VOLUME 88, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 18 MARCH 2002
111601-1Spontaneous Symmetry Breaking with Abnormal Numberof Nambu-Goldstone Bosons and Kaon Condensate
V. A. Miransky*Department of Applied Mathematics, University of Western Ontario, London, Ontario, Canada N6A 5B7
I. A. Shovkovy*School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455
(Received 23 August 2001; published 28 February 2002)
We describe a class of relativistic models incorporating a finite density of matter in which spontaneousbreakdown of continuous symmetries leads to a lesser number of Nambu-Goldstone bosons than thatrequired by the Goldstone theorem. This class, in particular, describes the dynamics of the kaon con-densate in the color-flavor locked phase of high density QCD. We describe the spectrum of low energyexcitations in this dynamics and show that, despite the presence of a condensate and gapless excitations,this system is not a superfluid.
DOI: 10.1103/PhysRevLett.88.111601 PACS numbers: 11.30.Qc, 12.38.Aw, 21.65.+fThe Goldstone theorem is a cornerstone of the phenome-non of spontaneous breakdown of continuous global sym-metries. It is applicable both to relativistic field theorieswith exact Lorentz symmetry  and to most condensedmatter systems  where this symmetry is absent. How-ever, there is an important difference between these twocases. While in Lorentz invariant systems, the Goldstonetheorem is universally valid, it is not so in condensedmatter systems. For example, it does not apply to con-densed matter systems with long range interactions .From the technical viewpoint, the difference is connectedwith a kinetic term, and derivative terms, in general, in aLagrangian density: while their form is severely restrictedby the Lorentz symmetry, it is much more flexible in sys-tems where this symmetry is absent.
In this Letter, we describe the phenomenon of sponta-neous symmetry breaking of continuous symmetries withan abnormal number of Nambu-Goldstone (NG) bosonstaking place at a sufficiently high density of matter in aclass of models without long range interactions. Here byabnormal we understand that the number of gapless NGbosons is less than the number of the generators in the cosetspace GH, where G is a symmetry of the action and His a symmetry of the ground state. On the other hand,as we shall see below, the degeneracy of the ground stateremains conventional: it is described by transformationsconnected with all the generators from the coset space. Itis noticeable that this class of models describes a recentlysuggested  scenario with a kaon condensate in thecolor-flavor locked (CFL) phase of high density QCD .
We illustrate this phenomenon in a toy model with thefollowing Lagrangian density:
L 0 1 imFy0 2 imF2 y2iF
yiF 2 m2FyF 2 lFyF2, (1)where F is a complex doublet field and y is a velocityparameter. Since here the Lorentz symmetry is brokenby the terms with the chemical potential m, the velocity0031-90070288(11)111601(4)$20.00y # 1 in general. The chemical potential m is providedby external conditions (to be specific, we take m . 0) .The above Lagrangian density is invariant under globalSU2 3 U1. The SU2 is treated as the isospin group Iand the U1 is associated with hypercharge Y . The electriccharge is Q I3 1 Y . This model describes the essenceof the dynamics of the kaon condensate  (see below).
When m , m, it is straightforward to derive the treelevel spectrum of the physical degrees of freedom. To thisend, we switch to the momentum space by decomposingall four real components of the F field in plane waves.Then, the quadratic part of the above Lagrangian densitytakes the following form:
L 2v,q 12
f1 f2 M
f1 f2 M
where the real and imaginary parts of each componentof the doublet were introduced, FT 1
p2 f1 1 if2,
f1 1 if2. Note that their Fourier transforms satisfyfi v, k fi2v,2k and f
i v, k fi2v,2k.
