Closing in on a perturbative fourth generation

Buchkremer, Mathieu; Gérard, Jean-Marc; Maltoni, Fabio

doi:10.1007/jhep06(2012)135

Cited by 23 publications

(28 citation statements)

References 66 publications

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“…Similar earlier analyses of this type may be found in [8][9][10], but new experimental data has appeared since these publications. Other similar studies have appeared recently [11][12][13].…”

Section: Introductionsupporting

confidence: 57%

Masses of a fourth generation with two Higgs doublets

et al. 2012

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We use sampling techniques to find robust constraints on the masses of a possible fourth sequential fermion generation from electroweak oblique variables. We find that in the case of a light (115 GeV) Higgs from a single electroweak symmetry breaking doublet, inverted mass hierarchies are possible for both quarks and leptons, but a mass splitting more than MW in the quark sector is unlikely. We also find constraints in the case of a heavy (600 GeV) Higgs in a single doublet model. As recent data from the Large Hadron Collider hints at the existence of a resonance at 124.5 GeV and a single Higgs doublet at that mass is inconsistent with a fourth fermion generation, we examine a type II two Higgs doublet model. In this model, there are ranges of parameter space where the Higgs sector can potentially counteract the effects of the fourth generation. Even so, we find that such scenarios produce qualitatively similar fermion mass distributions.

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Section: Introductionsupporting

confidence: 57%

Masses of a fourth generation with two Higgs doublets

et al. 2012

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“…HÞ nor from the Higgs decay rate into photons. Consequently, current LHC Higgs data put the SM4 under serious pressure [7][8][9][10]. In a recent publication [7], we presented a global fit of the SM4 parameters to EWPOs and Higgs signal strengths measured at Tevatron and the LHC.…”

Section: Methods and Inputsmentioning

confidence: 99%

Status of the fourth fermion generation before ICHEP2012: Higgs data and electroweak precision observables

et al. 2012

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We perform a global fit of the parameters of the Standard Model with a sequential fourth generation (SM4) to LHC and Tevatron Higgs data and electroweak precision data. Using several likelihood ratio tests, we compare the performance of the SM4 and the Standard Model (SM3) at describing the measured data. Since the SM3 and SM4 are not nested (i.e., the SM3 cannot be considered as a special case of the SM4 with some parameters fixed), the usual analytical formulae for p-values in likelihood ratio tests do not hold. We thus apply a new method to compute these p-values. For a Higgs mass of 126.5 GeV and fourth-generation quark masses above 600 GeV, we find that the SM4 is excluded at 3:1. I. METHOD AND INPUTSIn this paper, we study the Standard Model with a sequential fourth generation (SM4), which differs from the established Standard Model (denoted by SM3) by an additional fermion generation. We treat the masses of the extra fermions as free parameters and allow for arbitrary flavor mixings among the quarks of the four generations in our fits. Large mixings of the fourth-generation lepton doublet with those of the first three generations are ruled out [1] from data on lepton-flavor violating decays and lepton-flavor universality [2]. Recent NA62 data constrain these mixing angles even further [3]. Including lepton mixing within the allowed range has a negligible impact on the electroweak precision observables (EWPOs). In the absence of lepton mixing, the decay of the Higgs boson into neutrinos is invisible as long as the fourth-generation charged lepton is heavier than the corresponding neutrino. This invisible Higgs decay mode increases the total Higgs width and potentially counterbalances the effect of the enhanced gg ! H production mechanism [4,5], because the branching fractions into the observed final states are reduced [6,7]. Allowing for (even small) mixing of the fourth with the other lepton doublets can render the neutrino decay mode visible. Since we want to quantify the level at which the SM4 is ruled out, we may confine ourselves to the most conservative scenario with an unmixed fourth-generation lepton doublet. Like the SM3, the SM4 can be studied with Dirac or Majorana neutrinos. In the fits presented in this paper, we use Dirac neutrinos. In our conclusions, we briefly discuss the (marginal) changes in the results expected for Majorana neutrinos.From a model-building point of view, the hierarchy between three almost massless neutrinos and a fourth neutrino with mass of order of the electroweak scale can be motivated by a symmetry enforcing massless neutrinos in the exact symmetry limit: e.g., three right-handed neutrino fields might carry some U(1) charge while the fourth neutrino field and the left-handed lepton doublets are uncharged under this new symmetry. The Yukawa couplings are small spurions breaking this symmetry, leading to three tiny neutrino masses and tiny mixings between the fourth and the other generations.A sequential fourth generation of fermions decouples neither from the product...

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“…SM-like "sequential" fourth-generation quarks are disfavored, though not completely excluded [17][18][19][20]. On the other hand, so-called "vectorlike" quarks (VLQs), for which both the leftand right-handed fields transform identically under the SUð2Þ × Uð1Þ gauge transformations [21], remain viable.…”

Section: Introductionmentioning

confidence: 99%

Search for vectorlikeBquarks in events with one isolated lepton, missing transverse momentum, and jets ats=8 TeVwith the AT

Aad¹,

Abbott²,

Abdallah³

et al. 2015

Phys. Rev. D

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A search has been performed for pair production of heavy vectorlike down-type (B) quarks. The analysis explores the lepton-plus-jets final state, characterized by events with one isolated charged lepton (electron or muon), significant missing transverse momentum, and multiple jets. One or more jets are required to be tagged as arising from b quarks, and at least one pair of jets must be tagged as arising from the hadronic decay of an electroweak boson. The analysis uses the full data sample of pp collisions recorded in 2012 by the ATLAS detector at the LHC, operating at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb −1 . No significant excess of events is observed above the expected background. Limits are set on vectorlike B production, as a function of the B branching ratios, assuming the allowable decay modes are B → Wt=Zb=Hb. In the chiral limit with a branching ratio of 100% for the decay B → Wt, the observed (expected) 95% C.L. lower limit on the vectorlike B mass is 810 GeV (760 GeV). In the case where the vectorlike B quark has branching ratio values corresponding to those of an SUð2Þ singlet state, the observed (expected) 95% C.L. lower limit on the vectorlike B mass is 640 GeV (505 GeV). The same analysis, when used to investigate pair production of a colored, charge 5=3 exotic fermion T 5=3 , with subsequent decay T 5=3 → Wt, sets an observed (expected) 95% C.L. lower limit on the T 5=3 mass of 840 GeV (780 GeV).

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Closing in on a perturbative fourth generation

Cited by 23 publications

References 66 publications

Masses of a fourth generation with two Higgs doublets

Masses of a fourth generation with two Higgs doublets

Status of the fourth fermion generation before ICHEP2012: Higgs data and electroweak precision observables

Search for vectorlikeBquarks in events with one isolated lepton, missing transverse momentum, and jets ats=8 TeVwith the AT

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