Yang-Mills Theory in Three Dimensions as Quantum Gravity Theory

35 Pages·0.34 MB·English

by  Dmitri Diakonov

NORDITA-99/85 HE
Yang–Mills Theory in Three Dimensions
as Quantum Gravity Theory
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Dmitri Diakonov and Victor Petrov∗
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1 ⋄ NORDITA, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark

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v ∗ Petersburg Nuclear Physics Institute, Gatchina, St.Petersburg 188350, Russia
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E-mail: [email protected], [email protected]
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t Abstract
-
p
e We perform the dual transformation of the Yang–Mills theory in d = 3 dimensions
h
using the Wilson action on the cubic lattice. The dual lattice is made of tetrahedra
:
v triangulating a 3-dimensional curved manifold but embedded into a flat 6-dimensional
Xi space (for the SU(2) gauge group). In the continuum limit the theory can be refor-
r mulated in terms of 6-component gauge-invariant scalar fields having the meaning of
a
the external coordinates of the dual lattice sites. These 6-component fields induce a
metric and a curvature of the 3-dimensional dual colour space. The Yang–Mills theory
can be identically rewritten as a quantum gravity theory with the Einstein–Hilbert
action but purely imaginary Newton constant, plus a homogeneous ‘matter’ term. In-
terestingly, the theory can be formulated in a gauge-invariant and local form without
explicit colour degrees of freedom.
1 Lattice partition function
Though our objective is the continuum theory we start by formulating the SU(N ) gauge
c
theory on a cubic lattice. The partition function can be written as an integral over all link
variables being SU(N ) unitary matrices U with the action being a sum over plaquettes,
c
(β) = dU exp β (Tr U +c.c.) / 2Tr 1 (1)
link plaq
Z  
Z links plaquettes
Y X
 
where β is the dimensionless inverse coupling. The unitary matrix U is a product of four
plaq
link unitary matrices closing a plaquette.
Togettothecontinuum limitonewritesU = exp(iaAata)whereaisthelatticespacing
link µ
and Aata = A is the Yang–Mills gauge potential with ta being the generators of the gauge
µ µ
group normalized to Tr tatb = δab/2, and expands Tr U in the lattice spacing a. As a
plaq
result one gets for a plaquette lying in the (12) plane:
Tr U +c.c. Tr F2
β plaq = β 1 a4 12 +O(a6) , (2)
2 Tr 1 − 2Tr 1 !
where F = ∂ A ∂ A i[A A ] is the Yang–Mills field strength. Summing over all
µν µ ν ν µ µ ν
− −
plaquettes one obtains the partition function of the continuum theory,
1
= DA exp ddx Tr F2 , (3)
Zcont µ −2g2 µν!
Z d Z
with an obvious relation between the dimensionless lattice coupling β and the SU(N ) gauge
c
coupling constant in d dimensions, g2:
d
2N
c
β = . (4)
a4−dg2
d
In this paper we concentrate on the Euclidean SU(2) Yang–Mills theory in d = 3 dimen-
sions. In this case eq. (4) reads:
4
β = . (5)
ag2
3
The continuum limit of the d = 3 Yang–Mills theory given by the partition function (1) is
obtained as one takes the lattice spacing a 0 and β with their product g2 = 4/(aβ)
→ → ∞ 3
fixed. This quantity provides the theory with a mass scale. It is widely believed (though not
proven so far) thatthe theory possesses two fundamental properties: 1) theaverage ofa large
Wilson loop has an area behaviour with a string tension proportional to g4, 2) correlation
3
functions of local operators like F2 decay exponentially at large separations, with a ‘mass
µν
gap’ proportional to g2.
3
Our aim will be to rewrite the partition function (1) in dual variables and to study its
continuum limit.
2
2 Dual transformation
The general idea is to integrate over link variables U in eq. (1) and to make a Fourier
link
transformation in the plaquette variables U . This will be made in several steps, one for
plaq
a subsection.
2.1 Inserting a unity into the partition function
First of all, one needs to introduce explicitly integration over unitary matrices ascribed to
the plaquettes, U . This is done by inserting a unity for each plaquette into the partition
plaq
function (1):
1 = dU δ(U , U U U U ) (6)
plaq plaq 1 2 3 4
plaquettesZ
Y
where U arethe linkvariables closing into a given plaquette. Theδ-functionisunderstood
1...4
with the group-invariant Haar measure. Arealization of such a δ-functionis given by Wigner
D-functions:
δ(U,V) = (2J +1)DJ (U†)DJ (V). (7)
m1m2 m2m1
1 3
J=0,X2,1,2,...
This equation is known as a completeness condition for the D-functions [1]. The main
properties of the D-functions used in this paper are listed in Appendix A.
Eq.(7)shouldbeunderstoodasfollows: ifoneintegrates anyfunctionofaunitarymatrix
U with the r.h.s. of eq. (7) over the Haar measure dU one gets the same function but of the
argument V:
dU f(U) δ(U,V) = f(V). (8)
Z
Using the multiplication law for the D-functions (see Appendix A, eq. (75)) one can write
down the unity to be inserted for each plaquette in the partition function (1) as
1 = dU (2J +1) DJ (U† )DJ (U )DJ (U )DJ (U )DJ (U ) (9)
plaq m1m2 plaq m2m3 1 m3m4 2 m4m5 3 m5m1 4
Z J
X
where U are the corresponding link variables forming the plaquette under consideration.
1...4
2.2 Integrating over plaquette variables
Integrating over plaquette unitary matrices U becomes now very simple. For each pla-
plaq
quette of the lattice one has factorized integrals of the type
Tr U +Tr U† 2
dU exp β plaq plaq DJ (U† ) = δ I (β)T (β), (10)
plaq  2Tr 1  m1m2 plaq m1m2β 1 J
Z
 
