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ITEP-LAT-2002-21KANAZAWA-02-31Towards understanding structure of the monopole clusters M.N.Chernodub Institute of Theoretical and Experimental Physics,B.Cheremushkinskaja 25,Moscow,117259,Russia and Institute for Theoretical Physics,Kanazawa University,Kanazawa 920-1192,Japan V.I.Zakharov Max-Planck Institut f¨u r Physik,F¨o hringer Ring 6,80805M¨u nchen,Germany (Dated:February 1,2008)We consider geometrical characteristics of monopole clusters of the lattice SU (2)gluodynamics.We argue that the polymer approach to the ?eld theory is an ade-quate means to describe the monopole clusters.Both ?nite-size and the in?nite,or percolating clusters are considered.We ?nd out that the percolation theory allows to reproduce the observed distribution of the ?nite-size clusters in their length and radius.Geometrical characteristics of the percolating cluster re?ect,in turn,the basic properties of the ground state of a system with a gap.PACS numbers:14.80.Hv,11.15.Ha I.INTRODUCTION Explanation of the con?nement in terms of the monopole condensation was proposed as early as around the year 1974[1].Moreover,the idea is strongly supported by the lattice data [2].Nevertheless,understanding the lattice data in terms of the continuum theory still represents a challenge.Indeed,generically one usually thinks in terms of a Higgs-type model:S eff = d 4x (1
the MAP monopoles.In particular,(see[5]and references therein)the three dimensional monopole densityρmon does not depend on the lattice spacing and is given in the physical units:
ρmon=0.65(2)σ3/2
SU(2)≈const.,(2) whereσSU(2)is the string tension.
An important remark is in order here.While discussing the monopole density one should distinguish between?nite-size clusters and the percolating cluster[6,7].There is a spectrum of the?nite-size clusters,as a function of their length,while the percolating cluster is in a single copy.In other words,the percolating cluster?lls in the whole of the lattice and its length is proportional to the volume of the lattice V4:
L perc=ρmon·V4(3) The observation(2)refers only to the percolating cluster.
Also,upon identi?cation of the monopoles in the Abelian projection,one can measure the non-Abelian action associated with these monopoles.The results[8]turned in fact astonishing and can be explained only as a?ne-tuning[9].To explain the meaning of the ?ne tuning,let us remind the reader that the probability of?nding any?eld con?guration is a product of action-and entropy-factors.Then,it turns out that for the monopoles at the presently available lattices both factors diverge in the ultraviolet but cancel each other to a great extent:
P mon=exp(?S mon)×(entropy)~exp(?c1·L/a)·exp(+c2·L/a),(4)
|c1?c2|
8π
∞
a
B2d3r=const.
L
the simplest vacuum loop in the polymer representation.In Sect.V we comment on the properties of the percolating cluster as representing the ground state of the system.In Sect.VI we discuss measurements which could clarify further the structure of the e?ective theory describing the monopoles.In Sect.VII we present conclusions.
It is worth mentioning that in a few cases we reproduce in some detail argumentation well known in other?elds,in particular in the condense matter.And in this sense our presentation is not original in these cases.However,we believe that the justi?cation of the use and application of the condense-matter techniques to non-perturbative?uctuations of the Yang-Mills?elds is new.
II.MONOPOLE CLUSTERS AND PERCOLATION In this section we will outline interconnections between properties of monopole clusters, percolation theory[11]and?eld theory.The aim is to motivate our basic assumptions(see subsection II F)and to apply later the percolation theory to the monopole clusters.
A.Monopole condensation in compact U(1)
A two-line theory of the monopole condensation was presented?rst in Ref.[12]for the case of the compact(or lattice)U(1)theory.We will review the derivation here and dwell on its connection with percolation.
The role of the compactness of the U(1)lattice theory is to ensure that the Dirac string does not cost any energy(for a review see,e.g.,Ref.[4]).That is why in Eq.(6)we take into account only the energy of the radial?eld.Nevertheless,the monopoles are in?nitely heavy in the limit a→0and,at?rst sight,this precludes condensation since the probability to?nd a monopole trajectory of the length L is suppressed as
exp(?S)=exp ?c a .(7)
Note that the constant c depends on details of the lattice regularization but can be found explicitly in any particular case.
