This thesis presents a search for a doubly-charged Higgs boson in three and four lepton final states. The two production modes considered are the associated production mode with a doubly-charged Higgs boson produced in association with a singly-charged Higgs boson and the pair production mode with the simultaneous production of two doubly-charged Higgs bosons. The search is performed with 12.9 fb-1 of square root s = 13 TeV proton-proton collision data collected with the Compact Muon Solenoid at the Large Hadron Collider. In addition, this thesis presents the first measurement of the WZ production cross section in proton-proton collisions at square root s = 13 TeV. The measurement is performed in the WZ to [l][v][l]'[l]' decay mode where [l], [l]' = e, [mu] with an integrated luminosity of 2.3 fb-1.
This thesis describes a Standard Model (SM) cross section measurement of W+bb as well as a search for neutral Higgs bosons in the Minimal Supersymmetric Extension of the Standard Model (MSSM) decaying to tau pairs. The measurement of W+bb was performed using proton-proton collisions at sqrt(s) = 7 TeV in a data sample collected with the CMS experiment at the LHC corresponding to an integrated luminosity of 5.0 1/fb. The W+bb events are selected in the W to muon + neutrino decay mode by requiring a muon with transverse momentum pT>25 GeV and pseudorapidity absolute eta less than 2.1, and exactly two b-tagged jets with pT>25 GeV and absolute eta less than 2.4. The measured W+bb production cross section in the fiducial region, calculated at the level of final-state particles, is 0.53± 0.05(stat.) ± 0.09 (syst.) ± 0.06 (theory) ± 0.01 (lum.) pb, in agreement with the SM prediction. This measurement is a sensitive test of heavy quark production calculated with perturbative QCD. It also serves as an important benchmark in new physics searches which include a single isolated lepton and one or more b jets in the final state, as W+bb becomes an irreducible background. Also presented is a search for the CP-even MSSM Higgs bosons, H and h, and the CP-odd MSSM pseudoscalar, A, in their decays to tau pairs. This search is performed using events recorded by the CMS experiment at the LHC in 2011 and 2012 at a center-of-mass energy of 7 TeV and 8 TeV respectively. The dataset corresponds to an integrated luminosity of 24.6 1/fb, with 4.9 1/fb at 7 TeV and 19.7 1/fb at 8 TeV. To enhance the sensitivity to neutral MSSM Higgs bosons, the search includes the case where the Higgs boson is produced in association with a b-quark jet. No excess is observed in the tau-pair invariant-mass spectrum.
Zusammenfassung: Searches for charged Higgs bosons decaying to a tau lepton and a neutrino in fully hadronic final states are presented in this thesis. The analyses are based on 4.6fb^-1 and 19.5fb^-1 of proton-proton collision data taken with the ATLAS detector at the LHC at center-of-mass energies of 7TeV and 8TeV in 2011 and 2012, respectively.Charged Higgs bosons with masses less than the top quark mass are searched for in events consistent with top-quark pair production and charged Higgs bosons with masses greater than the top quark mass are searched for in associated production with a top quark.In the search based on data taken in 2011, all background contributions are estimated in data-driven ways. Only a minor background contribution is estimated using simulation in the searches based on data taken in 2012; all dominant background contributions are estimated using data-driven methods here as well. An embedding method is used in all searches to assess the dominant and irreducible background contributions containing true hadronically decaying tau leptons and described in detail in this thesis.No significant excess of the data compared to the background expectations due to Standard Model processes is observed. Statistical analyses lead to 95% confidence-level upper limits on the branching ratios B(t->bH^±)xB(H^±->tau^± nu) between 0.23% and 1.3% for charged Higgs boson masses between 80GeV and 160GeV and on the production cross section times branching ratio, sigma(pp->tH^±+X)xB(H^±->tau^± nu) ranging from 0.76pb to 4.5fb for charged Higgs boson masses between 180GeV and 1000GeV. The branching ratio and cross section limits are used to derive constraints on the parameter space of minimal supersymmetric extensions of the Standard Model in various benchmark scenarios. These limits are the so far strictest limits set by the ATLAS collaboration in the search for charged Higgs bosons in fully hadronic final states
