arXiv subjects

Camille Normand

Publications and source records attributed to Camille Normand.

3 recordsLinked to original sources

arXiv2023

Insights on the current semi-leptonic $B$-decay discrepancies -- and how $B_s \to μ^+ μ^- γ$ can help

$B_s \to μ^+ μ^- γ$, measured at high $q^2$ as a partially reconstructed decay, can probe the origin of the existing discrepancies in semi-leptonic $b \to s$ and $b \to c$ decays. We perform a complete study of this possibility. We start by reassessing the alleged discrepancies, with a focus on a unified EFT description. Using the SMEFT, we find that the tauonic Wilson coefficient required by $R(D^{(*)})$ implies a universal muonic Wilson coefficient of precisely the size required by semi-muonic BR data and, separately, by semi-muonic angular analyses. We thus identify reference scenarios. Importantly, $B_s \to μ^+ μ^- γ$ offers a strategy to access them without being affected by the long-distance issues that hamper the prediction of semi-leptonic $B$ decays at low $q^2$. After quantifying to the best of our knowledge the $B_s \to μ^+ μ^- γ$ experimental over the long haul, we infer the $B_s \to μ^+ μ^- γ$ sensitivity to the couplings relevant to the anomalies. In the example of the real-$δC_{9,10}$ scenario, we find significances below 3$σ$. Such figure is to be compared with other single-observable sensitivities that one can expect from e.g. BR and angular data, whether at low or high $q^2$, and not affected by long-distance issues such as narrow resonances or intermediate charmed di-meson rescattering.

hep-ph↗

arXiv2023

From $D_s \to γ$ in lattice QCD to $B_s \to μμγ$ at high $q^2$

We use a recent lattice determination of the vector and axial $D_s \to γ$ form factors at high squared momentum transfer $q^2$ to infer their $B_s \to γ$ counterparts. To this end, we introduce a phenomenological approach summarized as follows. First, we describe the lattice data with different fit templates motivated by vector-meson dominance, that is expected to hold in the high-$q^2$ region considered. We identify reference fit ansaetze with one or two physical poles, that we validate against alternative templates. Then, the pole residues can be unambiguously related to the appropriate couplings involving the pseudoscalar, the vector mesons concerned, and the photon -- or tri-couplings -- and the latter can be expressed as sums over quark magnetic moments, weighed by their e.m. charges. This description obeys a well-defined heavy-quark scaling, that allows to parametrically scale up the form factors to the $B_s \to γ$ case. We discuss a number of cross-checks of the whole approach, whose validation rests ultimately in a first-principle determination, e.g. in lattice QCD. Finally, we use our obtained form factors to reassess the SM prediction of $\mathcal{B}(B_s \to μ^+ μ^- γ)$ in the range $\sqrt{q^2} \in [4.2, 5.0]$ GeV, where an experimental measurement is awaited.

hep-ph↗

arXiv2021

On the effective lifetime of $B_s \to μμγ$

We consider the $B^0_s \to μ^{+} μ^{-} γ$ effective lifetime, and the related CP-phase sensitive quantity $A_{ΔΓ_s}^{μμγ}$, as a way to obtain qualitatively new insights on the current $B$-decay discrepancies. Through a fit comparing pre- to post-Moriond-2021 data we identify a few theory benchmark scenarios addressing these discrepancies, and featuring large CP violation in addition. We then explore the possibility of telling apart these scenarios with $A_{ΔΓ_s}^{μμγ}$, once resonance-modeling and form-factor uncertainties are taken into account. We do so in both regions of low and high invariant di-lepton mass-squared $q^2$. For low $q^2$, we show how to shape the integration range in order to reduce the impact of the $ϕ$-resonance modelling on the $A_{ΔΓ_s}^{μμγ}$ prediction. For high $q^2$, we find that the corresponding pollution from broad-charmonium resonances has a surprisingly small effect on $A_{ΔΓ_s}^{μμγ}$. This is due to a number of cancellations, that can be traced back to the complete dominance of semi-leptonic operator contributions for high $q^2$ -- at variance with low $q^2$ -- and to $A_{ΔΓ_s}^{μμγ}$ behaving like a ratio-of-amplitudes observable. Our study suggests that $A_{ΔΓ_s}^{μμγ}$ is -- especially at high $q^2$ -- a potentially valuable probe of short-distance CP-violating effects in the very same Wilson coefficients that are associated to current $b \to s$ discrepancies. Its discriminating power, however, relies on progress in form-factor uncertainties. Interestingly, high $q^2$ is the region where $B^0_s \to μ^{+} μ^{-} γ$ is already being accessed experimentally, and the region where form factors are more accessible through non-perturbative QCD methods.

hep-ph↗