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arXiv 1908.02328hep-phhep-exnucl-th
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从$b\to c$半轻子衰变的带电轻子能量分布进一步检验轻子味普适性：$Λ_b\to Λ_c \ell \barν_\ell$事例

Further tests of lepton flavour universality from the charged lepton energy distribution in $b\to c$ semileptonic decays: The case of $Λ_b\to Λ_c \ell \barν_\ell$

Neus Penalva, Eliecer Hernández, Juan Nieves

备注：
We show how NP scenarios predicting fully compatible ${\cal R}_D$, ${\cal R}_{D^*}$ and ${\cal R}_{Λ_c}$ ratios give rise to results for an observable, extracted from the charged lepton energy distribution in the $Λ_b \to Λ_c \ell \barν_\ell$ decay, which are separated by 5$σ$. Version accepted for publication in Phys. Rev. D
发表信息：
Phys. Rev. D 100, 113007 (2019)
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摘要：

在一个对任何$H\to H'\ell^-\barν_\ell$半轻子衰变都有效的一般框架下，我们分析了$d^2Γ/(dωd\cosθ_\ell)$和$d^2Γ/ (dωdE_\ell)$分布，其中$ω$是强子四速度的乘积，$θ_\ell$是在$W^-$质心系中带电轻子和末态强子的三动量所成的角，$E_\ell$是衰变强子静止系中带电轻子的能量.在标准模型（SM）中，$ d^2Γ/(dωdE_\ell)\propto \left(c_0(ω)+c_1(ω)E_\ell/M+c_2(ω)E^2_\ell/M^2\right)$，$M$是强子的初始质量。我们发现$c_2(ω)$与轻子味无关，因此它是寻找轻子味普适性（LFU）破坏的理想候选者。我们还发现了$a_2(ω)$结构功能之间的相关性，这决定了$d^2Γ/(dωd\cosθ_\ell)$和$c_2(ω)$的$(\cosθ_\ell)^2$依赖性。除了微不足道的运动学和质量因子，$a_2(ω)/c_2(ω)$的比率是一个普适函数，可以在任何半轻子衰变中测量，不仅涉及$b\to c$跃迁。这两个SM预测可以被用来作为新的测试在目前的搜索签名的LFU违反。我们还推广的形式主义，以考虑一些新的物理（NP）条款。最后，为了说明我们的研究结果，我们将我们的一般框架的$Λ_b\to Λ_c \ell \barν_\ell$衰变。我们表明，$τ$衰变的$c_2$（或$a_2$）的测量不仅是NP可能存在的直接测量，但它也将允许区分NP适合$b\to cτ\barν_τ$异常的介子部门，否则给出相同的总和差分$dΓ/dω$宽度。

英文摘要：

In a general framework, valid for any $H\to H'\ell^-\barν_\ell$ semileptonic decay, we analyze the $d^2Γ/(dωd\cosθ_\ell)$ and $d^2Γ/ (dωdE_\ell)$ distributions, with $ω$ being the product of the hadron four-velocities, $θ_\ell$ the angle made by the three-momenta of the charged lepton and the final hadron in the $W^-$ center of mass frame and $E_\ell$ the charged lepton energy in the decaying hadron rest frame. Within the Standard Model (SM), $ d^2Γ/(dωdE_\ell)\propto \left(c_0(ω)+c_1(ω)E_\ell/M+c_2(ω)E^2_\ell/M^2\right)$, with $M$ the initial hadron mass. We find that $c_2(ω)$ is independent of the lepton flavor and thus it is an ideal candidate to look for lepton flavor universality (LFU) violations. We also find a correlation between the $a_2(ω)$ structure function, that governs the $(\cosθ_\ell)^2$ dependence of $d^2Γ/(dωd\cosθ_\ell)$, and $c_2(ω)$. Apart from trivial kinematical and mass factors, the ratio of $a_2(ω)/c_2(ω)$ is a universal function that can be measured in any semileptonic decay, involving not only $b\to c$ transitions. These two SM predictions can be used as new tests in the present search for signatures of LFU violations. We also generalize the formalism to account for some new physics (NP) terms. Finally, in order to illustrate our findings, we apply our general framework to the $Λ_b\to Λ_c \ell \barν_\ell$ decay. We show that a measurement of $c_2$ (or $a_2$) for $τ$ decay would not only be a direct measurement of the possible existence of NP, but it would also allow to distinguish from NP fits to $b\to cτ\barν_τ$ anomalies in the meson sector, that otherwise give the same total and differential $dΓ/dω$ widths.

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