Productions Of Particles With Large Transverse Momentum In Relativistic Heavy Ion Collisions

In the ultra-relativistic heavy ion collisions at both Relativistic Heavy Ion Collid-er(RHIC)and Large Hadron Collider(LHC),a new state of matter called Quark Glu-on Plasma(QGP)has been produced,where the nuclear matter governed by Quantum Chromodynamics(QCD)reaches a temperature and baryochemical potential necessary to achieve the t,ransition of hadrons(bound states of quarks and gluons)to deconfined quarks and gluons.The QGP created in the laboratory only lasts few microseconds since QGP would expand and cool down to form the hadronic bound states where the quarks and gluons start to combine or fragment to the final-state hadrons.In the past two decades,a lot of observables/probes have been proposed to investigate and verify the formation and properties of this ephemeral matter.Jet quenching is one of vital probes to explore QGP,which describes the phenomenon that an energetic parton produced in the initial hard process would interact with the quark gluon plasma and radiates induced gluons when it traverses through the hot and dense medium.The measurement of the suppression of iden-tified hadron production,the transverse momentum asymmetry of dijet and gauge boson tagged jet,jet's sub-structure and other experimental observables have been applied to discuss the jet quenching effects.With the dramatically increasing center-of-mass colliding energies,jet quenching,as one of the most important hard probes,is attracting more and more attentions both in theory and in experiment.In this thesis,we will study several physical issues with a next-to-leading order(NLO)perturbative QCD(pQCD)improved parton model at high pT in heavy-ion collisions.Large transverse momentum(pT)hadrons originate primarily from the fragmentation of hard scattered quarks or gluons.In high energy p+p collisions this is well described in the framework of perturba.tive QCD.Large pT ?0 is the most abundant final-state hadron and the measurements of the suppressed hadron yields of ?0 in central Au+Au collisions at RHIC first indicate the existence of QGP.In this thesis,we calculate the production of the identified hadron,inclusive photon and isolated-?+ jet up to next-to-leading order(NLO)in proton-proton collision based on the framework of perturbative QCD.We have proved that this framework could predict the identified hadron yields well.We then calculate the transverse momentum spectra of two new identified mesons ? and K0s and one identified baryon ? at RHIC 200 GeV and LHC 2760 GeV in this article.In the factorisation theorem,the inclusive cross section can be calculated by convoluting three parts,namely PDFs,partonic cross section and fragmentation functions(FFs).We utilize the higher-twist approach of parton energy loss,which relates parton energy loss due to its multiple scattering in QCD medium and medium-induced gluon radiation to twist-four processes and shows that these processes may give rise to additional terms in QCD evolution equations and lead to the effectively medium-modified fragmentation functions(mmFFs).Thus the inclusive production of identified hadron in A+A can be obtained by replacing the FFs in vacuum with the mmFFs.Furthermore,we employ the state-of-art,event-by-event(2+1)viscous hydrodynamics description IEBE-VISHNU to give the space-time evolution information of the hot and dense medium in our model.In the NLO pQCD improved parton model,we calculate the inclusive productions of six identified mesols ?0,?,?0,?,? and K0s and their correspolding nuclear modification factors,then extract the best value of the initial jet transport parameter by a ?2 fit comparing the theoretical results and the available experimental data of all the different final hadrons.With the global extracted value of qo(RHIC:q0= 0.5(+0.15/-0.05)GeV2/fm and LHC:q0 = 1.2(+0.25/-0.15)GeV2/fm),we are able to further investigate the particle ratio of ? and Ks0 both at RHIC and LHC.We first calculate ?/?0 ratio as a function of pT and show the results in p+p and Au+Au,where the PHENIX experimental data on?/?0 ratio in p+p are also illustrated.An enhancement of the ratio in A+A relative to that in p+p is found in small pT region,whereas a small suppression in high pT regime.We also predict the ?/?0 ratio as a functioi of pT in p+p and Pb+Pb collisions with(?)= 2.76 TeV at LHC.In order to understand this feature deeper,we plot the gluon and quark(fragmentation)contribution fractions to ?(and ?0)in p+p and Au+Au at RHIC.One see in the p+p collision at RHIC,the production of ? is dominated by gluon fragmentation.In the A+A collisions,the gluon suffers more energy loss than the light quark due to its larger color factor,which decreases the gluon contribution fraction and increase the light quark contribution fraction,but the dominant gluon contribution fraction of ??60%is still observed up to pT=20 GeV in Au+Au at RHIC.On the other hand,?0 meson production is light quark fragmentation dominant in p+p,and jet quenching effect further enhance this dominance of light quark fragmentation to ?0 in A+A.Therefore,we see the yield ratio of ?