F-theory is a toolbox for describing type IIB string theory – including non-perturbative effects induced from the presence of D7-branes and (p,q)-strings – in terms of complex elliptic fibrations whose fiber modulus $\tau$ encodes the axio-dilaton (the coupling constant and the degree-0 RR-field) tranforming under the $SL(2, \mathbb{Z})$ S-duality/U-duality. See also at duality in string theory.
More technically, F-theory is what results when KK-compactifying M-theory on a elliptic fibration (which yields type IIA superstring theory compactified on a circle bundle) followed by T-duality with respect to one of the two cycles of the elliptic fiber. The result is (uncompactified) type IIB superstring theory with axio-dilaton given by the moduli of the original elliptic fibration.
Or rather, this is type IIB string theory with some non-perturbative effects included. With a full description of M-theory available also F-theory should be a full non-perturbative description of type IIB string theory, but absent that it is some kind of approximation. For instance while the modular structure group of the elliptic fibration in principle encodes (necessarily non-perturbative) S-duality effects, it is presently not actually known in full detail how this affects the full theory, notably the proper charge quantization law of the 3-form fluxes, see at S-duality – Cohomological nature of the fields under S-duality for more on that.
The following line of argument shows why first compactifying M-theory on a torus $S_1^A \times S_1^B$ to get type IIA on a circle and then T-dualizing that circle to get type IIB indeed only depends on the shape $\frac{R_A}{R_B}$ of the torus, but not on its other geometry.
By the dualities in string theory, 10-dimensional type II string theory is supposed to be obtained from the UV-completion of 11-dimensional supergravity by first dimensionally reducing over a circle $S^1_A$ – to obtain type IIA supergravity – and then applying T-duality along another circle $S^1_B$ to obtain type IIB supergravity.
To obtain type IIB sugra in noncompact 10 dimensions this way, also $S^1_B$ is to be compactified (since T-duality sends the radius $r_A$ of $S^1_A$ to the inverse radius $r_B = \ell_s^2 / R_A$ of $S^1_B$). Therefore type IIB sugra in $d = 10$ is obtained from 11d sugra compactified on the torus $S^1_A \times S^1_B$. More generally, this torus may be taken to be an elliptic curve and this may vary over the 9d base space as an elliptic fibration.
Applying T-duality to one of the compact direction yields a 10-dimensional theory which may now be thought of as encoded by a 12-dimensional elliptic fibration. This 12d elliptic fibration encoding a 10d type II supergravity vacuum is the input data that F-theory is concerned with.
A schematic depiction of this story is the following:
M-theory in $d = 11$ | F-theory in $d = 12$ | |
$\downarrow$ KK-reduction along elliptic fibration | $\downarrow$ axio-dilaton is modulus of elliptic fibration | |
IIA string theory in $d = 9$ | $\leftarrow$T-duality$\rightarrow$ | IIB string theory in $d = 10$ |
In the simple case where the elliptic fiber is indeed just $S^1_A \times S^1_B$, the imaginary part of its complex modulus is
By following through the above diagram, one finds how this determines the coupling constant in the type II string theory:
First, the KK-reduction of M-theory on $S^1_A$ yields a type IIA string coupling
Then the T-duality operation along $S^1_B$ divides this by $R_B$:
The general vacuum of type II superstring theory (including type I superstring theory) is an orientifold.
The target space data of an orientifold is a $\mathbb{Z}_2$-principal bundle/local system, possibly singular (hence possibly on a smooth stack). On the other hand, the non-singular part of the elliptic fibration that defines the F-theory is a $SL_2(\mathbb{Z})$-local system (being the “homological invariant” of the elliptic fibration).
An argument due to (Sen 96, Sen 97) says that the F-theory data does induce the orientifold data along the subgroup inclusion $\mathbb{Z}_2 \hookrightarrow SL_2(\mathbb{Z})$.
The degeneration locus of the elliptic fibration is that of D7-branes and O7-planes.
