The presence of in-medium effects which are manifested in the change of basic hadron properties is one of the central issues in the analysis of nuclear matter. Strange particles are essential probes for studying in-medium effects for hadrons produced in collisions at energies close to the production threshold. The influence of the nuclear medium on the production process of neutral strange particles, Λ hyperons and K^0 mesons, has been studied in pion-induced reactions at 1.15 GeV/c beam momentum with five different target nuclei (C, Al, Cu, Sn and Pb). The FOPI detector at the GSI (Darmstadt, Germany) was used as the experimental environment. The main part of the analysis presented in this work was done for Λ particles. The obtained invariant mass spectra for all five targets used in the experiment are presented. The results of other distributions important for this analysis are shown for reactions on carbon and lead targets. Moreover, relevant distributions gained form experimental data are compared with results produced by simulation, using two different simulation approaches. The reconstruction efficiency is determined and applied as a correction factor in the subsequent analysis and the computation of the invariant cross section. The experimentally determined Λ inclusive cross section and its dependence on the production target size (A) is presented for the first time at this energy. The investigation of the inclusive Λ production in the π- +A → Λ+K0 reaction in medium should contribute to a better understanding of in-medium effects and the general behaviour of neutral strange particles produced close to the threshold.

Jedno od najvažnijih pitanja u proučavanju svojstava nuklearne tvari jest njen utjecaj na hadrone koji se u njoj nalaze, a očituje se kroz promjene njihovih svojstava. Ta pojava poznata je pod nazivom učinci nuklearnog medija (engl. in-medium effects). Strane čestice prikladne su za proučavanje učinaka nuklearnog medija, budući da uz lagane (u i d) kvarkove ili antikvarkove, sadrže i strani (s) kvark te se utjecaj nuklearnog medija upravo na strane čestice može izravno uočiti. Stvaranje neutralnih stranih čestica, s naglaskom stavljenim na Λ česticu, proučavano je u pionski induciranim reakcijama pri 1.15 GeV/c za reakcijske sustave s pet nuklearnih meta (C, Al, Cu, Sn i Pb). Podaci korišteni za analizu prikupljeni su pomoću FOPI detektora koji se nalazi na institutu GSI (Darmstadt, Njemačka). Glavni cilj ovog istraživanja jest proučavanje stvaranja Λ hiperona u reakciji π- + A → Λ + K0 unutar nuklearnog medija (A je maseni broj mete jezgre). Analizom eksperimentalnih podataka i podataka dobivenih iz korištenih simulacijskih modela određene su ocjene inkluzivnog udarnog presjeka stvaranja Λ hiperona. Po prvi put za ovaj tip elementarnih reakcija to je učinjeno na energiji dostupnoj FOPI eksperimentima (od 1 do 2 AGeV, preciznije za ovaj eksperiment: √s = 1.75 GeV). Proučavanje inkluzivnog stvaranja Λ čestica, usporedba različitih izmjerenih i izračunatih fizikalnih veličina, uključujući i inkluzivni udarni presjek za pet nuklearnih meta čiji se maseni broj kreće od A = 12 (C) do A = 208 (Pb) doprinosi boljem razumijevanju in-medium učinaka i stvaranja stranih čestica unutar nuklerne tvari.