The matrices M and M in Eq. (2) readv2 1 m2 2 m2 2 y2q2 2imv
22imv v2 1 m2 2 m2 2 y2q2
The dispersion relations of the particles are determinedfrom the equation DetM 0. Explicitly, this equationreads
v 2 m2 2 m2 2 y2q2 3v 1 m2 2 m2 2 y2q2 0 ; (4)
i.e., the particles dispersion relations are
v1 v1 6pm2 1 y2q2 1 m , (5)
v2 v2 6pm2 1 y2q2 2 m . (6) 2002 The American Physical Society 111601-1
VOLUME 88, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 18 MARCH 2002Of course, the positive and negative values of energy cor-respond to creation and annihilation of excitations, respec-tively. Henceforth we consider only positive eigenvalues:for our purposes, no additional information is contained inthe eigenstates with negative eigenvalues.
Equations (5) and (6) imply that the particle spectrumcontains two doublets with the energy gaps m 1 m andm 2 m, respectively. In dense quark matter, the first dou-blet can be identified with K2, K0 and the second onewith K1,K0. It is important to note that the chemicalpotential causes splitting of the masses of particles andtheir antiparticles. This point is crucial for reducing thenumber of NG bosons in the asymmetric phase consideredbelow. The splitting is intimately connected with the factthat C, CP, and CPT symmetries are explicitly brokenin this system. In second quantized theory, the complexfield F describes creation and annihilation of K2, K0and K1,K0, respectively. This is quite unusual becausethe corresponding dispersion relations of these two dou-blets are not identical.
By studying the potential of the above model in tree ap-proximation, we could check that the perturbative groundstate becomes a local maximum when m . m [see Eq. (2)with v q 0]. At this point, the system experiencesan instability with respect to forming a condensate. In thenew phase, a vacuum expectation value, w0, of the field Foccurs:
f1 1 if2f1 1 if2
. (7)111601-2This choice of the neutral direction of the vacuum ex-pectation value corresponds to the conventional definitionof the electric charge, Q I3 1 Y .
By requiring that the new ground state is a minimum ofthe potential, we derive
w20 m2 2 m2
for the vacuum expectation value of the field. In thisground state, the initial SU2 3 U1 spontaneouslybreaks down to U1Q .
In the broken phase, the quadratic part of the Lagrangiandensity looks formally the same as in Eq. (2). The matricesM and M , however, are different. In particular, the firstone, describing two charged states, is
M v2 2 y2q2 2imv22imv v2 2 y2q2
while the other, describing two neutral states, is
M v2 2 2m2 2 m2 2 y2q2 2imv
22imv v2 2 y2q2
For the charged states, the dispersion relations are
v1,2 pm2 1 y2q2 6 m . (11)
For the neutral states, the dispersion relations are given byv1,2
r3m2 2 m2 1 y2q2 6
q3m2 2 m22 1 4m2y2q2 . (12)
All four dispersion relations are shown in Fig. 1. As is easy to check, one of the charged states with the relation
v2 pm2 1 y2q2 2 m (13)
and one of the neutral states with the relation
r3m2 2 m2 1 y2q2 2
q3m2 2 m22 1 4m2y2q2 , (14)describe NG bosons, i.e., gapless excitations whose energygoes to zero as q ! 0 (see Fig. 1). Indeed, in the farinfrared region, these relations take the following form:
sm2 2 m2
3m2 2 m2yq . (16)
No other gapless states appear. Thus, there are two gaplessNG bosons in the system. It comes as a real surprise.Indeed, since the initial global SU2 3 U1 symmetryspontaneously breaks down to U1Q , one should expectthe existence of three NG bosons. Where is the thirdone? To better understand the situation at hand, it isinstructive to consider the quadratic part of the potentialaround the ground state configuration in the broken phase.For the two sets of the fields, the corresponding quadraticforms are read from 2M and 2M , where v q 0is substituted. The eigenvalues of such matrices are
j1,2 j2 0 , (17)
j1 2m2 2 m2 . (18)
Therefore, we see that the potential part of the action hasthree flat directions in the broken phase, as it should. Thesethree directions would correspond to three NG bosons, re-lated to the three broken generators of the original SU2 3U1. The first order derivative terms in the kinetic partof the action, however, prevent the appearance of onecharged, K2, gapless mode. This fact is intimately con-nected with the point that the presence of the chemicalpotential leads to splitting of the energy spectra of K2
VOLUME 88, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 18 MARCH 20020.5 1 1.5 2 2.5 3q
FIG. 1. Dispersion relations of four particles in the brokenphase of the model in Eq. (1). The solid and dashed linesrepresent the dispersion relations of charged and neutral states,respectively. All quantities are given in units of m. To plot thefigure we used m 0.5m and y 1
Another noticeable fact is that the energy v2 of the K1
gapless mode in Eq. (15) is proportional to q2 rather thanto q. This implies that, despite the presence of the con-densate, the Landau criterion for superfluidity fails in thismodel . Therefore, this system is not a superfluid.
