Collider phenomenology of unparticle physics

Low-energy phenomenology of the unparticle physics associated with an exact scale invariant sector possessing a nontrivial infrared fixed point at a higher energy scale is explored for both electron-positron and hadronic colliders. Feynman rules for a spin 0, 1, or 2 unparticle coupled to a variety of standard model gauge invariant operators that are relevant to many low-energy processes involving either real emissions of unparticles or their virtual propagator effects are presented. Missing energy and/or recoil mass distributions of the unparticle in the associated production of the unparticle together with a photon or Z boson at LEP2 and ILC as well as in Z decay into an unparticle plus a fermion-antifermion pair are studied. In addition, monojet production with missing energy from the unparticle at hadronic collisions are explored. The complex phase in the unparticle propagator that can give rise to interesting interference effects between an unparticle-exchange diagram and the standard model amplitudes are studied in details for the Drell-Yan process as well as muon pair and diphoton production in electron-positron annihilation. These energy and/or recoil mass distributions (with the exception in hadron colliders) and interference effects are found sensitively depending not only on the scaling dimension but also on the spin of the unparticle. For the spin-2 unparticle, its physical effects are found to resemble that of a tower of Kaluza-Klein gravitons, which strongly indicates that the underlying unparticle physics may have root in a higher dimensional theory. A connection between unparticle physics and theories of large extra dimensions is speculated. Experimental constraints on the unparticle scale are deduced from the LEP2 data on monophoton production and from the 4-fermion contact interactions.