arXiv 2205.152122022-05hep-phhep-ex

解开$b\to s\ell^+\ell^-$跃迁中轻子味普适性和轻子味普适性破坏效应

Disentangling Lepton Flavour Universal and Lepton Flavour Universality Violating Effects in $b\to s\ell^+\ell^-$ Transitions

Marcel Algueró, Bernat Capdevila, Andreas Crivellin, Joaquim Matias

备注：: Version accepted for publication, footnote added. 9 pages, 5 figures
DOI：: 10.1103/PhysRevD.105.113007

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摘要：

在这封信中，我们提出了一个策略，辨别新的物理在威尔逊系数${\cal C}_{9μ}$是占主导地位的轻子味普适性违反或如果它包含一个相当大的轻子味普适组件（${\cal C}_9^{\rm U}$）。区分这两种情况，其中相关场景的模型独立拟合表现出类似的拉w.r.t.标准模型，是至关重要的，以促进我们的理解$B$异常。我们首先确定了这两种情况下的简并的起源，并指出关键的可观测量，可以打破它。特别是，虽然到目前为止，测试轻子味普适性的可观测量表现出类似的依赖于所有相关的威尔逊系数，即将测量的$Q_5=P_{5}^{\primeμ}-P_{5}^{\prime e}$是特别敏感的${\cal C}_{9μ}-{\cal C}_{9e}$。事实上，如果发现$Q_5$很小（即接近它的标准模型值），这将意味着一个小的${\cal C}_{9μ}-{\cal C}_{9e}$，但一个相当大的${\cal C}_{9}^{\rm U}$，给定的全局拟合的偏好，一个大的负面新物理贡献在${\cal C}_{9μ}$。我们讨论了可能的起源${\cal C}_9^{\rm U}$，特别是它如何可能起源于新的物理学。在这里，甚至可以将$b\to s \ell^+\ell^-$与$R_{D^{(*)}}$联系起来的一个有希望的方案是通过离壳光子企鹅图从tau环生成${\cal C}_9^{\rm U}$。该设置预测$B_s\toτ^+τ^-$和$B\to K^{(*)}τ^+τ^-$的分支比位于LHCb、CMS和Belle II的范围内。或者，在这些tau过程的非观测的情况下，我们表明，最自然的可能性，以产生${\cal C}_9^{\rm U}$是一个$Z^\prime$部分轻子味通用耦合。

英文摘要：

In this letter we propose a strategy for discerning if new physics in the Wilson coefficient ${\cal C}_{9μ}$ is dominantly lepton flavour universality violating or if it contains a sizable lepton flavour universal component (${\cal C}_9^{\rm U}$). Distinguishing among these two cases, for which the model independent fit of the related scenarios exhibits similar pulls w.r.t. the Standard Model, is crucial to advance our understanding of the $B$ anomalies. We first identify the origin of the degeneracy of these two cases and point out the key observables that can break it. In particular, while the observables measured so far that test lepton flavour universality exhibit similar dependencies to all the relevant Wilson coefficients, the forthcoming measurement of $Q_5=P_{5}^{\primeμ}-P_{5}^{\prime e}$ is particularly sensitive to ${\cal C}_{9μ}-{\cal C}_{9e}$. In fact, if $Q_5$ were found to be small (i.e. close to its Standard Model value), this would imply a small ${\cal C}_{9μ}-{\cal C}_{9e}$ but a sizable ${\cal C}_{9}^{\rm U}$, given the preference of global fits for a large negative new physics contribution in ${\cal C}_{9μ}$. We discuss the possible origins of ${\cal C}_9^{\rm U}$, in particular how it could originate from new physics. Here, a promising scenario, that could even link $b\to s \ell^+\ell^-$ to $R_{D^{(*)}}$, is the one in which ${\cal C}_9^{\rm U}$ is generated from a tau loop via an off-shell photon penguin diagram. This setup predicts the branching ratios of $B_s\toτ^+τ^-$ and $B\to K^{(*)}τ^+τ^-$ to lie within the reach of LHCb, CMS and Belle II. Alternatively, in case of a non-observation of these tauonic processes, we show that the most natural possibility to generate ${\cal C}_9^{\rm U}$ is a $Z^\prime$ with partially lepton flavour universal couplings.

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