This toy model illustrates a rather general phenomenon.While in Lorentz invariant systems with spontaneousbreakdown of continuous symmetries the degeneracy ofthe potential fixes the number of NG bosons being equalto the number of generators NGH in the coset space GH,in systems with a broken Lorentz symmetry the number ofgapless NG bosons can be lesser. The latter is connectedwith a form of the kinetic term which can include first orderderivative terms. In the systems under consideration, thechemical potential (leading here to such first order deriva-tive terms) splits up energy gaps of charged particles andtheir antiparticles both of which would be NG bosons oth-erwise. On the other hand, a neutral NG boson, a partnerof a field with a nonzero vacuum expectation value, al-ways survives. Therefore, the number of the gapless NGbosons reduces by Nch, where Nch is the number of charged(particle-antiparticle) pairs among the would-be NGbosons. In the present model NGH 3 and Nch 1,and as a result there are only two (one neutral and onecharged) NG bosons.
The choice of the simple model in Eq. (1) was notaccidental in this Letter. It describes the essence of amuch more complicated dynamics of spontaneous sym-metry breaking related to the kaon condensation in thecolor-flavor locked phase of dense quark matter . Letus turn to it.
We start by repeating briefly the analysis of Ref. .In the chiral limit, the ground state of the three flavordense quark matter corresponds to the CFL phase .The original SU3c 3 SU3L 3 SU3R symmetry ofthe microscopic action breaks down to the global lockedSU3c1L1R subgroup. The corresponding low energy ac-tion for the NG bosons was derived in Refs. [9,10]. Thecurrent quark masses break the original chiral symmetry of111601-3the model explicitly. As a result, nonzero gaps appear inthe spectra of the NG bosons, and they become pseudo-NGbosons. Their dynamics could still be described by thelow energy action which, for sufficiently small currentquark masses, could be derived from the microscopic the-ory . By making use of an auxiliary gauge symme-try, it was suggested in Ref.  that the low energy actionof Refs. [9,10] should be modified by adding a timelikecovariant derivative to the action of the composite field.
By neglecting a chemical potential of the electric charge,the low energy effective Lagrangian density of Ref.  (inMinkowski space) reads
4Tr=0S=0Sy 2 y2piSiS
0h02 2 y2h0 ih02
1 2cdetM TrM21Sep
23 ifh0 Ih 0 1 H.c. ,(19)
=0S 0S 1 iMMy
2pFS 2 iS
where pF is the quark Fermi momentum and M is aquark mass matrix chosen to be diagonal, i.e., M diagmu,md ,ms. By definition, I is a unit matrix inthe flavor space, and S is a unitary matrix field whichdescribes the octet of the NG bosons, transforming underthe chiral SU3L 3 SU3R group as follows:
S ! ULSUyR , (21)where UL,UR [ SU3L 3 SU3R . In Eq. (19), wealso took into account h0 field which couples to the octetwhen the quark masses are nonzero. The NG boson, re-lated to breaking the baryon number, was omitted, how-ever. Its dynamics is not affected much by the quarkmasses.
