We present the results of a search for a very light CP-odd Higgs boson a1° originating from top quark decays t → H{sup {+-}}b → W{sup {+-}(*)} a1° b, and subsequently decaying into??−. Using a data sample corresponding to an integrated luminosity of 2.7 fb−1 collected by the CDF II detector in p{bar p} collisions at 1.96 TeV, we perform a search for events containing a lepton, three or more jets, and an additional isolated track with transverse momentum in the range 3 to 20 GeV/c. Observed events are consistent with background sources, and 95% C.L. limits are set on the branching ratio of t → H{sup {+-}}b for various masses of H{sup {+-}} and a1°.
We present the results of a search for a very light CP-odd Higgs boson a10 originating from top quark decays t→H±b → W±(*)a10b, and subsequently decaying into [tau]+[tau]-. Using a data sample corresponding to an integrated luminosity of 2.7 fb-1 collected by the CDF II detector in pp̄ collisions at 1.96 TeV, we perform a search for events containing a lepton, three or more jets, and an additional isolated track with transverse momentum in the range 3 to 20 GeV/c. Observed events are consistent with background sources, and 95% C.L. limits are set on the branching ratio of t→H±b for various masses of H± and a10.
The Standard Model predicts the existence of one neutral scalar Higgs boson, which is a remnant of the mechanism that breaks the SU(2)[sub L]xU(1)[sub Y] electroweak symmetry and generates masses for the heavy vector bosons and fermions. Many extensions to the Standard Model predict two or more Higgs doublets, resulting in a larger spectrum of Higgs bosons including a charged Higgs boson (H[sup [+-]]). For a light charged Higgs boson mass, the top decay into a charged Higgs boson and bottom quark might occur. This thesis presents results of a direct search for this top quark decay mode via the charged Higgs decay to a tau lepton and tau-neutrino, using the hadronic decays of the tau leptons. The search data consist of 100 pb[sup -1] of Run 1 data collected between 1992-1995 at the CDF detector, from p[anti p] collisions at a center-of-mass energy of 1.8 TeV produced at Fermilab's Tevatron accelerator. A total of seven events are observed in two search channels with an expected background contribution of 7.4[+-]2.0 events coming from fake taus (5.4[+-]1.5), heavy vector boson decays with jets (1.9[+-]1.3) and dibosons(0.08[+-]0.06). Lacking evidence for a signal, we set limits on charged Higgs production at the 95% confidence level in the charged Higgs mass plane versus tan[beta](a parameter of the theory) for a top quark mass of 175 GeV/c[sup 2] and for top production cross sections ([sigma][sub t[anti t]]) of 5.0 and 7.5 pb, assuming the Type-II Two-Higgs-Doublet-Model. For large tan[beta], this analysis excludes a charged Higgs boson of mass below 147(158)GeV/c[sup 2] for [sigma][sub t[anti t]]=5.0(7.5)pb. Using the Standard Model measured top quark cross section from CDF, this limit increases to 168 GeV/c[sup 2] and we also exclude a branching fraction of top decays via this charged Higgs mode of greater than 43% for charged Higgs masses below 168 GeV/c[sup 2].
Abstract: In this thesis a search for a CP-odd Higgs boson A decaying into a Z boson and a neutral CP-even Higgs boson h with a mass of 125 GeV is described. The search is based on proton-proton collision data at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector in the years 2015 and 2016 corresponding to an integrated luminosity of 36/fb. Final states, where the Z boson decays into pairs of electrons, muons or neutrinos and the h boson into bottom-quark pairs, are considered. The presence of neutrinos is inferred by a momentum imbalance in the transverse plane, the bottom-quark pairs are reconstructed either as two small-radius or one large-radius calorimeter jet(s). The reconstructed invariant or transverse mass distributions of the Zh candidates are searched for the presence of localised excesses above the expectation from known Standard Model processes. No evidence for an A-boson signal is found. Upper limits on the A-boson production cross-section times the branching fractions for A-boson decays into Zh and h-boson decays to bottom-quark pairs are derived at the 95% confidence level for A bosons with masses between 220 GeV and 2 TeV; they range between 0.0019 pb and 0.81 pb. The results are interpreted in the context of Two-Higgs-Doublet Models. Furthermore, a novel method to calibrate the b-tagging efficiency for c-quark jets is described. It is demonstrated for the standard b-tagging algorithm of the ATLAS experiment in Run 1 using proton-proton collision data at a centre-of-mass energy of 7 TeV collected with the ATLAS detector in the year 2011. The calibration uses a sample of c jets obtained from events, where a W boson is produced in association with a single c quark; the resulting c jet is identified by a muon stemming from the semileptonic c-hadron decay. The measured c-jet tagging efficiencies are presented relative to the corresponding efficiencies in simulated W+c events as scale factors. The measured scale factors are extrapolated using a simulation-based and data-supported procedure to obtain scale factors valid for inclusive samples of c jets. Their uncertainties range between 5% and 13 %.
We report on the first direct search for charged Higgs bosons in decays of top quarks in p{bar p} collisions at √s = 1.96 TeV. The search uses a data sample corresponding to an integrated luminosity of 2.2 fb−1 collected by the CDF II detector at Fermilab, and looks for a resonance in the invariant mass distribution of two jets in the lepton+jets sample of t{bar t} candidates. We observe no evidence of charged Higgs bosons in top quark decays. Hence, 95% upper limits on the top quark decay branching ratio are placed at?(t → Hb)
A search for a very light Higgs boson decaying into a pair of tau leptons is presented within the framework of the next-to-minimal supersymmetric standard model(NMSSM). This search is based on a data set corresponding to an integrated luminosity of 19.7 fb-1 and 2.5 fb-1 of proton-proton collisions collected by the CMS experiment at a centre-of-mass energy of 8 TeV and 13 TeV respectively. The signal consists of the production of either of the two lightest scalars, h1 or h2, via gluon-gluon fusion and subsequent decay into a pair of the lightest Higgs bosons, a1 or h1. The h1 or h2 boson is identified with the observed state at a mass of 125 GeV. The analysis searches for decays of the a1(h1) states into pairs of tau leptons and covers a mass range for the a1(h1) boson of 4 to 15 GeV. The search reveals no significant excess in data above standard model background expectations, and an upper limit is set on the signal production cross section times branching fraction as a function of the a 1(h1) boson mass.
We report on the first direct search for charged Higgs bosons in decays of top quarks in p{bar p} collisions at √s = 1.96 TeV. The search uses a data sample corresponding to an integrated luminosity of 2.2 fb−1 collected by the CDF II detector at Fermilab, and looks for a resonance in the invariant mass distribution of two jets in the lepton+jets sample of t{bar t} candidates. We observe no evidence of charged Higgs bosons in top quark decays. Hence, 95% upper limits on the top quark decay branching ratio are placed at [Beta](t → H+b)
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.
A search for the standard model Higgs boson decaying into two Z bosons with subsequent decay into a final state containing two quark jets and two leptons, H to ZZ(*) to q q-bar l-l+ is presented. Results are based on data corresponding to an integrated luminosity of 4.6 inverse femtobarns of proton-proton collisions at sqrt(s)=7 TeV, collected with the CMS detector at the LHC. In order to discriminate between signal and background events, kinematic and topological quantities, including the angular spin correlations of the decay products, are employed. Events are further classified according to the probability of the jets to originate from quarks of light or heavy flavor or from gluons. No evidence for the Higgs boson is found, and upper limits on its production cross section are determined for a Higgs boson of mass between 130 and 600 GeV.
