REVIEW
The QCD Axion and Moduli Stabilisation
High Energy Physics - Theory · Submitted 22 Feb 2006 · 41 references
Pith's verdict: A clean no-go theorem for supersymmetric moduli stabilisation with unfixed axions, plus an honest but conditional outline of a non-supersymmetric large-volume construction with f_a ~ sqrt(M_SUSY M_P).
T0 · AI review
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Referee report
The paper
The paper claims
A QCD axion can coexist with stabilised string moduli only if supersymmetry is broken by the moduli potential, and then the axion decay constant is forced to the geometric mean of the Planck mass and the supersymmetry-breaking scale.
The reading
What carries the argument
The load-bearing object is the F-term scalar potential of N=1 supergravity, $V=e^K(K^{i\bar j}D_iW\,\overline{D_jW}-3|W|^2)$, together with the holomorphy of the superpotential. The no-go theorem turns on the observation that a Peccei-Quinn shift symmetry forces $K$ to depend on $T+\bar T$ while the axion is the imaginary part of $T$; a flat axionic direction then forces $W$ to be independent of that $T$, and the identity $\partial_{\tau_u}^2 V = -4 e^K|W|^2 K_{u\bar u}$ makes the size direction tachyonic. On the constructive side, the machinery is the exponentially large volume minimum of the $\alpha'^3$-corrected Kähler potential, stabilised by the competition between the nonperturbative superpotential and the $\xi/V^3$ correction; a subleading Kähler correction $\delta K = \epsilon\sqrt{\tau_2}/V+\epsilon\sqrt{\tau_3}/V$ creates a positive wall that stops the blow-up cycles from collapsing. The final relation $f_a = \sqrt{M_P m_{\rm soft}}$ follows from the volume scalings $f_a\sim M_P/\sqrt{V}$ and $m_{\rm soft}\sim M_P/V$.
Extended reading notes
Core claim
On the paper's own terms, the central result is a split verdict. First, a general no-go theorem: no supersymmetric minimum of the F-term potential can coexist with a flat axionic direction and fully stabilised moduli. If the axion $b_u$ is unfixed, holomorphy forces the superpotential to be independent of $T_u$, and then $D_{T_u}W=0$ forces either $\partial_{T_u}K=0$ or $W=0$; in the generic case the second derivative along the size modulus is $\partial_{\tau_u}^2 V = -4 e^K |W|^2 K_{u\bar u} < 0$, making that direction tachyonic, and even if the Breitenlohner-Freedman bound makes this stable in AdS, the required uplift to Minkowski space removes that protection. Second, the no-go can be bypassed: nonsupersymmetric minima with a massless axion exist in the exponentially large volume compactifications, provided a Kähler correction of the form $\delta K = \epsilon\sqrt{\tau_2}/V + \epsilon\sqrt{\tau_3}/V$ prevents the blow-up cycles from collapsing and provided instantons with two or four fermionic zero modes are absent on the QCD cycle. Third, in those vacua the axion decay constant is set by the volume, $f_a \sim M_P/\sqrt{V}$, while soft masses are $m_{\rm soft}\sim M_P/V$, so $f_a = \sqrt{M_P\, m_{\rm soft}}$—the geometric mean of the Planck scale and the supersymmetry-breaking scale.
Load-bearing premise
The proposed massless-axion vacuum depends on the existence of a specific Kähler correction of the form $\delta K = \epsilon\sqrt{\tau_2}/V + \epsilon\sqrt{\tau_3}/V$ having the right sign and magnitude to stop the blow-up cycles from collapsing, and on the complete absence of instantons with two or four fermionic zero modes on the QCD cycle; the paper adjusts $\epsilon$ phenomenologically and assumes the instanton absence by fiat.
Editorial extensions
If this is right
- Supersymmetric AdS constructions such as the KKLT scenario cannot hide a QCD axion in a flat direction: after uplift, the tachyonic Kähler modulus either destabilises the vacuum or produces an unobserved fifth force.
- A surviving QCD axion forces the moduli potential to break supersymmetry, biasing string model building toward gravity mediation and intermediate string scales.
- In large-volume compactifications the axion decay constant is set by the volume; to land in the allowed window, the internal volume must be exponentially large rather than string-sized.
- The relation $f_a \approx \sqrt{M_{\rm SUSY} M_P}$ predicts that TeV-scale supersymmetry breaking comes with $f_a \sim 10^{10}$ GeV, placing the axion in the range targeted by direct dark-matter searches.
- Higher-order instanton corrections on the QCD cycle must be absent or suppressed below $10^{-85} M_P^4$ for the axion to solve strong CP; the paper's tables give the precise bounds as a function of the string scale.
Reading between the lines
- The no-go theorem is stated in N=1 supergravity language and therefore also constrains axions in IIA flux compactifications and heterotic models; a systematic scan of known vacuum constructions could quantify just how few string vacua admit a QCD axion.
- If low-energy supersymmetry is discovered with soft masses around the TeV scale, the relation $f_a \approx \sqrt{M_P m_{\rm soft}}$ becomes a testable target: an axion search with sensitivity near $10^{10}$ GeV would either corroborate these large-volume vacua or force a different mechanism.
- The required Kähler correction, while phenomenologically adjusted here, is of the form expected from loop corrections computed in toroidal examples; computing it explicitly for a concrete Calabi-Yau orientifold would turn the construction from an existence proof into a predictive model.
