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Robert Fleischer

Publications and source records attributed to Robert Fleischer.

At least 37 records · Page 2Linked to original sources
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.

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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.

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arXiv2022

Using $B^0_s\to D_s^\mp K^\pm$ Decays as a Portal to New Physics

The system of $B^0_s\to D_s^\mp K^\pm$ decays offers a theoretically clean determination of the angle $γ$ of the Unitarity Triangle. A corresponding LHCb analysis resulted in a surprisingly large result, which is in tension with other determinations and global analyses of the Unitarity Triangle. Paying special attention to discrete ambiguities, we confirm this picture and resolve a final ambiguity. Moreover, we extract the branching ratios of the underlying $\bar{B}^0_s\to D_s^+K^-$ and $\bar{B}^0_s\to D_s^- K^+$ modes. Combining them with information from semileptonic $B_{(s)}$ decays, we arrive at another puzzling situation, which we obtain also for other decays with similar dynamics. These patterns could be footprints of New Physics in the $b\to c \bar{u} s$ and $b \to u \bar{c} s$ processes which govern the dynamics of the $\bar{B}^0_s\to D_s^\mp K^\pm$ channels. Employing a model-independent parametrisation, we present a strategy to reveal such effects. Applying it to the present data, we obtain strongly correlated New-Physics contributions with potentially large CP-violating phases. We find that new contributions sizeably smaller than the Standard Model amplitudes could actually accommodate the current data. This strategy offers an exciting probe for new sources of CP violation at the future high-precision frontier of $B$ physics.

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arXiv2022

Penguin Effects in $B_d^0\to J/ψK^0_S$ and $B_s^0\to J/ψϕ$

Controlling the contributions from doubly Cabibbo-suppressed penguin topologies in the decays $B_d^0\to J/ψK^0_S$ and $B_s^0\to J/ψϕ$ is mandatory to reach the highest possible precision in the measurement of the $B^0_q$--$\bar B^0_q$ ($q=d,s$) mixing phases $ϕ_d$ and $ϕ_s$. The penguin contributions can be determined using a strategy based on the $SU(3)$ flavour symmetry of QCD. Using the latest experimental data, we update our combined analysis of the decays $B_d^0\to J/ψK^0_S$, $B_s^0\to J/ψϕ$ and their control channels $B_s^0\to J/ψK^0_S$, $B_d^0\to J/ψπ^0$ and $B_d^0\to J/ψρ^0$. This allows us to simultaneously determine the penguin parameters and both mixing phases. We discuss how the branching fractions of these decays can be used to probe the size of non-factorisable $SU(3)$-breaking effects, which form the main theoretical uncertainty associated with our $SU(3)$-based strategy, and provide new insights into the factorisation approach.

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arXiv2022

Puzzles in the $B^0_s \rightarrow D_s^{\pm} K^{\mp} $ System

Non-leptonic $B^0_s \rightarrow D_s^{\pm} K^{\mp}$ transitions are particularly interesting processes to test the Standard Model. As these decays occur via pure tree diagrams, they allow a theoretically clean determination of the angle $γ$ of the unitarity triangle. Considering recent LHCb results, an intriguing picture arises, showing tension with the Standard Model. Extracting the branching ratios of the underlying $\bar{B}^0_s \rightarrow D_s^{\pm} K^{\mp}$ modes and combining them with information from semileptonic $B_{(s)}$ decays, we arrive at another puzzling situation, in accordance with similar decays. These patterns could be footprints of New Physics. We present a model-independent strategy to include such New-Physics effects and apply it to the data. Interestingly, new contributions of moderate size could accommodate the data. This formalism offers an exciting probe for new sources of CP violation at the future high-precision B physics era.

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arXiv2021

Searching for New Physics with $B^0_s \rightarrow D_s^{\pm} K^{\mp} $ Decays

Particularly interesting processes to test the Standard Model are non-leptonic $B^0_s \rightarrow D_s^{\pm} K^{\mp}$ transitions. As these decays occur via pure tree diagrams, they allow a theoretically clean determination of the angle $γ$ of the unitarity triangle. Considering recent LHCb results, an intriguing picture arises, showing tension with the Standard Model. Utilising the available experimental data, we perform a theoretical analysis in order to shed more light on these puzzling patterns. Do these puzzles actually indicate footprints of New Physics?

