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Number of Nambu-Goldstone bosons and its relation to charge densities

Haruki Watanabe1,2,* and Tomas Brauner3,4,

1Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan2Department of Physics, University of California, Berkeley, California 94720, USA

3Faculty of Physics, University of Bielefeld, 33615 Bielefeld, Germany4Department of Theoretical Physics, Nuclear Physics Institute ASCR, 25068 Rez, Czech Republic

(Received 30 September 2011; published 9 December 2011)

The low-energy physics of systems with spontaneous symmetry breaking is governed by the associated

Nambu-Goldstone (NG) bosons. While NG bosons in Lorentz-invariant systems are well understood, the

precise characterization of their number and dispersion relations in a general quantum many-body system

is still an open problem. An inequality relating the number of NG bosons and their dispersion relations to

the number of broken symmetry generators was found by Nielsen and Chadha. In this paper, we give a

presumably first example of a system in which the Nielsen-Chadha inequality is actually not saturated. We

suggest that the number of NG bosons is exactly equal to the number of broken generators minus the

number of pairs of broken generators whose commutator has a nonzero vacuum expectation value. This

naturally leads us to a proposal for a different classification of NG bosons.

DOI: 10.1103/PhysRevD.84.125013 PACS numbers: 11.30.Qc, 14.80.Va

I. INTRODUCTION AND SUMMARY

Spontaneous symmetry breaking (SSB) is a ubiquitousphenomenon in nature. In quantum many-body theory, itssignificance is further boosted by the fact that it provides uswith a rare example of a general exact result: the Goldstonetheorem [1,2]. The low-energy physics of systems exhib-iting SSB is governed by the associated soft excitations, theNambu-Goldstone (NG) bosons (see Ref. [3] for a com-prehensive early review). While NG bosons in Lorentz-invariant systems are well understood, the precise charac-terization of their number and dispersion relations in ageneral quantum many-body system is still an openproblem.The systematic study of NG bosons in Lorentz-

noninvariant systems was initiated by Nielsen andChadha in their seminal work [4]. They showed that undercertain technical assumptions, including rotational invari-ance and the requirement of absence of long-range inter-actions, the energy of the NG boson is proportional to aninteger power of momentum in the long-wavelength limit,"k / jkjn. The NG boson is then classified as type I if n isodd, and as type II if n is even. Nielsen and Chadha provedthat the number of type I NG bosons plus twice the numberof type II NG bosons is greater than or equal to the numberof broken symmetry generators.In Lorentz-invariant systems, the Nielsen-Chadha (NC)

inequality is trivial since it is well-known that the numberof NG bosons, all being naturally type I, equals the numberof broken generators. In Lorentz-noninvariant systems, theNC inequality allows the number of NG bosons to besmaller than the number of broken generators provided

some of them have nonlinear dispersion relation. A renownexample is the ferromagnet where two generators brokenby the spontaneous magnetization give rise to only one NGbosonthe magnonhaving a quadratic dispersion rela-tion at low momentum.It is perhaps worth emphasizing that the NC counting

rule is formulated as an inequality and that it does notconstrain in any way the power of momentum appearing inthe dispersion relation. In fact, a quick sociological surveyreveals that this generality of the Nielsen-Chadha theoremis not very well appreciated in publications citing theoriginal work [4]. The reason for this probably is that inthe concrete systems studied in literature, type I and type IING bosons always have linear and quadratic dispersionrelations, respectively, and the NC inequality is alwayssaturated. In Appendix A of this paper, we point out arather trivial class of theories where NG bosons withenergy proportional to an in principle arbitrarily highpower of momentum can appear, and the NC inequalityis not necessarily saturated [5]. This ultimately leads us tothe proposal for a different classification of NG bosons,and the evidence that this provides an exact equality fortheir count.Our work is based on the dependence of the number of

NG bosons and their dispersion relations on expectationvalues of conserved charges, which was first investigated atthe general level by Leutwyler [7] and Schafer et al. [8](see also Ref. [9] for a recent review). Leutwyler [7] usedeffective field theory to argue that, in the absence ofquantum anomalies, nonzero density of a non-Abelianconserved charge leads to the appearance of NG bosonswith a quadratic dispersion relation. This connection wasfurther elaborated in Ref. [10] where en exact equality(saturating the NC inequality) was established for the classof linear sigma models with chemical potential at tree

*hwatanabe@berkeley.edutbrauner@physik.uni-bielefeld.de

PHYSICAL REVIEW D 84, 125013 (2011)

