Closing in on a perturbative fourth generation

  • Mathieu Buchkremer
  • Jean-Marc Gérard
  • Fabio Maltoni


A perturbative new family of fermions is now severely constrained, though not excluded yet. We reconsider the current bounds (i.e., direct and from Higgs searches, R b, oblique parameters) on the fourth generation parameter space assuming the case of a small CKM mixing with the third generation. We identify viable scenarios featuring either a light or a heavy Higgs boson. A set of representative benchmark points targeted for LHC searches is proposed with a normal (inverted) quark mass hierarchy where \( {t^{\prime }} \to {b^{\prime }}W\left( {{b^{\prime }} \to {t^{\prime }}W} \right) \) are sizable. In the case where the fourth generation couplings to the lighter quark families are small, we suggest that search strategies at the LHC should include both pair (strong) and single (weak) production with bb + nW (n = 2, … , 6) final state signatures.


Higgs Physics Beyond Standard Model 


  1. [1]
    B. Holdom et al., Four statements about the fourth generation, PMC Phys. A 3 (2009) 4 [arXiv:0904.4698] [INSPIRE].ADSCrossRefGoogle Scholar
  2. [2]
    J. Alwall et al., Is V tb ≃ 1?, Eur. Phys. J. C 49 (2007) 791 [hep-ph/0607115] [INSPIRE].ADSCrossRefGoogle Scholar
  3. [3]
    E. Asılar et al., Oblique parameters and extra generations via OPUCEM, Eur. Phys. J. C 72 (2012)1966 [arXiv:1112.0507] [INSPIRE].ADSCrossRefGoogle Scholar
  4. [4]
    G.D. Kribs, T. Plehn, M. Spannowsky and T.M. Tait, Four generations and Higgs physics, Phys. Rev. D 76 (2007) 075016 [arXiv:0706.3718] [INSPIRE].ADSGoogle Scholar
  5. [5]
    M. Baak et al., Updated status of the global electroweak fit and constraints on new physics, Eur. Phys. J. C 72 (2012) 2003 [arXiv:1107.0975] [INSPIRE].ADSCrossRefGoogle Scholar
  6. [6]
    CDF collaboration, T. Aaltonen et al., Search for heavy bottom-like quarks decaying to an electron or muon and jets in pp collisions at \( \sqrt {s} = 1.96\;TeV \), Phys. Rev. Lett. 106 (2011) 141803 [arXiv:1101.5728] [INSPIRE].ADSCrossRefGoogle Scholar
  7. [7]
    CDF collaboration, J. Conway et al., Search for heavy top t  → W q in lepton plus jets events in 4.6 fb −1, CDF public conference note CDF-PUB-TOP-PUBLIC-10110, Fermilab, Batavia U.S.A. (2010).Google Scholar
  8. [8]
    CMS collaboration, Search for a heavy bottom-like quark in pp collisions at \( \sqrt {s} = 7\;TeV \), CMS-PAS-EXO-11-036, CERN, Geneva Switzerland (2011) [JHEP 05 (2012) 123] [arXiv:1204.1088] [INSPIRE].
  9. [9]
    CMS collaboration, Search for a heavy top-like quark in the dilepton final state in pp collisions at 7 TeV, CMS-PAS-EXO-11-050, CERN, Geneva Switzerland (2011) [arXiv:1203.5410] [INSPIRE].
  10. [10]
    CMS collaboration, Inclusive search for a fourth generation of quarks with the CMS experiment, CMS-PAS-EXO-11-054, CERN, Geneva Switzerland (2011).Google Scholar
  11. [11]
    ATLAS collaboration, G. Aad et al., Search for pair-produced heavy quarks decaying to W q in the two-lepton channel at \( \sqrt {s} = 7\;TeV \) with the ATLAS detector, CERN-PH-EP-2012-009, CERN, Geneva Switzerland (2012) [Phys. Rev. D 85 (2012) 092014] [arXiv:1203.3161] [INSPIRE].
  12. [12]
    J. Erler and P. Langacker, Precision constraints on extra fermion generations, Phys. Rev. Lett. 105 (2010) 031801 [arXiv:1003.3211] [INSPIRE].ADSCrossRefGoogle Scholar
  13. [13]
    A. Wingerter, Implications of the stability and triviality bounds on the Standard Model with three and four chiral generations, Phys. Rev. D 84 (2011) 095012 [arXiv:1109.5140] [INSPIRE].ADSGoogle Scholar
  14. [14]
