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Questions tagged [axion]

The axion is a hypothetical spinless particle. It underlies a conjectured solution of the "strong CP problem" of the standard model and makes for a good dark matter candidate.

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If dark matter is axion particles, how did such light particles slow down?

One candidate for dark matter is axions and there is tentative experimental evidence for their existence. Axions are very light, most models weigh them in at tiny fractions of an eV. It seems like ...
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Equivalence of axion to fermion couplings

In order to solve the Strong CP problem through the axion, we introduce the axion-gluon coupling $$ \dfrac{a(x)}{f_a} \text{Tr}\, G\tilde{G}.\tag{1} $$ In a similar fashion, we may introduce the axion-...
Gabriel Ybarra Marcaida's user avatar
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Axion domain wall number and heavy quarks

The domain wall number of a UV complete theory of axion is related to the number of PQ-charged heavy quarks that run in the loop. In the case of KSVZ model, $N_{\rm DW}=1$ while in DFSZ, $N_{\rm DW}&...
PhysicsStudy's user avatar
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Is there really a strong CP problem?

The strong CP problem is considered one of the major unsolved physics problems. A non-vanishing value of $\theta$ in QCD $\theta$-term violates CP-symmetry. A primary strong CP violating observable is ...
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Can axion fields make the gravitational constant to oscillate in harmonic way?

Axions (and more generally, I think scalar fields) can make fundamental constants to vary. Does it include the gravitational constant? And if so, can it be make to oscillate as a simple harmonic ...
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What is the (non-broken) Peccei-Quinn symmetry?

In the references I read, it's always said that the Peccei-Quinn (PQ) symmetry is a $U(1)$ symmetry, and the PQ Lagrangian has a potential term that, for a certain temperature, assumes a mexican-hat ...
Mauro Giliberti's user avatar
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Domain walls in theories of axions

I'm stuck in figuring out why some theories of axions predict the existence of domain walls. Axions are NG bosons associated with the chiral $U(1)_{PQ}$ symmetry, which was spontaneously broken at ...
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Scalar Axion Field Amplitude Calculation in localised laboratory / Earth

This question concerns the paper "Axion Dark Matter: What is it and Why Now?", in the Appendix A.3 regarding equations related to the Axion Field. It states that by the Friedmann Equation, $$...
Avis Yu's user avatar
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Slope in interaction strength vs mass for QCD axion

The theory models of QCD axion, i.e. those who solve the strong CP problem, all have a prediction that follows a band with a slope in the space interaction strength vs. axion mass. (1) What does cause ...
Stefano Barone's user avatar
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When to consider and when not to consider the loop contributions from light quarks?

Consider the following Lagrangian: $$ \tag 1 \mathcal{L} = \frac{\partial_{\mu}a(x)}{f}\sum_{q}\bar{q}\gamma^{\mu}\gamma_{5}q $$ This is a Lagrangian of the axion-like particles (ALPs) $a$ interacting ...
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Axion production from vacuum realignment and the strong CP problem

I am trying to understand axions as a possible candidate for dark matter. My understanding of one of the mechanisms by which energy could up in the axion field is by an initial misalignment between ...
Panopticon's user avatar
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Axion vs. WIMP / Is Axion a WIMP?

What is the difference between an Axion and a WIMP? A WIMP is just defined as a class of particles interacting via gravity and potentially via weak interaction (or a new force that is even weaker than ...
Welcome_Green's user avatar
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Axions as goldstone bosons of anomalous $U(1)$ symmetry

In the $m_q \rightarrow 0$ limit the QCD lagrangian has the symmetry $U(N)_V \times U(N)_A$. Including just the two lightest quarks, $N=2$, and looking at the $U(2)_A=SU(2)_A \times U(1)_A$ part, we ...
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Does a fermion loop regulate effective axion QED?

I'm studying these lectures by Falkowski, and when talking about the Euler-Heisenberg EFT he says consider a UV completion which is itself an effective theory. The Lagrangian contains a real scalar $...
Mauro Giliberti's user avatar
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Axion-photon coupling

The QCD axion couples to the photon through a term proportional to $a F_{\mu\nu} \tilde{F}^{\mu\nu}$. I understand that this term is due to the electromagnetic anomaly. I am interested in two ...
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What does it mean to add the wave function of axion particles together?

I have been recently following works studying axion field. In many works studying axions, the authors assume the axion field is a bunch of plane wave, for example in this paper, equation 7 (though ...
Tan Tixuan's user avatar
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Can ultralight dark matter particles be a massless relativistic field if they belong to a hidden dark sector?

