| According to the Big Bang theory,it is believed that a new form of matter——Quark Gluon Plasma(QGP)was created in the early universe where the temperature and density were extremely high,while the lattice Quantum Chromodynamics(IQCD)theoretically predicted that under the condition of low baryon number density and extremely high temperature,the ordinary hadronic matter will undergo the phase transition from quark confinement to quark deconfinement,and the quark and gluon reach local thermal equilibrium to form Quark Gluon Plasma.The relativistic heavy ion collider(RHIC)at the Brookhaven National Laboratory in the United States accelerates the nucleus to extremely high energy for collision.By using the collected data,we study the properties of the quark gluon plasma by measuring the information of the final particles produced after the collision.It provides a possible method for further study of matter interactions at partonic free degree.Since the rest mass of the heavy quark is much larger than the critical temperature of the quark gluon plasma,most of the heavy quarks are produced in the hard scattering process at the beginning of the collision,and have undergone the evolution of QGP until the heavy quark forms a new hadron.Therefore,the heavy quark is considered to be an excellent probe for studying the properties of QGP and the mechanism of hadronization.By measuring the invariant yield ratio of heavy hadrons(e.g.Ac±/D0,Ds±/D0)in Au+Au collisions and comparing it with the ratio in the pp collisions,we can study the similarities and differences of hadronization mechanism from Au+Au collisions and pp collisions(partonic fragmentation and hadronization).The DJ meson contains a charm quark and a strange quark.Since in the Au+ Au collisions it exists the strangeness enhancement with respect to the pp collisions,supposing the charm quark forms the hadrons by coalescence hadronization mechanism,we can predict that the Ds±/D0 ratio will show an enhancement which cannot be explained by the difference of energy loss between Ds± and D0 relative to the pp collisions.This thesis mainly analyzes the experimental data from Au+Au collisions at(?)=200 GeV collected by the STAR detector in 2016 and 2014.The Heavy Flavor Track was installed at that time.HFT has excellent resolution for the decay vertex position of heavy hadrons and accurately record the track information of the daughter particles that help us to accurately reconstruct the topological geometry from hadronic decay.We can reconstruct the heavy hadrons and greatly increase the raw yield of the reconstructed hadrons using HFT.In this analysis we mainly measured the pT spectrum of Ds± meson yield and further calculated the ratio of the invariant yield of Ds± and D0.Firstly,we use the Boosted Decision Tree from the Toolkit of Multivariate Analysis(TMVA)basing on machine learning algorithm to optimize the cut value of topological variables,and then we can extract Ds± raw yield with optimized significance for different centrality and pT bins;secondly we can estimate the efficiency of Ds± signal extraction by data-driven simulation applying same topological variable cuts with signal extraction;finally,the pT spectrum of Us± is obtained by dividing reconstruction efficiency on raw yield,with efficiency corrected spectra,we can furtherly calculate ratio of Ds± over D0.The experimental results show that the ratios of Ds±/D0 exist a larger enhancement(about 1.5-2 times)relative to the pp collisions and PYTHIA calculation.We think that possible reason is that in Au+Au collisions the hadronization mechanism of charm quark is obviously different from the parton fragmentation hadronization mechanism in PYTHIA setup.Meanwhile the strangeness enhancement was observed in Au+Au collisions.By comparing with the calculation of the coalescence hadronization model,It indicates that the calculation from the sequential coalescence hadronization model is in good agreement with the experimental data within the 4<pT<8 GeV/c.The measurement from the ALICE collaboration keeps good agreement with this measurement within the errors and especially,for low pT range,this measurement complement ALICE measurement.These results indicate that coalescence hadronization mechanism play an important role for the charm quark hadronization.This measurement also provides a proof for QGP creation with partonic degrees of freedom in Au+Au collisions. |
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