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Probing the flavour anomalies at LHCb

Applicant Mauri Andrea
Number 191121
Funding scheme Postdoc.Mobility
Research institution National Institute for Subatomic Physics
Institution of higher education Institution abroad - IACH
Main discipline Particle Physics
Start/End 01.05.2020 - 30.04.2022
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Keywords (4)

LHCb; experimental particle physics; lepton flavour universality; flavour anomalies

Lay Summary (Italian)

Il Modello Standard è la teoria che descrive le interazioni tra particelle elementari, queste sono i componenti più piccoli che formano tutta la materia dell’universo e vengono classificate tra quark e leptoni a seconda delle loro proprietà. Una delle principali proprietà del Modello Standard è che le diverse specie di leptoni presenti in natura (elettroni, muoni e tau) sono soggetto alle stesse interazioni e con la stessa intensità.Questa proprietà è stata dimostrata in molti esperimenti, tuttavia al momento la sua validità non è ancora stata provata per una tipologia di decadimenti (un quark b che decade in un quark s) che accadono con frequenza estremamente rara in natura (con una probabilità più bassa che uno su un milione).Una potenziale violazione di questa proprietà porterebbe alla scoperta di un nuovo tipo di forza.
Lay summary

L’obiettivo principale di questo studio è contribuire a una migliore comprensione delle proprietà (e possibili differenze) tra elettroni e muoni in decadimenti rari di particelle che contengono quark b. Queste particelle vengono prodotte in gran numero da collisioni di protoni a LHC, il più grande ed energetico acceleratore di particelle al mondo.

In particolare, analizzerò i dati ottenuti dall’esperimento LHCb al CERN, studiando diversi tipi di decadimento (tutti governati dallo stesso tipo di interazione fondamentale) così da stabilire se seguono le predizioni del Modello Standard e, in seguito, se vi siano differenze tra loro.

Inoltre, cercherò di facilitare l’interpretazione di questi risultati sperimentali da parte dei fisici teorici, soprattutto quando nuovi modelli teorici saranno disponibili in futuro.

Infine, contribuirò allo sviluppo di nuovi algoritmi per l’acquisizione di nuovi dati per l’upgrade dell’esperimento LHCb previsto a partire dal 2021.

Direct link to Lay Summary Last update: 31.03.2020

Responsible applicant and co-applicants


The Standard Model of particle physics (SM) aims to explain in a unified theorythe elementary constituents of the Universe and their interactions. The SM has been successfully tested with extraordinary precision in the last few decades, however, a series of experimental evidences not predicted by the SM have been seen in different fields.Therefore, the current theory is experimentally found to be incomplete.A number of extensions of the SM have been proposed and direct searches for New Physics (NP) effects are currently the dominant experimental effort of the particle physics community. Since no clear sign of NP has been observed yet, indirect searches, which are highly sensitive to new particles effects at higher scales, are becoming of crucial importance and can placestrong constraints in the phase space of NP contributions.A prime example is the flavour physics program, that recently showed a series of measurements that deviatefrom the SM predictions, known as flavour anomalies.Several of these intriguing results consist of measurements of semileptonic $b$-meson decays, varying from non-standard measurements of angular observables in $B^0 \to K^{*0} \mu^+ \mu^-$ decaysto a suppression of the muon channel in the ratio of branching fractions of$B^0 \to K^{*0} \ell^+ \ell^-$ and $B^+ \to K^+ \ell^+ \ell^-$ decays, with $\ell = \mu, e$.These deviations, although non statistically significant when considered separately, show a coherent pattern that can be interpreted as a hint of NP. Nevertheless, the weak knowledge of theoretical uncertainties in these decays and thelimited available statistics currently limit any claim of physics beyond the Standard Model.In order to achieve a conclusive understanding of these anomalies, novel approaches must be pursued both from the theoretical and experimental sides.My proposal aims to improve the understanding of the flavour anomalies in $b \to s \ell^+ \ell^-$ transitions and is divided in three sub-projects.The first part includes the analysis of three key decay channels, i.e. $B^{0(+)} \to K^{*0(+)} \ell^+ \ell^-$ and $B_s \to \phi \ell^+ \ell^-$ decays, with the data collected by the LHCb experiment.These decays undergo the same $b \to s \ell^+ \ell^-$ quark transition and provide complementary information on the underlying physics.As part of this project, I will perform an amplitude analysis to the decay rates with either muons or electrons in the final state.This method allows to exploit at best the sensitivity to NP enclosed in the dynamics of these decays, enables the direct determination of potential non-equal couplings of muons and electrons and goes beyond the current approaches based on angular observables and ratio of branching fractions.I expect with this result to either find evidence for new phenomena or to place stringent boundson the size of any NP effects, using the unprecedented dataset collected by the LHCb experiment.Secondly, the amplitude models developed for the analysis will be implemented in the zfit package,as part of a project developing a new fitting framework for particle physics that aims to unify on a single platform the tools required to handle amplitude analyses.The ultimate goal of this project is to bridge the gap between theory and experimental communities, favouring the interpretation of experimental results in light of new theoretical models.Finally, I propose the development of new isolation algorithms to be employed at LHCb Upgrades, when the increased instantaneous luminosity will challenge thecurrent data-taking and data-manipulation systems.These algorithms will contribute to the migration to a Real Time Analysis (RTA) approach, started with the fully software trigger system designed for the LHCb Run~III and intended to cover the totality of the detector operations.