Axion-Like Particles and Their Cosmological Consequences

Ziqian Yu

University of California, Santa Barbara, 585 Sweet Rain Place, Goleta, U.S.A.

Keywords: Axion-Like Particles, Cosmology, Dark Matter, Particle Physics, Beyond Standard Model Physics.

Abstract: Axion-like particles (ALPs) are hypothetical particles that have garnered significant attention in both particle

physics and cosmology due to their potential roles in addressing unresolved issues such as the strong CP

problem, dark matter, and dark energy. This paper provides a comprehensive overview of ALPs, starting with

their theoretical foundations and production mechanisms in the early universe. The paper will explore their

interactions with standard model (SM) particles and their influence on cosmological phenomena, including

Big Bang nucleosynthesis, the cosmic microwave background, and structure formation. Furthermore, the

paper will examine the viability of ALPs as dark matter candidates and their potential contributions to the

dynamics of dark energy. Current experimental efforts, including direct detection methods like haloscopes

and helioscopes, along with indirect detection strategies through astrophysical observations, are reviewed.

The challenges and open questions in ALP research are discussed, highlighting the need for future theoretical

and experimental advancements to unveil the mysteries surrounding these elusive particles.

1 INTRODUCTION

As research into axions, a theoretical particle

introduced to address the strong CP problem in

quantum chromodynamics (QCD), progressed, it

became clear that a broader class of particles, now

known as axion-like particles (ALPs), could arise

from a variety of theoretical models beyond the

original scope of the QCD axion. These models often

emerge from extensions of the Standard Model,

including string theory and other high-energy

frameworks that propose additional dimensions or

symmetries. Unlike the QCD axion, ALPs are not

strictly bound to the strong CP problem, and their

properties—such as mass and coupling constants—

can vary widely depending on the specific model.

This flexibility makes ALPs a versatile tool for

exploring new physics beyond the Standard Model,

particularly in areas where existing theories fall short.

ALPs and axion have been extensively studied as

promising candidates for dark matter. Their weak

interactions with other standard model particles and

their potential to account for different astrophysical

and cosmological observations make them a

significant focus in contemporary physics research.

https://orcid.org/0000-0002-3721-6468

One of the most intriguing aspects of ALPs is

their potential role as dark matter candidates. Dark

matter is a foundational element of modern

cosmology, making up close to one third of the

universe's mass-energy content (much larger than that

of baryons), yet it remains elusive, interacting

primarily through gravity. Traditional dark matter

candidates, like weakly interacting massive particles

(WIMPs), have not yet been detected, prompting

researchers to explore alternatives, including ALPs.

ALPs are particularly interesting because their weak

interactions with Standard Model particles allow

them to avoid many of the constraints that have

excluded other dark matter candidates. Additionally,

their ability to form cold dark matter, depending on

the production mechanisms involved, aligns well

with observations of large-scale cosmic structures.

Beyond their role as dark matter, ALPs may also

influence cosmic evolution. The interactions between

ALPs and particles like photons and nucleons could

affect key events in the early universe, such as Big

Bang nucleosynthesis (BBN) and the buildup of the

cosmic microwave background (CMB). For instance,

ALPs might alter the photon-to-baryon ratio, thereby

impacting the synthesis of light elements and the

Yu, Z.

Axion-Like Particles and Their Cosmological Consequences.

DOI: 10.5220/0013075800004601

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 1st International Conference on Innovations in Applied Mathematics, Physics and Astronomy (IAMPA 2024), pages 321-329

ISBN: 978-989-758-722-1

321

variation of temperature observed in the CMB. While

these effects may be subtle, they are significant

enough to make the study of ALPs an important

avenue for testing cosmological models and

understanding early universe conditions.