Reasoning like this might suggest that in generalization to how type II orientifolds involve $\mathbb{Z}_2$-equivariant K-theory (namely KR-theory), so F-theory should involve $SL_2(\mathbb{Z})$-equivariant elliptic cohomology. This was indeed conjectured in (Kriz-Sati 05, p. 3, p.17, 18). For more on this see at modular equivariant elliptic cohomology.
The duality between F-theory and heterotic string theory:
F-theory on an elliptically fibered K3 is supposed to be equivalent to heterotic string theory compactified on a 2-torus. An early argument for this is due to (Sen 96).
More generally, heterotic string theory on an elliptically fibered Calabi-Yau $Z \to B$ of complex dimension $(n-1)$ is supposed to be equivalent $F$-theory on an $n$-dimensional $X\to B$ with elliptic K3-fibers.
A detailed discussion of the equivalence of the respective moduli spaces is originally due to (Friedman-Morgan-Witten 97). A review of this is in (Donagi 98).
For F-theory a fairly advanced model building and string phenomenology has been developed. A detailed review is in (Denef 08).
Via the relation between supersymmetry and Calabi-Yau manifolds there is particular interest in F-theory compactied on Calabi-Yau spaces of (complex) dimension 4. For more on this see at F/M-theory on elliptically fibered Calabi-Yau 4-folds.
See also at flux compactification and landscape of string theory vacua.
F-theory KK-compactified on elliptically fibered complex analytic fiber $\Sigma$
$dim_{\mathbb{C}}(\Sigma)$ | 1 | 2 | 3 | 4 |
---|---|---|---|---|
F-theory | F-theory on CY2 | F-theory on CY3 | F-theory on CY4 |
The original article is
An early survey of its relation to M-theory with M5-branes is in
Lecture notes include
F-theory lifts of orientifold backgrounds were first identified in
Ashoke Sen, F-theory and Orientifolds (arXiv:hep-th/9605150)
Ashoke Sen, Orientifold Limit of F-theory Vacua (arXiv:hep-th/9702165)
with more details including
This is further expanded on in
A series of articles arguing for a relation between the elliptic fibration of F-theory and elliptic cohomology (see also at modular equivariant elliptic cohomology)
Robert Friedman, John Morgan, Edward Witten, Vector Bundles And F Theory (arXiv:hep-th/9701162)
Ron Donagi, ICMP lecture on heterotic/F-theory duality (arXiv:hep-th/9802093)
A large body of literature is concerned with particle physics string phenomenology modeled in the context of F-theory.
Frederik Denef, Les Houches Lectures on Constructing String Vacua, in String theory and the real world (arXiv:0803.1194)
Chris Beasley, Jonathan J. Heckman, Cumrun Vafa, GUTs and Exceptional Branes in F-theory - I (arxiv:0802.3391), II: Experimental Predictions (arxiv:0806.0102)
Martin Wijnholt, String compactification, PITP 2014 lecture notes (pdf, slides for lecture 1, slides for lecture 2, slides for lecture 3)
The image of the supergravity C-field from 11-dimensional supergravity to F-theory yields the $G_4$-flux.
Andres Collinucci, Raffaele Savelli, On Flux Quantization in F-Theory (2010) (arXiv:1011.6388)
Sven Krause, Christoph Mayrhofer, Timo Weigand, $G_4$ flux, chiral matter and singularity resolution in F-theory compactifications (arXiv:1109.3454)
Thomas Grimm, Denis Klevers, Maximilian Poretschkin, Fluxes and Warping for Gauge Couplings in F-theory (arXiv:1202.0285)
Sven Krause, Christoph Mayrhofer, Timo Weigand, Gauge Fluxes in F-theory and Type IIB Orientifolds (2012) (arXiv:1202.3138)
and with M5-brane instanton contributions:
Reviewed in
For more on this see also at F/M-theory on elliptically fibered Calabi-Yau 4-folds.