SEARCH FOR DIRECT PRODUCTION OF SUPERSYMMETRIC PAIRS OF TOP QUARKS AT √ S = 8 TEV, WITH ONE LEPTON IN THE FINAL STATE. Juan Pablo Gómez Cardona PhD Candidate Universidad de los Andes Carlos Avila Marcello Maggi Andres Florez Advisor Coadvisor Coadvisor Universidad de los Andes INFN Universidad de los Andes 1

2 1.To develop an analysis to discriminated between t* t* and t t (signal/main background) with very high efficiency. 2.To implement several tools to achieve the goal given in the last numeral: Chi Squared-based Method Merged Jet Method Splitted MET Method Matrix Elements Method 3.To run the analysis with the data collected by CMS at 8 TeV. 4.To analyze the results obtained. Objectives

3 This search is motivated by the fact that if SUSY is a natural solution to the hierarchy problem, the mass of the main squarks (stops) should be relatively lower than 1 TeV. Under this condition, the corrections to the Higgs field caused by a given boson, are almost entirely canceled by corrections due to its superpartner, since in this case the particle and superparticle masses are similar. This search focuses on two decay channels: t~ → tχ0 t~ → bχ± → bWχ0 which present a final state given by an isolated lepton with high amount of energy, 4 jets (two are the associated to b quarks) and MET. Motivation

4 Feynman Diagrams of the direct production of Stops and their main decays. Channels to Study The final state in both cases is conformed by 4 jets (2 of them are b- jets), 1 lepton and MET.

Main Backgrounds Semi-leptonic decay of tt~ 5 Leptonic decay of tt~ with lepton reconstructed as MET

6 “Events are required to contain one isolated lepton (e or ), no additional isolated track or hadronic t-lepton candidate, at least four jets with at least one b-tagged jet, and MET > 100 GeV. Signal regions are defined demanding MT > 120 GeV. “ Standard Analysis

7 This method is used to select the best permutation of jets that matches the decay process of the signal to be studied. For this purpose, it is used a variable based on Chi Square. The variable is constructed using relevant sub-variables such as: The Pt of the bjet associated with the leptonic branch (bLep). The difference in phi between the lepton and the bLep. The Chi-Square of the Invariant masses of the W and t in the hadronic branch (Whad and tHad). b-tagging. Basic Analysis (Chi Square based)

8 In order to find a good variable to select the best permutation, a study is required to find the weights, the means and the errors to be used. The variable is given by the following equation: V= S w i (sv i -mv i ) 2 /ev i Where: V: Variable used to select the best permutation sv i : Subvariable i used to construct V mv i : Mean of sv i mv i : Error of sv i W i : Weight assigned to the Chi Square of sv i Tunning Chi-Square

9 This results where obtained with the variable given by: V= ( IM_WHad-80. ) 2 / 15 + ( IM_tHad-173. ) 2 / 30 + ( IM_tLep-173. ) 2 / 45 Where: IM_ means Invariant Mass. Partial Results With Basic Analysis

10 Partial Results With Basic Analysis tt~SingleElec Wjets

11 Partial Results With Basic Analysis tt~SingleElec Wjets

12 Partial Results With Basic Analysis tt~SingleElec Wjets

13 Partial Results With Basic Analysis tt~SingleElec Wjets

14 This method is a generalization of the basic method. In this method each jet assigned to the process decay, can be composed by several jets reconstructed by the standard algorithm used at CMS (AK5). This method is expected to give a better resolution, because it can be used to have into account the final state radiation. In the same way it is possible to use this method to find the jets originated as a consequence of the initial state radiation. This method helps when two jets are generated together and only one is reconstructed. W is allowed to be reconstructed using one only jet!!! Analysis Using Merged Jets Method

15 This method calculates the likelihood that a certain event is observed at the detector as a consequence of a given process. The likelihood is given by: Predictions of the theory (matrix elements). Detector resolution. Analysis Using Matrix Elements Method

16 Matrix Elements is a method that is very time demanding, for such reason, it is a good idea to make a preselection in order to only use this method with a small fraction of all the different combinations. The preselection is made using a Chi Square-based variable. In this case, the idea is to find a subset of all permutations that have the biggest probability to match the process decay. Then MadWeight is used to select the best permutation from this subset. A study was done in order to find a good variable and the number of combinations to be used with MW. This study was done using the standard matching algorithm used at CMS. Study of Chi Square-based Preselection Method

17 Study of Chi Square-based Preselection Method (II)

18 In order to setup MW, several cards are used, among them we can find: -Process Card: It defines the process decay. -Transfer Function Card: It defines the transfer function to simulate the detector resolution. Cards Used to Setup MadWeight (MW)

19 Cards Used to Setup MadWeight (MW)

20 In order to construct the transfer function, the following steps were done: 1.Histograms showing the difference in Pt between the genParticle and the associated jet/lepton. This histograms were done for different ranges (20-40, 40-60, …, ) of the genParticle’s Pt. 2.A fit using a double-gaussian of the previous histograms. 3.A fit for each one of the six parameters w.r.t the mean value of the genParticle’s Pt (30, 50, 70, …, 310). 4.At the end we have a double gaussian where each one of the six parameters is a function of the genParticle’s Pt. Transfer Function (MW)

21 Transfer Function (MW)

22 Transfer Function (MW)

23 Transfer Function (MW)

24 Transfer Function (MW)

25 Transfer Function (MW)

26 This is the format of the files that are used as inputs to MW. LHCO Files

27 In order to reduce the time computing we made a study about the minimum number of points to be used in the integration process. Study About Integration Points

28 At this point we have developed several tools in order to be ready to used them in the analysis: Chi-Squared Method Merged Jets Method Matrix Elements Method We have found several optimal working points: Points of Integration Number of combinations to be used with MW. Variable to be used to select the subset of combinations. Now, we are in the tuning step: To find a good variable for the Chi-Square Method. We also need to finish the development of the code to be able to run the last two methods using the grid. Conclusions