0.0 0.2 0.4 0.6 0.8 1.0 0 100 200 300 400 - K - p STAR PHENIX pp AA central AA central Particle 〈 p$_{T}$ 〉 Err. 〈 p$_{T}$ 〉 Err. 〈 p$_{T}$ 〉 Err. π − 0.348 0.018 0.429 0.022 0.455 0.032 K − 0.605 0.073 0.717 0.050 0.677 0.068 K 0 0.600 0.030 0.800 0.050 p 0.686 0.041 1.104 0.11 0.949 0.085 ¯ p 0.683 0.041 1.105 0.069 0.959 0.084 φ 0.820 0.050 0.970 0.097 Λ 0.780 0.040 1.050 0.090 ¯ Λ 0.760 0.040 Ξ 0.924 0.130 1.110 0.090 ¯ Ξ 0.881 0.130 Ω + ¯ Ω 1.080 0.300 1.260 0.090 TABLE I: A summary of mid-rapidity 〈 p$_{T}$ 〉 in GeV/c for identified particles from the STAR [7–11] and the PHENIX [12] collaboration. FIG. 3: < p i T > as a function of the centrality for different particle species i. The PHENIX data [12] are presented by symbols, the calculations by lines. 3 0.5 0.6 0.7 0.8 0.9 1.0 1.1 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1.0 Star Data model f i core by f$_{core}$ in eq. 2 changes the curve only slightly. Its form is dominated by the difference between < p$_{T}$ >$_{core}$ and < p$_{T}$ >$_{corona}$ . Tab.I summarizes the experimental < p i T >$_{corona}$ and < p i T >$_{core}$ which enter our calcula- tions and which are taken from pp and central heavy ion collisions. PHENIX has measured very peripheral heavy ion collisions. The < p$_{T}$ > value there coincide within the error bars with the STAR pp data. We note that < p i T >$_{corona}$ / < p i T >$_{core}$ varies from .81 for pions to .62 for protons. By construction the core - corona model correlates the < p i T > values of peripheral heavy ion reactions with those observed in pp. This correlation is shown in fig.4. It displays < p i T >$_{peri}$ / < p i T >$_{core}$ as a function of < p i T >$_{corona}$ / < p i T >$_{core}$ , using eq.2, for different hadron species. The experiential data show this correla- tion as well, it is even stronger there. Thus also in exper- iment the value of < p i T > in peripheral reactions is small as compared to that in central collisions for those hadrons for which < p i T > measured in pp is small as compared to < p i T > measured in central heavy ion collisions. This correlation is far from being trivial. If, as often assumed, the heavy ion collision can be described by hydrodynam- ical calculations we do not expect any correlation be- tween peripheral heavy ion data and pp data. Therefore the correlations observed in fig. 4 support the validity of the core - corona approach and questions whether hy- drodynamical calculations are the proper tool to describe non-central heavy ion reactions. Corona particles do not form a thermal equilibrium with the core particles but decay like pp collisions and hence isotropically. Only the core particles feel the eccen- tricity of the overlap region. Because M charged particles core ≈ M charged particles corona , v$_{2}$/ϵ$_{part}$ is expected to be ∝ f$_{core}$ and therefore the expected centrality dependence of v$_{2}$/ϵ$_{part}$ is that of f$_{core}$ . This allows for some immediate pre- dictions. Because f$_{core}$ is similar in central CuCu and FIG. 4: The ratio of < p T > in peripheral to central reactions as compared to that of pp to central reactions. The data are from [7–11, 13]. Because the calculation uses the experimental data, the error bars of model predictions are of about the same size as that of the experimental points.