Randall-Sundrum Graviton Analysis

(Part of Thiago's Thesis studies)

Summary

The Randall-Sundrum RS model was proposed in 1999 by Lisa Randall and Raman Sundrum, in order to address what is known as the Hyerarchy Problem - the apparent discrepancy in between the Planck Scale (where quantum gravity effects are postulated to dominate the other interactions) and the Electroweak Scale (the scale where the Electroweak SU(2) X U(1) symmetry is broken, supposedly by the Higgs mechanism). Those two scales are separated by sixteen orders of magnitude - a ratio that, unless generated by Physics not described by the Standard Model, can only be explained by a fine tuning of the order of 1 / 10^32. The RS model proposes a braneworld-like 5-dimensional Universe, where the ratio in between the two scales is achieved by merit of a warped geometry for the 5th dimension. The model predicts an effective, perturbation-based quantum gravity theory where the graviton propagates in the extra dimension, leading to a Kaluza-Klein tower of states for it that can be detected as massive, universal-coupling spin-2 resonances. I study the decay of those resonances in all-hadronic final states, and the possibility of detecting those states at the Large Hadron Collider scheduled to come online later this year.

Theoretical Introduction

I will write a proper introduction in due time. For the time being, please refer to ReferencesED.

Monte Carlo studies

For my preliminary studies, I am investigating the decay G -> ZZ -> qqbar qqbar. The main characteristic of this decay is the high transverse boost that the Zs receive, dure to the high mass of the graviton (~1 TeV? ). Each of the boosted Zs decays in a quark anti-quark pair with very small separation in eta-phi space. This translates to a superposition of the partonic showers of each quark, which experimentally mean that each quark anti-pair will appear as a single jet in the detector, the signature of the decay as whole being two high-Et jets. My first task was to correlate the characteristics of each such jet - hereafter called "Zboosted jets" - with the related Z.

dR between Z daughters versus the Z pt.

Due to their nature of product of a qqbar pair overlapping showers, I would expect that these jets have roughly twice the number of tracks compared to a standard QCD jet. I also expect some underlying structure in the inside of the jet (jet shape variables), but as of now I have not found good handles as to differentiate those jets from standard QCD. One handle that I found, however, is that the Zboosted jets are massive, and the mass spectrum for each jet shows a peaking structure near the mass of the jet. However, for CaloJets? - jets constructed from standard CMS calorimetric towers - both the energy and the mass of the jets are systematically lower than the values for the corresponding Zs. Although these effects are expected due to the presence of the detector, I find unlikely that the standard jet corrections as developed by CMS will correct these quantities back to hadron level, due to the different nature of these jets. Nonstandard jet corrections will probably have to be developed by myself.

Mass of the 1st and 2nd most energetic jets. Notice the peak structure.

Graviton mass constructed from the two most energetic jets.

-- ThiagoTomei - 15 May 2008