Limiting fragmentation in heavy-ion collisions and percolation of strings

The observed limiting fragmentation of charged particle distributions in heavy ion collisions is difficult to explain as it does not apply to the proton spectrum itself. On the other hand, string percolation provides a mechanism to regenerate fast particles, eventually compensating the rapidity shift (energy loss) of the nucleons. However a delicate energy-momentum compensation is required, and in our framework we see no reason for limiting fragmentation to be exact. A prediction, based on percolation arguments, is given for the charged particle density in the full rapidity interval at LHC energy ( √ s = 5500 GeV ). Recentely, the phenomenon of limiting fragmentation, [1], or longitudinal scaling, was rediscovered in the framework of high-energy heavy ion collisions [2]. In general the inclusive particle distribution, dn/dy, is a function of the central rapidity y and of the centre of mass energy √ s, or of ∆ ≡ y− yb and yb, where yb is the beam rapidity, with √ s = mbe b. Limiting fragmentation essentially means that as ∆ becomes larger than some yb dependent threshold ∆0, ∆ ≥ ∆0(yb), dn/dy becomes a function only of ∆, dn dy (∆, yb) −→ ∆>∆0(yb) f(∆) , (1) Departamento de F́ısica, IST, Av. Rovisco Pais, 1049-001 Lisboa, Portugal CENTRA, Departamento de F́ısica, IST, Av. Rovisco Pais, 1049-001 Lisboa, Portugal IGFAE and Departamento de Fisica de Particulas, Univ. of Santigo de Compostela, 15706, Santiago de Compostela, Spain