Peccei-Quinn模型与IceCube中微子事件：早期衰变解析

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“Peccei-Quinn费米子的早期衰变和IceCube中微子事件” 在粒子物理学领域，Peccei-Quinn（PQ）机制是一个为解决强CP问题提出的理论框架。强CP问题指的是量子色动力学（QCD）中的电荷共轭和宇称（CP）对称性的违反程度为何比预期要小。PQ机制引入了一种新的对称性——Peccei-Quinn对称性，它通过一个被称为轴心场或轴子的假想粒子来解释CP对称性的恢复。轴子是一种轻量级、弱相互作用的粒子，可能参与宇宙早期的物理过程。 IceCube中微子天文台是位于南极冰层下的一个探测器，它在TeV到PeV能量范围内观测到高能中微子事件。这些中微子可能源于宇宙中的极端天体现象，如伽玛射线暴、活动星系核等。然而，另一种可能性是它们由早期宇宙中大量长寿命粒子的衰变产生，这种情景被称为“早期衰变场景”。 Yohei Ema和Takeo Moroi在文章中探讨了如何将Peccei-Quinn模型与这种早期衰变场景相结合。他们构建了一个包含大质量长寿命粒子的PQ模型，这种粒子在宇宙早期存在，然后逐渐衰变成包括中微子在内的各种粒子。他们通过计算粒子衰变产生的初始能量分布，进一步分析了这些粒子如何转化为现在的中微子通量，以此来解释IceCube观测到的现象。作者们指出，PQ模型中的长寿命粒子可以自然地适应早期衰变场景，因为它提供了一种机制，使得在宇宙早期形成的轴子或类似粒子能够以足够高的能量衰变，产生高能中微子。同时，这个模型还允许实现一个与观测到的宇宙历史相一致的背景，即这些长寿命粒子的存在不会破坏宇宙的大尺度结构形成和背景辐射的均匀性。此外，研究者们还讨论了这个模型如何与其他宇宙学观测结果，如大爆炸核合成（BBN）和宇宙微波背景（CMB）辐射的约束相协调。他们的工作不仅对理解高能中微子的来源有重要意义，还可能对轴子物理学和宇宙学的理解产生深远影响，尤其是在探索暗物质和宇宙暗能量等未解之谜方面。这项研究展示了Peccei-Quinn机制如何与粒子衰变和高能中微子观测相结合，提供了对宇宙早期物理过程的一种新视角，并可能为解决强CP问题以及理解宇宙的基本构成提供新的线索。

Physics Letters B 762 (2016) 353–361

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Early decay of Peccei–Quinn fermion and the IceCube neutrino events

Yohei Ema

∗

, Takeo Moroi

Department of Physics, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 133-0033, Japan

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

Article history:

Received

29 June 2016

Received

in revised form 21 September

2016

Accepted

21 September 2016

Available

online 28 September 2016

Editor: J.

Hisano

IceCube observed high-energy neutrino ﬂux in the energy region from TeV to PeV. The decay of a massive

long-lived particle in the early universe can be the origin of the IceCube neutrino events, which we call

an “early decay scenario.” In this paper, we construct a particle physics model that contains such a

massive long-lived particle based on the Peccei–Quinn model. We calculate the present neutrino ﬂux,

taking account of realistic initial energy distributions of particles produced by the decay of the massive

long-lived particle. We show that the early decay scenario naturally ﬁts into the Peccei–Quinn model,

and that the neutrino ﬂux observed by IceCube can be explained in such a framework. We also see

that, based on that model, a consistent cosmological history that explains the abundance of the massive

long-lived particle is realized.

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

1. Introduction

After the observations of high energy cosmic ray neutrino

events at the IceCube [1–5], the understanding of the origin of

such high energy neutrinos has become an important task in the

ﬁeld of astrophysics and particle cosmology. This is because, for

the energy range of O(10 TeV)  E

 O(1PeV), the observed ﬂux

is signiﬁcantly larger than that expected from the background, i.e.,

the atmospheric neutrinos. There have been many attempts to un-

derstand

the origin from astrophysical [6–23]

and particle physics

[26–41] points of view, although no scenario is considered to be

conclusive yet.

one of the possibilities, it was pointed out that a long-lived

particle, with its mass being much higher than the PeV scale and

its lifetime shorter than the present age of the universe, may be

the origin of ﬂux of high energy cosmic ray neutrinos [42].

(Here-

after,

such a scenario is called as the “early decay scenario.”) In

the early decay scenario, even though the neutrino produced by

the decay of the long-lived particle has much larger energy than

the PeV scale, which is the maximum energy scale of the de-

posited

energy observed by IceCube, the energy is red-shifted so

that the present energy can be O(10 TeV)  E

 O(1PeV). In

Refs. [42,44], it was discussed that the decay of a long-lived par-

ticle

with its mass of O(10

GeV) may well explain the IceCube

Corresponding author.

E-mail

address: ema@hep-th.phys.s.u-tokyo.ac.jp (Y. Ema).

For more on this respect, see, e.g. Refs. [24,25] and references therein.

See Ref. [43] as a similar scenario.

events, without specifying the particle-physics model for the long-

lived

particle.

considering the early decay scenario, one of the questions

is the origin of the mass scale of the long-lived particle. In par-

ticular,

the IceCube events may suggest the existence of a new

physics at such a scale, into which the long-lived particle is em-

bedded.

The suggested mass scale is, as we mentioned above,

∼ 10

GeV; one of the well-motivated new physics at such a scale

is the Peccei–Quinn (PQ) symmetry [45,46] as a solution to the

strong CP problem. The suggested scale of the PQ symmetry break-

ing

is 10

GeV  f

 10

GeV [see, e.g., Refs. [47–49]], which

well agrees with the mass scale required to realize the early de-

cay

scenario. Importantly, in hadronic axion models [50,51], the PQ

mechanism requires the existence of new colored fermions. Then,

if the model is embedded into grand uniﬁed theories (GUTs), there

exist GUT partners of the new colored fermions. Some of these

fermions (which we call PQ fermions) are stable if they do not

have any mixing with the standard model (SM) fermions. With

the mixing being suppressed enough, those fermions have long-

enough

lifetime required from the early decay scenario. As we will

see below, some of the PQ fermion may play the role of the long-

lived

particle whose decay explains the IceCube events.

this paper, we study the possibility of explaining the ﬂux

of cosmic ray high energy neutrinos observed by the IceCube us-

ing

the decay of the PQ fermion. We assume that a color- and

charge-neutral fermion (called X ) embedded into the PQ fermions

becomes long-lived, and that the decay of X becomes the dom-

inant

source of the cosmic ray neutrinos in the energy range of

O(10 TeV)  E

 O(1PeV). We concentrate on the case where

http://dx.doi.org/10.1016/j.physletb.2016.09.048

0370-2693/

SCOAP

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