What makes the condensation feasible,is an exponentially large enhancement factor due to the https://www.wendangku.net/doc/6b16664790.html,ly,a trajectory of length L for a point-like monopole can be realized on a cubic lattice in N L=7L/a various ways.To evaluate the N L let us notice that the monopole occupies the center of a cube and at each step the trajectory can be continued to an adjacent cube.In four dimensions there are8adjacent cubes.However,one of them is to be excluded from the counting since the monopole world-line is non-backtracking1.Thus the entropy factor is:
N L=exp ln7·L
1If a piece of the trajectory is covered in the both directions it is not observed on the lattice.Physically, this cancellation corresponds to the cancellation between monopole and anti-monopole.
and it cancels the suppression due to the action(7)if the coupling e2satis?es the condition of criticality:
e2crit=c/ln7≈1(9) where we quote the numerical value of e2crit for the Wilson action and cubic lattice.At e2crit any trajectory length L is allowed and the monopoles condense.This simple framework is veri?ed within about one percent accuracy as far as the prediction of e2crit is concerned[13, 14].
One can say that the coupling e2of the compact U(1)is to be?ne-tuned to trigger the phase transition.
B.Relation to percolation
In fact the derivation of the preceding subsection can be viewed as an application of the percolation theory[11].Moreover,one thinks in terms of the simplest percolation possible, that is,uncorrelated percolation.
Indeed,monopoles are observed as trajectories on the links of the dual lattice[2].Pos-tulate that the probability to occupy a link is given by:
p=exp(?c/e2),(10) compare Eq.(7)at L=a.The probability that(uncorrelated)links form a trajectory of length L is then given by Eq.(7).
Formation of an in?nitely long,or percolating cluster at a critical value p=p crit is a common feature of percolating systems.In our case,
p crit=1/7,(11) see Eq.(9).
It might worth mentioning that in text-books one would rather?nd p crit=1/8since the non-backtracking feature of the monopole trajectory is speci?c for charged particles. From the theoretical point of view the non-backtracking is a manifestation of the monopole charge conservation.Another consequence of this conservation law is the closeness of the trajectories which has not been taken into account so far.As we shall see in Sect.IV the closeness of the trajectories brings in only a pre-exponential factor and can be ignored at the moment for this reason.
C.Relation to?eld theory
The derivation in Section II A implies that the monopole condensation occurs when the monopole action is ultraviolet divergent.On the other hand,the onset of the condensation in the standard?eld theoretical language corresponds to zero mass of the magnetically charged ?eldφ.It is important to realize that this apparent mismatch between the two languages is not speci?c for the monopoles at all.Actually,there is a general kinematic relation(see, e.g.,Ref.[15,16])between the physical mass of a scalar?eld m2prop and the mass M de?ned in terms of the(Euclidean)action S,M≡S/L:
m2prop·a≈C2m·(M(a)?ln7
It is m2prop that enters the propagator of a scalar particle D(p2),
1
D sc(p2)~
Note that in the?eld theoretical language Eq.(16)would be interpreted as a power-like divergence well known in perturbation theory of charged scalar particles.Indeed,already on dimensional considerations one would conclude that:
|φ|2 perc~a(ρmon)perc~a?2,(17) (for a more careful derivation see Ref.[9]).
The behaviour(16)is in a sharp contrast with experimental data,see(2)and this is what we mean by the di?erence in the behaviour of a percolating system at p>p cr and of the monopoles in the con?ning phase.In other words,the?ne tuning exhibited by the monopoles in the con?ning phase is a speci?c feature of the vacuum state of the lattice gauge theory.
