Forward-backward asymmetry in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>B</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:msup><mml:mrow><mml:mi>D</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msup><mml:mo>ℓ</mml:mo><mml:mi>ν</mml:mi></mml:mrow></mml:math>: One more hint for scalar leptoquarks?
Alexandre Carvunis, Shireen Gangal, Andreas Crivellin, Diego Guadagnoli
Abstract
Experimental data have provided intriguing hints for the violation of lepton flavor universality (LFU), including $B\ensuremath{\rightarrow}{D}^{(*)}\ensuremath{\tau}\ensuremath{\nu}/B\ensuremath{\rightarrow}{D}^{(*)}\ensuremath{\ell}\ensuremath{\nu}$, the anomalous magnetic moment of the muon and $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ with a significance of $>3\ensuremath{\sigma}$, $>4\ensuremath{\sigma}$, and $>5\ensuremath{\sigma}$, respectively. Furthermore, in a recent reanalysis of 2018 Belle data, it was found that the forward-backward asymmetry ($\mathrm{\ensuremath{\Delta}}{A}_{\mathrm{FB}}$) of $B\ensuremath{\rightarrow}{D}^{*}\ensuremath{\mu}\overline{\ensuremath{\nu}}$ vs $B\ensuremath{\rightarrow}{D}^{*}e\overline{\ensuremath{\nu}}$ disagrees with the SM prediction by $\ensuremath{\approx}4\ensuremath{\sigma}$, providing an additional sign of LFU violation. We show that a tensor operator is necessary to significantly improve the agreement with data in $\mathrm{\ensuremath{\Delta}}{A}_{\mathrm{FB}}$ while respecting the bounds from other $b\ensuremath{\rightarrow}c\ensuremath{\ell}\ensuremath{\nu}$ observables. Importantly, this tensor operator can only be induced (at tree-level within renormalizable models) by a scalar leptoquark. Furthermore, among the two possible representations, the $SU(2{)}_{L}$-singlet ${S}_{1}$ and the doublet ${S}_{2}$, which can interestingly both also account for the anomalous magnetic moment of the muon, only ${S}_{1}$ can provide a good fit. Even though the constraints from (differences of) other angular observables prefer a smaller value of $\mathrm{\ensuremath{\Delta}}{A}_{\mathrm{FB}}$ than the current central one, this scenario is significantly preferred (nearly $4\ensuremath{\sigma}$) over the Standard Model hypothesis, and is compatible with constraints such as $B\ensuremath{\rightarrow}{K}^{*}\ensuremath{\nu}\ensuremath{\nu}$ and electroweak precision bounds. Therefore, if the $\mathrm{\ensuremath{\Delta}}{A}_{\mathrm{FB}}$ anomaly is confirmed, it would provide circumstantial evidence for scalar leptoquarks and pave the way for a natural connection with all other anomalies pointing toward LFU violation.