Search for Resonant X(3872) Formation in Electron Positron Annihilations and the Development of a Prototype Data Acquisition for the Crystal Zero Degree Detector at BESIII

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KochLeonard_2019_06_03.pdf (31.74 MB)

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2019

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DOI:
http://dx.doi.org/10.22029/jlupub-9806

Abstract

The X(3872) resonance is the most famous member of the family of exotic charmonium-like XYZ states. This class of hadrons lies in the charmonium mass region, but exhibits properties disfavoring their interpretation as conventional charmonium states. Many alternatives like compact tetraquarks or meson molecules are discussed while the inner structure of these states remain unclear. A property that might help to clarify the situation is the electronic width Gamma_ee of the X(3872) state. This quantity is accessible in the measurement of the so far unobserved e^+e^- -> X(3872) reaction.Within the framework of this thesis, the search for this reaction has been performed by analyzing electron positron collision data in the vicinity of the X(3872) mass. The data with an integrated luminosity of (322.3 ± 1.7) pb^-1 spreading over four different collision energies was recorded by the BESIII detector at the BEPCII accelerator in Beijing (China). The X(3872) state has been searched for via the decay of X(3872) -> pi^+pi^-J/psi and the subsequent J/psi -> l^+l^- decay, where l stands for e or mu.The cross section of e^+e^- -> pi^+pi^-J/psi has been extracted from maximum likelihood fits to the J/psi peak in the distributions of the invariant dilepton mass. There is no evidence for a X(3872) signal. An upper limit on the product of the electronic width times the branching fraction Gamma_ee × B(X(3872) -> pi^+pi^-J/psi) has been determined by a Bayesian likelihood analysis to be 7 × 10^-3 eV at 90 % confidence level. This is an improvement of a factor of about 20 upon existing limits.The second topic of this thesis is the development of an FPGA based prototype data acquisition for a small detector to be placed very closely to the beam line in the forward and backward direction of BESIII. This detector is designed to measure photons from initial state radiation which will be of great benefit for precise measurements of various hadron production cross sections in e^+e^- annihilations. The active material will be LYSO, a fast and dense inorganic scintillator, with silicon photomultipliers to collect the scintillation light.At the designated position, the detector will be exposed to an extremely high background rate in the MHz region. The implied design requirements on the data acquisition are realtime capabilities to detect pile-up and to correlate the events with the BESIII trigger.In the context of this thesis, a prototype data acquisition based on an already existing sampling ADC with FPGA support and a sampling frequency of 125 MHz has been developed. An additional FPGA board (already existing, too) acts as a buffer between the ADC and a PC which in turn writes the sampled data to disk. Available solutions for the high speed serial communication between the FPGAs as well as for monitoring/controlling and data transfer to the PC via Gigabit Ethernet have been complemented by custom designs. The prototype data acquisition has not performed event correlation or pile-up detection yet, but it provides means to record continuously sampled waveforms that can be analyzed offline.The prototype data acquisition was tested together with a prototype detector module in an electron beam at the MAMI accelerator in Mainz. The beam had a fixed energy of 855 MeV and a varying rate between 2.7 kHz and 13.9 MHz. Based on the recorded data, feature extraction and pile-up detection algorithms have been developed for the later implementation on the available FPGAs. It has been shown that about 90 % of pile-up events can be detected for rates of up to 6 MHz.

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