Precision measurements of the standard model; PDFs, pile-up and production of W+jets at 13 TEV with the ATLAS detector

The question of the nature of the universe at its most fundamental level is one which has been with humankind since ancient times. The development of the theoretical basis of the Standard Model of elementary particles during the 20th century, and its subsequent experimental validation, is a triumph of modern physics. However, the Standard Model is known to be incomplete, and, in the Large Hadron Collider era at the high-energy and high-intensity frontier, our understanding and modelling of the Standard Model has increasingly become limiting for searches for physics beyond it. In this thesis, a measurement performed using 36.2 fb−1 of integrated luminosity recorded by the ATLAS experiment during Large Hadron Collider Run 2 of a crucial benchmark process for Standard Model modelling is presented; W boson production in association with jets. This is the first such measurement in the W → μν decay channel at √s =13 TeV. Differential cross-sections, in observables sensitive to the modelling of quantum chromodynamics, are presented for both charge-independent and charge-separated W production, and compared to two cutting-edge calculations evolved to next-to-next-to-leading order in quantum chromodynamics. In addition, this process strongly constrains fits of the Parton Distribution Functions of the proton; an analysis of the structure of the proton using several previous ATLAS measurements, including W boson production in association with jets at √s=8 TeV, will also be presented

Real particle physics analysis by UK secondary school students using the ATLAS Open Data: an illustration through a collection of original student research

Since the 2020 release of 10fb-1 of integrated luminosity of proton–proton collision data to the public by the ATLAS experiment, significant potential for its use for youth engagement in physics and citizen science has been present. In particular, this article aims to address whether, if provided adequate training and resources, high school students are capable of leveraging the ATLAS Open Data to semi-autonomously develop their own original research projects. To this end, a repository of interactive Python Jupyter notebook training materials was developed, incrementally increasing in difficulty; in the initial instalments no prior knowledge of particle physics or Python coding is assumed, while in the latter stages students emulate the steps of a real Higgs boson search using ATLAS data. This programme was implemented in secondary schools throughout the UK during the 2022/23 academic year and is presented in this article through a collection of research projects developed by a selection of participating students

Wakakosha “You are Worth it”: Reported impact of a community-based, peer-led HIV self-stigma intervention to improve self-worth and wellbeing among young people living with HIV in Zimbabwe

Self-stigma - negative self-judgements or core beliefs - can result in feelings of shame, worthlessness and self-blame, and impacts social interaction, mental health and health service utilization among people living with HIV (PLHIV). Few interventions target self-stigma among PLHIV and, to our knowledge, none until now for adolescents and young people LHIV (AYPLHIV) in sub-Saharan Africa. We present qualitative findings on the perceived impact of a self-stigma intervention (Wakakosha, ‘You are worth it’)

Constraints on the Higgs boson self-coupling from single- and double-Higgs production with the ATLAS detector using pp collisions at √s=13 TeV

Constraints on the Higgs boson self-coupling are set by combining double-Higgs boson analyses in the bb̅bb̅, bb̅τ+τ− and bb̅γγ decay channels with single-Higgs boson analyses targeting the γγ, ZZ∗, WW∗, τ+τ− and bb̅ decay channels. The data used in these analyses were recorded by the ATLAS detector at the LHC in proton–proton collisions at √s = 13 TeV and correspond to an integrated luminosity of 126–139 fb−1. The combination of the double-Higgs analyses sets an upper limit of μHH <2.4 at 95% confidence level on the double-Higgs production cross-section normalised to its Standard Model prediction. Combining the single-Higgs and double-Higgs analyses, with the assumption that new physics affects only the Higgs boson self-coupling (λHHH ), values outside the interval −0.4 < κλ = (λHHH /λSMHHH) < 6.3 are excluded at 95% confidence level. The combined single-Higgs and double-Higgs analyses provide results with fewer assumptions, by adding in the fit more coupling modifiers introduced to account for the Higgs boson interactions with the other Standard Model particles. In this relaxed scenario, the constraint becomes −1.4 < κλ < 6.1 at 95% CL

ATLAS Run 2 searches for electroweak production of supersymmetric particles interpreted within the pMSSM

