Charting the flavor landscapeof MSSM-like Abelian heterotic orbifolds

# Charting the flavor landscape of MSSM-like Abelian heterotic orbifolds

Yessenia Olguín–Trejo111yess.olt@ciencias.unam.mx, Ricardo Pérez–Martínez222ricardoperezm@estudiantes.fisica.unam.mx and Saúl Ramos–Sánchez333ramos@fisica.unam.mx
Instituto de Física, Universidad Nacional Autónoma de México,
POB 20-364, Cd.Mx. 01000, México
Edificio A, Unidad Camporredondo, 25000, Saltillo, Coahuila, México
###### Abstract

Discovering a selection principle and the origin of flavor symmetries from an ultraviolet completion of particle physics is an interesting open task. As a step in this direction, we classify all possible flavor symmetries of 4D massless spectra emerging from supersymmetric Abelian orbifold compactifications, including roto-translations and non-factorizable tori, for generic moduli values. Although these symmetries are valid in all string theories, we focus on the heterotic string. We perform the widest known search of Abelian orbifold compactifications, yielding over 121,000 models with MSSM-like features. About 75.4% of these models exhibit flavor symmetries containing factors and only nearly 1.2% have factors. The remaining models are furnished with purely Abelian flavor symmetries. Our findings suggest that, should particle phenomenology arise from such a heterotic orbifold, it could accommodate only one of these flavor symmetries.

## 1 Introduction

The reason of the number of families in the standard model (SM) as well as the origin of fermion mixings may be clarified in extensions of the SM. The general structure of the quark-mixing matrix motivated the bottom-up introduction of ad hoc discrete flavor symmetries (see e.g. [1, 2] for a review) that, together with a number of extra fields transforming in non-trivial flavor representations, yield new phenomenology that may be contrasted with observations. Choosing the correct flavor symmetry among the different scenarios that render similar physics requires a selection principle that is not found in this field-theoretic approach.

It is in this sense that, given the constraints of string theory and its potential to provide an ultraviolet completion of the SM, we can try to identify a mechanism in string theory to restrict the admissible flavor symmetries, providing thereby their origin. This quest is not new. The seminal works were in the context of heterotic orbifold compactifications [3, 4], which sparked the study of the phenomenological consequences of some models [5, 6, 7, 8, 9], generalizations in models with magnetic fluxes [10] and relations with modular symmetries [11]. Flavor symmetries are associated in these works with geometric aspects of orbifolds, but they can also be related to larger continuous symmetries of the extra dimensions [12]. Also in D-brane compactifications, some sources of flavor symmetries have been identified and there is progress in the study of their phenomenology [13, 14, 15, 16].

Here we focus on the heterotic string compactified on all symmetric, toroidal, Abelian orbifolds444Beside the original works [17, 18], there are several good introductions to these constructions, see e.g. [19, 20, 21]. that yield 4D low-energy effective field theories, recently classified in ref. [22]. In these scenarios, the fact that most matter states are localized at the curvature singularities of the orbifold becomes instrumental to arrive at flavor phenomenology, because different singularities are assigned different localization numbers that can be interpreted as charges of a flavor symmetry in the 4D resulting model.

In this work, we present first a systematic classification of flavor symmetries in Abelian toroidal orbifolds, whose moduli have no special values, avoiding possible enhancements. These symmetries are completely determined by the orbifold space group, whose nature is purely geometric, and are thus independent of the string theory to be compactified. As the geometric structure of a toroidal orbifold can be more complicated than usually assumed, due to the presence of roto-translations or non-factorizable tori, this task can be challenging and lead to flavor symmetries not yet identified. Since we explore here all 6D orbifolds, this paper represents the completion of the work initiated in ref. [23].

Orbifolds are used in the heterotic strings to obtain models that reproduce the main features of the SM [24], its minimal supersymmetric extension [25, 26, 27, 28] (MSSM) and other non–minimal extensions [29], as well as many other observed and/or desirable properties of particle physics [30, 31, 32, 33, 34, 35, 36]. Aiming at gaining insight on the actual flavor symmetry of Nature, an interesting question we can pursue is: what flavor symmetries can these orbifolds have?

