Implications of the present bound on the width of theΘ+(1540)

The recently reported ${\Theta }^{+}(1540)$ exotic baryon seems to be very narrow: $\Gamma (\Theta )<1\mathrm{M}\mathrm{e}\mathrm{V}$ according to some analyses. Using methods of low energy scattering theory, we develop expectations for the width of the $\Theta$, an elastic resonance in $KN$ scattering in a theory where the characteristic range of interactions is $\sim 1\mathrm{F}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}$. If the $\Theta$ is a potential scattering resonance, generated by the forces in the $KN$ channel, its width is hard to account for unless the $KN$-channel orbital angular momentum is two or greater. If the $\Theta$ is generated by dynamics in a confined channel, its coupling to the scattering channel is at least an order of magnitude less than the coupling of the $\Lambda (1520)$ unless $\ell \ge 2$. Either way, if the $\Theta$ proves to be in the $KN$ $s$- or $p$-wave, new physics must be responsible for its narrow width.