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level. Every pair of broken generators whose commutatorhas a nonzero expectation value was found to give rise toone type II NG boson with a quadratic dispersion relation.On the other hand, Schafer et al. [8] related charge den-sities to the number of NG bosons by showing that thisnumber equals the number of broken generators providedthe expectation values of commutators of all pairs ofbroken generators vanish.Our present proposal can in a sense be understood as a

synthesis of these previous works. We argue that given thethree characteristics of SSB, that is, the number of NGbosons, their dispersion relations, and the expectation val-ues of charge densities, an exact equality can be obtainedby focusing on the relation between the number of NGbosons and the charge densities. Thus, our work general-izes the theorem of Schafer et al. [8]. This is in contrast toNielsen and Chadha [4] as well as Leutwyler [7] whoemphasized the role of the NG boson dispersion relations.In order that the generality of our proposal is not ob-

scured by the technical details, we formulate it here. LetQa be the set of (spontaneously broken) conserved chargesof the theory. The number of NG bosons nNG is related tothe number of broken symmetry generators nBS by theequality

nBS nNG 12 rank; (1)where the matrix of commutators is defined by iab lim!1 1 h0jQa;Qbj0i and is the space volume. Notethat rank is always even since is real and antisymmet-ric. Unfortunately, we were not able to prove Eq. (1) in thefull generality so it remains a conjecture at the moment.However, we can prove a one-sided inequality and provideevidence that the opposite inequality holds as well. Weshould also emphasize that our argument applies to con-tinuum field theories as well as to models defined on a(space) lattice.The paper is organized as follows. In Sec. II we make

some general remarks on SSB, basically setting up thestage for the discussion of our main result. We also in-troduce a class of uniform symmetries to which thestandard derivation of the Goldstone theorem applies. InSec. III we give a partial proof of the conjecture (1) andprovide evidence for the missing part of the proof. Finally,in Sec. IV we conclude and afford some speculations.Some technical details as well as specific examples thatdo not pertain to the general argument are relegated to theappendixes.

II. GENERAL REMARKS ON SSB AND THEGOLDSTONE THEOREM

In this section we will briefly review the notion of SSBand the Goldstone theorem. Although we will not repeat indetail its proof, we would like to make a number of com-ments explaining under what conditions and technicalassumptions this proof applies. This may at times look

like purely academic pedantry, yet the example of a sharpinequality in the NC counting rule presented inAppendix A shows that one should be prepared for theunexpected.Let us first, following Nielsen and Chadha [4], define

what we mean by the number of broken generators nBS. Wedemand that there is a set of conserved chargesQa and a setof (quasi)local fieldsax, or, on a space lattice, operatorsat; xi, where a 1; . . . ; nBS, such that the matrix

Mab h0jQa;b0j0i (2)is nonsingular (i.e., has a nonzero determinant). Moreover,we assume that the conserved charges can be expressed asa spatial integral or sum of local charge densities,

Qa Zddxj0at; x; or Qa

Xxi2lattice

at; xi: (3)

Strictly speaking, the broken charge operators are onlywell defined in a finite volume. Nevertheless, in the formalderivations they only appear in commutators which have awell-defined infinite-volume limit [3].

A. Uniform symmetries

The proof of the Goldstone theorem essentially proceedsby finding the spectral representation of the commutator(2). This heavily relies on the integral representation (3)and the following translational property of the chargedensities,

j0ax eiPxj0a0eiPx; orat; xi eiHtTyxia0Txi eiHt;

(4)

where P is the four-momentum operator, H theHamiltonian, and Txi is the operator of a finite (lattice)

translation by xi. Even though this property is usuallytaken for granted, it only holds provided that the chargedensity operator, as constructed in terms of the local op-erators of the theory, does not depend explicitly on thecoordinate. We will call symmetries whose charge den-sities satisfy this condition uniform.As an example, consider the simplest field theory exhib-

iting SSB and a NG boson: the free massless relativisticscalar field theory, defined by the Lagrangian

L 12@@: (5)The action of this theory is invariant under the set ofglobal transformations x ! x x with x a bx, where a, b are the parameters of the trans-formation. The corresponding five conserved Noethercurrents are given by

jx @x;j x x@x x:

(6)

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Thanks to the commutation relations with the field opera-tor, iQ;x 1 and iQ;x x, all five integralcharges Q, Q are spontaneously broken, while there isobviously only one massless mode in the spectrum. Theresolution of this apparent paradox is that the chargedensities j0x do not satisfy Eq. (4). In fact, a shortexplicit calculation gives j

x eiPxj 0eiPx

x@x. Thus the standard proof of the Goldstone theo-

rem does not apply to the charges Q, and we should nota priori expect additional NG bosons stemming from theirspontaneous breaking.An intuitive understanding of why there is only one NG

boson instead of the naively expected five can be gainedusing the argument of Low and Manohar [11]. Since theNG boson can be generated by a local broken symmetrytransformation acting on the order parameter for SSB, twospontaneously broken transformations will give rise to onlyone NG boson if their local forms coincide. This is exactlythe case of the transformations generated by Q, Q whichall have the same local form. A similar example is acrystalline solid where both continuous translations androtations are spontaneously broken, yet only NG bosonscorresponding to the translationsthe phononsappear inthe spectrum.Despite the above example of an internal nonuniform

