arXiv subjects

Méril Reboud

Publications and source records attributed to Méril Reboud.

At least 19 recordsLinked to original sources

arXiv2026

The $B^+ \to K^+ ν\bar ν$ decay as a QCD axion search: comparing reinterpretation approaches

Two recent independent analyses of Belle II $B^+ \! \to \! K^+ν\barν$ data yield limits on ${\mathcal B}(B^+ \! \to \! K^+ a)$ -- the two-body mode to a light invisible particle such as the QCD axion -- differing by a factor of roughly four; we trace this to the choice of kinematic variable space. The central figure of merit is the resolution in the reconstructed di-neutrino invariant mass $q^2_{\rm rec}$: fine-grained binning resolves the narrow axion signal, while coarse binning dilutes it into a background-dominated range. A BDT axis trained on $B^+ \! \to \! K^+ν\barν$ adds little discriminating power for $B^+ \! \to \! K^+ a$, as this axis is largely uncorrelated with $q^2$. These expectations are confirmed by a set of numerical tests. The subleading shape systematics omitted from our $q^2_{\rm rec}$-based approach {\em lower}, not raise, the $B^+ \! \to \! K^+ a$ limit: by better accommodating the $B^+ \! \to \! K^+ν\barν$ shape, they leave less room for the axion signal, making our $q^2_{\rm rec}$-based bound conservative, if anything. A dedicated reanalysis confirms that the kinematic-axes choice alone accounts for the factor-of-four sensitivity difference, and that the $B^+ \! \to \! K^+ a$ bound varies sizeably within the $q^2_{\rm rec}\timesη({\rm BDT}_2)$ space, depending on the SM-likeness of $B^+ \! \to \! K^+ν\barν$, thus losing the dual-probe feature of our $q^2_{\rm rec}$-based approach. These results point to a broader consideration: likelihoods dominated by BDT variables are of limited use for reinterpretations when the signal shape differs appreciably from the BDT's training signal. We therefore advocate that experimental collaborations publish likelihood projections in physical variable spaces alongside BDT-based likelihoods, to maximise the reinterpretability of their measurements.

hep-ph↗

arXiv2026

Extracting production fractions of $b$ hadrons from exclusive semi-leptonic decays

Ratios of production fractions of $b$ hadrons are a dominant source of uncertainty in many LHC analyses, in particular in measurements with $B_s$ mesons. The currently used value for $f_s/f_d$ is based on a combination of hadronic and semi-inclusive semi-leptonic decays, and relies in part on assumptions about the underlying decays that are hard to quantify. We propose an independent alternative method to obtain this quantity by measuring ratios of the exclusive semi-leptonic decays $\bar B_{(s)} \to D_{(s)}^{(*)} \ell \barν$. This method benefits from significant cancellations of both experimental and theoretical uncertainties, as well as robustness against potential contamination from heavy physics beyond the Standard Model. As a proof of principle, we show that current measurements constrain $f_s/f_d$ with an uncertainty of $7\%$, dominated by present experimental uncertainties. This method can also be applied to other ratios of production fractions involving heavier $b$ hadrons, such as $B_c$ or $Λ_b$.

hep-ph↗

arXiv2026

Advanced parametrisations for hadronic form factors

The rich analytic structure of hadronic form factors makes a theoretically consistent yet easily applicable parametrisation cumbersome. Consequently, most parametrisations are limited to reproducing the simplest analytic features sufficient to describe form factors on their first Riemann sheet. Here, we introduce two novel form factor parametrisations that allow resonance poles and left-hand cuts on the second Riemann sheet to be studied, while also making the connection to partial-wave amplitudes manifest.

hep-ph↗

arXiv2025

Form factors and phenomenology of $\boldsymbol{B_{(s)}}$ and $\boldsymbol{D_{(s)}}$ semileptonic decays to $\boldsymbolη$ and $\boldsymbol{η^\prime}$

