Session Y10: Top Quark and Electroweak Physics

Measurements of Top Quark Pair Spin Correlations in pp Collisions at a centre-of-mass energy of 13 TeV with CMS

Measurements of the top quark polarization and top quark pair spin correlations are presented using events containing two leptons produced in proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the CMS experiment at the CERN LHC during the full Run II and correspond to an integrated luminosity of 137/fb. A set of multi-differential cross section distributions are measured for each of the independent coefficients of the top-spin dependent parts of the production density matrix for the \mathrm{t \bar t}$ system, unfolded to particle and parton-level, and compared to standard model predictions from Monte Carlo simulations with next-to-leading-order (NLO) accuracy in quantum chromodynamics (QCD) at matrix-element level interfaced to parton-shower simulations. The spin coefficients are extracted from the measured distributions and compared to values from fixed order calculations at NLO in QCD with electroweak corrections. The measurement provides a precision test of the Standard Model of particle physics and probes for New Physics.   

Investigation of Entangled and Bound ttbar Pairs at the LHC with the CMS Detector

The top-antitop quark system offers a unique window into quantum information at the LHC. The mass of the top quark causes the top quark to have an exceptionally short lifetime before decaying. It is so short that typical estimates for the time scale for hadronization and spin-flip for the top quark are longer than the decay time. Therefore, we can use the top-antitop system to probe entanglement in an elementary particle system at energy scales previously unexplored in quantum information theory. On a related note, recently deviations in the low m_{ttbar} region have been observed by both ATLAS and CMS for dileptonic decays of the top-antitop system. One possible explanation for this would be the formation of a top-antitop bound state - toponium. This would alter the spin correlations and entanglement of the top-antitop pair near the threshold region.   

Probing anomalous Quartic Gauge Couplings using Muon Collider experiment

Standard Model (SM) is not a complete model, and we need to investigate further to search for new physics beyond SM. One way to do that is by exploring the high-energy behavior of electroweak interactions, the SM can be extended to allow for higher dimension operands. Doing these studies using muon collider benefits over LHC in that we will have access to full center of mass energy, and it serves as a gauge boson collider. We study the sensitivity of muon collider for anomalous quartic gauge couplings (aQGCs) of WWWW vertex. Vector boson scattering for the W+ W- ν ν as final state is analyzed at 3, 10 and 30 TeV and limits on aQGC sensitivity are set.   

Observation of the WWW Production in p-p Collision at √s = 13 TeV with the ATLAS Detector

WWW production was recently observed by the ATLAS collaboration using the full Run II data set with an integrated luminosity of 139 fb^-1 at √s =13 TeV. These first observation and cross-section measurements are presented. Measurements are performed in two final states. Events with two same-sign electrons or muons in association with two jets as well as events with three charged leptons with no same flavor opposite sign lepton pairs are selected. Machine learning techniques are used to improve the signal sensitivity by training different kinematic variables separately for each channel. Triboson WWW production is observed with a significance of 8.0σ, where the expectation is 5.4σ. The inclusive WWW production cross-section is measured to be 820 ± 100 (stat.) ± 80 (syst.) fb.   

Study of W±Z longitudinal-longitudinal interactions (𝑊± 0Ζ0) with Full Leptonic Final States (W±Ζ→ι±νι±ι∓) in ρρ collisions at √s=13 TeV with the ATLAS detector

In the Standard Model of particle physics, the spontaneous symmetry breaking of the complex Higgs field gives rise to the massive Higgs boson and three Goldstone bosons, which represent the longitudinal degree of freedom of the 𝑊^± and 𝑍 bosons. It is therefore critical to study the interactions of longitudinally-polarized 𝑊^± and 𝑍 bosons (𝑊^±₀ and 𝑍₀). Here, we present the first such analysis, in inclusive fully-leptonic final states (𝑊^±𝑍→𝑙^±ν𝑙^±𝑙^∓) with 139 fb⁻¹ of p-p collision data recorded with the ATLAS detector at 13 TeV of center of mass energy. We use kinematic variables like p_T^Zand p_T^𝑊±𝑍 to enhance the 𝑊^±₀𝑍₀ contribution. The so called radiation amplitude zero effect also enhances the 𝑊^±₀𝑍₀ contribution in the central region (cos𝜃_𝑉 ~ 0, where 𝜃_𝑉 is the scattering angle of the 𝑊^± or 𝑍 boson in the parton center-of-mass frame). A multivariate variable is developed to separate the longitudinal-longitudinal polarization from other polarizations. A template fit will be performed to extract polarization fractions.   

Measurement of collinear W boson emission off high transverse momentum jets using full ATLAS Run-2 data.

The production of a single electroweak vector boson in association with jets (V+jets) is one of the fundamental processes at the Large Hadron Collider (LHC) experiment. The leptonic decay modes of this process provides a clean experimental signature for measuring the electroweak sector of the Standard Model and the perturbative QCD accuracy in multi-jets final states.

In the talk, we will focus on the differential measurement of single W decaying to an electron or muon from a high transverse momentum jet with small angular separation, the so called collinear W+jets production. This measurement makes use of the LHC full Run-2 proton-proton collision datasets, corresponding to an integrated luminosity of 139.0 fb^-1 . The data is compared against newly develop state-of-the-art multi-jet merged setups accurate to next-to-leading order in the strong and weak coupling constants. The details of the generator configurations and their CPU costs will be discussed, and unfolded kinematic distributions at particle level in the collinear W+jets phase-space will be compared with the measured data cross sections.   

Study of WWZ tri-boson production at ATLAS

The Standard Model predicts self interactions between gauge bosons, including triple gauge boson couplings (TGC) and quartic gauge boson couplings (QGC). In addition, the interactions between the Higgs boson and gauge bosons are also of interest. In ATLAS group, events with four leptons (electrons or muons) in the final state are used to search for the production of WWZ -> lvlvll. The total luminosity used is 139 fb-1 at sqrt{s}=13 TeV. Events are further divided into three categories based on the invariant mass and the flavor of the two leptons from the decays of the two W bosons to increase the search sensitivity. A multivariate variable is further developed to increase the separation between the signal and background. I will present optimization on event selection and search sensitivity studies using simulated Monte Carlo events.