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Part of the book series: Lecture Notes in Physics ((LNP,volume 951))

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Abstract

In this chapter we discuss the basic features of non-linear sigma models with (0,2) supersymmetry. This is a large universe, and to circumscribe our explorations we mainly stick to the theories relevant to compactifications of the heterotic string. To elucidate the geometric structures it turns out easiest to start with (0,1) supersymmetry. The reader may find it useful to skim through the geometry appendix before diving into the details of this chapter.

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Notes

  1. 1.

    We will follow a standard abuse of terminology: for us, unless otherwise noted, a compact manifold will mean a compact manifold without boundary.

  2. 2.

    We will always stick to closed Riemann surfaces, but those interested in D-branes and open strings will want to add boundaries and boundary conditions for the fields.

  3. 3.

    Our predilection for a flat worldsheet has caused us to leave out the dilaton coupling that can be added to the action on a curved worldsheet; qualitatively it is of the form ΔS =∫ΣDil(ϕ)R Σ, where Dil(ϕ) is the dilaton field and R Σ is the Ricci form on Σ.

  4. 4.

    More precisely, the β function is an obstruction to scale invariance; it is possible to find non-linear sigma models that are scale invariant but not conformally invariant[244], but the examples given are all non-compact. This is consistent with the theorem that a two-dimensional unitary and compact scale-invariant theory is automatically conformal [326]. We will not discuss non-compact non-linear sigma models and so will conflate conformal invariance with scale invariance.

  5. 5.

    More precisely, X is a submanifold that represents the homology cycle \(H_3(M,{\mathbb {Z}})\) obtained by gluing \(\widehat {\pi }(T)\) and \(\widehat {\pi }'(T)\) along the common boundary. It is always possible to represent such a cycle by a smooth submanifold [92].

  6. 6.

    More details may be found in the geometry appendix.

  7. 7.

    The canonical bundle is trivial on the plane or cylinder (or torus), but it is generally non-trivial on a Riemann surface Σ; for instance, on \(\Sigma = {\mathbb {P}}^1\), we have \(K_\Sigma = {\mathcal {O}}(-2)\), so that \(\overline {K}^{1/2}_\Sigma = \overline {{\mathcal {O}}(-1)}\). On the plane the notation is simply a reminder that ψ is a right-moving (aka anti-holomorphic) fermion, but we keep the more general notation since we will want to define twisted versions of this non-linear sigma model on a compact Riemann surface.

  8. 8.

    If this is confusing, the reader can take a look ahead to where we explicitly make a similar manipulation for the right-moving fermions.

  9. 9.

    For reasons that will become clear when we discuss sigma model anomalies, E is an oriented vector bundle; without that assumption the general structure group would have been \(\operatorname {O{}}(r)\).

  10. 10.

    The reader should be aware of some notational differences in the literature: for us it is \({\mathcal {S}}^{-}\) that shows up in the action, and, as we will see shortly \({\mathcal {S}}^{+}\) that shows up in the Green–Schwarz anomaly. Sometimes conventions have it just the other way around.

  11. 11.

    There is a relation between local Lorentz anomalies and diffeomorphism anomalies—there are local counter-terms that can be added to shift the Lorentz anomaly into a diffeomorphism anomaly. The former is simpler to work with, so we will stick to that. A relevant discussion may be found in the timeless [10, 11].

  12. 12.

    Either a full contraction is impossible, or it is zero due to symmetry properties under a ↔ b.

  13. 13.

    This fundamental result is due to Hopf and should be understood in the larger context of representability of cycles as described by Thom: for \(H_k(M,{\mathbb {Z}})\) with \(\dim M = n\), if k ≤ 6 or if n − 2 ≤ k ≤ n, then every cycle can be represented by a submanifold. A readable summary and references may be found in [92].

  14. 14.

    We are again slightly abusing notation here and conflating the topological invariants \(c_i \in H^{2i}(M,{\mathbb {Z}})\), with the specific representatives in terms of the curvature forms. This usually does not cause trouble, but the reader should be aware of this abuse.

  15. 15.

    We restored units of α′ in restating the quantization condition.

  16. 16.

    We work this out explicitly in an abelian example in Sect. B.2.4.

  17. 17.