Usually, we believe that dark "matter" is fermionic. My question is simple: is there any reason why a SM-inert massless unknown boson could NOT be the thing we know as dark matter if it only ...
riemannium's user avatar
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Stückelberg mechanism and the axion

In the so-called Stückelberg mechanism we have the BF term $$ \sim \int m\; B_2\wedge F ~, $$ where the field $B_2$ is a 2-form and $F$ is the field strength arising from a $U(1)$. The Stückelberg ...
Nathanael Noir's user avatar
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Axions and magnetic field

From what I know about the hypothetical particles "axions": Axions are candidates to be cold dark matter particles Dark matter interacts only gravitationally Some experiments use strong ...
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How is energy balanced in the Primakoff effect for axions?

In the presence of a strong magnetic field, (theoretical) axions are converted to high energy photons via the Primakoff effect. How is energy balanced in this process? Is the mass of the axion being ...
Jackson Walters's user avatar
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Equation of motion for the axion field

I was reading Sikivie's paper on Axion Cosmology: https://arxiv.org/abs/astro-ph/0610440 . It says the equation of motion for the axion field $a(x)$ in Friedmann–Lemaître–Robertson–Walker space-time ...
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Unitarity in axion coupling to photons

I have been investigating about axion coupling to EM field, and I have found that it is usually described through the interaction Lagrangian: $$ \mathcal{L} = \frac{1}{4 M } a \tilde{F}_{\mu \nu} F^{\...
alexsanchezcast's user avatar
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Why would partial derivatives not commute inside an axionic cosmic string?

In this 1985 paper by Callan and Harvey, Eq. $11$ seems to claim that in the presence of an infinitely extended string-like topological defect, partial derivatives do not commute on the string: $$ [\...
Nanashi No Gombe's user avatar
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Orthogonality of curvature and isocurvature fluctuations, but only until horizon crossing?

I am theoretically investigating our universe with an additional scalar field called axion like particle (ALP). I now want to understand fluctuations in the ALP field, and I am confronted with the ...
Koschi's user avatar
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Scalar field displacement from the minimum of the potential gives rise to particles/dark matter, why?

In This paper (Kobayashi et al -- Lyman-alpha Constraints on Ultralight Scalar Dark Matter: Implications for the Early and Late Universe) it says, at the beginning of Section 3.1: A light scalar ...
RenatoRenatoRenato's user avatar
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The sign of axion $F$ $F$ dual term in Weinberg

Related to the earlier question $\gamma^5$ rotation of chiral fermion in (1) Peskin&Schroeder, (2) Weinberg, or (3) Srednicki. The sign of axion $F$ $F$ dual term in Weinberg (23.6.16) appears to ...
ann marie cœur's user avatar
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What is instanton effect in QCD?

In the context of the Peccei-Quinn Symmetry in solving the Strong CP problem, it is said that the Axion develops a mass due to QCD instanton effects below $\Lambda_{QCD}$, which is very much ...
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An exercise for axion from Mark Srednicki's QFT book

In Mark's QFT book, exercise 94.2(d) says In eq.(94.10), we should replace $\theta$ by $\theta+a/f...$. eq.(94.10) is just the mass term \begin{equation} M=Diag\{m_{u},m_d\}e^{-i\theta/2}, \end{...
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Can Peccei-Quinn $U(1)$ symmetry be a $U(1)_R$ symmetry in supersymmetric models?

Is there a possibility that the global $U(1)$ of Peccei-Quinn theory can be identified with $U(1)_R$ symmetry in supersymmetric extensions of the Standard Model?
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Why color anomaly is in the axion photon coupling?

The axion photon coupling is given by the expression $ g_{a\gamma\gamma}= \frac{\alpha}{2\pi f_a}(\frac{E}{N}-\frac{2}{3}\frac{4m_d+m_u}{m_d+m_u}) $, where $f_a$ PQ symmetry breaking scale, $E$ and $...
lIllIlllIIIl's user avatar
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Independent parameters for the model of axion-like particle

Consider the following model of an axion-like particle $a$ (or ALP): $$ \mathcal{L} = (\partial_{\mu}a)^{2}/2 - m_{a}^{2}a^{2}/2 + ag_{a\gamma\gamma}F_{\mu\nu}\tilde{F}^{\mu\nu}, $$ where $F_{\mu\nu}$ ...
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Why are axions considered cold dark matter? [duplicate]

Why are axions considered cold dark matter? They have a very low mass and if I'm not wrong they decouple before BBN, and in some models even earlier than neutrinos, they they should definitely be ...
Erontado's user avatar
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Lie derivative and Goldstone modes

Systems with a gravity dual might experience the spontaneous breaking of a symmetry. In such a case, the dynamics of the dual QFT is determined by the Goldstone bosons. But, what do the Goldstones ...
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How will the possibility of tritium contamination decay be removed from XENONnT data?

