For the central values of the relevant experimental inputs, that is the strong coupling constant and the top quark and Higgs masses, the effective Higgs potential displays two minima, one at the electroweak scale and a deeper one at high energies. We review the phenomenology of the Higgs inflation model, extending the Standard Model to include a non-minimal coupling to gravity; as recently shown, even configurations that would be metastable in the Standard Model, become viable for inflation if the non-minimal coupling is large enough to flatten the Higgs potential at field values below the barrier between the minima.
With no evidence of direct production of beyond the Standard Model (BSM) particles at the TeV scale, deviations from the Standard Model (SM) can be explored systematically through Effective Field Theories (EFTs) such as the Standard Model EFT (SMEFT). The SMEFT, a framework for probing BSM effects, extends the SM by introducing higher-dimensional operators parameterized by Wilson coefficients. This contribution highlights three recent analyses using the ATLAS Run-2 dataset at a center-of-mass energy of $\sqrt{s}$ = 13 TeV with an integrated luminosity of 140 $\text{fb}^{-1}$. Combined measurements enhance sensitivity to Wilson coefficients, exploring the potential of SMEFT in the top quark sector.
The Large Hadron Collider (LHC) offers a unique opportunity to investigate $\mathcal{CP}$ violation in the Yukawa coupling between the Higgs boson and the top quark by studying Higgs production in association with top quarks; this is of fundamental importance, seeing that the $\mathcal{CP}$ properties of the Higgs boson are yet to measure with high precision. To address this, the focus of this work has been an extension of the simplified template cross-section (STXS) framework, devised to be sensitive to $\mathcal{CP}$ effects. Our study focused on $\mathcal{CP}$-sensitive observables across multiple Higgs decay channels, comparing their performances. The result indicates that the most efficient extension of the current binning used in the STXS framework, which currently uses the Higgs boson's transverse momentum $p_{T,H}$, requires adding one further split using $\mathcal{CP}$-sensitive observables. Between these observables, one of the best is the Collins-Soper angle $|\cos\theta^*|$, a variable derived from momenta information of the top quarks. We have investigated the improvement brought by our two-dimensional STXS setup and compared it to the currently employed methodologies, finding an increase in performances at an integrated luminosity of $300 \mathrm{ fb}^{-1}$. Moreover, our results highlight that this advantage seems to be present also at $3000 \mathrm{ fb}^{-1}$.
Cross-section measurements of the associated production of a top quark pair and a photon ($t\bar{t}\gamma$) are performed with an integrated luminosity of 140 fb$^{-1}$ of proton−proton collisions at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the LHC. The measurement focuses on $t\bar{t}\gamma$ topologies where the photon is radiated from an initial-state parton or one of the top quarks. The differential cross-sections are measured for variables characterising the photon, lepton and jet kinematic properties. The distribution of the photon transverse momentum is used to constrain effective field theory operators related to the electroweak dipole moments of the top quark.
This talk presents the ATLAS Collaboration's first measurement of the inclusive cross-section for top-quark pair production in association with charm quarks. Using the full Run~2 proton--proton collision data sample at $\sqrt{s} = 13\,\text{TeV}$, collected with the ATLAS experiment at the LHC between 2015 and 2018, the measurement selects $t\bar{t}$ events with one or two charged leptons and at least one additional jet in the final state. A custom flavour-tagging algorithm is employed to simultaneously identify $b$-jets and $c$-jets. The fiducial cross-sections for $t\bar{t}+{≥}2c$ and $t\bar{t}+1c$ production are found to largely agree with predictions from various $t\bar{t}$ simulations, though all underpredict the observed values.
