Investigation of the Resonance-like <em>a<sub>1</sub></em>(1420) Signal and First Partial-Wave Decomposition of the <em>π<sup> -</sup>K<sub>S</sub><sup>0</sup>K<sub>S</sub><sup>0</sup></em> Final State at COMPASS
Creators: name: Wagner, Mathias
Publisher: name: Universitäts- und Landesbibliothek Bonn
Year: 2024
Subjects: subject: COMPASS
subject: a1(1420)
subject: pi1(1600)
subject: pi(1800)
subject: exotisch
subject: Meson
subject: Spektroskopie
subject: Dreiecksdiagramm
subject: Dreiecksamplitude
subject: Singularität
subject: Kshort
subject: Kaon
subject: diffractive Streuung
subject: Partialwellenprojektion
subject: komplexe Analysis
subject: Ereignisselektion
subject: Partialwellenanalyse
subject: Partialwellenzerlegung
subject: Resonanzmodell
subject: exotic
subject: spectroscopy
subject: rescattering
subject: triangle diagram
subject: triangle amplitude
subject: singularity
subject: diffractive scattering
subject: partial-wave projection
subject: complex analysis
subject: event selection
subject: partial-wave analysis
subject: partial-wave decomposition
subject: resonance model
subject: ddc:530

description: In 2014, a supernumerous light-meson state was found by the COMPASS collaboration. This discovery bore similarities to some of the <em>XYZ</em> states in the heavy-meson sector, discovered at <em>e<sup>+</sup>e<sup> -</sup></em> colliders, that did not fit the expectations from the quark model and hence were called exotic states. The partial-wave decomposition of the COMPASS data for the <em>π<sup> -</sup>π<sup>+</sup>π<sup> -</sup></em> final state, which is employing an isobar model, revealed a narrow signal with axial-vector quantum numbers in the <em>f<sub>0</sub></em>(980)<em>π</em> P-wave. Since it was found at an energy of 1.4 GeV by a resonance-model fit with a relativistic Breit-Wigner model, it was called <em>a<sub>1</sub></em>(1420). This thesis investigates the nature of the signal, following two avenues.<br> In the first part, it is shown how a rescattering of final-state particles can produce such a signal by explicitly performing a projection of a <em>K<sup>*</sup>K</em> intermediate state to the <em>f<sub>0</sub></em>(980)<em>π</em> final state. The resulting theoretical model is fitted to the COMPASS data and the quality is compared to a fit using a conventional relativistic Breit-Wigner model. Several systematic studies are performed. This includes a fit to each of the systematic studies of the underlying partial-wave decomposition, as well as a bootstrap analysis and modifications of the theoretical model. In all cases except one, the rescattering interpretation shows a better compatibility with the data.<br> In the second part, an event selection of the complementary <em>π<sup> -</sup>K<sub>S</sub><sup>0</sup>K<sub>S</sub><sup>0</sup></em> final state is performed, where the optimal vertex separation for the <em>K<sub>S</sub><sup>0</sup></em> mesons is determined by means of a significance study to reduce background coming from a possible 5<em>π</em> final state. A total of ~240000 exclusive events are selected, superseding the statistics of previous experiments of diffractive <em>π<sup> -</sup>K<sub>S</sub><sup>0</sup>K<sub>S</sub><sup>0</sup></em> production by a factor of ~240. Based on the selected data set, a first partial-wave decomposition is performed, revealing strong evidence for the <em>a<sub>1</sub></em>(1420) signal in the <em>f<sub>0</sub></em>(980)<em>π</em> P-wave of the <em>π<sup> -</sup>K<sub>S</sub><sup>0</sup>K<sub>S</sub><sup>0</sup></em> final state in form of an enhancement in the intensity and a phase motion in the relative phase to other waves, even for both resonance-spin projections <em>M=0</em> and <em>M=1</em>. At the same time, no resonance-like signal is visible at ~1.4 GeV in the <em>K<sup>*</sup></em>(892)<em>K<sub>S</sub><sup>0</sup></em> S-wave, which agrees well with the interpretation of a rescattering effect. The spin-exotic <em>π<sub>1</sub></em>(1600), a candidate for the lightest hybrid meson, manifests itself as a broad peak in the <em>K<sup>*</sup></em>(892)<em>K<sub>S</sub><sup>0</sup></em> P-wave, however, no significant evidence for a relative phase motion with other partial waves is observed. Another hybrid candidate with ordinary quantum numbers, the pseudoscalar <em>π</em>(1800), shows up as a clean narrow peak in the <em>f<sub>0</sub></em>(980)<em>π</em> S-wave. It is accompanied by a narrow resonance-like signal at 1.4 GeV, i.e. an enhancement in the intensity together with phase motion in the relative phase to other partial waves. This could be the first observation of a "<em>π</em>(1420)" rescattering effect, similar to the <em>a<sub>1</sub></em>(1420), but with pseudoscalar resonance quantum numbers.