    There is just one more exception in the case of Minkowski space heterotic string vacua— \({\mathbb {R}}^{1,2}\) with \({\mathcal {N}} = 1\) supersymmetry. All other Minkowski compactifications do require some amount of extended supersymmetry for the internal theory. More discussion of the exceptional \({\mathbb {R}}^{1,2}\) and \({\mathbb {R}}^{1,1}\) cases can be found in [303, 348]; exotic \({\mathbb {R}}^{1,1}\) supersymmetries are discussed in [166].

  18. 18.

    The presence of these operators of weight zero should not bother the reader, because, just as with a free compact boson, a ϕ by itself is not a well-defined local operator; those are constructed by pulling back tensors valued in the bundle E → M and contracting the indices with ∂ϕ, \(\bar {\partial }\phi \), ψ, and λ to construct gauge- and diffeomorphism-invariant operators.

  19. 19.

    Since we are now working with complex M, unless otherwise noted, T M will denote \(T_M^{1,0}\), the holomorphic tangent bundle.

  20. 20.

    This is discussed in the Sect. B.2.1.

  21. 21.

    Incidentally, these were some of the first constructions of complex non-Kähler manifolds [336].

  22. 22.

    Basic tensor identities such as \(g_{a\overline {a}} g_{b\overline {b}} {\epsilon }^{abc} = {\epsilon }_{\overline {a}\overline {b}\overline {c}} g^{\overline {c} c} \det g\) are useful in showing these relations.

  23. 23.

    G-structures and related notions of pure spinors play an important role in supersymmetric solutions of supergravity, both for type II and heterotic theories. We will not delve too far into these spacetime notions and refer the reader to some basic references. A very readable description of G-structures in the supergravity context is given in [190]; the connection to pure spinors and generalized geometry is discussed in many references; we refer to [203] for an introduction and references.

  24. 24.

    It is a familiar string–theoretic fact that the zero mode of the dilaton profile is related to the string coupling via e φ. We will be able to interpret the geometry as a perturbative string theory vacuum when the dilaton profile remains bounded on M and its zero mode is a modulus.

  25. 25.

    This can be contrasted with the result of Gauduchon that every compact complex manifold admits a hermitian metric with \(\partial \bar {\partial } \omega ^{n-1} = 0\).

  26. 26.

    A modern and thorough discussion of these statements is given in [127, 128].

  27. 27.

    As Exercise 4.2 indicates, the restriction to simply connected M is reasonable. When M is a compact Calabi–Yau manifold with exactly \(\operatorname {SU}(3)\) holonomy, then it cannot have continuous isometries and π 1(M) is a finite group. Therefore M has a cover \(\widetilde {M}\) that is simply connected [255]. Our statements then apply to the finite cover, and in principle computations for M can be obtained by taking an orbifold of \(\widetilde {M}\). As shown in, for instance, [34, 100], the orbifold can have important ramifications for the resulting SCFT.

  28. 28.

    This is a simple instance of a calibrated cycle: in this case the holomorphic curve σ( Σ) is said to be calibrated by the Kähler form. A more general discussion of calibrated geometry is given in [256].

  29. 29.

    Here ζ(3) is the Riemann zeta function, and χ(M) = 2(h 1, 1 − h 1, 2) is the Euler characteristic of M.

  30. 30.

    There is a similar ambiguity in the right-moving charge; however, if we flip both charges, we merely describe the moduli space in terms of the complex conjugate coordinates.

  31. 31.

    The Gepner models were classified in [288].

  32. 32.

    The reader may wish to review the spectral flow discussion in Chap. 2.

  33. 33.

    In pondering modular invariance it is useful to think of the internal (2,2) SCFT as “replacing” an \({\mathbb {R}}^6\) or T 6 in the usual construction of the ten-dimensional heterotic string. Spectral flow allows us to define NS/R sectors for the internal theory and show that they contribute in the same way to transformations of the worldsheet one-loop partition function as in the free theory. This is explained in detail in [194, 370].

  34. 34.

    It may be useful to review Exercise 2.9 on twisted fermions to see a convenient definition of fermion numbers and charges in the R sectors of the free theories.

  35. 35.

    We will suppress the omni-present momentum-dependent e ikX factor, and will not write explicit polarization tensors for the gravitino or gauginos.

  36. 36.

    The singularity need not be a singular SCFT: the correlation functions of the SCFT can remain perfectly finite as we approach the origin from either branch, as long as we keep track of the full SCFT spectrum, which will vary smoothly. If we artificially restrict attention to just the marginal operators, there will be a discontinuity at the origin. On the other hand, there are also singularities that cannot be understood in terms of the SCFT degrees of freedom, and in this case, the SCFT will be singular.

