Nuclear Modication Factor RAA
In the year 2010, data from the first heavy-ion collisions at the LHC have been recorded by the experiments. The ALICE experiment, which is well suited for the measurement of the properties of particles in high particle density environments, has measured the nuclear modification factor RAA for unidentified charged particles. RAA is a measure for the difference in particle production in pp and nucleus-nucleus collisions, taking into account the different collision geometries. RAA is defined as:
where Y(PbPb) and Y(pp) are the yields (or number of particles per event) in PbPb and pp collisions, respectively, and is the average number of so-called binary nucleon-nucleon collisions, which have taken place in the collision of two lead ions. Since each lead nucleus consists of 208 nucleons, i.e. protons and neutrons, it is clear that a typical PbPb collision can be seen as a superposition of many pp collisions. RAA, which can be determined in the experiment, measures how many pp collisions should be equivalent on average to one PbPb collision.
If the nuclear modification factor is equal to one, the production of particles in one PbPb collision on average is the same as in independent pp collisions. Naively, this could be interpreted such that the physics of pp and PbPb collisions is the same. RAA values other than one, showing enhancement or suppression of certain flavours, may indicate effects induced by the quark-gluon plasma (QGP).
A more detailed tutorial can be found here:
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