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Search for the standard model Higgs boson produced in association with a top-quark pair in pp collisions at the LHC  [PDF]
CMS Collaboration
Physics , 2013, DOI: 10.1007/JHEP05(2013)145
Abstract: A search for the standard model Higgs boson produced in association with a top-quark pair is presented using data samples corresponding to an integrated luminosity of 5.0 inverse femtobarns (5.1 inverse femtobarns) collected in pp collisions at the center-of-mass energy of 7 TeV (8 TeV). Events are considered where the top-quark pair decays to either one lepton+jets (t tbar to ell nu q q' b bbar) or dileptons (t tbar to ell(+) nu ell(-) nu b bbar), ell being an electron or a muon. The search is optimized for the decay mode H to b bbar. The largest background to the t tbar H signal is top-quark pair production with additional jets. Artificial neural networks are used to discriminate between signal and background events. Combining the results from the 7 TeV and 8 TeV samples, the observed (expected) limit on the cross section for Higgs boson production in association with top-quark pairs for a Higgs boson mass of 125 GeV is 5.8 (5.2) times the standard model expectation.
Azimuthal correlation among jets produced in association with a bottom or top quark pair at the LHC
Hagiwara, Kaoru;Mukhopadhyay, Satyanarayan
High Energy Physics - Phenomenology , 2013, DOI: 10.1007/JHEP05(2013)019
Abstract: Angular correlation of jets produced in association with a massive scalar, vector or tensor boson is crucial in the determination of their spin and CP properties. We study jet angular correlations in events with a high mass bottom quark pair or a top quark pair and two jets at the LHC, whose cross-section is dominated by the virtual gluon fusion sub-processes when appropriate kinematic selection cuts (vector-boson fusion cuts) are applied. We evaluate helicity amplitudes for sub-processes initiated by qq, qg and gg collisions in the limit where the intermediate gluons are collinear to the initial partons. We first obtain a general expression for the azimuthal angle correlations among the dijets and t t-bar or b b-bar, in terms of the gg to t t-bar or b b-bar helicity amplitudes in the real gluon approximation of the full matrix elements, and find simple analytic expressions in the two kinematic limits, the production of a heavy quark pair near the threshold, and in the relativistic limit where the invariant mass of the heavy quark pair is much larger than the quark mass. For b b-bar + 2 jets we find strong azimuthal angle correlations which are distinct from those expected for events with a CP-even or odd scalar boson which may decay into a b b-bar pair. For t t-bar + 2 jets we find that the angular correlations are similar to that of a CP-odd scalar+2 jets near the threshold M_(t t-bar) ~ 2 m_t, while in the relativistic limit they resemble the distribution for b b-bar + 2 jets. These correlations in the standard QCD processes will help establish the experimental technique to measure the spin and CP properties of new particles produced via gluon fusion at the LHC.
Azimuthal correlation among jets produced in association with a bottom or top quark pair at the LHC  [PDF]
Kaoru Hagiwara,Satyanarayan Mukhopadhyay
Physics , 2013, DOI: 10.1007/JHEP05(2013)019
Abstract: Angular correlation of jets produced in association with a massive scalar, vector or tensor boson is crucial in the determination of their spin and CP properties. We study jet angular correlations in events with a high mass bottom quark pair or a top quark pair and two jets at the LHC, whose cross-section is dominated by the virtual gluon fusion sub-processes when appropriate kinematic selection cuts (vector-boson fusion cuts) are applied. We evaluate helicity amplitudes for sub-processes initiated by qq, qg and gg collisions in the limit where the intermediate gluons are collinear to the initial partons. We first obtain a general expression for the azimuthal angle correlations among the dijets and t t-bar or b b-bar, in terms of the gg to t t-bar or b b-bar helicity amplitudes in the real gluon approximation of the full matrix elements, and find simple analytic expressions in the two kinematic limits, the production of a heavy quark pair near the threshold, and in the relativistic limit where the invariant mass of the heavy quark pair is much larger than the quark mass. For b b-bar + 2 jets we find strong azimuthal angle correlations which are distinct from those expected for events with a CP-even or odd scalar boson which may decay into a b b-bar pair. For t t-bar + 2 jets we find that the angular correlations are similar to that of a CP-odd scalar+2 jets near the threshold M_(t t-bar) ~ 2 m_t, while in the relativistic limit they resemble the distribution for b b-bar + 2 jets. These correlations in the standard QCD processes will help establish the experimental technique to measure the spin and CP properties of new particles produced via gluon fusion at the LHC.
Search for a standard model Higgs boson produced in association with a top-quark pair and decaying to bottom quarks using a matrix element method  [PDF]
CMS Collaboration
Physics , 2015, DOI: 10.1140/epjc/s10052-015-3454-1
Abstract: A search for a standard model Higgs boson produced in association with a top-quark pair and decaying to bottom quarks is presented. Events with hadronic jets and one or two oppositely charged leptons are selected from a data sample corresponding to an integrated luminosity of 19.5 inverse femtobarns collected by the CMS experiment at the LHC in pp collisions at a centre-of-mass energy of 8 TeV. In order to separate the signal from the larger t t-bar + jets background, this analysis uses a matrix element method that assigns a probability density value to each reconstructed event under signal or background hypotheses. The ratio between the two values is used in a maximum likelihood fit to extract the signal yield. The results are presented in terms of the measured signal strength modifier, mu, relative to the standard model prediction for a Higgs boson mass of 125 GeV. The observed (expected) exclusion limit at a 95% confidence level is mu < 4.2 (3.3), corresponding to a best fit value mu-hat = 1.2 +1.6 -1.5.