One should notice from the definition of the covariantderivative in Eq. (20) that the combination of the quarkmass matrices meff MMy2pF produces effectivechemical potentials for different flavor charges. Thesechemical potentials are of dynamical origin, and they areunavoidable.
The presence of the effective chemical potential meff hasfar-reaching consequences. In particular, if the mismatchof the quark masses of different flavors is large enough[e.g., ms * D2mu13], the perturbative CFL ground statebecomes unstable with respect to a kaon condensation.
The new ground state is determined by a rotated vac-uum expectation value of the S field,
Sap expial6 expipAlA
2f2p1 . . .
where a is determined by requiring that the correspond-ing ground state configuration is a global minimum of the111601-3
VOLUME 88, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 18 MARCH 2002potential energy. In the simplest case with mu md , wederive
cosa 4cp2Fmums 1 muf2p m2s 2 m2u2
, 1 , (23)
where (see Ref. )
21 2 8 ln236
The ground state with the kaon condensation which is de-termined by Eq. (22) breaks the SU2 3 U1Y symmetryof the effective action (19) down to U1Q. This is exactlythe symmetry breaking pattern that we encountered in thetoy model.
The derivation of the dispersion relations in the brokenphase with the kaon condensation involves rather tediouscalculations. Qualitatively, though, such a derivation issimilar to that in the model in Eq. (1). Additional diffi-culties come from more complicated particle mixing. Byomitting the details, we present the results.
With the choice of the vacuum expectation value inEq. (22), the original 9 degrees of freedom (pA withA 1, . . . , 8 and h0) group into two decoupled sets:p1,p2,p4,p5 and p3,p6,p7,p8 h,h0. The firstset of states contains all charged degrees of freedom (i.e.,p6 and K6), while the second set contains neutral ones(i.e., p0, K0, K0, h, and h0).
The dispersion relations for the charged degrees of free-dom are straightforward to derive. Our analysis shows thatthere is only one gapless NG boson in this set. In the farinfrared region q ! 0, its explicit dispersion relation reads
3m cosa1 1 cosa, (25)
where m m2s 2 m2u2pF . The analysis of the disper-sion relations of the other set is more difficult. But it isalso straightforward to show that there is only one gaplessNG boson there as well. Moreover, for q ! 0, the explicitdispersion relation reads
2 2 7 cos2a 1 9 cos4a
We see that, as in the case of the simple model in Eq. (1),there are only two gapless NG bosons, despite the fact thatthere are three broken symmetry generators in the phasewith the kaon condensate. One should note, however, thatas in the model in Eq. (1), the effective potential obtainedfrom (19) has the required three flat directions. It is thefirst order derivative terms that are responsible for produc-ing a nonzero energy gap in the spectrum of one of thecharged bosons. The corresponding opposite charge part-ner remains a gapless NG boson. It is crucial to note,111601-4though, that its dispersion relation behaves as v q2 forq ! 0. This implies that the criterion of superfluidity isnot satisfied in the phase of the dense quark matter withkaon condensation .
As has been argued in Refs. [4,5], there is a good chancethat the phase with a kaon condensate may exist in a coreof compact stars. Since the spectrum of low energy exci-tations in this phase derived in the present Letter is veryspecific, implying in particular that the matter is not su-perfluid, it could play an important role in detecting thisphase.
In conclusion, in this Letter the phenomenon of spon-taneous symmetry breaking with an abnormal number ofNG bosons was described. It admits a simple and clear in-terpretation. We expect that there exist wide applicationsof this phenomenon that deserve further study. In passing,we note that related systems in condense matter physics areferromagnets  and the superfluid 3He in the so-calledA phase .
We thank G. E. Volovik for bringing Ref.  to ourattention. The work of I. A. S. was supported by the U.S.Department of Energy Grant No. DE-FG02-87ER40328.
*On leave of absence from Bogolyubov Institute for Theo-retical Physics, 252143, Kiev, Ukraine.
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