th Discovery Discovery of of the the 4 4th generation generation Outline Outline Motivation Electroweak constraints Selected flavor constraints in B and D sector Conclusion 1
Introduction Introduction Focus here on chiral fourth generation neutrino either Majorana or Dirac (but mº > mz/2) New parameters: 2 x (2 masses + 3 mixing angles + 2 phases) = 14 Direct searches constraints (Tevatron pair production, decay to WWqq) (LEP) Invalidates the SM3 CKM unitarity and analyses based on it In SM3, top quark dominates many loop amplitudes Nondecoupling is crucial to search for 4th generation effects in virtual effects 2
Motivation Motivation Cosmology Cosmology Baryon asymmetry of the universe, a tiny amount of matter remains Successful baryogenesis requires three Sakharov conditions fulfilled (i) B violation, (ii) C and CP violation, (iii) thermal nonequilibrium 1st order EWPT 3
Motivation Motivation Cosmology Cosmology CP violation measure in SM3 is 10 orders of magnitude short of providing enough CPV. With 4th generation at EW scale (where md ms), the amount of CPV is [ChinJPhys47,2] The enhancement due to larger quark masses may be up to 14 orders It is not clear, however, whether the EWPT can be rendered first order to satisfy (iii), may be possible if the 4th generation is responsible for dynamical EWSB 4
Motivation Motivation Global Global EW EW fit, fit, flavor flavor puzzles puzzles Relaxes the Higgs mass to higher values, no longer in tension with LEP lower bound CKM4 provides additional phases (Bs mixing phase, AK¼ DCPV, ) To be explained in the rest of the talk... 5
Electroweak Electroweak constraints constraints Triviality Triviality and and stability stability Large Yukawa couplings of the 4th gen. drive the Higgs quartic coupling to a Landau pole not far above a TeV scale or drive slow: to negative values. Tunneling into another vacuum should be [Nucl.Phys.B609,387] Theory needs a UV completion Upper limit 500 GeV also on t' mass from unitarity [PRD76,075016] 6
Electroweak Electroweak constraints constraints Oblique Oblique corrections corrections Self energies of gauge bosons contain heavy fermion loops (Neutral currents) (Isospin violating) (Depends only on W mass and width, small in the 4th gen. case) U is negligible, fit the S and T parameters to EW precision observables 7
Electroweak Electroweak constraints constraints Oblique Oblique corrections corrections SM4 contributions to S and T (no CKM mixing with SM3 assumed!) Degenerate doublets excluded Higher mh is compensated by fermion contributions to S, T Compatible with the EW measurements 8
Electroweak Electroweak constraints constraints Oblique Oblique corrections corrections Constraints on the 4th generation fermion masses Again, degenerate case is excluded Mass splittings: quarks < 100 GeV leptons < 150 GeV Heavier t' and are preferred. 9
Electroweak Electroweak constraints constraints Oblique Oblique corrections corrections with with CKM4, CKM4, R Rbb Mass splittings are generally lower if mixing with first 3 generations is allowed (< 80 GeV for quarks) Degenerate quarks are allowed Furthermore, 3 4 mixing lowers [1005.3502] Using the Wolfenstein like power counting 10
Electroweak Electroweak constraints constraints R Rbb For large t' masses, Rb effectively constrains B3, i.e. s34 11
Quark Quark flavor flavor constraints constraints
Quark Quark flavor flavor constraints constraints SM3 cannot provide a CPV phase in effect) B Bss mixing amplitude (2 3 sigma Crucial feature nondecoupling of heavy fermions High sensitivity on 4th generation in FCNC processes Define and compare measured against measurements One finds 12
Quark Quark flavor flavor constraints constraints B Bss Representative t' mass, angles, and phase marginally compatible with experiment A analysis, taking into account mixing also in the S, T fit finds [1005.3502] Very unlikely Bs mixing phase favours lighter 4th gen. quark masses in tension with direct bounds An opportunity for LHC to rule out 4th gen. either increasing mass limits or improving 13
Quark Quark flavor flavor constraints constraints B Bss Interesting signatures in correlations between CPV in Bs mixing and decays' [PRD82,033009] 14
Quark Quark flavor flavor constraints constraints D D mixing mixing The only low energy window into b' quark mass and CKM mixings Vcb'* Vub' The most stringent value constraint comes from xd Not straightforward to relate to kaon or B observables [1004.4565] 15
Conclusions Conclusions Appealing framework from cosmo particle point of view No worse than SM3 in electroweak precision fits But Masses squeezed between direct lower bounds and perturbativity/unitarity constraints forces SM4 towards light quark masses, and is not a very likely explanation opportunity for both LHCb, SuperB and direct searches experiments Should not be too hard to exclude the perturbative domain of quark masses at the LHC 16
Precision EW observables constraints Around reference SM3: mh = 120 GeV Higher mh are allowed Caveat: CKM mixing neglected in the fit
Motivation 4th generation may be responsible for dynamical EW breaking Higgs mass is relaxed to higher values Can explain some flavor puzzles Couplings unification in the SU(5) framework A viable scenario, easy to confirm/rule out at LHC
Demotivation Very disparate scales Generally needs a UV completion below Planck scale Lacks beauty
Constraints from direct searches and cosmology From LEP L3 pair production From Tevatron Assuming
Constraints from unitarity/triviality Unitarity of S-wave scattering of gauge bosons Fixed point can explain the question of hierarchy
Precision EW observables constraints Vacuum polarization - S, T, U parameters (Peskin,Takeuchi) The Zbb vertex correction
Precision EW observables constraints Quark splitting below 80 GeV Degenerate doubletsare not allowed Caveat: CKM mixing neglected
Precision EW Directly accessible CKM elements are from tree level decays
CPV in Bs mixing D0 measured Large mixing angle 2-3 sigma off the SM prediction Crucial feature nondecoupling of heavy fermions High sensitivity on 4th generation in FCNC processes Compare measured against Mixing diagram
CPV in Bs mixing (Eberhardt, Lenz, Rohrwild) Only small part of parameter space is compatible with exp.
Probing b' through D mixing