arXiv subjects
Publications and source records attributed to J. A. Aguilar-Saavedra.
We discuss the eight independent spin observables for the $W$ boson in terms of its vector and tensor polarizations and identify the angular distributions and asymmetries able to separate them in collider experiments. The results are applied to the study of polarized top quark decays and diboson resonances. These novel observables are of great value for disentangling new physics mechanisms in $W$ boson production.
The ATLAS excess in fat jet pair production is kinematically compatible with the decay of a heavy resonance into two gauge bosons plus an extra particle. This hypothesis would explain the absence of such a large excess in the analogous CMS analysis of fat dijet final states, as well as the negative results of diboson resonance searches in the semi-leptonic decay modes. If the extra particle is the Higgs boson, this hypothesis might also explain -statistical fluctuations aside- why the CMS search for WH resonances in the semi-leptonic channel finds some excess while in the fully hadronic one it does not have a significant deviation.
We investigate existing and future hadron-collider constraints on the top dipole chromomagnetic and chromoelectric moments, two quantities that are expected to be modified in the presence of new physics. We focus first on recent measurements of the inclusive top pair production cross section at the Tevatron and at the Large Hadron Collider. We then analyse the role of top-antitop events produced at very large invariant masses, in the context of the forthcoming 13-14 TeV runs of the LHC, and at a future 100 TeV proton-proton collider. In this latter case, the selection of semileptonic decays to hard muons allows to tag top quarks boosted to the multi-TeV regime, strongly reducing the QCD backgrounds and leading to a significant improvement in the sensitivity to anomalous top couplings.
We review the asymmetries in top quark pair production at the Tevatron and the LHC. We summarize the experimental measurements and the interpretations of a possible excess in terms of new physics. We also review other top quark properties-emphasizing effects related to the $t \bar t$ asymmetries-as well as other collider signals.
We discuss top polarisation observables at the Tevatron and the LHC with special attention to some that have not been measured and provide new, independent information about the top polarisation.
New colour octets stand out among the new physics proposals to explain the anomalous forward-backward asymmetry measured in $t \bar t$ production by the CDF experiment at the Tevatron. We perform a fit to $t \bar t$ observables at the Tevatron and the LHC, including total cross sections, various asymmetries and the top polarisation and spin correlations, to find the most likely parameters of a light colour octet to be consistent with data. In particular, an octet coupling only to right-handed quarks gives a good fit to all measurements. The implications from the general fit are drawn in terms of predictions for top polarisation observables whose measurements are yet not very precise, and observables which simply have not been measured.
The CMS collaboration has recently reported a 2.8$σ$ excess of $eejj$ events with an invariant mass around 2 TeV. This observation can be explained in the context of standard model extensions with new gauge bosons $W'$, $Z'$ and heavy neutrinos coupling (mainly) to the electron. We discuss additional signals that allow to confirm or discard the $W'$ and $Z'$ hypotheses.
We revisit the relation between the asymmetries $A_{FB}$ and $A_{FB}^\ell$ in $t \bar t$ production at the Tevatron, using as new physics benchmark a colour octet. We find that $A_{FB}^\ell$ receives large contributions from the interference between $λ= \pm 1/2$ top helicity states, which has been ignored in some of the previous literature on the subject. The omission of these contributions results in a severe underestimation of the asymmetry, around $1/2$ and $1/50$ of the true value for right-handed and left-handed top couplings to the octet, respectively. Interference effects are closely related to a $\mathcal{O}(1)$ transverse top polarisation, as yet not considered in this context.
We define three orthogonal axes to investigate the top quark polarization in the $t$-channel single top process. We provide expressions for the polarization in these axes in terms of anomalous $Wtb$ couplings. It is found that the polarizations in the two axes orthogonal to the spectator quark axis are very sensitive to an anomalous coupling involving a $\bar b_L σ^{μν} t_R$ dipole term. In particular, an asymmetry based on the polarization normal to the production plane is more sensitive to the imaginary part of this coupling than previously studied observables.
We investigate a charge asymmetry in $t \bar t γ$ production at the LHC that provides complementary information to the measured asymmetries in $t \bar t$ production. We estimate the experimental uncertainty in its measurement at the LHC with 8 and 14 TeV. For new physics models that simultaneously reproduce the asymmetry excess in $t \bar t$ at the Tevatron and the SM-like asymmetry at the LHC, the measurement in $t \bar t γ$ at the LHC could exhibit significant deviations with respect to the SM prediction.
We argue why vector-like quarks are usually expected to mix predominantly with the third generation, and discuss about the expected size of this mixing and its naturalness.
We obtain constraints on the mixing of vector-like quarks coupling predominantly to the third generation. We consider all (seven) relevant types of vector-like quarks, individually. The constraints are derived from oblique corrections and Z -> b bbar measurements at LEP and SLC. We investigate the implications of these constraints on LHC phenomenology, concerning the decays of the heavy quarks and their single production. We also explore indirect effects of heavy quark mixing in top and bottom couplings. A remarkable effect is the possibility of explaining the anomalous forward-backward asymmetry in Z -> b bbar at LEP, with a hypercharge -5/6 doublet. We also study the impact of the new quarks on single Higgs production at the LHC and Higgs decay.
We present a reinterpretation of CMS searches for type-III seesaw lepton triplets at the LHC, both for the normal and the inverse seesaw. We find that, in contrast with previous expectations, these searches in the trilepton final state have a good sensitivity to triplets with predominant coupling to the tau lepton. We also show that the limits resulting from direct searches can be neatly presented for arbitrary masses and general flavour mixings. Thus, it turns out that the common (and often unrealistic) simplifying assumptions about the flavour couplings of the triplets used by experimental collaborations to present their results are unnecessary and should be dropped.
We present the current status of the Tevatron charge asymmetry and its sister asymmetry at the LHC. The relation between both is elucidated, using as framework the collider-independent asymmetries they originate from. Other related observables, such as the t tbar differential distribution and top polarisation, are also discussed.
We review the model-independent description of the couplings of the top quark to the Higgs and gauge bosons in theories beyond the Standard Model. Then we examine these couplings in the case of arbitrary heavy vector-like quarks mixing with the third family. We also discuss the couplings of these top partners, and comment on implications for LHC searches.
We compute, for top quark pair production at the Tevatron and the Large Hadron Collider, the collider-independent forward-backward asymmetries defined in [1] in the standard model at next-to-leading order in QCD, including also electromagnetic and weak corrections.
We introduce a new asymmetry in the decay t -> W b -> l nu b, which is shown to be directly proportional to the polarisation of the top quark along a chosen axis, times a sum of W helicity fractions. The latter have already been precisely measured at the Tevatron and the Large Hadron Collider. Therefore, this new asymmetry can be used to obtain a model-independent measurement of the polarisation of top quarks produced in any process at hadron or lepton colliders.
We introduce the forward-backward asymmetries A_u, A_d corresponding to u ubar, d dbar -> t tbar production, respectively, at hadron colliders. These are collider- and center-of-mass-independent observables, directly related to the forward-backward and charge asymmetries measured at the Tevatron and the LHC, respectively. We discuss how to extract these asymmetries from data. Because these asymmetries are collider-independent, their measurement at these two colliders could elucidate the nature of the anomalous forward-backward asymmetry measured at the Tevatron. Our framework also shows in a model-independent fashion that a positive Tevatron asymmetry exceeding the standard model expectation is compatible with the small asymmetry measured at the LHC.