1. Physics Motivation

1.1 The ALICE-Detector System

ALICE (A Large Ion Collider Experiment), one of the four large experiments at the CERN Large Hadron Collider, has been designed to study heavy-ion collisions. It also studies proton-proton (pp) collisions, which primarily provide reference data for the heavy ion collisions but also allow for a number of genuine proton-proton physics studies.

The ALICE detector has been designed to cope with the highest particle multiplicities anticipated for collisions of lead nuclei at the extreme energies that the LHC runs at. ALICE is composed of different sub-dectors. In the Central Barrel of the experiment, these are the Inner Tracking System (ITS), consisting of three detectors with 2 layers apeice, the Time Projection Chamber (TPC), the Transition Radiation Detector (TRD), the Time Of Flight detector (TOF), the High Momentum Particle Identification Detector (HMPID), the two electomagnetic calorimeters, EMCAL and PHOS, and ACCORDE, the dedicated cosmic ray detector. In the following analysis mainly the ITS, TPC, TRD and TOF will be discussed.

1.2 Nuclear Modification Factor R_AA

In 2010, the first heavy ion collisions at the LHC were recorded by the experiments. The ALICE experiment, which is well suited for measuring particle properties in high particle density environments, has published the nuclear modification factor R_AA for unidentified charged particles. R_AA is a measure of the difference in particle production in pp and PbPb collisions using the idea that a collision of two Pb ions is the superposition of $N$ pp collisions. R_AA is defined as:

R_AA = Y(PbPb)/(N_coll Y(pp))

with Y(PbPb) and Y(pp) being the yield in PbPb / pp collisions and N_coll the number of pp collisions, which have taken place in the collision of two lead ions.

If this nuclear modification factor is unity, the production of particles between pp and PbPb collisions is similar.

1.2 Ratio between Central and Peripheral Collisions R_CP

To quantify the suppression of high transverse momentum particles in heavy ion collisions, the yield in central A+A collisions is often compared to the yield in peripheral A+A collisions, scaled with the number of collisions.

R_Cp = (Y(PbPb,central)/N_coll,central)/ (Y(PbPb,peripheral)/N_{coll,peripheral})

The advantage in this case is, that most of the systematic uncertainties may cancel due to the same detector setup, since both data are taken in the same conditions but separated via the centrality. If no baseline in pp is measured, this measurement provides an important possibility to quantify the suppression even without pp reference.

R_CP shows a similiar behaviour to R_AA with transverse momentum.