CERN Accelerating science

Thesis
Report number	CERN-THESIS-2023-379
Title	Exploring the magnitude and phase of $b \to u$ quark transitions through open-charmed beauty decays
Related title	$b \to u$
Author(s)	Butter, Jordy (Cambridge U.)
Publication	2023 - 182.
Thesis note	PhD : Vrije U., Amsterdam : 2023-06-13
Thesis supervisor(s)	Tuning, Niels ; Merk, Marcel
Note	Presented 13 Jun 2023
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; LHCb
Abstract	Scientists worldwide have dedicated themselves to understanding the fundamental nature of the universe, leading to the development of the Standard Model. This model explains the building blocks of matter and their interactions. Atoms, composed of electrons and a nucleus containing quarks, are the basic units of matter. The Standard Model includes six types of quarks, with stable matter consisting of up and down quarks. The Standard Model allows for transitions between quark flavours through the weak interaction, but the probabilities of these transitions vary. One of the least probable transitions is the beauty-to-up quark transition ($b \to u$), which is 70,000 times less likely than a down-to-up quark transition ($d \to u$). The reasons behind the presence of only six quarks in the Standard Model and the varying probabilities of quark transitions remain unknown. The quark sector of the Standard Model also explains a phenomenon called $C\!P$ violation, which refers to the imbalance between matter and antimatter in the universe. Colliders produce equal amounts of matter and antimatter, but our observable universe consists mostly of matter. This implies a slight excess of matter over antimatter during the Big Bang, leading to the formation of our current universe. However, the observed $C\!P$ violation in the Standard Model is insufficient to account for this matter-antimatter asymmetry, posing one of the fundamental questions of our time. This thesis aims to measure the magnitude of the $b\to u$ quark transition ($V_{ub}$) and its complex phase $\gamma$, a key parameter describing $C\!P$ violation in the Standard Model. The research is conducted using the LHCb experiment at CERN's Large Hadron Collider. The study focuses on $b$ hadrons ($B^0$, $B_s^0$, and $\Lambda_b^0$) that decay into a $D_s^-$ meson and a charged pion, kaon, or proton. To quantify the branching fraction of $B^0 \to D_s^+ \pi^-$ and $\Lambda_b^0 \to D_s^- p$ decays (proportional to $V_{ub}$), the number of signal candidates is determined through reconstructed mass distributions using likelihood fits. Background contributions and selection requirements are considered, such as particle identification and classifiers to distinguish signal candidates from random combinations of final state tracks. The study finds that 1 in 52,000 $B^0$ mesons and 1 in 79,000 $\Lambda_b^0$ baryons decay into the respective final states. These decays, occurring through the $b\to u$ transition, are of interest due to their low probability and their potential for studying $V_{ub}$. Comparing yields of $B^0\to D^-\pi^+$ and $B_s^0\to D_s^-\pi^+$ decays provides information on the relative production of $B_s^0$ and $B^0$ mesons. This knowledge is crucial for branching fraction measurements with $B_s^0$ mesons. The similarity between $B^0 \to D_s^+ \pi^-$ and Cabibbo-suppressed $B^0 \to D^+ \pi^-$ decays allows for the calculation of a parameter essential for determining $C\!P$ violation in $B^0 \to D^\mp \pi^\pm$ decays. Finally, the $C\!P$ asymmetry in $B_s^0 \to D_s^\mp K^\pm$ decays is determined to obtain the complex phase $\gamma$. Decay-time-dependent asymmetries arise due to the oscillation and interference of $B_s^0$ and $\bar{B}_s^0$ mesons. By simultaneously fitting mass and decay-time distributions, accounting for detector effects using $B_s^0\to D_s^-\pi^+$ decays, the $C\!P$ parameters are measured. The results are currently blinded, and pending review. However, the analysis shows a four-fold increase in signal candidates compared to previous studies, leading to an expected reduction in the statistical uncertainty of $\gamma$ by a factor of two. The anticipation for a more precise determination of $\gamma$ using $B_s^0 \to D_s^\mp K^\pm$ decays is high, as it could help resolve discrepancies between the previous measurement using $B_s^0 \to D_s^\mp K^\pm$ decays and other determinations of $\gamma$.
ISBN	9789464198089
DOI	10.5463/thesis.256
Copyright/License	CC BY-ND 4.0

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