Searching for omega-mesic state in Carbon

Abstract

The existence of a bound pion-nucleus system has been experimentally demonstrated by embedding a charged pions in different nuclei like in a lead nucleus. This bound system, which is called deeply bound pionic atom, is held together by the interplay of Coulomb and strong interactions.This thesis concentrates on the investigation of the bound omega-mesic state in carbon nuclei. Since the meson does not carry an electric charge, this system, unlike the deeply bound pionic atoms, is governed by the strong interaction only. The study of such a system can lead to a better understanding of the basic meson-nucleus interaction.In March 2007, an experiment was carried out at the Elektronen-Stretcher-Anlage (ELSA) in Bonn to observe bound mesic states in carbon. The photon beam used for the experiment was produced in a thin radiator via electron bremsstrahlung. The Crystal Barrel (CB), the ForWard Plug (FW-Plug) and the Two Arm Photo-Spectrometer (MiniTAPS) calorimeters provided an almost 4pi coverage where the final-state photons were detected. The protons were identified with an aerogel Cherenkov detector and with the Time-of-Flight (TOF) technique.A bound mesic nuclear state can only be produced if the participating meson has a small momentum. The momentum distribution of the produced s measured in this experiment showed that slow mesons can be produced in quasi-free photoproduction in coincidence with knocked-out protons that were registered in very forward direction. Based on this result, only such events were used for the further analysis where a proton was detected in the MiniTAPS forward wall.Two reaction channels have been studied to explore bound mesons in nucleus:In the first analysis the reaction channel has been studied in which the slow meson produces a resonance by being absorbed by a nucleon. In this case the produced resonance has to be slow as well and it can decay into a meson-nucleon pair - where the meson is either pi0 or eta - with a relative angle of almost 180 between the decay products.The kinematic constraints of this reaction channel and the possible background channels have been studied by using Monte Carlo simulations. These simulations have shown that reactions with a charged pion in the final state are the prominent background channels because the charged pions are misinterpreted as protons. Since these background channels have a much larger production yield than the bound omega-mesic state, this background might overwhelm the possible signal.In the second analysis the neutral decay channel of the meson was investigated.If an meson decays into a pi0 and a photon and the pi0 decays further into two photons then the mass and energy of the can be reconstructed using the four-vectors of the final-state photons. A bound state is indicated by an extra yield in the kinetic energy histogram at E_{pi0gamma}-m_{omega}c^2 < 0 MeV together with an enhanced yield in the invariant mass histogram at m_{pi0gamma} < m_{omega} (m_{pi0gamma} and E_{pi0gamma} is the reconstructed invariant mass and energy of the three registered photons if the invariant mass of two of them gives the mass of the pi0, the m_{omega} is the mass of the omega in vacuum).Using a simultaneous fit of the two distributions, the presence of a surplus of 83 ± 32 counts has been shown in the lowest momentum bin (p_{pi0gamma} < 300 MeV/c) where the bound states were expected. The same kind of structure has not been observed either in the higher momentum bins or in the simulations where no bound state has been included.Different backgrounds of this reaction channel have also been studied by means of Monte Carlo (MC) simulations. Based on the results of these simulations, kinematical cuts have been determined to reduce these background channels. Applying these cuts in the analysis of the measured data, a change has been found in the yields of the observed structure and of the quasi-free peak.Theories predict a signature of the bound omega-mesic state in the kinetic energy distribution at E_{pi0gamma} - m_{omega}c^2 = -60 MeV. In this experiment no excess yield has been found in this region. An extra intensity has been, however, observed at E_{pi0gamma} - m_{omega} = -100 MeV where contributions from background channels are also expected although an effort has been made to suppress these background channels.Nevertheless, the observed yield is comparable with the theoretical predictions (16 counts). In view of statistical and systematic uncertainties the experimentally registered yield of 83±32 counts is considered as an upper limit. This yield corresponds to a a differential cross section of 4 µb/sr for the incident photon energy range of E = 1250 - 3113 MeVThis thesis has demonstrated that progress in the search for omega-mesic states requires two steps: a reference measurement on a LH2 target to understand and control the background and improved experiment on carbon with higher statistics and a high resolution momentum measurement of the forward going proton. Gebundene Pion-Kern Zustände konnten experimentell durch den Einschluss von geladenen Pionen in verschiedene Kerne wie z.B. Blei nachgewiesen werden. Solche Zustände werden auch tiefgebundene pionische Atome genannt. Sie werden durch ein Zusammenspiel der Coulomb- und der starken Kraft zusammen gehalten. Der Schwerpunkt dieser Arbeit liegt in der Untersuchung von gebundenen -mesischen Zuständen in Kohlenstoff. Im Gegensatz zu den tiefgebundenen pionischen Atomen spielt hier nur die starke Kraft eine Rolle, da die Mesonen keine elektrische Ladung tragen. Das Ziel dieser Untersuchungen ist ein besseres Verständnis der grundlegenden Meson-Kern Wechselwirkung zu erlangen.Das Experiment zur Untersuchung -mesischer Zustände in Kohlenstoff wurde an der Elektronen-Stretcher-Anlage (ELSA) in Bonn im März 2007 durchgeführt. Hierfür wurde ein energiemar-kierter Photonenstrahl mit einer maximalen Energie von E = 2.9 GeV verwendet, der über Bremsstrahlung Prozesse in einem dünnen Radiator erzeugt wurde. Das Detektorsystem bestehend aus den Kalorimetern Crystal Barrel (CB), ForWard Plug (FW-Plug) und Two Arm Photon-Spectrometer (TAPS) deckte nahezu den gesamten Raumwinkel von 4 ab und war damit ideal für den Nachweis der Photonen im Endzustand geignet. Des Weiteren konnten Protonen mithilfe eines Cherenkov Detektors und über Flugzeitmessungen (Time-of-Flight, ToF) identifiziert werden.