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We make a Monte Carlo study on compositeness of first generation quarks and leptons using the Drell-Yan distribution in the high dielectron mass region at the Tevatron and LHC energies. The current experimental lower limits on the compositeness scale, Lambda, vary from 2.5 to 6.1 TeV. In the present analysis, we assume that there will be no deviation of the dielectron mass spectrum from Standard Model prediction at center of mass energy 2 TeV (Tevatron) and 14 TeV (LHC). We then find that in the LL, RR, RL and LR chirality channels of the quark-electron currents, it is possible to extend the lower limits on Lambda (at 95% {CL}) to a range of 6 to 10 TeV for 2 fb^{-1} and 9 to 19 TeV for 30 fb^{-1} of integrated luminosity at Tevatron. At LHC, the corresponding limits extend to a range of 16 to 25 TeV for 10 fb^{-1} and 20 to 36 TeV for 100 fb^{-1} of integrated luminosity.
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We explore the possibility that the right-handed top quark is composite. We examine the consequences that compositeness would have on $t bar{t}$ production at the Tevatron, and derive a weak constraint on the scale of compositeness of order a few hundred GeV from the $t bar{t}$ inclusive cross section. More detailed studies of differential properties of $t bar{t}$ production could potentially improve this limit. We find that a composite top can result in an enhancement of the $t bar{t} t bar{t}$ production rate at the LHC (of as much as $10^3$ compared to the Standatd Model four top rate). We explore observables which allow us to extract the four top rate from the backgrounds, and show that the LHC can either discover or constrain top compositeness for wide ranges of parameter space.
This paper provides a review of the experimental studies of processes with a single top quark at the Tevatron proton-antiproton collider and the LHC proton-proton collider. Single top-quark production in the t-channel process has been measured at both colliders. The s-channel process has been observed at the Tevatron, and its rate has been also measured at the center-of-mass energy of 8 TeV at the LHC in spite of the comparatively harsher background contamination. LHC data also brought the observation of the associated production of a single top quark with a W boson as well as with a Z boson. The Cabibbo-Kobayashi-Maskawa matrix element |Vtb| is extracted from the single-top-quark production cross sections, and t-channel events are used to measure several properties of the top quark and set constraints on models of physics beyond the Standard Model. Rare final states with a single top quark are searched for, as enhancements in their production rates, if observed, would be clear signs of new physics.
In this paper searches are presented for the pair production of first and second generation scalar leptoquarks and limits are given on the quark-lepton compositeness scale from proton-antiproton collision data at a center-of-mass energy 1.96 TeV, collected with Run II D0 Detector in 2002-2004. No evidence for a leptoquark signal has been observed. From the upper bounds on the product of cross section times branching ratio beta=Br(LQ to lj), a lower mass limits of M(LQ1) > 241 GeV and M(LQ2) > 247 GeV for the first and second LQ generation are set for beta=1. These results, combined with those obtained by D0 in Run I at a center-of-mass energy of 1.8 TeV, allow to exclude scalar LQ masses up to 256 GeV and 251 GeV (for beta=1) for the first and second generation, respectively. The dilepton mass spectra in pp -> l+l-+X interactions are studied using dielectron (dimuon) data samples, corresponding to an integrated luminosity of 271 pb-1 (406 pb-1). The mass spectra being a probe for new physics are examined for new interactions of quarks and leptons from a common composite structure. No excess of events is found over the expectation from Standard Model processes. The current experimental lower limits on the compositeness scale vary, for different chirality channels, from 3.6 to 9.1 TeV for the (eeqq) and from 4.2 to 9.8 TeV for the (mumuqq) contact interaction.
In composite Higgs (CH) models, large mixings between the top quark and the new strongly interacting sector are required to generate its sizeable Yukawa coupling. Precise measurements involving top as well as left-handed bottom quarks therefore offer an interesting opportunity to probe such new physics scenarios. We study the impact of third-generation-quark pair production at future lepton colliders, translating prospective effective-field-theory sensitivities into the CH parameter space. Our results show that one can probe a significant fraction of the natural CH parameter space through the top portal, especially at TeV centre-of-mass energies.
This note is an attempt to interpret some excesses, not yet significant due to systematics, observed by ATLAS and CMS in various analyses related to the standard channels ttH, ttZ and ttW. It is argued, within a composite interpretation of top particles, that such excesses are not necessarily related to these channels themselves, although this is not excluded, but due to the underlying presence of either vector-like heavy quarks tprime,bprime or to final states as predicted in composite theories. The outcome of this discussion is that although it will not be easy to reach an exclusive interpretation, the data collected at 13 TeV may establish the origin of this effect as coming from the four top quark topology.
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