arXiv subjects

Robert Fleischer

Publications and source records attributed to Robert Fleischer.

At least 19 recordsLinked to original sources

arXiv2026

A Standard Model analysis of $D^0 \to \pi^- \pi^+, K^- K^+, K_{\rm S}^0 K_{\rm S}^0$

We investigate the Standard Model (SM) description of the singly Cabibbo-suppressed charm decays $D^0\to \pi^-\pi^+$, $D^0\to K^-K^+$ and $D^0\to K_{\rm S}^0K_{\rm S}^0$. Using factorisation together with isospin symmetry, we constrain non-factorisable effects and the associated $U$-spin breaking required by the measured branching fractions. We find that corrections of order $50\%$ are sufficient to describe all modes, which although sizeable, are not unexpected for hadronic charm decays. Using the constraints from the branching fractions, we derive SM predictions for the direct CP asymmetry in $D^0\to K_{\rm S}^0K_{\rm S}^0$, finding it to be at most at the per-mille level in our benchmark scenario, providing motivation for future precision measurements.

hep-ph↗

arXiv2026

Probes for CP Violation in B Decays at the FCC: A Theorist's Perspective

CP violation offers powerful probes to explore the quark-flavour sector, where decays of B mesons have been key players since decades. I discuss a variety of probes ranging from non-leptonic to rare B decays, offering exciting opportunities at the FCC in the era after the HL-LHC and Belle II.

hep-ph↗

arXiv2026

CP Violation in $B_{(s)}\to\phi K$ Decays: Standard Model Benchmarks and Isospin-Breaking New Physics

The penguin loop-suppressed $B\to\phi K$ decays are highly sensitive to contributions of hypothetical heavy new particles. Particularly interesting probes for testing the Standard Model and revealing such phenomena are provided by CP violation in the $B^0_d\to\phi K_{\rm S}$ decay. Standard-Model estimates for the corresponding CP-violating observables are theoretically limited by doubly Cabibbo-suppressed penguin contributions. We study these effects using a factorization approach, and provide predictions for CP asymmetries, to be contrasted with future measurements. To gain additional insight into these hadronic effects, we propose the $B_s^0\to\phi K_{\rm S}$ decay as a new channel. We predict the observables for this decay for which currently no measurements exist. By comparing $B^0_d\to\phi K_{\rm S}$ and $B^+ \to \phi K^+$, we further derive state-of-the-art constraints on isospin observables within the Standard Model. The same framework enables probing of possible New-Physics contributions, including general effects and those with non-trivial isospin structure. Interesting prospects arise for the high-precision era of flavour physics ahead.

hep-ph↗

arXiv2025

Standard Model Benchmarks for $D^0\to K^- K^+, \pi^-\pi^+, K^0_{\rm S} K^0_{\rm S}$ Decays

The non-leptonic $D^0\to K^- K^+$ and $D^0\to \pi^-\pi^+$ decays are powerful probes of the Standard Model and are related to each other through the $U$-spin symmetry of the strong interaction. Using lattice QCD inputs we calculate the corresponding colour-allowed tree amplitudes in factorisation and demonstrate that non-factorisable contributions and $U$-spin-breaking effects at the level of 50% allow us to accommodate the measured branching ratios in the Standard Model. An exciting direct probe of such non-factorisable and $U$-spin breaking effects is provided by the $D^0\to K^0_{\rm S} K^0_{\rm S}$ channel. This decay is governed by non-factorisable exchange topologies and essentially vanishes in the $U$-spin limit, although it is experimentally well established with a prominent branching ratio. Extrapolating our $D^0\to K^- K^+$ results using the isospin symmetry, we find a consistent benchmark picture. Specifically, we can accommodate the measured $D^0\to K^0_{\rm S} K^0_{\rm S}$ branching ratio with $U$-spin-breaking effects at the 50% level and exchange amplitudes at the level of 50% of the colour-allowed $D^0\to K^- K^+$, $D^0\to \pi^-\pi^+$ tree contributions. Finally, we explore the resulting range for direct CP violation in $D^0\to K^0_{\rm S} K^0_{\rm S}$, obtaining upper bounds in our benchmark scenarios of a few per mille, offering an exciting target for future measurements.

hep-ph↗

arXiv2025

How to tame penguins: Advancing to high-precision measurements of $\phi_d$ and $\phi_s$