Motivated by more precise recent measurements of the $B \to η\ell ν$ and $D_{(s)} \to η^{(\prime)}\ell ν$ decays, we employ state-of-the-art parametrizations to describe the $B_{(s)}, D_{(s)} \to η^{(\prime)}$ form factors across the full $q^2$ range, fitting them to the latest light-cone sum rule results. Using these results, we compute the branching ratios for all relevant decays and compare them with experimental data, finding good agreement. Additionally, we examine the validity and precision of the extracted $η$-$η'$ mixing angle in the context of heavy meson decays. By combining our predictions for the $B, D, D_s \to η^{(\prime)}$ form factors with recent semileptonic decay measurements, we extract the CKM elements $|V_{ub}|$, $V_{cs}$, and $V_{cd}$. We also provide tests of the lepton flavour universality in the $B, D, D_s \to η^{(\prime)} \ell ν$ decays and present results for the forward-backwards asymmetries in these decays. Our findings indicate that the extracted values are becoming comparable in precision to those obtained from more conventional semileptonic decay analyses.

hep-ph↗

arXiv2025

Communicating Likelihoods with Normalising Flows

We present a machine-learning-based workflow to model an unbinned likelihood from its samples. A key advancement over existing approaches is the validation of the learned likelihood using rigorous statistical tests of the joint distribution, such as the Kolmogorov-Smirnov test of the joint distribution. Our method enables the reliable communication of experimental and phenomenological likelihoods for subsequent analyses. We demonstrate its effectiveness through three case studies in high-energy physics. To support broader adoption, we provide an open-source reference implementation, nabu.

hep-ph↗

arXiv2025

A Simple Parametrisation of the Pion Form Factor

We discuss a novel and simple parametrisation of the pion vector form factor that transparently connects spacelike and timelike regions of the momentum transfer $q^2$. Our parametrisation employs the framework of conformal mapping and respects the known analyticity properties of the form factor, accounting explicitly for the $ρ(770)$-meson pole. The parametrisation manifestly fulfils the normalisation condition at $q^2 = 0$ as well as further restrictions at the pion production threshold and in the limit $|q^2| \to \infty$. In contrast to the widely used Omnès parametrisation, our approach does not use the pion-pion scattering phase shift as input. We confront the parametrisation with experimental data from $πH$ scattering and $τ^- \to π^-π^0ν$ decay. We already find a good description of the data with only five free parameters, which include the pole mass and decay width of the $ρ(770)$.

hep-ph↗

arXiv2025

Dispersive Analysis of $B\to K^{(*)}$ and $B_s\to ϕ$ Form Factors

We perform the first simultaneous dispersive analysis of the $B\to K$, $B\to K^*$, and $B_s\to ϕ$ form factors. By means of an improved parametrization, we take into account the form factors' below-threshold branch cuts arising from on-shell $\bar{B}_s π^0$ and $\bar{B}_s π^0 π^0$ states, which so far have been ignored in the literature. In this way, we eliminate a source of hard-to-quantify systematic uncertainties. We provide machine-readable files to obtain the full set of the $\bar{B}\to \bar{K}^{(*)}$ and $\bar{B}_s\to ϕ$ form factors in and beyond the entire semileptonic phase space.

hep-ph↗

arXiv2024

Constraining $|V_{cs}|$ and physics beyond the Standard Model from exclusive (semi)leptonic charm decays

We study the available data on exclusive leptonic and semileptonic $c\to s\ell^+ν$ decays within the Standard Model and beyond. Our analysis accounts for theory correlations between the relevant hadronic matrix elements through application of dispersive bounds. We find that, within a global analysis, the dispersive bounds are generally well respected and only mildly affect the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $|V_{cs}|$. Assuming Standard Model dynamics, we obtain $$ |V_{cs}| = 0.957 \pm 0.003\,, $$ which is compatible with the HFLAV/PDG reference value $|V_{cs}| = 0.975 \pm 0.006$ at the $2.7\,σ$ level. Our findings lead to significant deficits in the second-row and second-column unitarity relations of the CKM matrix. Allowing for beyond the SM contributions in the Weak Effective Theory, we find very strong constraints on potential (pseudo)scalar and tensor effects. However, the data still permits sizeable CP-violating right-handed currents.

hep-ph↗

arXiv2024

Constructing model-agnostic likelihoods, a method for the reinterpretation of particle physics results

