Precision Study On Signatures Of Littlest Higgs Models At High Energy Colliders

In July 2012,both the ATLAS and CMS collaborations at CERN's Large Hadron Collider(LHC)announced the observation of a new boson with mass of about 125 GeV.The present analyses indicate that this boson is consistent with the Higgs boson predicted by the Standard Model(SM).This discovery is a tremendous achievement in the history of particle physics.However,it is just the first step in understanding the electroweak symmetry breaking.Establishing the properties of the Higgs boson,especially its couplings to the SM particles is one of the primary missions of the current LHC run and demands further precise investigation.At the meanwhile,the so-called naturalness problem is still a haunting nightmare and is the driving force for new physics beyond the SM at the TeV energy scale.Many models have been proposed to solve the naturalness problem,such as the supersymmetry,extra dimensions,little Higgs,technicolor and so on.Among these models,the Littlest Higgs models gain widely attention.In the Littlest Higgs model with T parity(LHT),the SM particle partners are introduced to ameliorate the Higgs mass hierarchy problem;these new particles are the core part of the LHT,and searching for these new particles will provide direct information for the correctness of LHT.In the LHT,the ZH-pair production with subsequent decay ZH?AHh can offer an interesting di-Higgs boson production mechanism.Moreover,the di-Higgs boson production is often discussed as a probe of new physics,which plays a key role in probing the Higgs self-coupling and the existence of heavier states coupled to the Higgs boson.Based on this,we examined the QCD NLO correction for the pp?ZHZH+X process at the LHC,and also presented the detailed kinematic distributions for final products by considering the subsequent decays.The numerical results showed that the QCD NLO correction can obviously enhance the total cross section and the differential cross sections.We also found that it is possible to select the signature of the ZH pair production from possible SM background by taking proper kinematic cuts.Among the LHT heavy gauge boson pair production channels at the LHC,WHZH owns the largest production rate,as a result,WHZH is the most important production channel for the studying of heavy gauge pair production in LHT.We investigated the total cross section for pp?WHZH + X and the kinematic distributions for the final states of pp?WHZH?WhAHAH??v?hAHAH+X.The results showed that the NLO QCD correction is considerable.At CLIC,we studied the???q-q-+ X process at the QCD NLO;the analysis of the total and the differential cross sections showed that the NLO QCD correction is numerically important for the phenomenological studies;it is also found that by adopting proper kinematic cuts the signature of q-q-production can be discriminated from possible SM background.HZ associated production is one of the most important Higgs production channels at LHC,and it is a direct process to investigate the HZZ coupling.As indirect searches for the Littlest Higgs model,we studied the HZ associated production and investigated the effects of the Littlest Higgs model at the QCD NLO and NLL level including the lowest contribution from gg-fusion channel.We investigated the renormalization/factorization scale dependence of the total cross section,the pT distribution and invariant mass distribution of the HZ system,and found that the QCD NLO+NLL corrected results owns the best scale dependence,compared to the QCD NLO corrected and the LO results.We presented the discrepancy between the Littlest Higgs model and the SM.The results showed that the discrepancy is suppressed by the resummation effects.The innovations of the dissertation are listed as follows:We firstly presented the NLO QCD correction for the ZHZH/WHZH pro-duction at the LHC and the q-q-production at the CLIC,and provided the total cross section and kinematic distributions for the final state particles.These analyses can be very useful in searching for new particles at the high energy colliders.We studied the effects of the Littlest Higgs model effects on the HZ pro- duction by applying the resummation formalism for the first time.At the QCD NLO+NLL level,we provided the total cross section for the HZ pro-duction,as well as the pT distribution and invariant mass distribution for the HZ system.The results can provide important theoretical information for investigating new physics beyond the SM at the TeV energy scale.