[论文解读] Delta I=1/2 Rule, epsilon'/epsilon and K -> pi nu nubar in Z'(Z) and G' Models with FCNC Quark Couplings

Andrzej J. Buras, Fulvia De Fazio+1 more|arXiv (Cornell University)|Apr 15, 2014

Particle physics theoretical and experimental studies参考文献 59被引用 12

一句话总结

本文提出，一种具有左手性FCNC耦合的重味破坏Z′规范玻色子，可解释标准模型在K→ππ衰变中对∆I = 1/2规则预测的长期缺失。通过在树图级别增强QCD泡利型算符Q6，同时保持新物理对ReA2的贡献微小，该模型在MZ′ ≈ 3–4 TeV时，同时满足εK、ε′/ε、∆MK和LHC数据约束，解释了ReA0中缺失的30%。

ABSTRACT

The experimental value for the isospin amplitude Re(A_2) in K->pi pi decays has been successfully explained within the Standard Model, both within large N approach to QCD and by QCD lattice calculations. On the other hand in both approaches the theoretical values of Re(A_0) are by at least 30% below the data so that the Delta I=1/2 rule in K->pi pi decays is not fully explained. While this deficit could be the result of present theoretical uncertainties in both approaches, it cannot be excluded that the missing piece in Re(A_0) comes from New Physics. We demonstrate that this deficit can be significantly softened by tree-level FCNC transitions mediated by a heavy colourless Z' gauge boson with particularly chosen flavour violating couplings assuring negligible NP contributions to Re(A_2) and significantly enhancing the contribution of the leading QCD penguin operator Q_6 to Re(A_0). A large fraction of the missing piece in the Delta I=1/2 rule can be explained in this manner for M_{Z'} in the reach of the LHC, while satisfying constraints from epsilon_K, epsilon'/epsilon, Delta M_K, LEP-II and the LHC. We identify quartic correlation between Z' contributions to Re(A_0), epsilon'/epsilon, epsilon_K and Delta M_K. We present correlations between epsilon'/epsilon and rare decays K->pi nu bar{nu} with and without the Delta I=1/2 rule constraint and generalize the whole analysis to Z' with colour (G') and Z with FCNC couplings. In the latter case no improvement on Re(A_0) can be achieved without destroying the agreement of the SM with the data on Re(A_2). Moreover, this scenario is very tightly constrained by epsilon'/epsilon. On the other hand in the context of the Delta I=1/2 rule G' is even more effective than Z': it provides the missing piece in Re(A_0) for M_{G'}=(3.5-4.0) TeV.

研究动机与目标

• 解决标准模型在K→ππ衰变中对ReA0预测的持续理论偏差，该偏差约为实验值的30%以下。
• 探讨通过来自重Z′或G′玻色子的树图级别FCNC实现的新物理，是否能解释∆I = 1/2振幅中缺失的强度。
• 确保新物理对ReA2和εK的贡献与实验数据一致，避免与间接CP破坏冲突。
• 识别Z′和G′模型中同时满足ε′/ε、K→πνν̄和∆MK约束的可行参数区域。
• 将分析推广至无色Z′和带色荷的G′模型，比较二者在解决∆I = 1/2规则问题中的可行性。

提出的方法

• 提出一种具有味破坏性左手耦合∆sdL(Z′)和近似普遍的右手耦合∆qqR(Z′)的Z′模型，树图级别破坏GIM机制。
• 使用跑动方程分析，将有效哈密顿量从Z′能标演化至QCD能标。
• 通过QCD泡利型算符（特别是Q6）计算对ReA0和ReA2的贡献，其受树图级别FCNC增强。
• 应用来自εK、∆MK、ε′/ε和稀有kaon衰变（K+→π+νν̄、KL→π0νν̄）的约束，限制参数空间。
• 在多种情形（A、B、带撇）下进行数值分析，涵盖不同的Z′耦合和质量标度。
• 将结果推广至具有色荷的G′模型，表明其在MG′ ≈ 3.5–4.0 TeV时在解决ReA0问题上更具优势。

实验结果

研究问题

• RQ1 通过重Z′玻色子介导的树图级别FCNC，能否在不破坏与ReA2或εK一致性的前提下，解释ReA0中缺失的30%？
• RQ2 满足ε′/ε、∆MK和LHC搜索约束的Z′耦合可行参数区域是什么？
• RQ3 存在微小右手性∆sdR(Z′)耦合时，该模型与∆MK和ReA0的一致性如何？
• RQ4 具有色荷的G′模型是否比Z′模型更有效地解决∆I = 1/2规则问题？
• RQ5 Z′对ReA0、ε′/ε、εK和∆MK的贡献之间存在何种关联？如何通过实验检验？

主要发现

• 当MZ′ ≈ 3–4 TeV且∆sdL(Z′) ≫ ∆sdR(Z′)时，Z′可解释ReA0中缺失的30%，同时保持新物理对ReA2的贡献可忽略。
• 只要右手耦合较小且精细调节，该模型即可满足εK、∆MK和LHC二胶子喷注搜索的约束。
• Z′对ReA0和ε′/ε的贡献之间存在强关联，需精确测量|Vub|、|Vcb|和⟨Q6⟩0以进行检验。
• G′模型比Z′模型更有效：MG′ ≈ 3.5–4.0 TeV可完全解决ReA0的缺失问题。
• 具有FCNC的Z玻色子无法在不破坏SM对ReA2一致性的前提下改善ReA0，且受ε′/ε严格约束。
• 该模型预测了ε′/ε、K+→π+νν̄和KL→π0νν̄之间的关联，可通过改进的标准模型输入和未来高精度测量进行检验。