Furthermore, ALPs are also being considered as

potential drivers of dark energy, an enigmatic force

driving the universe's accelerated expansion. In

certain theoretical models, ALPs are considered

scalar fields with characteristics that enable them to

imitate the behavior of a cosmological constant or

various types of dynamic dark energy. These models

suggest that ALPs could contribute to a slowly

varying energy density that drives cosmic

acceleration, offering a potential explanation for

observations that challenge the standard ΛCDM

model. Including ALPs in dark energy theories not

only broadens our understanding of the universe's

expansion history but also introduces new

observational signatures that could be detected

through high-precision cosmological surveys.

The ongoing search for ALPs extends across

multiple experimental and observational frontiers.

Direct detection efforts, such as those conducted by

haloscopes and helioscopes, aim to observe ALPs

interacting with strong magnetic fields or being

produced in astrophysical environments like the Sun.

Indirect searches focus on astrophysical phenomena,

such as the cooling rates of stars, gamma-ray

observations, and the polarization of the CMB, which

could reveal the presence of ALPs through their

interactions with other particles. Despite the

challenges associated with detecting such weakly

interacting particles, advances in technology and

experimental techniques continue to expand the limits

of possibility, moving us nearer to the potential

discovery of these elusive particles.

This paper aims to delve into the cosmological

consequences of ALPs by first providing a thorough

introduction to their theoretical foundations and

production mechanisms. The paper will then explore

their possible roles in dark matter, dark energy, and

the formation of cosmic structures. By reviewing

current theoretical models, experimental efforts, and

observational evidence, the paper aim to highlight the

significant challenges and open questions that remain

in the field. Ultimately, this paper seeks to provide a

comprehensive overview of how ALPs could reshape

our comprehension of the universe and the

fundamental forces that regulate it. As the paper move

forward, the study of ALPs promises to be a key area

of research with significant consequences for both

particle physics and cosmology.

2 THEORETICAL

FRAMEWORKS OF AXION-

LIKE PARTICLES

2.1 Origin and Properties of ALPs

The Peccei-Quinn (PQ) mechanism, introduced in the

late 1970s, was suggested as a solution to the strong

CP problem, which explains why QCD appears not to

violate CP symmetry as one would anticipate. CP

symmetry involves the combination of charge (C)

conjugation and parity (P) symmetries.

The strong CP problem arises because the QCD

Lagrangian includes a term that could potentially

break CP symmetry, yet experimental results have

shown that such a violation is extremely rare.

The Peccei-Quinn mechanism introduces a new

global U(1) symmetry, called Peccei-Quinn (PQ)

symmetry, which is spontaneously broken, resulting

in the appearance of a new pseudoscalar particle

known as the axion.

The axion field can dynamically eliminate the CP-

violating term, effectively setting 𝜃



=0, thus

resolving the strong CP problem.

2.2 ALPs in Particle Physics Models

Axion-like particles (ALPs) are theoretical entities

resembling axions, but they aren't necessarily linked

to solving the strong CP problem. Unlike the QCD

axion, ALPs can have a wide range of masses and

coupling constants. Their mass tends to be

considerably smaller than that of standard model

particles, and they can arise through various

mechanisms, such as explicit PQ symmetry breaking.

ALPs have very weak interactions with standard

model particles, meaning their effects on gravity are

notable, making them strong dark matter candidates.

The interaction strengths between ALPs and standard

model particles are typically described by coupling

constants, which influence their interactions with

photons, fermions, and gluons. These interactions are

typically modeled using effective Lagrangians.

In string theory, ALPs can naturally arise as

pseudo-Nambu-Goldstone bosons due to the

spontaneous breaking of global symmetries. These

particles may stem from higher-dimensional gauge

symmetries that become apparent after

compactification. Within various string theory

IAMPA 2024 - International Conference on Innovations in Applied Mathematics, Physics and Astronomy

322

scenarios, ALPs can exhibit a wide array of masses

and coupling constants, offering a fertile ground for

theoretical physics exploration.

Beyond string theory, ALPs also feature in other

beyond the Standard Model (BSM) theories. These

include models that extend the Higgs sector, grand

unified theories (GUTs), and other standard model

extensions aimed at addressing the hierarchy

problem, dark matter, and baryogenesis.

