Flavor changing neutral currents involving heavy quarks with four generations

Abstract

We study various flavor changing neutral currents (FCNC) involving heavy quarks in the Standard Model (SM) with a sequential fourth generation. After imposing B → X s γ, B → X s l + l − and Z → bb constraints, we find B(Z → sb +sb) can be enhanced by an order of magnitude to 10 −7 , while t → cZ, cH decays can reach 10 −6 , which are orders of magnitude higher than three generation SM. However, these rates are still not observable for the near future. With the era of Large Hadron Collider approaching, we focus on FCNC decays involving fourth generation b ′ and t ′ quarks. We calculate the rates for loop induced FCNC decays b ′ → bZ, bH, bg, bγ, as well as t ′ → tZ, tH, tg, tγ. If |V cb ′ | is of order |V cb | ≃ 0.04, tree level b ′ → cW decay would dominate, posing a challenge since b-tagging is less effective. For |V cb ′ | ≪ |V cb |, b ′ → tW would tend to dominate, while b ′ → t ′ W * could also open for heavier b ′ , leading to the possibility of quadruple-W signals via b ′b′ → bbW + W − W + W − . The FCNC b ′ → bZ, bH decays could still dominate if m b ′ is just above 200 GeV. For the case of t ′ , in general t ′ → bW would be dominant, hence it behaves like a heavy top. For both b ′ and t ′ , except for the intriguing light b ′ case, FCNC decays are typically in the 10 −4 − 10 −2 range, and are quite detectable at the LHC. For a possible future International Linear Collider, we find the associated production of FCNC e + e − → bs, tc are below sensitivity, while e + e − → b ′b and t ′t can be better probed. Tevatron Run-II can still probe the lighter b ′ or t ′ scenario. LHC would either discover the fourth generation and measure the FCNC rates, or rule out the fourth generation conclusively. If discovered, the ILC can study the b ′ or t ′ decay modes in detail.

Standard model with four generations: Selected implications for rareBandKdecays

“…To account for these discrepancies here we consider the effect of sequential fourth generation quarks [20,[42][43][44][45]. In the SM, the relevant effective Hamiltonian describing the decay modes B → πK is given by…”

“…Several of these observables like FB asymmetry in B → K * l + l − [39], CP asymmetry in B s → ψφ [40] and the decay rate of K L → π 0 νν [41] have also been studied before, as well as many other interesting aspects of SM4 by Hou and collaborators [42][43][44][45], see also [46].…”

RareB−→Λp¯νν¯decay

“…Experimental searches for these rare decays could shed light to find new physics. For example, as the current experimental upper bound on the decay branching ratio of B − → K − νν is 14 × 10 −6 [1,2], while its SM prediction is (4.5 ± 0.7) × 10 −6 [3], there would exist some kind of new physics [3][4][5][6][7] between the sandwiched area. Moreover, for the decay of B → K * νν with an on-shell K * → Kπ, some physical observables [8] are also available to test T-violating effects beyond the SM.…”

Flavor-Changing Bottom-Strange Associated in the Littlest Higgs Model with T-parity at the ILC

“…The final states of bs can be produced via the FC interactions. At the linear collider, the production process e + e − → bs has been investigated in the SM [10], the topcolor-assistant multiscale technicolor model [11], and the the 4 generation Standard Model [12]. In addition, the production process pp → h → bs(h = h 0 , H 0 , A 0 ) was also discussed in the Minimal Supersymmetric Standard Model (MSSM) at the large hadron collider (LHC) [13].…”