To answer this question, we perform a search of semi-realistic heterotic orbifolds with help of the orbifolder [37]. We then study their flavor symmetries, which build subgroups of the symmetries we classify in section 3. We expect that a statistical analysis of these findings may hint towards the family structure that particle physics emerging from strings can have.

This paper is organized as follows. After reviewing the aspects of heterotic orbifolds that are crucial for our study on flavor symmetries, we proceed in section 3 to discuss how flavor symmetries arise in Abelian toroidal orbifolds. We then classify all flavor symmetries that can arise from these orbifolds, independently of the string theory one may compactify. In section 4, we show the results of the most comprehensive search of semi-realistic heterotic orbifolds so far. Section 5 is devoted to the discussion of the flavor symmetries that arise in the promising models we found, which are summarized in the tables presented in the appendix. In section 6, we provide our summary and outlook.

## 2 Orbifold compactifications

### 2.1 Toroidal orbifolds

In order to introduce our notation and the main aspects of our constructions, let us first study the structure of 6D toroidal orbifolds in the context of 4D models resulting from the supersymmetric heterotic strings.

In general, a 6D toroidal orbifold is defined as the quotient space that results from dividing a 6D torus by the so-called orbifolding group . The torus can be embedded in by dividing this space by a lattice with basis vectors , corresponding to identifying all points of connected by translations , such that for some integers .

Alternatively, one can produce the same orbifold by moding by the space group , which is a discrete group of isometries of the torus , including the translations in the lattice . For our purposes, this description of an orbifold turns out to be more useful. That is, we shall consider here a 6D toroidal orbifold defined as

 O=R6/S. (1)

The elements have the general structure , where the operators are in general elements of that form a discrete, Abelian or non-Abelian point group of , and is a vector in , which may or may not be an element of the torus lattice, although it can always be written in the basis of (with arbitrary coefficients). The action of on is defined by

 x \lx@stackrelg⟼ gx=ϑx+μ, (2)

that is, denotes a rotation, reflection or inversion of whereas denotes a translation vector.

It is said that the action of is trivial on the torus only if it amounts to a lattice translation. This is because , i.e. the torus is obtained by the identification , . It follows that, if is an element of the torus lattice, , the only component of that exerts a non-trivial action on the torus is , since and are identified on a toroidal orbifold.

When in eq. (2) is chosen to be a more general vector, , the space-group element is called a roto-translation. In this case, both and act non-trivially on the 6D torus. One of the purposes of this work is to study this case with more attention, attempting to pave the path towards phenomenology of orbifolds with roto-translations.

In an orbifold, the space group defining the orbifold consists of a finite number of elements called space group generators, their products, computed according to

 g′′=gg′=(ϑ,μ)(ϑ′,μ′)=(ϑϑ′,μ+ϑμ′),g,g′,g′′∈S, (3)

and their conjugations. All elements of a space group can be grouped in different conjugacy classes . All elements of a conjugacy class are equivalent. Note that an element , with or null, can be rewritten as . Therefore, the space group generators can be pure transformations, roto-translations or translations.

An additional property of orbifold generators is that each of them has an integer order , such that is trivial on the torus, that is with . We point out that this restricts the shape of the translation vectors of roto-translations . The trivial action of on the torus implies that and . Notice that, for example, if , then the translation vector is given as a fraction of lattice vector, .

Let us focus now on Abelian orbifolds, which are the scope of this work. Complexifying the orbifold generators , eq. (2) becomes

 z \lx@stackrelg⟼ gz=ϑz+μ,z,μ∈C3, (4)

with the complex coordinates of related by , , with the real coordinates . In Abelian orbifolds, the complexified elements of the space group generators can be simultaneously diagonalized and represented as matrices of the form , with . The vector is commonly called twist vector.

#### 2.1.1 Fixed points and roto-translations

Space group generators with non-trivial twist have a non-free action on . This implies that, in these cases, some points are left unaltered or fixed under , which correspond to curvature singularities of the compact space. The simplest example of such a fixed point is for the space group element with a rotation in six dimensions, but there are frequently more than one fixed points in these cases. The number and localization of the fixed points depend on the details of the torus (or, equivalently, the lattice ) and the space group element under consideration. There are as many inequivalent fixed points as conjugacy classes of with non-trivial twist.