The complex phases $\phi_d$ and $\phi_s$, associated with the mixing between neutral $B_q^0$ and $\bar B_q^0$ mesons ($q\in\{d,s\}$), are key observables to test the Standard Model and search for contributions from new physics. They are conventionally determined from the measurements of mixing-induced CP violation in the decays $B_d^0\to J/\psi K^0$, $B_s^0\to J/\psi\phi$ and $B_s^0\to D_s^+D_s^-$. To reach the highest possible precision on $\phi_d$ and $\phi_s$, it is crucial that corrections from next-to-leading order effects - primarily associated with penguin decay topologies - are accounted for. The strategy adopted in this paper uses the $SU(3)$ flavour symmetry of QCD to relate the unknown contributions from penguin topologies to their counterparts in suitably-chosen control modes, where their effects are enhanced. Utilising new CP asymmetry measurements from LHCb on the decays $B_s^0\to D_s^+D_s^-$, $B_d^0\to D^+D^-$, $B^+\to J/\psi K^+$ and $B^+\to J/\psi\pi^+$, as well as from Belle-II on the decay $B_d^0\to J/\psi\pi^0$, we present the current state-of-the-art picture on controlling the penguin contributions and extract $\phi_d$ and $\phi_s$ from the corresponding observables. We explore the prospects for the end of the Belle-II and HL-LHC flavour physics programmes, and demonstrate the importance of measuring the control modes with future data.

hep-ph↗

arXiv2025

Probing New Physics Through CP Violation in $B_{(s)}\to V\mu^+\mu^-$ Decays

Rare decays of the kind $B\to K^*\mu^+\mu^-$ and $B_s\to \phi\mu^+\mu^-$ are key players for testing the Standard Model. The current experimental data for their decay rates and angular observables show tensions with the theoretical predictions that may be indications of New Physics. We present a strategy to extract the relevant short-distance coefficients in the presence of new sources of CP violation, utilizing a synergy with $B\to K\mu^+\mu^-$ decays. Using the current data as a guideline, we illustrate the new method to determine the complex coefficients $C_9^{(\prime)}$ and $C_{10}^{(\prime)}$ using only four angular observables. Interestingly, the current experimental picture leaves significant room for CP-violating New Physics. We discuss also the link to leptonic $B^0_s\to\mu^+\mu^-$ decays. We are looking forward to the implementation of these strategies at the future high-precision frontier of flavour physics.

hep-ph↗

arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

hep-ph↗

arXiv2025

Taming Penguins: Towards High Precision Measurements in $\phi_d$ and $\phi_s$

Experimentally, the phases $\phi_d$ and $\phi_s$ are determined from CP asymmetry measurements in the "golden modes" $B_d^0\to J/\psi K_{\mathrm{S}}^0$ and $B_s^0\to J/\psi\phi$. At leading order, the theoretical interpretation of these measurements is straightforward. However, to reach high precision determinations of $\phi_d$ and $\phi_s$, which is essential in view of the searches for signs of beyond the SM physics, corrections from next-to-leading order effects need to be accounted for. These corrections primarily originate from so-called penguin topologies. Using the $SU(3)$ flavour symmetry, these corrections can be determined using suitably chosen control modes. Recent new CP asymmetry measurements from LHCb in $B\to DD$ and Belle-II in $B_d^0\to J/\psi\pi^0$ decays greatly improve our knowledge on the parameters describing the contribution from penguin topologies. These proceedings will discuss the current constraints on the penguin parameters in $B\to J/\psi X$ and $B\to DD$ decays, provide corrected determinations for $\phi_d$ and $\phi_s$, and highlight what can be expected at the end of the HL-LHC and Belle-II programmes.

hep-ph↗

arXiv2024

Targeting (Pseudo)-Scalar CP Violation with $B_s \to \mu^+\mu^-$

The leptonic decay $B_s \to \mu^+\mu^-$ is both rare and theoretically clean, making it an excellent probe for New Physics searches. Due to its helicity suppression in the Standard Model, this decay is particularly sensitive to new (pseudo)-scalar contributions. We present a new strategy for detecting CP-violating New Physics contributions of this kind, exploiting two observables: $\mathcal{A}_{\Delta\Gamma_s}^{\mu\mu}$, which is accessible due to the sizeable decay width difference of the $B_s$ system, and the mixing-induced CP asymmetry $\mathcal{S}_{\mu\mu}$. The strategy also uses information from $B\to K^{(*)} \mu^+\mu^-$ and $B_s\to \phi \mu^+\mu^-$ decays. We find remarkably constrained regions in the $\mathcal{A}_{\Delta\Gamma_s}^{\mu\mu}$-$\mathcal{S}_{\mu\mu}$ plane that serve as promising targets for future measurements.

hep-ph↗

arXiv2024

CP Violation in $B \to K\ell^+\ell^-$ Decays: New Opportunities in the High-Precision Era

