Long-range Correlations of Charged Hadrons in Nucleus–Nucleus Collisions at the CERN SPS

Abstract It has been believed since the 1960s that hadrons — most artificially-produced particles as well as protons and neutrons making up atomic nuclei — consist in fact of even smaller particles called quarks. At the same time it is believed that it is impossible to free a quark from inside a hadron; this phenomenon is called confinement and has so far been confirmed by all experimental observations. On the other hand the opposite effect, asymptotic freedom, has led physicists to believe that under appropriate environmental conditions quark matter could undergo a phase transition into states in which quarks along with gluons (carriers of the strong force) could be considered deconfined; one of such states, characterised by high temperature, is called quark-gluon plasma (QGP). Even though the QGP is thought no longer to naturally exist in our universe, we are capable of recreating appropriate conditions through the means of high-energy collisions of heavy atomic nuclei, hurried to relativistic speeds in particle accelerators. Experimental programs involving searches for the QGP have been in progress in a number of accelerator facilities around the world since the 1970s; one of such programs was launched in early 1990s at the Super Proton Synchrotron at CERN and involved a variety of experiments dedicated to searching for quark- gluon plasma in Pb+Pb collisions at the beam energy of 158 A GeV, including the experiment NA49. With minuscule droplet sizes and extremely short lifetime, experimentally-produced quark- gluon plasma cannot be observed directly and must be searched for by examining observables expected to be possible to trace back to when it existed — either by examining them in sys- tems believed to contain the QGP and looking for discrepancies with respect to systems which do not, or by tracking their behaviour as a function of beam energy and looking for qualitative changes which could signify phase transition. One such observable is the shape of jets, colli- mated showers of particles originating from interactions of individual quarks/gluons rather than whole hadrons (the so-called hard scatterings) and as such expected to be sensitive to the pres- ence of deconfined quarks and gluons. In turn, one of the methods which can be used to observe jet shapes in collisions takes advantage of their particles’ collimation to produce multiparticle azimuthal-correlation functions. Its application to analysing data from experiments at the BNL RHIC has been a resounding success, however until 2005, when first results on the subject were shown by the CERES Collaboration, no analysis of this sort was performed in the energy range of the SPS — not least due to the fact the realm of hard processes is not as easy to access there as at the RHIC, with other correlation sources potentially capable of distorting or obscuring the jet signal. NA49 is a large-acceptance hadronic spectrometer which between 1995 and 2002 collected information about a variety of different collision events, with several types of collided particles and for a wide range of collision energies. Its primary components are four large-volume Time Projection Chambers, providing kinematic information about event particles as well as identi- fying them by ionisation energy loss ( dE/dx ). Two Time of Flight walls complement particle identification at low momentum and around mid-rapidity. The set-up also contains a plethora of small beam-position and triggering detectors, as well as a Veto Calorimeter used to deter- mine centrality of nucleus–nucleus collisions through measurements of beam-remnant energy. In spite of its non-uniform azimuthal coverage, with its large acceptance and many different observed systems NA49 can be considered a good source of data for correlation studies. In the study described in this dissertation we have produced two-particle azimuthal corre- lation functions for p+p , central Si+Si as well as central and mid-central Pb+Pb collisions at 158 A GeV, along with central Pb+Pb events at 80 A , 40 A , 30 A and 20 A GeV; moreover, for central Pb+Pb collisions at 158 A GeV we have also produced two-particle ( Δ η , Δ φ ) functions. Event and track cuts used to improve quality of the observed signal, along with procedures for calculating and/or estimating statistical and systematic errors, have been described and dis- cussed. The functions have been used in a number of scans attempting to establish trends of their behaviour with changing centrality, selection of charge or transverse momentum of paired particles, system size and beam energy. Results from the CERES experiment at the SPS have been used for reference for heavy-nucleus collisions at the top SPS energy, whereas functions from the PHENIX experiment at the RHIC have allowed us to extend the energy scan beyond the SPS. Finally, two-particle azimuthal correlation functions from p+p events at 158 GeV as well as from central Pb+Pb events at different collision energies, along with two-particle ( Δ η , Δ φ ) functions from central Pb+Pb collisions at 158 A GeV for a number of transverse-momentum bins, have been compared to the output of the string-hadronic model UrQMD Our results show the shape and amplitude away-side peak of two-particle azimuthal cor- relation functions to depend strongly on system size but only weakly on collision energy or transverse-momentum selection, moreover they agree with UrQMD regardless of whether jet production is enabled in the model or not. This is at odds with expectations regarding such correlations should they originate from jets and their modification by quark-gluon plasma but is consistent with effects of global momentum conservation. Furthermore no ridge-like structure, visible in RHIC results and associated with the QGP, has been observed in two-particle ( Δ η , Δ φ ) correlations. On the other hand, the amplitude of the near-side peak in two-particle azimuthal correlations drops with decreasing energy and turns into a depletion around 40 A GeV. This phe- nomenon is not visible in simulated functions, suggesting the possibility of its association with the onset of deconfinemen