Symmetric-asymmetric collision comparison: disentangling nuclear structure and subnucleonic structure effects for small system flow

Abstract

Previous flow measurements in small collision systems were mostly based on highly asymmetric collisions ($p$+Pb, $p$+Au, $d$+Au, $^{3}$He+Au), where both nuclear structure and subnucleonic fluctuations are important. Comparing these asymmetric systems with the newly available symmetric $^{16}$O+$^{16}$O collisions at RHIC and LHC provides a unique opportunity to disentangle these two contributions. Using Glauber models incorporating both nucleon and quark-level substructure, we analyze multiplicity distributions and initial-state estimators: eccentricities $\varepsilon_n$ for anisotropic flow $v_n$ and inverse transverse size $d_{\perp}$ for radial flow. We find that subnucleonic fluctuations impact O+O collisions differently from asymmetric systems, creating specific patterns in flow observables that enable disentangling the competing contributions. Such experimental comparisons will reduce uncertainties in the initial conditions and improve our understanding of the properties of the QGP-like medium produced in small systems.

Figures (6)

• Figure 1: Charged particle multiplicity distributions $p(N_{\mathrm{ch}})$ in $p$+Au, $d$+Au, $^{3}$He+Au and $^{16}$O+$^{16}$O collisions at $\sqrt{s_{\mathrm{NN}}}$=200 GeV calculated using nucleon Glauber (left), three-quark Glauber (middle), and seven-quark Glauber (right) models. The enhanced sensitivity of symmetric O+O collisions to subnucleonic fluctuations is evident in the significantly broadened high-multiplicity tails when quark substructure is included, while asymmetric systems show minimal changes across the three models.
• Figure 2: Elliptic eccentricity as a function of $N_{\mathrm{ch}}$ in $p$+Au, $d$+Au, $^{3}$He+Au, and $^{16}$O+$^{16}$O collisions. Results are shown for nucleon Glauber (left), three-quark (middle), and seven-quark (right) Glauber models. From top to bottom: $\varepsilon_{2}\{2\}$, $\varepsilon_{2}\{4\}$, and $\varepsilon_{2}\{2\}/\varepsilon_{2}\{4\}$.
• Figure 3: Fluctuation-driven component of elliptic eccentricity $\delta_{\varepsilon_2}$ Eq. \ref{['eq:5']} (top row) and $\varepsilon_{3}\{2\}$ (bottom row) for $p$+Au, $d$+Au, $^{3}$He+Au, and $^{16}$O+$^{16}$O collisions. Results are shown for nucleon Glauber (left), three-quark (middle), and seven-quark (right) Glauber models.
• Figure 4: Radial flow estimators as a function of $N_{\mathrm{ch}}$ in $p$+Au, $d$+Au, $^{3}$He+Au, and $^{16}$O+$^{16}$O collisions. Top row: inverse transverse size $d_{\perp}$ which is expected to correlate with event-wise mean transverse momentum $[p_{\mathrm{T}}]$. Bottom row: variance of $d_{\perp}$, denoted as $\sigma_{d_{\perp}}$, which should correlate with fluctuations in $[p_{\mathrm{T}}]$. Results are shown for the nucleon-level Glauber model (left), the three-quark Glauber model (middle ), and the seven-quark Glauber model (right).
• Figure 5: The $N_{\mathrm{ch}}$ distribution for the three Glauber models in $p$+Au (top-left), $d$+Au (top-right), $^{3}$He+Au (bottom-left) and $^{16}$O+$^{16}$O (bottom-right) collisions at $\sqrt{s_{\mathrm{NN}}}$=200 GeV. The insensitivity of asymmetric collision systems to subnucleonic fluctuations, in contrast to symmetric O+O collisions, is clearly evident.