Doubly charmed baryon searches at the LHCb experiment

Seman Bobulska, Dana (2022) Doubly charmed baryon searches at the LHCb experiment. PhD thesis, University of Glasgow.

Abstract

The main analysis presented in this thesis is a search for the doubly charmed baryon Ξ⁺cc in the Ξ⁺c π ⁻π ⁺ final state, where the Ξ⁺c baryon is reconstructed in the pK⁻π ⁺ final state. The search is based on the data collected at the LHCb experiment in 2016–2018 at a centre-ofmass energy of 13 TeV, corresponding to about 5.4 fb⁻¹ of integrated luminosity. Since the Ξ⁺cc baryon has previously not been observed and this analysis presents the first search for this baryon in the Ξ⁺c π ⁻π ⁺ final state, the Ξ⁺c π ⁺π ⁻ invariant-mass region from 3.3 to 3.8 GeV/c² was not examined until the full analysis was finalised in order to avoid any bias of the decisions made during the analysis.

No significant signal for the Ξ⁺cc baryon is observed in the invariantmass range from 3.4 to 3.8 GeV/c² , therefore upper limits are set on the ratio of production cross-section times branching fraction with respect to the already observed Ξ⁺⁺cc → (Ξ⁺c → pK⁻π ⁺ )π ⁺ decay, as a function of the assumed Ξ⁺cc mass for different Ξ⁺cc lifetime hypotheses, in the rapidity range from 2.0 to 4.5 and the transverse momentum range from 2.5 to 25 GeV/c. For the Ξ⁺cc lifetime hypothesis of 80 fs, the evaluated upper limit on the ratio of production cross-section times branching fraction with respect to the Ξ⁺⁺cc → (Ξ⁺c → pK⁻π ⁺ )π ⁺ decay varies between 2 and 5 at 95% confidence level in the invariantmass range from 3.4 to 3.8 GeV/c² .

The results are combined with the previously published search for the Ξ⁺cc baryon in the Λ ⁺c K ⁻π ⁺ final state at LHCb. A maximum local significances of 4.0 standard deviations, including systematic uncertainties, is found for the combination of the two decay channels around the mass of 3623 MeV/c². Taking into account the lookelsewhere effect in the invariant-mass range from 3.5 to 3.7 GeV/c² , the combined global significance is 2.9 standard deviations, including systematic uncertainties.

A detector performance study described in this thesis is devoted to hit-resolution corrections of the VELO detector in the simulation, in order to improve the agreement with the corresponding resolutions measured in data. The method uses the hit resolutions in data to generate the hit uncertainties in simulation. A significant improvement in the resolutions of 2 and 3-strip clusters between the corrected simulation and data is achieved. As the method is not applicable on 1-strip clusters, a strategy to correct the 1-strip clusters is developed, in order to improve the agreement for the 1-strip clusters in the future.

Lastly, an inclusive software trigger line is developed for the Ξ⁰c baryon based on the multivariate selection, which was implemented for the 2018 data taking at the LHCb experiment. The developed selection is aimed to be efficient for the Ξ⁰c → pK⁻K ⁻π ⁺ decay, independent of the origin of the Ξ⁰c baryon. The efficiency of the developed trigger line is improved with respect to the corresponding existing cutbased trigger line for the Ξ⁰c baryon, in particular for the Ξ⁺cc → Ξ⁰cπ ⁺ decay, which is important for future searches and studies of the doubly charmed baryon Ξ⁺cc.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Soler, Professor Paul and Eklund, Professor Lars
• Date of Award: 2022
• Depositing User: Theses Team
• Unique ID: glathesis:2022-83205
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 20 Oct 2022 13:27
• Last Modified: 27 Oct 2022 15:40
• Thesis DOI: 10.5525/gla.thesis.83205
• URI: https://theses.gla.ac.uk/id/eprint/83205