Search for Unparticles in Z and Missing Transverse Energy with the CMS Detector

Many extensions of the Standard Model suggest a hidden valley at high energies. The model of Unparticles, which was proposed 2007 by H. Georgi [1], is one of them. The basic idea is a scale invariant sector, which at high energies couples directly to the Standard Model particles. The nature of this coupling must not be defined in detail, but one of the consequences of this scale invariant field would be particles with a continuous mass spectrum. Since normal particles have a defined mass pole this “stuff” would not be particles in the narrow sense of the word, therefore the name Unparticles was coined. One of the advantages of the Unparticle model is that it is a formalism to describe all scale invariant high energy fields in a low energy limit. The result is always one new (Un)particle. The low energy limit in this context means energies accessible at the LHC. The most impor- tant parameters of this theory are the scaling dimension dU and the effective energy cutoff parameter ΛU. The requirement of scale invariance does not fix the spin of the Unparticle. Therefore scalar, bosonic and fermionic Unparticles are possible. At the LHC with 7 TeV centre of mass energy Unparticles could be produced in association with Z bosons. The neutral Unparticle is assumed to be stable and would leave the detector. Therefore one of the observable signatures would be Z + ETmiss. The focus of this analysis is on scalar Unparticles in associated production with the Z. For this analysis only the muon decay channel of the Z is considered. To increase the signal an events with in the Z mass region are selected and the back-to-back kinematic for Unparticle and Z is used. The discriminating variable between the Drell-Yan and associated Unparticle production is ETmiss. From the renaming Standard Model backgrounds the tt¯ process is the the largest. This can be reduced by jet specific requirements by a factor of two. In 5 fb−1 of the proton-proton data recorded by the CMS experiment in 2011 no sign of Un- particles could be detected. Therefore the excluded parameter region for scalar Unparticles was expanded. Unparticles are excluded for the scaling dimension of dU = 1 to 1.95 for an effective energy scale of ΛU = 1 TeV.