symmetry, typical representatives of the class of nonuni-form symmetries according to our definition are spacetimesymmetries such as rotational or conformal invariance. Infact, the only example of a uniform spacetime symmetry isobviously translational invariance. In the following, wewill assume that there is at least a discrete unbroken trans-lational invariance, such as in crystalline solids. This isnecessary in order to have quasiparticles with well-defined(real or crystal) momentum. With the above in mind, ourresult will apply to all spontaneously broken global con-tinuous uniform symmetries. Equation (4) then holdswhether continuous translational invariance is spontane-ously broken or not.One might think that the restriction to uniform symme-

tries could be avoided by resorting to the variant of theproof of the Goldstone theorem using the quantum effec-tive potential or action [2], which does not rely on theoperator identity (4). As it may be of general interest, weprovide in Appendix B a modification of this proof thatapplies to nonuniform symmetries, leading essentially tothe same conclusion as the classical argument of Ref. [11].However, this method of proof only gives the number offlat directions of the effective potential (or equivalently thenumber of zero modes of the inverse propagator of thetheory at zero momentum), which in Lorentz-noninvarianttheories is in general not equal to the number of NGbosons. This means that while it can be used to makestatements within Lorentz-invariant theories, it can tell usvery little about the number of NG bosons in Lorentz-noninvariant theories.

B. Conserved charges and their densities

Here we make a number of other short comments relatedto the conserved charges required by the Goldstone theo-rem. First, despite the fact that one (including ourselves)usually speaks of broken generators, it should be notedthat the operators Qa in Eq. (2) need not be generators ofsymmetry transformations in the Noether sense [12]. Infact, the only two properties needed for the proof of theGoldstone theorem are that Qa are integrals or sums ofcharge densities as in Eq. (3) and that Qa are time-independent. Of course, the most convenient way to ensurethat Qa is time-independent is starting from a four-currentja x which satisfies the continuity equation, @ja 0.Second, our conjecture (1) is formulated in terms of a

commutator of charges. Since generators of symmetrytransformations span a Lie algebra, this strongly suggeststhat ab is actually a linear combination of expectationvalues of the charges themselves. However, this woulddisregard the possibility that the representation of the Liealgebra of conserved charges on the Hilbert space pos-sesses a set of central charges. One can then expect ageneralized commutation relation of the type Qa;Qb icab1 ifabcQc. This phenomenon is certainly rather raresince the central charges cab can be removed by a suitableredefinition of the generators Qa for all semisimple Liealgebras [13]. Yet, once central charges do appear, it caneven happen that ab is nonzero although at the classicallevel the Lie algebra of conserved charges is commutative.The simplest example is perhaps the free nonrelativisticparticle which can be interpreted as a type II NG boson ofan extended ISO(2) symmetry [9].Finally, it may be instructive to emphasize that any proof

of the Goldstone theorem or its generalizations, includingours, can be applied not only to the set of all brokengenerators of a given theory, but also to its subsets closedunder commutation. Any conclusion about the number ofNG bosons then refers to those NG bosons that couple tothe broken charges considered.

III. CHARGE COMMUTATOR ANDTHE NUMBER OF NG BOSONS

The aim of this section is to provide evidence for ourconjecture (1). We will actually prove a one-sided inequal-ity and give a physical argument why the opposite inequal-ity also holds. We will essentially follow the strategy ofNielsen and Chadha [4] and complement it with the rela-tion to the charge commutator matrix ab.The NG mode with zero momentum corresponding to

the broken generator Qa will be represented as jai Qaj0i. This is a consistent definition of zero-momentumNG states. On the one hand, all states jai are created by thebroken charges and have zero energy. On the other hand,assume that there is another zero-energy state which isorthogonal to all jais. By construction, it then has a zerocoupling to all broken charges, hence does not contribute to

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the commutator (2), and thus should not be counted amongthe NG bosons. Consequently, the number of NG bosons isequal to the number of linearly independent vectors jai.Note that this argument uses the implicit assumption thatthe NG states form a vector space, which is rigorouslyjustified at least at zero momentum.

A. Proof of nBS nNG 1=2 rankSince out of the nBS vectors jai, there are only nNG

linearly independent ones, there must be a set of complexcoefficients Cpa, where p 1; . . . ;n and n nBS nNG, such that Cpajai 0 with rank C n (summationsover repeated indices are implied). As pointed out byNielsen and Chadha [4], the n vectors Cpajai are linearlyindependent. Indeed, if this were not the case, there wouldbe a set of coefficients Dp such that DpC

pajai 0. Then,

DpCpaMab DpCpah0jQa;b0j0i hajCpaDpb0j0i h0jb0DpCpajai 0; (7)

where the matrix M is defined in Eq. (2). Equation (7)would imply that the matrix M has a zero eigenvalue,which would contradict the assumption of SSB that M isnonsingular.We have observed that CpaQaj0i 0 for all p

