长寿Stau、中微子暗物质与右 Slepton 谱系

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"寿命长的stau，中微子暗物质和右链子谱" 这篇研究论文探讨了在超对称标准模型（SUSY MSSM）的扩展版本中，引入右手性中微子（right-chiral neutrinos）所带来的理论可能性。这种模型可能导致中微子成为最轻的超对称粒子（LSP），并可能作为非热暗物质的候选者。尤其当中微子仅具有狄拉克质量时，这一假设更加引人关注。在这种场景下，如果最轻的MSSM粒子是stau（一种超对称τ轻子），那么大型强子对撞机（LHC）上的SUSY信号会表现为长寿命的带电轨迹，它们在内部跟踪器和μon腔之间会有时间延迟，这有助于与背景信号区分。文章着重研究了在这种特定情况下，如何确定第一、第二代中性ino（中性超对称粒子）与右链子（right-sleptons）之间的质量关系。作者采用了之前研究中发展的中性ino重构技术，并结合smuon（超对称μ子，或称选择子）衰变中的变量M T 2的终点来实现这一目标。结果显示，这种方法可以用来确定smuons（选择子）和中性ino的质量层次，精度达到5-10%，并且能探测到高达1 TeV的质量范围。这篇论文发表于JHEP（Journal of High Energy Physics）2018年9月刊，由Springer出版，SISSA（国际高等科学研究所）授权。论文经过多次修订，最终于2018年9月接受并发表。作者包括Shankha Banerjee、Geneviève Bélanger、Avirup Ghosh和Biswarup Mukhopadhyaya，分别来自英国达勒姆大学的粒子物理现象学研究所、法国格勒诺布尔阿尔卑斯大学的理论物理学实验室以及印度哈里什-查克拉瓦蒂研究所以及区域加速器基础粒子物理中心。通过这项工作，研究者为理解超对称性、中微子物理以及暗物质的性质提供了新的洞察，特别是在LHC实验背景下，对于发现和研究长寿命stau以及相关超对称粒子提供了重要的理论框架和技术手段。

JHEP09(2018)143

addition of the RH neutrino superﬁeld entails the following additional soft terms in the

MSSM Lagrangian:

− L

soft

⊃ m

˜ν

|˜ν

+ (y

L ˜ν

+ h.c.), (2.3)

where A

plays a role in the left-right mixing in the sneutrino sector. The sneutrino mass

matrix is deﬁned as

˜ν

−y

v sin β (µ cot β − A

)

−y

†

v sin β (µ

∗

cot β − A

∗

) m

˜ν

(2.4)

where m

˜ν

and m

˜ν

are respectively the soft scalar masses for the left- and right-chiral

sneutrinos. One then ﬁnds that the left-right sneutrino mixing angle,

Θ, can be written as

tan 2

Θ =

2 y

v sin β |µ cot β − A

˜ν

− m

˜ν

, (2.5)

thus implying that the admixture of SU(2) doublets in the ˜ν

-dominated mass eigenstates

are limited by the neutrino Yukawa couplings.

As mentioned in the introduction, the present study focuses on scenarios with the

lighter stau (˜τ

) as the NLSP. Such a stau, upon production at the LHC, will eventually

decay into the right sneutrino LSP through modes such as ˜τ

→ W

(∗)

˜ν

, driven, as

expected, by the neutrino Yukawa coupling. For m

˜τ

> m

˜ν

+ m

, the width of the

above two-body decay is given by

˜τ

' Γ

˜τ

→ W ˜ν

32 π

( ˜τ

)

˜τ

1 −

2(m

˜ν

+ m

)

˜τ

˜ν

− m

)

˜τ

3/2

, (2.6)

where g is the SU(2)

gauge coupling, m

the W -boson mass and U

( ˜τ

)

parametrises the

left-right mixing of the staus. Assuming A

is of the same order as the other trilinear cou-

plings, the ˜τ

s are fairly long-lived with a typical life-time of O(1) sec for all the benchmark

points that we will consider in section 4. Thus, the decay length of ˜τ

is large compared to

the typical collider scale. All processes at the LHC, which are initiated with the production

of superparticles, will ultimately lead to the production of a pair of quasi-stable ˜τ

s which

will travel all the way up to the muon-chamber. In addition to making the NLSP stable at

the collider scale, the smallness of y

also implies an out-of-equilibrium decay of the NLSP

in the early universe into the ˜ν

LSP. The contribution to the ˜ν

relic density has two

components. The ﬁrst of which arises from the decay of the stau after it freezes out, and

can be estimated as

Ω

F O

˜ν

˜τ

Ω

˜τ

, (2.7)

where Ω

˜τ

is the (thermal) relic density of the quasi- stable NLSP when it freezes

out. This contribution can be calculated via Ω

˜τ

using a standard package such as

microOMEGAs [51].

– 4 –

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