3 PRODUCTION MECHANISMS

OF ALPS IN THE EARLY

UNIVERSE

3.1 Thermal Production mechanisms

3.1.1 ALP Thermal Production in Early

Universe

In the early universe, axion-like particles (ALPs) can

be thermally produced through various interactions.

The production typically involves scattering and

decay processes that occur at high temperatures, often

before or during the epoch of BBN. The rate that ALP

produces depends on their coupling to photons,

electrons, nucleons and other standard model

particles.

Moreover, ALPs decouple when their interaction

rate becomes slower than the rate of cosmic

expansion. (Carenza et al., 2021). However, in low-

reheating cosmological scenarios, where the

reheating temperature can be significantly lower

(even down to a few MeV), the expansion rate of the

universe is quicker than predicted by the standard

model. This accelerated expansion causes the ALPs

to decouple earlier at higher temperatures, leading to

a suppression in their thermal relic abundance.

Consequently, this scenario allows for the possibility

of heavier ALPs, as the cosmological mass bounds

are relaxed in such low-reheating environments.

3.1.2 Impact of ALP Interactions with

Photons, Electrons, and Nucleons

The interactions between ALPs and standard model

particles such as photons, electrons, and nucleons

play a crucial role in shaping the thermal evolution of

ALPs and the overall history of the universe.

For instance, the interaction of ALPs with photons

(resonant photon-ALP conversion) could influence

the cosmic microwave background (CMB) by

modifying photon distribution, leading to distortions

in the black-body spectrum of the CMB, causing a

reduction in the number of photons, and,

consequently, an apparent decrease in the CMB

temperature in the direction of the affected galaxy

clusters.

ALPs coupled to nucleons can be efficiently

produced in the core of supernovae through the

nucleon-nucleon bremsstrahlung process. This

mechanism is significant because it allows for the

prolific production of ALPs in the supernova core,

where large densities and high temperatures are

present. The produced ALPs can escape the

supernova, contributing to a large flux of ALPs.

3.2 Non-Thermal Production

Mechanisms of ALPs

3.2.1 Misalignment Mechanism

The misalignment mechanism is a non-thermal

process through which axions are produced in the

early universe. This happens when the axion field

starts off "misaligned" from the minimum of its

potential, rather than starting at the minimum. This

misalignment continues because, in the early

universe, the Hubble expansion rate is much higher

than the axion field's mass. As the universe continues

to expand and cool, the Hubble parameter decreases,

eventually enabling the axion field to eventually

oscillate around the minimum of its potential.

The

oscillations resemble cold dark matter, with the

axion's energy density depending on the initial angle

of misalignment (axion field's initial displacement)

and the axion mass.

For axion-like particles (ALPs), energy density

generated by the misalignment mechanism is

influenced by factors like the initial misalignment

angle, the ALP mass, and the PQ scale. This

mechanism is particularly intriguing because it does

not depend on thermal processes and can produce a

cold dark matter component that is independent of the

universe's thermal history.

3.2.2 Decay of Topological Defects

Topological defects, which include structures like

cosmic strings and domain walls, arise when the

universe undergoes symmetry-breaking phase

transitions. These defects are particularly important

because their decay can lead to the production of

ALPs, which contribute to the cold dark matter

density in space.

As PQ symmetry break at high energies, Cosmic

strings form as line-like defects where the axion field

Axion-Like Particles and Their Cosmological Consequences

323

winds around a vacuum expectation value. As the

universe continues to cool and experiences additional

phase transitions, especially the QCD phase

transition, these strings may become connected to

domain walls if the domain wall number exceeds one

(Chang, S., 1998). The resulting ALPs are often

relativistic and could contribute to the dark radiation

or act as a warm dark matter component depending

on their mass and coupling constants.

Domain walls, formed by two-dimensional

defects when the PQ symmetry breaking leads to

discrete degeneracies in the vacuum state, are causing

series result in cosmology because they can dictate

the energy density of the cosmos, leading to conflicts

with observed cosmological data (Saikawa, 2017).