1; . . . ;n, and that fCpaQaj0ignp1 are linearly indepen-dent. This implies that Ap CpaQa (Ayp CpaQa) acts asthe annihilation (creation) operator of a NG boson labeled

by p. Since fAypj0ignp1 are linearly independent, thenn matrix G defined as

Gpq h0jApAyq j0i h0jAp; Ayq j0i CgCypq (8)is Hermitian and positive definite; the Hermitian matrix ghere is defined as gab hajbi. The matrix G can bediagonalized by a unitary matrix U so that UGUy diag1; . . . ; n. From Eq. (8) we can see that the diag-onalization is achieved by the replacement C! UC. Thus,we can chooseC from the beginning in such a way thatG isalready diagonal. We then have

h0jjAp; Ayq j0i pqq no sum over q: (9)Note that this relation is reminiscent of a commutationrelation between the annihilation and creation operators,

Ap; Ayq / pq.Now we are ready to show the desired inequality. Let us

define

Qp 12 Ap Ayp ReCpaQa;

Qp 12i Ap Ayp ImCpaQa:

(10)

These make 2n independent broken generators. Inorder to obtain a basis of the space spanned by thebroken generators fQagnBSa1, we need to add nBS 2nother broken generators, Q0s BsaQa, with real Bsa.Introducing finally the notation for the new basis

Q1; Q2; . . . ; QnBS Q1 ; Q1 ; . . . ; Qn; Qn; Q01; . . . ; Q0nBS2n; (11)

we infer that the antisymmetric matrix , defined byi ab lim!1 1 h0j Qa; Qbj0i, equals

lim!1

1

2

0 1

1 0 0. ..

0 0 n

n 0

0BBBBBBBBBBBB@

1CCCCCCCCCCCCA; (12)

where the asterisks indicate in turn 2n nBS 2n,nBS 2n 2n, and nBS 2n nBS 2nblocks of unknown, possibly nonzero entries. Since thetransformation f QagnBSa1 $ fQagnBSa1 is nothing but a changeof basis, must have the same rank as ; hence

rank rank 2n; (13)which was to be proven.

B. Explanation of nBS nNG 1=2 rank and therelation to the NC classification

In Ref. [14], Nambu pointed out that h0jQa;Qbj0i 0implies that the corresponding zero modes behave likecanonical conjugates of each other; the same observationwas made in a special case in Ref. [8]. We have seen

this above; Qp and Qp excite the same mode Aypj0i, and

they have the commutation relation h0jQp ;Qp j0i i=2h0jAp; Aypj0i 0 [see Eqs. (9) and (10)].By construction, fQp gnp1 [ fQ0sgnBS2ns1 (or equiva-

lently fQp gnp1 [ fQ0sgnBS2ns1 ) excite linearly independentmodes. So, if we assume that Nambus argument is valid,all of h0jQp ; Q0sj0i, h0jQp ; Q0sj0i, and h0jQ0s; Q0s0 j0ishould vanish, which means all the asterisks in Eq. (12)should be replaced with zero block matrices. We thenarrive at the equality nBS nNG 1=2 rank.As for the dispersion relation "pk of the mode Aypj0i,

we can apply the same analyticity argument as Nielsen andChadha [4] and conclude that "pk is an analytic functionof k. In the special case of an unbroken rotational invari-ance, we then have "pk / jkj2mp where mp is a positiveinteger. Thus, all Aypj0i modes are type II NG bosons. How

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about the modes excited by fQ0sgnBS2ns1 ? Based on theexamples discussed in literature, we can say that theyusually have a linear dispersion relation, that is, are typeI. However, there is no general argument that would guar-antee this. It is possible to modify the dispersion relation toa quadratic one by fine-tuning the parameters of the theory.Indeed, this is demonstrated by the example we propose inAppendix A. All in all, the number of NG bosons with aneven power of momentum in the dispersion relation is atleast n. This leads immediately to the NC inequality.In practice, the classification of the various types of NG

bosons can be significantly simplified by choosing anappropriate basis of the broken generators. To that end,note that one can always choose the ground state in such away that only mutually commuting charges have nonzerovacuum expectation value. In other words, only generatorsfrom the Cartan subalgebra of the Lie algebra of thesymmetry group can have a nonzero vacuum expectationvalue. A detailed proof of this statement is given inAppendix C at the level of generality sufficient for allpractical purposes. By means of the general root decom-position of Lie algebras, it then follows immediately thatthe set of broken generators splits into pairs whose com-mutator has possibly nonzero vacuum expectation value(see Appendix D for details).A cautious reader may have noticed that, strictly speak-

ing, the above choice of basis may not be compatible withthe choice made in Eq. (11). Therefore, the material ofAppendix C cannot be directly used in our proof. However,as already stressed, it is of great practical utility, and inaddition, it can be used for an alternative derivation of theinequality nBS nNG 1=2 rank, which is given inAppendix D. This derivation provides us with a new insightinto the nature of the NG bosons associated with chargedensity.

