Investigating the pion emission source in pp collisions using the AMPT model with sub-nucleon structure

Abstract

The measurement of momentum correlations of identical pions serves as a fundamental tool for probing the space-time properties of the particle emitting source created in high-energy collisions. Recent experimental results have shown that, in pp collisions, the size of the one-dimensional primordial source depends on the transverse mass (\mt) of hadron pairs, following a common scaling behavior, similar to that observed in Pb--Pb collisions. In this work, a systematic study of the \pipi source function and correlation function is performed using the multiphase transport model (AMPT) to understand the properties of the emitting source created in high multiplicity pp collisions at $\sqrt{s}=13$ TeV. The \mt scaling behavior and pion emission source radii measured by ALICE experiment can be well described the model with sub-nucleon structure. These studies shed new light on the understanding of the effective size of the \pipi emission source and on studying the intensity interferometry in small systems with a transport model.

Figures (11)

• Figure 1: A sketch illustrates the modification of the coordinate $\vec{x_1}$ to $\vec{x'_1}$ for particle $a$ (blue disk), which is generated at time $t_1$, due to the different freeze-out time compared to particle $b$ (gray disk), generated at time $t_2$, in pairing the two particles based on the AMPT framework. The coordinate system is defined by the rest frame of the two particles and is consistent with Eq. \ref{['Eq:Koonin-Pratt']}, where $\vec{r^*}$ represents their relative distance (dash-dotted lines).
• Figure 2: (Color online) A schematic view of the AMPT evolution from the space-time perspective. Panel (a) illustrates the initial partonic distribution. Panels (b) and (d) depict the core source radii for a pair of primordial hadrons, boosted before and after the ART stage, respectively, with an emission time parameter $\tau$ and each particle's velocity $\vec{\beta}$. Panel (c) is the same as (d) but with $\tau=0$. The figure is inspired by CECA.
• Figure 3: (Color online) The $\pi\hbox{--}\pi$ source functions before and after the ART stage from three initial partonic distributions. See the text for details.
• Figure 4: (Color online) The source function in the $k_\text{T} \in [0.15, 0.3)$ and $[0.9, 1.5)$ GeV/$c$ intervals before and after ART stages within the "3 quarks" AMPT model. Fitting with Gaussian and Cauchy functions are represented by solid and dashed lines, respectively. The shaded bands are the core radii from Ref. SourceMaxi.
• Figure 5: (Color online) The $\pi\hbox{--}\pi$ correlation function in the $k_\text{T}$ interval 0.15–0.30 GeV/$\it{c}$ before and after the ART stages for three initial partonic distributions in AMPT+CATS framework Dimi_CATS.