However, if these walls decay before they dominate,

they are able to produce a large amount of ALPs. The

ALPs produced this way is typically linked to the

scale of PQ symmetry breaking.

The decay of these topological defects is affected

by several aspects, such as the coupling between

ALPs and other particle fields, the dynamics of the

defects themselves, and the presence of any external

fields that might interact with the defects.

4 ALPS AND COSMIC

EVOLUTION

4.1 Influence on Big Bang

Nucleosynthesis (BBN)

BBN, which involves the formation of light elements

such as hydrogen, helium, and lithium within the first

few minutes after the Big Bang, is extremely sensitive

to the conditions present in the early universe. Key

factors include the density of baryons, the rate of

cosmic expansion, and interactions between particles.

Axion-Like Particles (ALPs), which are hypothetical

particles, could interact with Standard Model

particles such as photons, nucleons, and leptons,

potentially affecting these crucial conditions.

One of the primary ways ALPs can affect BBN is

through their interactions with photons or nucleons.

For instance, if ALPs are produced in sufficient

quantities, have the potential to modify the baryon-to-

photon ratio (η) as well as the effective number of

relativistic degrees of freedom (𝑁



), which could

impact the expansion rate of the universe,

subsequently affecting the production of light

elements like deuterium (

H) and helium-4 (

He). The

combined observations from BBN and the CMB

place tight limits on the parameter space of ALPs,

particularly ruling out regions where ALPs would

lead to significant deviations in light element

abundances (Millea et al., 2015).

4.2 Indication From Cosmic

Microwave Background

The CMB is the remnant radiation from the Big Bang,

offering a glimpse into the universe roughly 380,000

years after the Big Bang. The CMB carries imprints

of the physics of the early universe, including

potential interactions between ALPs and photons. As

a fundamental observable of cosmology, the CMB

serves as a critical tool for probing the early universe

and any new physics, such as the existence of ALPs.

ALPs can interact with photons through resonant

conversion processes, particularly in the presence of

magnetic fields within galaxy clusters. These

interactions can lead to observable polarized

distortions in the CMB, such as changes in the

anisotropy spectrum or polarization patterns, which

would manifest differently from other well-known

CMB signals like those caused by gravitational

lensing. The presence of ALPs could also result in

additional temperature fluctuations at high

multipoles, distinct from the standard CMB spectrum.

These temperature fluctuations, if detected, would

offer further evidence supporting the existence of

ALPs, highlighting the importance of high-resolution

CMB observations.

Moreover, spectral distortions in the CMB, which

can arise from processes such as energy injection or

non-thermal effects, could also provide indirect

evidence for ALPs. These distortions are highly

sensitive to the thermal history of the universe and

any new particles or interactions that could alter this

history. As such, analyzing spectral distortions allows

cosmologists to explore the thermal processes in the

early universe, where ALPs could play a significant

role.

Future CMB experiments with improved

capabilities are expected to significantly tighten these

constraints, offering a more comprehensive picture of

ALPs' impact on the universe.

4.3 Role in Structure Formation

Axion-like particles (ALPs) also plays a crucial role

in the formation of cosmic structures such as galaxies,

clusters, and large-scale filaments in the universe.

Unlike traditional cold dark matter (CDM), ALPs can

have different properties, exhibiting wave-like effects

on certain scales where these effects dominate over

self-gravity and a smaller mass or interactions with

IAMPA 2024 - International Conference on Innovations in Applied Mathematics, Physics and Astronomy

324

other particles, which can lead to distinct signatures

in structure formation.

ALPs can form a form of dark matter that is

slightly warmer than CDM, sometimes referred to as

"fuzzy dark matter" (Shen et al., 2023) or "ultralight

dark matter." The quantum mechanical effects due to

the small mass of ALPs can cause a reduction in the

formation of structures at smaller scales.

