Search for Radion decay into two Higgs bosons with the production of a photon pair and a b quark pair in the final state within the Randall-Sundrum model for extradimension

Alexandre Nikitenko

The Standard Model is not a complete unified theory since it cannot include the gravitational force as a fundamental interaction. There are other shortcomings despites its experimental success. Many extensions of the model have been formulated and the possible presence of extra dimensions seems to cure the hierarchy problem. The hierarchy problem consists of the important energy scale difference between the typical symmetry breaking scale (of the order of 200 GeV) and the scale at which all interactions of the Standard Model are unified (10¹⁶ GeV). In the Randall-Sundrum model the Standard Model particles would live in a given 4-dimensional hyper- surface (brane) and the world of the high energy scale (called the Planck scale) would live in another. These two worlds would be linked by a 5th dimension. This approach gives birth to a new particle, the radion, which is able to propagate through this extra dimension. The radion can interact with particles in the Standard Model brane, and even decay into Higgs bosons. The resulting signal can be sought in the CMS detector. The Imperial College group has been studying this particular signature and developing ways of using the excellent tracker and electromagnetic calorimeter to search for the radion in various final states.

This signal has been sought in three final states: the radion would decay in two Higgs boson and one of the Higgs boson would decay in a pair of b quarks while the other would decay in either a pair of photons or tau leptons or another pair of b quarks. Each of the channels has been studied separately using simulated data. All possible backgrounds have been generated. An efficient selection of events at the trigger level was formulated; the reconstruction method of the Higgs mass and then the radion invariant mass has been defined. The final states containing two photons or two tau leptons benefit of the ECAL and the knowledge of tau properties. The final state including only b quarks would suffer of the large background and would then be less sensitive to the signature. However, the combined analysis allows achieving a better sensitivity. All uncertainties coming from experimental and theoretical sources were considered in order to provide a precise result on the discovery potential of that signal with the CMS detector.

Only the search of that signature can be performed in the two photons and two b quarks final states. The two taus and two b quarks final state can help exploring other range of the model parameters space. The final state including only b quarks cannot be used. Results obtain when applying this analysis with the CMS data would help constraining this type of models which depend on an important number of free parameters. A sufficient amount of data could also lead to the discovery of this particular particle and thus validating the Randal Sundrum approach.

More information on the results obtained are available on the Higgs Group website.