FASER：LHC中的轴子探测与光子束流研究

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"FASER（ForwArd Search ExpeRiment）是大型强子对撞机（LHC）的一个实验项目，旨在利用ATLAS或CMS实验的远距离区域来寻找轻、弱相互作用的新粒子。研究者们在ATLAS交互点（IP）下游480米的未使用服务隧道内找到了一个理想位置。该位置对于发现如深色光子、深色希格斯玻色子和重中性轻子等新粒子具有潜力。在此研究中，他们探讨了FASER作为‘光子束流收集器’的能力，即新粒子不是在IP处产生，而是在更下游的探测器元件中通过碰撞生成。特别关注的是轴状颗粒（Axion-like Particles, ALPs），这些ALPs可以通过aγγ相互作用与标准模型耦合。" ALPs是理论中的一种假设粒子，它们在高能物理中扮演着潜在的角色，特别是在暗物质研究和解决量子色动力学（QCD）的CP问题中。当在IP处产生的TeV级光子与大约130米下游的TAXN中性粒子吸收器碰撞时，可能会通过Primakoff效应产生ALPs。之后，这些ALPs会在FASER探测器中衰变为两个光子。研究者们证明，FASER有可能发现质量范围在ma约为30至400 MeV且与光子耦合强度gaγγ在10^-6至10^-3 GeV^-1之间的ALPs。他们还详细讨论了ALP信号的特征以及实现这一发现所需的探测器要求。这项工作不仅展示了FASER实验的独特能力，也为我们理解宇宙中的暗物质和未知粒子提供了新的途径。通过这种“束流”功能，FASER能够探索标准模型之外的物理现象，为粒子物理学的研究开辟了新的方向。ALPs的潜在发现将对粒子物理学的标准模型提出挑战，并可能为暗物质的性质提供线索，这在粒子物理学和宇宙学中都是极其重要的问题。此外，FASER实验的低成本和小规模设计使其成为探索新物理现象的高效工具，尤其是在LHC的现有基础设施中找到未被充分利用的区域进行实验。这表明，即使在已有的大型实验设施中，仍然有创新和发现的空间，只要我们能够以新的视角和方法来利用它们。 FASER实验的轴状颗粒研究不仅扩展了我们对粒子物理学的理解，也强调了在现有科学装置中寻找新物理现象的重要性。通过深入研究这些轻弱相互作用粒子，科学家们可能能够揭示宇宙的基本结构，包括暗物质的性质，以及可能存在的超出标准模型的新物理理论。

production in rare meson decays, and the angular accep-

tance function for FASER are given in Appendices A, B,

and C, respectively.

II. PROPERTIES OF AXIONLIKE PARTICLES

We consider a low-energy effective theory in which an

ALP a couples to vector bosons through the dimension-5

interactions

int

¼ −

aBB

μν

−

aWW

μν

A;μν

; ð1Þ

where B

μν

and W

μν

are the Uð1Þ

and SUð2Þ

field strength

tensors, respectively, and g

aBB

and g

aWW

are the corre-

sponding coupling constants with dimension GeV

−1

.If

such interactions are generated by physics with coupling α

that was integrated out at some heavy scale f, one expects

aBB

aWW

∼

2πf

: ð2Þ

This is the case for axions [23–26], for example, and more

generally for pseudo-Goldstone bosons with nonvanishing

axial anomalies.

After electroweak symmetry breaking, the couplings

aBB

and g

aWW

induce couplings of the ALP to γγ , γZ,

ZZ, and W

−

. In this study, we focus on the γγ coupling

and neglect the others. The other couplings typically have a

small effect on our signal; for example, the subdominant

production process of ALPs from meson decays can be

enhanced by W

−

couplings [32]. More important are

their effects on other observables. For example, a non-

vanishing γZ coupling induces the exotic decay Z → aγ.

Although this process does not contribute significantly to

the ALP production rate in the far-forward region, it can

be searched for in high-p

experiments at the LHC. (See

Refs. [27,33] for some future projections.)

ALPs may also couple through dimension-5 operators to

gluons and fermions, as well as through dimension-6

couplings to the Higgs boson. For a recent review see,

e.g., Ref. [27]. In this study, we assume that the effects of

these other couplings on our signal processes are negli-

gible. This is the case when these couplings are relatively

small, or, for example, when the couplings are to heavy

particles, such as third-generation fermions, and so their

impact on ALP production and decay is suppressed. It is

important to note, however, that gluon and fermion cou-

plings generate diphoton couplings through loops and vice

versa, and so to analyze a specific underlying ALP model in

detail, one would in general have to include all of these

couplings in a unified way. Here, we take a more model-

independent, phenomenological approach.

With these simplifying assumptions, we therefore focus

on the ALP effective Lagrangian

L ⊃

∂

a∂

a −

−

aγγ

μν

; ð3Þ

where F

μν

is the field strength tensor of electromagnetism.

The resulting parameter space is very simple, as it is

spanned by two parameters: the ALP mass m

and the

diphoton coupling g

aγγ

With this Lagrangian, the ALP decay width is

≡ Γða → γγÞ¼

aγγ

64π

: ð4Þ

The cubic dependence on m

, resulting from the fact that

the decay is mediated by a dimension-5 operator, implies

that, as the ALP mass decreases, the ALP lifetime increases

rapidly. The ALP decay length is

≈ 630 m



−4

GeV

−1

aγγ





TeV



50 MeV



;

ð5Þ

where we have normalized to currently viable values of g

aγγ

and m

. For these values and ALP momenta p

∼ TeV, the

ALP decay length is naturally hundreds of meters, i.e., in

the range relevant for FASER searches for LLPs.

Although the ALP decays primarily into pairs of

photons, it is possible that one of the photons converts

into an electron pair leading to the decay a → e

−

γ. The

branching fraction for this decay is [34]

Bða → e

−

γÞ¼

6π

jF ðq

Þj



1 −

4 m





1 þ

2 m



1 −



; ð6Þ

where q

is the invariant mass of the electron pair, and

Fðq

Þ ≈ 1. For ALP masses between m

¼ 10 MeV and

1 GeV, the branching fraction ranges from Bða →

−

γÞ¼0.4% to 1.7%. Note that this branching fraction

peaks at low q

, implying that most of the ALP energy will

be carried by the photon, while the electrons will typically

be softer.

III. ALP PRODUCTION AND DECAY IN FASER

A. Mechanisms for ALP production

in the forward region

In the dominant ALP-photon coupling scenario, ALPs

can be produced in any process involving photons by

radiating an ALP off a photon line. However, for FASER,

we are primarily interested in the production of highly

energetic ALPs in the very forward region. The dominant

production mechanism is then the Primakoff process, in

which a photon converts into an ALP when colliding with a

nucleus. This can happen when photons produced at the

AXIONLIKE PARTICLES AT FASER: THE LHC AS A … PHYS. REV. D 98, 055021 (2018)

055021-3

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