Given a space group generator , it follows from eq. (4) that the associated fixed points satisfy the condition

 gzf=ϑzf+μ=zf+λf,λf∈Λ, (5)

where the lattice translations are needed because the identity must happen in the torus. In order to obtain all inequivalent fixed points associated with , one can take different choices of and then select only those that are not related by space group elements. We note that, by using the product rule (3), defining leads to the identity . The space group element is typically called the constructing element associated with the singularity .

Let us illustrate the fixed point structure of an orbifold by using a orbifold with roto-translations.555In terms of the classification of ref. [22], we refer here to a 2D subsector of the non-local geometry of a 6D toroidal orbifold. By itself, the non-orientable geometry induced by this space group cannot be used to compactify a 6D field theory as it cannot sustain chiral fermions. Belonging to a larger 6D orbifold solves this issue. We thank H.P. Nilles for this observation. We define the orbifold through the space-group roto-translation generators and , where are the orthogonal lattice generators of the torus and the generators are given by

 θ=(100−1),ω=(−1001), (6)

satisfying , such that and have a trivial action on the torus, as expected for a orbifold. Omitting the translational generators and their conjugations, the space group comprises the conjugacy classes of the elements .

Let us first focus on the element . By applying eq. (5), we find four fixed points in the fundamental domain of the torus: . One can easily verify that only two of these points are inequivalent; is related to and is related to in the torus by acting on them with or . Thus, one can choose the fixed points and as the inequivalent fixed points associated with . These points are depicted with bullets in fig. 1. The constructing elements associated with the fixed points are given by and .

We consider now the element . For this element, it turns out that eq. (5) has no solution, revealing that there are no fixed points associated with this space group element. The same is true for . This observation will be useful when figuring out the geometric discrete symmetries of the compactification.

For reasons that shall be clearer in sec. 2.2, each set of fixed points is named a sector. From our previous discussion, we note that, ignoring the trivial sector of the identity element, in the orbifold worked out here there are two empty sectors and one sector with two fixed points. The appearance of empty sectors is related to the existence of roto-translation space group elements in toroidal Abelian orbifolds. In general, orbifolds without roto-translations do not exhibit empty sectors.

The global geometric structure of the orbifold is obtained by inspecting the action of all the space group generators. From the sector associated with , we find that the space group reduces the fundamental domain of to of the torus fundamental domain, as illustrated in fig. 1. We see that the combined action of and identifies the singularities depicted at the top with those in the bottom, sharing the symbols and . This crossed identification also affects the “boundaries” of the fundamental domain of the orbifold, which are also identified according to the types of arrows in the figure. From this description, we observe that this orbifold is equivalent to the well-known 2D cross-capped pillow, with p-rectangular Bravais lattice (see e.g. table B.2 of ref. [22]), also called singular (real) projective plane.

The structure of fixed points in an orbifold allows to determine its geometric symmetries. Notice, in our example, that the cross-capped pillow is symmetric under the exchange of its inequivalent singularities. Consequently, this orbifold is invariant under transformations. Analogous (permutation) symmetries arise in orbifolds with a different number of singularities.

### 2.2 Heterotic Abelian orbifolds

The degrees of freedom of a string theory emerge from the left and right-moving vibrational modes of a string. The observation that they are independent led to the heterotic strings, which are the mixture of the right-moving modes, and , of a 10D supersymmetric string with the left-moving modes, of a 26D bosonic string. The 16 extra bosonic degrees of freedom , , are compactified on a torus , whose lattice vectors are constrained by anomaly cancellation to be those of the or root lattice,666If one does not demand the resulting theory to be supersymmetric, there is a third option, the root lattice. revealing the structure of an or gauge group on a 10D supersymmetric space-time. We focus here on the heterotic string with gauge group.

Heterotic orbifolds are constructed by compactifying six spatial dimensions of the 10D space-time of a heterotic string on a toroidal orbifold. Right and left-moving modes, and , mix to build the (bosonic) coordinates of the space-time, , but they can still be taken as independent degrees of freedom. As a consequence, one can in principle choose different compactification schemes for each mode. However, for simplicity, we focus here on so-called symmetric heterotic orbifolds, in which both modes are compactified on the same orbifold. As already mentioned, we can also complexify these coordinates, so that we have two uncompactified complex dimensions, corresponding to those of the observed space-time, and three complex dimensions compactified on an Abelian orbifold.

