Produkcja cząstek naładowanych w oddziaływaniach ołów-ołów przy energii √SNN=2.76 TeV i proton-ołów przy energii √SNN=5.02 TeV w eksperymencie ATLAS
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Date
2015
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Institute of Nuclear Physics Polish Academy of Sciences
Abstract
The LHC (Large Hadron Collider) at CERN is dedicated to physics of proton-proton collisions at the highest energies. Since 2010 it provides data at new maximal energies obtained in the laboratory, currently at 13 TeV. In addition to proton beams the lead beams are
also accelerated for some fraction of time, which allowed to measure P b + P b collisions at √ sNN= 2.76 TeV and p+P b collisions at √ sNN
= 5.02 TeV - which are analysed in this work. The main objective of the high-energy heavy ion physics programme is the search and
studies of properties of the Quark-Gluon Plasma (QGP). This state of matter was present in the early Universe (before 10− 12 s) and can be created in the collisions of heavy nuclei. One of the basic properties of these collisions and subject of one of the first measurements
is the charged particle multiplicity. It can be used to estimate the energy density of the system created in the collision and provides a basic test of theoretical models. Before first measurements at the LHC the number of produced charged particles followed a logarithmic
dependence on the energy of collisions, but new observations revealed a faster growth of this observable. This thesis describes a new method for a measurement of the charged particle mul-
tiplicity and the pseudorapidity density, applied to the data from the ATLAS experiment, which is based on counting signals (pixel clusters) from the first layer of the pixel detector. The main advantage of this method is that is fully uses the acceptance of the innermost part
of the pixel detector, which is larger in pseudorapidity η than that available for other methods based on track reconstruction. In addition, this method allows to include particles with very small transverse momenta, which give signal only in the first layer of the pixel detector because either they stop before reaching even the second layer or their path is too curved by the magnetic field. The main disadvantage of this method is a strong dependence on corrections from Monte Carlo simulations. Using the pixel cluster method the charged particle density in the pseudorapidity range | η | <2.5 for P b + P b collision and | η | < 3.1 for p + P b collisions is measured. Charged particle multiplicity and angular distribution of particles are obtained separately for different intervals of the collision centrality. The charged particle multiplicity in the | η | < 0.5 range in P b + P b collisions increases fast with centrality, starting from dN ch /dη = 28.66 (centrality
75-80%) up to 1742.84 (centrality 0-1%). Mean number of charged particles per a pair of nucleons participating in the collision increases slowly with centrality of P b + P b collisions. Pseudorapidity density distributions for different centralities are similar while small differences between them can be attributed to a lower longitudinal momentum of particles produced in the central collisions than in the peripheral collisions. The charged particle density in p + P b collisions increases with the centrality of the collision. Also the shape of it changes due to an enhanced particle production in the Pb- going region (η > 0). In the central collisions (0-1%) charged particle density in Pb-going side reaches 70 particles while in the p-going side it is about 35. For peripheral collisions (60-90%)dN ch /dη is nearly flat in the full η range | η | < 3.1 and does not exceed 10. The average value of the charged particles density per a pair of nucleons participating in the collision is strongly depend on the method which is used to calculate N part for p + P b collisions. Three versions of the Glauber’s model used give very different dependences of this observable on N part . All presented results from the pixel clusters method are consistent with results obtained by the ATLAS experiment using other methods and with results of the ALICE experiment.