Testing flavor-changing neutral currents in the rare top quark decayst→cViVj

Abstract

We discuss the flavor-changing neutral current ͑FCNC͒ decays of the top quark t→cV i V j (V i ϭ␥,Z,g) in the framework of the standard model ͑SM͒ and in a two-Higgs-doublet model ͑2HDM͒ with tree-level FCNC couplings. While in the SM the expected branching ratios are extremely small, in the 2HDM they may be sizable, of order 10 Ϫ4 -10 Ϫ5 , and thus accessible at the CERN Large Hadron Collider. We conclude with the interesting observation that the FCNC decay modes t→cV i V j may not be equally suppressed as their corresponding decays t→cV i in this 2HDM. ͓S0556-2821͑99͒04521-X͔

Width effects on near threshold decays of the top quarkt→cWW,cZZand of neutral Higgs bosons

“…On the other hand, when FWE are "turned on", a huge enhancement to the width arises within a large range of the Higgs mass. In particular, for 100 GeV < ∼ m h 0 < ∼ 165 GeV, we 9 The dependence of BR(t → cW + W − ) and BR(t → cZZ) on the Higgs mixing angle α in Model III can be found in [10,11]. The maximum of these branching ratios with respect to α takes place at α = π/8 (due to the dependency of the Higgs width on α) and not at α = π/4 which is used through out our analysis.…”

“…FWE in these decay modes will be studied within the SM (in the case of t → cW W ) and in the context of the type III Two Higgs Doublet Model (in both t → cW W and t → cZZ), which admits flavor changing neutral currents (FCNC) at the tree-level. The three-body top decays t → cW W, bW Z, cZZ have been considered before, without including FWE, in the SM [7,8,9], in 2HDM's [9,10,11,12,13], in a generic formalism including scalar, vector or fermion exchanges [14] and in topcolor-assisted Technicolor model [15]. In addition, the top decays t → bW h 0 and t → bW A 0 have been analyzed in the context of a general 2HDM [16].…”

New Higgs signals induced by mirror fermion mixing effects

“…Most applications have focused on the light fermions, and only recently the possible implications for the top quark have been studied [8,9]. Given the fact that coming stages of Tevatron (Run-II) will produce about 10 4 top pairs, it is interesting to study rare decays of the top as possible tests of the SM [10,11]. Furthermore, since Higgs bosons could also be produced at significant rates at Tevatron, it is also important to search for all the relevant Higgs signatures that could be detected at Run-2; future colliders (LHC, VLHC, linear colliders) will have the chance to extend further the search range.…”

Rare top decay $t \to c \bar{l}l$ as a probe of new physics

“…The top-charm coupling with the SM Higgs φ 0 could be induced at one-loop level with a resulting branching ratio BR(t → c + φ 0 ) = 10 −15 . The FCNC top decays involving a pair of vector bosons in the final state, t → cV V , can also be of interest [9]. Although one could expect such modes to be even more suppressed than the ones with a single vector boson, the appearance of an intermediate scalar resonance, as in the previous case, could enhance the branching ratio.…”