The abstract of the new arXiv preprint Observation of Excess Electronic Recoil Events in XENON1T contains the following sentence: The excess can also be explained by $\beta$ decays of tritium, which ...
uhoh's user avatar
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What corresponds to the radial component of the axion potential?

An axion can be understood as an oscillation of the axion field in some potential. In the Higgs mechanism, there are 4 degrees of freedom. The Higgs boson is the radial oscillation and 3 remaining ...
alessandro's user avatar
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Physical significance of the string anti-symmetric tensor field?

I've been looking at David Tong's Lectures on String Theory. Is the anti-symmetric tensor field $B_{\mu\nu}$ (Eqn. 2.29) of string theory related to the electromagnetic field? Also I've seen $B_{\mu\...
John Eastmond's user avatar
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3 answers
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Is a pseudo-Goldstone boson always a pseudoscalar particle?

There are several examples of pseudo-Goldstone bosons which are CP-odd particles, such as the pion, as well as many axion-inspired models. If we invert the logic, Are all pseudo-Goldstone boson of ...
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What is the defect in the standard model with quark interactions

I am watching the Master Class Axions: Ghost Riders in the Sky as well as another lecture, this one given by Michael Turner. In his lecture, he states the Axion idea came about because of a defect in ...
Rick's user avatar
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Thermalization of BEC dark matter

In the context of BEC/superfluid dark matter, one of the important requirements is that - within galaxies - dark matter thermalizes [1]. Axions are appropriate in this regieme, as their self-...
Jack Hughes's user avatar
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Obtaining the effective Lagrangian for axions from the spontaneous breakdown of Peccei-Quinn symmetry

I am reading Section $3$ of this review titled The Strong CP Problem and Axions by R. D. Peccei and also the post here. A famous solution of the Strong CP problem in QCD is offered by the proposal of ...
SRS's user avatar
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How come Goldstone boson, PQWQ axion, be able to have mass at all?

Quote: Goldstone's theorem: For every spontaneously broken continuous symmetry, there is a massless particle created by the symmetry current. However, under $U(1)_{PQ}$ symmetry, I read that PQWW ...
ShoutOutAndCalculate's user avatar
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2 answers
359 views

Do axions have an anti-particle?

Axions are the particles that let energy flash between a matter particle and its anti-particle. But according to quantum field theory, every particle has an anti-particle (besides the majorana fermion)...
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D7-brane in Type IIB and axio-dilaton

In the context of Type IIB string theory with one D7-brane we have the following situation. The axio-dilaton complex scalar is defined by $\tau:= C_0+i e^{-\phi}$ where $C_0$ is the axion and $\phi$ ...
Singular Lotus's user avatar
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Why is the $\theta$-term sometimes proportional to $g_s$?

In some sources, the CP violating $\theta$-term is written as $$ \frac{g_s^2 \theta}{32\pi^2} F \tilde F \, ,$$ while others write it as $$ \frac{ \theta}{16\pi^2} F \tilde F $$ or $$ \frac{ \...
jak's user avatar
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Pion mass in theta vacuum

Does the mass of the charged pion depend on the QCD vacuum angle? I've seen it said---e.g., in these TASI lectures---that when the quark mass matrix is real and there is a nonzero QCD vacuum angle $\...
plusplusplus's user avatar
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Why does the axion solve the CP problem?

I don't understand why the CP violating term in QCD vanishes when we introduce the axion. The term is $$\left(\frac{a}{f_a}+\theta\right)G\tilde{G}$$ It is claimed that $\left\langle a\right\rangle=-\...
Joshua Tilley's user avatar
4 votes
1 answer
198 views

What paper experimentally ruled out the original Peccei-Quinn axion?

Could someone please refer me to the paper that ruled out Peccei-Quinn axions experimentally. Most literature mentions this happening but doesn't refer to the original paper where it was proven so.
6 votes
1 answer
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How do we see that the axion is a pseudoscalar?

The axion is the pseudo-Goldstone boson associated to the breaking of the conjectured Peccei-Quinn Abelian symmetry. The axion couples to the SM gauge fields in a CP-invariant manner (e.g. $aF\tilde F ...
Nanashi No Gombe's user avatar
11 votes
2 answers
546 views

If neutrinos are disfavoured as DM candidates why aren't axions?

Numerical simulations of observed large-scale structure formation work best with Cold Dark Matter (CDM; see the answer here). Neutrinos are candidates for Hot Dark Matter (HDM), and hence they cannot ...
SRS's user avatar
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Is it sufficient to consider the uncharged dark matter and a $\mathbb{Z}_2$ symmetry to prevent its decay into photons?

Higgs boson, though electrically neutral, can decay to produce a pair of photons i.e., have electromagnetic interactions. See the answer here. Therefore, it appears that making dark matter ...
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