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Kristof De Bruyn

Publications and source records attributed to Kristof De Bruyn.

15 recordsLinked to original sources
arXiv2026

How to tame penguins: Advancing to high-precision measurements of $ϕ_d$ and $ϕ_s$

The complex phases $ϕ_d$ and $ϕ_s$, 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 $B_d^0\to J/ψK^0$, $B_s^0\to J/ψϕ$ and $B_s^0\to D_s^+D_s^-$. To reach the highest possible precision on $ϕ_d$ and $ϕ_s$, 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 $B_s^0\to D_s^+D_s^-$, $B_d^0\to D^+D^-$, $B^+\to J/ψK^+$ and $B^+\to J/ψπ^+$, as well as from Belle-II on the decay $B_d^0\to J/ψπ^0$, we present the current state-of-the-art picture on controlling the penguin contributions and extract $ϕ_d$ and $ϕ_s$ 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↗
arXiv2026

Looking forward to $B^+\to τ^+ ν_τ$ and $B_c^+\to τ^+ ν_τ$

These proceedings present the outcome of a feasibility study using RapidSim simulation software that demonstrates that the LHCb experiment will be capable of observing the decays $B^+\to τ^+ ν_τ$ and $B_c^+\to τ^+ ν_τ$ using the data that is being collecting during Run 3 of the LHC. The proposed analysis exploits the small distance of only 5.1 millimetres between the sensing elements of LHCb's innermost silicon pixel detector, the VELO, and the LHC's proton beams to identify direct pixel hits in the VELO that can be associated with the charged $B^+$, $B_c^+$ or $τ^+$ particles. By using this extra information, the limitations due to the missing momentum and vertex information will be significantly reduced. This provides enough statistical power to pursue the measurements of these two decay channels at the LHC. In particular for the decay $B_c^+\to τ^+ ν_τ$, which has been identified by the high energy physics community as a key objective for experiments at the planned next-generation particle accelerators, this means we do not need to wait for the 2030s or beyond to get first experimental constraints.

hep-ex↗
arXiv2025

Taming Penguins: Towards High Precision Measurements in $ϕ_d$ and $ϕ_s$

Experimentally, the phases $ϕ_d$ and $ϕ_s$ are determined from CP asymmetry measurements in the "golden modes" $B_d^0\to J/ψK_{\mathrm{S}}^0$ and $B_s^0\to J/ψϕ$. At leading order, the theoretical interpretation of these measurements is straightforward. However, to reach high precision determinations of $ϕ_d$ and $ϕ_s$, 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 $B_d^0\to J/ψπ^0$ 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 $B\to J/ψX$ and $B\to DD$ decays, provide corrected determinations for $ϕ_d$ and $ϕ_s$, and highlight what can be expected at the end of the HL-LHC and Belle-II programmes.

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

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.

hep-ph↗
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.

hep-ph↗
arXiv2015

Anatomy of $B\rightarrow D\bar{D}$ Decays

The decays $B_d^0\rightarrow D_d^-D_d^+$ and $B_s^0\rightarrow D_s^-D_s^+$ probe the CP-violating mixing phases $ϕ_d$ and $ϕ_s$, respectively. The theoretical uncertainty of the corresponding determinations is limited by contributions from penguin topologies, which can be included with the help of the $U$-spin symmetry of the strong interaction. We analyse the currently available data for $B^0_{d,s}\rightarrow D_{d,s}^-D_{d,s}^+$ decays and those with similar dynamics to constrain the involved non-perturbative parameters. Using further information from semileptonic $B_d^0\rightarrow D_d^-\ell^+ν_{\ell}$ decays, we perform a test of the factorisation approximation and take non-factorisable $SU(3)$-breaking corrections into account. The branching ratios of the $B_d^0\rightarrow D_d^-D_d^+$, $B_s^0\rightarrow D_s^-D_d^+$ and $B_s^0\rightarrow D_s^-D_s^+$, $B_d^0\rightarrow D_d^-D_s^+$ decays show an interesting pattern which can be accommodated through significantly enhanced exchange and penguin annihilation topologies. This feature is also supported by data for the $B_s^0\rightarrow D_d^-D_d^+$ channel. Moreover, there are indications of potentially enhanced penguin contributions in the $B_d^0\rightarrow D_d^-D_d^+$ and $B_s^0\rightarrow D_s^-D_s^+$ decays, which would make it mandatory to control these effects in the future measurements of $ϕ_d$ and $ϕ_s$. We discuss scenarios for high-precision measurements in the era of Belle II and the LHCb upgrade.

