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Register a new userThe effects of large extra dimensions on associated $tbar{t} h^0$ production at linear colliders
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In the framework of the large extra dimensions (LED) model, the effects of LED on the processes rrtth and eetth at future linear colliders are investigated in both polarized and unpolarized collision modes. The results show that the virtual Kaluza-Klein (KK) graviton exchange can significantly modify the standard model expectations for these processes with certain polarizations of initial states. The process rrtth with $sqrt{s}=3.5 TeV$ allows the effective scale $Lambda_T$ to be probed up to 7.8 and 8.6 TeV in the unpolarized and $P_{gamma} = 0.9$, J=2 polarized $gamma gamma$ collision modes, respectively. For the eetth process with $sqrt{s}=3.5 TeV$, the upper limits of $Lambda_T$ to be observed can be 6.7 and 7.0 TeV in the unpolarized and $P_{e^+} = 0.6$, $P_{e^-} = 0.8$, $-+$ polarized $e^+e^-$ collision modes, respectively. We find the rrtth channel in J=2 polarized photon collision mode provides a possibility to improve the sensitivity to the graviton tower exchange.
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In the frameworks of the littlest Higgs($LH$) model and its extension with T-parity($LHT$), we studied the associated $tbar th^0$ production process $e^+ e^- to gammagamma to t bar t h^0$ at the future $e^+e^-$ linear colliders up to QCD next-to-leading order. We present the regions of $sqrt{s}-f$ parameter space in which the $LH$ and $LHT$ effects can and cannot be discovered with the criteria assumed in this paper. The production rates of process $gammagamma to t bar t h^0$ in different photon polarization collision modes are also discussed. We conclude that one could observe the effects contributed by the $LH$ or $LHT$ model on the cross section for the process $e^+ e^- to gammagamma to t bar t h^0$ in a reasonable parameter space, or might put more stringent constraints on the $LH$/$LHT$ parameters in the future experiments at linear colliders.
We report on our recent work on electroweak corrections to $tbar{t}$ production at hadron colliders. Specifically, we discuss the weak-interaction contributions to the top quark transverse momentum and $t bar{t}$ invariant mass distributions and an induced parity-violating top-spin asymmetry.
We consider QCD radiative corrections to the associated production of a heavy-quark pair ($Q{bar Q}$) with a generic colourless system $F$ at hadron colliders. We discuss the resummation formalism for the production of the $Q{bar Q}F$ system at small values of its total transverse momentum $q_T$. The perturbative expansion of the resummation formula leads to the explicit ingredients that can be used to apply the $q_T$ subtraction formalism to fixed-order calculations for this class of processes. We use the $q_T$ subtraction formalism to perform a fully differential perturbative computation for the production of a top-antitop quark pair and a Higgs boson. At next-to-leading order we compare our results with those obtained with established subtraction methods and we find complete agreement. We present, for the first time, the results for the flavour off-diagonal partonic channels at the next-to-next-to-leading order.
In the following we present our recent results on the resummation of soft gluon corrections to the $pprightarrow tbar{t}H$ cross section at the LHC. The resummation was carried out at next-to-next-to-leading-logarithmic (NNLL) accuracy using the Mellin space technique. Obtained results were matched to the NLO cross section. We show that the resummation leads to reduction of scale-variation uncertainty of the total $pprightarrow tbar{t}H$ cross section.
The resummation of multiple soft gluon emission affects the production rate and kinematic distributions of $W^+h$ (where h is a Higgs boson) and $t bar b$ pairs at the Tevatron with $sqrt{s}=2$ TeV. Using the Collins-Soper-Sterman resummation formalism, the production rate is enhanced over the next-to-leading-order (NLO) prediction by 2-3% for the $W^+h$ process, for Higgs boson masses between 80-120 GeV, and over 3% for the $tbar b$ process for $m_t=175$ GeV. After resummation, the $tbar b$ rate changes by 12-13% when $m_t$ is varied by $pm 5$ GeV. Various kinematic distributions are presented for the individual final state particles and for the pair. The explicit radiation of hard gluons in NLO QCD is included also for the $tbar b$ final state.
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