Measurements Of J/? Polarization In P+p Collisions At (?)=200 GeV With The STAR Experiment

Quantum chromodynamics(QCD)as a part of Standard Model(SM)is used to study the strong reaction between elementary particles.It has not been fully understood yet.The asymptotic freedom allows the accurate calculation at high energy transfer process with a perturbative approach.However,QCD is poorly known in the low en-ergy transfer process which is non-perturbative.Due to the mass of the heavy quark,quarkonium production mechanism involving different energy scales provides an avail-able means for QCD study.In hadronic collisions,the quarkonium production process can be factorized into two parts.One is the production of heavy quark pairs involv-ing high energy transfer.The other is the hadronization to a heavy quarkonium as a long-distance process.The mainly difference of the different models(and the theoreti-cal difficulties)are on the second part which needs to establish models.Thus,studying heavy quarkonium states provide an inside look at the interplay between perturbative and non-perturbative QCD.J/? as a bound state of c and c is the most popular quarkonium and has been studied for more than forty years.However,there are still some unknowns to be investigated.There are three popular models:Color Singlet Model(CSM),Color Evaporation Model(CEM)and Color Octet Model(COM also known as Non-Relativistic QCD(NRQCD)approach).The mainly difference of the different models(and the theoretical difficul-ties)are on the soft part.All of them can describe measured cross-sections reasonably well.Thus,a posteriori test of predictions is needed.Polarization as one of the funda?mental connection to the production mechanisms is used as a key test for different model predictions.Besides,there is still not a global theoretical description for both cross sections and polarizations.In addition,the previous polarization measurements are in?complete and ambiguous.The CDF polarization measurement using RunI((?)=1.8 TeV)and Run?((?)=1.96 TeV)data indicate different polarization trends.At RHIC,the STAR previous measurements have a longitudinal polarization trend with increas-ing transverse momentum(PT)but only for one reference frame.The possible reason leading to these differences is that the angular distributions are correlated on one hand and on the other hand different reference frames are needed.The new measurement of polarization with improved and more robust analysis techniques are needed.This is the first polarization measurement for inclusive J/???+?-decays in p+p collisions at(?)=200 GeV at mid-rapidity at STAR.Due to the limit of the statistics and to have a stable measurement,a simultaneous fitting to 1-dimensional polar(cos?)and azimuthal angular((?))distributions is applied.A Monte Carlo simulation having the STAR detector geometry and performance(embedding)is used to correct the effect of the acceptance and efficiency of the detector.Considering the correlation of the 1-dimensional cos?-and ?-acceptances,one angular distribution strongly depend on the input of the others.The difficulty is that the J/? polarization is not known a priori in the simulation.An iterative procedure for the correction is adopted.Another simulation to check the performance of the iterative procedure is also performed showing that the correct value can be reached when a convergence occurs.The acceptance values as a function of one variable strongly depend on the input distribution used for the other variable.The measurements are performed in the continuous J/? transverse momentum range of 0-5 GeV/c.Both th ?? and ?? parameters are extracted in both the helicity and the Collins-Soper frames.The new results are consistent with 0,i.e.,no significant polarizations can be seen in both frames.The frame-independent parameters ?inv as a function of PT are also measured to cross-check the performance of the measurement in different frames.The results are consistent in both frames.The CSM,improved CEM(ICEM)and COM predictions are used to compare with the data.The CSM predicts no polarization for ?? in helicity frame with large uncertainties.The ICEM predicts transverse polarization and only touches the upper limits of the three data points.The COM approaches using two LDMEs with different rapidity region show different pre-dictions especially at low pT.These findings provide new insight in the dynamics of J/? production at RHIC energy and give a hint that the polarization measurement can be used to further constrain the calculations.Additionally,at the beginning of my study,I have joined in the end-cap time-of-flight(ETOF)upgrade project in the Beijing Spectrometer ?(BES?).I am respon-sible for the quality control and batch testing of the total 72 mass-produced Multi-gap Resistive Plate Chamber(MRPC).The momentum(energy)uncertainty of cosmic ray muons lead to an observable time-of-flight fluctuation in the measurement as a system uncertainty.I calculated/corrected the influence of MPRC vertical position on the time resolution measurement and fixed it as a uniform system uncertainty in batch testing.The time resolution of MRPC modules can reach to 60 ps after the correction.The up-dated ETOF system is commissioned into BESIII since January 2016.The upgrade of BESIII ETOF is successful.