ATLAS

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:

Processes with τ$$\tau $$-leptons in the final state are important for Standard Model measurements and searches for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider observes τ$$\tau $$-leptons produced in proton–proton collisions only through their decay products. Data analyses involving hadronically decaying τ$$\tau $$-leptons face challenges due to backgrounds from jets misidentified as τ$$\tau $$-leptons that are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such misidentified backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from dedicated regions. This paper describes a refined technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for a signal region by using fake factors from samples enriched in different sources of jets misidentified as τ$$\tau $$-leptons (light-quark, gluon, b-quark, and pile-up jets). Each fake factor is calculated as a linear combination of fake factors measured in these different enriched samples. For the full Run 2 data set, the systematic uncertainty of the calculated fake factors, evaluated using W(μν)$$W(\mu \nu )$$ enriched event sample, ranges from 15 to 35% depending on the τ$$\tau $$-lepton’s transverse momentum and charged-particle decay multiplicity.

Abstract:

Higgs boson production cross-sections via gluon–gluon fusion and vector-boson fusion in proton–proton collisions are measured in the H→WW∗→ℓνℓν$$H\rightarrow WW^*\rightarrow \ell \nu \ell \nu $$ decay channel. The Large Hadron Collider delivered proton–proton collisions at a centre-of-mass energy of 13TeV$$13\,\text {TeV}$$ between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of 140fb-1.$$140\,\text {fb}^{-1}.$$ The total cross-sections for Higgs boson production by gluon–gluon fusion and vector-boson fusion times the H→WW∗$$H\rightarrow WW^*$$ branching ratio are measured to be 12.4-1.2+1.3pb$$12.4^{+1.3}_{-1.2}\,\text {pb}$$ and 0.79-0.16+0.18pb,$$0.79^{+0.18}_{-0.16}\,\text {pb},$$ respectively, in agreement with the Standard Model predictions. Higgs boson production is further characterised through measurements of Simplified Template Cross-Sections in a total of fifteen kinematic fiducial regions. A new scheme of kinematic fiducial regions has been introduced to enhance the sensitivity to CP-violating effects in Higgs boson interactions. Both schemes are used to constrain CP-even and CP-odd dimension-six operators in the Standard Model effective field theory

Abstract:

ATLAS, a general-purpose experiment at the Large Hadron Collider (LHC), makes use of a large internationally-distributed computing infrastructure, including over 106$$10^6$$ TB of managed data on disk and tape and almost one million simultaneously running CPU cores. Upgrades for the High-Luminosity LHC (HL-LHC) will increase the required computing resources by a factor of 3–4 by the beginning of the 2030s, and by an order of magnitude before the conclusion of data taking at the beginning of the 2040s. These resources are spread over around 100 computing sites worldwide. Efforts are underway within the experiment to evaluate and mitigate various aspects of the environmental impact of the sites, with the additional long-term goal of making recommendations to the sites that will significantly reduce the total expected environmental impact in the HL-LHC era. These efforts take several forms: building awareness in the experiment community, adjusting aspects of the computing policy, and modifications of data center configurations, either in ways that take advantage of particular features of ATLAS workloads or in generic ways that reduce the environmental impact of the computing resources. This paper describes the ongoing investigations and approaches that have already provided useful and actionable outcomes.

Abstract:

A search for the dimuon decay of the Higgs boson is presented based on $pp$ collision data recorded by ATLAS during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of $165{\mathrm{fb}}^{-1}$ at $\sqrt{s}=13.6\mathrm{TeV}$ . To enhance the sensitivity, the results are combined with those from Run 2. An excess of events over the background is observed with a significance of $3.4\sigma$ ( $2.5\sigma$ expected). The best-fit signal strength is $\mu =1.4\pm 0.4$ . This result provides evidence for the $H\to \mu \mu$ decay with ATLAS data and offers a direct probe of the Higgs-boson Yukawa coupling to second-generation fermions.