- The same argument applies to any exactly massless axion-like particle, not just the QCD axion, suggesting that string-derived ALPs in stabilised vacua are either heavy or require special geometric conditions.
Original abstract
We investigate the conditions for a QCD axion to coexist with stabilised moduli in string compactifications. We show how the simplest approaches to moduli stabilisation give unacceptably large masses to the axions. We observe that solving the F-term equations is insufficient for realistic moduli stabilisation and give a no-go theorem on supersymmetric moduli stabilisation with unfixed axions applicable to all string compactifications and relevant to much current work. We demonstrate how nonsupersymmetric moduli stabilisation with unfixed axions can be realised. We finally outline how to stabilise the moduli such that f_a is within the allowed window 10^9 GeV < f_a < 10^{12} GeV, with f_a ~ \sqrt{M_{SUSY} M_P}.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We investigate the conditions for a QCD axion to coexist with stabilised moduli in string compactifications. We show how the simplest approaches to moduli stabilisation give unacceptably large masses to the axions. We observe that solving the F-term equations is insufficient for realistic moduli stabilisation and give a no-go theorem on supersymmetric moduli stabilisation with unfixed axions applicable to all string compactifications and relevant to much current work. We demonstrate how nonsupersymmetric moduli stabilisation with unfixed axions can be realised. We finally outline how to stabilise the moduli such that f_a is within the allowed window 10^9 GeV < f_a < 10^{12} GeV, with f_a ~ \sqrt{M_{SUSY} M_P}.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We investigate the conditions for a QCD axion to coexist with stabilised moduli in string compactifications. | contribution 0.74 | 0.83 | 0.05 | unclear 0.54 |
| s1 | We show how the simplest approaches to moduli stabilisation give unacceptably large masses to the axions. | result 0.98 | 0.86 | 0.11 | asserted 1.00 |
| s2 | We observe that solving the F-term equations is insufficient for realistic moduli stabilisation and give a no-go theorem on supersymmetric moduli stabilisation with unfixed axions applicable to all string compactifications and relevant to much current work. | result 0.85 | 0.91 | 0.60 | asserted 1.00 |
| s3 | We demonstrate how nonsupersymmetric moduli stabilisation with unfixed axions can be realised. | contribution 0.69 | 0.91 | 0.18 | asserted 0.99 |
| s4 | We finally outline how to stabilise the moduli such that f_a is within the allowed window 10^9 GeV < f_a < 10^{12} GeV, with f_a ~ \sqrt{M_{SUSY} M_P}. | result 0.51 | 0.88 | 0.35 | asserted 0.86 |
What was examined, and what was not
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References
How later papers used it
203 later papers cite it
1 later paper by other authors is listed below: 1 compared its own results with it.
Compares with it 1 of 2 papers with this label
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Showed that tachyons can be avoided in most instances in the simplest LVS set-up with an arbitrary number of odd moduli, which it contrasted with this paper's tachyonic no-go for SUSY minima.
Systematics of type IIB moduli stabilisation with odd axions (2021) · 0.95 · paraphrased: the licence does not allow quoting
How the literature uses it
191 arXiv papers cite this in 264 sentences
Papers citing it · 145 sentences without a confident label
Compares with it 2 papers
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By studying the simplest LVS set-up with an arbitrary number of odd moduli, we show that tachyons can be avoided in most instances which needs to be contrasted with the tachyonic no-go of [ref] for SUSY minima.
Systematics of type IIB moduli stabilisation with odd axions · Non-perturbative superpotentials · 0.95
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The crucial difference to the setting in [ref] is that we are not stabilizing all fields supersymmetrically, i.e. supersymmetry is spontaneously broken in the $T$ direction and so the argument of [ref] does not apply in our case.
A sufficient condition for de Sitter vacua in type IIB string theory · Moduli masses · 0.88
Cites it as support 1 paper
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As emphasized in [ref] , this requires that they obtain a mass primarily from Kähler potential effects, rather than nonperturbative superpotential terms.
Extra-Dimensional Axion Expectations · Outlook · 0.97
Cites it as background 106 papers
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Starting from the ten-dimensional low energy Lagrangian of the heterotic string [ref] , and reducing to four dimensions by compactifying a six-dimensional manifold $Z$ whose volume is $V_Z$, an effective Lagrangian describing the field $ϕ= a/f_a$ is found to be [ref] : = f_a^2/2\(∂^μϕ\)\(∂_μϕ\) - Λ_a^4/2ϕ^2, where, in various string compactification models [ref] , the axion decay constant satisfies $f_a ≲x/S$, with $x$ a factor of $O(1)$ [For example, $x=√(2)/2$ in the context of the model-independent heterotic string.] and $S$ is the (dimensionless) action of the instantons that break the axion shift symmetry, generating the axion potential.
Cosmological window onto the string axiverse and the supersymmetry breaking scale · Axions in string theory · 0.86
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We show the so-called KKLT model [ref] with [equation] where $W_0 ≪1$.
Note on moduli stabilization, supersymmetry breaking and axiverse · KKLT · 0.89
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String theory and M-theory predict that there should be a large number of as-yet-undiscovered light bosonic degrees of freedom, including hidden $U(1)$ gauge fields, moduli, and axions [ref] .
Black Hole Spin Constraints on the Mass Spectrum and Number of Axion-like Fields · Introduction · 0.96
Discussion
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