arXiv subjects

Ruben Jaarsma

Publications and source records attributed to Ruben Jaarsma.

13 recordsLinked to original sources

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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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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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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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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arXiv2017

In Pursuit of New Physics with $B^0_{s,d}\to\ell^+\ell^-$

Leptonic rare decays of $B^0_{s,d}$ mesons offer a powerful tool to search for physics beyond the Standard Model. The $B^0_{s}\toμ^+μ^-$ decay has been observed at the Large Hadron Collider and the first measurement of the effective lifetime of this channel was presented, in accordance with the Standard Model. On the other hand, $B^0_{s}\toτ^+τ^-$ and $B^0_{s}\to e^+e^-$ have received considerably less attention: while LHCb has recently reported a first upper limit of $6.8\times10^{-3}$ (95% C.L.) for the $B^0_s\toτ^+τ^-$ branching ratio, the upper bound $2.8\times 10^{-7}$ (90% C.L.) for the branching ratio of $B^0_s\to e^+e^-$ was reported by CDF back in 2009. We discuss the current status of the interpretation of the measurement of $B^0_{s}\toμ^+μ^-$, and explore the space for New-Physics effects in the other $B^0_{s,d}\to\ell^+\ell^-$ decays in a scenario assuming flavour-universal Wilson coefficients of the relevant four-fermion operators. While the New-Physics effects are then strongly suppressed by the ratio $m_μ/m_τ$ of the lepton masses in $B^0_s\toτ^+τ^-$, they are hugely enhanced by $m_μ/m_e$ in $B^0_s\to e^+e^-$ and may result in a $B^0_s\to e^+e^-$ branching ratio as large as about 5 times the one of $B^0_{s}\toμ^+μ^-$, which is about a factor of 20 below the CDF bound; a similar feature arises in $B^0_{d}\to e^+e^-$. Consequently, it would be most interesting to search for the $B^0_{s,d}\to e^+e^-$ channels at the LHC and Belle II, which may result in an unambiguous signal for physics beyond the Standard Model.

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arXiv2016

Towards New Frontiers in the Exploration of Charmless Non-Leptonic $B$ Decays

Non-leptonic $B$ decays into charmless final states offer an important laboratory to study CP violation and the dynamics of strong interactions. Particularly interesting are $B^0_s\to K^-K^+$ and $B^0_d\toπ^-π^+$ decays, which are related by the $U$-spin symmetry of strong interactions, and allow for the extraction of CP-violating phases and tests of the Standard Model. The theoretical precision is limited by $U$-spin-breaking corrections and innovative methods are needed in view of the impressive future experimental precision expected in the era of Belle II and the LHCb upgrade. We have recently proposed a novel method to determine the $B_s^0$-$\bar{B}_s^0$ mixing phase $ϕ_s$ from the $B_s^0\to K^-K^+$, $B_d^0\to π^-π^+$ system, where semileptonic $B^0_s\to K^-\ell^+ν_\ell$, $B^0_d\to π^-\ell^+ν_\ell$ decays are a new ingredient and the theoretical situation is very favourable. We discuss this strategy in detail, with a focus on penguin contributions as well as exchange and penguin-annihilation topologies which can be probed by a variety of non-leptonic $B$ decays into charmless final states. We show that a theoretical precision as high as ${\cal O}(0.5^\circ)$ for $ϕ_s$ can be attained in the future, thereby offering unprecedented prospects for the search for new sources of CP violation.

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arXiv2016

New Strategy to Explore CP Violation with $B^0_s\to K^-K^+$

The $U$-spin symmetry provides a powerful tool to extract the angle $γ$ of the Unitarity Triangle and the $B_s^0$-$\bar{B}_s^0$ mixing phase $ϕ_s$ from CP violation in the $B^0_s\to K^-K^+$, $B^0_d\toπ^-π^+$ system. LHCb has obtained first results with uncertainties at the $7^\circ$ level. Due to $U$-spin-breaking corrections, it will be challenging to reduce the uncertainty below ${\cal O}(5^\circ)$ at Belle II and the LHCb upgrade. We propose a new strategy, using $γ$ as input and utilizing $B^0_s\to K^-\ell^+ν_\ell$, $B^0_d\to π^-\ell^+ν_\ell$ decays, which allows an extraction of $ϕ_s$ with a future theoretical precision of up to ${\cal O}(0.5^\circ)$, thereby matching the experimental prospects. Since $B^0_s\to K^-K^+$ is dominated by penguin topologies, new sources of CP violation may be revealed.

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