E.Fine tuning and Coulomb-like interaction
Let us emphasize again that in the present note we treat the?ne tuning of the monopoles as a pure observation and look for its implications.We are not in position at all to track its origin back to the non-Abelian Lagrangian which after all determines the results of all the lattice measurements.One of very few theoretical points which we can nevertheless add is that a long-range,Coulomb-like force is a necessary ingredient of a?ne-tuned theory.Of
course,in case of color by long distances we mean r?a but r≤Λ?1
QCD .
To substantiate the point,let us consider the con?ning phase of the compact U(1). The relative simplicity of this case is that the dynamics is explicitly known and,moreover, determined by the value of the electric charge e which can be tuned arbitrarily“by hand”(while in the non-Abelian case the coupling runs and is not under control in this sense).
Thus,one can introduce two scales in the con?ning phase of the compact U(1)in the following way:
R UV=a,R IR=
a
a
,
and nothing can prevent development of a quadratic divergence,see discussion above.
On the other hand it is known from the lattice measurements(see,e.g.,Ref.[18]and references therein)that the IR scale does coexist with the UV one.And,indeed,there is the loophole in our logic that introducing the tachyonic mass would immediately result in a quadratic divergence.This would be true if the interaction were local.However,imagine that there are other monopoles at a distance of order R IR.Then these monopoles can modify the Coulomb?eld of the monopole considered by order unit at r~R IR.This,in turn,can be interpreted as a change in the mass(due to interactions):
|δM(a)|~?
no power-like UV divergences(see discussion above).This double-face interpretation of(19) as IR and UV e?ects is a unique feature of the Coulomb-like interaction.
Note that we are not really giving a proof that the introduction of R IR is self-consistent. We just argue that without long-range forces the?ne tuning would be not possible at all.In case of non-Abelian theories there are many more open questions since the monopole action is not bounded from below(see,e.g.,Ref.[4])and the fact that the action is tuned to the entropy is even more di?cult to explain theoretically.
F.Fine tuning andλφ4models
It is worth to emphasize that the?ne tuning which is observed for the lattice monopoles is of the same generic type considered so mystifying in case of the Standard Model.There is,however,a peculiarity in our case.Indeed,if we compare the“natural”estimate forρmon (16)with the data(2)we see that there are three powers of a?1that are“tuned away”,not two powers as in the standard?eld-theoretical language.And,indeed,the data(2)imply in the“naive”a→0limit[9]:
lim
a→0 |φ|2 ~ρmon·a→0,
lim
a→0
m2prop~lim a→01a→∞(20)
lim
a→0
m2gluon~g2 |φ|2 →0,
where m gluon is the(dual)gluon mass which arises in theories of the type(1).
Eqs.(20)are of course not what one would expect for the standardλφ4theory with spontaneous symmetry breaking.It is worth emphasizing,however,that the limit a→0in Eqs.(20)should be understood with some reservations.What we mean actually in Eqs.(20) by a→0limit is“the lattice spacing a as small as possible within availability on present lattices”.The behaviour(20)can actually change in the academic limit a→0(i.e.at lattice spacings much smaller than those presently available).Subsequent considerations in the present paper would actually suggest such a possibility(see Sect.V).
Although Eqs.(20)may not survive at smaller a,they do answer questions why new point-like?uctuations(implied by the?ne-tuning)do not disturb the standardβ-function of the SU(2)gluodynamics at existing lattices.Indeed,according to(20)all the scalar degrees of freedom are actually removed from the physical spectrum if a→0.
The properties(20)imply also that the potential energy V(|φ|2)scales in the limit a→and our e?ective theory can well be a useful approximation to study the vacuum properties. In particular,the monopole con?ning mechanism survives in the limit[9]a→0.
G.Formulating the main hypothesis
After all these preliminary discussions we are set to formulate our main hypothesis. Namely,we will assume that we can consider the point
p=p crit(21) as adequately describing the physics in the con?ning phase of the non-Abelian gauge theory. The justi?cation for this hypothesis is the?ne tuning observed on the lattice and discussed in length above.
