Author: Ruth Magdalena JacobsPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Probing New Physics with Boosted H→bb̄ Decays with the ATLAS Detector at 13 TeV
Author: Ruth Magdalena JacobsProbing New Physics with Boosted H→b ̄b Decays with the ATLAS Detector at 13 TeV
Author: Ruth Magdalena JacobsMeasurement of the TtH Production Cross-section with a Collimated H -> Bb Decay in Pp Collisions at Sqrt(s)
Author: Eftychia TzovaraPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
The predictions of the Standard Model (SM) of particle physics have been probed with remarkable accuracy, so far. The Large Hadron Collider (LHC) at CERN has significantly contributed to this quest. A remarkable achievement of the ATLAS and CMS experiments at the LHC was the discovery of the Higgs boson in 2012, the last missing piece of the SM. With the increasing amount of proton-proton collisions delivered by the LHC, more precise measurements of the Higgs boson are now possible, while rare processes are accessible as well. A property of the Higgs boson that is of particular importance is its coupling to the top quark, which is expected to be the strongest in the SM due to the high mass of the top quark. Therefore, its precise measurement is a stringent test of the SM. A direct measurement of the top-quark Yukawa coupling can be assessed through the Higgs-boson production in association with a pair of top quarks (ttH). This thesis presents the measurement of the ttH process with a subsequent Higgs-boson decay to a pair of b-quarks (H -> bb), the decay mode with the largest branching ratio. The measurement is performed with data collected by the ATLAS detector, corresponding to an integrated luminosity of 139 fb^-1 at a center-of-mass energy of 13 TeV. Events with one or two charged leptons from the tt decay in the final state are considered to the measurement. The main challenge of the ttH(H -> bb) channel emerges from the large SM backgrounds from the production of top-quark pairs with additional jets (tt+jets). Also the many jets coming from b-hadrons (b-jets) in the final state cause combinatorial ambiguities. Thus, the identification of such jets is decisive in order to determine the signal and reject many background processes. The ttH events are split into exclusive analysis regions, based on the number of leptons, jets, and jets tagged as b-jets, providing regions enhanced in signal, or in the main background components. Specifically in the single-lepton channel, a boosted category is defined by selecting events in which the Higgs boson and possibly also the hadronically decaying top quark are produced with high transverse momentum (pT), with their decay products being collimated in large-radius jets. The single-lepton boosted channel targets events with Higgs-boson candidate pT >= 300 GeV and is the main scope of this thesis. To identify the reconstructed objects with the underlying particles and to maximise the discrimination of the ttH signal from the overwhelming tt+jets background events in the signal-enriched regions, machine-learning algorithms are employed. The background is dominated by a tt process with an additional gluon in the final state which further splits into a pair of b-quarks (tt+bb). Besides, a large number of heavy-flavour jets in the final state is not well modelled, thus many systematic uncertainties have to be considered, decreasing the sensitivity of the measurement. All the defined analysis regions are analysed together in a combined profile likelihood fit to test for the presence of signal. The fit simultaneously determines the event yields for the signal and the most important background component, while constraining the overall background model within the assigned systematic uncertainties. Eventually, the ratio of the measured ttH cross section to the SM expectation in the inclusive cross-section measurement is found to be 0.35 +0.36,-0.34}, corresponding to an observed (expected) significance of 1.0 (2.7) standard deviations. A ttH signal strength larger than the SM prediction is excluded at 95% confidence level. The measurement uncertainty is dominated by systematic uncertainties, mainly regarding the theoretical knowledge of the tt +>= 1b background process. Finally, to further test the SM, the cross-section is measured differentially as a function of the generator-level Higgs-boson pT, taking advantage of the reconstruction of the Higgs-boson kinematics.
Search for New Phenomena in Dijet Angular Distributions at √s = 8 and 13 TeV
Author: Lene Kristian BryngemarkPublisher: Springer
ISBN: 3319673467
Category : Science
Languages : en
Pages : 177
Book Description
This thesis describes the use of the angular distributions of the most energetic dijets in data recorded by the ATLAS experiment, at CERN’s Large Hadron Collider (LHC), the goal of which is to search for phenomena beyond what the current theory of Particle Physics (the Standard Model) can describe. It also describes the deployment of the method used in ATLAS to correct for the distortions in jet energy measurements caused by additional proton–proton interactions. The thesis provides a detailed introduction to understanding jets and dijet searches at the LHC. The experiments were carried out at two record collider centre-of-mass energies (8 and 13 TeV), probing smaller distances than ever before. Across a broad momentum transfer range, the proton constituents (quarks and gluons) display the same kinematical behaviour, and thus still appear to be point-like. Data are compared to predictions corrected for next-to-leading order quantum chromodynamics (NLO QCD) as well as electroweak effects, demonstrating excellent agreement. The results are subsequently used to set limits on parameters of suggested theoretical extensions to the Standard Model (SM), including the effective coupling and mass of a Dark Matter mediator.