IV. SOME SPECULATIONS AND OUTLOOK

In this paper we pointed out the intimate connectionbetween the number of NG bosons in a quantum many-body system exhibiting SSB and the densities of conservedcharges in the ground state. Our main result is summarizedin Eq. (1). The method used to (partially) prove it, follow-ing closely Ref. [4], suggests that a different classificationof NG bosons than that based on the dispersion relationmight be useful. Let us denote a NG boson that couples tosome combination of broken charges CpaQa introduced inSec. III as type C (charged), and the remaining NGbosons as type N (neutral). The argument in Sec. IIIthen immediately proves that

nN 2nC nBS; (14)with obvious notation. Moreover, our conjecture (1) canthen be cast in the form nC 1=2 rank.As follows from the proof of the Nielsen-Chadha theo-

rem [4], all type C NG bosons are necessarily type II. On

the other hand, as pointed out already in Sec. III B, type NNG bosons can be both type I and type II. In the systemsusually discussed in literature, all type N NG bosonsactually are type I; hence the two classifications coincideand the NC inequality is saturated as a consequence ofEq. (1). However, the explicit example in Appendix Ashows that there may exist NG bosons which are simulta-neously type N and type II. We may say that such NGbosons are accidentally type II since their nonlinear,typically quadratic, dispersion relation is achieved by tun-ing the parameters of the model. On the contrary, all type CNG bosons are robustly type II due to the connectionwith the conserved charge densities. The physical origin ofthese two different kinds of type II NG bosons is obviouslyvery different.Of course, in order that this classification of NG bosons

is consistently defined, one first has to prove that it does notdepend on the choice of the operatorsQa. For instance, thetype II NG boson of Appendix A would be classified astype N. However, one could imagine that there is anotherconserved charge which couples to the same NG boson andhas a nonzero commutator with the Noether charge of theshift symmetry of the theory (A1). The same NG bosonwould then be classified as type C. Of course, the conjec-ture (1) would be satisfied in both cases. That is why weonly propose this classification here in the form of aspeculation. It would be interesting to investigate this issuefurther.Another interesting problem is what happens when sym-

metry whose spontaneous breaking gives rise to a reducednumber of NG bosons is gauged. How many massivespin-one bosons appear in the spectrum as a consequenceof the Anderson-Higgs mechanism? Is their number equalto nNG or rather to nBS? This question has been addressedin Refs. [15,16], but so far only for one particular model,whose global version was introduced earlier inRefs. [8,17]. The problem would certainly deserve amore general investigation.Finally, note that in this paper we concentrated solely on

NG bosons of spontaneously broken uniform symmetries.While a generalization of the Goldstone theorem to spon-taneously broken nonuniform symmetries, which ensuresthe existence of at least one NG boson, was given inAppendix B, the issue of the count of the NG bosons insuch a case is still open. Any result with the same level ofgenerality as well as rigor as the Goldstone theorem wouldsignificantly improve our understanding of spacetime aswell as other nonuniform symmetries.

ACKNOWLEDGMENTS

We thank H. Abuki, H. Aoki, T. Hatsuda, T. Hayata, T.Kanazawa, and T. Kugo for fruitful discussions. Part ofthe work was carried out during the stay of H.W. atthe University of Bielefeld. H.W. acknowledges theUniversity of Tokyo exchange program for a partial

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support of his visit to Bielefeld. The work of T. B. wassupported by the Sofja Kovalevskaja program of theAlexander von Humboldt Foundation.Note added.After this work was completed, we

learned that the result proven in Appendix C is a specialcase of the following general theorem known in mathe-matical literature: If G is a compact connected Lie groupand T is a maximal torus, then each element of G isconjugate to a member of T (theorem 4.36 in Ref. [22]).We are indebted to S. Naito and R. Sakamoto for bringingthis theorem to our attention.

APPENDIX A: INEQUALITY IN THE NCCOUNTING RULE

Here we propose an example in which a strict inequalityin the NC counting rule holds. Similarly to the example inSec. II A, let us consider a class of free scalar field theoriesdefined by the Lagrangian density

L 12@0@0 12D; (A1)where D P1n1 cnr2n. This Lagrangian is invariantunder a (real) constant shift of the field, x ! x .By choosing the ground state j0i for simplicity as the

Fock vacuum, we get

h0jjiQ;xj0i 1: (A2)Thus, the shift symmetry is always spontaneously broken.The corresponding NG boson is nothing but the one-particle state in the Fock space of this noninteracting field

theory, jnki ayk j0i with the dispersion relation "k Pncnjkj2n

q. For the usual Lorentz-invariant case (cn

n;1), we have "k jkj which represents a type I NGboson so that the NC inequality is saturated. However,despite being somewhat pathological, it is also possibleto choose cn / n;2. In this case, "k / jkj2 which de-scribes a type II NG boson; hence a strict inequality occursin the NC counting rule. The punch line is that the power ofmomentum in the dispersion relation of the NG boson canbe modified by tuning the parameters of the Lagrangian.We expect the same is also true in other models such as theHeisenberg ferromagnet with a sufficient number of cou-pling constants.