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arXiv2021

Mapping out the Space for New Physics with Leptonic and Semileptonic $B_{(c)}$ Decays

Decays of $B$ mesons with leptons in the final state offer an interesting laboratory to search for possible effects of physics from beyond the Standard Model. In view of puzzling patterns in experimental data, the violation of lepton flavour universality is an interesting option. We present a strategy, utilising ratios of leptonic and semileptonic $B$ decays, where the elements $|V_{ub}|$ and $|V_{cb}|$ of the Cabibbo--Kobayashi--Maskawa (CKM) matrix cancel, to constrain the short-distance coefficients of (pseudo)-scalar, vector and tensor operator contributions. The individual branching ratios allow us then to extract also the CKM matrix elements, even in the presence of new-physics contributions. Bounds on unmeasured leptonic and semileptonic decays offer important additional constraints. In our comprehensive analysis, we give also predictions for decays which have not yet been measured in a variety of scenarios.

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arXiv2021

In Pursuit of New Physics with $B_d^0\to J/ψK^0$ and $B_s^0\to J/ψϕ$ Decays at the High-Precision Frontier

The decays $B_d^0\to J/ψK^0_{\text{S}}$and $B_s^0\to J/ψϕ$ play a key role for the determination of the $B^0_q$-$\bar B^0_q$ ($q=d,s$) mixing phases $ϕ_d$ and $ϕ_s$, respectively. The theoretical precision of the extraction of these quantities is limited by doubly Cabibbo-suppressed penguin topologies, which can be included through control channels by means of the $SU(3)$ flavour symmetry of strong interactions. Using the currently available data and a new simultaneous analysis, we discuss the state-of-the-art picture of these effects and include them in the extracted $ϕ_q$ values. We have a critical look at the Standard Model predictions of these phases and explore the room left for new physics. Considering future scenarios for the high-precision era of flavour physics, we illustrate that we may obtain signals for physics beyond the Standard Model with a significance well above five standard deviations. We also determine effective colour-suppression factors of $B_d^0\to J/ψK^0$, $B_d^0\to J/ψK^0_{\text{S}}$ and $B_d^0\to J/ψπ^0$ decays, which serve as benchmarks for QCD calculations of the underlying decay dynamics, and present a new method using information from semileptonic $B_d^0\toπ^-\ell^+ν_{\ell}$ and $B_s^0\to K^-\ell^+ν_{\ell}$ decays.

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arXiv2019

Testing Lepton Flavour Universality with (Semi)-Leptonic $D_{(s)}$ Decays

Data in $B$-meson decays indicate violations of lepton flavour universality, thereby raising the question about such phenomena in the charm sector. We perform a model-independent analysis of NP contributions in (semi)-leptonic decays of $D_{(s)}$ mesons which originate from $c \to d \bar{\ell} ν_{\ell}$ and $c \to s \bar{\ell} ν_{\ell}$ charge-current interactions. Starting from the most general low-energy effective Hamiltonian containing four-fermion operators and the corresponding short-distance coefficients, we explore the impact of new (pseudo)-scalar, vector and tensor operators and constrain their effects through the interplay with current data. We pay special attention to the elements $|V_{cd}|$ and $|V_{cs}|$ of the Cabibbo--Kobayashi--Maskawa matrix and extract them from the $D_{(s)}$ decays in the presence of possible NP decay contributions, comparing them with determinations utilizing unitarity. We find a picture in agreement with the Standard Model within the current uncertainties. Using the results from our analysis, we make also predictions for leptonic $D_{(s)}^+ \to e^+ ν_e$ modes which could be hugely enhanced with respect to their tiny Standard Model branching ratios. It will be interesting to apply our strategy at the future high-precision frontier.

