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?


1 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.

  • $\begingroup$ Thank you for the reference. After having a closer look it seems to be the average number of pp interactions per bunch crossing which you list. The number for Pb-Pb is listed a bit lower on the same page and it is 10^-5-10^-3. However, the report is somewhat old - any idea about present numbers? $\endgroup$
    – AltLHC
    Commented Aug 14, 2015 at 8:54
  • $\begingroup$ Sorry, the numbers are somewhat old. For Pb-Pb they stem from 2010/2011. $\endgroup$
    – AltLHC
    Commented Aug 14, 2015 at 11:56

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