| 1. |
- Bokan, Petar
(författare)
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Pair production of Higgs bosons in the final state with bottom quarks and τ leptons in the ATLAS experiment : Search results using LHC Run 2 data and prospect studies at the HL-LHC
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- After the discovery of the Higgs boson, the ultimate test of the electroweak symmetry breaking and the Standard Model (SM) of particle physics is to establish evidence of Higgs boson self-coupling, which can be achieved by searching for pair production of Higgs bosons. In addition, many theories beyond the SM predict heavy resonances that could decay into pairs of Higgs bosons. A search for non-resonant and resonant pair production of Higgs bosons in the final state with two bottom quarks and two τ leptons (bb̄τ+ τ−) is presented for 36.1 fb-1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded by the ATLAS experiment at the Large Hadron Collider (LHC). The observed (expected) 95% confidence level (CL) upper limit on the non-resonant Higgs boson pair production cross-section times the bb̄τ+τ- branching ratio corresponds to 12.7 (14.8) times the SM prediction. The ratio of the Higgs boson self-coupling to its SM expectation, κλ, is observed (expected) to be constrained to κλ ∈ [-7.4, 15.7] ([-8.9, 16.8]) at 95% CL. The sensitivity is extrapolated to a 14 TeV centre-of-mass energy and 3000 fb-1, which is the target integrated luminosity of the High-Luminosity LHC (HL-LHC). Various extrapolation assumptions are taken into account. The estimated expected signal significance for the SM Higgs boson pair production in the bb̄τ+τ- final state at the HL-LHC reaches 2.1 standard deviations, while the expected allowed κλ interval is κλ ∈ [-0.8, 8.8] at 95% CL, assuming the SM Higgs boson couplings. Furthermore, potential improvements of the analysis techniques are discussed in the context of searches for pair production of Higgs bosons with the full Run 2 ATLAS data.
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| 2. |
- Madsen, Alexander, 1987-
(författare)
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Hunting the Charged Higgs Boson with Lepton Signatures in the ATLAS Experiment
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents searches for a charged Higgs boson (H±) in proton-proton collisions with center-of-mass energies of 7 TeV and 8 TeV, using data collected by the ATLAS experiment at the Large Hadron Collider at CERN. Multiple search channels are used with the common characteristic of at least one charged lepton (electron or muon) that effectively reduces the multi-jet background and is used for efficient triggering. Charged Higgs bosons decaying to a tau lepton and a neutrino are searched for using final states with two charged leptons, or one charged lepton and a hadronically decaying tau. A significant background originates from quark- or gluon-initiated jets that may be misidentified as hadronic tau decays. Methods to estimate this background are presented, including a largely data-driven matrix method. Signal processes with a charged Higgs boson mass below or above that of the top quark are considered. With the dataset collected at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb-1, upper limits at 95% confidence level are placed on the branching fraction B(t→bH±)×B(H±→τν) in the range 1.1–0.3% for charged Higgs boson masses between 80 GeV and 160 GeV, and on the top-quark associated H± production cross section in the range 0.93 to 0.03 pb for charged Higgs boson masses between 180 GeV and 1 TeV.
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| 3. |
- Pelikan, Daniel, 1985-
(författare)
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Searches for a Charged Higgs Boson in ATLAS and Development of Novel Technology for Future Particle Detector Systems
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The discovery of a charged Higgs boson (H±) would be a clear indication for physics beyond the Standard Model. This thesis describes searches for charged Higgs bosons with the ATLAS experiment at CERN’s Large Hadron Collider (LHC). The first data collected during the LHC Run 1 is analysed, searching for a light charged Higgs boson (mH±<mtop), which decays predominantly into a tau-lepton and a neutrino. Different final states with one or two leptons (electrons or muons), as well as leptonically or hadronically decaying taus, are studied, and exclusion limits are set.The background arising from misidentified non-prompt electrons and muons was estimated from data. This so-called "Matrix Method'' exploits the difference in the lepton identification between real, prompt, and misidentified or non-prompt electrons and muons. The Matrix Method is used in all charged Higgs boson searches in this thesis.In 2024 the LHC will be upgraded into a High Luminosity LHC (HL-LHC). The ATLAS detector is expected to collect around 300 fb-1 of collision data until 2022, whereas the HL-LHC will deliver about 250-300 fb-1 of data per year. This will increase the mean number of interactions per bunch crossing, resulting in larger particle fluxes. This puts challenging requirements on the electronics. In order to keep trigger and data rates at manageable levels, new trigger concepts require more intelligence at early stage which possibly results in more cables and connectors, inside the detector which lead to degraded performance of the detector system.This thesis presents new concepts using wireless technology at 60 GHz, in order add more data links inside the detector system without adding much material. Patch antennas have been developed, operating at 60 GHz. Manufacture methods have been investigated, and the fabrication tolerances and bandwidth of these antennas have been studied. Also, concepts of using passive repeaters have been investigated, to make the 60 GHz signal pass boundaries. These repeaters can be used to connect intelligence inside the detector, but also for reading out data from the whole detector radially.
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| 4. |
- Dimitriadi, Christina
(författare)
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Double Higgs τrouble : Searches for Higgs boson pairs in the ATLAS experiment and their interpretations
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the discovery of the Higgs boson by the ATLAS and CMS experiments at the Large Hadron Collider (LHC), extensive measurements of its properties have confirmed the predictions of the Standard Model (SM). However, the Higgs boson self-coupling, which is related to the shape of the Higgs potential, still remains loosely constrained experimentally. Searches for the production of Higgs boson pairs (HH) are of great interest, especially for measuring the Higgs boson self-coupling. While the SM predicts a very small event rate for this process, modifications of the Higgs boson self-coupling or new couplings introduced in effective field theories (EFTs) can lead to enhancements of the HH cross-section. This thesis focuses on searches for, and interpretations of, non-resonant Higgs boson pair production in the final state with two b-quarks and two τ-leptons using 140 fb-1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded by the ATLAS experiment. This final state offers a sizeable branching ratio and moderate background rates, making it one of the most appealing search channels for HH production. No statistically significant signal excess is found above the expected background, therefore upper limits are set on the HH signal strength. The search yields an observed (expected) upper limit of 5.9 (3.3) times the SM cross-section prediction at 95% confidence level. In turn, the ratio of the Higgs boson self-coupling to its SM value, κλ, is constrained to an observed (expected) 95% confidence interval of [-3.1, 9.0] ([-2.5, 9.3]). The results are also interpreted within two distinct EFT frameworks, the Higgs EFT (HEFT) and Standard Model EFT (SMEFT), through constraints on the respective Wilson coefficients and upper limits on the HH cross-section for seven HEFT shape benchmarks with representative features in the HH invariant mass spectrum. Looking forward, the sensitivity of the search is explored at the High-Luminosity LHC by extrapolating the results to 14 TeV and 3000 fb-1. The projected signal significance assuming SM kinematics reaches 2.8σ, while κλ is expected to be constrained to the 2σ confidence interval [-0.3, 7.4].
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