The purpose of this section is to briefly review the Green–Schwarz (GS) formulation of the superstring. This is not done in a self-contained way, but rather as a very swift presentation of the features that will turn out to be universal in the formulation of brane effective actions.

There exist two distinct formulations for the (super)string:

- The worldsheet supersymmetry formulation, called the Ramond–Neveu–Schwarz (RNS) formulation
^{2}, where supersymmetry in 1 + 1 dimensions is manifest [432, 404]. - The GS formulation, where spacetime supersymmetry is manifest [256, 257, 258].

The RNS formulation describes a 1 + 1 dimensional supersymmetric field theory with degrees of freedom transforming under certain representations of some internal symmetry group. After quantisation, its spectrum turns out to be arranged into supersymmetry multiplets of the internal manifold, which is identified with spacetime itself. This formulation has two main disadvantages: the symmetry in the spectrum is not manifest and its extension to curved spacetime backgrounds is not obvious due to the lack of spacetime covariance.

The GS formulation is based on spacetime supersymmetry as its guiding symmetry principle. It allows a covariant extension to curved backgrounds through the existence of an extra fermionic gauge symmetry, kappa symmetry, that is universally linked to spacetime covariance and supersymmetry, as I will review below and in Sections 3 and 4. Unfortunately, its quantisation is much more challenging. The first volume of the Green, Schwarz and Witten book [260] provides an excellent presentation of both these formulations. Below, I just review its bosonic truncation, construct its supersymmetric extension in Minkowski spacetime, and conclude with an extension to curved backgrounds.

Just as point particles can be charged under gauge fields, strings can be charged under 2-forms. The coupling to this extra field is minimal, as corresponds to an electrically-charged object, and is described by a Wess–Zumino (WZ) term

where the charge density was introduced and stands for the pullback of the -dimensional bulk 2-form , i.e., Thus, the total bosonic action is: Notice the extra coupling preserves worldsheet diffeomorphism invariance and spacetime covariance. In the string theory context, this effective action describes the propagation of a bosonic string in a closed string background made of a condensate of massless modes (gravitons and Neveu–Schwarz Neveu–Schwarz (NS-NS) 2-form ). In that case, where stands for the length of the fundamental string. For completeness, let me stress that at the classical level, the dynamics of the background fields
(couplings) is not specified. Quantum mechanically, the consistency of the interacting theory
defined in Eq. (6) requires the vanishing of the beta functions of the general nonlinear sigma
models obtained by expanding the action around a classical configuration when dealing with
the quantum path integral. The vanishing of these beta functions requires the background to
solve a set of equations that are equivalent to Einstein’s equations coupled to an antisymmetric
tensor^{3}.
This is illustrated in Figure 2.

In analogy with the covariant superparticle [118], consider the action

This uses the Polyakov form of the actionEven though the constructed action is supersymmetric and 2d diffeomorphic invariant, the number of on-shell bosonic and fermionic degrees of freedom does not generically match. To reproduce the supersymmetry in the spectrum derived from the quantisation of the RNS formulation, one must achieve such matching.

The current standard resolution to this situation is the addition of an extra term to the action while still preserving supersymmetry. This extra term can be viewed as an extension of the bosonic WZ coupling (4), a point I shall return to when geometrically reinterpreting the action so obtained [294]. Following [260], it turns out the extra term is

Invariance under global supersymmetry requires, up to total derivatives, the identity for . This condition restricts the number of spacetime dimensions and the spinor representation to be- and is Majorana;
- and is Majorana or Weyl;
- and is Weyl;
- and is Majorana–Weyl.

Let us focus on the last case, which is well known to match the superspace formulation of
type IIA/B^{5}
Despite having matched the spacetime dimension and the spinor representation by the requirement of
spacetime supersymmetry under the addition of the extra action term (11), the number of on-shell bosonic
and fermionic degrees of freedom remains unequal. Indeed, Majorana–Weyl fermions in have 16
real components, which get reduced to 8 on-shell components by Dirac’s equation. The extra
gives rise to a total of 16 on-shell fermionic degrees of freedom, differing from the 8
bosonic ones coming from the 10-dimensional vector representation after gauge-fixing worldsheet
reparameterisations.

The missing ingredient in the above discussion is the existence of an additional fermionic
gauge symmetry, kappa symmetry, responsible for the removal of half of the fermionic degrees of
freedom.^{6}
This feature fixes the fermionic nature of the local parameter and requires to transform by some
projector operator

The purpose of going over this explicit construction is to reinterpret the final action in terms of
a more geometrical structure that will be playing an important role in Section 3.1. In more
modern language, one interprets as the action describing a superstring propagating
in super-Poincaré [259]. The latter is an example of a supermanifold with local coordinates
. It uses the analogue of the superfield formalism in global supersymmetric field theories
but in supergravity, i.e., with local supersymmetry. The superstring couples to two of these superfields, the
supervielbein and the NS-NS 2-form superfield , where the index M stands for curved
superspace indices, i.e., , and the index A for tangent flat superspace indices, i.e.,
^{7}.

In the case of super-Poincaré, the components are explicitly given by

These objects allow us to reinterpret the action in terms of the pullbacks of these bulk objects into the worldsheet extending the bosonic construction Notice this allows us to write both Eqs. (9) and (11) in terms of the couplings defined in Eq. (15). This geometric reinterpretation is reassuring. If we work in standard supergravity components, Minkowski is an on-shell solution with metric , constant dilaton and vanishing gauge potentials, dilatino and gravitino. If we work in superspace, super-Poincaré is a solution to the superspace constraints having non-trivial fermionic components. The ones appearing in the NS-NS 2-form gauge potential are the ones responsible for the WZ term, as it should for an object, the superstring, that is minimally coupled to this bulk massless field.It is also remarkable to point out that contrary to the bosonic string, where there was no a priori reason why the string tension should be equal to the charge density , its supersymmetric and kappa invariant extension fixes the relation . This will turn out to be a general feature in supersymmetric effective actions describing the dynamics of supersymmetric states in string theory.

It is important to stress that in the GS formulation, kappa symmetry invariance requires the background fields to be on-shell, whereas in the RNS formulation, it is quantum Weyl invariance that ensures this self-consistency condition, as illustrated in Figure 2.

The purpose of Section 3.1 is to explain how these ideas and necessary symmetry structures to achieve a manifestly spacetime covariant and supersymmetric invariant formulation extend to different half-BPS branes in string theory. More precisely, to M2-branes, M5-branes and D-branes.

Living Rev. Relativity 15, (2012), 3
http://www.livingreviews.org/lrr-2012-3 |
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