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

Unparticles ($\U$) 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 $\Lambda_\U$ are very sensitive to the dimension $d_\U$ of the unparticles. From invisible $Z$ and $V$ decays, we find that with $d_\U$ close to 1 for vector $\U$, the unparticle scale $\Lambda_\U$ can be more than $10^4$ TeV, and for $d_\U$ around 2, the scale can be lower than one TeV. From invisible neutrino decays, we find that if $d_\U$ is close to 3/2, the scale can be more than the Planck mass, but with $d_\U$ 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.