Insisting on preserving supersymmetry in 4D after compactifying the heterotic strings on 6D toroidal orbifolds, restricts a number of properties of these constructions. First, it is known that preserving requires that the point group be a subgroup of . Recalling that the point group elements of Abelian orbifolds can be written as , we immediately find that the condition for each diagonalized generator leads to obtain in 4D. Furthermore, more than two independent generators of would not leave any invariant supersymmetry generator; thus, only one or two distinct point group generators of orders and can be considered, corresponding to cyclic or point groups. It is customary to label the orbifold by the name of its point group (also called class). In general, there is more than one (couple of) generator(s) that can yield the same point group, but any choice can be diagonalized in terms of the same twist vector.

Secondly, demanding that the space group elements be torus isometries further restricts both the choice of the tori and the space groups. For each choice of generators of a given point group, there are different torus lattices that are left invariant under the point group. If we allow for a number of moduli to take any values and consider the lattices so related to be equivalent, they build a so-called class. Each point group admits different classes.

Finally, once the point group and a torus lattice have been chosen for the compactification, one has still the freedom to consider different values of the translations of the space group generators , which may be equivalent up to affine transformations or not. Equivalent translations together with the corresponding lattice and point group generators define an affine class.

In summary, all space groups useful for orbifold compactifications are obtained by classifying the admissible combinations of point groups and their and affine classes. This has been done systematically in ref. [22], from which we learn that there exist 138 admissible Abelian space groups for 6D supersymmetric orbifold. All possible point groups with their corresponding twist vectors and the number of compatible and affine classes are listed in table 1. We shall explore the phenomenology of all 138 space groups.

Once one space group has been chosen to compactify the heterotic strings, the geometric features of the orbifold in 6D are completely defined, and, due to the conformal structure of string theory, these properties determine some aspects of the spectrum of matter in the resulting 4D supersymmetric field theory. In particular, modular invariance of the heterotic string requires that the orbifold action be embedded into the gauge degrees of freedom of the string. This means that the space group must be translated into an equivalent group acting in the 16D space associated with the gauge group, the so-called gauge twisting group.

The simplest such an embedding is defined (in the bosonic formulation) by two kinds of translations of the gauge degrees of freedom. The point group elements are embedded as shifts , whereas the torus lattice vectors are embedded as so-called Wilson lines (WLs) , . Let us explain the details by using orbifolds as our working example. A generic space group element of a orbifold with generators and can be embedded into the gauge degrees of freedom as

 g=(θnωm,μiei)↪Vg≡nV+mW+μiAi,n,m∈\mathbbmZ,μi∈R, (7)

where and are the 16D shift vectors of fractional entries that encode in the gauge group the respective action of and in the 6D orbifold; are non-integers or integer numbers, depending on whether the space group element is a roto-translation or not; and the six WLs are also 16D fractional vectors. represents the gauge embedding of the space group .

Under this gauge embedding, the action of a space group element is such that in six of the ten dimensions of the space-time of the heterotic string, and the bosonic left-moving coordinates associated with the gauge degrees of freedom of the heterotic strings are transformed according to

 XIL⟼XIL+2πVIg,I=1,…,16. (8)

It is convenient to discuss the details of the states associated with the string excitations in the dual momentum space. If we focus on the gauge momentum contribution to the states with momentum , its behavior under the action of a space group element is dominated by the left-moving contribution to the full vertex operator (see e.g. eq. (2.5) of ref. [38]). Under the action of , this operator becomes , which means that the momentum state acquires a phase under ,

 |p⟩L→e2πip⋅Vg|p⟩L,p∈ΛE8×E8, (9)

where denotes the (self-dual and integer) root lattice of the gauge group.

The gauge embedding is subject to some constraints. First, since the point group generators of a orbifold satisfy , the action of the shift vectors corresponding to and must be trivial in the gauge degrees of freedom. This implies that, according to eq. (9), the shift vectors are constrained to satisfy because the lattice is integer (i.e. the inner product of different lattice vectors is an integer). Secondly, WLs must be consistent with the torus geometry and the orbifold action on it. For a given point group generator, in general, for some integer coefficients . This implies that the WLs must fulfill the relations up to lattice translations in . The set of resulting equations of this type can be reduced to conditions for the WLs; some of them must vanish and other WLs have a non-trivial order , such that (without summation over ).