    H. Murayama, V. Rentala, J. Shu and T.T. Yanagida, Saving fourth generation and baryon number by living long, Phys. Lett. B 705 (2011) 208 [arXiv:1012.0338] [INSPIRE].ADSCrossRefGoogle Scholar
  15. [15]
    G. Burdman and L. Da Rold, Electroweak symmetry breaking from a holographic fourth generation, JHEP 12 (2007) 086 [arXiv:0710.0623] [INSPIRE].ADSCrossRefGoogle Scholar
  16. [16]
    K. Ishiwata and M.B. Wise, Fourth generation bound states, Phys. Rev. D 83 (2011) 074015 [arXiv:1103.0611] [INSPIRE].ADSGoogle Scholar
  17. [17]
    T. Enkhbat, W.-S. Hou and H. Yokoya, Early LHC phenomenology of Yukawa-bound heavy QQ mesons,Phys. Rev. D 84 (2011)094013 [arXiv:1109.3382][INSPIRE].ADSGoogle Scholar
  18. [18]
    L3 collaboration, P. Achard et al., Search for heavy neutral and charged leptons in e + e annihilation at LEP, Phys. Lett. B 517 (2001) 75 [hep-ex/0107015] [INSPIRE].ADSCrossRefGoogle Scholar
  19. [19]
    Particle Data Group collaboration, K. Nakamura et al., Review of particle physics, J. Phys. G 37 (2010) 075021 [INSPIRE].ADSCrossRefGoogle Scholar
  20. [20]
    H. Lacker and A. Menzel, Simultaneous extraction of the Fermi constant and PMNS matrix elements in the presence of a fourth generation, JHEP 07 (2010) 006 [arXiv:1003.4532] [INSPIRE].ADSCrossRefGoogle Scholar
  21. [21]
    O. Cobanoglu, E. Ozcan, S. Sultansoy and G. Unel, OPUCEM: a library with error checking mechanism for computing oblique parameters, Comput. Phys. Commun. 182 (2011) 1732 [arXiv:1005.2784] [INSPIRE].ADSCrossRefMATHGoogle Scholar
  22. [22]
    CMS collaboration, Combined results of searches for the standard model Higgs boson in pp collisions at \( \sqrt {s} = 7\;TeV \), CMS-HIG-11-032, CERN, Geneva Switzerland (2012) [Phys. Lett. B 710 (2012) 26] [arXiv:1202.1488] [INSPIRE].
  23. [23]
    CMS collaboration, Search for the standard model Higgs boson decaying into two photons in pp collisions at \( \sqrt {s} = 7\;TeV \), CMS-HIG-11-033, CERN, Geneva Switzerland (2012) [Phys. Lett. B 710 (2012) 403 ] [arXiv:1202.1487] [INSPIRE].
  24. [24]
    ATLAS collaboration, An update to the combined search for the Standard Model Higgs boson with the ATLAS detector at the LHC using up to 4.9 fb −1 of pp collision data at \( \sqrt {s} = 7\;TeV \), ATLAS-CONF-2012-019, CERN, Geneva Switzerland (2012).Google Scholar
  25. [25]
    ATLAS collaboration, Search for the Standard Model Higgs boson in the diphoton decay channel with 4.9 fb −1 of ATLAS data at \( \sqrt {s} = 7\;TeV \), ATLAS-CONF-2011-161, CERN, Geneva Switzerland (2011).Google Scholar
  26. [26]
    CMS collaboration, Search for Standard Model Higgs boson in pp collisions at \( \sqrt {s} = 7\;TeV \), CMS-PAS-HIG-11-011, CERN, Geneva Switzerland (2011) [INSPIRE].
  27. [27]
    CMS collaboration, Combination of SM, SM4, FP Higgs boson searches, CMS-PAS-HIG-12-008, CERN, Geneva Switzerland (2012).Google Scholar
  28. [28]
    ATLAS collaboration, Update of the combination of Higgs boson searches in pp collisions at \( \sqrt {s} = 7\;TeV \) with the ATLAS experiment at the LHC,ATLAS-CONF-2011-135,CERN, Geneva Switzerland (2011).Google Scholar
  29. [29]
    S. Cetin, T. Cuhadar-Donszelmann, M. Sahin, S. Sultansoy and G. Unel, Impact of the relatively light fourth family neutrino on the Higgs boson search, Phys. Lett. B 710 (2012) 328 [arXiv:1108.4071] [INSPIRE].ADSCrossRefGoogle Scholar
  30. [30]
    L.M. Carpenter, Higgs search constraints on fourth generation scenarios with general lepton sectors, arXiv:1110.4895 [INSPIRE].
  31. [31]
    G. Guo, B. Ren and X.-G. He, LHC evidence of a 126 GeV Higgs boson from H → γγ with three and four generations, arXiv:1112.3188 [INSPIRE].
