Top-antitop Pair Research Articles

Direct Bound on the Total Decay Width of the Top Quark inpp¯Collisions ats=1.96 TeV

We present the first direct experimental bound on the total decay width of the top quark using 955pb^-1 of the Tevatron's ppbar collisions recorded by the Collider Detector at Fermilab. We identify 253 top-antitop pair candidate events. The distribution of reconstructed top quark mass from these events is fitted to templates representing different values of the top quark width. Using a confidence interval based on likelihood ratio ordering, we extract an upper limit at 95% C.L. of Gamma_t <13.1 GeV for an assumed top quark mass of 175 GeV/c^2.

Influence of graviton on top-antitop production at the LHC

We examine the top quark production in the brane world scenario. We study two typical models - the model proposed by Arkani-Hamed, Dimopoulos and Dvali (ADD) and the model of Randall and Sundrum (RS). In addition to the Standard Model processes, there is a new contribution to the top-antitop pair production process mediated by graviton Kaluza-Klein modes in the s-channel. We calculated the density matrix for the top-antitop pair production including the new contribution. With a reasonable parameter choice, we find a sizable deviation of the top-antitop quark spin correlations from those in the Standard Model.

Top Quarks Spin Correlations with Graviton in ADD and RS Models at the Large Hadron Collider

In LHC physics we study the spin correlation of top-antitop pairs production to investigate the production mechanism of heavy quarks[F. Hubard et al. Eur. Phys. J. C 44 (2006) 13]. The s -channel process mediated by graviton Kaluza-Klein modes in ADD model with several extra dimensions[N. Arkani-Hamed, S. Dimopoulos and G. Dvali, Phys. Lett. 429B (1998) 263, hep-ph/9803315 ] or in the Randall-Sundrum model with only one extra dimension[L. Randall and R. Sundrum, Phys. Rev. Lett. 83 (1999) 3370 hep-ph/9905221 ] contribute to the top-antitop pair production and affects the resulting top spin correlations. We calculated the full density matrix for the top-antitop pair production. We find a sizable deviation of the top spin correlations from the Standard.

Independent measurement of the top quark mass and the light- and bottom-jet energy scales at hadron colliders

A method for the simultaneous determination of the energy scales for b-quark jets and light jets, the jet energy resolution, and the top quark mass at hadron colliders is presented. The method exploits the unique kinematics of events with top-antitop pair production, where one of the top quarks involves a leptonic and one a hadronic W boson decay. The paper shows a feasibility study of how this simultaneous measurement can be performed at the upcoming LHC experiments ATLAS and CMS.

Top-squark-mediated annihilation scenario and direct detection of dark matter in compressed supersymmetry

Top-squark-mediated annihilation of $b$-ino-like neutralinos to top-antitop pairs can play the dominant role in obtaining a thermal relic dark matter abundance in agreement with observations. In a previous paper, it was argued that this can occur naturally in models of compressed supersymmetry, which feature a running gluino mass parameter that is substantially smaller than the $W$-ino mass parameter at the scale of apparent gauge coupling unification. Here I study in some more detail the parameter space in which this is viable, and compare to other scenarios for obtaining the observed dark matter density. I then study the possibility of detecting the dark matter directly in future experiments. The prospects are consistently very promising for a wide variety of model parameters within this scenario.

Tevatron constraint on the Kaluza-Klein gluon of the Bulk Randall-Sundrum model

The Bulk Randall-Sundrum model, where all Standard Model particles except the Higgs are free to propagate in the bulk, predicts the existence of Kaluza-Klein (KK) modes of the gluon with a large branching into top-antitop pairs. We study the production of the lowest KK gluon mode at the Tevatron energy and use the data on the top cross-section from the Run II of Tevatron to put a bound on the mass of the KK gluon. The resulting bound of 800 GeV, while being much smaller than the constraints obtained on the KK gluon mass from flavour-changing neutral currents, is the first, direct collider bound which is independent of the specificities of the model.

