Types of quantum field thories
This page reviews some of the relation between the existence of supersymmetries in a spacetime quantum field theory which arises as the effective quantum field theory of some string 2d SCFT, the special geometry of that spacetime (such as Calabi-Yau manifold structure) as well as increased worldsheet supersymmetry of the superstring.
(Here the notion of covariant derivative includes the usual Levi-Civita connection, but also in general torsion components and contributions from other background gauge fields such as a Kalb-Ramond field and the RR-fields in type II supergravity or heterotic supergravity.)
Of particular interest to phenomenologists around the turn of the millenium (but maybe less so today with new experimental evidence) has been in KK-compactification solutions of spacetime manifolds of the form for the locally observed Minkowski spacetime (that plays a role as the background for all available particle accelerator experiments) and a small closed 6-dimensional Riemannian manifold .
In the absence of further fields besides gravity, the condition that such a configuration has precisely one Killing spinor and hence precisely one global supersymmetry turns out to tbe precisely that is a Calabi-Yau manifold. One such remaining global supersymmetry in the low energy effective field theory in 4-dimensions is or was believed to be of relevance in phenomenology, see for instance the supersymmetric MSSM extension of the standard model of particle physics. This is where all the interest into these Calaboi-Yau manifolds in string theory comes from. (Notice though that nothing in the theory itself demands such a compactification. It is only the phenomenological assumption of the factorized spacetime compactification together with supersymmetry that does so).
Alternatively, if one starts the KK-compactification not from 10-dimensional string theory but from 11-dimensional supergravity/M-theory, then the condition for the KK-compactification to preserved precisely one global supersymmetry is that it be on a G2-manifold. For more on this see at M-theory on G2-manifolds.
On the other hand, the enhanced global supersymmetry of target space is also reflected in enhanced local supersymmetry on the worldsheet of the string. For instance for the heterotic string whose worlsheet 2d SCFT apriori has supersymmetry, the target space theory has supersymmetry precisely if the worldsheet theory’s supersymmetry enhanced to . (BDFF 88). For more on this see at 2d (2,0)-superconformal QFT.
Similar comments apply to type II superstring theory, where target space supersymmetry enhanced the worldheet symmetry from to . This is reflected notably in the mirror symmetry of the targt Calabi-Yau manifolds.
The above analysis in string perturbation theory? is expected to find a non-perturbative lift to M-theory/F-theory. Under this lift, compactification on a Calabi-Yau complex-3-fold (CY3) lifts to compactification on a G2-manifold/CY4-fold, respectively:
|perspective||KK-compactification with supersymmetry|
|M-theory||M-theory on G2-manifolds|
|F-theory||F-theory on CY4-manifolds|
|heterotic string theory||heterotic string theory on CY3-manifolds|
The idea originates in
where in the introduction it says the following
Recently, the discovery  of anomaly cancellation in a modified version of supergravity and superstring theory with gauge group or has opened the possibility that these theories might be phenomenologically realistic as well as mathematically consistent. A new string theory with gauge group has recently been constructed  along with a second theory.
For these theories to be realistic, it is necessary that the vacuum state be of the form , where is four-dimensional Minkowski space and K is some compact six-dimensional manifold. (Indeed, Kaluza-Klein theory – with its now widely accepted interpretation that all dimensions are on the same logical footing – was first proposed  in an effort to make sense out of higher-dimensional string theories). Quantum numbers of quarks and leptons are then determined by topological invariants of and of an or gauge field defined on . Such considerations, however, are far from uniquely determining .
In this paper, we will discuss some considerations, which, if valid, come very close to determining uniquely. We require
(i) The geometry to be of the form , where is a maximally symmetric spacetime.
(ii) There should be an unbroken supersymmetry in four dimensions. General arguments  and explicit demonstrations  have shown that supersymmetry may play an essential role in resolving the gauge hierarchy or Dirac large numbers problem. These arguments require that supersymmetry is unbroken at the Planck (or compactification) scale.
(iii) The gauge group and fermion spectrum should be realistic.
These requirements turn out to be extremely restrictive. In previous ten-dimensional supergravity theories, supersymmetric configurations have never given rise to chiral fermions – let alone to a realistic spectrum. However, the modification introduced by Green and Schwarz to produce an anomaly-free field theory also makes it possible to satisfy these requirements. We will see that unbroken supersymmetry requires that have, for perturbatively accessible configurations, holonomy and that the four-dimensional cosmological constant vanish. The existence of spaces with holonomy was conjectured by Calabi  and proved by Yau .
(Of course later it was understood that Calabi-Yau spaces, even those of complex dimension 3, are not “very close to unique”.)
Further original references include
Andrew Strominger, Special Geometry, Comm. Math. Phys. 133 (1990) 163.
Philip Candelas and X. De la Ossa, Moduli Space of Calabi-Yau Manifolds, Nucl. Phys. B355 (1991) 455.
and chapters 12 - 16 of
A canonical textbook reference for the role of Calabi-Yau manifolds in compactifications of 10-dimensional supergravity is
Pierre Deligne, Pavel Etingof, Dan Freed, L. Jeffrey, David Kazhdan, John Morgan, D.R. Morrison and Edward Witten, eds. , Quantum Fields and Strings, A course for mathematicians, 2 vols. Amer. Math. Soc. Providence 1999. (web version)
Lecure notes in a more general context of string phenomenology include
Discussion of generalized Calabi-Yau backgrounds is for instance in