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Register a new userZ Polarization in $e^+e^-to tbar t Z$ for testing the top quark mass structure and the presence of final interactions
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Authors
Fernand M Renard
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We show that the measurement of the $Z$ polarization in the $e^+e^-to tbar t Z$ process would allow an interesting determination of the role of the top quark mass. This can be used for testing the possibility of top compositeness or of the occurence of final state interactions related to the mass generation in particular the interaction with dark matter.
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We show what type of effects on the rate of $Z_L$ production in the $gammagammato tbar t Z$ and $gluon~gluonto tbar t Z$ processes could be produced by an effective scale dependent top mass or by final state interactions between massive particles for example generated by the presence of dark matter.
The effect of anomalous chromomagnetic (mu) and chromoelectric couplings (d) of the gluon to the top quark are considered in e+ e- --> t tbar, with unpolarized and longitudinally polarized electron beams. The total cross section, as well as t and tbar polarizations are calculated to order alpha_s in the presence of the anomalous couplings. One of the two linear combinations of t and tbar polarizations is CP even, while the other is CP odd. The limits that could be obtained at a typical future linear collider with an integrated luminosity of 50 1/fb and a total c.m. energy of 500 GeV on the most sensitive CP-even combination of anomalous couplings are estimated as -3 < Re(mu) < 2 for Im(mu) = 0 = d, and sqrt{Im(mu)^2 + |d|^2} < 2.25 for Re(mu) = 0. There is an improvement by roughly a factor of 2 at 1000 GeV. On the other hand, from the CP-odd combination, we derive the possible complementary bounds as -3.6 < Im(mu^* d) < 3.6 for Im(d) = 0, and -10 < Im(d) < 10 for Im(mu^* d) = 0, at a c.m. energy of 500 GeV. The corresponding limit for 1000 GeV is almost an order of magnitude better for Im(mu^* d), though somewhat worse for Im(d). Results for the c.m. energies 500 GeV and 1000 GeV, if combined, would yield independen limits on the two CP-violating parameters of -0.8 < Im(mu^* d) < 0.8 and -11 < Im(d) < 11.
We consider the issue of the top quark Yukawa coupling measurement in a model in dependent and general case with the inclusion of CP-violation in the coupling. Arguably the best process to study this coupling is the associa ted production of Higgs boson along with a $tbar t$ pair in a machine like the International Linear Collider (ILC). While detailed analyses of the sensitivity of the measurement assuming a Standard Model (SM) - like coupling are available in the context of ILC, conclude that th e coupling could be pinned down at about 10% level with modest luminosity, our investigations show that the scenario could be different in case of a more general coupling. The modified Lorentz structure resulting in a changed functional dependence of the cross section on the couplin g, along with the difference in the cross section itself leads to considerable deviation in the sensitivity. Our studies with an ILC of center of mass energies of 500 GeV, 800 GeV and 1000 GeV show that moderate CP-mixing in the Higgs sector could change the sensitivity to about 20 %, while it could be worsened to 75% in cases which could accommodate more dramatic changes in the coupling. While detailed considerations of the decay distributions point to a need for a relook at the analysis strategy followed for the case of SM such as for a model independent analysis of the top quark Yukawa coupling measurement. This study strongly suggests that, a joint analysis of the CP properties and the Yukawa coupling measurement would be the way forward at the ILC and that caution must be excercised in the measurem ent of the Yukawa couplings and the conclusions drawn from it.
The effect of anomalous chromoelectric couplings of the gluon to the top quark are considered in e+ e- --> t tbar. The total cross section, as well as t and tbar polarizations are calculated to order alpha_s in the presence of the anomalous couplings. One of the two linear combinations of t and tbar polarizations is CP even, while the other is CP odd. Limits that could be obtained at a future linear collider on CP-odd combinations of anomalous couplings are determined.
We investigate the sensitivity to new physics of the process e+e- -> t bar{t} when the top polarization is analyzed using leptonic final states e+e- -> t bar{t} -> l+l- b bar{b} nu_l bar{nu}_l. We first show that the kinematical reconstruction of the complete kinematics is experimentally tractable for this process. Then we apply the matrix element method to study the sensitivity to the Vtbar{t} coupling (V being a vector gauge boson), at the tree level and in the narrow width approximation. Assuming the ILC baseline configuration, sqrt{S}=500 GeV, and a luminosity of 500 fb^{-1}, we conclude that this optimal analysis allows to determine simultaneously the ten form factors that parameterize the Vtbar{t} coupling, below the percent level. We also discuss the effects of the next leading order (NLO) electroweak corrections using the GRACE program with polarized beams. It is found that the NLO corrections to different beam polarization lead to significantly different patterns of contributions.
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