3.5 keV X射线线：轻轴距解释的挑战

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"这篇文章是关于对3.5 keV X射线线的一种可能解释的探讨，即通过轴心轴（axino）理论。作者们在《Physics Letters B》上发表的研究详细分析了7 keV质量的轴心轴能否解释这个未识别的X射线线，该线可能源于星系团的暗物质衰变。他们提出了在现有模型中被忽视的一些问题，这些问题使得轴心轴作为解释这一现象的候选粒子变得不太可能。文章还提出了一种可能的解决方案，即预测存在轻亚稳的中性分子，它们的质量在0.1到10 GeV之间，寿命在10^3到10^4秒之间。" 这篇研究的主要内容包括以下几个知识点： 1. **3.5 keV X射线线**：这是一种在星系团X射线观测中发现的未知线谱，可能暗示着新的物理现象。尽管它的存在仍然需要进一步验证，但科学家们已经在探索可能的解释，其中包括暗物质的衰变。 2. **暗物质**：宇宙中大量存在但无法直接观测到的物质，它对星系旋转曲线和大尺度结构的形成有重要影响。暗物质的性质仍然是天体物理学的一个重大谜题，其可能的组成包括各种粒子，如轴心轴。 3. **轴心轴（Axino）**：轴心轴是一种超对称理论中的假设粒子，作为暗物质的潜在候选者。在本文中，研究人员探讨了7 keV质量的轴心轴是否能够解释3.5 keV的X射线线。 4. **模型独立性**：为了尽可能地减少模型依赖，研究者试图通过基本物理原理来探讨轴心轴的可能性，而不是基于特定的理论框架。 5. **障碍与挑战**：文章指出了几个在现有文献中被忽视的问题，这些问题使得轴心轴作为解释3.5 keV线的候选粒子显得困难。这些障碍可能涉及到轴心轴的性质、衰变机制以及与其他物理过程的相互作用。 6. **轻亚稳中性分子**：作为替代解释，研究者提出可能存在一种新的粒子，即轻亚稳的中性分子，它们可能具有适中的质量和较长的寿命，这使得它们能产生所观测到的X射线线。 7. **理论与实验结合**：研究者的工作强调了理论研究与实验观测的紧密结合，通过分析观测数据并提出理论预测，以期揭示宇宙的深层次秘密。 8. **科学交流与同行评审**：文章的发表和接收过程体现了科学研究的严谨性，包括提交、修订和接受的过程，以及由A.Ringwald编辑的同行评审，这些都是保证科研质量的重要环节。这篇研究揭示了在探索暗物质和新物理学领域中，理论与观测之间的复杂关系，以及如何通过提出和测试假设来推进科学理解。虽然轴心轴可能不是解释3.5 keV X射线线的最佳候选，但这种探索推动了我们对暗物质世界的认识，并可能启发未来的新发现。

Physics Letters B 750 (2015) 107–111

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Heavy concerns about the light axino explanation of the 3.5 keV

X-ray line

Stefano Colucci

, Herbi K. Dreiner

, Florian Staub

, Lorenzo Ubaldi

c,∗

Physikalisches Institut der Universität Bonn, Bethe Center for Theoretical Physics, Nußallee 12, 53115 Bonn, Germany

Theory Division, CERN, 1211 Geneva 23, Switzerland

Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel

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

Article history:

Received

27 July 2015

Received

in revised form 13 August 2015

Accepted

2 September 2015

Available

online 7 September 2015

Editor:

A. Ringwald

An unidentiﬁed 3.5 keV line from X-ray observations of galaxy clusters has been reported recently. Al-

though

still under scrutiny, decaying dark matter could be responsible for this signal. We investigate

whether an axino with a mass of 7 keV could explain the line, keeping the discussion as model inde-

pendent

as possible. We point out several obstacles, which were overlooked in the literature, and which

make the axino an unlikely candidate. The only viable scenario predicts a light metastable neutralino,

with a mass between 0.1 and 10 GeV and a lifetime between 10

−3

and 10

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

1. Introduction

There has been interest recently in an unidentiﬁed 3.5 keV line

in X-ray observations of galaxy clusters [1,2]. Despite its physi-

cal

origin being subject to debate [3–8], there is still some room

to speculate that decaying dark matter is responsible for this sig-

nal.

The ﬁrst obvious dark matter (DM) candidate in this context

is a 7 keV sterile neutrino [1], but many other alternatives have

been proposed. Some authors have pointed out that a decaying

axino could explain the line [9–12]. In this paper we examine care-

fully

the conditions under which a 7 keV axino would produce

the observed X-ray line. Due to the large number of parameters

at disposal in supersymmetric models, it is hard to exclude with

certainty the axino scenario. However we show that various con-

straints

leave almost no room available in the parameter space of

these models. Therefore we deem the axino an unlikely candidate

to explain the line. The authors of Ref. [13] also mentioned the ax-

ino

as an unnatural explanation of the X-ray line. In this work we

elaborate on the physical arguments that lead to such a conclusion.

There

are a few reasons why it is appealing to consider a model

with a light axino. First, introducing the axion multiplet (axion, ax-

ino,

saxion) in models of supersymmetry (SUSY) solves the strong

CP problem [14]. Second, the axion and the axino can both be

Corresponding author.

E-mail

addresses: colucci@th.physik.uni-bonn.de (S. Colucci),

dreiner@th.physik.uni-bonn.de (H.K. Dreiner), ﬂorian.staub@cern.ch (F. Staub),

ubaldi.physics@gmail.com (L. Ubaldi).

DM candidates. Third, if the axino has a mass in the keV range,

it is warm DM and it could help reconcile some small-scale struc-

ture

issues [15–17] of cold DM. In the context of R-parity violating

(RPV) SUSY [18], a light axino is unstable. Its lifetime is still longer

than the age of the universe, but it can decay into a neutrino and

a photon. It is this channel that would produce the 3.5 keV line.

RPV models also have the virtue of explaining the null-results for

SUSY searches at the LHC without introducing a little hierarchy

problem: the limits on the masses of the sparticles become much

weaker [19–23]. The axino could also produce an X-ray photon in

R-parity conserving SUSY. This is possible if the lightest neutralino

is very light or even massless. The axino can then decay into the

neutralino plus a photon [12]. We comment also on this possibility,

below.

This

letter is organized as follows. In Section 2, we review how

the axino abundance depends on the reheating temperature, in

Section 3 we discuss various constraints that the 3.5 keV line puts

on models with a decaying axino. We conclude in Section 4.

2. Axino relic density

Any supersymmetric model with an axino contains also an ax-

ion.

The latter, as an invisible axion, is always a good DM candi-

date,

while the former is suitable only if it is suﬃciently long lived.

We are here interested in a scenario where the axino constitutes

almost the entire DM budget of the universe. This is typically re-

alized

for low values of the axion decay constant ( f

∼10

GeV),

in which case the axion DM component is suppressed and can be

neglected. See e.g. Ref. [24] for a review on axion DM.

http://dx.doi.org/10.1016/j.physletb.2015.09.009

0370-2693/

SCOAP

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