The final constraint on the gauge embedding comes from modular invariance, which is a string theoretical requirement ensuring that the compactified field theory is anomaly free. In the most general case of Abelian heterotic orbifolds, modular invariance requires that [39]

 N(V2−v2)=0mod2, Ni(V⋅Ai)=0mod2,i=1,…,6, (10) M(W2−w2)=0mod2, Ni(W⋅Ai)=0mod2, M(V⋅W−v⋅w)=0mod2, Ni A2i=0mod2, gcd(Ni,Nj)(Ai⋅Aj)=0mod2,i≠j.

Here we consider and in terms of the two twist vectors, and .

The space group together with the corresponding gauge twisting group, fulfilling all the previous requirements, builds up an admissible symmetric, Abelian orbifold compactification of a heterotic string.

The properties of the space group and a compatible gauge twisting group completely determine the matter content of the emerging 4D field theory. The matter fields in a heterotic orbifold correspond to the quantum states of (left and right-moving) closed string modes, that are left invariant under the action of all elements of the space and gauge twisting groups. String modes that are not invariant under the orbifold do not build admissible states of the compactification. Closed strings in an orbifold are of two kinds: untwisted and twisted strings. Untwisted strings are closed strings found among the original strings of the 10D heterotic theory and that are not projected out by the orbifold action. Twisted strings are special. They arise only because of the appearance of the orbifold singularities and are thus attached to them.

As in the uncompactified theory, 4D effective states consist of a left and a right-moving component. Both components must fulfill the so-called level-matching condition, , whose origin is that there is no preferred point on a closed string. For non-zero masses of string states are few times the string scale , which is close to the Planck scale, any massive state is too massive to be observed at low energies and, therefore, decouples from the observable matter spectrum of the compactification. In string compactifications aiming at reproducing the physics of our universe, one must thus focus on the study of massless (super)fields, .

Since in 10D the only massless closed strings found in the heterotic theory are those corresponding to the superfields and the gravity supermultiplet, the untwisted closed-string states that are invariant under the orbifold represent first the unbroken 4D gauge superfields that generate the unbroken gauge group , and the 4D gravity multiplet. Additionally, they correspond to the (untwisted) moduli, which parametrize the size and shape of the orbifold, and some (untwisted) matter fields that transform non-trivially under . The gauge properties of heterotic string fields are determined by their left-moving momentum, which for untwisted fields is just a vector of the root lattice of the 10D gauge group, . Those states whose momenta satisfy belong to the gravity multiplet, the gauge multiplets or are moduli; the rest of the states have non-trivial gauge quantum numbers and build therefore matter fields.

The twisted states correspond to closed strings whose center of mass is at the orbifold singularities. Their left and right-moving momenta depend on the constructing element associated with the singularity to which they are attached, according to our discussion in sec. 2.1.1. The matter spectrum of string states of an orbifold is mostly populated by twisted fields. The gauge momentum of a string attached to the fixed point associated with the constructing element is given by , where is defined in eq. (7). The corresponding states remain in the orbifold spectrum only if they are invariant under the action of all centralizer elements , such that . It is thus clear that, when some WL is chosen to vanish, for some fixed (up to lattice translations), 4D matter fields located at the singularities with constructing elements and are identical concerning their quantum numbers under , as long as their centralizers are equivalent. Following the final remarks in sec. 2.1.1, those states would nevertheless be related under the internal geometric (permutation) symmetry of the orbifold. However, if , at various singularities differ, breaking the permutation symmetry. These are key observations to arrive at the flavor symmetries, as we now proceed to discuss.

## 3 Flavor symmetries in Abelian heterotic orbifolds

### 3.1 Symmetries from string selection rules

As long as the strings are not deformed by the space-time curvature, conformal field theory (CFT) is a useful tool to compute, for example, the amplitude of the interactions among the fields related to the string states [40, 41, 42, 43]. Since orbifolds are flat everywhere but at isolated points, the description of the string dynamics is just as in the original uncompactified theory, even after compactification on these spaces. This is a great advantage of orbifold compactifications because we must not rely on a supergravity approximation, which might break the connection between string theory and the 4D effective model.