Higgs Boson Decays into a Pair of Bottom Quarks
Author: Cecilia TosciriPublisher: Springer Nature
ISBN: 3030879380
Category : Science
Languages : en
Pages : 171
Book Description
The discovery in 2012 of the Higgs boson at the Large Hadron Collider (LHC) represents a milestone for the Standard Model (SM) of particle physics. Most of the SM Higgs production and decay rates have been measured at the LHC with increased precision. However, despite its experimental success, the SM is known to be only an effective manifestation of a more fundamental description of nature. The scientific research at the LHC is strongly focused on extending the SM by searching, directly or indirectly, for indications of New Physics. The extensive physics program requires increasingly advanced computational and algorithmic techniques. In the last decades, Machine Learning (ML) methods have made a prominent appearance in the field of particle physics, and promise to address many challenges faced by the LHC. This thesis presents the analysis that led to the observation of the SM Higgs boson decay into pairs of bottom quarks. The analysis exploits the production of a Higgs boson associated with a vector boson whose signatures enable efficient triggering and powerful background reduction. The main strategy to maximise the signal sensitivity is based on a multivariate approach. The analysis is performed on a dataset corresponding to a luminosity of 79.8/fb collected by the ATLAS experiment during Run-2 at a centre-of-mass energy of 13 TeV. An excess of events over the expected background is found with an observed (expected) significance of 4.9 (4.3) standard deviation. A combination with results from other \Hbb searches provides an observed (expected) significance of 5.4 (5.5). The corresponding ratio between the signal yield and the SM expectation is 1.01 +- 0.12 (stat.)+ 0.16-0.15(syst.). The 'observation' analysis was further extended to provide a finer interpretation of the V H(H → bb) signal measurement. The cross sections for the VH production times the H → bb branching ratio have been measured in exclusive regions of phase space. These measurements are used to search for possible deviations from the SM with an effective field theory approach, based on anomalous couplings of the Higgs boson. The results of the cross-section measurements, as well as the constraining of the operators that affect the couplings of the Higgs boson to the vector boson and the bottom quarks, have been documented and discussed in this thesis. This thesis also describes a novel technique for the fast simulation of the forward calorimeter response, based on similarity search methods. Such techniques constitute a branch of ML and include clustering and indexing methods that enable quick and efficient searches for vectors similar to each other. The new simulation approach provides optimal results in terms of detector resolution response and reduces the computational requirements of a standard particles simulation.
Measurements of WH and ZH Production in the H->bb Decay Channel in Pp Collisions at 13TeV with the ATLAS Detector
Author: ATLAS Collaboration CERNPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Abstract: Measurements of the Standard Model Higgs boson decaying into a bb ̄ pair and produced in association with a W or Z boson decaying into leptons, using proton-proton collision data collected between 2015 and 2018 by the ATLAS detector, are presented. The measurements use collisions produced by the Large Hadron Collider at a centre-of-mass energy of s√=13TeV, corresponding to an integrated luminosity of 139fb−1. The production of a Higgs boson in association with a W or Z boson is established with observed (expected) significances of 4.0 (4.1) and 5.3 (5.1) standard deviations, respectively. Cross-sections of associated production of a Higgs boson decaying into bottom quark pairs with an electroweak gauge boson, W or Z, decaying into leptons are measured as a function of the gauge boson transverse momentum in kinematic fiducial volumes. The cross-section measurements are all consistent with the Standard Model expectations, and the total uncertainties vary from 30% in the high gauge boson transverse momentum regions to 85% in the low regions. Limits are subsequently set on the parameters of an effective Lagrangian sensitive to modifications of the WH and ZH processes as well as the Higgs boson decay into bb ̄
Searching for New Physics in Decays of the Higgs Boson with the ATLAS Detector
Author: Elliot Thomas James ReynoldsPublisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
A Beauty-ful Boson
Author: Giulia Di GregorioPublisher: Springer Nature
ISBN: 3031200136
Category : Science
Languages : en
Pages : 229
Book Description
The analysis described in this thesis is the search for the Higgs boson, decaying into bb pair, in the associated production with a vector boson, in the extreme Higgs boson transverse momentum region where the Higgs boson is reconstructed using the large-R jet technique. The use of the large-R jets allows to add a part of the phase space unexplored so far, which is particularly sensitive to possible new physics. The analysed data have been collected at LHC by the ATLAS detector between 2015 and 2018 at a centre-of-mass energy of √s = 13 TeV. The same dataset has been used to perform the differential pp → ZH and pp → WH cross-section measurements used to extract the information on the Higgs couplings and to put limits on Beyond the Standard Model effects. Furthermore the analysis has been re-used to perform a cross-section measurement of the diboson ZZ and WZ processes because the diboson and the Higgs processes have a similar topology. For the first time the ZZ(bb) and WZ(bb) cross-sections are measured at √s = 13 TeV and the observed cross-section measurements are consistent with the Standard Model predictions.
Electroweak Interactions and Unified Theories
Author: J. Thanh Van TranPublisher: Atlantica Séguier Frontières
ISBN: 9782863320556
Category : Dark matter (Astronomy)
Languages : en
Pages : 616
Book Description
Observation of H→bb Decays and VH Production with the ATLAS Detector, Anglais
Author: Yanhui MaPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
A search for the Standard Model Higgs boson produced in association with a W or Z boson, and decaying to a bb pair has been performed with ATLAS detector. The data were collected in proton-proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV, and correspond to an integrated luminosity of 79.8 fb−1. Three channels containing zero, one and two charged leptons (electrons or muons) have been considered to target each of the leptonic decays of the W or Z boson, Z->vv, W->lv et Z->ll, referred to as as the 0-lepton, 1-lepton and 2-lepton channels, respectively. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 4.9 standard deviations, compared to an expectation of 4.3 standard deviations. The ratio of the measured signal events to the Standard Model expectation equal 1.16 +0.27/-0.25 = 1.16 +/-0.16(stat) +0.21/-0.19(syst). The result is also combined with the other results from the searches for the Higgs boson in the bb 18 decay mode in Run 1 and Run 2, the combination yields an observed (expected) significance of 5.4 (5.5) 20 standard deviations, and therefore provides a direct observation of the Higgs boson decay into a bb pair. In addition, a combination of Run 2 results searching for the Higgs boson produced in association with a W or Z boson yields an observed (expected) significance of 5.3 (4.8) standard deviations, and therefore provides a direct observation of Higgs boson being produced in association with a W or Z boson.