Study of the Top-quark Pair Production in Association with a Bottom-quark Pair from Fast Simulations at the LHC  [PDF]
Young Kwon Jo,Su Yong Choi,Tae Jeong Kim,Youn Jung Roh
Physics , 2015, DOI: 10.3938/jkps.67.807
Abstract: A large number of top quarks will be produced at the Large Hadron Collider (LHC) for Run II period. This will allow us to measure the rare processes from the top sector in great details. We present the study of the top-quark pair production in association with a bottom-quark pair (ttbb) from fast simulations for the Compact Muon Solenoid (CMS) experiment. The differential distributions of ttbb are compared with the top-quark pair production with two additional jets (ttjj) and with the production in association with the Higgs (ttH), where the Higgs decays to a bottom-quark pair. The significances of ttbb process in the dileptonic and semileptonic decay mode are calculated with the data corresponding to an integrated luminosity of 10 fb-1, which is foreseen to be collected in the early Run II period. This study will be an important input in searching for new physics beyond the standard model as well as in searching for ttH process where the Yukawa coupling with the top quark can be directly measured.
Top quark pair production in association with a jet: QCD corrections and jet radiation in top quark decays  [PDF]
Kirill Melnikov,Andreas Scharf,Markus Schulze
Physics , 2011, DOI: 10.1103/PhysRevD.85.054002
Abstract: We consider top quark pair production in association with a hard jet through next-to-leading order in perturbative QCD. Top quark decays are treated in the narrow width approximation and spin correlations are retained throughout the computation. We include hard jet radiation by top quark decay products and explore their importance for basic kinematic distributions at the Tevatron and the LHC. Our results suggest that QCD corrections and jet radiation in decays can lead to significant changes in shapes of basic distributions and, therefore, need to be included for the description of ttbar+jet production. We compare the shape of the transverse momentum distribution of a top quark pair recently measured by the D0 collaboration with the result of our computation and find reasonable agreement.
Top Quark Pair Production in Association with a Jet with NLO QCD Off-Shell Effects at the Large Hadron Collider  [PDF]
G. Bevilacqua,H. B. Hartanto,M. Kraus,M. Worek
Physics , 2015,
Abstract: We present a complete description of top quark pair production in association with a jet in the dilepton channel. Our calculation is accurate to next-to-leading order in QCD (NLO) and includes all non-resonant diagrams, interferences and off-shell effects of the top quark. Moreover, non-resonant and off-shell effects due to the finite W gauge boson width are taken into account. This calculation constitutes the first fully realistic NLO computation for top quark pair production with a final state jet in hadronic collisions. Numerical results for differential distributions as well as total cross sections are presented for the Large Hadron Collider (LHC) at 8 TeV. With our inclusive cuts, NLO predictions reduce the unphysical scale dependence by more than a factor of 3 and lower the total rate by about 13% compared to leading order QCD (LO) predictions. In addition, the size of the top quark off-shell effects is estimated to be below 2%.
Multilepton Signatures of the Higgs Boson through its Production in Association with a Top-quark Pair  [PDF]
Pankaj Agrawal,Somnath Bandyopadhyay,Siba Prasad Das
Physics , 2013, DOI: 10.1103/PhysRevD.88.093008
Abstract: We consider the possible production of the Higgs Boson in association with a top-quark pair and its subsequent decay into a tau-lepton pair or a W-boson pair. This process can give rise to many signatures of the Higgs boson. These signatures can have electrons, muons, tau jets, bottom jets and/or light flavour jets. We analyze the viability of some of these signatures. We will look at those signatures where the background is minimal. In particular, we explore the viability of the signatures "isolated 4 electron/muon" and "isolated 3 electron/muon + a jet" The jet can be due to a light flavour quark/gluon, a bottom quark, or a tau lepton. Of all these signatures, we find that "isolated 3 electron/muon + a tau jet", with an extra bottom jet, can be an excellent signature of this mode of the Higgs boson production. We show that this signature may be visible within a year, once the Large Hadron Collider (LHC) restarts. Some of the other signatures would also be observable after the LHC accumulates sufficient luminosity.
Search for a dark matter candidate produced in association with a single top quark in $p\bar{p}$ collisions at $\sqrt{s} = 1.96$ TeV  [PDF]
CDF Collaboration
Physics , 2012, DOI: 10.1103/PhysRevLett.108.201802
Abstract: We report a new search for dark matter in a data sample of an integrated luminosity of 7.7 fb$^{-1}$ of Tevatron $p\bar{p}$ collisions at $\sqrt{s} = 1.96$ TeV, collected by the CDF II detector. We search for production of a dark matter candidate, $D$, in association with a single top quark. We consider the hadronic decay mode of the top quark exclusively, yielding a final state of three jets with missing transverse energy. The data are consistent with the standard model; we thus set 95% confidence level upper limits on the cross section of the process $p\bar{p}\rightarrow t+D$ as a function of the mass of the dark-matter candidate. The limits are approximately 0.5 pb for a dark-matter particle with mass in the range of 0-150 GeV/$c^{2}$.
Observation of top quark pairs produced in association with a vector boson in pp collisions at sqrt(s) = 8 TeV  [PDF]
CMS Collaboration
Physics , 2015,
Abstract: Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 inverse-femtobarns, collected by the CMS experiment at the LHC. Final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from ttW or ttZ decays. The ttW cross section is measured to be 382 +117 -102 fb with a significance of 4.8 standard deviations from the background-only hypothesis. The ttZ cross section is measured to be 242 +65 -55 fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the ttW and ttZ cross sections.
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