A study of the pp to ZZ 6́2 llll final state is conducted using 19.6 inverse femtobarn of 8 TeV proton-proton collisions at the LHC using the CMS detector. The Standard Model pp 6́2 ZZ production cross section is measured to be 7.7+0.5 -0.5 (stat.) +0.6 -0.5(sys.) ± 0.3(lumi) pb. Evidence of a new Higgs-like boson is observed with a significance of 6.1ï3 at 126 GeV. 95% confidence level upper limits are set on the neutral anomalous triple gauge couplings; -0.004
This first open access volume of the handbook series contains articles on the standard model of particle physics, both from the theoretical and experimental perspective. It also covers related topics, such as heavy-ion physics, neutrino physics and searches for new physics beyond the standard model. A joint CERN-Springer initiative, the "Particle Physics Reference Library" provides revised and updated contributions based on previously published material in the well-known Landolt-Boernstein series on particle physics, accelerators and detectors (volumes 21A, B1,B2,C), which took stock of the field approximately one decade ago. Central to this new initiative is publication under full open access
This concise primer reviews the latest developments in the field of jets. Jets are collinear sprays of hadrons produced in very high-energy collisions, e.g. at the LHC or at a future hadron collider. They are essential to and ubiquitous in experimental analyses, making their study crucial. At present LHC energies and beyond, massive particles around the electroweak scale are frequently produced with transverse momenta that are much larger than their mass, i.e., boosted. The decay products of such boosted massive objects tend to occupy only a relatively small and confined area of the detector and are observed as a single jet. Jets hence arise from many different sources and it is important to be able to distinguish the rare events with boosted resonances from the large backgrounds originating from Quantum Chromodynamics (QCD). This requires familiarity with the internal properties of jets, such as their different radiation patterns, a field broadly known as jet substructure. This set of notes begins by providing a phenomenological motivation, explaining why the study of jets and their substructure is of particular importance for the current and future program of the LHC, followed by a brief but insightful introduction to QCD and to hadron-collider phenomenology. The next section introduces jets as complex objects constructed from a sequential recombination algorithm. In this context some experimental aspects are also reviewed. Since jet substructure calculations are multi-scale problems that call for all-order treatments (resummations), the bases of such calculations are discussed for simple jet quantities. With these QCD and jet physics ingredients in hand, readers can then dig into jet substructure itself. Accordingly, these notes first highlight the main concepts behind substructure techniques and introduce a list of the main jet substructure tools that have been used over the past decade. Analytic calculations are then provided for several families of tools, the goal being to identify their key characteristics. In closing, the book provides an overview of LHC searches and measurements where jet substructure techniques are used, reviews the main take-home messages, and outlines future perspectives.
Searches are presented for heavy scalar (H) and pseudoscalar (A) Higgs bosons posited in the two doublet model (2HDM) extensions of the standard model (SM). These searches are based on a data sample of pp collisions collected with the CMS experiment at the LHC at a center-of-mass energy of √s=8 TeV and corresponding to an integrated luminosity of 19.5 fb-1. The decays H→hh and A→Zh, where h denotes an SM-like Higgs boson, lead to events with three or more isolated charged leptons or with a photon pair accompanied by one or more isolated leptons. Our search results are presented in terms of the H and A production cross sections times branching fractions and are further interpreted in terms of 2HDM parameters. We place 95% C.L. cross section upper limits of approximately 7 pb on ?B for H→hh and 2 pb for A→Zh. Furthermore, the results of a search for the rare decay of the top quark are presented; this results in a charm quark and an SM Higgs boson,t→ch, the existence of which would indicate a nonzero flavor-changing Yukawa coupling of the top quark to the Higgs boson. We place a 95% C.L. upper limit of 0.56% on B(t→ch).
