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Register a new userSearch for standard-model Z and Higgs bosons decaying into a bottom-antibottom quark pair in proton-antiproton collisions at 1.96 TeV
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The Collider Detector at Fermilab collected a unique sample of jets originating from bottom-quark fragmentation ($b$-jets) by selecting online proton-antiproton ($pbar{p}$) collisions with a vertex displaced from the $pbar{p}$ interaction point, consistent with the decay of a bottom-quark hadron. This data set, collected at a center-of-mass energy of $sqrt{s}=$1.96 TeV, and corresponding to an integrated luminosity of $5.4~rm{fb}^{-1}$, is used to measure the $Z$-boson production cross section times branching ratio into $bbar{b}$. The number of $Zrightarrow bbar{b}$ events is determined by fitting the dijet-mass distribution while constraining the dominant $b$-jet background, originating from QCD multijet events, with data. The result, $sigma(pbar{p} rightarrow Z) times mathcal{B}(Z rightarrow bbar{b})= 1.11pm 0.08(text{stat}) pm 0.14(text{syst})~text{nb}$, is the most precise measurement of this process, and is consistent with the standard-model prediction. The data set is also used to search for Higgs-boson production. No significant signal is expected in our data and the first upper limit on the cross section for the inclusive $pbar p rightarrow Hrightarrow bbar b$ process at $sqrt{s}=$1.96 TeV is set, corresponding to 33 times the expected standard-model cross section, or $sigma = 40.6$ pb, at the 95% confidence level.
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A search for heavy pseudoscalar ($A$) and scalar ($H$) Higgs bosons decaying into a top quark pair ($tbar{t}$) has been performed with 20.3 fb$^{-1}$ of proton--proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of $sqrt{s} = 8$ TeV. Interference effects between the signal process and Standard Model $tbar{t}$ production, which are expected to distort the signal shape from a single peak to a peak--dip structure, are taken into account. No significant deviation from the Standard Model prediction is observed in the $tbar{t}$ invariant mass spectrum in final states with an electron or muon, large missing transverse momentum, and at least four jets. The results are interpreted within the context of a type-II two-Higgs-doublet model. Exclusion limits on the signal strength are derived as a function of the mass $m_{A/H}$ and the ratio of the vacuum expectation values of the two Higgs fields, $tanbeta$, for $m_{A/H} > 500$ GeV.
We present a search for the standard model Higgs boson produced in association with a $Z$ boson, using up to 7.9 fb$^{-1}$ of integrated luminosity from $pbar{p}$ collisions collected with the CDF II detector. We utilize several novel techniques, including multivariate lepton selection, multivariate trigger parametrization, and a multi-stage signal discriminant consisting of specialized functions trained to distinguish individual backgrounds. By increasing acceptance and enhancing signal discrimination, these techniques have significantly improved the sensitivity of the analysis above what was expected from a larger dataset alone. We observe no significant evidence for a signal, and we set limits on the $ZH$ production cross section. For a Higgs boson with mass 115 GeV/$c^2$, we expect (observe) a limit of 3.9 (4.8) times the standard model predicted value, at the 95% credibility level.
We search for high-mass resonances decaying into Z boson pairs using data corresponding to 6 fb^-1 collected by the CDF experiment in pbar{p} collisions at sqrt{s}=1.96 TeV. The search is performed in three distinct final states: ZZ --> l^+l^-l^+l^-, ZZ --> l^+l^- u u, and ZZ --> l^+l^-jj. For a Randall-Sundrum graviton G*, the 95% CL upper limits on the production cross section times branching ratio to ZZ, sigma(pbar{p} --> G^* --> ZZ), vary between 0.26 pb and 0.045 pb in the mass range 300 < M_{G*} < 1000 GeV/c^2.
A search for the production and non-standard decay of a Higgs boson, h, into four taus through intermediate pseudoscalars, a, is conducted on 683 pb-1 of data collected by the ALEPH experiment at centre-of-mass energies from 183 to 209 GeV. No excess of events above background is observed, and exclusion limits are placed on the combined production cross section times branching ratio, xi^2 = sigma(e+e- --> Zh)/sigma_{SM}(e+e- --> Zh) x B(h --> aa)x B(a --> tau^+tau^-)^2. For mh < 107 GeV/c2 and 4 < ma < 10 GeV/c2, xi^2 > 1 is excluded at the 95% confidence level.
We report on a search for a spin-zero non-standard-model particle in proton-antiproton collisions collected by the Collider Detector at Fermilab at a center-of-mass-energy of 1.96 TeV. This particle, the $phi$ boson, is expected to decay into a bottom-antibottom quark pair and to be produced in association with at least one bottom quark. The data sample consists of events with three jets identified as initiated by bottom quarks and corresponds to $5.4~text{fb}^{-1}$ of integrated luminosity. In each event, the invariant mass of the two most energetic jets is studied by looking for deviations from the multijet background, which is modeled using data. No evidence is found for such particle. Exclusion upper limits ranging from 20 to 2 pb are set for the product of production cross sections times branching fraction for hypothetical $phi$ boson with mass between 100 and 300 GeV/$c^2$. These are the most stringent constraints to date.
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