Strangeness production in a Quark Gluon Plasma

The threshold energy required to produce a pair of $s\bar s$ quarks is just the mass of two strange quarks. Due to the high temperature involved in the QGP phase, the thermal production of $s\bar s$ pairs becomes possible. Another source of enhancement of $s\bar s$ pairs comes from the process of Pauli blocking of the light quarks. As all quarks are fermions, they obey the Pauli Exclusion Principle. Therefore, as more and more light quarks are produced in the collision, they fill up the available low energy levels and it becomes favourable to create $s\bar s$ pairs. The production of anti-strange and multi-strange baryons will be enhanced as well.

Even if an enhancement of strangeness occurs in the QGP, there are some difficulties in quantifying the magnitude of this enhancement. The lifetime of the QGP phase is unknown, it is impossible to compute the actual values of particle production. An enhancement is expected to occur in A+A collisions compared to scaled p+p collisions (secondary collisions). Therefore, the unanswered theoretical question is what is the normal enhancement expected in A+A collisions. This can only be extracted from experimental results.

In order to summarize, before one can apply strangeness enhancement as a probe in nuclear collisions, it is important to fulfill a sequence of steps:

• study the elementary production process in p-p to determine its behaviour in the absence of any nuclear effects,
• determine its behaviour in the confined nuclear matter,
• check if in A-B collisions there are deviations from the ``normal'' behaviour observed in the confined nuclear matter.

Therefore pA experiments are a very important intermediate step, which can help to understand the behaviour of strange matter before to make any conclusions about the signature of QGP.