本論⽂報告了|η| < 1.1範圍內、中⼼度 (centrality) 介於 0-70% 的各中⼼度區間 (centrality classes) 之dNch /dη分布。對於中心度最高的 0-3% 之碰撞事件，中快度區域 (mid-rapidity) 的帶電強子贗快度密度 dNch /dη(|η|<0.3) 測得為723.45±45.3。此外，本研究進一步探討以參與碰撞核子對數目 (number of participating nucleon paris) 歸一化後的中快度帶電強子多重度，如何隨參與碰撞核子數目 (Npart) 而變化。本論文使用 sPHENIX 數據所量測的結果與過去的 RHIC 量測結果一致，並在中等中心度區域 (mid-centrality region) 提供了更小的不確定度 (uncertainty)。除了其物理上的重要性之外，此項量測亦作為一項以 sPHENIX tracking system 為基礎的標準燭光量測 (standard candle measurement)，用以驗證偵測器效能、重建演算法及分析框架，進而為更廣泛的 sPHENIX 物理研究計畫奠定了重要基礎。;This thesis presents the measurement of the charged-hadron pseudorapidity density， $\dndeta$， at midrapidity in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200~\mathrm{GeV}$ with the sPHENIX detector. The charged-hadron pseudorapidity density is a critical global observable for studying the bulk properties of relativistic heavy-ion collisions， as it reflects the overall particle production and provides constraints on the collision geometry， initial conditions， and subsequent evolution of the produced QCD medium.
The \dndeta measurement in sPHENIX is performed primarily using data collected by the Intermediate Silicon Tracker (INTT)， a two-layer barrel silicon strip tracker. As a newly constructed detector， the performance of the INTT must be evaluated before its application to physics measurements. This evaluation is conducted through a test beam experiment at RARiS in Japan. The beam test analysis demonstrates that the INTT achieves a hit-detection efficiency above 99\%， providing an essential foundation for the sPHENIX \dndeta measurement.
In the \dndeta analysis with Au+Au collisions， INTT data are used to determine the beam spot and reconstruct the per-event z-vertex. Charged-hadron yields are extracted from tracklets formed by pairing clusters in the two layers of INTT barrel， with corrections applied for detector acceptance， reconstruction efficiency， and contributions from leptons and secondary decays. Two independent analysis methods are developed， and their results are found to be in good agreement.
The measured $\dndeta$ distributions are reported within $|\eta|<1.1$ for centrality classes ranging from 0-70\%. For the 3\% most central collisions， the mid-rapidity charged-hadron pseudorapidity density， $\left.dN_{\mathrm{ch}}/d\eta\right|_{|\eta|<0.3}$， is measured to be $723.45 \pm 45.3$ (syst.). The mid-rapidity charged-hadron multiplicity， normalized by the number of participating nucleon pairs， is further studied as a function of the number of participating nucleons. The results are consistent with all previous RHIC measurements and provide improved uncertainties in the mid-centrality region. In addition to its physics importance， this measurement serves as a tracking-based standard candle for validating detector performance， reconstruction algorithms， and the analysis framework， thereby supporting the broader sPHENIX physics program.
