The GERDA experiment is searching for the neutrinoless double beta (0vbb) decay of Ge-76.
By that, it tries to answer two long standing questions about the neutrino: 'How large is the neutrino mass?' and 'Is the neutrino either Dirac or Majorana particle?'.
Additionally, an observation would imply that lepton number is not conserved, which is an important puzzle piece for theories explaining the asymmetry between matter and anti-matter in the universe.
The effective Majorana electron neutrino mass can be extracted from the half-life of the 0vbb -decay.
However, during that conversion large uncertainties are added through nuclear matrix elements, that are calculated by a variety of theoretical models.
Experimental input is required to constrain such models and their parameters to improve the reliability of the calculations.
Additional input can be obtained by comparing the model predictions for the two neutrino double beta (2vbb) decay to the ground state, but also for decay modes to excited states of the daughter nuclide with measurements.
The latter decay modes have not yet been observed in the case of Ge-76.
The event signature of transitions to excited states is enhanced by de-excitation gamma-rays.
The GERDA experiment employs an array of bare germanium semi-conductor detectors in a liquid argon cryostat.
This array is suited to search for excited state transition in the 2vbb and 0vbb -decay modes using data with coincident energy depositions in multiple detectors.
This work presents the preparation and characterisation of this data set, which includes the evaluation and correction of crosstalk between detector channels, the determination of the energy resolution of the detectors and the modelling of background.
In an analysis combining 22 kgyr of Phase I data with the first 35 kgyr of Phase II data of GERDA, no signal has been observed for 2/0vbb -decays of Ge-76 to the energetically lowest three excited states in Se-76.
New limits have been set for the two neutrino decay modes at
T1/2(2v)(0+g.s. to 0+1) > 3.1x10^23 yr,
T1/2(2v)(0+g.s. to 2+1) > 3.4x10^23 yr
T1/2(2v)(0+g.s. to 2+2) > 2.5x10^23 yr
with 90% credibility using a Bayesian approach, improving upon the limits obtained in Phase I. The corresponding sensitivities are 3.6x10^23 yr, 6.7x10^23 yr and 3.7x10^23 yr, respectively. First limits are set for the neutrinoless decay modes in the order of (10^24-10^25) yr. Reaching the desired Phase II exposure of 100 kgyr, the sensitivities will increase by up to 50%.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:38032 |
| Date | 29 January 2020 |
| Creators | Wester, Thomas |
| Contributors | Zuber, Kai, Caldwell, Allen, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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