Massive Gauge Boson Production As A Probe Of Cold Nuclear Matter Effects And Influences Of Coherent Pion Emission On Observables

In relativistic heavy-ion collisions,the final-state-particle signals can be essentially clas?sified as the "hard" and the "soft",according to their transverse momentum magnitudes.The hard particles with high transverse momentum(pT>>2 GeV/c)are mostly produced through the hard scattering processes in the early stage of the collisions,and part of them pass through the hot/dense QCD matter(e.g.QGP)before being detected.The low-transverse-momentum(pT?2 GeV/c)soft particles are mainly emitted(or decoupled)from the evolv-ing hot/dense medium.The two kinds of signals can shed light on the properties of QGP in different ways,and are thus very valuable.In this thesis,we focus on the initial-state cold nuclear matter(CNM)effects in hard scattering processes as well as the effects of the coherent soft-paticle emissions on the final-state observables.The initial-state CNM effects can be related to the alteration of the nuclear paton dis-tribution functions(nPDFs)relative to the PDFs of the free nucleon.For the hard probes of QGP(e.g.jet),the initial-state CNM effects together with the hot nuclear medium effects like the energy loss,underlie the experimental observable RAA The study of CNM effects is useful for better understanding the hot nuclear medium effects.In addition,the CNM effect itself is also an important issue in high-energy nuclear physics.Largely impeded by the non-perturbative mechanism therein,it is hard to fully compute the nuclear parton dis-tributions or the CNM effect from first principle.Conventionally,the nPDFs in the form of parametrization are extracted from global fits to the experimental(e.g.DIS)data.How-ever,the differences among various nowaday paametrizations are still considerable so far.More valuable probes and theoretical supports are needed to provide more constraints on the nPDFs parametrizations.With the running of the LHC,the massive gauge boson(W±/Z0)production through the Drell-Yan mechanism in heavy-ion collisions is expected to provide a new excellent probe of the CNM effects.Since the lepton pair in the final state of Drell-Yan process does not participant in the strong interaction,it will carry a clean initial-state CNM-effect information.In the context of the QCD perturbation theory,the Drell-Yan process is one of the best understood hard scattering processes.The systematic study of W±/Z0 production in heavy-ion collisions in the framework of perturbative QCD will be helpful for us to disentangle various nPDFs parametrization sets,and to constrain them with the experimental data.On the other hand,nPDFs or CNM-effect models based on the microscopic nuclear-effect mechanism(e.g.,the meson exchange current,the off-shell correction of the bound nucleon PDFs,and so on)have been developed in the past several decades,and the KP model is one of the mature nPDFs models.With the help of this kind of nPDFs models,one can reveal the underlying physical mechanisms of the experimental data.In the aspect of soft particle signals,people usually learn the information about the QCD-medium evolution and the medium property(e.g.,the viscosity of QGP)form observ-ables like the particle transverse-momentum spectrum and azimuthal anisotropy.With great efforts,people find that the relativistic hydrodynamics is an effective theory of the macro-scopic evolution of QGP,and estimate that the QGP created at the RHIC and LHC is strongly coupled and behaves like a perfect fluid.The models base on hydrodynamics can well ex-plain most of the experimental data,like the transverse-momentum spectrum and azimuthal anisotropy.On the other hand,the transport models based on the microscopic mechanism can also describe the behaviors of most of the data.It is noteworthy that people usually consider the hot/dense medium as a chaotic par-ticle emitting source in the conventional study,namely,the particle emissions at different space-time positions are incoherent.However,possible signals of source coherence have been observed in the measurements of HBT correlations.Especially in the recent measurements of three-and four-pion HBT correlations performed by ALICE,the extracted coherent fraction in pion emission is approximately 30%,with other correlations excluded effectively.This observation further indicates the partial coherence of pion emitting source in heavy-ion col-lisions.However,the mechanism of coherent pion emission is not yet clear.The study of the effects of coherent emission on observables like transverse-momentum spectrum and az-imuthal anisotropy will be helpful for us to learn more details of the pion emission and the mechanism of the coherent pion emission.In this thesis,we study the W±/Z0 production in heavy-ion collisions at the LHC and future facilities with higher colliding energies,within the framework of perturbative QCD.By utilizing several nPDFs parametrization sets as well as the KP nuclear paxton distribution model,we investigate the CNM effects on various observables and compare the results with the recent LHC data.It is found that the rapidity distribution of massive boson is a good probe of the nuclear quark distributions,while the its transverse-momentum distribution is sensitive to the nuclear modifications of the gluon density.With the KP model,we show the role played by each individual underlying mechanism of CNM effects in the LHC p+Pb data.The results with KP model axe in an excellent agreement with the LHC p+Pb data,including the W± charge asymmetry,which shows deviations from the predictions of the nPDFs parametrization sets.Our results also provide an evidence of the presence of CNM effects in W/Z production in p+Pb collisions.On the other hand,we investigate the influences of source coherence on the pion transverse-momentum spectrum,azimuthal anisotropy,and HBT correlations,by considering the Bose-Einstein condensate in a static pion gas and a more general description of coherent pion emission of a relativistically expanding source.We find the momentum distributions of co-herent and chaotic emissions behave differently.For coherent emission,the pion transverse-momentum spectrum and azimuthal anisotropy arise from and are sensitive to the initial geometry of the coherent source,but are not sensitive to the source collective expansion.However,the chaotic emission is significantly affected by the source collective dynamics.With a partially coherent source model,constructed by 67%chaotic pion emission of a vis-cous hydrodynamical source and 33%coherent pion emission,we reproduce the experimental data of the pion transverse-momentum spectrum,elliptic anisotropy,as well as the four-pion HBT correlations,in Pb+Pb collisions at the LHC.