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Angular analysis of the decays ?b -> ?* l+l- with the LHCb detector

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General information

Reference : UMR9012-SLADRA1-004
Workplace : ORSAY
Date of publication : Thursday, July 23, 2020
Scientific Responsible name : Yasmine Amhis
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2020
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

The LHCb experiment is one of the four detectors of the LHC at CERN in Geneva. The main purpose of LHCb is to search for new physics in an indirect way using the decays of beauty and charm hadrons. A few anomalies have been observed by the LHCb expriment in rare decays of the type b -> sl+l- [arXiv :1512.04442]. These decays offer a very powerful way to seach for New Physics since particles beyond the Standard Model can contribute to these loop diagrams. Two studies using B+ [arXiv:1406.6482] and B0 [arXiv:1705.05802] mesons have shown that Lepton Universality might differ from the Standard Model prediction. The current sensitivity of these measurements does not allow to conclude for the presence of New Physics.

The first test of lepton universality using b-baryon decays désintegrations Λb -> Λ* l+l– (l = or m) is expected to be publish in Decembre 2019. The LAL group is playing a leading role of this analaysis. An untermediate result of this publication which is based on part of the data collected by LHCb is the first observation of the rare mode Λb -> Λ* (-> pK) e+e–.
Recent theoretical studies on Λb -> Λ*l+l– decays, arXiv :1908.00448, predict for the first time the expected behaviour of angular observavbles in the Standard Model as well as their sensitivity to New Physics.
The topic of the thesis is to peform an angular analysis of the decays Λb -> Λ* l+l–. After designing and optimizing a selction of interesting candidates, the student will build a fitting program which will allow to extract the physics obvservables of interest, taking into account the effect of angular acceptances, and evaluating the impact of backgrounds.
The student will have access to the legacy RUN1 [2011-2012] and RUN 2 [2015-2017] LHCb data.

In parallel of the physics analysis the student will be involved in the Upgrade of the calorimeter. Indeed, an upgrade of the LHCb detector is foreseen for the start of data taking in 2021. Many aspects of this Upgrade will impact the electron and photon reconstruction. Two subdetectors of the current calorimeter (Scintillating Pad Detector and PreShower) located in front of the electromagnetic calorimeter (ECAL) will be removed. Moreover, the LHCb detector will be running at a higher instantaneous luminosity causing a significant increase of the event multiplicity which will affect the track-cluster association in the ECAL. This association is crucial for the separation of neutral (photons) and charged (electrons) electromagnetic objects. In the context of the thesis the student will participate to the improvement of the identification of electromagnetic objects and to the commissioning of the new detector in 2020.
The thesis will be hosted in the LHCb group of LAL. The team is involved in both the design and running of the detector (electronics, trigger) and in physics analyses. The group is composed of about 10 physicists (permanent researches, postdocs and PhD students). During the thesis, the student will have the opportunity to go regularly to CERN to participate to physics analysis meetings and tot he detector commissioning. The PhD thesis will be supervised by Yasmine Amhis. They will be working in close collaboration with MH Schune (CNRS) and Carla Marin Benito (Postdoc) in a group focusing on b sl+l- decays

Work Context

Located on the campus of the Faculty of Sciences of Orsay, Irène Joliot-Curie Lab - IJCLab, a joint research unit of the CNRS, the University of Paris-Saclay and the University of Paris which includes around 750 people, is structured around fundamental physics research poles, from nuclear physics to astrophysics, cosmology, high energy physics, accelerator physics and their applications (health, energy).
The student will work in the LHCb group of the Laboratory which is involved both in the design of the detector (calorimeter electronics, first level triggering, etc.) and in the physics analyzes. The group includes around ten physicists (permanent researchers, post-docs and doctoral students). This thesis involves a regular presence at CERN as part of the LHCb collaboration

Constraints and risks

Shifts at CERN.

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