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Search for the PSEUDOSCALAR A Higgs boson in its decay into the light Higgs (h) and the Z gauge boson WITHIN THE MSSM Georgios Daskalakis Introduction of a new symmetry Supersymmetry (SUSY) introduces a symmetry that turns bosons into fermions and vice-versa. It is appealing to theorists as it stabilizes the radiative corrections of scalar particles like the Higgs boson in the Standard Model (SM) and provides coupling unification if found at the TeV scale. Each particle of the SM has a supersymmetric counterpart. That symmetry is obviously broken since only half of the spectrum is visible in our accelerators. The MSSM is a supersymmetric extension of the SM with the minimal particle spectrum and soft supersymmetry breaking terms in order to parameterize the unknown mechanism that breaks this new symmetry. The Higgs sector of this model requires two SU(2) complex Higgs doublets. After electroweak symmetry breaking five Higgs bosons remain: the light higgs h, H, a charged Higgs pair H± and a neutral pseudoscalar A. The Higgs sector is completely determined by two parameters: a common choice is the ratio of vacuum expectation values of the two doublets (Tanβ) and the mass of the pseudoscalar Higgs boson A. Exploring the Higgs sector In this analysis, the pseudo-scalar Higgs (A) decay into a Z boson and the lighter scalar Higgs (h) is studied. The final state under study involves the production of two leptons from the Z decay and a pair of b quarks from the disintegration of the light Higgs. Thus, this channel provides an interesting way to detect A and h simultaneously. The motivation of the study is the fact that at LEP data do not exclude completely the low Tanβ region especially if the mass of the top quark is around 180 GeV/c² and the SUSY scale around 2 TeV. Analysis strategy Background signals: This analysis has been performed with fully simulated events from CMS. The main backgrounds in this analysis are the irreducible Zbb and the top pair production. Other potential backgrounds are the gauge boson pair production ZZ and ZW as well as the Z+jets events. Trigger: The triggers selected for this channel are the di-muon and di-electron triggers since there will always be a real Z in the event decaying into two high PT electrons or muons. We have tested the single muon/electron streams and the combination of single/double muon/electron streams does not bring any improvement. Basic selection: The baseline selection requires two opposite sign high PT isolated leptons (electron or muon) and two high PT tagged b-jets which have a separation of DR > 0.7 from both leptons. Muons must have |η| < 2.1 and electrons should be in ECAL fiducial region. The event is required to have small missing transverse energy and a reconstructed invariant mass of the leptons close to the Z mass in order to reject a significant fraction of the top background. Looking for two Higges After optimizing the cuts on each variables such as the significance is maximized, the search for the signal is performed in the (MA,Mh) plan. The optimization procedure has been done for a relevant range of the possible mass of the A Pseudoscalar. All possible combinations of Higgs masses can be scanned and the result can be interpreted within various models. Fig .1 shows the distribution of the b quark pair invariant mass after all selection cuts being applied. Fig 2. shows the distribution of the lepton pair and b quark pair invariant mass.
The plot in Fig .3 shows the 5 σ contours in the MA as a function of Tanβ plan for 30 and 60 fb-1 of integrated luminosity. For the calculation of the significance the signal and background events were counted in mass windows of one σ around the reconstructed mass of Mh and MA. Since only three different A masses and two Tanβ values are available, the estimations for the rest of MA,Tanβ parameter space was done using extra/interpolations of the signal efficiencies from the available parameter points.
More details on the results presented can be found here and the corresponding notes and publications here. |
