**2.2 Dualities and Nongeometry**

**2.2.2 T-Duality and Mirror Symmetry**

In addition to S-duality, string theory contains new types of dualities which originate purely from the extended nature of strings. Among the most important ones is a class of highly non-trivial relations between different target space geometries, known by the name T-duality.

Circular Compactifications of the Bosonic String

To illustrate the idea, we begin by considering a closed bosonic string in D= 26
dimen-sions, with one direction X^{25} compactified on a circle of radius R [95]. While sharing
many similarities with the settings discussed in section 2.1, a particular property of this
model is that closed strings can wind around the internal direction. Such different
wind-ing states can be uniquely described by a so-calledwinding number p˜(see also figure 2.1),
which, due to the topology of the circle, is invariant under continuous transformations.

They thus define a conserved charge and are an inherent property of the theory. Depend-ing on the number of windDepend-ings around the circle, a closed strDepend-ing has to satisfy different boundary conditions

X^{25}(τ, σ+ 2π) = X^{25}(τ, σ) + 2πpR˜ (2.2.8)
ensuring periodicity along internal direction. From here, one can proceed by splitting the
bosonic fields into linear combinations of left- and right-movers,

X^{M}(τ, σ) =X_{L}^{M} (τ +σ)±X_{R}^{M}(τ −σ), M ∈ {0,1, . . . ,25}, (2.2.9)

and perform mode expansions analogously to the previously discussed settings. After tak-ing into account the boundary condition (2.2.8) and a somewhat lengthy calculation [95], one eventually arrives at the mass formula

m^{2} =p
R

2

+ pR˜

α^{0}
2

+ 2
α^{0}

N +Ne−2

(2.2.10) and the level-matching condition

N −Ne =p˜p (2.2.11)

ensuring worldsheet reparameterization invariance under constant shifts of σ. Here, p
denotes the (quantized) total momentum of the string along the internal direction, N
and Ne describe the left- respectively right-moving oscillation modes of the string, and
we included the slope parameter α^{0} = 1 explicitly for pedagogical reasons. Taking a
closer look at (2.2.10), one can see that the mass spectrum contains contributions of a
zero-point energy term, the internal momentum, the winding number and the left- and
right-moving oscillation modes.

Figure 2.1: Various topologically distinct configurations of a string winding around a two-dimensional cylinder. From top to bottom: ˜p=−1,0,1,2.

At this point, one might notice that the contributions originating from the internal momentumpand the winding number ˜pshow a very similar structure, the only difference being their inverse scaling behavior with respect to the radius R. This analogy extends so far that the two equations get mapped onto themselves when exchanging the roles of

2.2 Dualities and Nongeometry 23

p and ˜p while at the same time inverting the radius,
p↔p,˜ R↔ α^{0}

R. (2.2.12)

A remarkable feature of this result is that the inversion of the radius renders small and
large compactification spaces physically equivalent. In particular, there exists a self-dual
radius R =α^{0}, which defines a lower bound for all physically-distinct values of R. This
is a simple example of a T-duality transformation and nicely demonstrates how some of
the most fundamental concepts of geometry can break down in string theory.

Buscher Rules

Circular compactifications describe only one particular instance of T-duality in string theory, and there exist various approaches to generalize the idea. One concept which will be of essential importance for the upcoming sections and chapters are the so-called Buscher rules [96, 97]. To illustrate the idea, we follow the lines of [98] and consider a slightly generalized version of the Polyakov action (1.1.1) with a possibly non-vanishing Kalb-Ramond two-form field B. Employing conformal gauge hαβ = diag(−1,1) and complex worldsheet coordinates, the action takes the form

S = 1 2π

Z

Σ

d^{2}z (g_{mn}+B_{mn})∂x^{m}∂x¯ ^{n}, (2.2.13)
where the bosonic fieldsx^{m} can again be interpreted as coordinates of theD-dimensional
target space M. Assume now that there exists an Abelian 2π-periodic isometry for g,
generated by a corresponding Killing vector field k with Lkg = 0. Furthermore, let
L_{k}B =dω for some one-formω onM. One can then show that the transformation given
byδx^{m} =k^{m} is a symmetry of the action.

