夸克胶子等离子体中部分子能量损失随碰撞系统的路径长度依赖性：对带电粒子RAA的贝叶斯分析，与从O+O到Pb+Pb的通用指数一致
Path-length dependence of parton energy loss across collision systems: a Bayesian analysis of charged-particle RAA, consistent with a universal exponent from O+O to Pb+Pb
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中文总结 AI 辅助
研究夸克胶子等离子体中部分子能量损失随路径长度的依赖性，通过联合分析四个碰撞系统的带电粒子核修正因子\(R_{AA}\)，用贝叶斯分析得出有效系统大小指数，支持辐射机制，给出能量损失幅度，为相关研究提供重要数据和结论。
中文摘要 AI 辅助
夸克胶子等离子体（QGP）中部分子能量损失如何随介质路径长度\(L\)缩放，编码了其机制：碰撞（\(\Delta E \propto L\)）、辐射（\(\Delta E \propto L^2\)）或强耦合（\(\Delta E \propto L^3\)）。利用欧洲核子研究中心大型强子对撞机的新轻离子数据，我们从系统大小本身提取这种缩放，联合分析CMS在四个系统——O+O、Ne+Ne、Xe+Xe和Pb+Pb（质量数\(A = 16\)到\(208\)）中的带电粒子核修正因子\(R_{AA}\)。采用数据驱动的光谱基线和蒙特卡罗格劳伯几何的贝叶斯分析得出有效系统大小指数\(n = 1.78 \pm 0.15\,\mathrm{(统计)} \pm 0.05\,\mathrm{(系统)}\)。嵌套采样模型选择决定性地支持辐射值（\(n = 2\)）附近的有效指数，而不是碰撞（\(n = 1\)）和强耦合（\(n = 3\)）值，这一结论在所有160种分析变体中都很稳定。由于涨落只会使有效指数低于其微观对应值，该测量在固定几何条件下从下方限制了后者，排除了纯碰撞能量损失。介质密度和路径长度在系统大小上是简并的，所以我们将有效指数作为主要结果引用。贝叶斯因子检验发现小系统和大系统之间没有机制变化，与通用指数一致；相同框架为仅在O+O中有非零能量损失提供了决定性证据，量化了最小系统中抑制的开始。能量损失幅度对应于\(\hat{q}/T^3 \approx 2\) - \(5\)，与JETSCAPE的测定一致。
英文摘要
How parton energy loss in the quark-gluon plasma (QGP) scales with the in-medium path length $L$ encodes the mechanism: collisional ($ΔE \propto L$), radiative ($ΔE \propto L^2$), or strong-coupling ($ΔE \propto L^3$). Exploiting the new CERN LHC light-ion data, we extract this scaling from the system size itself, jointly analysing CMS charged-particle nuclear modification factors $R_{AA}$ in four systems - O+O, Ne+Ne, Xe+Xe and Pb+Pb - spanning mass number $A = 16$ to $208$. A Bayesian analysis with a data-driven spectral baseline and a Monte-Carlo Glauber geometry yields an effective system-size exponent $n = 1.78 \pm 0.15\,\mathrm{(stat)} \pm 0.05\,\mathrm{(syst)}$. Nested-sampling model selection decisively favours an effective exponent near the radiative value ($n = 2$) over the collisional ($n = 1$) and strong-coupling ($n = 3$) values, a conclusion stable across all 160 analysis variants. Because fluctuations can only lower the effective exponent below its microscopic counterpart, the measurement bounds the latter from below at fixed geometry, excluding purely collisional energy loss. The medium density and the path length are degenerate across system size, so we quote the effective exponent as our primary result. A Bayes-factor test finds no change of regime between small and large systems, consistent with a universal exponent; the same framework gives decisive evidence for non-zero energy loss in O+O alone, quantifying the onset of suppression in the smallest system. The energy-loss magnitude corresponds to $\hat{q}/T^3 \approx 2$--$5$, consistent with the JETSCAPE determination.