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arXiv2018

Probing Lepton Universality with (Semi)-Leptonic B decays

The most recent measurements of the observables $R_{D^{(*)}}$ are in tension with the Standard Model offering hints of New Physics in $b\rightarrow c \ell \barν_{\ell}$ transitions. Motivated by these results, in this work we present an analysis on their $b\rightarrow u \ell \barν_{\ell}$ counterparts (for $\ell=e, ~μ, ~τ$). Our study has three main objectives. Firstly, using ratios of branching fractions, we assess the effects of beyond the Standard Model scalar and pseudoscalar particles in leptonic and semileptonic $B$ decays ($B^-\rightarrow \ell^- \barν_{\ell}$, $\bar{B}\rightarrow π\ell \barν_{\ell}$ and $\bar{B}\rightarrow ρ\ell \barν_{\ell}$). Here a key role is played by the leptonic $B$ processes, which are highly sensitive to new pseudoscalar interactions. In particular, we take advantage of the most recent measurement of the branching fraction of the channel $B^-\rightarrow μ^-\barν_μ$ by the Belle collaboration. Secondly, we extract the CKM matrix element $|V_{ub}|$ while accounting simultaneously for New Physics contributions. Finally, we provide predictions for the branching fractions of yet unmeasured leptonic and semileptonic $B$ decays.

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arXiv2018

Probing New Physics in $B\toπK$ Decays

Over the last two decades, $B \to πK$ modes have received a lot of attention. They are particularly interesting since the main contribution to these decays comes from QCD penguin topologies. Furthermore, electroweak penguin amplitudes enter at a level comparable to the tree topologies. In the past, a discrepancy was found in the correlation between the CP asymmetries of $B_d^0 \to π^0 K_{\rm S}$. We provide the state-of-the-art picture of this situation and consider new constraints, finding that the effect has become larger. An attractive explanation is offered by a modified electroweak penguin sector. We propose a new method to determine the relevant parameters. It employs an isospin relation between the amplitudes of the $B \to πK$ decays, and makes only minimal use of the $SU(3)$ flavour symmetry. Using current data as well as future scenarios, we demonstrate how the mixing-induced CP asymmetry of $B_d^0 \to π^0 K_{\rm S}$ plays a key role in this analysis. The application of our strategy at the next generation of $B$-physics experiments may establish New Physics and reveal new sources of CP violation in the electroweak penguin sector.

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arXiv2018

Decoding (Pseudo)-Scalar Operators in Leptonic and Semileptonic $B$ Decays

We consider leptonic $B^-\to \ell^- \barν_\ell$ and semileptonic $\bar B \to π\ell^- \barν_\ell$, $\bar B \to ρ\ell^- \barν_\ell$ decays and present a strategy to determine short-distance coefficients of New-Physics operators and the CKM element $|V_{ub}|$. As the leptonic channels play a central role, we illustrate this method for (pseudo)-scalar operators which may lift the helicity suppression of the corresponding transition amplitudes arising in the Standard Model. Utilising a new result by the Belle collaboration for the branching ratio of $B^-\to μ^- \barν_μ$, we explore theoretically clean constraints and correlations between New Physics coefficients for leptonic final states with $μ$ and $τ$ leptons. In order to obtain stronger bounds and to extract $|V_{ub}|$, we employ semileptonic $\bar B \to π\ell^- \barν_\ell$ and $\bar B \to ρ\ell^- \barν_\ell$ decays as an additional ingredient, involving hadronic form factors which are determined through QCD sum rule and lattice calculations. In addition to a detailed analysis of the constraints on the New Physics contributions following from current data, we make predictions for yet unmeasured decay observables, compare them with experimental constraints and discuss the impact of CP-violating phases of the New-Physics coefficients.

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arXiv2018

Exploring $B\toππ, πK$ Decays at the High-Precision Frontier

The $B\toππ,πK$ system offers a powerful laboratory to probe strong and weak interactions. Using the isospin symmetry, we determine hadronic $B\toππ$ parameters from data where new measurements of direct CP violation in $B^0_d\toπ^0π^0$ resolve a discrete ambiguity. With the help of the $SU(3)$ flavour symmetry, the $B\toππ$ parameters can be converted into their $B\toπK$ counterparts, thereby allowing us to make predictions of observables. A particularly interesting decay is $B^0_d\toπ^0 K_{\rm S}$ as it exhibits mixing-induced CP violation. Using an isospin relation, complemented with a robust $SU(3)$ input, we calculate correlations between the direct and mixing-induced CP asymmetries of $B^0_d\toπ^0 K_{\rm S}$, which are the theoretically cleanest $B\toπK$ probes. Interestingly, they show tensions with respect to the Standard Model. Should this $B\toπK$ puzzle originate from New Physics, electroweak penguins offer an attractive scenario for new particles to enter. We present a strategy to determine the parameters characterising these topologies and obtain the state-of-the-art picture from current data. In the future, this method will allow us to reveal the $B\toπK$ dynamics and to obtain insights into the electroweak penguin sector with unprecedented precision.

