part of the dilute surfaces of Au and Cu (corona) overlap. []{data-label="fig:cartoon"}](Cartoon_CollisionGeometry.pdf){width="1.0\linewidth"}

   Centrality interval   $\left\langle T_{AB} \right\rangle$ (mb$^{-1}$)   $\left\langle N_{\rm part} \right\rangle$
  --------------------- ------------------------------------------------- -------------------------------------------
      Minimum Bias                       $2.54\pm 0.19$                                  $61.1\pm 2.7$
         0%–10%                           $8.8\pm 0.6$                                  $177.2\pm 5.2$
         10%–20%                          $6.0\pm 0.4$                                  $132.4\pm 3.7$
         0%–20%                           $7.5\pm 0.5$                                  $154.8\pm 4.1$
         20%–40%                          $3.1\pm 0.2$                                   $80.4\pm 3.3$
         40%–60%                         $1.00\pm 0.12$                                  $34.9\pm 2.8$

  : \[tab0\] The average values of the nuclear thickness function $\left\langle 
  T_{AB} \right\rangle$ and the numbers of nucleons participating in the nuclei interaction $\left\langle N_{\rm part} \right\rangle$ in different Cu$+$Au centrality intervals.

A detailed description of the PHENIX experimental set-up can be found elsewhere [@Adcox:2003zm]. Beam-beam counters (BBC, $3.0<|\eta|<3.9$) [@Allen:2003zt] located downstream in both beam directions (north and south), each consisting of 64 Čerenkov-radiator counters, provide the minimum-bias (MB) trigger [@Adler:2003zu] and are also used to determine the event centrality and vertex position along the beam axis ($z_{\rm BBC}$). The MB trigger is formed if two or more BBC counters on each side detect the passage of charged particle(s). The MB trigger efficiency in is 93% of total inelastic collisions. The event centrality is defined by the total charge observed in the BBC. The mean number of participating nucleons (), binary collisions () and 