Model-independent extraction of ∣Vtq∣|V_{tq}| matrix elements from top-quark measurements at hadron colliders

Current methods to extract the quark-mixing matrix element $|V_{tb}|$ from single-top production measurements assume that $|V_{tb}|\gg |V_{td}|, |V_{ts}|$: top quarks decay into $b$ quarks with 100% branching fraction, s-channel single-top production is always accompanied by a $b$ quark and initial-state contributions from $d$ and $s$ quarks in the $t$-channel production of single top quarks are neglected. Triggered by a recent measurement of the ratio $R=\frac{|V_{tb}|^{2}}{|V_{td}|^{2}+|V_{ts}|^{2}+|V_{tb}|^{2}}=0.90 \pm 0.04$ performed by the D0 collaboration, we consider a $|V_{tb}|$ extraction method that takes into account non zero d- and s-quark contributions both in production and decay. We propose a strategy that allows to extract consistently and in a model-independent way the quark mixing matrix elements $|V_{td}|$, $|V_{ts}|$, and $|V_{tb}|$ from the measurement of $R$ and from single-top measured event yields. As an illustration, we apply our method to the Tevatron data using a CDF analysis of the measured single-top event yield with two jets in the final state one of which is identified as a $b$-quark jet. We constrain the $|V_{tq}|$ matrix elements within a four-generation scenario by combining the results with those obtained from direct measurements in flavor physics and determine the preferred range for the top-quark decay width within different scenarios.Comment: 36 pages, 17 figure

Measurement of the charge asymmetry in dileptonic Decays of top quark pairs in pp collisions at √ s = 7 TeV using the ATLAS detector

A measurement of the top-antitop (tt) charge asymmetry is presented using data corresponding to an integrated luminosity of 4.6 fb −1 of LHC pp collisions at a centre- of-mass energy of 7 TeV collected by the ATLAS detector. Events with two charged leptons, at least two jets and large missing transverse momentum are selected. Two observables are studied: A tt/C, based on the reconstructed tt final state. The asymmetries are measured to be A ll/C = 0.024 +/- 0.015 (stat.) +/- 0.009 (syst.) Att/C = 0.021 +/- 0.025 (stat.) +/- 0.017 (syst.) The measured values are in agreement with the Standard Model predictions

Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Results of a search for H → τ τ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb−1 and 20.3 fb−1 at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV respectively. All combinations of leptonic (τ → `νν¯ with ` = e, µ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of µ = 1.43 +0.43 −0.37 is consistent with the predicted Yukawa coupling strength in the Standard Model

Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at s√=8 TeV with the ATLAS detector

The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy s√=8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb−1. No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/R c = 950 GeV for a cut-off scale times radius (ΛR c) of approximately 30

Measurement of the top pair production cross section in 8 TeV proton-proton collisions using kinematic information in the lepton plus jets final state with ATLAS

A measurement is presented of the $t\bar{t}$ inclusive production cross-section in $pp$ collisions at a center-of-mass energy of $\sqrt{s}=8$ TeV using data collected by the ATLAS detector at the CERN Large Hadron Collider. The measurement was performed in the lepton+jets final state using a data set corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The cross-section was obtained using a likelihood discriminant fit and $b$-jet identification was used to improve the signal-to-background ratio. The inclusive $t\bar{t}$ production cross-section was measured to be $260\pm 1{\textrm{(stat.)}} ^{+22}_{-23} {\textrm{(syst.)}}\pm 8{\textrm{(lumi.)}}\pm 4{\mathrm{(beam)}}$ pb assuming a top-quark mass of 172.5 GeV, in good agreement with the theoretical prediction of $253^{+13}_{-15}$ pb. The $t\bar{t}\to (e,\mu)+{\mathrm{jets}}$ production cross-section in the fiducial region determined by the detector acceptance is also reported.Comment: Published version, 19 pages plus author list (35 pages total), 3 figures, 2 tables, all figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2013-06

Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at s√=8 TeV with the ATLAS detector

The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1fb−1 of proton–proton collision data at √s = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via t˜→tχ˜01 or t˜→ bχ˜±1 →bW(∗)χ˜01 , where χ˜01 (χ˜±1 ) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of t˜ → tχ˜01 . For a branching fraction of 100%, top squark masses in the range 270–645 GeV are excluded for χ˜01 masses below 30 GeV. For a branching fraction of 50% to either t˜ → tχ˜01 or t˜ → bχ˜±1 , and assuming the χ˜±1 mass to be twice the χ˜01 mass, top squark masses in the range 250–550 GeV are excluded for χ˜01 masses below 60 GeV

Search for Higgs boson decays into a pair of light bosons in the bbμμ final state in pp collision at √s=13 TeV with the ATLAS detector

A search for decays of the Higgs boson into a pair of new spin-zero particles, H→aa, where the a-bosons decay into a b-quark pair and a muon pair, is presented. The search uses 36.1fb−1of proton–proton collision data at √s=13 TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% confidence level are placed on the branching ratio (σH/σSM) ×B(H→aa →bbμμ), ranging from 1.2 ×10−4to 8.4 ×10−4in the a-boson mass range of 20–60GeV. Model-independent limits are set on the visible production cross-section times the branching ratio to the bbμμ final state for new physics, σvis(X) ×B(X→bbμμ), ranging from 0.1fb to 0.73fb for mμμ between 18 and 62GeV

Search for supersymmetry in final states with charm jets and missing transverse momentum in 13 TeV pp collisions with the ATLAS detector

A search for supersymmetric partners of top quarks decaying as t ~ 1 →cχ ~ 0 1 and supersymmetric partners of charm quarks decaying as c ~ 1 →cχ ~ 0 1, where χ ~ 0 1 is the lightest neutralino, is presented. The search uses 36.1 fb −1 pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider and is performed in final states with jets identified as containing charm hadrons. Assuming a 100% branching ratio to cχ ~ 0 1, top and charm squarks with masses up to 850 GeV are excluded at 95% confidence level for a massless lightest neutralino. For m t ~ 1 ,c ~ 1 −m χ ~ 0 1 < 100 GeV, top and charm squark masses up to 500 GeV are excluded

Measurement of the Higgs boson mass in the H→ZZ ∗ →4ℓ and H→γγ channels with √s =13 TeV pp collisions using the ATLAS detector

The mass of the Higgs boson is measured in the H→ZZ ∗ →4ℓ and in the H→γγ decay channels with 36.1 fb −1 of proton-proton collision data from the Large Hadron Collider at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector in 2015 and 2016. The measured value in the H→ZZ ∗ →4ℓ channel is m ZZ ∗ H =124.79±0.37 GeV, while the measured value in the H→γγ channel is m γγ H =124.93±0.40 GeV. Combining these results with the ATLAS measurement based on 7 TeV and 8 TeV proton-proton collision data yields a Higgs boson mass of m H =124.97±0.24 GeV

Correlated long-range mixed-harmonic fluctuations measured in pp, p+Pb and low-multiplicity Pb+Pb collisions with the ATLAS detector

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