In the CFT, one determines the coupling strength of interactions among, say, effective fields , , by computing the -point correlation functions of the vertex operators associated with the interacting fields,

 A=⟨V(1)−1/2V(2)−1/2V(3)−1V(4)0⋯V(r)0⟩, (11)

where denotes a fermionic vertex operator in the -ghost picture and denote bosonic vertex operators in the or -ghost pictures. The explicit expressions are written in terms of the quantum numbers of the string states (cf. e.g. [38]), revealing that there is a number of conditions that those quantum numbers must satisfy in order for the interaction amplitudes (11) to be non-vanishing. These conditions are known as selection rules [44, 45, 46, 47, 38, 48, 49]. The selection rules, beside gauge invariance, include -charge conservation and space-group invariance, which deserve a discussion because they lead to discrete symmetries that may be important for flavor physics.

##### R-charge conservation.

In addition to the left-moving momentum that contains the information about its gauge charges, a string state has the so-called -momentum in the three compactified, complex dimensions . In the bosonic formulation of the heterotic string, the entries of the -momentum are fractional numbers that depend on whether they correspond to the description of a fermion or a boson, differing by units. This momentum, together with the number of left and right-moving oscillator perturbations acting on the vacuum, build the so-called -charge (see e.g. [48, 49]), which, in contrast to pure -momentum, is invariant under the ghost picture-changing operation.777 The ghost picture or ghost charge of the vertex operators is given as subindex in eq. (11). The total ghost charge must be to cancel the ghost charge of the sphere on which is computed. However, all different ghost-charge assignations or pictures yielding the same total ghost charge provide equivalent results. Thus, it is natural to demand that physical charges be invariant under ghost-picture changing.

By computing CFT correlation functions (11), one can demonstrate that weakly-coupled strings interact only if the total -charge of the coupling satisfies a conservation principle stated as [48, 49]

 r∑ℓ=1R(ℓ)a=−1modNa,a=1,2,3, (12)

where each integer denotes the order of the point group generators acting on the -th complex coordinate of the 6D torus, i.e. such that . If one normalizes the charges to be integers by multiplying by , eq. (12) provides the discrete symmetry group .

On the other hand, since these -charges distinguish the bosonic and fermionic components of the 4D effective superfields, the discrete symmetry arising from this invariance principle can be only an symmetry, explaining why they are called -charges. We assume here that flavor symmetries are not symmetries, thus the discrete, symmetry of -charges cannot be part of a flavor symmetry.

##### Space-group invariance.

In the compactified theory, interactions must be invariant under the space group that defines the orbifold compactification. This implies that the joint action of the composition of the constructing elements of the interacting strings must be trivial on the orbifold (rather than on the torus). This condition is the so-called space-group selection rule. If we denote the constructing element of the fixed point of the sector as , the space-group selection rule is given by

 r∏ℓ=1g(ℓ)f=r∏ℓ=1(ϑℓ,μ(ℓ)f)\lx@stackrel!=(\mathbbm1,⋃ℓ~Λℓ),~Λℓ=(\mathbbm1−ϑℓ)Λ, (13)

where e.g. for orbifolds and denotes the invariant sublattice of fixed points. The invariant sublattice of fixed points is such that, if the fixed point has constructing element and with arbitrary , then is the fixed point associated with the constructing element which is in the conjugacy class of and refers thus to the same fixed point in the orbifold.

In order to satisfy the space-group selection rule, we must impose first that , which for orbifolds amounts to demanding

 r∑ℓ=1qℓ\lx@stackrel!=0modN,r∑ℓ=1wℓ\lx@stackrel!=0modM. (14)

These relations suggest that the effective fields can be considered to transform under a discrete symmetry with charges . Nonetheless, as we shall shortly see, these two symmetries are not always independent, yielding sometimes a smaller symmetry.

The second part of the space-group selection rule can be rewritten as

 μ(1)f+r∑ℓ=2(ℓ−1∏ℓ′=1ϑℓ′)μ(ℓ)f\lx@stackrel!=r∑ℓ=1~λ(ℓ),~λ(ℓ)∈~Λℓ. (15)

Since all vectors and can be expressed in terms of the basis vectors , , eq. (15) becomes a set of (up to) six independent conditions similar to those of eq. (14), which depend on the specifics of the space group elements. I.e. the 4D fields are charged under additional , , that depend on the space group.

