Strange particle production in proton-proton collisions at 158 GeV

A new experimen tal procedure to reconstruct neutral strange particles (so-called V 0 s, like §, §) and multiply strange particles ( cascades , like • ° and ≠ ° ) in the NA49 experimen t at the CERN SPS is presen ted in this thesis, together with the results obtained by this metho d in proton-proton collisions at 158 GeV beam energy . The NA49 experimen t, its detector system and reconstruction software is brie∞y describ ed. The way from the detector signals to reconstructed particle trajectories is ex- plained, with special emphasis on V 0 and cascade reconstruction. The newly developed procedure is presen ted together with another metho d. These two metho ds are com- pared, and their performance is studied in detail. Argumen ts for the need of the newly developed procedure (and for the need of two indep enden t metho ds) are given. The results of my work, presen ted in this thesis (obtained by combining the results of the two separate metho ds) are as follows: ≤ Inclusiv e longitudinal and transv erse momen tum distributions of § and § in proton- proton collisions are presen ted, providing a large impro vemen t in qualit y compared to the previously existing results. The shap e of the transv erse momen tum distri- bution is found to be indep enden t from the longitudinal momen tum. ≤ Previously existing experimen tal data on § and § production in p-p collisions are compiled, and compared to the NA49 results. ≤ Inclusiv e longitudinal and transv erse momen tum distributions of the • ° and • + particles in proton-proton collisions are presen ted. These are the Ørst measuremen t of • production cross sections in proton-proton collisions. ≤ The ≠ ° particle is observ ed in proton-proton collisions for the Ørst time. Its production cross section in the central region is estimated to be (8 : 5 ß 1 : 3) ¢ 10 ° 5 per inelastic event and unit of rapidit y. The upper limit for the ≠ + = ≠ ° ratio is estimated to be 0.5 with 95% conØdence level. Antibary on to baryon ratios are presen ted and compared to model predictions as a function of strangeness. These results contradict to the predictions of string models. ≤ The transv erse momen tum distributions of §s and protons are compared at midra- pidit y, and are found to have a very similar shap e. §s and antiprotons also have a very similar p T distribution, but with smaller inverse slope. ≤ Bary on-an tibary on correlations are studied. The rapidit y distribution of §s created in conjunction with §s or ps has a maxim um at midrapidit y (contrary to the same distribution in minim um-bias p-p collisions), which is attributed to baryon pair production. The shap e of the p T distribution of §s created together with §s or ps has the same shap e as in minim um bias proton-proton events. ≤ Strangeness enhancemen t in proton-n ucleus and nucleus-n ucleus collisions is stud- ied, using the results of the NA49 experimen t in proton-proton collisions as a reference, and results of the NA49 and WA97 experimen ts in proton-n ucleus and nucleus-n ucleus collisions. Enhancemen t (about a factor of 2) is seen already in p-Be collisions. Larger enhancemen t is seen for larger number of collisions per par- ticipan t, and the enhancemen ts in proton-n ucleus and nucleus-n ucleus collisions are found to be of similar magnitude.