Study of ordered hadron chains with the ATLAS detector

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A search for resonances decaying into a Higgs boson and a new particle X in the XH→qqbb final state with the ATLAS detector

A search for heavy resonances decaying into a Higgs boson ($H$) and a new particle ($X$) is reported, utilizing 36.1 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} =$ 13 TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. The particle $X$ is assumed to decay to a pair of light quarks, and the fully hadronic final state $XH \rightarrow q\bar q'b\bar b$ is analysed. The search considers the regime of high $XH$ resonance masses, where the $X$ and $H$ bosons are both highly Lorentz-boosted and are each reconstructed using a single jet with large radius parameter. A two-dimensional phase space of $XH$ mass versus $X$ mass is scanned for evidence of a signal, over a range of $XH$ resonance mass values between 1 TeV and 4 TeV, and for $X$ particles with masses from 50 GeV to 1000 GeV. All search results are consistent with the expectations for the background due to Standard Model processes, and 95% CL upper limits are set, as a function of $XH$ and $X$ masses, on the production cross-section of the $XH\rightarrow q\bar q'b\bar b$ resonance

Study of the material of the ATLAS inner detector for Run 2 of the LHC

Instituto de Física La Plat

Search for the Flavor-Changing Neutral Current, tt →\rightarrow to qqγ\gamma, in Top Pair Events Using the ATLAS Detector

This dissertation presents the search for flavor-changing neutral currents in processes with top pairs where one top decays through the flavor-changing neutral current decay mode (to an up-type quark and a photon) and the other decays through the most common Standard Model mode to a b-quark and a W boson. The W boson then decays leptonically, defining the channels (electron+jets and muon+jets) searched for. This search uses the entire Run-2 dataset of √s = 13 TeV data collected using the ATLAS experiment between 2015 and 2018, corresponding to a total integrated luminosity of 139 fb$^{−1}$. Observation of the flavor-changing neutral current decays in top quarks would be an unambiguous sign of physics beyond the Standard Model. A neural network was developed to separate signal and background events using both low-level kinematic variables as well as physics-guided high-level variable combinations as inputs. The candidate signal events contain an isolated very high $_{pT}$ photon, a lepton (either an electron or muon), a b-tagged jet, at least one more light jet, and missing transverse energy. Various data-driven techniques were used to estimate contributions to the background from events with a hard scatter photon or a photon faked by either a jet or an electron. As there is no sensitivity for observation of the Standard Model flavor-changing neutral current process, upper limits on the observed (expected) branching ratio and cross section of these processes are set: BR($t$ → $q$$\gamma$)< 9.6 × 10$^{−5}$(11.0 × 10$^{−5}$) and σ($pp$ → $t$$\bar{t}$ → $bWq$$\gamma$) < 50(60)fb. This dissertation includes previously unpublished co-authored material

Viscoelastic hydrodynamics and holography

We formulate the theory of nonlinear viscoelastic hydrodynamics of anisotropic crystals in terms of dynamical Goldstone scalars of spontaneously broken translational symmetries, under the assumption of homogeneous lattices and absence of plastic deformations. We reformulate classical elasticity effective field theory using surface calculus in which the Goldstone scalars naturally define the position of higher-dimensional crystal cores, covering both elastic and smectic crystal phases. We systematically incorporate all dissipative effects in viscoelastic hydrodynamics at first order in a long-wavelength expansion and study the resulting rheology equations. In the process, we find the necessary conditions for equilibrium states of viscoelastic materials. In the linear regime and for isotropic crystals, the theory includes the description of Kelvin-Voigt materials. Furthermore, we provide an entirely equivalent description of viscoelastic hydrodynamics as a novel theory of higher-form superfluids in arbitrary dimensions where the Goldstone scalars of partially broken generalised global symmetries play an essential role. An exact map between the two formulations of viscoelastic hydrodynamics is given. Finally, we study holographic models dual to both these formulations and map them one-to-one via a careful analysis of boundary conditions. We propose a new simple holographic model of viscoelastic hydrodynamics by adopting an alternative quantisation for the scalar fields