{+}\ell ^{-}$$ production in the region of high dilepton invariant mass with effective field theory (EFT) interpretations are presented. They are performed using final states with three isolated leptons (electrons or muons) and are based on s=13$$\sqrt{s} = 13$$ TeV$$\text {T}\text {e}\hspace{-1.00006pt}\text {V}$$ proton–proton collision data with an integrated luminosity of 140fb-1$$140\,\textrm{fb}^{-1}$$, recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. Measurements of the tt¯ℓ+ℓ-$$t\bar{t}\,\ell ^+\ell ^-$$ signal strength and cross-section upper-limits are performed inclusively in lepton flavour and separately for electrons and muons. The study also aims to probe anomalous four-fermion interactions including to test for possible lepton flavor universality violation. No significant deviations from the Standard Model predictions are observed and the measurements are interpreted through the EFT formalism to provide new constraints on the relevant operators.

Abstract:

A search for R-parity-conserving supersymmetry in events with large missing transverse momentum, jets and at least one hadronically decaying τ$$\tau $$-lepton is presented. Both gluino and squark pair production are considered, with the cascade decay of each gluino or squark producing either a τ$$\tau $$-slepton or a τ$$\tau $$-sneutrino. Three channels are examined, requiring either exactly one hadronically decaying τ$$\tau $$-lepton and no other leptons, exactly one hadronically decaying τ$$\tau $$-lepton and at least one other lepton, or two or more hadronically decaying τ$$\tau $$-leptons. Analyses in the three channels are optimised independently and combined statistically. Two separate analysis strategies, either a cut-and-count or machine-learning approach, are used. The search uses 140 fb-1$$\textrm{fb}^{-1}$$ and 51.8 fb-1$$\textrm{fb}^{-1}$$ of pp collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015–2018 at s=13$$\sqrt{s} = 13$$ TeV and 2022–2023 at s=13.6$$\sqrt{s} = 13.6$$ TeV, respectively. Gluino masses below 2.25 TeV and squark masses up to 1.7 TeV are excluded