Constraints on Unparticle Interactions from Invisible Decays of Z, Quarkonia and Neutrinos

Unparticles ($\text{U}$) interact weakly with particles. The direct signature of unparticles will be in the form of missing energy. We study constraints on unparticle interactions using totally invisible decay modes of $Z$, vector quarkonia $V$ and neutrinos. The constraints on the unparticle interaction scale ${\mathrm{\Lambda }}_{\text{U}}$ are very sensitive to the dimension $d_{\text{U}}$ of the unparticles. From invisible $Z$ and $V$ decays, we find that with $d_{\text{U}}$ close to 1 for vector $\text{U}$, the unparticle scale ${\mathrm{\Lambda }}_{\text{U}}$ can be more than ${10}^4$ TeV, and for $d_{\text{U}}$ around 2, the scale can be lower than one TeV. From invisible neutrino decays, we find that if $d_{\text{U}}$ is close to 3/2, the scale can be more than the Planck mass, but with $d_{\text{U}}$ around 2 the scale can be as low as a few hundred GeV. We also study the possibility of using $V (Z)\to \gamma + \U$ to constrain unparticle interactions, and find that present data give weak constraints.