227 research outputs found

    Higgs Boson Physics at ATLAS

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    The discovery of a new boson with the ATLAS detector at the LHC proton-proton collider is confirmed using the full data set collected at centre-of-mass energies of 7 and 8 TeV. The spin and parity properties of the boson are consistent with that of a scalar particle with positive parity. Comparison of the JP=0+J^{P}=0^+ hypothesis to alternatives JP=0,1+,1,2+J^{P} = 0^-,1^{+},1^-,2^+ result in exclusion of these other choices at 97.8\%, 99.97\%, 99.7\%, and 99.3\% CL. The Higgs-boson Mass is mH=125.5±0.2(stat.)0.5+0.5(syst.)m_H = 125.5 \pm 0.2 {\rm (stat.)} ^{+0.5}_{-0.5} {\rm (syst.)} \GeV. Evidence for production of the Higgs boson by vector boson fusion is obtained in a model-independent approach by comparing the signal strengths μ\mu of vector boson fusion and production associated with a vector boson to to that for gluon fusion including associated production of top quark pairs: μVBF+VH/μggF+ttH=1.40.3+0.4(stat.)0.4+0.6(syst.)\mu_{\rm VBF+VH}/\mu_{\rm ggF+ttH}= 1.4 ^{+0.4}_{-0.3} \rm{(stat.)} ^{+0.6}_{-0.4} \rm{(syst.)} which is 3.3 Gaussian standard deviations from zero

    Higgs at ATLAS

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    A review of the ATLAS results for Higgs to WW, gamma gamma, tau tau and ZZ is given with the present analysis of couplings, spin and mass. This is to be presented at the 16th Lomonosov conference

    Search for supersymmetric particles with R-parity violation in Z decays

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    Searches for supersymmetric particles produced in e+e− interactions at the Z peak have been performed under the assumptions that R-parity is not conserved, that the dominant R-parity violating coupling involves only leptonic fields, and that the lifetime of the lightest supersymmetric particle can be neglected. In a data sample collected by the ALEPH detector at LEP up to 1993, and corresponding to almost two million hadronic Z decays, no signal was observed. As a result, supersymmetric particle masses and couplings are at least as well constrained as under the usual assumption of R-parity conservation

    Search for charginos and neutralinos with R-parity violation at s**(1/2) = 130-GeV and 136-GeV

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    Searches for charginos and neutralinos produced in e+e− collisions at centre-of-mass energies of 130 and 136 GeV have been performed under the assumptions that R-parity is not conserved, that the dominant R-parity violating coupling involves only leptonic fields, and that the lifetime of the lightest supersymmetric particle can be neglected. In the 5.7 pb−1 data sample collected by ALEPH, no candidate events were found. As a result, chargino and neutralino masses and couplings are constrained and the domains previously excluded at LEP1 are extended

    Measurement of B0bbar0 Mixing At the Fermilab Tevatron Collider

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    The B0B0BAR average mixing parameter-chi has been extracted from e-mu and ee events produced in ppBAR collisions at square-root = 1.8 TeV. In a sample of 900 e-mu-events, the like-sign to opposite-sign charge ratio R is measured to be 0.556 +/- 0.048(stat)-0.042(+0.05)(syst). In the absence of mixing, the expected value of R would be 0.23 + 0.06. The corresponding number for 212 ee events is 0.573 +/- 0.116(stat) +/- 0.047(syst) with an expected nonmixing value of 0.24 +/- 0.07. The observed excess in R leads to a combined determination of chi = 0.176 +/- 0.031 (stat + syst) +/- 0.032(model), where the last uncertainty is due to Monte Carlo modeling

    Measurement of the Production and Muonic Decay-rate of W and Z Bosons In P(p)bar Collisions At Root-s = 1.8 Tev