  37. 37.

    We refer to [28] and references therein for results obtained in the large radius limit. D-branes in type II theories also offer an example with some similar features, where the general open-close string deformation theory is encoded in a superpotential, which can be directly studied in the framework of topological field theory [40].

  38. 38.

    This was first discussed in the context of large radius compactifications in [384], but the idea applies to general deformations of (0,2) SCFTs.

  39. 39.

    A classic reference for this analysis is [144]; although it uses the non-linear sigma model for the internal SCFT, the worldsheet analysis is essentially identical. The reader may find it instructive to take a look and compare the abstract A/2 and B/2 structures to the non-linear sigma model realizations. We will have more to say on this in the next section.

  40. 40.

    This is a convention for the 8 c and 8 s representations.

  41. 41.

    This discussion closely follows [302].

  42. 42.

    This is not true for a general Hermitian metric, nor for the \({\mathcal {R}}_+\) curvature of the connection with torsion, and it is another indication that the equations themselves must be corrected in the α′ expansion.

  43. 43.

    The decomposition is discussed in Sect. B.3.4.

  44. 44.

    We depart from the notation Γ for the fermi multiplets to avoid confusion with the Levi-Civita connection. The chiral fermi multiplets ΛA will have the components λ A and L A that parallel the γ A and G A familiar from Sect. 1.8.

  45. 45.

    Some examples of these were investigated in unpublished work by McOrist and the author. For instance, one concrete non-trivial scenario involves a rank r = 4 compactification of the \(\operatorname {E}_8\times \operatorname {E}_8\) string on a K3 manifold. The structure group is enlarged as \(\operatorname {SU}(4)\to \operatorname {SO}(7)\), and the spacetime Higgs mechanism is \(\operatorname {\mathfrak {so}}(10) \to \operatorname {\mathfrak {so}}(9)\). A discussion of an r = 3 case on a Calabi–Yau 3-fold, where the terms give another parametrization of the \(\operatorname {\mathfrak {e}}_6\to \operatorname {\mathfrak {so}}(10)\) Higgs branch described above, can be found in [1]. Another study of these couplings, in the context of an orbifold compactification, is given in [317].

  46. 46.

    There is another equivalence relation possible as well— we can shift \({\mathcal {O}} \to {\mathcal {O}}+ \partial X'\), where ∂X′ is a well-defined chiral operator. The resulting “deformation” is a total derivative. Such operators are intimately tied to the right-moving chiral currents and should be considered in discussing theories with (0,4) or larger superconformal invariance, but they are not present in the \(\operatorname {SU}(3)\) structure non-linear sigma model of interest here [299].

  47. 47.

    Results highlighting the differences in this deformation theory and the more familiar Calabi–Yau case are presented in [251].

  48. 48.

    We confront here the confusion between the six-dimensional and two-dimensional supersymmetry labels. We will try to stick to (1,0) as the supersymmetry with eight chiral supercharges in six dimensions.

  49. 49.

    When R i is real or complex, N i is the number of hypermultiplets; when R i is pseudoreal, then representation is actually 2N i \({\textstyle \frac {1}{2}}\)-hypermultiplets, each in representation R i.

  50. 50.

    There is a nice presentation of these ideas in the Appendix of [327].

  51. 51.

    Taking a look at the characteristic classes discussion above, it is easy to see that for a holomorphic bundle over any complex surface \(\operatorname {ch}({\mathcal {E}}\otimes {\mathcal {E}}^\ast ) = r^2 +r(c_1({\mathcal {E}})^2-2 c_2({\mathcal {E}})).\) To obtain the counting above it is also important to remember that the worldsheet states count \({\textstyle \frac {1}{2}}\)-hypermultiplets.

  52. 52.

    We denote the intersection product \(H^2(M,{\mathbb {Z}}) \times H^2(M,{\mathbb {Z}}) \to H^4(M,{\mathbb {Z}})\) by ⋅; the product is symmetric and has signature (3,  19).

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Melnikov, I.V. (2019). Heterotic Non-linear Sigma Models. In: An Introduction to Two-Dimensional Quantum Field Theory with (0,2) Supersymmetry. Lecture Notes in Physics, vol 951. Springer, Cham. https://doi.org/10.1007/978-3-030-05085-6_4

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