Analysis of the decay D0 → K0S K+ K− with the BESIII experiment
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Abstract
The study of mesons and baryons containing one or more charm quark is referred to as open charm physics. Mesons containing one charm and one light quark (u,d,s) are called D mesons. The lightest D mesons, D0 and D±, can only decay via a weak transition c → (d,s). The charm quark is together with the u and t quark a so-called ‘up-type’ quark. Since the u quark is the lightest quark and the t quark is too heavy to form bound states, D decays provide a unique laboratory to study ‘up-type’ quark transitions. The weak transition of bound states is also influenced by strong interaction effects which are important to understand, among others, the oscillation of D0 and D0bar. The precise measurement of branching fractions can help to improve the theoretical understanding in this field. Furthermore, D mesons are expected to decay via subsequent two-body decays, and in decays with three or more final state particles intermediate resonances can be studied. Due to the phase space limitations of D decays so-called ‘light mesons’ can be observed.
The Beijing Electron-Positron collider II (BEPCII) is located at the Institute for High Energy Physics (IHEP) in Beijing. It provides collisions in the energy range from 2 GeV to 4.6 GeV. At an energy of 3.773 GeV the ψ(3770) resonance is produced which predominantly decays to DDbar. The BESIII detector has collected a data sample of 2931.8 pb^−1 at this energy which corresponds to approximately 10.7 × 10^6 D0D0bar and 8.5 × 10^6 D+D- decays. Each event contains two recoiling D decays in a almost background free environment. D0D0bar pair’s are furthermore produced in a quantum entangled state. This provides the possibility to ‘tag’ properties of one D decay from the reconstruction of the opposite D. In this work the favour of the signal decay is obtained in this way.
The topic of this thesis is the study of the D0 → K_S K+ K− Dalitz plot using a favour tagged data sample. Using an untagged event sample, the branching fraction is measured.
The Dalitz plot analysis is performed using 1856 ± 45 flavour tagged signal events with a purity of 96.4 %. We find that the Dalitz plot is well described by a set of 4 resonances: a0(980)0, φ(1020), a0(980)+ and a2(1320)−. We determine their magnitudes, phases and fit fractions. Furthermore we measure the a0(980) coupling to KK to be gKK =(2.88 ± 0.25 (stat.) ± 0.56 (sys.))GeV. The branching fraction is measured with 11 384 ± 115 untagged signal decays and we obtain a value of (4.45 ± 0.05 (stat.) ± 0.18 (sys.))×10^−3. The measurement yields a relative precision of 4.21 % which improves the current best measurement significantly. Both measurements are limited by the systematic uncertainty. As part of this work the reconstruction of displaced vertices is studied using J/ψ → ΛΛ decays. A comparison of data and simulation yields no significant differences.