APPENDIX B: GOLDSTONE THEOREM FORNONUNIFORM SYMMETRIES

The alternative proof of the Goldstone theorem [2]usually makes use of the quantum effective potential,which is a function of classical uniform fields, and thuscannot be applied directly to nonuniform symmetry trans-formations. We offer here a generalization of the argumentthat relies on the full quantum effective action.Following essentially Weinberg [18], we consider a

theory of a set of (not necessarily scalar) fields, ix,

and assume that the classical action of the theory is invari-ant under the infinitesimal shift ix Fix;. Here is a parameter of the transformation, and Fi is in generala functional of the fields and a function of the coordinate x.This classical symmetry is shared by the quantum effectiveaction, , that is,

Zd4y

jyFjy; 0; (B1)

provided Fi is linear in the fields. Taking now the varia-tional derivative of Eq. (B1) with respect to ix andsetting the field equal to its expectation value,h0jixj0i vix, we obtain

Zd4y

2vixjyFjy;v 0: (B2)

Finally, we assume for the sake of simplicity full transla-tional invariance of the vacuum. The second derivative ofthe effective action can then be expressed in terms of theinverse propagator of the theory, G1ij k.For uniform symmetries we can immediately infer that

G1ij 0Fjv 0. In other words, SSB implies the exis-tence of a gapless pole in the propagator. This is equivalentto the more common formulation of the proof using theeffective potential. Our argument is nevertheless moregeneral since it also applies to theories with a nonlocalaction which cannot be expressed as a spacetime integral ofa local Lagrangian density.For nonuniform symmetries, we have to make one fur-

ther step, that is, multiply Eq. (B2) by eikx and integrateover the coordinate x, which yields

G1ij kZd4yeikyFjy;v 0: (B3)

(Nonzero momentum k is used just to ensure the conver-gence of the integral.) Once again, in the limit k! 0, theintegral of Fjy;v represents a zero mode of the inversepropagator, which establishes the existence of a masslessparticle in the spectrum.Applying Eq. (B3) to the example discussed in Sec. II A,

we can immediately see that all five spontaneously brokencharges of Eq. (6) give rise to the same zero mode of theinverse propagator. Generally, it follows from Eq. (B3) thattwo broken symmetries that have the same local form willgive the same zero mode of the inverse propagator, that is,the same NG boson. This is in accord with the conclusionof Low andManohar [11]. Nevertheless, in contrast to theirclassical field theory argument, our derivation actuallyproves the existence of the massless mode in the spectrumof the quantum theory, and is only limited by the assumedtranslational invariance and the linearity of the symmetrytransformation in the fields.

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APPENDIX C: CHARGE DENSITIES AND THECARTAN SUBALGEBRA

In this appendix we will discuss in detail the statementmade in Sec. III B that one can always choose the groundstate in such a way that only mutually commuting chargeshave nonzero vacuum expectation value. As explainedthere, this is of great practical help in the classification ofthe NG bosons. Even though we have not been able toprove this claim in the full generality, we give below adetailed proof that applies to all compact semisimple Liegroups which are given by direct products of classicalsimple Lie groups. This is sufficient for virtually all prac-tical purposes. Our claim can be reformulated mathemati-cally as the statement that every adjoint orbit of thesymmetry group passes through the Cartan subalgebra ofits Lie algebra. We will prove it in turn for all classes ofclassical simple Lie groups and address the generalizationto semisimple groups in the end.

Let us first introduce some notation. By Ta and Q^a wewill denote the generators of the symmetry group in itsdefining representation, and its representation on theHilbert space of the quantum system, respectively. Thecorresponding finite symmetry transformations will then

be U eiaTa and U^ eiaQ^a . Finally, R willstand for the adjoint representation of the same transfor-mation. Note that for the sake of clarity, we use in thisappendix hats to distinguish operators on the Hilbert spacefrom finite-dimensional matrices.We now introduce the charge expectation values,

qa h0jQ^aj0i, and construct the matrix M qaTa be-longing to the Lie algebra of the symmetry group. Upon asymmetry transformation, the ground state j0i becomesji U^j0i. The charges qa then transform to [19]

q0a hjQ^aji h0jU^yQ^aU^j0i h0jR1ab Q^bj0i Rbaqb: (C1)

HenceM transforms to

M 0 q0bTb qaRabTb UMUy: (C2)We can see thatM transforms in the adjoint representa-tion, which provides a link between the physical and themathematical formulation of the problem; the fact that onlyq0as corresponding to mutually commuting generators arenonzero simply means that the matrixM0 lies in the Cartansubalgebra. In the following, we will use the more concisemathematical formulation of the problem.