hep-ph↗
arXiv2015

A Roadmap to Control Penguin Effects in $B^0_d\to J/ψK_{\rm S}^0$ and $B^0_s\to J/ψϕ$

Measurements of CP violation in $B^0_d\to J/ψK_{\rm S}^0$ and $B^0_s\to J/ψϕ$ decays play key roles in testing the quark-flavour sector of the Standard Model. The theoretical interpretation of the corresponding observables is limited by uncertainties from doubly Cabibbo-suppressed penguin topologies. With continuously increasing experimental precision, it is mandatory to get a handle on these contributions, which cannot be calculated reliably in QCD. In the case of the measurement of $\sin2β$ from $B^0_d\to J/ψK_{\rm S}^0$, the $U$-spin-related decay $B^0_s\to J/ψK_{\rm S}^0$ offers a tool to control the penguin effects. As the required measurements are not yet available, we use data for decays with similar dynamics and the $SU(3)$ flavour symmetry to constrain the size of the expected penguin corrections. We predict the CP asymmetries of $B^0_s\to J/ψK_{\rm S}^0$ and present a scenario to fully exploit the physics potential of this decay, emphasising also the determination of hadronic parameters and their comparison with theory. In the case of the benchmark mode $B^0_s\to J/ψϕ$ used to determine the $B^0_s$-$\bar B^0_s$ mixing phase $ϕ_s$ the penguin effects can be controlled through $B^0_d\to J/ψρ^0$ and $B^0_s\to J/ψ\bar{K}^{*0}$ decays. The LHCb collaboration has recently presented pioneering results on this topic. We analyse their implications and present a roadmap for controlling the penguin effects.

hep-ph↗
arXiv2012

Exploring $B_s \to D_s^{(*)\pm} K^\mp$ Decays in the Presence of a Sizable Width Difference $ΔΓ_s$

The $B_s \to D_s^{(*)\pm} K^\mp$ decays allow a theoretically clean determination of $ϕ_s+γ$, where $ϕ_s$ is the $B^0_s$-$\bar B^0_s$ mixing phase and $γ$ the usual angle of the unitarity triangle. A sizable $B_s$ decay width difference $ΔΓ_s$ was recently established, which leads to subtleties in analyses of the $B_s \to D_s^{(*)\pm} K^\mp$ branching ratios but also offers new "untagged" observables, which do not require a distinction between initially present $B^0_s$ or $\bar B^0_s$ mesons. We clarify these effects and address recent measurements of the ratio of the $B_s\to D_s^\pm K^\mp$, $B_s\to D_s^\pmπ^\mp$ branching ratios. In anticipation of future LHCb analyses, we apply the SU(3) flavour symmetry of strong interactions to convert the $B$-factory data for $B_d\to D^{(*)\pm}π^\mp$, $B_d\to D_s^{\pm}π^\mp$ decays into predictions of the $B_s \to D_s^{(*)\pm} K^\mp$ observables, and discuss strategies for the extraction of $ϕ_s+γ$, with a special focus on untagged observables and the resolution of discrete ambiguities. Using our theoretical predictions as a guideline, we make simulations to estimate experimental sensitivities, and extrapolate to the end of the planned LHCb upgrade. We find that the interplay between the untagged observables, which are accessible thanks to the sizable $ΔΓ_s$, and the mixing-induced CP asymmetries, which require tagging, will play the key role for the experimental determination of $ϕ_s+γ$.

