Ausrichtung des ALICE Übergangsstrahlungsdetektors sowie Zweiteilchenintensitätsinterferometrie identischer Pionen aus p+p Kollisionen bei LHC Energien von 900 GeV und 7 TeV

This PhD thesis deals with results from the Large Hadron Collider (LHC), which provided first data in September 2009. Intrinsically it was foreseen that first collisions should already occur end of 2008. On September the 10th the first beam event was partially successful, but after a week of running an accident in the sectors 3 and 4 spoiled some of the magnets of the synchrotron. This caused a delay of almost one year due to the necessary repairs. After repairing and some further test for the purpose of calibration and alignment of the subsystems with cosmic rays on the 23rd of November and ultimately with stable beam on the 6/7th of December 2009 collisions of protons (p+p) at an energy of √sNN = 900 GeV took place. After an intended winter stop the first long run of the LHC commenced on the 30th of March 2010. Like in the previous years protons with an increased world record energy of 7 TeV were brought to collision. After 7 month of continuous data taking in p+p the last month of running was dedicated to lead collisions (Pb+Pb) at 2.76 TeV per nucleon. This will be the foreseen program for the next years of data taking with the LHC. On the long run the energy will be increased to 14 TeV in p+p and 5.5 TeV per nucleon in Pb+Pb. A Large Ion Collider Experiment (ALICE) is one of the four major detectors at the LHC and the only one dedicated to heavy ion physics. It is divided into 13 subsystems. One of these is the ALICE Transition Radiation Detector (TRD), which is installed around the Time Projection Chamber (TPC) at a distance of 3 m to the beam pipe. The acceptance in ϕ covers the complete 360◦. The coverage of the TRD in pseudorapidity is ±0.84, comparable to an angle of 45°. This subsystems concept is intentionally modular, being composed of 18 supermoduls, each containing 30 chambers. Always 6 such small units in a row (in r direction) are called a stack. Altogether the TRD is made up of 522 chambers, each of them being able to work as a self-sustaining small Transition Radiation Detector. Because of the limited precision while the integration of the supermoduls into the ALICE detector, they are shifted in average by 10 mm with reference to their ideal dedicated position. Furthermore the chambers are misaligned up to 1 mm relative to their stack. This imprecise installation together with time dependant changes of the real positions relative to the ideal geometry leads to a loss in detector efficiency and resolution. Some tracks are badly reconstructed concerning true trajectory others are even not found. Aim of the alignment of the TRD is the minimization of the above described geometrical uncertainties while the conversion of the detector signals into digital position informations, the so called reconstruction. This leads to a better tracking efficiency and resolution in the TRD. For this purpose the AliROOT alignment framework was developed. In the first step, the tracks which can come from cosmic rays or collisions in the LHC are examined concerning there usefulness for the alignment. Some cuts are applied. With the tracks which pass these cuts it is possible to calculate the necessary corrections of the positions for the supermoduls and chambers of the TRD. The supermoduls are aligned relative to the TPC which is suggested to be in an ideal position and the chambers are positioned concerning the remaining 5 chambers of their stack. As a result one receives six correction parameters (alignment parameters) for each alignable module of the TRD (supermoduls and chambers) in the chosen reference frame. These parameters are the three shifts along the axis in the local frame - z shift, rϕ-shift and r-shift, as well as the three rotations or tilts around these axis - z-tilt, ϕ-tilt and r-tilt. The extracted correction parameters are stored in the of Offline Condition Data Base (OCDB) and used when doing a new reconstruction cycle. In the end the efficiency and resolution of the TRD are monitored. The final position uncertainty of the supermoduls concerning the TPC was below 1000 μm. The position uncertainty of the chambers within their stacks appears to be around 300 μm. This is below the resolution (tracks relative to tracklets) of the TRD which reveals values higher than 400 μm. The second part of this thesis deals with two particle intensity interferometry. HBT, named after Robert Hanbury Brown (1916-2002) and Richard Twiss (1920-2005), reveals one possible way of gaining access to the space time evolution of the particle emitting source in p+p as well as in heavy ion collisions. The data of ALICE where analysed with this method, which first ideas trace back to the fifties of the last century. The systems for intensity interferometry of identical pions (π+π+ und π−π−) which we analysed were p+p at √sNN = 900 GeV as well as 7 TeV, and heavy ion collisions in Pb+Pb at √sNN= 2.76 TeV per nucleon. The first data we looked at were identical 900 GeV p+p pions [Aamodt, K. et al., 2010b]. This data set was collected in December 2009 as well as in April 2010. The one dimensional HBT radius obtained was 0.83±0.05(sys.) ±0.07(stat.) fm. It was remarkable that, like in preceding experiments in small systems, one was not able to fit the correlation function with a simple gaussian form (like in heavy ion systems). An exponential function was used to gain acceptable results. Furthermore we saw long range correlations at higher invariant pair momenta. A scaling behaviour with KT could not be verified by implication. Looking at the three dimensional parametrization one gets the HBT radii Rout = 0.79±0.09(sys.)±0.05(stat.) fm, Rside = 0.62±0.08(sys.)±0.06(stat.) fm as well as Rlong = 1.24±0.07(sys.)±0.10(stat.). Their dependency on the event multiplicity dNch/dη was obvious, whereas the scaling with the pair momentum kT was not as strong as in prior studies. In addition this dependency of the HBT radii on the pair momenta depends on the subtraction of the nonfemtoscopic background. The world record 7 TeV p+p data of the LHC were analysed the same way. In the Centre of Mass System (CMS) the invariant one dimensional HBT radius was RInv = 1.02± 0.04(sys.)±0.04(stat.) fm. The results of a three dimensional treatment of the data were Rout = 0.82±0.04(sys.)±0.01(stat.) fm, Rside = 0.75±0.03(sys.)±0.02(stat.) fm and Rlong = 1.41±0.05(sys.)±0.04(stat.) fm. Like in the 900 GeV data a pronounced dependency of the radii on the event multiplicity was discoverable. Another very interesting result that appeared was that the transverse dimensions of the pion emitting region do not significantly differ in the two observed energies (looking at equal multiplicity bins). In the end with the 7 TeV data it was possible to establish a link in multiplicity between proton collisions and peripheral heavy ion collisions at RHIC energies [Aamodt, K. et al., 2011d].