Longitudinal quark polarization in e+e- --> t tbar and chromoelectric and chromomagnetic dipole couplings of the top quark

TL;DR

This paper investigates how anomalous chromomagnetic and chromoelectric couplings of the top quark affect e+e- collisions producing top-antitop pairs, providing potential bounds on these couplings at future linear colliders.

Contribution

It offers the first detailed calculation of top quark polarization and cross sections including anomalous couplings at order alpha_s, with estimated bounds for future collider experiments.

Findings

Potential bounds on Re(mu) and Im(mu) at 500 GeV and 1000 GeV.

Constraints on Im(d) and Im(mu* d) from CP-odd polarization measurements.

Improved limits at higher collision energies.

Abstract

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.