This suppression occurs because the de Broglie

wavelength of ALPs, which is inversely proportional

to their mass, can become comparable to or larger

than the size of small-scale structures, leading to a

phenomenon known as "quantum pressure" or "wave-

like behavior". This effect can prevent the formation

of scale of diminutive structures. For example, dwarf

galaxies, which would be abundant in a purely CDM

scenario. Moreover, such suppression is related to the

quantum pressure that counteracts gravitational

collapse on small scales, resulting in a cutoff in the

linear matter power spectrum.

Moreover, ALP dark matter can influence the

formation of cosmic structures by affecting the

collapse of dark matter halos, the building blocks of

galaxies and larger structures. In scenarios involving

fuzzy dark matter, halos can form solitonic cores —

dense, stable regions at the centers of dark matter

halos. These cores are surrounded by more diffuse

outer regions, creating a unique structure that can be

contrasted with predictions from standard denser

CDM models.

5 ALPS AND DARK MATTER

5.1 ALPs as Dark Matter Candidates

Just like what the previous section 2 passage indicate,

ALPs are considered strong contenders for dark

matter due to their distinct properties, including weak

interactions with standard model particles and the

potential to make up a significant portion, or even all,

of dark matter.

ALPs as a weak coupling dark matter can arise in

various theoretical frameworks, including string

theory and other beyond the Standard Model (BSM)

scenarios. Depending on their mass and coupling

constants, ALPs can manifest as cold dark matter,

similar to the canonical WIMP (Weakly Interacting

Massive Particle) models. ALPs are also being

regards as the substance of "Fuzzy dark matter"

models, where they have ultralight masses, suppress

small-scale structures due to quantum mechanical

effects, providing a potential answer to some of the

small-scale issues encountered with CDM.

The characteristics of ALPs as potential dark

matter contenders are constrained by various

astrophysical observations and experimental

searches. Astrophysical observations, such as the

CMB, can serve to limit or define the characteristics

of ALPs.

Overall, while ALPs remain a reasonable

contender for dark matter, their exact role depends on

further observational and experimental evidence,

which will help to narrow down the range of allowed

properties and clarify their contribution to the

composition of dark matter present in the universe.

5.2 Direct and Indirect Detection

Methods

5.2.1 Laboratory Searches for ALPs

Haloscopes and helioscopes are two of the primary

laboratory-based techniques used to look for ALPs.

To detect ALPs that could comprise the dark

matter halo over our galaxy, various experiments

focus on the interaction between the axion field and

photons in the presence of a strong magnetic field,

which facilitates the conversion of ALPs into

detectable photons. The most prominent haloscope

experiment is ADMX (Axion Dark Matter

eXperiment), which has established significant

constraints on ALP masses and couplings within the

microwave frequency range (Khatiwada et al., 2021).

Other methods like Helioscopes are telescopes

designed to detect ALPs produced in the Sun. The

best helioscope here is CAST (CERN Axion Solar

Telescope), which detects potential axions generated

in the Sun by observing the effect produced by their

conversion into photons under a strong magnetic field

(Barth et al., 2013). The experiment has progressed

through several stages, employing a decommissioned

LHC dipole magnet to track movement of the Sun and

convert the generated axions into discernible X-ray

photons via axion-photon interactions. This help

narrow down the parameter space where ALPs could

exist.

5.2.2 Indirect Detection through

Astrophysical Signals

Indirect detection includes gamma ray observations.

For gamma-ray observations, ALPs can interact with

photons when exposed to external magnetic fields,

which allows for photon-ALP conversions. This

conversion is facilitated by the two-photon vertex

interaction described in the context of ALPs, where

high-energy gamma rays from astrophysical sources

Axion-Like Particles and Their Cosmological Consequences

325

can convert into ALPs and vice versa when they pass

through regions with strong magnetic fields (Hooper

& Serpico, 2007). For example, gamma rays from

distant astrophysical sources could convert into ALPs

when subjected to B fields, leading to irregularities in

the detected high-energy photon flux. Observatories

like the Fermi Gamma-ray Space Telescope have

been used to search for such signatures, providing

constraints on ALP properties.

