Unravelling LEP-era Lepton Flavour Universality discrepancy with ATLAS

Abstract: A remarkable feature of the Standard Model (SM) is that each lepton flavour (electron, muon, tau) is equally likely to interact with a W boson. This is known as Lepton Flavour Universality. In a recent ATLAS measurement, a novel technique using events with top-quark pairs has been exploited to test the ratio of the probabilities for tau leptons and muons to be produced in W boson decays, R(τ/μ). In the SM, R(τ/μ) is expected to be unity, but a longstanding tension with this prediction has existed since the LEP era, where, from a combination of experiments, R(τ/μ) was measured to be 2.7σ higher than the SM expectation. If the LEP result were confirmed to O(1%) precision it would correspond to an unambiguous discovery of beyond the SM physics. This latest ATLAS measurement obtains this level a precision - join the RPM to discover whether or not it agrees with LEP!

Top: ATLAS results from precision to rarity

Abstract: The abundant production of top quark events at the LHC allows testing the standard model in unprecedented ways. The relative probability for W bosons to decay to a muon or a tau-lepton is measured, selecting events in which a top quark pair decays to two opposite-sign leptons plus jets through W bosons. The measurement confirms the universality of tau and muon couplings to the W boson with the highest precision, to date. In a second analysis, charged leptons are again used to select di- and multi-lepton final states with high jet multiplicity. The kinematic, multiplicity and flavour properties are input to a multivariate discriminant to recognise and establish evidence for the extremely rare production of four top quarks in an event.

FASER status report and the FASERnu proposal

Abstract:

Prospects for Higgs & precision SM physics at HL-LHC with the ATLAS detector

Abstract: The Large Hadron Collider (LHC) has been successfully delivering proton-proton collision data at the unprecedented center of mass energy of 13 TeV. An upgrade is planned to increase the instantaneous luminosity delivered by LHC in what is called HL-LHC, aiming to deliver a total of about 3000/fb of data to the ATLAS detector at a center of mass energy of 14 TeV. To cope with the expected data-taking conditions ATLAS is planning major upgrades of the detector.

In this contribution we present an overview of the precision physics measurement of the Standard Model, with particular focus on the electro-weak and Higgs sectors. Prospects for precision determination of Standard Model fundamental parameters, as the weak mixing angle, the Higgs couplings, di-Higgs observation, vector boson scattering processes, as well as measurement for rare Higgs decays are presented.

Such studies formed the basis of the ATLAS Collaboration input to the recent HL/HE-LHC Yellow-Report. An executive summary of this report was then submitted as input to the European Strategy process.

Measurements of V+jets with the ATLAS detector

Abstract: The production of jets in association with vector bosons is an important process to study QCD in a multi-scale environment. The ATLAS collaboration has performed measurements of vector boson+jets cross sections, differential in several kinematic variables, in proton-proton collision data taken at center-of-mass energies of 8TeV and 13TeV. The measurements are compared to state-of-the art theory predictions and can be used to constrain the proton structure.

We have also studied the jet production rates at different resolution scales. In particular, we present a measurement of the splitting scales in the kt jet-clustering algorithm for final states containing a Z-boson candidate at a centre-of-mass energy of 8 TeV. The data are corrected for detector effects and are compared to state-of-the-art Monte Carlo predictions.

Tackling gluon splitting to bb

Abstract: Many LHC measurements and searches exploit final states that include b-tagged jets. In several such analyses the modelling of heavy flavour production can be a vital component of the background estimation and a significant source of systematic uncertainty. In many cases, e.g. VH(→bb) and ttH(→bb), the dominant modelling uncertainty comes from the theory prediction of g→bb. We make the most precise measurement of g→bb at the LHC yet, with a new technique that examines b-hadron pair production in the B→J/ψ(→μ+μ−)+X and B→μ+X decay mode. The measurement exploits the excellent angular and momentum resolution for muon reconstruction of the ATLAS detector using 11.4 fb−1 of p-p collisions recorded at √s = 8 TeV. The total cross section as well as several kinematic variables sensitive to b-hadron production modes are measured. These measurements are compared to MC predictions from widely used event generators, providing new constraints on heavy flavour production and the possibility to reduce the associated systematic uncertainties.

Tackling gluon splitting to bbbar with a measurement of b-hadron pair production in the J/ψ+μ final state at ATLAS

Abstract: Many LHC measurements and searches exploit final states that include b-tagged jets. In several such analyses the modelling of heavy flavour production can be a vital component of the background estimation and a significant source of systematic uncertainty. In many cases, e.g. VH(→bb) and ttH(→bb), the dominant modelling uncertainty comes from the theory prediction of g→bb. We make the most precise measurement of g→bb at the LHC yet, with a new technique that examines b-hadron pair production in the B→J/ψ(→μ+μ−)+X and B→μ+X decay mode. The measurement exploits the excellent angular and momentum resolution for muon reconstruction of the ATLAS detector using 11.4 fb−1 of p-p collisions recorded at √s = 8 TeV. The total cross section as well as several kinematic variables sensitive to b-hadron production modes are measured. These measurements are compared to MC predictions from widely used event generators, providing new constraints on heavy flavour production and the possibility to reduce the associated systematic uncertainties.

Search for ttH and tH production (not including H→bb) at the LHC