1. Proof for the groups SUNSince the Cartan subalgebra is in this case formed by all

real diagonal traceless matrices, our claim follows triviallyfrom the well-known fact that every Hermitian matrix canbe diagonalized by a suitable (unimodular) unitary trans-formation. To the best of our knowledge, the proof in this

case was given for the first time in Ref. [20] using anargument essentially identical to ours.

2. Proof for the groups SONThe Cartan subalgebra in this case can be chosen as the set

of all (purely imaginary and antisymmetric) matrices whoseonly nonzero entries are localized in 2 2 blocks along thediagonal. Our task is to prove that every purely imaginaryantisymmetric matrixM can be brought into such block-diagonal form by a special orthogonal transformation.We know that the (real) eigenvalues of a purely imagi-

nary antisymmetric matrix come in pairs with oppositesigns and that the corresponding eigenvectors are relatedby complex conjugation. Let us denote these eigenvectorsas uk, u

k. Every such pair of vectors spans an invariant

subspace ofM. We can trade them for real vectors vk uk uk=

2

pand wk iuk uk=

2

p. (In case of odd

N, there will be an additional real eigenvector associatedwith zero eigenvalue.) Since the basis of eigenvectors

fuk; ukgN=2k1 is orthonormal, so is the basis fvk;wkgN=2k1.It is real and hence connected to the basis we startedfrom by a real orthogonal transformation. In this basis,M takes the desired block-diagonal form. Finally, byflipping the sign of one of the basis vectors if necessary,we can ensure that the similarity transformation leading to

fvk;wkgN=2k1 is unimodular.

3. Proof for the groups Sp2NRecall that the group Sp2N consists of all 2N 2N

unitary matricesU such thatUIUT I where I i2 1NN with 2 the second Pauli matrix. We can say thatSp2N contains all unitary matrices which are simulta-neously I-orthogonal. Likewise, the generators of Sp2Nare Hermitian and I-antisymmetric, TTa ITaI1. As aconsequence, the generators of Sp2N take the form

T A BBy AT

; (C3)

where A is a Hermitian and B a symmetric complex N Nmatrix. The Cartan subalgebra then consists of all matricesof this form with A real and diagonal and B 0.We are now to prove that every I-antisymmetric (and

Hermitian) matrix M can be diagonalized by a unitarysymplectic transformation. First, observe that if uk is itseigenvector with the (real) eigenvalue k, then its I con-jugate, Iuk, is also an eigenvector with the eigenvaluek. Hence the eigenvalues of M once again come inpairs with opposite sign and we can thus construct anorthonormal basis of eigenvectors and organize them inthe columns of a unitary matrix [21],

P u1; . . . ; uN; Iu1; . . . ; IuN: (C4)In this basis,M acquires the desired diagonal form and asimple calculation shows that P itself is symplectic asrequired. This completes the proof.

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4. Proof for semisimple groups

Let us prove our claim for a direct product of two groupsof the type discussed in the preceding three subsections.The general case will then follow by induction. Thetwo (commuting) sets of generators will be denoted Ta 1 and 1 T, where we use Latin and Greek indicesto distinguish the two groups. The matrix M thenbecomes

M qaTa 1 1 qT: (C5)The transformations from the two groups are labeledby independent parameters a and , that is, U eiaTa and V eiT . Under a general transformationfrom the product group,U V, the matrixM trans-forms to

M 0 U VMU Vy UqaTaUy 1 1 VqTVy:

(C6)

Obviously, one can choose the transformations in the twogroups independently to diagonalize the respective parts ofM so that, in the end,M0 will lie in the Cartan subalgebraof the product group, which is just a direct sum of the twoCartan subalgebras.

APPENDIX D: CLASSIFICATION OF NG BOSONSVIA CARTAN DECOMPOSITION

In order to view the NG bosons associated with chargedensities from a different perspective, let us recall the rootdecomposition from the theory of Lie algebras. (Our argu-ment will apply to all Lie algebras to which the result ofAppendix C applies, that is, to all compact semisimple Liealgebras which are given by direct products of classicalsimple Lie algebras.) There, one constructs linear combi-nations of the Lie algebra generators in the form of root

generators E, such that E; Ey iHi, where isthe associated root vector and Hi is the basis of the Cartansubalgebra.Using the result of Appendix C, we now choose the basis

of the Lie algebra so that only the generators Hi canhave a nonzero vacuum expectation value. Concretely,let us consider all the roots c, c 1; . . . ;n0 such that

0c cih0jHij0i 0. Defining analogously to Eq. (10)the set of 2n0 generators Qc as

Qc 12 Ec Eyc; Qc

1

2iEc Eyc; (D1)

and adding nBS 2n0 generators Q0s in order to obtain acomplete basis of generators, the commutator matrix becomes

lim!1

1

2

0 0101 0 0

. ..