There are important reservations to be https://www.wendangku.net/doc/6b16664790.html,ly,the?ne tuning does imply that the physics is so to say“frozen”at p=p cr as far as the UV scale is concerned.However,as far as the dependence on the scaleΛQCD is concerned it could be di?erent than at the point (21).Moreover,we can give a more quantitative meaning to the scale“ΛQCD”in the case considered.The point is that the percolating cluster has self-crossings and the length of the trajectory between these self-crossings can be considered as the monopole free-path length. Direct measurements show[19]that this length(measured along the trajectory)scales:
L free≈1.6fm.(22) Thus,we can apply our hypothesis as long as
L free?L?a.(23) In the next section we exploit the percolation theory to describe the structure of the ?nite-size monopole clusters satisfying(23).
III.FINITE-SIZE CLUSTERS AND PERCOLATION
A.Data and percolation picture
Detailed data on the structure of the monopole clusters were obtained in[7].As was mentioned above,there is a single percolating cluster,whose length grows with the lattice volume,and?nite-size clusters.In this section we will concentrate on the?nite-size clusters satisfying the condition(23).These clusters are characterized,?rst of all,by their length. It was found that the length spectrum is described by a power law:
N(L)=
c4
L L/a
i=1(x i?ˉx)2=a2
L.(27)
Thus,our problem is to clarify whether the data(25)and(27)can be understood within the percolation theory(and our main hypothesis,see subsection II F).It is encouraging to observe that even without any dynamical input we can conclude that the lattice data on
the?nite-size monopole clusters reveal a picture typical for percolating systems.Indeed,a generic form of the spectrum,for p
exp(?μ·L)
N(L)~
+1,(31)
D fr
where d is the dimension of space-time and D fr is in fact D fr(p=p cr).
In our case,
τ≈3,D≡4,D fr≈2.(32) Thus,Eq.(31)is satis?ed within the error bars of the lattice measurements and this observation is one of our main results.In view of its importance,we will later rederive (32)and some generalizations of it in the language of?eld theory.
C.Fractal dimension
As is mentioned above,the relation between the length and radius of the cluster is deter-mined by the fractal dimension,see Eq.(27).The fractal dimension,in turn,is determined by the kind of the walk.
In fact we are dealing with the monopole walk which,to the best of our knowledge,has not been studied in detail in the literature.However,the characteristics of the monopole walk are so to say?anked by the characteristics of the well known random and self-avoiding walks.Indeed,the monopole walk chooses freely one of7directions available at each step. This is in common with the random walk.On the other hand,choosing the eighth direction would result in an immediate self-crossing and this is forbidden.The latter feature is in common with the self-avoiding walk.However,in contradistinction from the self-avoiding walk,self-crossings are allowed for the monopole trajectories at later stages.
The observation central for this section is that in D=4the fractal dimension D fr=2 both for the random and self-avoiding walks.Therefore we can predict D fr=2for the monopole walk as well.
In more detail,for the random walk one has in any number of space dimensions2:
(D fr)random=2.(33) For the self-avoiding walk one has(the so called Flory’s fractal dimension):
D+2
(D fr)self?avoiding=
2This relation,in connection with the monopole clusters,is in fact mentioned in Ref.[7].
3We are grateful to D.Diakonov for a discussion on this point and providing us with a reference to[21].
The partition function for a closed polymer is:
Z= d4x∞ N=11
2π2a3 (36) This partition function(35)contains a summation over all atoms of the polymer weighted by the Boltzmann factors.Theδ–functions in(36)ensure that each bond in the polymer has length a.The starting point of the polymer is x0and the ending point is x f≡x N.
Note that there is a pre-exponential factor1/N in Eq.(35).This is due to consider-ing closed trajectories.Indeed,the factor is introduced to compensate for the N-multiple counting of the same closed trajectory in the partition function(35)since any atom on this trajectory can be considered as the initial and?nal point.As we shall see later,our?nal result crucially depends on the pre-exponential factors.