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arXiv2018

Utilising $B \to πK$ Decays at the High-Precision Frontier

For about twenty years, $B\toπK$ decays are in the focus of $B$-decay studies. We show that a correlation between the CP asymmetries of $B^0_d\toπ^0K_{\rm S}$ reveals a tension with the Standard Model. Should it be due to New Physics, a modified electroweak penguin sector provides particularly interesting possibilities. We present a new method to determine the electroweak penguin parameters, which uses an isospin relation and requires only minimal $SU(3)$ input. We apply it to the current data for $B\toπK$ decays and discuss the prospects for utilizing CP violation in $B^0_d\toπ^0K_{\rm S}$. The strategy has the exciting potential to establish New Physics in the electroweak penguin sector in the high-precision era of $B$-physics.

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arXiv2018

New Probes of New Physics with Leptonic Rare B Decays

Decays of the kind $B^0_{s,d}\to\ell^+\ell^-$ belong to the most favourable processes for probing the flavour structure of the Standard Model, with outstanding sensitivity to new (pseudo)-scalar contributions. While the branching ratio of $B^0_s\toμ^+μ^-$ has already been measured at the LHC in the ballpark of the Standard Model expectation, there is still significant room for New-Physics effects. We discuss how these may be revealed in the future super-high precision era of $B$-decay studies by utilising new theoretically clean observables, including CP-violating asymmetries. Another promising decay is $B^0_{s}\to e^+e^-$, which has received little attention in view of its enormously helicity suppressed Standard Model branching ratio, with the most recent experimental upper bound dating back to 2009. Using the current constraints on New Physics from $B^0_s\toμ^+μ^-$ as a guideline, we show that the $B^0_s\to e^+e^-$ branching ratio may be hugely enhanced through new (pseudo)-scalar contributions up to the regime of $B^0_{s}\toμ^+μ^-$.

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arXiv2017

CP Violation in Leptonic Rare $B^0_s$ Decays as a Probe of New Physics

The decay $B^0_s\toμ^+μ^-$ is a key probe for the search of physics beyond the Standard Model. While the current measurements of the corresponding branching ratio agree with the Standard Model within the uncertainties, significant New-Physics effects may still be hiding in $B^0_s\toμ^+μ^-$. In order to reveal them, the observable $\mathcal{A}^{μμ}_{ΔΓ_s}$, which is provided by the decay width difference $ΔΓ_s$ of the $B^0_s$-meson system, plays a central role. We point out that a measurement of a CP-violating observable ${\cal S}_{\rm μμ}$, which is induced through interference between $B^0_s$-$\bar B^0_s$ mixing and $B_s\toμ^+μ^-$ decay processes, is essential to obtain the full picture, in particular to establish new scalar contributions and CP-violating phases. We illustrate these findings with future scenarios for the upgrade(s) of the LHC, exploiting also relations which emerge within an effective field theory description of the Standard Model, complemented with New Physics entering significantly beyond the electroweak scale.

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arXiv2017

Towards New Frontiers with $B\toπK$ Decays

Exploring correlations between the CP asymmetries of $B^0_d\toπ^0K_{\rm S}$ following from an isospin relation, we uncover new tensions with the Standard Model in data for neutral $B\toπK$ decays. Should this intriguing picture originate from New Physics, a modified electroweak penguin sector provides a key scenario. It includes models with extra $Z'$ bosons, which offer attractive ways to resolve anomalies in $B\to K^{(*)}\ell^+\ell^-$ measurements. We present a new strategy to reveal the underlying physics, apply it to current $B\toπK$ data, and discuss the excellent prospects for Belle II.

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