Particles density at central rapidity in Au+Au

• dNch/dy constraints mechanisms of initial multi-particle production:

• dNch/dy (per participant pair) increases faster than linearly with Npart :

• Particle density at y=0 well described by pQCD- & CGC- based models alike:

•  "Soft + hard" (string + pQCD “minijet”): increased hard contribution (Ncoll )

•  Initial-state gluon saturation (CGC): dNch/dy ~ dNgluon/dy ~ 1/as ~ Npartln(Npart )