Top quark spin correlations in the Randall-Sundrum model at the CERN Large Hadron Collider

In the Randall-Sundrum model, we study top-antitop pair production and top spin correlations at the Large Hadron Collider. In addition to the standard model processes, there is a new contribution to the top-antitop pair production process mediated by graviton Kaluza-Klein modes in the $s$-channel. We calculate the density matrix for the top-antitop pair production including the new contribution. With a reasonable parameter choice in the Randall-Sundrum model, we find a sizable deviation of the top-antitop pair production cross section and the top spin correlations from those in the standard model. In particular, resonant productions of the graviton Kaluza-Klein modes give rise to a remarkable enhancement of such a deviation.

Pair production of heavy MSSM charged and neutral Higgs bosons in multi-TeVe+e−collisions at the Compact Linear Collider

In the ATLAS experiment at the Large Hadron Collider (LHC) at CERN, direct searches for the elusive Higgs boson will be conducted, as well as for physics beyond the Standard Model. The charged Higgs boson (H±) is interesting both as a part of the Higgs sector, and as a clear sign of new physics. This thesis focuses on H± searches, with H± production in top-antitop pair events, and in particular the bW± bH±, H±→τhadν, W±→qq channel. Its potential was investigated as part of a larger study of the expected performance of the entire ATLAS experiment. Full simulation of the ATLAS detector and trigger was used, and all dominant systematics considered. It was shown to be the most promising H± discovery channel for mH±<mt.As hadronic τ decays are important for H± searches, their correct identification is critical. Possibilities of improving tau-jet identification in pile-up and top-antitop pair events were investigated. Redundant or even performance-reducing variables in the default likelihood identification were identified, as were new variables showing discriminatory power. This allows for increased rejection of QCD jets in these environments, and higher robustness of the method.Before any physics studies, a commissioned and well-understood detector is required. The Lorentz angle of the ATLAS Semi-Conductor Tracker (SCT) barrel was measured using 2008 cosmic-ray data. It is an important observable for the performance of several detector aspects. Potential sources of systematics were investigated and evaluated. The Lorentz angle in the SCT barrel was measured as θL = 3.93 ± 0.03(stat) ± 0.10(syst) degrees, agreeing with the model prediction.The Compact Linear Collider (CLIC) is a proposed successor to the LHC. The potential for charged and heavy neutral Higgs bosons at CLIC was investigated, in terms of both discovery and precision measurement of parameters like tanβ or the Higgs masses, up to and beyond 1 TeV, which would be challenging at the LHC

Another step towards a no-lose theorem for NMSSM Higgs discovery at the LHC

We show how the LHC potential to detect a rather light CP-even Higgs boson of the NMSSM, H1 or H2, decaying into CP-odd Higgs states, A1A1, can be improved if Higgs-strahlung off W bosons and (more marginally) off top–antitop pairs are employed alongside vector boson fusion as production modes. Our results should help extracting at least one Higgs boson signal over the NMSSM parameter space.

Weak corrections to gluon-induced top–antitop hadro-production

Abstract We calculate purely weak virtual one-loop corrections to the production cross section of top–antitop pairs at the Large Hadron Collider via the gluon–gluon fusion subprocess. We find very small negative corrections to the total cross section, of order − 0.6 % , but significantly larger effects to the differential one, particularly in the transverse momentum distribution, of order − 5 % to − 10 % (in observable regions). In case of parity-conserving spin-asymmetries of the final state, α S 2 α W corrections are typically of a few negative percent, with the exception of positive and negative peaks at + 12 % and − 5 % , respectively (near where the tree-level predictions change sign), while those arising in parity-violating asymmetries (which are identically zero in QCD) are typically at a level of a few permille.