  32. [32]
    A. Denner et al., Higgs production and decay with a fourth Standard-Model-like fermion generation, Eur. Phys. J. C 72 (2012) 1992 [arXiv:1111.6395] [INSPIRE].ADSCrossRefGoogle Scholar
  33. [33]
    A. Djouadi and A. Lenz, Sealing the fate of a fourth generation of fermions, arXiv:1204.1252 [INSPIRE].
  34. [34]
    E. Kuflik, Y. Nir and T. Volansky, Implications of Higgs searches on the four generation Standard Model, arXiv:1204.1975 [INSPIRE].
  35. [35]
    O. Eberhardt et al., Joint analysis of Higgs decays and electroweak precision observables in the Standard Model with a sequential fourth generation, arXiv:1204.3872 [INSPIRE].
  36. [36]
    C. Anastasiou, S. Buehler, F. Herzog and A. Lazopoulos, Inclusive Higgs boson cross-section for the LHC at 8 TeV, JHEP 04 (2012) 004 [arXiv:1202.3638] [INSPIRE].ADSCrossRefGoogle Scholar
  37. [37]
    P. Gonzalez, J. Rohrwild and M. Wiebusch, Electroweak precision observables within a fourth generation model with general flavour structure, Eur. Phys. J. C 72 (2012) 2007 [arXiv:1105.3434] [INSPIRE].ADSCrossRefGoogle Scholar
  38. [38]
    T. Yanir, Phenomenological constraints on extended quark sectors, JHEP 06 (2002) 044 [hep-ph/0205073] [INSPIRE].ADSCrossRefGoogle Scholar
  39. [39]
    J. Bernabeu, A. Pich and A. Santamaria, Top quark mass from radiative corrections to the Z → bbdecay,Nucl. Phys. B 363 (1991)326 [INSPIRE].ADSCrossRefGoogle Scholar
  40. [40]
    B.A. Kniehl and J.H. Kuhn, QCD corrections to the Z decay rate, Nucl. Phys. B 329 (1990) 547 [INSPIRE].ADSCrossRefGoogle Scholar
  41. [41]
    M.E. Peskin and T. Takeuchi, Estimation of oblique electroweak corrections, Phys. Rev. D 46 (1992)381 [INSPIRE].ADSGoogle Scholar
  42. [42]
    O. Eberhardt, A. Lenz and J. Rohrwild, Less space for a new family of fermions, Phys. Rev. D 82 (2010) 095006 [arXiv:1005.3505] [INSPIRE].ADSGoogle Scholar
  43. [43]
    J.R. Forshaw, D. Ross and B. White, Higgs mass bounds in a triplet model, JHEP 10 (2001) 007 [hep-ph/0107232] [INSPIRE].ADSCrossRefGoogle Scholar
  44. [44]
    LEP Electroweak Working Group webpage, status of August 2009,
  45. [45]
    M.S. Chanowitz, Bounding CKM mixing with a fourth family, Phys. Rev. D 79 (2009) 113008 [arXiv:0904.3570] [INSPIRE].ADSGoogle Scholar
  46. [46]
    P.H. Frampton, P. Hung and M. Sher, Quarks and leptons beyond the third generation, Phys. Rept. 330 (2000) 263 [hep-ph/9903387] [INSPIRE].ADSCrossRefGoogle Scholar
  47. [47]
    J.M. Campbell, R. Frederix, F. Maltoni and F. Tramontano, Next-to-leading-order predictions for t-channel single-top production at hadron colliders, Phys. Rev. Lett. 102 (2009)182003 [arXiv:0903.0005] [INSPIRE].ADSCrossRefGoogle Scholar
  48. [48]
    J.M. Campbell and R.K. Ellis, Next-to-leading order corrections to W + 2 jet and Z + 2 jet production at hadron colliders, Phys. Rev. D 65 (2002) 113007 [hep-ph/0202176] [INSPIRE].ADSGoogle Scholar
  49. [49]
    Y. Chao et al., Threshold effects in the decay of heavy band tquarks, Phys. Rev. D 84 (2011)014029 [arXiv:1101.0592] [INSPIRE].ADSGoogle Scholar
  50. [50]
    A. Dighe, D. Ghosh, R.M. Godbole and A. Prasath, Large mass splittings for fourth generation fermions allowed by LHC Higgs exclusion, arXiv:1204.3550 [INSPIRE].

Copyright information

© SISSA, Trieste, Italy 2012

Authors and Affiliations

  • Mathieu Buchkremer
    • 1
  • Jean-Marc Gérard
    • 1
  • Fabio Maltoni
    • 1
  1. 1.Centre for Cosmology, Particle Physics and Phenomenology (CP3)Université catholique de LouvainLouvain-la-NeuveBelgium

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