Strangeness enhancement

  • Posted on: 20 January 2015
The strange quark is heavier than u and d, yet close enough in mass to undergo production and modification processes in similar manner. That, and the relative abundance of the strange quark in high-energy interactions, make the s-quark a very useful study tool for proton-proton and heavy nucleus collisions.

Strange particles, K-mesons (Kaons, made up of a strange and a non-strange quark pair), Lambda (uds), Xi (dss), and Omega (sss) baryons have an appreciable lifetime before they decay into ordinary matter. These decays have a characteristic geometrical configuration, which allows an effective reconstruction of strange particles. The strangeness data obtained in pp collisions are particularly important to improve the overall modeling of those collisions. PYTHIA is a software package that is able to generate events from a model. The model parameterizes the low-momentum processes taking place in elementary collisions, and calculates the higher energy processes up to the next to leading order perturbative term in the perturbative QCD expansion.

The latest PYTHIA versions describe the general properties of real collisions fairly closely, but significantly underestimate the yields of strange particles: the more the strangeness content of the particle, the worse is the discrepancy. One of the recent PYTHIA versions, PYTHIA Perugia-2011, made significant modifications to its s-quark cross-section, and as a result came very close to reproducing the yields of baryons with multiple strange quarks, especially at higher transverse momentum. ALICE is the only experiment at the LHC to have measured Omega baryon yields in pp collisions. In addition, the pp measurement serves as a baseline for the measurements in Pb-Pb collisions, where we expect to produce the Quark-Gluon-Plasma (QGP). An enhancement in the production of particles with strange quarks has long been thought to be a signature of extra degrees of freedom available in the QGP. Indeed, this enhancement has been seen at lower energies as well: the larger the volume of the collision, the more the number of Lambda, Xi and Omega baryons increases with respect to the baseline (a pp or a Be-Be collision). This is also observed at 2.76 TeV Pb-Pb collisions, however, with a caveat: the enhancement is smaller than that at lower energies! We think this is due to the complexity of our baseline: at these high energies, pp collisions to which we compare are more complex and produce much more strangeness than events at lower energies.