Improved Pion-Kaon Identification in Heavy-Ion Collisions with a Two-Dimensional Transformation

Abstract

Accurate identification of charged pions and kaons is essential for precision measurements in relativistic heavy-ion collisions, but becomes increasingly challenging at intermediate and high transverse momentum due to the overlap between time-of-flight mass-square ($m^{2}$) and ionization energy loss ($nσ$) distributions. In this work, we present a two-dimensional shift and rotation method that exploits the correlated information between $m^{2}$ and $nσ$ to enhance particle identification performance. The method is validated using Au+Au collision events generated with the AMPT model, where detector response effects are incorporated through a data-driven smearing procedure tuned to reproduce the particle identification performance of the STAR experiment. The reconstructed pion and kaon transverse momentum distributions show excellent agreement with the AMPT input, maintaining a purity exceeding 98\% at high $p_T$ and extend the reliable identification range up to $p_T \approx$ 3 GeV/$c$. The extracted elliptic flow $v_2$ remains consistent with the input over the extended $p_T$ range, demonstrating that the proposed method provides a robust framework for high precision identified hadron measurements.

Figures (7)

• Figure 1: Transverse momentum ($p_T$) dependence of the smearing parameters used to model the detector response in the particle identification variables: (a) mean of the time-of-flight mass-squared ($m^{2}$), (b) width ($\sigma$) of the $m^{2}$ distribution, (c) mean of $n\sigma_{\pi}$, and (d) width ($\sigma$) of the $n\sigma_{\pi}$ distribution.
• Figure 2: Two-dimensional particle identification distributions for charged pions and kaons before and after applying the shift and rotation transformation. Panels (a) and (c) show the raw $m^{2}$ versus $n\sigma_{\pi}$ distributions in the transverse momentum intervals $2.0 < p_T < 2.2$ and $2.8 < p_T < 3.0$ GeV/$c$, respectively. Panels (b) and (d) present the corresponding distributions in the transformed coordinate system ($x$, $y$) after the two-dimensional shift and rotation, for the same $p_T$ intervals.
• Figure 3: Simulated $m^{2}$ distribution of charged particles in the transverse momentum interval $2.4 < p_T < 2.6$ GeV/$c$. The red dashed line represents the multi-Gaussian fit to the inclusive $m^{2}$ distribution, while the blue and gray solid lines indicate the pion and kaon components extracted from the fit, respectively.
• Figure 4: Projections of the transformed two-dimensional distributions for charged pions and kaons. (a) and (b) Projections onto the new $x$ and $y$ axes, respectively, for $2.0 < p_T < 2.2$ GeV/$c$. (c) and (d) Corresponding projections for $2.8 < p_T < 3.0$ GeV/$c$. The red dashed curves denote multi–Student-$t$ fits, while the blue and gray solid lines represent the pion and kaon components.
• Figure 5: Transverse momentum ($p_{T}$) distribution of charged pions and kaons from the AMPT input (gray bands) and the two-dimensional PID extraction (red solid circles). The lower panels present the corresponding PID purity versus $p_T$.