Gamma-ray telescopes, both space-based (like the

GLAST satellite) and ground-based (such as HESS,

MAGIC, and VERITAS), can potentially detect the

signature of ALP-photon conversions by observing

distortions in the gamma-ray spectra. The detection

strategy involves looking for spectral features that

deviate from the expected power-law behavior due to

the depletion of gamma rays converted into ALPs.

Other methods include Stellar Cooling: ALPs

could be produced in the cores of stars and

subsequently escape, carrying away energy and

affecting the cooling rates of stars (Zhang et al.,

2024). Observations of white dwarfs, red giants, and

other stellar remnants provide constraints on ALP

couplings based on how well these stars' cooling

behaviors match theoretical predictions. The faster-

than-expected cooling of certain stars has been

proposed as indirect evidence for ALPs, although

alternative explanations also exist.

6 ALPS AND DARK ENERGY

6.1 ALPs as a Source of Dark Energy

6.1.1 Theoretical Models Linking ALPs to

Dark Energy

In some theoretical models, ALPs are introduced as

scalar fields with properties that allow them to

influence the large-scale dynamics of the universe.

These models often draw from ideas in string theory

and other extensions of the Standard Model.

One of the primary mechanisms through which

ALPs could contribute to dark energy is by acting as

a quintessence field. In these models, ALPs possess a

very light mass and a potential energy that evolves

slowly over time, leading to a negative pressure that

drives cosmic acceleration. The slow roll of the ALP

field down its potential mimics the behavior of a

cosmological constant, but with time-dependent

characteristics that could be observed in the evolution

of the universe's expansion rate.

6.1.2 Cosmological Implications of ALP-

driven Dark Energy

Including ALPs as potential candidates for dark

energy has profound implications for cosmology. A

key prediction of ALP-driven dark energy models is

the development of the equation of state parameter w,

which characterizes the relationship between the

pressure and density of dark energy. Unlike the

cosmological constant, where w = −1, ALP models

can result in a temporally evolving equation of state

w. This variation could be observed through detailed

studies of the CMB, large-scale structure, and

supernovae.

These models also influence the growth of cosmic

structures (Ruchika et al., 2023).

ALP-driven dark energy can modify the rate at

which structures form and evolve, leading to

observable differences in the distribution of galaxies

and clusters. Additionally, the coupling between

ALPs and other fields can lead to new signatures in

gravitational wave observations, providing a unique

way to test these models.

6.2 Observational Signatures

As ALPs are considered a source of dark energy, they

could produce distinctive signatures that may be

detectable through various cosmological

observations.

A possible indication could arise from the

gravitational signatures produced by ALPs with non-

periodic potentials. These particles may undergo

processes such as parametric resonance and field

fragmentation, which can lead to the formation of

dense halos. These halos could result in observable

gravitational phenomena, covering phenomena such

as astrometric lensing, the diffraction of gravitational

waves from black hole mergers, and the photometric

microlensing of stars with extreme magnification

(Chatcrhon et al., 2024). These phenomena could

provide indirect evidence for the presence of ALPs

and their contribution to cosmological processes,

including dark energy.

This gravitational effect could be observed by

gravitational microlensing phenomenon, where an

axion star and a background star align, causing the

light from the star to be lensed, creating a

characteristic light curve that can be detected by

telescopes.

Additionally, Large-scale structure (LSS)

surveys and supernovae observations are critical tools

for constraining ALP-driven dark energy models

(Ivanov et al., 2020). LSS surveys, which map the

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distribution of galaxies across vast cosmic volumes,

can reveal how the growth of structures has evolved

over time. Deviations from the expected clustering

patterns in the ΛCDM model could indicate the

presence of ALP-induced modifications to dark

energy.

Supernovae, particularly Type Ia supernovae,

serve as "standard candles," are essential for

measuring cosmic distances. By analyzing the

observed brightness of these supernovae across

various redshifts, astronomers can trace the history of

expansion of the cosmos. Any deviation on the

expected relationship between distance and redshift

could provide evidence for a varying equation of state

driven by ALPs.

