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Particles and Fields
Eur. Phys. J. C 11, 485-493
DOI 10.1007/s100529900089

Longitudinal quark polarization in $e^+e^- \to t\overline{t}$ and chromoelectric and chromomagnetic dipole couplings of the top quark

S.D. Rindani1 - M.M. Tung2

1 Theory Group, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India
2 Institute for Theoretical Physics, State University of New York, Stony Brook, NY 11794-3840, USA

Received: 3 September 1998 / Revised version: 15 February 1999 / Published online: 22 October 1999

Abstract
The effect of anomalous chromomagnetic ($\mu$) and chromoelectric couplings (d) of the gluon to the top quark is considered in $e^+e^- \to t\overline{t}$, with unpolarized and longitudinally polarized electron beams. The total cross section, as well as t and $\overline{t}$ polarizations, are calculated to order $\alpha_s$ in the presence of the anomalous couplings. One of the two linear combinations of t and $\overline{t}$ 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 fb-1 and a total c.m. energy of 500 GeV on the most sensitive CP-even combination of anomalous couplings, are estimated as $-3 \stackrel{<}{~}{\rm Re}(\mu) \stackrel{<}{~}2$, for ${\rm Im}(\mu)=0=d$, and $\sqrt{{\rm Im}(\mu)^2 + \vert d \vert ^2} \stackrel{<}{~}2.25$, for ${\rm Re}(\mu)=0$. 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 $-3.6 < {\rm Im} (\mu^* d) < 3.6$, for ${\rm Im} (d) = 0$, and $-10 < {\rm Im} (d) < 10$, for ${\rm Im} (\mu^* d) = 0$, for a c.m. energy of 500 GeV. The corresponding limit for 1000 GeV is almost an order of magnitude better for ${\rm Im} (\mu^* d)$, though somewhat worse for ${\rm Im} (d)$. Results for the c.m. energies 500 GeV and 1000 GeV, if combined, would yield independent limits on the two CP-violating parameters of $-0.8 < {\rm Im} (\mu^* d) < 0.8$ and $-11 < {\rm Im} (d) < 11$.


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