Baseline Study For The Net-Proton Fluctuation In The Relativistic Heavy Ion Collisions

The heavy ion collision experiment is an effective method for obtaining information on matter under high energy density.Measuring the thermodynamic properties of high-density nuclear matter in experiments is crucial for understanding the strong interactions of microscopic matter.The physical properties of matter in accelerators go beyond previous knowledge and can help scholars enrich their understanding of the composition of matter,the origin and evolution of the universe,and other aspects.The phase structure of quantum chromodynamics(QCD)matter is an important issue in accelerator experiments,which had been carried out in facilities such as RHIC and LHC,with plans for more data collection.Experimental resultshave been publicly released for Au+Au collision experiments with(?)of 3.0-200 GeV.In the publications of released experimental results,conserved quantities such as event-by-event net-baryon number,net-charge number,and net-strangeness number fluctuation are used to detect the variation of correlation length in QCD phase transitions.The fluctuation of net baryon number is most sensitive to phase transitions among these quantum numbers.The net-baryon number and its surrogate quantity,net-proton number,are calculated in the experiment as an observable associated with fluctuations and correlations at each point in the phase diagram.In the existing experimental results,it has been found that the fluctuations of net-proton number in the high energy and low baryon density regions can be well described by physical models that only contain ideal gas interaction.Based on the previous work by others,this study has replicated the model research on net-proton number,reproduced and compared the behavior of experimental results with the reference baseline.In the region of low energy and high baryon density,existing experimental studies have found energy dependent deviates from the baseline,which are more consistent with the predictions of the model with QCD phase transition.This study summarizes the similarity of these models with experimental results in these regions.In the experimental measurement of net-proton number,the correction and processing of data are important.This study discusses the correction tools of experimental data.Efficiency correction is related to the efficiency loss of data acquisition and reconstruction.The sensitivity of high order moments to phase transitions is restricted by acceptance effects.Therefore,this study discusses the impact of these effects and summarizes the efficiency correction methods in experiments.As an example,this study discusses the impact of non-binomial efficiency on high order moment calculations and attempts to use Bayesian unfolding methods to correct non-binomial efficiency.Heavy ion collisions are experiments with collision parameter fluctuations,and changes in collision parameters contribute to distortions in high order moments.This study discusses the centrality volume fluctuation and the uncertainty in determining centrality volume by using the reference multiplicity.The study also provides methods for analyzing the centrality volume fluctuation in other literature and summarizes their applications in experiments.The main content of this study is divided into three parts.The first part is a background introduction,which discusses the information on the structure of strong interactions and introduce that quantum chromodynamics is the theoretical framework for description of strong interactions.The QCD theory predicts that there is a critical point in the T-μ phase structure,which is the endpoint of the first-order phase transition line into the continuous phase transition region from hadronic phase to quark-gluon plasma phase.Experimental measurements are important for finding evidence of phase structures.In recent years,heavy ion collision experiments have found some phenomena similar to the predictions of the model with QCD phase transition at high energy regions,which can be used as evidence to support the existence of a critical point.This work will introduce observable that used to characterize the thermodynamic critical point in theory and experiments,such as fluctuations in net-baryon number,charge number,and strangeness number,and their higher order correlation functions.The second part of this study is about the baseline and model research on event-by-event cumulants calculation.This study will introduce theoretical models with/without critical contributions.Comparisons between experimental and baseline values will provide possible evidence for interesting physics.This part will show the comparison between experimental with baseline and model calculations of cumulants.Differences between the experimental results and the possible QCD phase transition signal will be analyzed in order to find the possible link to the signal of critical point.The last part of this study will introduce the most likely background effects that may appear in cumulants analysis,such as global charge number conservation,statistical bias caused by finite acceptance of data collection and reconstruction,and non-critical physical effects,such as the impact of resonance decay.The limited performance of detectors will also be considered,including the nonlinear efficiency response of detectors and the finite resolution to classify events.This work will discuss the impact of efficiency fluctuation.We also introduce efficiency correction methods based on solving this problem such as bin method and unfolding method.The discussion about Bayesian unfolding will go there.The centrality volume fluctuation is introduced by the fluctuation and uncertainties of initial collision geometry.This work will discuss the origins of the problem,and will use theory tool such is independent produce model to study the problem.The deviation arise from volume fluctuation on cumulants will be discussed.It is a challenge work to calculate cumulants in a situation where centrality has serious overlap,we will introduce the effects and assumption from other work to suppress this uncertainty.At last,we will introduce another analysis framework for the volume fluctuation problem.The last part of this work will be a summary.