hep-ph↗
arXiv2012

Branching Ratio Measurements of $B_s$ Decays

We have just entered an era of precision measurements for $B_s$-decay observables. A characteristic feature of the $B_s$-meson system is $B^0_s$--$\bar B^0_s$ mixing, which exhibits a sizable decay width difference. The latter feature leads to a subtle complication for the extraction of branching ratios of $B_s$ decays from untagged data samples, leading to systematic biases as large as O(10%) that depend on the dynamics of the considered decay. We point out that this effect can only be corrected for using information from a time-dependent analysis and suggest the use of the effective $B_s$ decay lifetime, which can already be extracted from the untagged data sample, for this purpose. We also address several experimental issues that can play a role in the extraction of effective lifetimes at a hadron collider, and advocate the use of the $B_s$ branching ratios, as presented in this note, for consistent comparisons of theoretical calculations and experimental measurements in particle listings.

hep-ph↗
arXiv2012

Probing New Physics via the $B^0_s\to μ^+μ^-$ Effective Lifetime

We have recently seen new upper bounds for $B^0_s\to μ^+μ^-$, a key decay to search for physics beyond the Standard Model. Furthermore a non-vanishing decay width difference $ΔΓ_s$ of the $B_s$ system has been measured. We show that $ΔΓ_s$ affects the extraction of the $B^0_s\to μ^+μ^-$ branching ratio and the resulting constraints on the New Physics parameter space, and give formulae for including this effect. Moreover, we point out that $ΔΓ_s$ provides a new observable, the effective $B^0_s\to μ^+μ^-$ lifetime $τ_{μ^+μ^-}$, which offers a theoretically clean probe for New Physics searches that is complementary to the branching ratio. Should the $B^0_s\to μ^+μ^-$ branching ratio agree with the Standard Model, the measurement of $τ_{μ^+μ^-}$, which appears feasible at upgrades of the LHC experiments, may still reveal large New Physics effects.

hep-ph↗
arXiv2010

Extracting gamma and Penguin Parameters from B^0_s -> J/psi K_S

The B^0_s -> J/psi K_S decay has recently been observed by the CDF collaboration and will be of interest for the LHCb experiment. It is the U-spin partner of the golden B^0_d -> J/psi K_S channel and offers a determination of the angle gamma of the unitarity triangle. Moreover, it allows us to control the hadronic penguin effects in the extraction of the B^0_d-\bar B^0_d mixing phase phi_d through a measurement of CP violation in B^0_d^0 -> J/psi K_S. We discuss the picture emerging for these measurements from an LHCb feasibility study. While LHCb will be able to extract gamma from the CP violation in B^0_d -> J/psi K_S, the main application of this channel will be the determination of hadronic penguin parameters. Such an analysis is actually needed in order to fully exploit LHCb's impressive experimental precision for the determination of phi_d from B^0_d -> J/psi K_S. We give also the target regions for the effective lifetime of B^0_s -> J/psi K_S and its CP-violating observables for improved measurements by CDF and the 2011 data taking at LHCb.

hep-ph↗
arXiv2010

Extracting gamma and Penguin Topologies through CP Violation in B_s^0 -> J/psi K_S

The B_s^0 -> J/psi K_S decay has recently been observed by the CDF collaboration and will be of interest for the LHCb experiment. This channel will offer a new tool to extract the angle gamma of the unitarity triangle and to control doubly Cabibbo-suppressed penguin corrections to the determination of sin(2beta) from the well-known B_d^0 -> J/psi K_S mode with the help of the U-spin symmetry of strong interactions. While any competitive determination of gamma is interesting, the latter aspect is particularly relevant as LHCb will enter a territory of precision which makes the control of doubly Cabibbo-suppressed Standard-Model corrections mandatory. Using the data from CDF and the e^+e^- B factories as a guideline, we explore the sensitivity for gamma and the penguin parameters and point out that the B_s^0-\bar B_s^0 mixing phase phi_s, which is only about -2 deg in the Standard Model but may be enhanced through new physics, is a key parameter for these analyses. We find that the mixing-induced CP violation S(B_s^0 -> J/psi K_S) shows an interesting correlation with sin(phi_s), which serves as a target region for the first measurement of this observable at LHCb.

hep-ph↗