Use of W-boson longitudinal-transverse interferencein top quark spin-correlation functions

Most of this paper consists of the derivation of general beam-referenced stage-two spin-correlation functions for the analysis of top-antitop pair production at the Tevatron, at the Large Hadron Collider, and/or at an International Linear Collider. However, for the charged-lepton plus jets reaction $q \bar{q} \rightarrow t \bar{t} \rightarrow (W^ + b) (W^- \bar{b} ) \rightarrow (l^{ + } \nu b ) (W^- \bar{b} )$ , there is a simple three-angle spin-correlation function for the determination of the relative sign of or for the measurement of a possible non-trivial phase between the two dominant λb=-1/2 helicity amplitudes for the t → W+b decay mode. For the CP-conjugate case, there is an analogous function and tests for $\bar{t} \rightarrow W^{-} \bar{b}$ decay. These results make use of W-boson longitudinal-transverse interference.

Top quark spin correlations in theories with large extra dimensions at the CERN Large Hadron Collider

In theories with large extra dimensions, we study the top-spin correlations at the Large Hadron Collider. The $s$-channel process mediated by graviton Kaluza-Klein modes contributes to the top-antitop pair production in addition to the standard model processes, and affects the resultant top-spin correlations. We calculated the full density matrix for the top-antitop pair production. With the fundamental scale of the extra dimensional theory below 2 TeV, we find a sizable deviation of the top-spin correlations from the standard model one.

Analytic results for $$e^ + e^ - \to \overline {tt} $$ and $$\gamma \gamma \to \overline {tt} $$ observables near the threshold up to the next-to-next-to-leading order of NRQCD

We present an analytical description of top-antitop pair production near the threshold in $e^+e^-$ annihilation and $\g\g$ collisions. A set of basic observables considered includes the total cross sections, forward-backward asymmetry and top quark polarization. The threshold effects relevant for the basic observables are described by three universal functions related to S wave production, P wave production and S-P interference. These functions are computed analytically up to the next-to-next-to-leading order of NRQCD. The total $e^+e^-\to t\bar t$ cross section near the threshold is obtained in the next-to-next-to-leading order in the closed form including the contribution due to the axial coupling of top quark and mediated by the Z-boson. The effects of the running of the strong coupling constant and of the finite top quark width are taken into account analytically for the P wave production and S-P wave interference.

Top-Antitop Pair Production Close to Threshold Synopsis of Recent NNLO Results

Using non-relativistic effective theories, new next-to-next-to-leading order (NNLO) QCD corrections to the total $$t{\bar t}$$ production cross section at the Linear Collider have been calculated recently. In this article the NNLO calculations of several groups are compared and the remaining uncertainties are discussed. The theoretical prospects for an accurate determination of top quark mass parameters are discussed in detail. An outlook on possible future improvements is given.

Top-antitop pair production via b-quark initiated processes at the Large Hadron Collider

We study production via subprocesses initiated by b-quarks at the Large Hadron Collider. Both QCD and electroweak interactions are included in the elementary scattering amplitudes for . Since the additional jet in the final state (arising from the bottom quark) is in most cases at very low transverse momentum and very high pseudorapidity, it tends to escape detection. Therefore, such a process can act as a background to double-top as well as to single-top channels exploited in top-quark phenomenology at present and future hadron colliders. Furthermore, if the additional b-jet can be tagged then samples can be exploited in constraining possible effects of new physics. The relevance of this reaction in such contexts is discussed and various total and differential rates of phenomenological interest are given.

WHERE IS TOP?