0

0 0 0n00n0 0

0 0

0BBBBBBBBBBBBB@

1CCCCCCCCCCCCCA:

(D2)

All nonzero entries of the matrix are located in n0 2 2blocks along the diagonal. Ordering the generators as inEq. (11), we next choose the first 2n0 interpolating fieldsa in the commutator (2) as (j

01 ; j

01 ; . . . ; j

0n0 ; j

0n0). For

s 2n0 1; . . . ; nBS and a 1; . . . ; 2n0, we now haveMsa 0, and the SSB condition detM 0 demands thatdetM0 0 whereM0st is the nBS 2n0 nBS 2n0lower-right corner of Mab. Finally, we observe that sincethe expectation values of commutators of all pairs of gen-erators Q0s vanish, according to the theorem of Schaferet al. [8] there must be nBS 2n0 NG bosons as inter-mediate states in the commutatorM0st. At the same time, atleast one NG boson must appear as an intermediate state inthe commutator h0jQc ; j0c 0j0i for each c 1; . . . ;n0. Therefore, the number of NG bosons asbounded from below as nNG nBS 2n0 n0 nBS n0 nBS 1=2 rank, as proved by a differentmethod in Sec. III A. While the above argument uses thenotation for charge densities introduced in Sec. II forsystems with continuous translational invariance, it goesthrough without change if we replace j0a with aa chargedensity on a spatial latticeas in Eq. (3).

[1] J. Goldstone, Nuovo Cimento 19, 154 (1961).[2] J. Goldstone, A. Salam, and S. Weinberg, Phys. Rev. 127,

965 (1962).[3] G. S. Guralnik, C. R. Hagen, and T.W. B. Kibble, in

Advances in Particle Physics, edited by R. L. Cool andR. E. Marshak (Wiley, New York, 1968), Vol. 2, p. 567.

[4] H. B. Nielsen and S. Chadha, Nucl. Phys. B105, 445(1976).

[5] The power of momentum in the dispersion relation isbounded from above by a value depending on the spacedimension d though. Generalizing naively the argument ofthe classic paper by Coleman [6] to Lorentz-noninvariant

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systems, the correlation function of the NG field woulddiverge at large distance if the power of momentumexceeded this limit.

[6] S. R. Coleman, Commun. Math. Phys. 31, 259(1973).

[7] H. Leutwyler, Phys. Rev. D 49, 3033 (1994).[8] T. Schafer, D. T. Son, M.A. Stephanov, D. Toublan,

and J. J.M. Verbaarschot, Phys. Lett. B 522, 67(2001).

[9] T. Brauner, Symmetry 2, 609 (2010).[10] T. Brauner, Phys. Rev. D 72, 076002 (2005).[11] I. Low and A.V. Manohar, Phys. Rev. Lett. 88, 101602

(2002).[12] To avoid misunderstanding, let us emphasize that every

conserved charge of course generates a symmetry trans-formation on the phase space of the classical theory.Nevertheless, it may not be possible to obtain this chargeas a consequence of an off-shell invariance of the action inthe Lagrangian formalism. This is what we mean bysaying that the charge is not of the Noether type.

[13] S. Weinberg, The Quantum Theory of Fields (CambridgeUniversity Press, Cambridge, UK, 1995), Vol. 1.

[14] Y. Nambu, J. Stat. Phys. 115, 7 (2004).[15] V. P. Gusynin, V.A. Miransky, and I. A. Shovkovy, Phys.

Lett. B 581, 82 (2004).

[16] Y. Hama, T. Hatsuda, and S. Uchino, Phys. Rev. D 83,125009 (2011).

[17] V. Miransky and I. Shovkovy, Phys. Rev. Lett. 88, 111601(2002).

[18] S. Weinberg, The Quantum Theory of Fields (CambridgeUniversity Press, Cambridge, UK, 1996), Vol. 2.

[19] The last step here makes use of the assumption that theadjoint representation matrices R are orthogonal, whichin turn follows from the full antisymmetry of the structureconstants of the Lie algebra. This can be achieved for anycompact semisimple Lie algebra by a proper choice ofbasis.

[20] K. Rajagopal and A. Schmitt, Phys. Rev. D 73, 045003(2006).

[21] While the orthogonality is trivial in the case of a nonzeroeigenvalue with multiplicity one, in general one has to be abit careful. First, note that when k 0, Iuk is orthogonalto uk by construction, thanks to the antisymmetry of I .Second, suppose that some eigenvalues are equal, say,k1 k. We can then choose the eigenvector uk1 tobe orthogonal to both uk and Iuk. This already guaranteesthat Iuk1 will be orthogonal to all three of these vectors.One can thus construct an orthonormal basis by induction.

[22] A.W. Knapp, Lie Groups beyond an Introduction(Birkhauser, Boston, 2005), 2nd ed.

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