Browsing by Organization "Center for Particle Physics"

In the Standard Model of particle physics, the strong dynamics of hadronic particles such as the B meson are governed by Quantum Chromodynamics (QCD). Currently, there exists no single method capable of accurately predicting all phenomena associated with QCD. Rather, various specialized methods have emerged to address specific phenomena. Among these approaches, QCD Factorization and QCD Light-Cone Sum Rules are used for exclusive, energetic B decays. Therein, the B-meson Light-Cone Distribution Amplitudes (LCDAs) systematically represent the internal structure of the B meson, and as such, the LCDAs are (presently) inaccessible from first principles. In this thesis, we develop a new systematic framework for phenomenological analyses, focusing on the leading-twist B-meson LCDA which leads to the dominant contribution in predictions. Our framework allows for the integration of various theoretical and experimental constraints to infer the LCDA and derive predictions. While models have been used for the same purpose, our approach makes systematic uncertainties quantifiable and provides greater transparency in the implementation of constraints. We derive a new systematic parametrization of the leading-twist B-meson LCDA that fulfills established mathematical properties and satisfies a parameter bound to address the issue of truncation in the expansion. We discuss certain practical aspects, such as renormalization group evolution, the implementation of the known short-distance behavior as a fit constraint, and more. In addition, we update the known short-distance behavior with the effect of a non-zero spectator quark mass to improve the effectiveness of our approach for applications with the Bs meson. For this purpose, we perform a generic matching calculation, which further yields a new result for the short-distance behavior of the subleading 2-particle LCDA. To demonstrate the practical utility of our framework, we perform a detailed analysis of the decay mode B meson to photon, lepton, and neutrino, which serves as a benchmark for probing the leading-twist B-meson LCDA. To that end, we extended the EOS software with experimental observables of this decay and pseudo-observables to accommodate the theoretical short-distance constraint. This enabled a proof-of-concept study using mock data and the Bayesian analysis tools in EOS which underscores the utility of the measurement of this decay mode and further demonstrates the effectiveness of the parameter bound in managing truncation errors. This analysis highlights the significant potential of our approach for future research.

A first measurement of the top quark pair charge asymmetry ($A_{C}$) is presented in top quark pair (tt̄) production in association with a photon (tt̄γ production), using the data of ATLAS experiment with an integrated luminosity of 139 $fb^{-1}$ collected at proton-proton collisions of √s=13 TeV at the Large Hadron Collider at CERN. The measurement is performed using the semileptonic tt̄ decay channel in fiducial phase space at particle level using a profile likelihood unfolding method on the distribution of the difference between the absolute rapidity of the two top quarks. The $A_{C}$ is measured to be -0.003 +- 0.029, in agreement with the Standard Model prediction. The measurement is expected to be more sensitive to potential new physics than the same measurement in tt̄ production. However, currently, it is limited by the available statistics and serves as a stepping stone for future analyses with more data.

Searching for new physics not described by the Standard Model (SM) is of great importance to enhance our understanding the universe. One possibility to discover new physics is to perform precision tests of the SM by comparing experimental measurements with theoretical predictions. Taking advantage of the vast set of experimental results, we first perform global fits to test and constrain a class of extensions to the SM referred to as Two-Higgs-Doublet models (2HDMs). In addition, we discuss a motivation for collider searches for the rare process Bs → e+e− which can be greatly enhanced in a 2HDM without conflicting with other data. Large uncertainties from non-perturbative hadronic observables limit the ability to constrain the parameter space of 2HDMs or other new physics scenarios. Next we focus at improving predictions for hadronic observables by non-perturbatively calculating matrix elements of four-quark operators for neutral meson mixing and heavy meson lifetimes using lattice quantum chromodynamics (QCD). In particular, we perform a pilot study establishing a non-perturbative renormalisation procedure via the gradient flow and its short-flow-time expansion allowing us to quote final results in the MS scheme. Considering for simplicity a neutral charm-strange (Ds) meson on the lattice, we determine the O1 operator for neutral meson mixing as well as the O1 and T1 operators describing heavy meson lifetimes. Our short-distance contribution to mixing for a neutral Ds meson is BMS s (3 GeV) = 0.787(5) where the quoted error accounts for statistical and perturbative truncation uncertainties only. This value is in good agreement with literature results for short-distance D0 mixing where only the spectator quark differs. In addition we pioneer the calculation of the four-quark operators O1 and T1 for heavy meson lifetimes in lattice QCD. Essential for this determination is that our gradient flow procedure suppresses the mixing with operators of lower mass dimension on the lattice. Although our lattice calculation does not include all diagrams and also the perturbative matching to the MS scheme is still incomplete, we observe that our values BMS 1 = 1.110(2) and ϵMS 1 = 0.119(1) have the same order of magnitude as corresponding determinations based on HQET sum rules.

