CERN Accelerating science

Report number CERN-THESIS-2020-004
Title Search for radiative B decays to orbitally excited mesons at LHCb
Author(s) Quintana, Boris Julien (Centre National de la Recherche Scientifique (FR))
Publication 173 p.
Thesis note PhD : Université Clermont Auvergne : 2020-02-11
Thesis supervisor(s) Deschamps, Olivier ; Tisserand, Vincent
Note Presented 25 Nov 2019
Subject category Particle Physics - Experiment
Accelerator/Facility, Experiment CERN LHC ; LHCb
Abstract This document presents studies of radiative decays of $B^0$ and $B_s$ mesons to orbitally excited states, produced in proton collisions at the Large Hadron Collider (LHC) of CERN (European Council for Nuclear Research). It takes place in the context of the search for new physics processes beyond the standard model of particle physics. The work is based on the data collected at the LHCb experiment during the Run 1(2011-2012) and the Run 2 (2015-2018) of the LHC, which corresponds to an integrated luminosity of about 8.7 $fb^{-1}$. First, an optimised selection of the final states comprising of two charged tracks and a high energy photon is put in place, regrouping a large samples of B hadron radiative decays. Then, a mass model is fit to the data distribution, allowing to statistically subtract the background contributions in the data set. A first amplitude analysis is then developed to study the hadronic structure in the $B_s\rightarrow K^{+}K^{-}\gamma$ decay mode. Resulting from this analysis is the first observation of the $B_s\rightarrow f'_2\gamma$ mode together with a relative branching ratio measurement: \begin{equation} \frac{\mathcal B(B_s\rightarrow f'_2\gamma)}{\mathcal B(B_s\rightarrow\phi\gamma)}= (24.1 \pm {0.9}~\text { (stat) }^{+1.6}_{-0.8} \pm {0.6} ~\text {(BR))} \times 10^{-2} \nonumber \end{equation} It is only the second radiative decay mode observed for the $B_s$ meson. A similar analysis is performed on the $B^0\rightarrow K^{\pm}\pi^{\mp}\gamma$ mode, allowing a preliminary measurement of the relative branching ratio of $B^0\rightarrow K^{*0}_2(1430)\gamma$: \begin{equation} \nonumber \frac{\mathcal B(B^0 \to K^{*0}_2(1430)\gamma)}{\mathcal B(B^0 \rightarrow K^{*0}(892)\gamma)}=({36.4} \pm {0.4}~\text{(stat)} \pm {1.3}\mathrm{(BR)})\% \end{equation} However, a detailed systematics study is needed to assess its competitivity with previous measurements of this decay mode. Making use of a similar selection strategy, a preliminary study of $B\rightarrow K^{0}_{S}hh\gamma$ modes is also set up during the thesis to estimate the available statistics for these modes at LHCb, which are particularly interesting to measure the photon polarisation in the $b\to s\gamma$ transition. This work will be the starting point of an analysis comprising the future data that will be collected during the Run 3 of the LHC.

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 Record created 2020-02-17, last modified 2020-03-02

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