Possibilities are discussed for determining the top-quark mass mt from observations on the decay processes for top-antitop pairs produced in antiproton-proton collisions at Tevatron energies, assuming that the t→bW+ decay channel is dominant and much faster than hadronization. The final states [Formula: see text] provide the most striking signal, with little background, but they are rare (≈2/81). If all three candidate events prove to be from [Formula: see text], an estimate follows for P(mt| rate ), the probability distribution for mt. The one reported configuration allows an independent estimate for P (mt|μ± e∓ 2 jets ). These two distributions are compatible and can be combined to give an mt estimate of about 122 GeV. Decay events “1 energetic lepton(ℓ)+4 jets” should appear 12 times as often as “ μ± e∓ 2 jets ” events and can be analyzed to give estimates for P(mt|ℓ 4 jets ); this is illustrated for a fictitious event. There may be background from nontop events but suitable cuts on the data and our analysis procedure together reduce this to a low level. The rate observed for these events does not appear to be as large as the factor 12. Identification of either or both of the [Formula: see text] jets would be a great step forward, separating out the “[Formula: see text]” and “[Formula: see text]” events of importance with little background. We advocate an energetic approach to the analysis of individual events [whether 2, 1 or no (b or [Formula: see text]) jets identified] on an event-by-event basis, with the hope of finding a subgroup of events with a common mass estimate.

Z 0, γ production in association with top quark pairs

The production of neutral vector bosons, Z 0 and γ, in association with top-antitop pairs at LHC and SSC is studied. These reactions could provide direct information on the electroweak couplings of the top. Their relevance as a background for vector boson pair production is discussed.

Decay and polarization properties of the top quark.

Polarization and angular distributions in the decay sequence {ital t}{r arrow}{ital bW}{sup +},{ital W}{sup +}{r arrow}{ital l}{sup +}{nu}{sub {ital l}} are discussed for the standard model. Top quarks from {ital e}{sup +}{ital e{minus}}{r arrow}{ital t{bar t}} are predicted to have large polarization but, even if not, the parity-violating effects in this decay chain are large and will test closely the detailed spin structure of the electroweak interactions involving the top quark. A means of analyzing {ital {bar t}t} decays following {ital {bar t}t} production in hadronic interactions is developed, leading to an illuminating construction. Its application is illustrated by the analysis of the candidate for top-antitop pair creation in {ital {bar p}p} collisions found by the Collider Detector at Fermilab (CDF) at 1.8 TeV center-of-mass energy. If this is really {ital {bar t}t} production, then the top-quark mass would be 125{sub {minus}11}{sup +19} GeV/{ital c}{sup 2}.

Prompt photon pair production in association with top-antitop pairs. An important background to intermediate mass Higgs detection

The reaction pp→ t t ̄ γγ is studied for 80⩽ M γγ ⩽140 GeV, as a possible background to the detection of an intermedia te mass standard model Higgs in the rare ℓ νγγ final state. If the top is not too heavy the prompt photon production, integrated over a window of 6 GeV in M γγ around the Higgs mass, can be larger than the production of photon pairs from Higgs decay. Standard isolation cuts can effectively dispose of this background for m t⩾150 GeV. For m t∼100 GeV approximately the same nu mber of background and signal events pass the cuts.

Associated production of Higgs bosons with tt-bar pairs.

Associated production of Higgs bosons with top-antitop pairs at hadron colliders, pp\ensuremath{\rightarrow}Htt\ifmmode\bar\else\textasciimacron\fi{}X, is examined. By tagging on an isolated secondary lepton, e or \ensuremath{\mu}, from the t or t\ifmmode\bar\else\textasciimacron\fi{} decay, discovery of an intermediate-mass Higgs boson, ${\mathit{m}}_{\mathit{H}}$\ensuremath{\simeq}80--150 GeV, via H\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma} is rendered feasible at the Superconducting Super Collider (SSC) (\ensuremath{\surd}s =40 TeV). Allowing a factor-of-0.4 signal reduction for isolation cuts and selection efficiencies leaves about 21 events per SSC year for ${\mathit{m}}_{\mathit{H}}$\ensuremath{\simeq}80--130 GeV, and somewhat less for 130--150 GeV, above an anticipated small background. At the CERN Large Hadron Collider (\ensuremath{\surd}s =16 TeV), the cross section is reduced by about a factor of 1/8.