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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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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.
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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 ...
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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 ...
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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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Correction to fermion mass in Supergravity mediated supersymmetry-breaking

I recently came across this paper about axino (fermion superparter of axion) mass in supergravity: https://doi.org/10.1016/0370-2693(92)90547-H In this paper, they think of a specific superpotential (...
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Overlap between experimental searches for axion and WIMP dark matter

Are there experiments which search for WIMPs that also set limits for the axion? I'm doing a literature review on axions, and I've been told I should include a brief overview of WIMP searches as they ...
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Exclusion limits for axion-like particle

Suppose the axion-like field $\theta$ with lagrangian $$ \tag 1 L = \frac{1}{2}(\partial_{\mu}\theta)^{2}-\frac{1}{2}m_{\theta}^{2}\theta^{2} - \frac{1}{4}\frac{\theta}{f_{\gamma}}F_{\mu\nu}\tilde{F}^...
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Why is is the vacuum expectation value (VEV) of the axion field such that it cancels the theta angle of QCD?

Why does the vacuum expectation value of the axion field $a(x)$ take on the value $\langle a \rangle = -\theta f/\xi \sim -\theta$, i.e. the vacuum theta angle of QCD? (Sources or proof if possible.) ...
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Effective field theory with coordinate dependent “axion”

Suppose the theory of fermions interacting with EM field and the axial 4-vector $b_{\mu}$: $$ S = \int d^{4}x \bar{\psi}\gamma^{\mu}(i\partial_{\mu} + eA_{\mu} +\gamma_{5}b_{\mu})\psi $$ We want to ...
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Is this article about the SMASH hypothesis a fair assessment?

New Scientist is gushing as usual about a new development in physics. This one, called SMASH, looks truly interesting, though. Apparently it's a small extension to the Standard Model that explains "...
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Axions and complex fields

I have been reading on axions and pseudo-Nambu-Goldstone Bosons (pNGBs) and I was wondering the following: A complex field has two degrees of freedom, meaning that one can write the following non-...
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A question from Coleman's "aspects of symmetries''

I know this question is a bit specialized and require rather deep background on Coleman's book Aspects of Symmetries. And I am sorry for that. But I still would like to ask the question and hope some ...
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Axion-like bounds from star cooling

Suppose the axion-like particle (ALP) $a$ with the lagrangian $$ L = \frac{1}{2}(\partial_{\mu} a)^2-\frac{m^2}{2} a^2 - g_{a\gamma\gamma}aF\tilde{F} $$ Here $F$ is EM field strength, and $\tilde{F}$ ...
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Why would be axions be a candidate for warm/cold dark matter?

Axions are the current main favoured candidate for the identity of dark matter. While their masses are sub-$\mathrm{eV}$, why is it that they would be cold or possibly warm dark matter candidates ...
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CP-Symmetry of the axion field and decay constant

In the $U(1)$-Peccei-Quinn-Symmetry an axion field $a$ is introduced in order to solve the strong CP-Problem. It is said, that below a certain scale $f_{a}$ this symmetry is broken and you are able to ...
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Does the Peccei-Quinn (PQ) mechanism require fine-tuning?

The strong CP problem: the QCD theta parameter can not be arbitrarily small due to 't Hooft's doctrine of naturalness (a parameter can be arbitrarily small only when putting it to zero provides an ...
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How can I prove that the following Lagrangian is Lorentz-invariant iff the axionic field is a pseudoscalar?

My professor has given me the following action stating that $a(x)$ is an axionic field and told us in class that for this action to be Lorentz invariant the field must be a pseudoscalar. $$ S = -\int ...
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Fermion decay into axions

Do you know any literature on fermion decay into an axion and a lighter fermion? I am especially interested in the dependence of the decay width on the initial fermion momentum. The momentum should ...
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82 views

Global cosmic strings evolution

Recently I've read about axion string. It can be shown that the energy per unit length of the string located along $z$ axis is $$ \mu = 2 \pi f_{a}^{2}\ln\left( \frac{L}{\delta}\right), $$ where $L$ ...
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Axion strings and spontaneously broken symmetry

I have two question about axion strings: Why their appearance is connected with spontaneously broken symmetry? How to demonstrate that? Why they are stable topological configurations (look to the "...
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Why is the strong CP term $ \theta \frac{g^2}{32 \pi^2} G_{\mu \nu}^a \tilde{G}^{a, \mu \nu}$ never considered for $SU(2)$ or $U(1)$ interactions?

The Lagrangian one would write down naively for QCD is invariant under CP, which is in agreement with all experiments. Nevertheless, if we add the term \begin{equation} \theta \frac{g^2}{32 \pi^2} ...
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What is the current state of axion research theoretically?

Is the problem (the strong CP problem) that is solved by axions still considered a really big problem, like the missing mass terms that could be solved by the Higgs mechanism? Or is it more a "problem"...
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Do most physicists believe that axions exist? [closed]

Is the theoretical justification behind proposing axions sufficiently robust, like, say the Higgs boson prior to its detection, that one should consider the existence of axions as generally accepted (...
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Why is a tensor times its dual a derivative?

Why is a term like $$F \tilde{F}$$ or $$G \tilde{G}$$ considered to be a derivative? Where are the derivatives? This is said in a PRD article. Check section IV page 5.