Študija meritve radiativnih razpadov mezonov B z detektorjem Belle II

Cucek, Jernej (author), Golob, Boštjan (mentor), Šantelj, Luka (comentor)

V magistrskem delu na simuliranem vzorcu podatkov detektorja Belle II opravimo študijo meritve razvejitvenih razmerij redkih radiativnih razpadov $B^{\scriptscriptstyle 0}\rightarrow \rho^{\scriptscriptstyle 0}\gamma$ in $B^{\scriptscriptstyle0}\rightarrow K^{\scriptscriptstyle *0}\gamma$ ter od tu določimo razmerje elementov matrike CKM $|V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts}|$. Vzorec velikosti $375$ fb$^{-1}$ vsebuje simulirane podatke trkov elektronov in pozitronov pri veliki luminoznosti nadgrajenega trkalnika eksperimenta Belle II. Eksperiment bo tekom delovanja v naslednjih letih zbral vzorec velikosti $50$ ab$^{-1}$, kar pomeni 50-krat več razpadov mezonov $B$, kot jih je trenutno na voljo. Iz simuliranih podatkov najprej ekskluzivno rekonstruiramo oba izbrana radiativna razpada. Nato iz vzorca z uporabo pionskega veta in metod strojnega učenja izločimo čim več dogodkov ozadja. Iz kinematičnih porazdelitev določimo signalni donos obeh razpadov ter ocenimo učinkovitosti rekonstrukcij. Izkoristki rekonstrukcije imajo vrednosti $\epsilon_{\scriptscriptstyle \rho^{\scriptscriptstyle 0}}=9\%$ in $\epsilon_{\scriptscriptstyle K^{\scriptscriptstyle \ast 0}}=22.7\%$, ki so v primerjavi z dosedanjimi študijami na podatkih iz merjenega vzorca detektorja Belle višji. Iz signalnih donosov izračunane vrednosti razvejitvenih razmerij imajo vrednosti $\mathcal{B}(B^{\scriptscriptstyle 0}\longrightarrow\rho^{\scriptscriptstyle 0} \gamma)=(12\pm3)\cdot10^{-7}$ in $\mathcal{B}(B^{\scriptscriptstyle 0}\longrightarrow K^{\scriptscriptstyle \ast0}\gamma)=(4.19\pm0.08)\cdot10^{-5}$. Rezultati se v okviru statistične negotovosti ujemajo s teoretičnimi napovedmi in s svetovnim povprečjem; za razpadni način $\rho^{\scriptscriptstyle 0}$ je relativna statistična negotovost primerljiva z rezultati študij izvedenih na meritvah detektorja Belle in znaša 26\%, medtem ko je za razpadni način $K^{\scriptscriptstyle \ast 0}$ izboljšana in znaša 1.9\%. Vrednost izračunanega razmerja elementov matrike CKM znaša $\left|V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts}\right|= 0.268\pm 0.041$. Rezultati imajo glede na aktualno svetovno povprečje večjo statistično negotovost, h kateri prispevajo slabši rezultati razpadnega načina $\rho^{\scriptscriptstyle 0}$, vendar na podlagi naše študije ocenjujemo, da bo statistična negotovost na vzorcu, ki ustreza velikosti $50$ ab$^{-1}$, izboljšana za velikostni red. Negotovost razmerja $\left|V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts}\right|$ bo tako končno omejena predvsem z negotovostmi nekaterih parametrov potrebnih za izračun $\left|V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts}\right|$, ki so določeni teoretično ali iz drugih meritev.

• Language: Slovenian
• Keywords: SuperKEKB, Belle II, Standardni Model, Nova Fizika, matrika Cabibbo–Kobayashi–Maskawa, simulacija Monte Carlo, radiativni pingvinski razpadi, ekskluzivna rekonstrukcija, mezoni s kvarkom $b$
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2020
• PID: 20.500.12556/RUL-124058
• COBISS.SI-ID: 43467267
• Publication date in RUL: 23.12.2020

CUCEK, Jernej, 2020, Študija meritve radiativnih razpadov mezonov B z detektorjem Belle II [online]. Master’s thesis. [Accessed 30 April 2025]. Retrieved from: https://repozitorij.uni-lj.si/IzpisGradiva.php?lang=eng&id=124058

Secondary language

• Language: English
• Title: Study of measurement of B meson radiative decays with the Belle II detector

Abstract

Using a simulated data sample of the Belle II detector measurements with an integrated luminosity of $375$ fb$^{-1}$ we performed a study of measurement of branching ratios for two rare $B$ meson radiative decays $B^{\scriptscriptstyle 0}\rightarrow \rho^{\scriptscriptstyle 0}\gamma$ and $B^{\scriptscriptstyle 0}\rightarrow K^{\scriptscriptstyle*0}\gamma$. The obtained branching ratios were then used to determine the value of $\lvert V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts} \rvert$ ratio of the Cabibbo–Kobayashi–Maskawa matrix elements. The sample contained simulated data of electron-positron collisions from an upgraded high luminosity Belle II experiment, which will in its future operation period accumulate $50$ ab$^{-1}$ of data. From the data sample, we first performed na exclusive reconstruction of events for both decay chains. We proceeded with the elimination of background events with the usage of pion veto and machine learning techniques. We then obtained signal yields from fits to kinematic distributions of events and determined the reconstruction efficiencies, which were found to be $\epsilon_{\scriptscriptstyle \rho^{\scriptscriptstyle 0}}=9\%$ and $\epsilon_{\scriptscriptstyle K^{\scriptscriptstyle \ast 0}}=22.7\%$. In comparison to the results from older Belle studies, our reconstruction efficiencies are higher. The branching ratios determined from the signal yields have values of $\mathcal{B}(B^{\scriptscriptstyle 0}\longrightarrow\rho^{\scriptscriptstyle 0} \gamma)=(12\pm3)\cdot10^{-7}$ and $\mathcal{B}(B^{\scriptscriptstyle 0}\longrightarrow K^{\scriptscriptstyle \ast0}\gamma)=(4.19\pm0.08)\cdot10^{-5}$, and are within the statistical uncertainty in agreement with theoretical predictions and world average values; in our case the relative statistical uncertainty for the $\rho^{\scriptscriptstyle 0}$ decay mode has a value of 26\% and is comparable with results from older studies with Belle data, while the uncertainty of the $K^{\scriptscriptstyle *0}$ mode branching ratio has a value of 1.9\% and is improved in same comparison. Finally, we calculated the value of ratio CKM elements $\left|V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts}\right|= 0.268\pm 0.041$ which is also in agreement with the world average values, albeit statistical uncertainty is larger descending from results obtained from $\rho^{\scriptscriptstyle 0}$ mode. Nevertheless, we estimate that the statistical uncertainty will be improved by an order of magnitude when the sample of size $50$ ab$^{-1}$ will be used. We conclude, that in the future, the uncertainty of the ratio $\left|V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts}\right|$ will be dominated by uncertainties of some of the parameters used to calculate the value of $\left|V_{\scriptscriptstyle td}/V_{\scriptscriptstyle ts}\right|$ that are calculated theoretically or determined from other measurements.

• Keywords: SuperKEKB, Belle II, Standard Model, New Physics, Cabibbo–Kobayashi–Maskawa matrix, Monte Carlo simulation, radiative penguin decays, exclusive reconstruction, bottom mesons