An example: the Z 3 orbifold

Before we turn to theZ2×Z4orbifold in the next chapter, we complete the discussion on theZ3orbifold which is one of the most classic string compactification[34,35] space. We choose the SU(3)³root lattice as the torus lattice and the orbifold twist and gauge twist to be given as

v₃=(1 3,1

3,−2

3) V₃=(1 3,1

3,−2

3,0⁵)(0⁸), (2.65)

where we have chosen the standard embedding. Due to the identifications

e^2πivae_2a−1=e_2a fora=1,2,3, (2.66) we can switch on one order three Wilson line in thea-th torus which however would bring us away from the standard embedding. We find only one independent twisted sector generated byg(θ,0) because the second twisted sector is simply the inverseg⁰(θ²,0)= g⁻¹and carries the CPT conjugated states of the first one.

In the untwisted sector the mass equation is not shifted and this corresponds to the original 10D theory reduced by the orbifold projection. We have summarized the states that survive the projection in Table 2.1. We find indeed only one gravitino as we expect for anN =1 theory. We note that there are 9 moduli.

In particular the three diagonal moduli that are given when the−1 inqis at the j-th position,parametrize continuous deformations of the three torus radii. Furthermore we find six more off-diagonal Kähler

2.3 Heterotic Strings on Orbifolds

D=10 state projection condition resulting states Name

|qi ⊗αⁱ₋₁|0i q·v₃=0 mod 1

| ±1,0,0,0i ⊗α^µ₋₁|0i Gravity Multiplet

±|¹₂,¹₂,¹₂,¹₂i ⊗α^µ₋₁|0i Gravitino

|0,−1,0,0i ⊗α₋₁^j |0i 9 (bosonic) Kähler moduli

| − ¹₂,−¹₂,¹₂,¹₂|0i ⊗α₋₁^j |0i 9 (fermionic) Kähler moduli Table 2.1: Orbifold of the 10D SUGRA multiplet.

deformations that correspond to lattice deformations that respect theZ3orbifold action⁸. Also note that there are no complex structure moduli corresponding to|0,1,0,0i ⊗α₋₁^j |0ifields.

When we turn to the gauge group we observe that a 4D vector stateq_v=(±1,0,0,0) has only one SUSY partner qs = ±(¹₂,¹₂,¹₂,¹₂) on the left movers side, hence we confirm that the gauge multiplet contains only one gaugino, as we expect it from anN =1 theory in 4D. The projection on the right moving states is given by the additional constraintV₃·P=0 mod 1 and leads to the following states:

|q_vi ⊗α^I₋₁|0i 16 Cartan Elements,

|q_vi ⊗ |P₁i 6 SU(3) roots withP₁ =(1,−1,0,0⁵),

|q_vi ⊗ |P₁i 40 roots withP₁=((0,0,0),(±1,±1,0,0,0)),

|q_vi ⊗ |P˜₁i 32 roots with ˜P₁=(±(1 2,1

2,1 2),(±1

2)⁵),

|q_vi ⊗ |P₂i P₂240 roots ofE²₈.

In total the first E8breaks down to SU(3)×E6while the second one stays intact.

The untwisted chiral matter on the other hand is obtained from the right-moverq_s=(−¹₂,−¹₂,¹₂,¹₂) states that have a non-trivial weightPto satisfy for the constraintq·v−P·V₃=0 mod 1. The resulting states and their quantum numbers are summarized in Table2.2.

We can proceed in a similar manner for the twisted sector states that have the constructing elements g(θ,n_ae_a). As there is no Wilson line, we find the same shift, and the same projection conditions for every fixed point and hence we find the same spectrum at every fixed point. The invariant left moving shifts are given as

q^bos_sh =(0,1 3,1

3,1

3), q^fer_sh =(1 2,−1

6,−1 6,−1

6). (2.67)

The positive first factor in q^fer_sh indicates that we have found a right-handed fermion and its bosonic superpartner. For the above right mover, we find 27 left movers that survive the orbifold projection making up a (27,1,1) representation. Furthermore there are three oszillator solutions involving three P_shsolutions that survive the orbifold projection. These states lead to three (1,3,1) states.

Before we sum up all the states, we also discuss the flavor symmetry along the lines of [40]. For this it is sufficient to consider only one T²/Z3 plane. Lets consider first states from the first twisted sector. Here a state corresponds to an constructing elementg(θ,ne₁)rand from the point group element it becomes clear that an invariant coupling must involve at least three of them in order to be allowed.

In the following it is convenient to rewrite the generating elements as sublattice (11−θ)Λequivalent vectors. AsΛis generated bye₁ ande₂the sublattice is generated bye₁−e₂ and 3e₂. Hence we can

8The CPT complementary states are given by an overall sign change of the states.

represent the three fixed point generating elements asg(θ,0),g(θ,e₁) andg(θ,e₁+e₂) just viag(θ,me₁) with m=0,1,2.

