四代关联：Bq0→μ+ μ-与KL→π0νν̅ 衰变的新探索

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本文主要探讨了在高能物理领域，特别是在大型强子对撞机（Large Hadron Collider, LHC）的第二运行期（LHC Run 2）期间，关于四代粒子模型下B meson（Bq0）衰变与K介子（KL）衰变之间可能的关联性研究。具体关注的是两个关键衰变模式：Bq0→μ+ μ-（μ子对）和KL→π0ννbar（π介子和中微子对）。首先，实验结果显示Bq0→μ+ μ-模式虽然最终被观察到，但其发生率低于标准模型（Standard Model, SM）预测值的1.2σ，这表明存在一定的理论偏差或可能暗示着新物理的存在。Bd0→μ+ μ-这一更罕见的衰变则表现出不同寻常的行为，其中心值接近SM期望值的4倍，但仅具有2.2σ的显著性，这意味着虽然存在增强的趋势，但其统计意义还不足以确认是新物理效应。同时，CP违反相位ϕs的测量达到了SM的敏感程度，这是对基础粒子物理的重要验证。随着LHC的进一步优化，研究者们不仅关注Bd0→μ+ μ-的后续分析，还同步推进了对KL→π0ννbar和K+→π+ννbar这两种衰变的研究。这些衰变的测量对于理解基本粒子的性质以及潜在的四代粒子模型具有重要意义。作者们的研究结果表明，即使Bd0→μ+ μ-的强度有所改变，KL→π0ννbar的衰变仍然有可能增强，并且这种关联可能是探索新物理现象的关键线索。他们的工作旨在通过比较这些衰变模式，寻找可能的四代粒子存在的证据或者解释现有的数据离SM预测的偏离。这篇文章深入探讨了高能物理实验中的细致观测和理论解读，特别是在多世代粒子模型框架下的Bq0和KL衰变现象，为未来粒子物理学的研究提供了新的思考角度和实验依据。同时，它也提醒我们，即使在标准模型内，也需要持续的精确测量来排除或验证潜在的新物理效应。

Physics Letters B 751 (2015) 458–463

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Correlating B

→μ

−

and K

→π

ν decays with four generations

Wei-Shu Hou

, Masaya Kohda

a,b

, Fanrong Xu

a,c,∗

Department of Physics, National Taiwan University, Taipei 10617, Taiwan

Department of Physics, Chung-Yuan Christian University, Chung-Li 32023, Taiwan

Department of Physics, Jinan University, Guangzhou 510632, China

a r t i c l e i n f o a b s t r a c t

Article history:

Received

2 June 2015

Received

in revised form 22 September

2015

Accepted

27 October 2015

Available

online 2 November 2015

Editor: J.

Hisano

The B

→ μ

−

mode has ﬁnally been observed, albeit at rate 1.2σ below Standard Model (SM) value,

while the rarer B

→μ

−

decay has central value close to 4 times SM expectation but with only 2.2σ

signiﬁcance. The measurement of CP violating phase φ

has ﬁnally reached SM sensitivity. Concurrent

with improved measurements at LHC Run 2, K

→ π

ν and K

→ π

ν decays are being pursued in

a similar time frame. We ﬁnd, whether B

→ μ

−

is enhanced or not, K

→ π

ν can be enhanced

up to the Grossman–Nir bound in the fourth generation model, correlated with some suppression of

→μ

−

, and with φ

remaining small.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

The 7-and-8 TeV run (Run 1) of the LHC has been a great suc-

cess,

but no New Physics (NP) has emerged. The hint of NP in the

forward–backward asymmetry of B → K

∗



−

decay [1] from the

B factories was eliminated by LHCb [2] early on. The mild hint

at the Tevatron [3,4] for large CP violating (CPV) phase φ

in B

mixing was also swiftly eliminated by LHCb [5,6], vanquishing the

suggested possible correlation [7] with large direct CPV difference

A

K π

≡ A(B

→ K

) − A(B

→ K

−

) [8]. Finally, the hot

pursuit for B

→ μ

−

at the Tevatron culminated in the re-

cent

observation by the LHCb [9] and CMS [10] experiments, albeit

again consistent with the Standard Model (SM).

The

combined LHC result for B

→μ

−

is [11],

B(B

→μ

−

) =(2.8

+0.7

−0.6

) × 10

−9

, (1)

B(B

→μ

−

) =(3.9

+1.6

−1.4

) × 10

−10

. (2)

At 6.2σ , the B

→μ

−

mode is established, but SM expectation

is 7.6σ . The B

→ μ

−

mode deviates from SM expectation of

(1.06 ±0.09) ×10

−10

[12] by 2.2σ , with central value more than 3

times

the SM value. Thus, B

→ μ

−

should be keenly followed

at the up and coming LHC Run 2 (13 and 14 TeV).

The

1fb

−1

LHCb update for φ

[13] is:

Corresponding author.

E-mail

address: frxu@hep1.phys.ntu.edu.tw (F. Xu).

=0.01 ±0.07 ±0.01 (1fb

−1

, LHCb), (3)

which combines two results opposite in sign,

=0.07 ±0.09 ±0.01 (1fb

−1

J/ψ K

K , LHCb), (4)

=−0.14

+0.17

−0.16

±0.01 (1fb

−1

J/ψ ππ, LHCb). (5)

Eq. (3) dominates the Heavy Flavor Averaging Group (HFAG) com-

bination [14] of

all experiments,

=0.00 ±0.07 (PDG2014), (6)

adopted by the Particle Data Group (PDG) [15], and is in good

agreement with SM value of φ

−0.04. A preliminary result [16]

CMS,

=−0.03 ±0.11 ±0.03 (20 fb

−1

, CMS), (7)

is not included in PDG2014, but is fully consistent.

Also

not making PDG2014 is the 3fb

−1

update by LHCb for

→ J /ψ π

−

with full Run 1 data [17],

=0.070 ±0.068 ±0.008 (3fb

−1

J/ψ ππ, LHCb), (8)

with sign change from Eq. (5), becoming same sign with Eq. (4).

Because the analysis is done simultaneously with 

measure-

ment,

the B

→ J /ψ K

K mode took much longer. Intriguingly, it

also switched sign [18],

=−0.058 ±0.049 ±0.006 (3fb

−1

J/ψ φ, LHCb), (9)

and the combined 3fb

−1

result is,

http://dx.doi.org/10.1016/j.physletb.2015.10.080

0370-2693/

SCOAP

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