To illustrate the conditions that follow from eq. (15), let us consider the orbifold with the point-group generators given by eq. (6), ignoring the rest of the 6D heterotic orbifold (see footnote 5). A generic element is written as with . Let us suppose that we are considering couplings among states arising only from the sector since no fixed points appear in the and sectors of this orbifold. Because of eq. (14) and for all massless twisted states we consider, we learn that the number of fields that an admissible coupling can have in this orbifold is even. A general element of the corresponding invariant sublattice is given by . Thus, we see that eq. (15) takes the form

 μ(1)−μ(2)+μ(3)−μ(4)+…−μ(r)\lx@stackrel!=2λ, (16)

where the sign in the last vector is a consequence of being even. Rewriting the constructing elements as and , so that the field in a coupling may have the constructing element with or , we find that eq. (16) yields two (apparently) independent conditions

 n(1)1+n(2)1+n(3)1+n(4)1+…+n(r)1 \lx@stackrel!= 0mod2, (17) n(1)2+n(2)2+n(3)2+n(4)2+…+n(r)2 \lx@stackrel!= 0mod2,

where we have used that the integer can be replaced by , , without loss of generality. Another important observation is that the orbifold sectors corresponding to the generators and do not lead to fixed points. This implies that there are no massless twisted states related to those sectors. Thus, if one focuses on massless twisted states, our previous considerations are enough to arrive at the flavor symmetry in the effective theory.

From our discussion, one could be tempted to conclude that the discrete symmetry emerging from the space group is . This is wrong. The correct discrete symmetry that massless states support is only a . The reason is as follows. First, since the point-group charges of these states satisfy , if , then too. That is, we obtain only one independent from these conditions. Similarly, the second eq. of (17) is automatically fulfilled once has been imposed because we have chosen or . However, there exists a non-trivial condition yielding a that does contribute to the flavor symmetry of massless states in the sample orbifold that we study here.888It is possible to show that is the only charge that is independent of the choice of the constructing elements we take (from their conjugacy classes). We thank Patrick K.S. Vaudrevange for very useful discussions and insight on this topic.

It must be stressed that the symmetries that we have discussed are only related to massless states. Massive string states can wind on a torus even if it has no fixed points. In our example, this case would correspond to constructing elements such as or . When all elements of the space group are taken into account, the corresponding symmetry becomes larger and the charges associated with the point-group generators and translations combine. Nevertheless, in this paper we shall only consider massless states and leave the general discussion for future work [50].

### 3.2 General structure of flavor symmetries

In orbifold compactifications (of any string theory), flavor symmetries can arise from the properties of the space group. In particular, in heterotic orbifolds, they emerge as a result of combining the geometric properties of the extra dimensions and the symmetries emerging from the selection rules that we examined in the previous section.

As we have illustrated in sec. 2.1.1, if the global structure of an orbifold contains fixed points, the compact space exhibits an permutation symmetry, which indicates that geometrically all singularities are equivalent. From the perspective of the gauge quantum numbers, 4D effective fields located at the singularities do not display any difference as long as the gauge embeddings associated with the singularities are equal (see eq. (7) and final remarks in sec. 2.2). Under these conditions, the 4D twisted fields build up non-trivial representations.

In the case of factorizable orbifolds, i.e. when can be decomposed as , each subtorus has at least a Kähler modulus that allows for differences in the effective theory of the fields originated in different tori. Thus, considering a number of singularities in each torus, the full permutation symmetry of the orbifold is the product , where each factor corresponds to the permutation symmetry among the fixed points localized at each of the various tori.

Invariance under the full permutation group holds only if all WLs have trivial values. When some WLs are non-trivial, (at least some) twisted states with identical gauge quantum numbers located at various fixed points get different gauge properties and some others do not change. Hence, the 4D field theory is not invariant under the full permutation symmetry anymore, but only under (at most) a (permutation) subgroup thereof. Therefore, the permutation symmetry is said to be explicitly broken by non-trivial WLs in heterotic orbifold compactifications. The permutation symmetry is completely broken when all WLs have non-trivial values.

In order to identify the permutation symmetries, it is important to notice which singularities prevail in the global structure of the orbifold. In simple prime orbifolds, the same singularities appear in all sectors. However, in less trivial orbifolds, different sectors (corresponding to inequivalent space group elements) have in general different singularities. It is the intersection of all sectors what determine the global structure of the orbifold. This means that only the singularities appearing in all sectors must be regarded to determine the permutation symmetries. These fixed points, which include points in invariant subtori (like those of orbifolds), exhibit equivalent centralizers and thus the associated twisted states are equal.

