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Target

In the $Pb$ run, we used $Pb$ targets made in the shape of a 1cm diameter disk of 1.15 $g/cm^2$ or 2.3 $g/cm^2$ thickness. Interaction probability in the target can be estimated, scaling the known  [27] nuclear inelastic cross-section ( $\sigma(pPb)=1.77$ $barn$) in proportion to the number of primary collisions between individual participants (1 for $p$, 208 for $Pb$):

$$\frac{\sigma(PbPb)}{\sigma(pPb)} = \left(\frac{2 \times 208^\frac{1}{3}}{1^\frac{1}{3} + 208^\frac{1}{3}}\right)^2 \approx 2.93$$ (1)

When the interaction cross-section $\sigma$ is known, the interaction probability is $\sigma N_A d/ \mu$, where $d$ is target thickness in $g/cm^2$, $N_A$ is the Avogadro's number, and $\mu$ is molar mass of the material in $g/mol$. For the two target thicknesses mentioned, the interaction probabilities would be, respectively, 1.7 and 3.4 %.