At next we take the product ofnconstructing elements of states

n

Y

r

g(θ,m^re₁)=g(θⁿ,

n

X

r

m^re₁), (2.68)

where the lattice part can only be trivial whenPn

i =0 mod 3 and hence we also get aZ3family symmetry here. By the above rewriting, we give the fixed points a label i.e. a family quantum number. But when there is no Wilson line switched on, all fixed points are equivalent up to permutations. This is why we have to take thesemi-directproduct with the permutation groupS₃. Hence we find the resulting group

S₃n(Z3×Z3)= ∆(54). (2.69) The untwisted sector fields give trivial 1 representations and the twisted ones form triplet states 3.

Similarly as for SU(3)⁹there is an invariant combination1∈3·3·3given by an anti-symmetrization.

The total flavor group is obtained by recombining the parts of each orbifold plane. Hence we obtain three ∆(54) factors. However we have to remember that there is only one independent Z3 orbifold rotation. Thus we have to divide by two of them such that the total flavor group is given by∆(54)³/Z²₃ that lie in the centers of the∆54 . The final part is the R-symmetry that is generated by the individual sub lattice rotationse^2iπ3 in each torus, demanding the 4D super potential to have R-symmetry charge

Q_R(W)=(−1,−1,−1) mod 3. (2.70) However, we note that in 4D there is only one independent R-charge and that the orbifold identification should remove one discrete symmetry that one can choose for example as the diagonal sum of the three symmetries. Then there are two residual non-R symmetries whereas one is redundant by the orbifold action. The whole spectrum, including all quantum numbers is summarized in Table2.2. Clearly the

E₆×SU(3)×E₈rep. ∆(54)³/Z3flavor Rep. R-charge

(27,3,1) (1,1,1) (−1,0,0)

(27,3,1) (1,1,1) (0,−1,0)

(27,3,1) (1,1,1) (0,0,−1)

(27,1,1) (3,3,3) (−¹₃,−¹₃,−¹₃) (1,3,1) (3,3,3) (²₃,−¹₃,−¹₃) (1,3,1) (3,3,3) (−¹₃,²₃,−¹₃) (1,3,1) (3,3,3) (−¹₃,−¹₃,²₃)

Table 2.2: Summary of the charged spectrum of the standard embeddingZ3orbifold.

above spectrum and geometry is very simple which makes this orbifold a nice playground to study the heterotic string and its features. In Chapter4we come back to this particular geometry and study its Landau-Ginzburg Phase.

However this simple example provides already all kinds of symmetries that are interesting for particle physics, like flavor-and R-symmetries. Especially we have learned, that the number of family

represent-9Note that∆(54) is a discrete subgroup of SU(3).

2.3 Heterotic Strings on Orbifolds

ation i.e. the27’s are strongly coupled to the amount of fixed points and that certain Yukawa couplings can be invoked by the localization properties of the fields i.e. untwisted (bulk) fields VS. twisted (local-ized) fields. However, for (semi-) realistic string model building purposes, the above geometry is clearly unsuitable: There are way to many families and the E₆gauge group has to be broken down to the one of the standard model. By all these reasons it is necessary, to consider other orbifold geometries and to leave the standard embedding by considering shifts and Wilson lines that break directly down a stand-ard model gauge factor and breaks the fixed point degeneracy in a suitable way. These (semi-) realistic model building attempts will be the topic of the next chapter with an particular focus on theZ2 ×Z4

orbifold geometry.

CHAPTER 3

Particle physics from orbifolds: The mini-Landscape and its extensions

The vast amount of symmetries and the enormous computational control of the orbifold CFT made it a very fruitful playground to explore and obtain realistic models of particle physics. In particular, when the computer power grew there have been efforts to systematically scan over all orbifold geometries and gauge embeddings using the C++orbifolder [41]. The first systematic searches, made in theZ6−II

orbifold made it possible to explicitly construct O(200) string vacua with the features of the MSSM, the so called Mini-Landscape[24]. Another orbifold worth mentioning is based on theZ2×Z2orbifold [42]. In this orbifold, the Wilson line associated to a freely acting involution was used to break the SU(5) GUT group to that of the standard model but keeping the GUT structure at the fixed points. In this section we carry on with this program and consider model building on theZ2×Z4orbifold geometry and concentrate on its phenomenological properties.

In order to do so it is inevitable to first introduce the compactification geometry and its symmetries in Section3.1. We have seen in the chapter before that we have to embed the space group action into the gauge degrees of freedom. To anticipate the full potential of that geometry we cannot only rely on the standard embedding and hence we perform a full classification of all shift embeddings in Section 3.1.3. Finally we work out a combinatorial strategy that incorporates phenomenological requirements with the geometric properties that we have classified before in Section3.2. We highlight the validity of our strategy that can also explain the findings of the Mini-Landscape searches a posteriori with a Benchmark.

3.1 The Z

₂

× Z

₄

orbifolded String

To lay the grounds for model building in the next section we discuss the heterotic string on this orbifold in some detail. We start by introducing the geometry itself including an extensive discussion of all twisted sectors and their fixed points. After that we can discuss the flavor structure induced by the fixed points as well as the R-symmetries. Finally we give a discussion on how we obtaine all gauge embeddings and give an exploration of the action of Wilson lines for all fixed points.

Im Dokument Universität Bonn (Seite 34-40)