Experimental data on rare $B$-meson decays indicate deviations from Standard Model predictions. In studies of these decays, possible new sources of CP violation are often neglected. We discuss CP violation in the rare $B$-meson decays $B\to K\ell^+\ell^- (\ell = \mu,e)$ and point to two phenomena that arise when new sources of CP violation are included. First, the Wilson coefficients $C_{9\ell}$ and $C_{10\ell}$ become complex, and we show how we can extract their values from measurements of direct and mixing-induced CP asymmetries. Second, new sources of CP violation can generate nontrivial lepton flavour universality violation. Such a violation is usually measured through ratios like $R_K$ and $R_{K^*}$, but we show that measuring only these ratios leaves a large parameter space unexplored. These results bring exciting opportunities to reveal New Physics effects in the high-precision era.

hep-ph↗

arXiv2024

CP Violation in B Decays: Recent Developments and Future Perspectives

CP violation in B decays provides a powerful tool to probe physics from beyond the Standard Model. A theoretical overview of recent developments of benchmark channels is given, ranging from non-leptonic to rare leptonic and semileptonic modes, opening up exciting perspectives for the future high-precision era of flavour physics and the pursuit of New Physics.

hep-ph↗

arXiv2023

New Probes of Electron--Muon Universality in $B \to K\ell^+\ell^-$ Decays

In the pursuit of physics beyond the Standard Model, a promising path is the study of B-meson decays caused by the transition $b \to s\ell^+\ell^-$. A key observable in such decays is the ratio $R_K$, which measures electron--muon universality in $B \to K μ^+μ^-/e^+e^-$. At first sight, the recent LHCb measurement of $R_K \sim 1$ may seem to largely constrain deviations from universality in these decays. However, we show that this is actually not the case: new sources of CP violation allow for significant universality violation consistent with $R_K \sim 1$. This provides an exciting new opportunity to search for New Physics by measuring differences between CP asymmetries in $B \to Kμ^+μ^-$ and $B \to K e^+e^-$.

hep-ph↗

arXiv2023

Revealing New Physics in $B^0_s\to D_s^\mp K^\pm$ Decays

The $B^0_s\to D_s^\mp K^\pm$ system offers a determination of the Unitarity Triangle angle $γ$. Intrigued by an LHCb analysis showing a surprisingly large result in tension with information on the Unitarity Triangle and other $γ$ measurements, we make a transparent study of the measured observables, confirming the LHCb picture. The corresponding $γ$ puzzle at the $3σ$ level would require CP-violating contributions of New Physics, which should also manifest themselves in the corresponding decay branching ratios. Indeed, we find that the rates of the individual $B^0_s\to D_s^\mp K^\pm$ channels show puzzling patterns, in accordance with similar decays, with tensions up to $4.8σ$, thereby making the situation much more exciting. We present a formalism to include New-Physics effects in a model-independent way and apply it to the data to constrain the corresponding parameters. Interestingly, new contributions of moderate size could accommodate the data. Utilising this formalism in the future high-precision $B$ physics era may allow us to finally establish new sources of CP violation.

hep-ph↗

arXiv2023

New Perspectives for Testing Electron-Muon Universality

Intriguing results for tests of the universality of electrons and muons through measurements of rates of $B\to K \ell^+ \ell^-$ and similar decays have been in the spotlight for years. The LHCb collaboration has recently reported new results which are in agreement with Lepton Flavour Universality, while the individual decay rates are found below their Standard Model predictions. In view of this new situation, we explore how much space is left for a violation of electron-muon universality. Considering new sources of CP violation and taking the new LHCb measurements into account, we show that significant differences between the short-distance coefficients for electronic and muonic final states are actually allowed by the current data. These patterns can be revealed through CP asymmetries in neutral and charged $B\to K \ell^+ \ell^-$ decays. We obtain correlations between these observables and map them to the short-distance coefficients. This results in regions in New Physics parameter space with large differences between CP asymmetries of the decays with final-state electrons and muons, thereby leaving a lot of room for possible surprises in the future high-precision era.

hep-ph↗

arXiv2023

New Physics in $B_q^0-\bar B_q^0$ Mixing: Present Challenges, Prospects, and Implications for $B_q^0\toμ^+μ^-$