Das Verzweigungsverhältnis des inklusiven Zerfalls B̄ → Xs γ ist seit mehreren Jahrzehnten ein sehr aktives Feld der Teilchenphysik und funktioniert durch hohe theoretische und experimentelle Genauigkeit als eine Standardkerze für das Stan- dardmodell der Teilchenphysik (SM). Durch das hohe Interesse an einer genauen theoretischen Vorhersage liegen heutzutage eine fast vollständige Berechnung auf nächst-zu-führender Ordnung und eine hohe Anzahl an Beiträgen auf nächst-zu- nächst-zu-führender Ordnung vor. Die stetig steigende Präzision von Messungen an Beschleunigern ruft auch nach einer steigenden Präzision der theoretischen Vorher- sagen. In der vorliegenden Arbeit berechnen wir die verbleibenden Beiträge von Vier- Körper-Zerfällen eines b-Quark in ein s-Quark, ein Photon γ und zwei zusätzliche Quarks qq̄ zum Verzweigungsverhältnis in nächst-zu-führender Ordnung in der star- ken Kopplung αs. Dieser Ein-Schleifen-Prozess b → sγqq̄, der ein virtuelles Gluon g beinhaltet, muss durch den entsprechenden reellen Abstrahlungsprozess b → sγqq̄g ergänzt werden, um durch das Gluon hervorgerufenene Infrarot-Divergenzen zu eli- minieren. Ein Fokus dieser Arbeit liegt auf der Berechnung der auftretenden Vier- und Fünf- Teilchen Phasenraum-Integrale und den dafür benötigten Rechentechniken. Diese beinhalten aktuelle Methoden der Reduktion durch partielle Integration (IBP) mit denen wir jeweils unsere Basis an Masterintegralen identifiziert haben. Zudem be- schreiben wir die Lösung dieser Masterintegrale mithilfe von Differentialgleichungen und die verschiedenen Arten, auf die wir die hierfür benötigten Randbedingungen bestimmt haben. Weiterhin legen wir unsere Methodik für Renormierung und Regularisierung dar. Da wir die Quarks im Endzustand als masselos behandeln, führt unsere Nutzung von dimensionaler Regularisierung zu verbleibenden kollinearen Divergenzen. Für diese verbleibenden, divergenten Ausdrücke wechseln wir das Regularisierungssche- ma mittels Splitting-Funktionen und tauschen die verbleibenden Pole gegen einen natürlicheren Regulator, d.h. Logarithmen der Quark-Massen, ein. Dabei geben wir einen Überblick über die momentan noch laufende Berechnung des letzten Teils der Splitting-Funktion auf nächst-zu-führender Ordnung, der für eine Vervollständigung der perturbativen Beiträge auf O(αs ) in Zukunft benötigt wird. Unter Nutzung dieser Techniken geben wir hier analytische Ergebnisse für unse- ren Beitrag zur Zerfallsbreite von B̄ → Xsγ als einen der letzten fehlenden Teile für einen vollständiges perturbatives Resultat für das NLO Verzweigungsverhältnis. Un- sere finalen Ausdrücke werden online im Mathematica-Format öffentlich zugänglich gemacht. Als letztes diskutieren wir noch die nächsten Schritte, die nötig sind um eine numerische Vorhersage über das Verzweigungsverhältnis machen zu können.

In extensive air showers induced by ultra-high-energy (UHE) cosmic rays, secondary particles are produced with energies far above those accessible by other means. These extreme energies can be used to search for new physics beyond the current Standard Model of particle physics. In this work, the effects of isotropic, nonbirefringent Lorentz violation in the photon sector are investigated. In the case of a photon velocity smaller than the maximum attainable velocity of standard Dirac fermions, vacuum Cherenkov radiation becomes possible. For photon velocities greater than the maximum attainable velocity of standard Dirac fermions, photon decay is introduced together with a change in the decay time of neutral pions. Implementing these Lorentz-violating effects in air-shower simulations, a significant reduction of the average atmospheric depth of the shower maximum ⟨Xmax⟩ is obtained in both cases. Based on measurements of ⟨Xmax⟩ and its shower-to-shower fluctuations σ(Xmax), a new two-sided bound on Lorentz violation exploiting the high energies of particles in air showers is presented. The impact of Lorentz violation on the number of muons in air showers is also discussed.