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Register a new userHeavy Vector-like Top Partners at the LHC and flavour constraints
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We consider the phenomenology at the Large Hadron Collider of new heavy vector-like quarks which couple mainly to the third generation quarks via Yukawa interactions, with special emphasis on non-standard doublet representations which are less constrained from present data. We also discuss in detail the flavour limits at tree level and loop level and implications of a generalised CKM mixing matrix to these cases.
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Vector-like quarks (VLQs) that are partners of the heavy top and bottom quarks are predicted in many extensions of the Standard Model (SM). We explore the possibility that these states could explain not only the longstanding anomaly in the forward-backward asymmetry in $b$-quark production at LEP, $A_{rm FB}^b $, but also the more recent $sim 2sigma$ deviation of the cross section for the associated Higgs production with top quark pairs at the LHC, $sigma(ppto tbar t H)$. Introducing three illustrative models for VLQs with different representations under the SM gauge group, we show that the two anomalies can be resolved while satisfying all other theoretical and experimental constraints. In this case, the three different models predict VLQ states in the $1-2$ TeV mass range that can be soon probed at the LHC. In a second step, we discuss the sensitivity on the VLQ masses and couplings that could be obtained by means of a percent level accuracy in the measurement of ratios of partial Higgs decay widths, in particular $Gamma(H ! to! gammagamma)/Gamma(H ! to! ZZ^*)$ and $Gamma(H ! to ! bbar b)/Gamma(H ! to ! WW^*)$. We show that top and bottom VL partners with masses up to $sim 5$ TeV and exotic VLQs with masses in the $10$ TeV range can be probed at the high-luminosity LHC.
In scenarios that stabilize the electroweak scale, the top quark is typically accompanied by partner particles. In this work, we demonstrate how extended stabilizing symmetries can yield scalar or fermionic top partners that transform as ordinary color triplets but carry exotic electric charges. We refer to these scenarios as hypertwisted since they involve modifications to hypercharge in the top sector. As proofs of principle, we construct two hypertwisted scenarios: a supersymmetric construction with spin-0 top partners, and a composite Higgs construction with spin-1/2 top partners. In both cases, the top partners are still phenomenologically compatible with the mass range motivated by weak-scale naturalness. The phenomenology of hypertwisted scenarios is diverse, since the lifetimes and decay modes of the top partners are model dependent. The novel coupling structure opens up search channels that do not typically arise in top-partner scenarios, such as pair production of top-plus-jet resonances. Furthermore, hypertwisted top partners are typically sufficiently long lived to form top-partnerium bound states that decay predominantly via annihilation, motivating searches for rare narrow resonances with diboson decay modes.
A summary of results in heavy flavour physics from Run 1 of the LHC is presented. Topics discussed include spectroscopy, mixing, CP violation and rare decays of charmed and beauty hadrons.
We explore signatures related to squark decays in the framework of non-minimally flavour-violating Supersymmetry. We consider a simplified model where the lightest squark consists of an admixture of charm and top flavour. By recasting the existing LHC searches for top and charm squarks, we show that the limits on squark masses from these analyses are significantly weakened when the top-charm mixing is sizeable. We propose a dedicated search for squarks based on the $tc+{E_{mathrm{T}}^{mathrm{miss}}}$ final state which enhances the experimental sensitivity for the case of high mixing, and we map its expected reach for the forthcoming runs of the LHC. We emphasize the role of analyses requiring a jet tagged as produced by the fragmentation of a charm quark in understanding the squark mixing pattern, thus providing a novel handle on new physics. Our results show that, in order to achieve full coverage of the parameter space of supersymmetric models, it is necessary to extend current experimental search programmes with analyses specifically targeting the cases where the lightest top-partner is a mixed state.
In this work we reappraise the collider constraints from leptonic final states on the vectorlike colored top partners taking into account the impact of exotic colored vector resonances. These colored states are intrinsic to a broad class of models that employ a strongly interacting sector to drive electroweak symmetry breaking. We translate the recent results in the {sl monolepton + jets} channel as reported by CMS with an integrated luminosity of 35.8 fb$^{-1}$, and {sl dilepton + jets} and {sl trilepton + jets} channels as reported by ATLAS with an integrated luminosity of 36.1 fb$^{-1}$ to constrain the parameter space of these class of models. We also comment on the impact and modification of the derived constraints due to the expected fatness of the colored vector resonance, when accounted for beyond the narrow-width approximation by simulating the full one-particle irreducible resummed propagator.
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