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B Meson Decays and Lepton Flavour Universality Violation

English title B Meson Decays and Lepton Flavour Universality Violation
Applicant Crivellin Andreas
Number 176884
Funding scheme SNSF Professorships
Research institution Physik-Institut Universität Zürich
Institution of higher education University of Zurich - ZH
Main discipline Theoretical Physics
Start/End 01.01.2019 - 31.12.2022
Approved amount 1'516'179.00
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All Disciplines (2)

Theoretical Physics
Particle Physics

Keywords (9)

Lepton flavour violation; Lepton Flavour Universality Violation; Higher dimensional operators; Physics beyond the Standard Model; Loop calculations; B meson decays; Flavour; Dark Matter; Effective Field Theories

Lay Summary (German)

Die fundamentalen Bausteine der Materie (Fermionen) kommen in drei Generationen genannt "Flavour" vor. Teilchen unterschiedlichen Flavours unterscheiden sich nur durch ihre Masse, ansonsten haben sie identische Eigenschaften. Z.B. das Muon welches in der kosmischen Strahlung vorkommt, ist die schwere Version des Elektrons. Die Flavourphysik untersucht die Übergänge zwischen verschiedenen Generationen. Dazu werden die Vorhersagen des Standardmodells der Teilchenphysik mit den experimentellen Messungen verglichen. Eventuelle Differenzen deuten dabei auf neue Physik, also auf neue, noch unentdeckte Teilchen hin.
Lay summary

Ziel und Inhalt des Forschungsprojekts

Das übergeordnete Ziel des Forschungsprojekts ist ein detaillierteres Verständnis des Einflusses neuer Physik in Flavourobservablen sowohl im Lepton- als auch im Quarksektor. Das Forschungsprojekt beschäftigt sich hierbei besonders mit den Hinweisen auf neue Physik welche in den letzten Jahren in B Meson Zerfallen gefunden wurden. Zum einen werden Modelle neuer Physik konstruiert. Zum anderen werden in diesen Modellen Observablen berechnet und eine phänomenologische Analyse durchgeführt. Eine Verknüpfung mit Dunkler Materie ist angedacht.

Wissenschaftlicher und Gesellschaftlicher Kontext

Dieses Projekt trägt zum tieferen wissenschaftlichen Verständnis der fundamentalen Wechselwirkungen zwischen den Materieteilchen bei. Dadurch können die experimentellen Ergebnisse der Hochenergie- (LHC) und der Präzisions-Teilchenphysik (MEG, BELLE II, etc.) gedeutet werden, was einen signifikanten Einfluss auf unser Weltbild hat.

Direct link to Lay Summary Last update: 23.10.2018

Responsible applicant and co-applicants


Associated projects

Number Title Start Funding scheme
175449 Flavourphysik im Standardmodell und dessen Erweiterungen 01.11.2017 Project funding (Div. I-III)
154834 Flavour Physics beyond the Standard Model 01.01.2016 Ambizione


The ultimate goal of particle physics is to uncover the fundamental constituents and interactions of matter. Currently, this is described by the Standard Model (SM) which has been tested extensively in the last decades and its last missing piece, the famous Higgs particle, was discovered at CERN in 2012. Now the focus of High Energy Physics is on the discovery of new particles and new laws of physics beyond the SM. In this way one hopes to answer great open questions, e.g. why our universe consists of matter and not antimatter or the origin and nature of Dark Matter.In addition to direct searches for new physics (NP) at the LHC searching for new resonances, precision experiments are searching for quantum effects of new particles. In this context flavour experiments are especially interesting: matter particles appear in three generations which differ by their flavour quantum number, i.e. while being otherwise identical, they differ in mass. All matter particles are grouped into three . Flavour physics studies the transitions between these three generations. In the SM the only source of flavour violation in the quark sector is the Cabibbo-Kobayashi-Maskawa (CKM) matrix while in the limit of vanishing neutrino masses, lepton flavour is exactly conserved and lepton flavour universality (LFU) violation originates only from tiny couplings of leptons to the Higgs. Therefore, flavour violation is highly suppressed in the SM and thus very sensitive to NP.For generic NP, flavour observables probe much higher energy scales than LHC searches and one can expect to find NP here, before discovering it directly in high energy collisions. In fact, while the LHC did not directly observe new particles so far, in the last years, several experiments accumulated very interesting indications for NP in semi-leptonic B meson (bound states involving a heavy bottom quark) decays. In b->c tau nu processes a combined deviation from the SM prediction of ~4 sigma has been observed and a global fit to b->s mu mu observables even prefers NP compared to the SM with a significance above the 5 sigma level. Furthermore, there is the long standing anomaly in the anomalous magnetic moment of the muon (a_mu) at the 3 sigma level. In my view, these measurements are the most compelling indication for a breakdown of the SM at high energies (disregarding neutrino masses and dark matter which can be accommodated in the SM context without significantly affecting collider observables) since its foundation over 40 years ago.The goal of this proposal is to study the implications of these intriguing hints for LFU violating NP in great detail. The model independent effective field theory approach, in which the SM is supplemented with higher dimensional operators, already uncovered possible patterns of NP contributions but has of course to be continuously updated with forthcoming experimental data. Concerning concrete models, leptoquarks (LQs), i.e. hypothetical new particles which couple quarks to leptons, provide a natural explanation of measured data in B and also for a_mu. Such LQs occur for example in Grand Unified Theories and a large part of this proposal is devoted to a comprehensive study of the effects of these particles. Models with Z' bosons will be examined in detail since they provide a valid explanation to the b->s mu mu anomalies, but alternative solutions will also be pursued. This research plan is thus divided into seven sub-projects: A) Leptoquarks, B) Z' models, C) Flavour in extended Pati-Salam models, D) Loop effects in b-> s mu mu and a_mu, E) Leptoquarks and Supersymmetry, F) alpha_s corrections to mu->e gamma, G) Review article: ``New Physics in the Flavour Sector''. By examining the intriguing hints for LFU violation using model building, precision calculations and phenomenological analysis, this project has the unique potential of uncovering the model of NP superseding the SM.