2011 QCD and High Energy Interactions - Rencontres de Moriond ...

EPOS 2 and LHC Results
TanguyPierog (1) , Iu.Karpenko (2,3) , and K.Werner (3)
(1) Karlsruhe Institut of Technology, Institut für Kernphysik, Postfach 3640, 76021 Karlsruhe Germany
(2) Bogolyubov Institute for Theoretical Physics, Kiev 143, 03680, Ukraine
(3) SUBATECH, University of Nantes – IN2P3/CNRS– EMN, Nantes, France
To study the effects related to the formation of a Quark-Gluon-Plasma in heavy ion collisions,
the proton-proton collisions are usually used as a reference. If this approach seems to be valid
at RHIC or for very high transverse momentum at LHC, it might be different at intermediate
pt for LHC energies, where some collective effects might be visible already in pp data (ridge,...).
Within a global model such as EPOS 2, where light and heavy systems are treated using the
same physics, we can test the collective effects both in pp and lead-lead collisions at LHC. In
this paper, it will be shown that the collective effect are not negligible already for pp at LHC.
1 Introduction
The CMS collaboration published recently results 1 on two particle correlations in ∆η and ∆φ,
in pp scattering at 7 TeV. Most remarkable is the discovery of a ridge-like structure around
∆η = 0, extended over many units in ∆η, referred to as “the ridge”, in high multiplicity pp
events. A similar structure has been observed in heavy ion collisions at RHIC, and there is little
doubt that the phenomenon is related to the hydrodynamical evolution of matter. This “fluid
dynamical behavior” is actually considered to be the major discovery at RHIC 2 .
1.5
0.5
-0.5
∆φ
R( ∆η,
∆φ)
1
0
4
3
2
1
0
-1
-3
-2
-1
0
1
2
3
∆η
R( ∆η,
∆φ)
Figure 1: (Color online) Two particle correlation function R versus ∆η and ∆φ for high multiplicity events in pp
collisions at 7 TeV, as obtained from a hydrodynamical evolution based on flux tube initial conditions (lefthandside).
On the righthand-side the calculation is done without hydro evolution .i.e. particle production directly
from string (flux tube) decay.We consider particles with pt between 1 and 3 GeV/c.
So does pp scattering provide as well a liquid, just ten times smaller than a heavy ion
1.5
∆φ
1
0.5
0
-0.5
4
3
2
1
0
-1
-3 -2
-1
0
1
2
3
∆η