nLab orientifold



String theory

Differential cohomology



An orientifold (Dai-Leigh-Polchinski 89, p. 12) is a target spacetime for string sigma-models that combines aspects of 2\mathbb{Z}_2-orbifolds with orientation reversal on the worldsheet, whence the name.

In type II string theory orientifold backgrounds (inducing e.g. type I string theory, and the Sugimoto string) with 2\mathbb{Z}_2-fixed points – called O-planes (see there for more) – are required for RR-field tadpole cancellation. This is a key consistency condition in particular for intersecting D-brane models used in string phenomenology.

Where generally (higher gauge) fields in physics/string theory are cocycles in (differential) cohomology theory and typically in complex oriented cohomology theory, fields on orientifolds are cocycles in genuinely 2\mathbb{Z}_2-equivariant cohomology and typically in real-oriented cohomology theory. For instance, the B-field, which otherwise is a (twisted) cocycle in (ordinary) differential cohomology, over an orientifold is a cocycle in (twisted) HZR-theory, and the RR-fields, which usually are cocycles in (twisted differential) K-theory, over an orientifold are cocycles in KR-theory (Witten 98).

An explicit model for B-fields for the bosonic string on orientifolds (differential HZR-theory) is given in (Schreiber-Schweigert-Waldorf 05) and examples are analyzed in this context in (Gawedzki-Suszek-Waldorf 08). See also (HMSV 16, HMSV 19).

The claim that for the superstring the B-field is more generally a cocycle with coefficients in the Picard infinity-group of complex K-theory (super line 2-bundles) and a detailed discussion of the orientifold version of this can be found in (Distler-Freed-Moore 09, Distler-Freed-Moore 10) with details in (Freed 12).

The quadratic pairing entering the 11d Chern-Simons theory that governs the RR-field here as a self-dual higher gauge field is given in (DFM 10, def. 6).


Orientifold backreaction?

Essentially all existing results on orientifolds (such as O-plane-charges and RR-field tadpole cancellation) are derived for string sigma-models on flat orbifold (toroidal orbifold) or orientifolded Calabi-Yau manifold target spacetimes, or for orientifoldings of algebraically defined rational CFT string vacua (Gepner models, non-geometric vacua).

No string-theory results on back-reacted orientifolds

There are to date no results in actual string theory for what one might expect to be the curved back-reacted geometry of orientifolds, analogous to the curved near horizon geometry that is well-known for the case of black D-branes.

From Banks-van den Broeck 06:

The discussion of these [[orientifold]] compactifications is generally carried out in low energy effective field theory [[3a, 3b ]] [[4]], despite the fact that they all contain orientifold singularities. Further, there is no perturbative world sheet treatment of these backgrounds.


The inevitable orientifold of flux compactifications is one potential barrier to an effective field theory treatment [[footnote 1 : G. Moore and S. Ramanujam have emphasized to us the problems with the back reaction of the orientifold [...[...] ]]

From Cordova-de Luca-Tomasiello 19 (p. 2 and p. 30):

To assess the validity of these [[assumed effective orientifold]] solutions in string theory, one should ideally use the full string theory action, or switch to a dual description. Unfortunately neither of these options is available


the presence of O-planes is inferred by comparison with their flat-space behavior. Since the [[spacetimes with orientifolds considered]] have strong curvature and coupling, stringy corrections come into play, and it is impossible to decide with supergravity alone whether the solutions are valid.

It is important to stress that this will be so for any solution with O-planes. It would be important, then,to develop techniques to decide whether a solution with O-planes will survive in full string theory. In other words, it would be important to understand what conditions one needs to impose near the O-plane singularities.


We clearly need alternative procedures that are better justified physically.

A popular assumption in effective field theory

Nevertheless, motivated from the fact that the computations for flat orbifolds show that the O-planes there are similar to (while clearly different from!) D-branes (not in their back-reacted form as black branes, though!) with negative tension (i.e. negative energy density) it may seem plausible that the low energy effective field theory of perturbative string theory vacua including O-planes is a modification of supergravity where negative-energy source-terms are added to the equations of motion much like one might add black D-brane-contributions, but simply equipped with a negative sign prefactor.

This ad hoc effective field theory picture of orientifold backgrounds has been advocated, seminally, in Giddings-Kachru-Polchinski 01, in a one-sentence argument (below (2.19)):

String theory does have such [[negative tension]] objects, and so evades the no-go theorem [[which rules out certain warped solutions of supergravity]].

There this is followed by reference to a presumed example considered earlier in Verlinde 00, where the statement is introduced in a similar manner (above (9)):

In a more complete treatment, we must also include the backreaction of the 64 orientifold planes. These have a negative tension equal to −1/4 times the D3-brane tension, which need to be taken into account. We can write an explicit form for the background metric [[just as for D-branes but with a minus sign included]].

One may feel this is plausible – and it might even be right, sometimes – but there does not exist a derivation of this statement from actual perturbative string theory (see above), beyond the hand-waving leap of faith extrapolating from perturbative string theory on flat orientifolds to curved backreacted orientifold throat geometries (if such indeed exist).

Its use in the landscape/swampland literature

Nevertheless, in the wake of the discussion of the landscape (or not) of de Sitter string theory vacua and of the “swampland conjectures” it became popular to rely on the handwaving argument of Giddings-Kachru-Polchinski 01 and behave as if it is established that questions about low energy effective orientifold string theory vacua may be answered using a modification of supergravity where the equations of motion are changed – by hand – simply by including negative-tension source terms of some form.

