A search for R-parity violating Supersymmetric top decays at CMS with \(\sqrt{s} = 8\) TeV


A search for a supersymmetric top decay assuming a 100% branching ratio of \(\widetilde{t}\widetilde{t}^* \rightarrow \mu^+ \mu ^- b \overline{b}\) is presented using a minimally supersymmetric model at an integrated luminosity of \(19.5\ \mathrm{fb}^{-1}\). The datasets were recorded with the CMS detector at the LHC. Using Baysian marginalization, an upper limit on the cross section of this process is computed and a cut off point is calculated below which the data does not support the presence of the target decay. This cut off point was calculated to be around 780 GeV.



With the discovery of the Higgs, the Standard Model seems to offer a complete tally of the fundamental particles with which all of the matter around us is made.(Aad 2012) However, with this answer comes more questions as phenomena such as the origin of dark matter and the seemingly unnatural Higgs mass remain unanswered, and cause theorists to look for physics beyond the Standard Model.

Supersymmetry is one such theory and proposes an extension to space-time which allows for a symmetry relating the two groups of fundamental particles: fermions and bosons, where each of the particles from one group have a corresponding supersymmetric partner differing by half-integer spin. These supersymmetric partners provide opposite quantum corrections to the mass of the Higss and thus yield a natural explanation for the Higss mass. Associated with this extension is a quantum number \(R\), defined as

\[R = (-1)^{(3B + L + 2S)}\]

where S, B, and L, are the spin, baryon, and lepton quantum numbers of the particle.(Huh 2009)

It is important to note that due to the definition of this quantum number, particles in this theory must be created in a pair-wise fashion. Because of this, this quantum number is refered to as ’R-parity.’ It is also important to note that Supersymmetric particles have an R value of -1 whereas particles that are of the Standard Model have R = 1.

In order for energy and R-parity to be conserved, Supersymmetric particles must decay pair-wise through a series of intermediate processes and have an eventual final state of lighter Standard particles and some number of the lightest supersymmetric particle (LSP).(Berger 2013) However, if R-parity is not conserved, a Supersymmetric particle can have a final state which consists only of particles found in the Standard Model.

While there have been a few searches for R-parity violating decays in the past, R-parity conserving theories are more widely researched as they provide an explanation for the massive prevalence of seemingly weakly interacting dark matter.(Trotta 2007)(Lahanas 2007) However, there is little reason to believe a priori that spacetime would behave in such a way as to conserve \(R_p\). Coupling this with the increased capacity for data collection available to the Compact Muon Solenoid (CMS) at LHC at CERN, it is necessary to perform searches for these decays using new techniques which might uncover possibly interesting results.

This paper describes a search for such a decay of the form:

\[\widetilde{t}\widetilde{t}^* \rightarrow \mu^+ \mu ^- b \overline{b}\]

Since the signature of this decay resembles that of many common Standard Model vertices, various data-based background estimation techniques were implemented in conjunction with Monte Carlo simulations to obtain a number of expected events. For a detailed description of these techniques see section \ref{sect:eventSelection}.