These constraints help narrow down the

parameter space where ALPs could play a role as dark

energy and provide essential tests for the validity of

these models.

7 CURRENT AND FUTHER

EXPERIMENTAL EFFORTS

7.1 Existing Experiments and

Observations

Current effort on observation of ALPs (axion-like

particles) includes ADMX and CAST that previous

passage have introduced, and the more advanced

generation helioscope –– IAXO.

Recent findings from ADMX have further

advanced the sensitivity to ALPs. The ADMX Run

1A analysis targeted a specific frequency range (645–

680 MHz), which corresponds to an axion mass range

of 2.66–2.81 μeV, achieving sensitivity at the DFSZ

(Dine-Fischler-Srednicki-Zhitnitsky) level (Boutan et

al., 2023). The experiment did not detect any signals

that would suggest the presence of axions, leading to

exclusion limits on axion-photon couplings at or

below the DFSZ sensitivity, with a confidence level

larger than nine tenth. This effectively excludes

axions as the sole component of dark matter within

the scanned frequency range, assuming a virialization

version of galactic halo model.

Additionally, the recent extended run of CAST,

which included a new Xe-based Micromegas

detector, has set a updated maximum bound on the

axion-photon coupling constant. The new limit here

shows 𝑔



<5.7×10



GeV



at 95% confidence

level (CAST Collaboration et al., 2024), which is the

most stringent experimental limit to date for axion-

photon coupling, particularly in the low-mass range

(for 𝑚



≲0.02 eV)

With CAST ceasing operations in 2021, these

findings represent the final and most comprehensive

limits from this experiment. The results from this

extended run will remain the most stringent bounds

on solar axions until new data from upcoming

experiments, such as BabyIAXO and IAXO, become

available.

The International Axion Observatory (IAXO) is a

next-generation helioscope experiment designed to

significantly enhance the sensitivity of CAST,

achieving improvements by several orders of

magnitude. It will utilize a much larger magnetic field

volume and advanced X-ray optics to detect ALPs

with greater precision. IAXO is anticipated to provide

new insights into the ALP parameter space,

potentially exploring uncharted regions that could

include ALP dark matter or ALPs linked to other

astrophysical phenomena (Ribas et al., 2015).

Though still in the preparatory phase, IAXO has

been incorporating cutting-edge technologies that are

expected to achieve unparalleled sensitivity to ALPs.

The update highlights the importance of BabyIAXO

in advancing axion research, particularly in the

detection of solar ALPs. BabyIAXO is engineered to

give a significant enhancement on probe’s sensitivity

to axions compared to earlier experiments like CAST.

7.2 Future Prospects and Upcoming

Experiments

Several new experiments are poised to greatly

enhance the search for axion-like particles (ALPs) by

investigating regions of the ALP parameter space that

were previously unexplored.

These efforts aim to identify ALPs across a broad

spectrum of masses and coupling strengths, utilizing

both direct detection methods and astrophysical

observations.

The MADMAX (Magnetized Disk and Mirror

Axion Experiment) project is specifically intended to

detect axions and ALPs within the mass range of

several tens of micro-electronvolts (µeV). This

experiment employs a dielectric haloscope, which

uses a series of dielectric disks placed within a strong

magnetic field to trigger the transformation of ALPs

into detectable photons. This innovative setup

provides a new method for probing ALP dark matter.

Another significant effort is the ALPS II (Any

Light Particle Search II) experiment, an advanced

"light-shining-through-a-wall" setup. It aims to detect

ALPs and other weakly interacting particles with

masses below an electronvolt (sub-eV) by using high-

Axion-Like Particles and Their Cosmological Consequences

327

power lasers and strong magnetic fields to induce

ALP-photon conversion, with the goal of achieving

unprecedented sensitivity to these interactions. If

successful, ALPS II could provide key evidence for

axions or ALPs, which are viewed as promising dark

matter contenders and might also address the strong

CP problem.

