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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.

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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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} G_{\mu \nu}^a \tilde{G}^{a, \mu…
Tim
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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 account for the total dark matter abundance of…
SRS
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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 $\left\langle a\right\rangle=-\theta f_a$ which cancels the term, but why is the…
Joshua Tilley
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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 $\langle a \rangle = -\theta f_a/\xi \sim -\theta$, i.e. the vacuum theta angle of QCD? (Sources or proof if possible.) In other words, why does this VEV minimize the…
L. K.
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Peccei-Quinn-symmetry and effective Lagrangian for the Axion field

To solve the strong CP-problem Peccei and Quinn suggested the use of a new $U(1)$-symmetry called the PQ-symmetry. For this symmetry they constructed an effective Lagrangian involving the Nambu-Goldstone-boson of the spontanoues broken PQ-symmetry:…
Alpha001
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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 extended symmetry in the theory). Putting QCD theta…
siddhartha
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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 was initially not considered, at $3.2\sigma$…
uhoh
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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 the Peccei-Quinn symmetry and its spontaneous…
SRS
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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. $aF\tilde F \sim a\ \vec E\cdot \vec B$), which means that…
Nanashi No Gombe
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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 write the field as $a = \langle a\rangle …
F.ert
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Axion effective potential

I am reading this https://arxiv.org/abs/hep-ph/0607268 paper, the Strong CP Problem and Axions by R. D. Peccei. Here they say that $$\bigg< \frac{\partial V_{eff}}{\partial a}\bigg> = - \frac{\xi}{f_a} \frac{g_s^2}{32\pi^2}\left
RoTheory73
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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: T. Goto, M. Yamaguchi, "Is axino dark matter possible in supergravity?", Phys. Lett. B 276 (1992) 103-107. In this paper, they think of a specific…
Gould67
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Is there really a strong CP problem?

The strong CP problem is considered as 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 the electric dipole moment of the neutron…
MadMax
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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 mechanism allows to introduce a scalar field $\eta$…
Nathanael Noir
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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 magnetic fields to convert axions to photons and vice versa…
Halo
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