A summary of the constraints from searches performed by the ATLAS collaboration for the electroweak production of charginos and neutralinos is presented. Results from eight separate ATLAS searches are considered, each using 140 fb−1 of proton-proton data at a centre-of-mass energy of √ = 13 TeV collected at the Large Hadron Collider during its second data-taking run. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, where R-parity conservation is assumed and the lightest supersymmetric particle is assumed to be the lightest neutralino. Constraints from previous electroweak, flavour and dark matter related measurements are also considered. The results are presented in terms of constraints on supersymmetric particle masses and are compared with limits from simplified models. Also shown is the impact of ATLAS searches on parameters such as the dark matter relic density and the spin-dependent and spin-independent scattering cross-sections targeted by direct dark matter detection experiments. The Higgs boson and Z boson ‘funnel regions’, where a low-mass neutralino would not oversaturate the dark matter relic abundance, are almost completely excluded by the considered constraints. Example spectra for non-excluded supersymmetric models with light charginos and neutralinos are also presented

ATLAS flavour-tagging algorithms for the LHC Run 2 pp collision dataset

The flavour-tagging algorithms developed by the ATLAS Collaboration and used to analyse its dataset of √s = 13 TeV pp collisions from Run 2 of the Large Hadron Collider are presented. These new tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77% b-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model tt¯ events; similarly, at a c-jet identification efficiency of 30%, a light-jet (b-jet) rejection factor of 70 (9) is obtained

The impact of ATLAS VV+jet measurements on PDF fits

The production of $W$/$Z$-bosons in association with jets is an important test of perturbative QCD predictions and also yields information about the parton distribution functions of the proton. We present fits to determine PDFs using inclusive $W$/$Z$-boson and $W$/$Z$+jets measurements from the ATLAS experiment at the LHC. The ATLAS measurements are used in combination with deep-inelastic scattering data from HERA. An improved determination of the sea-quark densities at high Bjorken-$x$ is seen, while confirming a strange-quark density similar in size to the up- and down-sea-quark densities in the range $x<0.02$ found by previous ATLAS analyses.The production of $W$/$Z$-bosons in association with jets is an important test of perturbative QCD predictions and also yields information about the parton distribution functions of the proton. We present fits to determine PDFs using inclusive $W$/$Z$-boson and $W$/$Z$+jets measurements from the ATLAS experiment at the LHC. The ATLAS measurements are used in combination with deep-inelastic scattering data from HERA. An improved determination of the sea-quark densities at high Bjorken-$x$ is seen, while confirming a strange-quark density similar in size to the up- and down-sea-quark densities in the range $x<0.02$ found by previous ATLAS analyses

QCD Measurements in ATLAS

The ATLAS experiment has performed a range of QCD measurements. The recoil of the Z-boson is sensitive to quark and gluon emissions and is used to determine the strong coupling constant in a novel approach. Measurements of transverse energy-energy correlation in multijet events are compared to state-of-the-art NLO and NNLO predictions. used to determine the strong coupling constant. Jet cross-section ratios between inclusive bins of jet multiplicity are measured differentially in variables that are sensitive to either the energy-scale or angular distribution of hadronic energy flow in the final state. Several improvements to the jet energy scale uncertainties are described, which result in significant improvements of the overall ATLAS jet energy scale uncertainty. The measurements are compared to state-of-the-art NLO and NNLO predictions. Finally, we present fits to determine parton distribution functions (PDFs) using a diverse set of measurements from the ATLAS experiment at the LHC in combination with deep-inelastic scattering data from HERA

PDFs@LHC

Slides for talk 'PDFs@LHC' to be given at SM@LHC 202

Proton parton distribution functions using ATLAS data

Precise knowledge of proton parton distribution functions is a crucial element of accurate predictions of both Standard Model and Beyond Standard Model physics at hadron colliders such as the LHC. We present a PDF fit at next-to-next-to-leading order in QCD demonstrating the constraining power of a diverse range of ATLAS measurements, in combination with deep-inelastic scattering data from HERA, on the parton distributions within the proton. Careful consideration is made of the correlation of systematic uncertainties within and between the ATLAS datasets. The resulting set of parton distribution functions, named ATLASpdf21, is evaluated for two choices of $\chi^2$ tolerance and compared to a range of global PDF fits