Precise Study On The Vector Boson Associated Productions At Hadron Colliders

The Standard Model is considered as the most successful model in particle physics,especially in 2012 both the ATLAS and CMS at CERN Large Hadron Collider(LHC)announced the observation of a new boson,so far all experimental analyses indicated that this particle has all the same properties with the SM Higgs.Although the SM has won tremendous success,it is still regarded as an effective theory in the present observed energy region,the SM fails to explain the asymmetry of matter and antimatter in the universe,also the SM itself has some theoretical defects like fine tuning problem.For the sake of testing the SM and looking for new physics,further precise studies about the Higgs properties and gauge principle are in great interests.Particularly the studies of vector boson pair and their associated productions draw much attention because that they serve as the background processes of Higgs production and also help to test the gauge coupling directly.In this thesis,we systematically studied the pp → ZZ + jet/γ productions in next-to-leading order(NLO)QCD and electroweak(EW)accuracy,the numerical results of total cross sections and kinematic distributions with and without vector boson decays were presented.At LHC the ZZ+jet/γ productions are important background processes of Higgs production and it’s also helpful to test the Triple and Quartic Gauge Couplings of SM.We concluded that NLO QCD correction enhanced the LO cross section with reduced scale uncertainties.Compared to the NLO QCD correction,the EW correction had less impacts on the total cross section but imposed strong suppression effect in high energy region due to the EW Sudakov logarithms,thus in the high energy region,espe-cially in TeV scale,the ignoring of EW correction may lead to wrong conclusions.In the real emission process of pp → ZZ+ jet + γ,the final-state photon and jet separation is involved in the NLO calculations of ZZ + jet/γ productions.We give two solutions by adopting both the quark-to-photon fragmentation function and the Frixione isolation method to give an infrared-safe observations.In the analytical and numerical calcu-lations of the IR divergence,we use both the two cutoff phase space slicing method(TCPSS)and the Catani-Seymour dipole subtraction with higher numerical precision.The calculation of the off-shell produced multi-body final-states would be confronted with great difficulties,we use the MADSPIN to handle with unstable vector boson de-cays which can retain the off-shell and spin-correlation effects at LO accuracy as far as possible.In the region of low transverse momentum or threshold of final-state system,the large logarithms due to soft gluon radiation will weaken the prediction capability in fix-order calculations.In this thesis the resummation technique under CSS scheme was applied in studying the pp→ZH production both in the Standard Model and Littlest Higgs Model(LHM),the numerical results demonstrated that the NLO+NLL calcula-tion improved the distribution shapes at low pT and invariant mass distributions of ZH system and also the scale uncertainties,moreover the deviation between SM and LHT models was decreased to some extent.Some innovations of this dissertation are listed as follows:●For the first time,we present the NLO EW calculation to pp→ ZZ + jet/γproductions at Large Hadron Collider,and then combined with NLO QCD cor-rection to give the up-to-date theoretical prediction with higher precision.Due to the importance in studying Higgs background and gauge principle,the precise calculations of ZZ + jet/γ productions attract attention of theoretical and experi-mental researchers and is part of the desired items in 2013 and 2015 Les Houches wishlists.We accomplished this task with high precision,the publication about pp→ ZZ + jet/γ calculation has been cited multiple times by far,and recently the released of 2017 Les Houches wishlist also cited our publications.● The final-state photon-jet separation is one of the difficulties in the NLO calcula-tion of pp→ZZ + jet/γ productions,looking for reasonable schemes for event classifications is closely related to the theoretical IR safe observable definitions.Two solutions with both the quark-to-photon fragmentation function and the Frix-ione isolation methods are based on different photon produced mechanisms,that can be easily extended to all the NLO calculations of processes accompanied by photons or jets at LO.● Compared to the NLO QCD correction,we found that the NLO EW relative cor-rection with respect to LO cross section showed strong dependence of kinemati-cal distribution(considerable correction can be seen in high energy region),thus the mixed QCD and EW corrections need appropriate estimation,in this thesis we used additive and multiplicative approaches to approximate this contribution.In addition we used MADSPIN package to handle the final-state unstable vector boson decays,which retained the off-shell and spin correlation effects as far as possible.● We applied the resummation technique in the research of pp → ZH production at LHC,the NLO+NLL calculation was found to improve the distribution shapes at low pT and invariant mass distributions of ZH system and also the scale un-certainties,besides the related new physics effects were weakened to a certain degree.