This step happens for instance around (2.2) in Junghans 20 where it is justified, without references, by the words “as is standard in the literature” (footnote 5 in Junghans 20).

Lifts to M-theory F-theory

Lifts of orientifold background from type II string theory to F-theory go back to (Sen 96, Sen 97a).

Lifts of type IIA string theory orientifolds of D6-branes to D-type ADE singularities in M-theory (through the duality between M-theory and type IIA string theory) goes back to (Sen 97b). See at heterotic M-theory on ADE-orbifolds.

A more general scan of possible lifts of type IIA orientifolds to M-theory is indicated in (Hanany-Kol 00, around (3.2)), see (Huerta-Sati-Schreiber 18, Prop. 4.7) for details.

For instance the O4-plane lifts to the MO5-plane.




The concept originates around

Early accounts include

Traditional lecture notes include

Textbook discussion is in

and specifically in the context of intersecting D-brane models with an eye towards string phenomenology in


  • Marcus Berg, Introduction to Orientifolds (pdf, pdf)

The original observation that D-brane charge for orientifolds should be in KR-theory is due to

and was re-amplified in

In terms of KO-theory

Discussion of orbi-orienti-folds in terms of equivariant KO-theory is in

  • N. Quiroz, Bogdan Stefanski, Dirichlet Branes on Orientifolds, Phys.Rev. D66 (2002) 026002 (arXiv:hep-th/0110041)

  • Volker Braun, Bogdan Stefanski, Orientifolds and K-theory, Braun, Volker. “Orientifolds and K-theory.” Progress in String, Field and Particle Theory. Springer, Dordrecht, 2003. 369-372 (arXiv:hep-th/0206158)

  • H. Garcia-Compean, W. Herrera-Suarez, B. A. Itza-Ortiz, O. Loaiza-Brito, D-Branes in Orientifolds and Orbifolds and Kasparov KK-Theory, JHEP 0812:007, 2008 (arXiv:0809.4238)

A definition and study of orientifold bundle gerbes, modeling the B-field background for the bosonic string (differential HZR-theory), is in

see also

An elaborate formalization in terms of differential cohomology in general and twisted differential K-theory in particular that also takes the spinorial degrees of freedom into account is briefly sketched out in

based on stuff like

Details on the computation of string scattering amplitudes in such a background:

Related lecture notes / slides include

A detailed list of examples of KR-theory of orientifolds and their T-duality is in

A formulation of some of the relevant aspects of (bosonic) orientifolds in terms of the differential nonabelian cohomology with coefficients in the 2-group AUT(U(1))AUT(U(1)) coming from the crossed module [U(1) 2][U(1) \to \mathbb{Z}_2] is indicated in

More on this in section 3.3.10 of

Examples and Models

Specifically K3 orientifolds (𝕋 4/G ADE\mathbb{T}^4/G_{ADE}) in type IIB string theory, hence for D9-branes and D5-branes:

Specifically K3 orientifolds (𝕋 4/G ADE\mathbb{T}^4/G_{ADE}) in type IIA string theory, hence for D8-branes and D4-branes:

The N\mathbb{Z}_N action with even NN contains an order 2 element [...][ ...] Then there will be D8-branes in the type IIA D4-brane theory. Since the concept of intersecting D-branesinvolves use of the same dimensional D-branes, we restrict ourselves to the case that the order NN of N\mathbb{Z}_N is odd. (p. 4)

The Witten-Sakai-Sugimoto model on D4-D8-brane bound states for QCD with orthogonal gauge groups on O-planes:

Specifically D5 brane models T-dual to D6/D8 models:

Specifically for D6-branes:

  • S. Ishihara, H. Kataoka, Hikaru Sato, D=4D=4, N=1N=1, Type IIA Orientifolds, Phys. Rev. D60 (1999) 126005 (arXiv:hep-th/9908017)

  • Mirjam Cvetic, Paul Langacker, Tianjun Li, Tao Liu, D6-brane Splitting on Type IIA Orientifolds, Nucl. Phys. B709:241-266, 2005 (arXiv:hep-th/0407178)

Specifically for D3-branes/D7-branes:

Specifically 2d toroidal orientifolds:

2d toroidal orientifolds:


  • Dieter Lüst, S. Reffert, E. Scheidegger, S. Stieberger, Resolved Toroidal Orbifolds and their Orientifolds, Adv.Theor.Math.Phys.12:67-183, 2008 (arXiv:hep-th/0609014)

Orientifold Gepner models

Orientifolds of Gepner models

  • Brandon Bates, Charles Doran, Koenraad Schalm, Crosscaps in Gepner Models and the Moduli space of T 2T^2 Orientifolds, Advances in Theoretical and Mathematical Physics, Volume 11, Number 5, 839-912, 2007 (arXiv:hep-th/0612228)

Specifically string phenomenology and the landscape of string theory vacua of Gepner model orientifold compactifications:

Lift to M-theory

Lifts of orientifolds to M-theory (MO5, MO9) and F-theory are discussed in

The MO5 is originally discussed in

See also:

The classification in Hanany-Kol 00 (3.2) also appears, with more details, in Prop. 4.7 of

The “higher orientifold” appearing in Horava-Witten theory with circle 2-bundles replaced by the circle 3-bundles of the supergravity C-field is discussed towards the end of

Orientifold backreaction

Last revised on January 25, 2024 at 09:48:09. See the history of this page for a list of all contributions to it.