[SUNY BING 4/22/98] hep-ph/9806373 MEASUREMENT OF HELICITY PARAMETERS IN TOP QUARK DECAY

The standard model(SM) predicts only a gV−A coupling in t → W+b decay. However, if additional Lorentz structures exist, they can manifest themselves in high energy processes such as this via non-SM values of the helicity parameters describing t → W+b. Plots and tables of the values of these helicity parameters are obtained for various coupling strengths. Three phase-type ambiguities are uncovered: gV−A + gS+P with effective-mass scale ΛS+P ∼ −35GeV , gV−A+ gfM+fE with ΛfM+fE ∼ 53GeV , and an arbitrary sign-flip in the bL amplitudes AX(λb = −1/2) = −AV−A(λb = −1/2). These ambiguities are analyzed so that they might be partially resolved when experiments resume at the Fermilab Tevatron. Electronic address: cnelson @ bingvmb.cc.binghamton.edu 1 Helicity Amplitudes and α, β, γ Relative Phases For t→Wb decay, the four on-shell helicity amplitudes A(λW+, λb) can be uniquely determined by measurement of four moduli and three relative phases. In Fig. 1, measurements in the right and left columns are respectively of order O(L) and O(R). The interference measurements between the two columns are of order O(LR). L and R denote the b quark’s helicity λb = ∓1/2. The values of A(λW+ , λb) for the standard model(SM) are given in the top row of Table 1. For the pure V − A coupling of the standard model, the left-handed helicity λb = −1/2 amplitudes dominate by 1 to 2 orders of magnitude for mb ∼ 4.5GeV . Intrinsic and relative signs of these helicity amplitudes are determined by the Jacob-Wick phase convention. The layout of the corners in Fig. 1 has been chosen to reflect the layout in the following probability plots for P (WL) versus P (bL) where P (WL) = Probability W + is longitudinally polarized, λW+ = 0 P (bL) = Probability b is left-handed, λb = −1/2 The focus of this paper is on direct measurements of these bL amplitudes by forthcoming experiments at the Fermilab Tevatron[1] and at the CERN LHC[2] relative to the anticipated pure V −A predictions. If, for instance, sizable bR amplitudes were found to occur by these experiments, further analysis of coupling and phase-type ambiguities may be warranted. Likewise, when more precise measurements become possible in later experiments at the LHC or near the tt̄ threshold at a linear eē collider, higher order QCD and EW corrections must be included, see [3]. Plots and tables of the values of the helicity parameters are given in terms of a “(V − A) + additional Lorentz structure”. Generically, we denote these additional couplings by gTotal ≡ gL + gX (1)