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    An analysis of proton-antiproton collisions at square-root s = 1.8 TeV recorded with the Collider Detector at Fermilab (CDF) yields sigma(ppBAR --> WX)B(W --> mu-nu) = 2.21 +/- 0.22 nb and sigma(ppBAR --> ZX)B(Z --> mu+mu-) = 0.226 +/- 0.032 nb. The ratio is R(mu) = sigma(W)B(W --> mu-nu)/sigma(Z)B(Z --> mu+mu-) = 9.8 +/- 1.2. Combining with previous CDF electron results gives sigma(W)B(W --> l-nu) = 2.20 +/- 0.20 nb, sigma(Z)B(Z- l+l-) = 0.214 +/- 0.023 nb, and R(l) = 10.0 +/- 0.8. We extract the ratios of the coupling constants g(mu)/g(e) and g(tau)/g(mu). Using standard model assumptions we deduce the inverse branching ratio B-1(W --> l-nu), the width GAMMA(W), and a decay-mode-independent lower bound on the top quark mass of 45 GeV/c2 (95% C.L.)

    Tau hadronic branching ratios

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    From 64492 selected tau-pair events, produced at the Z0 resonance, the measurement of the tau decays into hadrons from a global analysis using 1991, 1992 and 1993 ALEPH data is presented. Special emphasis is given to the reconstruction of photons and pi(o)'s, and the removal of fake photons. A detailed study of the systematics entering the pi(o) reconstruction is also given. A complete and consistent set of tau hadronic branching ratios is presented for 18 exclusive modes. Most measurements are more precise than the present world average. The new level of precision reached allows a stringent test of tau - mu universality in hadronic decays, g(tau)/g(mu) = 1.0013 +/- 0.0095, and the first measurement of the vector and axial-vector contributions to the non-strange hadronic tau decay width: R(tau,V) = 1.788 +/- 0.025 and R(tau,A) = 1.694 +/- 0.027. The ratio (R(tau,V) - R(tau,A))/(R(tau,V) + R(tau,A)), equal to (2.7 +/- 1.3) %, is a measure of the importance of QCD non-perturbative contributions to the hadronic tau decay width

    Measurement of the Dijet Mass-distribution In P(p)over-bar Collisions At Root-s = 1.8 Tev

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    The dijet invariant mass distribution has been measured in the region between 120 and 1000 GeV/c2, in 1.8-TeV ppBAR collisions. The data sample was collected with the Collider Detector at Fermilab (CDF). Data are compared to leading order (LO) and next-to-leading order (NLO) QCD calculations using two different clustering cone radii R in the jet definition. A quantitative test shows good agreement of data with the LO and NLO QCD predictions for a cone of R = 1. The test using a cone of R = 0. 7 shows less agreement. The NLO calculation shows an improvement compared to LO in reproducing the shape of the spectrum for both radii, and approximately predicts the cone size dependence of the cross section

    Observation of Rapidity Gaps In (p)over-bar-p Collisions At 1.8 Tev

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    In pp̄ collisions at s=1.8 TeV we find jet events with a rapidity gap topology. The number of hadrons in the rapidity interval ΔηD between leading-jet cones was sampled by charged tracks with PT>400 MeV/c. We find excess trackless events beyond that expected in a smooth multiplicity distribution. In a control region outside ΔηD we see no excess. For ΔηD>0.8, the fraction of excess trackless events, consistent with estimates based on exchange of color-singlet digluons, is R(gap)=σjet(gap) σjet=0.0085±0.0012(stat)-0.0012+0.0024(syst). © 1995 The American Physical Society

    A Measurement of R(b) using a lifetime mass tag

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    ALEPH's published measurement of using a lifetime tag is updated using the full LEP 1 data sample. Considerable effort has been devoted to understanding systematic effects. Charm background is better controlled by combining the lifetime tag with a tag based on the hadron mass difference. Furthermore, the algorithm used to reconstruct the event primary vertex is designed so as to reduce correlations between the two hemispheres of an event. The value of Rb is measured to be 0.2167 ± 0.0011(stat) ± 0.0013(syst)
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