Analysis of diffractive dissociation of K– into K–π+π– on a liquid hydrogen target at the COMPASS spectrometer

The systematic exploration of excited meson and baryon states was the central topic of the COmmon Muon Proton Apparatus for Structure and Spectroscopy (COMPASS) physics program in the years 2008 and 2009 at the CERN facility. Particularly states non fitting a constituent quark model were searched for, identified by their exotic quantum numbers which are forbidden by a simple q¯q scheme. A high energetic hadron-beam on a fixed target yielded in resonances decaying into a large variety of final states. The diffractive and central production mechanisms allowed for a clean exclusive selection of channels such as -p ! +-+precoil, which then were analysed with a high precision and a huge number of events. Apart from the main pion component in the negative hadron beam a small fraction of kaons of about 2.5% allowed the study of light strange resonances in the K-+- decay channel. The best measurement in this channel was quoted to be so far the measurement of theWA03 experiment at CERN. The ACCMOR-collaboration has the data recorded and analysed with an experimental set-up very similar to the COMPASS-spectrometer [1]. COMPASS had therefore not only the ability to remeasure this channel with a significantly higher number of events and better precision to cross-check those over 30 years old results. Moreover, improved analysis methods together with new evidences from other experiments, are expected to enlighten our present picture of the hadron spectrum. Chapter 1 introduces the concept of meson spectroscopy. The formation of bound quark anti-quark systems allows to interpret our measurements in terms of coupled quantum numbers, with properties as spin, charge conjugation and parity. Particularly kaonic isospin I = 1 2 resonances are classified as proposed by group theory and differences to I = 1 and I = 0 states, formed mainly by u and d quarks and their anti-quarks, are pointed out. That chapter introduces also the production mechanisms used to access those resonances and summarizes briefly the results obtained so far from previous experiments. Special emphasis is put on single diffraction of beam particles at high beam energies. Important observables as well as the main properties of the production mechanisms are defined, to understand how excited beam particles were identified in the COMPASS spectrometer. Chapter 2 describes the most important parts of the COMPASS spectrometer as it was assembled in the years 2008 and 2009 under light meson spectroscopy considerations. The beam line and beam properties of the hadron beams are discussed since beam divergence was affecting the kaon identification in the initial state significantly. Detectors used to measure the large variety of processes are presented as well as information about triggers, the DAQ and event reconstruction is given. As strategies had been developed for this analysis, to distinguish kaons from pions with the CEDAR detectors in the initial channel as well as with the RICH detector in the final states, those detectors are treated of in two separate chapters. The CEDAR detector, designed to identify one kind of particles in the beam is described in chapter 3. The set-up, performance and measurement of it is discussed as well as stability issues of those detectors filled with pressurized helium are pointed out. The calculation of the separation-purity is described and alternative methods to analyse the signals of this detector are shown. Chapter 4 deals with the final state PID at COMPASS with the help of the RICH detector. The PID, based on individual likelihood cuts, is explained and a measurement of the performance of this detector for the data of the year 2008 is presented. This is particularly important as the resulting efficiency and purity distributions were used directly as an input to the MC acceptance simulations. The strategy for the selection of diffractively produced K-+- events is shown in chapter 5. Apart from the detailed explanation of cuts applied to the data, quality studies showing the impact by initial state pions are presented. The model of a two-body decay of resonances into the final states is already motivated by observations of resonances in the invariant mass distributions of K-+and +-track combinations as well as the corresponding Dalitz plots. About 270 000 events served finally as an input to PWA. As Partial Wave Analysis (PWA) needs further information on the available K-+- phase space and the spectrometer acceptance of it, a detailed MC simulation study was performed as depicted in chapter 6. There, the methods to simulate the signal response of several detectors are presented, used to process in total 44 million MC decays in the range of 0.8 6 m(K-+-) [GeV/c2] < 3.0. Finally all information from event selection and MC studies was combined in the PWA to observe resonances, suspected to contribute to the invariant K-+- mass spectrum. The continuous momentum basis of final states was expanded in terms of discreet states with spin, parity and orbital angular momentum. A partial wave set was found with the help of mass independent fit algorithms, able to describe the observed K-+- spectrum and it’s dynamics in narrow bins of the invariant K-+- mass. Individual partial wave resonances are discussed and a proposal for a partial wave amplitude parametrization is made. The results of the mass independent analysis studies are the fundamental basis for a prospective mass dependent fit giving finally the information about poles, widths and coupling constants of contributing resonances.