Search For Heavy Higgs And Majorana Neutrino Production Signal

The Standard Model(SM)of particle physics,completed by the discovery of the 125 GeV Higgs boson at the Large Hadron Collider(LHC),has been a great success in describing the interactions among fundamental particles.Still,the SM does not seem to be a complete description of Nature for many good reasons.For example,the neu-trino oscillation observed at various neutrino observatories indicates that neutrinos have masses,which requires a modification to the SM.Another key physical phenomena is the existence of dark matter,indicated by the cosmological observations,for which the SM provides no candidates.Experimental facts like these,as well as internal theoretical problems,such as the hierarchy problem and the(in)compatibility issue with gravity,provide a clear hint of new physics beyond the SM.In order to answer these questions,various extensions of the SM have been proposed and studied in literature,where new elementary particles are typically predicted.Study and search for some of these new elementary particles are the main focus of the thesis presented here.Ever since the discovery of the 125 GeV Higgs boson at LHC,a detailed investi-gation of its properties,as well as searching for new Higgs beyond sm,remains among the major research topics of the current and future LHC.The two Higgs Doublet Model(2HDM),as one of the simplest possible extensions of SM,predicts the existence of several heavy Higgs with different properties under the Charge and Parity transforma-tion(CP),which thus leads to a richer Higgs phenomenology.In this thesis we present our analysis on searching for heavy Higgs production signals at hadron colliders within the framework of the so-called Type-II 2HDM model.An outline of the contents of heavy Higgs is as follows.·The top quark,the heaviest known fundamental particle,has a decay width about 1.5 GeV,corresponding to a lifetime?5×10-25 s.Since the top decay width is much larger than the QCD hadronization scale,top quarks decay before forming hadronic bound states.As an axial vector,the spin of top quark remains unchanged under theparity transformation.The polarization effect of top quark can be very useful to study its parity-violating interactions.In the SM,the dominant decay mode of the top quark is t?bw?blv,with the tbW vertex of the form of V-A charged-current interaction.Information about the polarization of the top quark can be retrieved from the angular distributions of its decay products,and the best spin-analyser is the charged lepton in the decay.Angular distributions of the charged leptons can thus be used to investigate the structure of the tt?k interaction.Through this thesis,we study systematically the semi-leptonic decay of top quark pairs produced via the quark annihilation and gluon fusion processes,i.e.pp?tt?k?bl+vbl-v?k,and also the background processes at LHC.Suitable top spin-correlation observables are defined to study the properties of the scalar,pesudoscalar and scalar-pesudoscalar mixing Higgs through the tt?k interaction.In particular,these spin-correlation observables depend on the Higgs masses,as well as the PDF of the initial partons.Our work shows that normalized differential distributions of observables are quite sensitive to CP properties of the Higgs,as well as the coupling strength to tt,which can thus be used to distinguish different Higgs and also different new physics models.·Study on the various decay modes of Higgs is of great importance to the reconstruc-tion of Higgs events at hadron colliders.Both the theoretical calculations and detector simulations related to this have progressed a lot in the recent years.A direct observation of Higgs decay to a pair of bottom quarks at LHC was achieved recently through the associated production with a Z and/or W boson.Compared to the decay into bb,the?+?-decay mode provides a clean experimental channel and is also favored in event selection,although its branching ratio is smaller than the former.Within the framework of 2HDM,we study the associated production of a Higgs boson with a pair of bottom quarks at LHC by reconstructing the Higgs through the decay modes of h?bb and h??+?-.Different combinations of kinematic cuts are examined,from which we select the one that is most efficient in suppressing the backgrounds.In addition,the rapidity distribution of the associated Higgs productions,bbH(bbA)and tt H(ttA),at LHC are computed.Our results show that the CP-violating coupling exhibit a strong dependence on the CP properties of the heavy Higgs,and are proportional to the mass ratio of the quark to the heavy Higgs.Therefore,this observable can be used to discrim-inate different Higgs-top coupling structures.We perform a scan of the signal strength in the two dimensional parameter plane of Higgs mass and tan ? in order to search for the possible parameter space for new physics allowed by the current experiments.Through our study,it is shown that bbbb and bb?+?-events could play an important role in the search for heavy Higgs particles productions at the LHC and also future hadron colliders.Neutrino mass,indicated by the observed neutrino oscillations,is one direct evi-dence of new physics beyond the SM.The seesaw model is one of the natural extensions of SM to describe the tiny neutrino mass.The existence of Majorana neutrino is pro-posed by the type(I)seesaw model,which can be investigated through the B meson rare decays.As a second topic of the thesis,the branching ratio of the B meson rare decays B+?K(*)?+?-are computed with the contributions from the Majorana neutrino exchange included.And we find that the results are still in agreement with the latest LHCb data within the uncertainties.In addition,we compute also the lepton-number-violating same-sign decay process B+?K(*)?+?+.Constrains on heavy Majorana neutrino mass and neutrino mixing matrix element |V?N|2 can be deduced from the upper limits that the LHC experiments impose on the branching ratio of this decay process.Our result in this is in agreement with the LHCb(BABAR)measurements in 2012(2014).At last,the dileption invariant mass distribution and angular distribution of B+?K(*)-?+?+ are predicted,which are useful for future B meson experiments at LHCb.