Using diffeomorphism-invariance, the coordinates can be chosen in a way that the
isometry k acts as translation in one particular direction x^{r}. Similarly, the B-field can
be brought to a form satisfying L_{k}B = _{∂r}^{∂}B = 0 via spacetime gauge transformations
B 7→ B +dχ with χ ∈ Ω^{1}(M), such that both fields g and B do not depend on the
isometric direction. Under these assumptions, the original action (2.2.13) can be obtained
from another sigma-model where the isometry appears as a gauge symmetry^{1}. More
precisely, consider the “master action”

S_{Master}= 1
2π

Z

Σ

d^{2}z

g_{rr}AA¯+ (g_{rp}+B_{rp})A∂x¯ ^{p}+ (g_{pr}+B_{pr})∂x^{p}A+¯
+ (g_{pq}+B_{pq})∂x^{p}∂x¯ ^{q}+θ ∂A¯−∂A¯

,

(2.2.14)

with some gauge field A = A(z)dz + ¯A(¯z)d¯z, a Lagrange multiplier θ and the indices p, q running over all values except for r. Now there are two routes to follow. Integrating out θ yields the equations of motion

∂A¯−∂A¯ = 0, (2.2.15)

1This method is referred to as “gauging the isometry” in most literature.

which in topologically trivial worldsheets Σ can be solved by letting the fields A and ¯A become pure gauge,

A=∂θ , A¯= ¯∂θ . (2.2.16)

Inserting these relations into (2.2.14) restores the original action (2.2.13), with θ taking
the role of the coordinatex^{r}. On the other hand, integrating out the gauge field A gives
rise to the equations of motion

(g_{rp} +B_{rp}) ¯∂x^{p}+g_{rr}A¯= 0,

(g_{pr}+B_{pr})∂x^{p}+g_{rr}A = 0, (2.2.17)
and their solutions

A¯=−g_{rp}+B_{rp}
g_{rr}

∂x¯ ^{p}− 1
g_{rr}

∂θ,¯
A =−g_{pr} +B_{pr}

g_{rr} ∂x^{p}+ 1
g_{rr}∂θ.

(2.2.18)

Substituting these expressions back into the master action (2.2.14) leads to a dual action S˜= 1

2π Z

Σ

d^{2}z(˜g_{mn}+ ˜B_{mn})∂x^{m}∂x¯ ^{n}, (2.2.19)
where the newly-introduced fields ˜g and ˜B are related to g and B by theBuscher rules

˜

g_{rr} = 1

g_{rr}, g˜_{rq} = B_{rq}

g_{rr}, g˜_{pq} =g_{pq}− g_{pr}g_{rq}+B_{pr}B_{rq}
g_{rr} ,
B˜_{rq} = g_{rq}

g_{rr}, B˜_{pq} =B_{pq}− g_{pr}B_{rq}+B_{pr}g_{rq}
g_{rr} .

(2.2.20)

Using a more involved approach, the above transformations can be generalized to sigma-models with non-trivial dilaton background. Computing the Buscher rules for such set-tings commonly requires consideration of path integrals at one loop, for which one obtains the transformation behaviour [96, 97]

φ=φ− 1

2lng_{rr}. (2.2.21)

Taking up the previous example of circular compactifications, the isometric direction is given by the azimutal angleϕ,and the above transformations correctly reproduce the T-duality mappings (2.2.12) we encountered earlier. In the case of D-dimensional tori, the T-duality transformations along the D different directions span the group O(D, D;Z), which builds the basis for the construction of double field theory. This will be discussed in more detail in chapter 4.

We should at this point remark that we kept our discussion of the Buscher roles on a somewhat superficial level, and there exist several subtleties that have to be taken into account to show that the dual models are truly equivalent as conformal field theories.

This was addressed in more detail in [98]. A generalization of the above approach to non-Abealian isometries can furthermore be found in [99].

2.2 Dualities and Nongeometry 25

Mirror Symmetry

While usually more intricate in nature, it is a common property of target-space du-alities to draw connections between geometries that, at first glance, seem completely unrelated or even antithetic. A particularly important example for this is Mirror Sym-metry [100,58], which relates the complex and K¨ahler structures of Calabi-Yau manifolds and is extensively studied various fields of pure mathematics [58–60]. Interestingly, this highly complex duality could be traced back to simple T-duality transformations by using standard string-theoretic methods. This postulated equivalence is known as the SYZ-conjecture [61].

As we will see in the following sections, T-duality not only unifies different concepts geometry, but also gives rise to new structures which cannot be described in terms of the widely used frameworks. Much of this thesis is devoted to studying recently-developed ex-tensions of field theory and geometry which allow for an integration of suchnongeometric phenomena.