How many pairs of nuclei collide in heavy ion collisions?

Question

As each bunch of heavy ions consist of a large number of nuclei it does not seem unlikely that multiple binary ion collisions will occur as it does in p-p collisions. However, should this be the case, I do not see how a possible azimuthal anisotropy could be related to the elliptic flow - which to my understanding is defined for the individual binary nuclei collisions and in terms of the reaction plane of the collision.

So, either the luminosity is tuned to allow for maximally one collision per crossing or there is a way to separate the individual collisions from each other.

So, my question is two-fold:

1. How many pairs of nuclei collide in heavy ion collisions (at the LHC)?

2. If $n>1$; how are they distinguished from each other?

Answer 1

ALICE is a heavy ion experiment at CERN.

Here is a lead lead collision

One of the LHC's first lead-ion collisions, as recorded by the ALICE detector.

Thanks to the advances of computing the vertex is determined by the tracks , measured and pointing back to it, even though there are thousands of tracks from each vertex. Certain tolerance assumptions have to be used to identify a track to a vertex, of course.

The average number of interactions per bunch crossing varies from 0.05 to 0.3. (page 9 in link)

So the experiment is designed to have one main vertex (one colliding pair of ions) per crossing by having a low luminosity.