The phenomenon of $B^0_q-\bar B^0_q$ mixing ($q=d,s$) provides a sensitive probe for physics beyond the Standard Model. We explore the corresponding space for New Physics left through the current data, having a careful look at analyses of the Unitarity Triangle that are needed for the Standard Model predictions of the $B_q$ mixing parameters. In particular, we explore the impact of tensions between inclusive and exclusive determinations of the CKM matrix elements $|V_{ub}|$ and $|V_{cb}|$. Moreover, we focus on the angle $γ$ of the Unitarity Triangle, comparing measurements from $B\to DK$ and $B\toππ$, $ρπ$, $ρρ$ decays, where the latter are typically interpreted in terms of the angle $α$. We discuss various scenarios and present the corresponding state-of-the-art constraints on the New Physics parameters of $B_q^0-\bar B_q^0$ mixing. We point out that these results have an interesting application in the analysis of rare $B_q^0\to μ^+μ^-$ decays, allowing us to minimise the impact of CKM parameters in the search for New Physics. In view of the high-precision era, we make future projections. Interestingly, we find that for the extraction of the New Physics parameters in the $B_d$ system the determination of the apex of the Unitarity Triangle results in a key limiting factor. By contrast, the corresponding impact is negligible for the $B_s$ system, making it a promising candidate to reveal sources of New Physics.

hep-ph↗

arXiv2023

Studies of New Physics in $B^0_q-\bar{B}^0_q$ Mixing and Implications for Leptonic Decays

The phenomenon of $B^0_q$-$\bar{B}^0_q$ mixing ($q=d,s$) provides a sensitive probe for physics beyond the Standard Model. We have a careful look at the determination of the Unitarity Triangle apex, which is needed for the Standard Model predictions of the $B_q$ mixing parameters, and explore how much space for New Physics is left through the current data. We study the impact of tensions between inclusive and exclusive determinations of the CKM matrix elements $|V_{ub}|$ and $|V_{cb}|$, and focus on the $γ$ angle extraction. We present various future scenarios and discuss the application of these results for leptonic rare $B$ decays, which allows us to minimise the CKM parameter impact in the New Physics searches. Performing future projections, we explore and illustrate the impact of increased precision on key input quantities. It will be exciting to see how more precise data in the future high-precision era of flavour physics can lead to a much sharper picture.

hep-ph↗

arXiv2022

Fingerprinting CP-violating New Physics with $B \to Kμ^+μ^-$

The $B \to K \ell^+ \ell^-$ system is particularly interesting to explore physics beyond the Standard Model. Such effects could provide new sources of CP violation, thereby giving rise to Wilson coefficients which could not only be real, as assumed in most analyses, but also be complex. We propose a new strategy to extract the complex $C_{9 μ}$, $C_{10 μ}$ from the data, utilising the complementarity of direct CP violation in $B^+\to K^+ μ^+μ^-$ and mixing-induced CP violation arising - in addition to direct CP violation - in $B^0_d \to K_{S} μ^+μ^-$, as well as the differential rate in appropriate $q^2$ bins. The long-distance effects, which play a key role to generate the direct CP asymmetries, cannot be reliably calculated due to their non-perturbative nature. In order to describe them, we apply a model by the LHCb Collaboration employing experimental data. Assuming specific scenarios, we demonstrate the fingerprinting of CP-violating New Physics and the distinction between ambiguities in the model of the long-distance contributions. Finally, New Physics could couple differently to muons and electrons, as probed through the lepton flavour universality ratio $R_K$. We discuss these effects in the presence of CP violation and present a new way to measure the direct CP asymmetries of the $B \to K e^+ e^-$ channels.

hep-ph↗

arXiv2022

Zooming into CP violation in $B_{(s)}\to hh$ Decays

The LHCb collaboration has recently reported the first observation of CP violation in the penguin-dominated $B^0_s\to K^-K^+$ decay and further new measurements, indicating differences between the direct CP asymmetries of both the $B^0_s\to K^-K^+$, $B^0_d\toπ^-K^+$ and the $B^0_d\to π^-π^+$, $B^0_s\to K^-π^+$ modes. We show that these puzzling differences can be accommodated through sizeable penguin annihilation and exchange topologies in the Standard Model, and constrain them. Utilising the $U$-spin symmetry, we extract the angle $γ$ of the unitarity triangle from the CP asymmetries in the $B^0_s\to K^-K^+$, $B^0_d\to π^-π^+$ system alone, finding $γ=(65^{+11}_{-7})^\circ$, in perfect agreement with the determination from tree-level $B\to DK$ decays. The $B^0_s$--$\bar B^0_s$ mixing phase $ϕ_s$ can be extracted from CP violation measurements in $B^0_s\to K^-K^+$ in a clean way. We present a new strategy and extract $ϕ_s=-(3.6\pm 5.4)^\circ$. This result is in agreement with the determination from $B^0_s\to J/ψϕ$ decays. New CP-violating contributions would influence these determinations differently. Hence it is interesting to keep monitoring both as the experimental picture sharpens.

hep-ph↗