/?0 in A+A should be suppressed relative to that in p+p due to energy loss effect,and it separates with the one in p+p even at very high pT.We also compute the Ks0/?0 ratio as a function of pT both at RHIC and LHC.We find the curves in A+A and in p+p are approaching to each other with pT increasing,and an obvious coincidence of these two curves is seen at LHC.We show the gluon and quark contribution fractions to Ks0(and ?0 meson)as functions of pT.We find in p+p collision,the productions of both Ks0 and ?0 at very large transverse momenta are dominated by quark fragmentation.In A+A collisions,the gluon contribution shall be further suppressed because gluon generally loses more energy.Thus,both in p+p and A+A collisions,the ratio Ks0/?0 should be largely determined by the ratio of quark FFs for Ks0(DqKs0(zh,Q2))to quark FFs for ?0(Dq?0(zh,Q2))at very high pT,where these FFs vary slowly with the momentum fraction zh,very similar to the case of ?0/?0 at high pT.Even though in A+A collisions,jet quenching effect can shift zh of quark FFs,if quark FFs have a rather weak dependence on zh and pT,we can see at very high-pT regime the curves for Ks0/?0 in A+A and p+p are coming close to each other,and even coincide at LHC.In our calculations for(p+p)/(?+?-)and(?+?)/2Ks0,we notice the baryon anomaly could be caused by the different fragmentation functions for each identified hadron and the hydro description for the hot/dense medium.In the heavy ion collisions,the initial-state cold nuclear matter(CNM)effects,that caused by nuclear medium,would also affect the signals of QGP in the experiments,then we also discuss the CNM effects for better understanding the hot/dense nuclear medium effects in our thesis.Also the study on nPDFs is one of the main topics in high energy nuclear physics.The CNM effects can be considered as the alteration of nuclear parton distribution functions(nPDFs)relative to the free nucleon parton distribution function-s(PDFs)with a parametric formulism.Due to the non-perturbative mechanism,we can not easily calculate the parametric formulism or nPDFs from first principle,but extract the parameters needed in the parametrization from the global fitting to the experimental data(e.g.DIS).In fact,various parametrizations for nPDFs behave quite differently,so we need more experimental results and theoretical researches to constraint the parametriza-tion.Moreover we are trying to get a good quantity to study CNM effects effectively.The production of gauge boson tagged jets has been expected as a excellent tool to detect the CNM effects.Since the gauge boson is back to back with its associated jet in the hard scattering at leading order(LO)and the gauge boson or its lepton pair does not participant in the strong interaction,then it will carry the entire initial colliding information.And we choose the photon+jet production as the tool to investigate the CNM effects in our researches.Considering photons have many sources and the direct photons is our concern,we apply the same isolation cut used in experiments to reject the background of the frag-mentation photons and the secondary decay photons.A photon is isolated if the amount of deposited hadronic transverse energy is smaller than the threshold value in the cone of radius in pseudo-rapidity and azimuthal angle around the photon direction.The production of prompt photons in hadronic collisions proceeds through two mech-anisms.In the first one,which is usually referred to as "direct",the photon takes part in the hard process and is well separated from any hadronic activity.In the other one,which is usually referred to as "fragmentation",the photon results from the collinear fragmenta-tion of a coloured high pT parton.We discuss the influence of isolated cut in the photon production from both two mechanisms in our studies.And we point out the require-ments that the isolated cut needs to fulfil to preserve the factorisation theorem,since the isolation criterion would bring on additional phase-space restrictions in the calculations.We investigate the cold nuclear matter(CNM)effects on the productions of the isolated prompt photon and ?+ jet in proton-lead collisions at 8.16 TeV under the next-to-leading order(NLO)pQCD calculations with four parametrizations for nPDFs,i.e.DSSZ,EPP-S16,nCTEQ15,nIMParton16.We calculate the dependence of the nuclear modification factor of isolated photon on transverse momentum pT? and pseudo-rapidity ?? at very forward and backward rapidity regions,and demonstrate that the forward-to-backward yield asymmetries YpPbasym as a function of pT? with different nPDFs parametrizations have diverse behaviors.Furthermore,the nuclear modification factor of isolated-?+jet RpPbJet as a function of ?+jet's pseudo-rapidity ??Jet =1/2(????Jet)at different average transverse momentum pTavg = 1/2(pT? + pTJet)has been discussed,which can facilitate a tomography study of CNM effects with precise locations in a rather wide kinematic region by varying the transverse momenta and rapidities of both isolated photon and jet in p+A collisions.We discuss the Monte-Carlo methods and the applying in high energy nuclear physics.Then a Monte-Carlo event generator PYTHIA is introduced for p+p collisions.Moreover we introduce a Monte-Carlo event generator HIJING for heavy ion collisions and the related C++ upgraded progress.Since HIJING is constructed on the fortran-programmed PYTHIA6 to simulate heavy ion collision,our main task is how to build up the HIJING++framework based on c++-programmed PYTHIA8.Firstly we compare these two versions of PYTHIA to figure out which part we shall change in the new framework,such as the way we define the default values for the input parameters.Then we modularize the HIJING and divide different functionality into classes in HIJING++,like class HijPhysics.Furthermore we incorporate these classes with PYTHIA8 to be packed as Hijing class.Besides these,we also have changed the original random seeds sequence in HIJING++ and considered the scale dependence of shadowing factor and so on.At last,the results from the updated HIJING++ have been presented.