The crucial step to relate(36)to a free particle path integral is the so called coarse–https://www.wendangku.net/doc/6b16664790.html,ly,the N–sized polymer is divided into m units by n atoms(N=mn),and the limit is considered when both m and n are large while a and
√
2π2a3
δ(|x i?x i+1|?a)→ 2n a2(x(ν+1)n?xνn)2 ,(37)
where the index i,i=νn···(ν+1)n?1,labels the atoms inνth unit.The polymer partition function becomes[22]:
Z N(x0,x f)=const· m?1 ν=1d4x 2na2
·exp ?m ν=1n·aμ .(38)
The x i’s have been re-labeled so that xνis the average value of x in at the coarser cell. Note also that at this stage there appears the chemical potentialμrelated to the original parameter M through
μ=M?ln7/a,(39) as is discussed above.
Using the variables:
s=1
8
a2N,m2prop=
8μl x(0)=x(l)=x Dx exp
?l
1
Note that the mass renormalization in Eq.(40)is consistent
with Eqs.(12,39).After
these preliminary
steps we can readily derive the distribution in the length of the trajectories in the massless case.To this end,let us rescale x μand s in such a way,that there is no l dependence left in the action if m prop =0:
L =l/a,?s =s/l,?x μ=x μ/
√L
·I ,(43)where
I ≡ ?x (0)=?x (l )=?x
D ?x exp ?1
L 1+D/2,(45)
where V D is the volume in D -dimensional space and c D is a constant.Although we are interested only in the D =4case we kept D as variable to emphasize that we rederive in fact the hyperscaling relation (31).Note that D fr =2is implicit in our derivation and is encoded in fact in the transformation (37).
B.Coulomb-like interaction
So far we considered approximation of free particles which corresponds to the dominance of the monopole self-energy.We expect,however,that the monopoles interact also Coulomb-like.Other,e?ective interactions are not ruled out either.The Coulomb-like interaction can readily be included into the action in the polymer representation.The corresponding extra piece in the action is given by:
S Coulomb =g 2M
2 10 10d ?s 1d ?s 2˙?x 1,μD μν(?x 1??x 2)˙?x 2,ν,(47)
and,therefore,the1/L3behaviour of the spectrum(45)should be still true for4D=4upon inclusion of this interaction5.
C.Finite temperatures,?nite volume lattices
Formalism of Section IV A can readily be generalized to the case of a non-zero tempera-ture.Indeed,we should rewrite now in terms of a length-of-the-trajectory distribution the propagator of a massless particle at?nite temperature.
As usual,?nite temperature in Euclidean space corresponds to a compacti?ed fourth direction.Thus,we simply write the L-distribution corresponding to an ensemble of non-interacting particles with masses m2=(2πn)2T2in d=3space–time:
N(L)=c3V3
1
L·a)and we come back to (45)upon proper normalization of the constant c3,
c3=2c4
√
L
4
μ=1 nμ∈Z Z exp{?(2πnμ/Xμ)2L·a},(49)
where
c0=16π2c4a2.
V.INFRARED CLUSTER AND PROPERTIES OF THE GROUND STATE In this section we discuss geometrical properties of the percolating cluster.It was sug-gested already in[15,22]that the percolating cluster corresponds to theφ-?eld condensate
in the classical approximation6.However,it is only the phenomenon of the?ne tuning that makes the properties of the vacuum state non-trivial.Indeed,we have now two coexisting
scales,a andΛ?1
QCD .Moreover,we will see that the properties of the percolating cluster
di?er radically from the properties of the?nite-size clusters.
https://www.wendangku.net/doc/6b16664790.html,ttice data
We have already mentioned that the density of the monopoles in the percolating cluster scales,see(2).Recently,further measurements on the geometrical elements of the perco-lating cluster have been performed[19].In more detail,the cluster consists of self-crossings and segments connecting the crossings.It was found that the average distance between the crossings measured along the trajectory scales,see Eq.(22).Violations of the scaling in L free are negligible for all the lattices tested.
One can also measure the average of the shortest,or Euclidean distance, d ,between the two crossings connected by segments.In this case violations of the scaling are more signi?cant and,roughly,the data can be approximated as:
d √σ)?0.25]),(50) wher