We modify the Sakai-Sugimoto model of chiral symmetry breaking to take into account the open string tachyon which stretches between the flavour D8-branes and D8-branes. There are several reasons of consistency for doing this: (i) Even if it might be reasonable to ignore the tachyon in the ultraviolet where the flavour branes and antibranes are well separated and the tachyon is small, it is likely to condense and acquire large values in the infrared where the branes meet. This takes the system far away from the perturbatively stable minimum of the Sakai-Sugimoto model; (ii) The bifundamental coupling of the tachyon to fermions of opposite chirality makes it a suitable candidate for the quark mass and chiral condensate parameters. We show that the modified Sakai-Sugimoto model with the tachyon present has a classical solution satisfying all the desired consistency properties. In this solution chiral symmetry breaking coincides with tachyon condensation. We identify the parameters corresponding to the quark mass and the chiral condensate and also briefly discuss the mesonic spectra.

Polarized electronic absorption spectra of a nematic liquid crystal (4-butyl-4′-ethoxyazobenzene, BEAB) were measured in sandwich-type cells to which no rubbing treatment was applied. These polarized spectra were utilized to evaluate the mean squares of the direction cosines of the transition moment, those of the molecular orientation axis, and the three-dimensional absorption components as a function of the cell gap. It has been shown that the inclinations of the transition moment for the 354 nm band (long-axis polarized) and the molecular orientation axis to the cell plane increase with the decrease in the cell gap while the absorption component for the 354 nm band normal to the cell plane decreases. These observations provide a quantitative support for the molecular orientation model proposed previously that the molecular long axis tends to be oriented parallel to the cell plane in the interface region and perpendicular in the bulk region. The thickness of the interface region has been estimated to be about 0.02 μm by a model calculation based on a discrete model for the orientation of the BEAB molecules in the cell.