Both the permutation symmetry and the Abelian space-group symmetries build a large set of symmetry generators, usually denoted by . The multiplicative closure of the elements of this set constitutes the flavor symmetry perceived by the 4D effective fields. In most cases, the product of Abelian discrete symmetries originated from the space group, , is a normal subgroup of , which implies that the flavor group is given by . Only in a few cases, the resulting symmetry requires extra generators, leading to a symmetry that differs from this structure. This is important when non-trivial WLs are considered.

##### Flavor symmetries in orbifolds with roto-translations.

If the generators of the space group include roto-translations, some sectors may not exhibit fixed points. As a consequence, no massless states can appear in those sectors and, therefore, the sectors can be ignored to determine the flavor symmetries of the massless spectrum.

As an illustration, let us study again our example defined by the generators around eq. (6). In that case, only the sector has two inequivalent fixed points. The sectors and do not exhibit fixed points and thus cannot support massless states. The global geometric structure of the orbifold is just that of the projective plane with two singularities, allowing, in the absence of WLs, for an permutation symmetry of the twisted states. In addition, as we discussed in section 3.1, there is a symmetry due to the space group selection rule. That is, we observe that the 4D effective theory must be invariant under the set of symmetries. It is possible to verify that the group remains invariant under elements, so it is a normal subgroup, which implies that the multiplicative closure of the set of symmetries is . Therefore, the corresponding flavor symmetry is , which coincides with the emerging flavor symmetry when only one dimension is compactified on an orbifold.

### 3.3 Flavor symmetries of Abelian orbifolds without Wilson lines

One of the outcomes of our study is a full classification of the flavor symmetries emerging from 6D Abelian orbifold compactifications without WLs. Interestingly, these symmetries do not depend on the specific string theory to be compactified. They correspond to the flavor symmetries perceived by 4D massless closed-string states attached to the orbifold singularities. Thus, without any further elements (such as D-branes, orientifolds and open strings), the flavor symmetries we find are common to all 4D supersymmetric models arising from orbifold compactifications in generic points of their moduli space.

In these orbifolds, all states associated with fixed points of a particular sector have identical gauge quantum numbers and only their localization in different, independent tori, , distinguish them. By applying the tools explained in the previous section, we determine the flavor symmetries of all 138 admissible Abelian orbifolds.

Our findings are presented in table 2. Following the notation of ref. [22], we label each Abelian orbifold, presenting its point group symmetry, and, in parentheses, the labels of the corresponding and affine classes, as introduced in section 2.2. These space group labels are presented in the first and third columns. In the second and fourth columns of table 2 we display the corresponding flavor symmetries at massless level.

There are three space groups which do not lead to any flavor symmetries. The reason is that no fixed points and thus no twisted states appear in those orbifolds. Further, there are 71 orbifolds that yield only Abelian symmetries. The origin of this simplicity in those cases is that only one fixed point is common to all sectors and thus only one point appears in the global structure of the orbifold, avoiding permutation symmetries. We also observe that 45 cases include factors, whereas 19 space groups lead to flavor-symmetry factors, three exhibit and only one contains . In some cases the structure of the flavor symmetry follows a factorizable pattern, that is, the resulting flavor symmetry is the direct product of two or more independent non-Abelian symmetries; see e.g. the geometry of orbifolds. However, most of the resulting flavor symmetries are more complicated products and quotients of several permutation and cyclic groups.

As expected from previous studies [3], flavor factors appear in orbifolds whereas is present in orbifolds. However, we see that also other symmetries arise in those cases. Thus, only the careful study of the space groups that we carry out here reveals the flavor symmetries of the 4D effective theories arising from orbifold compactifications.

Note that, given a point group, the largest flavor symmetry arises for , because the space groups with correspond to non-factorizable 6D tori and/or include roto-translations. Both features reduce the number of fixed points in the orbifold with respect to the space group, avoiding large permutation symmetries. Yet there are two exceptional cases: the flavor symmetries of –I and –I orbifolds are smaller than those for . This follows from the fact that the point group induces only a symmetry for the twisted states due to their localization in the case.