Effects of mixing with quark singlets

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Analysis of events with b-jets and a pair of leptons of the same charge in pp collisions at √ s = 8 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.chAbstract: An analysis is presented of events containing jets including at least one btagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb −1 of pp collisions at √ s = 8 TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced fourtop-quark production, pair production of chiral b -quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral b -quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models. The ATLAS collaboration 35 IntroductionThe Standard Model (SM) has been repeatedly confirmed experimentally. Nonetheless there is a need for physics beyond the SM (BSM) at about the TeV scale, with additional features that explain the baryon asymmetry of the universe, specify the nature of dark matter, and provide a mechanism to naturally stabilize the Higgs boson mass at its observed value of approximately 125 GeV [1,2]. This paper reports on a search for BSM physics resulting in pairs of isolated high-transverse-momentum (high-p T ) leptons 1 with the same electric charge, hereafter denoted as same-sign leptons, (or three or more leptons of any charge) missing transverse momentum, and b-jets. This is a promising search channel since the SM yields of such events are small, and several types of BSM physics may contribute. Among the models that predict enhanced same-sign lepton production are those that postulate the existence of vector-like quarks, an enhancement of the four-top-quark production cross section, the existence of a fourth generation of chiral quarks, or production of two positively charged top quarks. A common data sample is used to search for each of 1 Only electrons and muons are considered in the search. Tau leptons are not explicitly reconstructed, but electrons and muons from τ decay may enter the selected samples. Table ...

Section: Introductionmentioning

confidence: 99%

Analysis of events with b-jets and a pair of leptons of the same charge in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Aad¹,

Abbott²,

Abdallah³

et al. 2015

Self Cite

“…(This assumption is also in agreement with stringent experimental constraints arising from flavour-changing processes at low energies [53][54][55][56].) Therefore, we can ignore the first two generations and write the relation between weak eigenstates and mass eigenstates as 4) JHEP06(2017)095…”

Section: Interactions For Two Scalar Doubletsmentioning

confidence: 49%

Novel signatures for vector-like quarks

Aguilar-Saavedra

López-Fogliani

Muñoz

2017

Abstract:We consider supersymmetric extensions of the standard model with a vectorlike doublet (T B) of quarks with charge 2/3 and −1/3, respectively. Compared to nonsupersymmetric models, there is a variety of new decay modes for the vector-like quarks, involving the extra scalars present in supersymmetry. The importance of these new modes, yielding multi-top, multi-bottom and also multi-Higgs signals, is highlighted by the analysis of several benchmark scenarios. We show how the triangles commonly used to represent the branching ratios of the 'standard' decay modes of the vector-like quarks involving W , Z or Higgs bosons can be generalised to include additional channels. We give an example by recasting the limits of a recent heavy quark search for this more general case.

“…A tighter constraint originates from the unitarity of the CKM mixing matrix and flavour physics: however such bound is not applicable here because it does not take into account the effect of the heavy fermions, and it is also sensible to the contribution of other particles in the model (see [24] for a detailed study of bounds for singlet quarks and [25] for a vector-like up-type quark or a fourth generation). The couplings of the Z to the bottom are also directly measured and very constrained [26]: in the left-handed coupling a +1% and −0.2% deviations are allowed; in the right-handed one, +20% and −5%.…”

Section: Jhep11(2010)159mentioning

confidence: 99%

Bounds and decays of new heavy vector-like top partners

Cacciapaglia

Deandrea

Harada

et al. 2010

120

Abstract:We study the phenomenology of new heavy vector-like fermions that couple to the third generation quarks via Yukawa interactions, covering all the allowed representations under the standard model gauge groups. We first review tree and loop level bounds on these states. We then discuss tree level decays and loop-induced decays to photon or gluon plus top. The main decays at tree level are to W b and/or Z and Higgs plus top via the new Yukawa couplings. The radiative loop decays turn out to be quite close to the naive estimate: in all cases, in the allowed perturbative parameter space, the branching ratios are mildly sensitive on the new Yukawa couplings and small. We therefore conclude that the new states can be observed at the LHC and that the tree level decays can allow to distinguish the different representations. Moreover, the observation of the radiative decays at the LHC would suggest a large Yukawa coupling in the non-perturbative regime.