Vector Boson Scattering and Electroweak Production of Two Like-Charge W Bosons and Two Jets at the Current and Future ATLAS Detector

The scattering of electroweak gauge bosons is closely connected to the electroweak gauge symmetry and its spontaneous breaking through the Brout-Englert-Higgs mechanism. Since it contains triple and quartic gauge boson vertices, the measurement of this scattering process allows to probe the self-interactions of weak bosons. The contribution of the Higgs boson to the weak boson scattering amplitude ensures unitarity of the scattering matrix. Therefore, the scattering of massive electroweak gauge bosons is sensitive to deviations from the Standard Model prescription of the electroweak interaction and of the properties of the Higgs boson. At the Large Hadron Collider (LHC), the scattering of massive electroweak gauge bosons is accessible through the measurement of purely electroweak production of two jets and two gauge bosons. No such process has been observed before. Being the channel with the least amount of background from QCD-mediated production of the same final state, the most promising channel for the first measurement of a process containing massive electroweak gauge boson scattering is the one with two like-charge W bosons and two jets in the final state. This thesis presents the first measurement of electroweak production of two jets and two identically charged W bosons, which yields the first observation of a process with contributions from quartic gauge interactions of massive electroweak gauge bosons. An overview of the most important issues in Monte Carlo simulation of vector boson scattering processes with current Monte Carlo generators is given in this work. The measurement of the final state of two jets and two leptonically decaying same-charge W bosons is conducted based on proton-proton collision data with a center-of-mass energy of √s = 8 TeV, taken in 2012 with the ATLAS experiment at the LHC. The cross section of electroweak production of two jets and two like-charge W bosons is measured with a significance of 3.6 standard deviations to be σ(W± W±jj−EW[fiducial]) = 1.3 ± 0.4(stat.) ± 0.2(syst.) fb in a fiducial phase space region selected to enhance the contribution from W W scattering. The measurement is compatible with the Standard Model prediction of σ(W±W± jj−EW[fiducial]) = 0.95 ± 0.06 fb. Based on this measurement, limits on anomalous quartic gauge couplings are derived. The effect of anomalous quartic gauge couplings is simulated within the framework of an effective chiral Lagrangian unitarized with the K-matrix method. The limits for the anomalous coupling parameters α4 and α5 are found to be −0.14 < α4 < 0.16 and −0.23 < α5 < 0.24 at 95 % confidence level. Furthermore, the prospects for the measurement of the electroweak production of two same-charge W bosons and two jets within the Standard Model and with additional doubly charged resonances after the upgrade of the ATLAS detector and the LHC are investigated. For a high-luminosity LHC with a center-of-mass energy of √s = 14 TeV, the significance of the measurement with an integrated luminosity of 3000 fb^−1 is estimated to be 18.7 standard deviations. It can be improved by 30 % by extending the inner tracking detector of the atlas experiment up to an absolute pseudorapidity of |η| = 4.0

Integration of NEMO into an existing particle physics environment through virtualization

With the ever-growing amount of data collected with the experiments at the Large Hadron Collider (LHC) (Evans et al., 2008), the need for computing resources that can handle the analysis of this data is also rapidly increasing. This increase will even be amplified after upgrading to the High Luminosity LHC (Apollinari et al., 2017). High-Performance Computing (HPC) and other cluster computing resources provided by universities can be useful supplements to the resources dedicated to the experiment as part of the Worldwide LHC Computing Grid (WLCG) (Eck et al., 2005) for data analysis and production of simulated event samples. Computing resources in the WLCG are structured in four layers – so-called Tiers. The first layer comprises two Tier-0 computing centres located at CERN in Geneva, Switzerland and at the Wigner Research Centre for Physics in Budapest, Hungary. The second layer consists of thirteen Tier-1 centres, followed by 160 Tier-2 sites, which are typically universities and other scientific institutes. The final layer are Tier-3 sites which are directly used by local users. The University of Freiburg is operating a combined Tier-2/Tier-3, the ATLAS-BFG (Backofen et al., 2006). The shared HPC cluster »NEMO« at the University of Freiburg has been made available to local ATLAS (Aad et al., 2008) users through the provisioning of virtual machines incorporating the ATLAS software environment analogously to the bare metal system at the Tier-3. In addition to the provisioning of the virtual environment, the on-demand integration of these resources into the Tier-3 scheduler in a dynamic way is described. In order to provide the external NEMO resources to the user in a transparent way, an intermediate layer connecting the two batch systems is put into place. This resource scheduler monitors requirements on the user-facing system and requests resources on the backend-system

Double Higgs boson production and Higgs self-coupling extraction at CLIC

AbstractThe Compact Linear Collider (CLIC) is a future electron–positron collider that will allow measurements of the trilinear Higgs self-coupling in double Higgs boson events produced at its high-energy stages with collision energies from $$\sqrt{s}$$ s  = 1.4 to 3 TeV. The sensitivity to the Higgs self-coupling is driven by the measurements of the cross section and the invariant mass distribution of the Higgs-boson pair in the W-boson fusion process, $$\text {e}^{+}\text {e}^{-}\rightarrow {\text {H}\text {H}\nu \bar{\nu }}$$ e + e - → HH ν ν ¯ . It is enhanced by including the cross-section measurement of ZHH production at 1.4 TeV. The expected sensitivity of CLIC for Higgs pair production through W-boson fusion is studied for the decay channels $$\mathrm{b}\bar{\mathrm{b}}\mathrm{b}\bar{\mathrm{b}}$$ b b ¯ b b ¯   and $$\mathrm{b}\bar{\mathrm{b}}\mathrm{W}\mathrm{W}^{*}$$ b b ¯ W W ∗   using full detector simulation including all relevant backgrounds at $$\sqrt{s}$$ s = 1.4 TeV with an integrated luminosity of $$\mathcal {L}$$ L  = 2.5 ab$$^{-1}$$ - 1 and at $$\sqrt{s}$$ s = 3 TeV with $$\mathcal {L}$$ L  = 5 ab$$^{-1}$$ - 1 . Combining $$\text {e}^{+}\text {e}^{-}\rightarrow {\text {H}\text {H}\nu \bar{\nu }}$$ e + e - → HH ν ν ¯ and ZHH  cross-section measurements at 1.4 TeV with differential measurements in $$\text {e}^{+}\text {e}^{-}\rightarrow {\text {H}\text {H}\nu \bar{\nu }}$$ e + e - → HH ν ν ¯ events at 3 TeV, CLIC will be able to measure the trilinear Higgs self-coupling with a relative uncertainty of $$-8\%$$ - 8 % and $$+11\%$$ + 11 % at 68% C.L., assuming the Standard Model. In addition, prospects for simultaneous constraints on the trilinear Higgs self-coupling and the Higgs-gauge coupling HHWW are derived based on the $${\text {H}\text {H}\nu \bar{\nu }}$$ HH ν ν ¯ measurement.</jats:p

Dynamic Virtualized Deployment of Particle Physics Environments on a High Performance Computing Cluster

The NEMO High Performance Computing Cluster at the University of Freiburg has been made available to researchers of the ATLAS and CMS experiments. Users access the cluster from external machines connected to the World-wide LHC Computing Grid (WLCG). This paper describes how the full software environment of the WLCG is provided in a virtual machine image. The interplay between the schedulers for NEMO and for the external clusters is coordinated through the ROCED service. A cloud computing infrastructure is deployed at NEMO to orchestrate the simultaneous usage by bare metal and virtualized jobs. Through the setup, resources are provided to users in a transparent, automatized, and on-demand way. The performance of the virtualized environment has been evaluated for particle physics applications

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements