Search for Higgs Bosons Beyond the Standard Model in Early ATLAS Data PDF Download

Author: Miika Anttoni Klemetti
Publisher:
ISBN: 9780494786536
Category :
Languages : en
Pages :

View

Book Description

Author: Giulia Di Gregorio
Publisher: Springer Nature
ISBN: 3031200136
Category : Science
Languages : en
Pages : 229

View

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.

Author: Marc Christopher Thomas
Publisher: Springer
ISBN: 3319434527
Category : Science
Languages : en
Pages : 113

View

Book Description
This thesis studies collider phenomenology of physics beyond the Standard Model at the Large Hadron Collider (LHC). It also explores in detail advanced topics related to Higgs boson and supersymmetry – one of the most exciting and well-motivated streams in particle physics. In particular, it finds a very large enhancement of multiple Higgs boson production in vector-boson scattering when Higgs couplings to gauge bosons differ from those predicted by the Standard Model. The thesis demonstrates that due to the loss of unitarity, the very large enhancement for triple Higgs boson production takes place. This is a truly novel finding. The thesis also studies the effects of supersymmetric partners of top and bottom quarks on the Higgs production and decay at the LHC, pointing for the first time to non-universal alterations for two main production processes of the Higgs boson at the LHC–vector boson fusion and gluon–gluon fusion. Continuing the exploration of Higgs boson and supersymmetry at the LHC, the thesis extends existing experimental analysis and shows that for a single decay channel the mass of the top quark superpartner below 175 GeV can be completely excluded, which in turn excludes electroweak baryogenesis in the Minimal Supersymmetric Model. This is a major new finding for the HEP community. This thesis is very clearly written and the introduction and conclusions are accessible to a wide spectrum of readers.

Author: Hongtao Yang
Publisher:
ISBN:
Category :
Languages : en
Pages : 156

View

Book Description
With 4.8~$\rm{fb}^{-1}$ of proton-proton collision data collected at $\sqrt{s}=7~\rm{TeV}$ in 2011, and 5.9~$\rm{fb}^{-1}$ collected at $\sqrt{s}=8~\rm{TeV}$ in 2012 by the ATLAS detector at the Large Hadron Collider, an excess of 4.5 standard deviations from the background-only hypothesis is observed near 126.5~GeV in the diphoton invariant mass spectra. Along with the excesses observed in the $H \rightarrow ZZ^{(*)}\rightarrow \ell\ell\ell\ell$ and $H \rightarrow WW^{(*)}\rightarrow \ell\nu\ell\nu$ channels, the observation of a Higgs-like particle is established at 6.0 standard deviations level. With more data accumulated during LHC Run~1, the measurements of Higgs boson couplings and mass in the $H\to\gamma\gamma$ channel are conducted by the ATLAS experiment based on 4.5~$\rm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=7~\rm{TeV}$ collected in 2011, and 20.3~$\rm{fb}^{-1}$ at $\sqrt{s}=8~\rm{TeV}$ collected in 2012. The combined signal strength, defined as number of observed Higgs boson decays to diphoton divided by the corresponding Standard Model prediction, is measured to be $1.17 \ ^{+0.28}_{-0.26}$ assuming the Higgs boson mass being 125.4~$\rm{GeV}$. The signal strengths for individual Higgs boson production processes are also measured, and are found to be in good consistency with the Standard Model. The mass of the Higgs boson is measured in $H\to\gamma\gamma$ channel by the ATLAS experiment to be $125.98 \pm 0.50$~\GeV. This measurement is combined with the ones from ATLAS $H \rightarrow ZZ^{(*)}\rightarrow \ell\ell\ell\ell$ as well as CMS $H\to\gamma\gamma$ and $H \rightarrow ZZ^{(*)}\rightarrow \ell\ell\ell\ell$. The Higgs boson mass measured from the combination is $125.09\pm0.24~\rm{GeV}$. With LHC center-of-mass energy increased to 13~TeV, a search for high mass Beyond the Standard Model scalar resonance is performed in the diphoton decay channel based on 15.4~$\rm{fb}^{-1}$ of proton-proton collision data collected by the ATLAS detector during 2015 and 2016. While a notable wide excess was first observed in the diphoton invariant mass spectrum from the 2015 data (3.2~$\rm{fb}^{-1}$) with mass near 750~GeV, it is not confirmed by the 2016 data with much higher statistics (12.4~$\rm{fb}^{-1}$). Limits on the production cross section times branching ratio of such resonances are set.

Author: Aleandro Nisati
Publisher: World Scientific
ISBN: 981442546X
Category : Science
Languages : en
Pages : 470

View

Book Description
The recent observation of the Higgs boson has been hailed as the scientific discovery of the century and led to the 2013 Nobel Prize in physics. This book describes the detailed science behind the decades-long search for this elusive particle at the Large Electron Positron Collider at CERN and at the Tevatron at Fermilab and its subsequent discovery and characterization at the Large Hadron Collider at CERN. Written by physicists who played leading roles in this epic search and discovery, this book is an authoritative and pedagogical exposition of the portrait of the Higgs boson that has emerged from a large number of experimental measurements. As the first of its kind, this book should be of interest to graduate students and researchers in particle physics.

Author: Brian Moser
Publisher: Springer Nature
ISBN: 3031394429
Category : Science
Languages : en
Pages : 250

View

Book Description
Precision measurements of the Higgs boson’s properties are a powerful tool to look for deviations from the predictions of the Standard Model (SM) of particle physics. The 139/fb of proton-proton collision data which have been collected by the ATLAS experiment during Run 2 of the LHC, offer an opportunity to investigate rare Higgs-boson topologies, which are particularly sensitive to new physics scenarios but experimentally difficult to access. Several such measurements, which target Higgs-boson decays to heavy-flavour quarks, as well as their combinations are presented in this thesis. A novel analysis that measures Higgs-boson production in association with a heavy vector boson V (VH, with V=W,Z) at high energies is presented. Dedicated Higgs-boson reconstruction techniques are applied to reconstruct the highly Lorentz-boosted Higgs-boson decays into pairs of bottom quarks. The measurement is subsequently combined with a VH cross-section measurement at low and intermediate pT(V) to provide a differential cross-section measurement in kinematic fiducial volumes over the largest possible pT(V) range. All cross-section measurements agree with the SM predictions within relative uncertainties that range from 30% to 300%. The results are furthermore interpreted as limits on the parameters of a SM effective field theory. Finally, a combination of measurements of Higgs decays to heavy-flavour quarks is used to experimentally determine that the Higgs-boson coupling to charm quarks is weaker than to bottom quarks, as predicted by the SM. The target audience for the thesis are physicists and physics students, in particular those with a background in high energy physics.

Author: Sean Gasiorowski
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

View

Book Description
This thesis discusses searches for pair production of Higgs bosons in the bbbb final state using data recorded by the ATLAS detector from $\sqrt{s} = 13$ TeV proton-proton ($pp$) collisions during the full period of LHC Run 2. It develops two separate analysis strategies: one targeting resonant pair production of Higgs bosons, in which Beyond the Standard Model resonances are produced which subsequently decay to two Higgs bosons, and one targeting non-resonant pair production of Higgs bosons, which is sensitive to Standard Model $HH$ production as well as to variations of the Higgs trilinear self-coupling. In the resonant searches, no significant excesses are seen, and upper limits on cross section are set on both spin-0 and spin-2 resonant hypotheses. Such limits are competitive with other leading ATLAS full Run 2 searches, and represent a significantly stronger statement than previous, beating the early Run 2 combined ATLAS results above 350 GeV and leading the ATLAS full Run 2 sensitivity above 700 GeV. In the non-resonant, no evidence of Standard Model $HH$ production is seen, but upper limits on cross section of $pp\rightarrow HH$ via gluon-gluon fusion are set to be 4.4 (5.9) observed (expected) times the value predicted by the Standard Model. Such limits represent a significant improvement in sensitivity over the early Run 2 bbbbresults, achieving a 30 (40)~\% additional gain in sensitivity beyond that predicted from a pure increase in dataset size. These limits are competitive with other leading ATLAS full Run 2 searches. Cross section limits are additionally set for a range of values of the Higgs self coupling, parametrized via its ratio to the value predicted by the Standard Model, $\kappa_{\lambda} = \lambda_{HHH}/\lambda_{HHH}^{SM}$. This is restricted to have values $-4.9 \leq \kappa_{\lambda} \leq 14.4$ observed ($-3.9 \leq \kappa_{\lambda} \leq 10.9$ expected). An excess of data over background is seen for values of $\kappa_{\lambda} \geq 5$, with maximum local significance of $3.8\sigma$ at $\kappa_{\lambda} = 6$. Such an excess is demonstrated to be due to low mass, where the bbbb channel has limited sensitivity, and is not seen in more sensitive channels in this region. Results on the development of two methods for the improvement of hadronic shower modeling in ATLAS fast calorimeter simulation are also presented.

Author: Daniel Salerno
Publisher: Springer Nature
ISBN: 3030312577
Category : Science
Languages : en
Pages : 207

View

Book Description
The work presented in this PhD dissertation is the first search at CMS for Higgs bosons produced in association with top quarks (ttH) in a final state consisting of only jets. The results presented in this book uncover a new class of ttH events that will help us elucidate our understanding of the Yukawa sector interactions between the Higgs boson and the top quark. Despite this being the most common decay signature for ttH, a large contamination of SM backgrounds makes it the most challenging for extracting a signal from data. The PhD thesis presents many sophisticated tools and techniques that were developed in order to overcome these challenges. These tools pave the way for future analyses to investigate other standard model and beyond-standard model physics.

Author: Cecilia Tosciri
Publisher: Springer Nature
ISBN: 3030879380
Category : Science
Languages : en
Pages : 171

View

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.

Author: Grigore Tarna
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

View

Book Description
Proton-proton collisions at a center of mass energy of 13 TeV recorded during 2015-2017 with the ATLAS detector at LHC are analyzed in this thesis. The first part of the thesis details electron efficiency measurements with a Tag-and-Probe method in Z->ee events. The measured electron reconstruction efficiency varies between 97\% (15E_T25 GeV) up to 99\% at higher E_T with an uncertainty of around 1\% and per-mile level, respectively. The electron efficiency of a new algorithm (PLI and PLV) designed to reduce the electrons originating from heavy flavor decays is found to be 60-70\% at low E_T (E_T20 GeV) increasing up to 95\% at higher E_T. The second part of the thesis covers a search for flavor-changing neutral currents (FCNC) in top decays to an up-type quark (u or c) and a Higgs boson (t-Hu or t-Hc) using the 2015-2016 data. These processes are strongly suppressed in the Standard Model (SM), however larger enhancements are predicted in new physics models. Multivariate analyses (MVA) are used to improve signal-background separation in multilepton final states. The measured branching ratio for the t-Hu (t->Hc) process is found to be compatible with zero and a 95\% confidence level upper limit is set at 0.19\% (0.16\%) with an expected limit of 0.15\% (0.15\%), the best limit in a single channel to date.The Higgs boson production in association with a top quark pair (ttH) allows direct access to the top Yukawa coupling. The ttH production mode was observed by the ATLAS and CMS experiments, independently, by combining several decay channels. Further efforts to measure ttH in the two leptons same-sign channel are discussed in the last part of the thesis.