Experimental High Energy Physics has entered an era of precision measurements. However, measurements of many of the accessible processes assume that the final states' underlying kinematic distribution is the same as the Standard Model prediction. This assumption introduces an implicit model-dependency into the measurement, rendering the reinterpretation of the experimental analysis complicated without reanalysing the underlying data. We present a novel reweighting method in order to perform reinterpretation of particle physics measurements. It makes use of reweighting the Standard Model templates according to kinematic signal distributions of alternative theoretical models, prior to performing the statistical analysis. The generality of this method allows us to perform statistical inference in the space of theoretical parameters, assuming different kinematic distributions, according to a beyond Standard Model prediction. We implement our method as an extension to the pyhf software and interface it with the EOS software, which allows us to perform flavor physics phenomenology studies. Furthermore, we argue that, beyond the pyhf or HistFactory likelihood specification, only minimal information is necessary to make a likelihood model-agnostic and hence easily reinterpretable. We showcase that publishing such likelihoods is crucial for a full exploitation of experimental results.

hep-ph↗

arXiv2024

Maximising the physics potential of $B^\pm\toπ^\pmμ^+μ^-$ decays

We present a method that maximises the experimental sensitivity to new physics contributions in $B^\pm\toπ^\pmμ^+μ^-$ decays. This method relies on performing an unbinned maximum likelihood fit to both the measured dimuon $q^2$ distribution of $B^\pm\toπ^\pmμ^+μ^-$ decays, and theory calculations at spacelike $q^2$, where QCD predictions are most reliable. We exploit the known analytic properties of the decay amplitude and employ a dispersion relation to describe the non-local hadronic contributions across spacelike and timelike $q^2$ regions. The fit stability and the sensitivity to new physics couplings and new sources of $CP$-violation are studied for current and future data-taking scenarios, with the LHCb experiment as an example. The proposed method offers a precise and reliable way to search for new physics in these decays.

hep-ph↗

arXiv2024

Analysis of the $ψ(3770)$ resonance in line with unitarity and analyticity constraints

We study the inclusive and exclusive cross sections of $e^+e^-\to \text{hadrons}$ for center-of-mass energies between 3.70 GeV and 3.83 GeV to infer the mass, width, and couplings of the $ψ(3770)$ resonance. By using a coupled-channel K-matrix approach, we setup our analysis to respect unitarity and the analyticity properties of the underlying scattering amplitudes. We fit several models to the full dataset and identify our nominal results through a statistical model comparison. We find that, accounting for the interplay between the $ψ(2S)$ and the $ψ(3770)$, no further pole is required to describe the $ψ(3770)$ line shape. In particular we derive from the pole location $M_{ψ(3770)} = 3778.8 \pm 0.3$ MeV and $Γ_{ψ(3770)} = 25.0 \pm 0.5$ MeV. Moreover, we find the decay to $D^+D^-$ and $D^0\bar{D}^0$ to be consistent with isospin symmetry and derive an upper bound on the branching ratio $\mathcal{B}(ψ(3770) \to \textrm{non-}D\bar{D}) < 6\%$ at $90\%$ probability.

hep-ph↗

arXiv2024

Computing Tools for Effective Field Theories

In recent years, theoretical and phenomenological studies with effective field theories have become a trending and prolific line of research in the field of high-energy physics. In order to discuss present and future prospects concerning automated tools in this field, the SMEFT-Tools 2022 workshop was held at the University of Zurich from 14th-16th September 2022. The current document collects and summarizes the content of this workshop.

hep-ph↗

arXiv2024

Prospects for searches of $b \to s ν\barν$ decays at FCC-ee

We investigate the physics reach and potential for the study of various decays involving a $b \to s ν\barν$ transition at the Future Circular Collider running electron-positron collisions at the $Z$-pole (FCC-ee). Signal and background candidates, which involve inclusive $Z$ contributions from $b\bar{b}$, $c\bar{c}$ and $uds$ final states, are simulated for a proposed multi-purpose detector. Signal candidates are selected using two Boosted Decision Tree algorithms. We determine expected relative sensitivities of $0.53\%$, $1.20\%$, $3.37\%$ and $9.86\%$ for the branching fractions of the $B^{0} \to K^{*0} ν\barν$, $B^{0}_{s} \to ϕν\barν$, $B^{0} \to K^{0}_{S} ν\barν$ and $Λ_{b}^{0} \to Λ^{0} ν\barν$ decays, respectively. In addition, we investigate the impact of detector design choices related to particle-identification and vertex resolution. The phenomenological impact of such measurements on the extraction of Standard Model and new physics parameters is also studied.

