The Study On The Polarization Effects Of The Lambda Hyperon In High Energy Scattering

The spin structure of hadron is one of the important topics in particle physics.In the last decades,a series of spin dependent physical observables have been measured in high energy experiments.In particular,experimental physicists observed that the?produced from hadron collision p+p??+X is substantially polarized,although the initial hadrons are unpolarized.Convention wisdom suggested that the mean polarization of the final state?hyperon should be zero.This unexpected spin effect has triggered a lot of theoretical studies and phenomenological analyses.In recent years,understanding the fragmentation mechanism behind the?polarization from the fundamental principle of quantum chromodynamics?QCD?has become an frontier study in hadronic physics community.Since the fragmentation of quarks is a typical hadronization process,in which the hadronic scale is involved,it is not possible to calculate this kind of process directly from perturbative QCD.In order to apply QCD in high energy process involving hadrons to the most extent,the-orists developed the factorization theorem of QCD.In this framework,the differential cross section of the high energy process in which hadrons participate can be decomposed to two parts:one is the hard scattering processes with large momenta transferring and can be calculat-ed by perturbation QCD?pQCD?,the other is the nonperturbative part containing the internal structure of hadron or the hadronization information in the fragmentation region.The later one corresponds to the distribution functions or fragmentation functions of hadrons,which can be extracted phenomenologically from experimental measurements.And their scale dependence is determined from the QCD evolution equations.PDFs and FFs are both import inputs in the high energy processes involving hadrons.Compared to PDFs,detailed studies have not been carried out on FFs from theory and experimental measurements.Particularly,polarized FFs are almost unknown.Therefore,it is necessary to do further study on FFs.This thesis is dedicated to investigate polarized FFs theoretically and phenomenologically.We will mainly consider the fragmentation process of quark to a spin-1/2 hadron?such as?hyperon?and focus on the production of the?hyperon in semi-inclusive deep inelastic scattering process?SIDIS?and electron positron annihilation process.Moreover,we apply the spectator model to calculate the transversely polarized fragmentation function.Besides,we also investigate the electromagnetic form factor of the?hyperon.The main content of the thesis are organized as follows:Firstly,on the basis of the spectator quark model,we perform model calculation on the twist-3 time-reversal-odd?T-odd?FFs G^?and?G^?for?meson,the numerical results are also shown.Using the Collins FF from previous study,we test the relation among them connected by the equation of motion relation.The FF?G^?originate from quark-gluon-quark correlation and may give rise to the single spin asymmetries?SSAs?A_LU^sin?and A_UL^sin?,through the coupling with the unpolarized distribution function f₁and longitudinally polarized distribution function g₁.We find that the magnitudes of G^?and?G^?are substantial in the spectator model.Thus it is necessary to consider their effects in phenomenological study for the asymmetry G^?and?G^?.Secondly,we study the semi-inclusive leptoproduction of polarized?hyperon:l+N-?l^?+?+X,in which an unpolarized lepton beam collides on an unpolarized nucleon tar-get.In the framework of transverse momentum dependent factorization,we have presented the parton-model result of the cross-section for the process up to twist-3 level.We find that six twist-three level structures can be expressed as convolutions of twist-2 and twist-3 transverse momentum dependent PDFs and FFs.We give some discussions on the properties of the T-odd FFs.Our study thus can provide new insights on the fragmentation mechanism and polarization phenomena of the?hyperon throughl+N-?l^?+?+X process.Thirdly,we perform the calculation on the spin-dependent T-odd fragmentation functions D^?_1Tand H₁^?using the diquark spectator model.We estimate the transverse polarization P_?Tof the?hyperon with our numerical result of D^?_1Tin both SIDIS and e⁺e^-annihilation processes.We find that in these two processes the polarizations are negative.The magnitudes increase with increasing momentum fraction z.We also study the contribution of the Collins function H₁^?for?hyperon and?meson to azimuthal asymmetries A₁₂in e⁺e^-????X and e⁺e^-???X processes.We studied the QCD evolution effects of the Collins function and find that the evolu-tion effects significantly change the shape and size of the asymmetries.Our results show that the asymmetries are sizable and measurable at the kinematical configurations of Belle and BABAR experiments.Similarly,we compute the twist-3 distribution functions h,f^?,g^?and twist-2fragmentation functions H₁,G_1Tand estimate their contribution to the sin?_Shasymmetry for the proton production as well as for the?production at JLab and COMPASS kinematics region.The predictions show that the asymmetry in the production of transversely polarized proton are measurable at JLab and COMPASS,but that of transverse polarized?hyperon is small.Fourthly,we performed an analysis on the electromagnetic form factors of the?hyperon in the time-like reaction e⁺e^-????applying two different models.One is the pQCD inspired parametrization,the other is the modified vector meson dominance?VMD?model.The study of the electromagnetic form factors may provide the information on the distribution of the electric charge and magnetic moment inside the?hyperon.We find that,after taking into account the contribution of excited states of vector meson,the model can well reproduce the data of the?hyperon EMFFs.