Probing $α$ clustering in $^{12}\mathrm{C}$ at CSR energies using the Jet AA Microscopic Transport Model

TL;DR

The paper investigates whether intrinsic nuclear structure, specifically $\alpha$ clustering in $^{12}\mathrm{C}$, leaves observable imprints in low-energy heavy-ion collisions. Using the JAM transport model, it compares Woods--Saxon and triangular $\alpha$-cluster initial configurations for $^{12}$C in $\mathrm{C+C}$ and $\mathrm{C+Pb}$ at $\sqrt{s_{NN}}=2.36$ GeV to map initial geometry onto final-state observables. It finds that $\alpha$ clustering produces a more compact participant zone and enhances proton $\langle p_T \rangle$ and mean-flow magnitudes at large $N_{\mathrm{part}}$, while size and eccentricity fluctuations are only weakly affected and many final-state correlations show limited separation. Symmetric cumulants of initial eccentricities reveal clustering sensitivity, underscoring the complementary role of radial observables and flow-based correlations for constraining clustering in light nuclei, with significant motivation for CSR and HIAF measurements.

Abstract

We investigate the sensitivity of low-energy nuclear collisions to intrinsic nuclear structure by studying the interplay between initial-state geometry and final-state observables in C+C and C+Pb collisions at $\sqrt{s_{NN}}=2.36$~GeV, relevant for experiments at the Cooling Storage Ring (CSR) facility in Lanzhou and forthcoming experiments at the High Intensity heavy-ion Accelerator Facility (HIAF) in Huizhou. Calculations are performed within the Jet AA Microscopic Transport Model (JAM) using Woods--Saxon and triangular $α$-clustered configurations for the $^{12}C$ nucleus. The initial geometry is characterized in terms of transverse size, compactness, eccentricities, and their event-by-event fluctuations. We find that $α$ clustering leads to a more compact participant configuration than the Woods--Saxon case, while transverse-size and eccentricity fluctuations show only weak sensitivity to clustering. At this beam energy, radial observables remain sensitive to geometric compactness, with the proton mean transverse momentum $\langle p_T \rangle$ enhanced for $α$-clustered configurations, whereas pions show little sensitivity. The anisotropic response is examined using flow harmonic coefficients. We find an enhancement of the mean flow magnitudes, $\langle v_n \rangle = \sqrt{\langle v_n^2\rangle}$, for $α$-clustered configurations at large $N_{part}$, while the event-by-event fluctuation strength of individual harmonics remains small. Symmetric cumulants of the initial-state eccentricities show sensitivity to clustering, whereas the corresponding correlations among final-state flow harmonics do not exhibit a comparably strong separation. These results indicate that radial observables and correlation-based flow measurements provide complementary probes of $α$ clustering in low-energy nuclear collisions.

Figures (21)

• Figure 1: Radial distribution of the point-proton density of $^{12}\mathrm{C}$ obtained from JAM initial configurations for Woods--Saxon and $\alpha$-clustered nuclei. The densities are normalized to the total proton number.
• Figure 2: The transverse size ($\langle r^{2} \rangle$), compactness ($C$) and event-by-event transverse-size fluctuations as a function of $N_{\mathrm{part}}$ for Woods--Saxon and triangular $\alpha$-clustered initial configurations for C+C collisions at $\sqrt{s_{\mathrm{NN}}}=2.36$ GeV.
• Figure 3: The transverse size ($\langle r^{2} \rangle$), compactness ($C$) and event-by-event transverse-size fluctuations as a function of $N_{\mathrm{part}}$ for Woods--Saxon and triangular $\alpha$-clustered initial configurations for C+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.36$ GeV
• Figure 4: Event averaged participant eccentricities and their event-by-event fluctuations as a function of $N_{\mathrm{part}}$ for Woods--Saxon and triangular $\alpha$-clustered initial configurations for C+C collisions at $\sqrt{s_{\mathrm{NN}}}=2.36$ GeV.
• Figure 5: Event averaged participant eccentricities and their event-by-event fluctuations as a function of $N_{\mathrm{part}}$ for Woods--Saxon and triangular $\alpha$-clustered initial configurations for C+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.36$ GeV. Results are calculated using C participants only (see text for details)