hep-ex↗

arXiv2023

Testable Likelihoods for Beyond-the-Standard Model Fits

Studying potential BSM effects at the precision frontier requires accurate transfer of information from low-energy measurements to high-energy BSM models. We propose to use normalising flows to construct likelihood functions that achieve this transfer. Likelihood functions constructed in this way provide the means to generate additional samples and admit a ``trivial'' goodness-of-fit test in form of a $χ^2$ test statistic. Here, we study a particular form of normalising flow, apply it to a multi-modal and non-Gaussian example, and quantify the accuracy of the likelihood function and its test statistic.

hep-ph↗

arXiv2023

Toward a complete description of $b \to u \ell^- \barν$ decays within the Weak Effective Theory

We fit the available data on exclusive semileptonic $b\to u\ell^-\barν$ decays within the Standard Model and in the Weak Effective Theory. Assuming Standard Model dynamics, we find $|V_{ub}| = 3.59^{+0.13}_{-0.12} \times 10^{-3}$. Lifting this assumption, we obtain stringent constraints on the coefficients of the $ub\ellν$ sector of the Weak Effective Theory. Performing a Bayesian model comparison, we find that a beyond the Standard Model interpretation is favoured over a Standard Model interpretation of the available data. We provide a Gaussian mixture model that enables the efficient use of our fit results in subsequent analyses beyond the Standard Model, within and beyond the framework of the Standard Model Effective Field Theory.

hep-ph↗

arXiv2023

Dispersive analysis of $Λ_b \to Λ(1520)$ local form factors

We perform an analysis of $Λ_b\toΛ(1520)$ local form factors. We use dispersive techniques to provide a model-independent parametrisation of the form factors that can be used in the whole kinematic region. We use lattice QCD data to constrain the free parameters in the form factors expansion, which is further constrained by endpoint relations, dispersive bounds, and SCET relations. We analyse different scenarios, where we expand the form factors up to different orders, and their viability. Finally, we use our results to obtain predictions for some observables in $Λ_b\toΛ(1520)\ell^+\ell^-$ decays, as the differential branching ratio, the forward-backwards lepton asymmetry and the branching ratio of $Λ_b\toΛ(1520)γ$. Finally, we provide a python notebook based on the software EOS to reproduce our result.

hep-ph↗

arXiv2022

Improved Theory Predictions and Global Analysis of Exclusive $\boldsymbol{b\to sμ^+μ^-}$ Processes

We provide improved Standard Model theory predictions for the exclusive rare semimuonic processes $B\to K^{(*)}μ^+μ^-$ and $B_s\toϕμ^+μ^-$. Our results are based on a novel parametrization of the non-local form factors, which manifestly respects a recently developed dispersive bound. We critically compare our predictions to those obtained in the framework of QCD factorization. Our predictions provide, for the first time, parametric estimates of the systematic uncertainties due to non-local contributions. Comparing our predictions within the Standard Model to available experimental data, we find a large tension for $B\to Kμ^+μ^-$. A simple model-independent analysis of potential effects beyond the Standard Model yields results compatible with other approaches, albeit with larger uncertainties for the $B\to K^*μ^+μ^-$ and $B_s\to ϕμ^+μ^-$ decays. Our approach yields systematically improvable predictions, and we look forward to its application in further analyses beyond the Standard Model.

hep-ph↗

arXiv2022

EOS -- A Software for Flavor Physics Phenomenology

I present EOS, an open-source software dedicated to a variety of tasks in the processing of flavor physics observables. EOS is written in C++ and offers both a C++ and a Python interface. It is developed for three main tasks, the production of theoretical predictions for flavor physics observables; the inference of theoretical parameters from an extensible database of likelihoods; and the production of Monte Carlo samples of flavor processes for sensitivity studies.

hep-ph↗