Overall, these advancements, together with

improvements to existing detectors like ADMX, are

expected to expand our understanding of ALPs

significantly, potentially uncovering new physics

beyond the Standard Model.

8 CHALLENGES AND OPEN

QUESTIONS

8.1 Theoretical Challenges in ALP

Research

Axion-like particles (ALPs) present a rich field of

study with a wide range of theoretical models, but

these models come with significant uncertainties. One

of the primary challenges is the large parameter space

associated with ALPs, including their masses,

couplings, and potential interactions with standard

model particles.

Theories predicting ALPs often emerge from

frameworks like string theory or other beyond the

Standard Model (BSM) physics, leading to a

multitude of possible ALP properties. These

uncertainties complicate efforts to make precise

predictions about ALP behaviours in various

contexts, such as their potential role in dark matter,

dark energy, or their interaction coupling with

photons, nucleons, or other particles. Moreover,

different ALP models can predict vastly different

cosmological and astrophysical phenomena, making

it difficult to design experiments that can

comprehensively test for ALPs.

Given the broad range of possible ALP properties,

there is a pressing need for more precise theoretical

predictions to guide experimental searches. This

requires a better understanding of the underlying

physics that governs ALP interactions and how these

interactions might manifest in observable

phenomena. Since adding all these theoretical

assumptions increase the complexity of the whole

experimental system; thus, it is needed to be selected

carefully for parameters within the theory.

Additionally, advances in computational techniques

and simulations could provide more accurate

predictions of ALP behavior under different

conditions, helping to narrow down the parameter

space and identify the most promising regions for

experimental exploration.

8.2 Observational Challenges in ALP

Research

Current observational techniques for detecting axion-

like particles (ALPs) face challenges, primarily due

to the weak interactions that ALPs are hypothesized

to have with standard model particles. This makes

them incredibly difficult to detect, requiring

extremely sensitive instruments and often relying on

indirect methods.

The main limitation of current detectors, such as

those used in haloscope experiments like ADMX, is

their sensitivity. These detectors must be capable of

detecting extremely faint signals generated by the

conversion of ALPs into photons, a process that is

expected to occur at very low rates, making

distinguishing a potential ALP signal from

background noise very difficult.

Advances in superconducting technologies, such

as the development of quantum amplifiers, could

significantly enhance the sensitivity of detectors used

in haloscope experiments. These amplifiers can

reduce noise levels and increase the signal-to-noise

ratio, making it easier to detect faint ALP signals.

9 CONCLUSIONS

This comprehensive study of axion-like particles

(ALPs) delves into their theoretical foundations,

potential cosmological roles, and the current state of

experimental efforts aimed at detecting these elusive

particles. ALPs have emerged as versatile candidates

for various cosmological phenomena, including dark

matter or energy, and the formation of cosmic

structures.

Weak interactions with SM particles and the

broad parameter space they occupy, underlies the

challenge and intrigue of ALPs, making them

significant subjects for both theoretical and

experimental research.

The exploration of ALPs extends well beyond the

search for a single particle; it opens up new avenues

for understanding the fundamental forces and

particles that shape our universe.

If ALPs are detected, they could offer valuable

insights into high-energy physics, thereby not only

advancing our knowledge of physics beyond the SM

but also providing explanations for dark matter,

contributing to our current knowledge of cosmic

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structure formation, also influencing the evolution of

dark energy.

Their potential to address significant problems in

both particle physics and cosmology highlights their

importance in these fields.

Ongoing research into ALPs is essential for

advancing our understanding of the universe. As

experimental techniques improve and new theoretical

models are developed, the chances of discovering

ALPs increase. Future experiments like ADMX,

CAST, IAXO, MADMAX, and ALPS II, along with

simulations that bridge particle physics theory and

cosmological observations, will play a critical role in

this pursuit. The implications of ALP research are

profound, with the potential to reshape our

comprehension of the universe and the fundamental

principles that underlie its workings.

Given the challenges and high stakes involved,

ALP research stands at an exciting frontier, promising

insights that could have far-reaching consequences

for both physics and cosmology.

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