where T (β) is the ratio of the modified Bessel functions [2],
J
3
I (β) 2J(J +1)
2J+1
T (β) = exp at β . (11)
J
I1(β) −→ "− β # → ∞
The quantity T (β) is the ‘Fourier transform’ of the Wilson action; since in the lattice
J
formulation the dynamical variables have the meaning of Euler angles and are therefore
compact, the Fourier transform depends on discrete values J = 0, 1/2, 1, 3/2,.... However,
as one approaches the continuum limit (β ) the essential values of the plaquette angular
→ ∞
momenta increase as J √β and their discreteness becomes less relevant. Strictly speaking,
∼
the continuum limit is achieved at plaquette angular momenta J 1.
≫
We would like to make a side remark on this occasion. The quantity T (β) gives the
J
probability that plaquette momenta J is excited, for given β. For a typical value used in
latticesimulationsβ = 2.6(in4dimensions)wefindthattheprobabilitiesofhavingplaquette
excitations with J = 0, 1/2, 1, 3/2 and 2 are 56%, 29%, 11%, 3% and 1%, respectively. It
means that lattice simulations are actually dealing mainly with J = 0, 1/2 and 1 with a
tiny admixture of higher excitations. It would be important to understand why and how
continuum physics is reproduced by lattice simulations despite only such small values of
plaquette J’s are involved.
We get, thus, for the partition function:
#of plaquettes
2
= I (β) (2J +1) T (β)
Z "β 1 # P JP
XJP plaqYuettes
dU DJP (U ) DJP (U ) DJP (U ) DJP (U ) (12)
× l m1m2 1 m2m3 2 m3m4 3 m4m1 4
linkslZ
Y
where U are link variables forming a plaquette with angular momentum J .
1−4 P
2.3 Integrating over link variables
The difficulty in performing integration over link variables in eq. (12) is due to the fact
that any link enters several plaquettes. In d = 2 dimensions every link is shared by two
plaquettes, hence one has to calculate integrals of the type
1
dUDJ1(U)DJ2 (U†) = δ δ δ (13)
kl mn 2J +1 J1J2 kn lm
Z 1
for all links on the lattice. We shall consider this case later, in section 4.
In d = 3 dimensions every link is shared by four plaquettes, hence the integral over link
variables is of the type
dUDJA (U)DJB (U)DJC (U)DJD (U) (14)
m1m2 m3m4 m5m6 m7m8
Z
where J are angular momenta associated with four plaquettes intersecting at a given
A,B,C,D
link U, and m are ‘magnetic’ quantum numbers, to be contracted inside closed plaquettes.
1−8
In d = 4 dimensions there will be six plaquettes intersecting at a given link but we shall not
consider this case here.
4
Figure 1: ‘Even’ cubes in checker board order.
The general strategy in calculating the link integrals (14) is (i) to divide by a certain rule
four D-functions into two pairs and to decompose the pairs of D-functions in terms of single
D-functions using eq. (82) of Appendix A, (ii) to integrate the resulting two D-functions
using eq. (13) and, finally, (iii) to contract the ‘magnetic’ indices. Since all‘magnetic’ indices
will be eventually contracted we shall arrive to the partition function written in terms of the
invariant 3nj symbols.
There are several different tactics how to divide four D-functions into two pairs, even-
tually leading to anything from 6j to 18j symbols. In this paper we take a route used in
refs.[3, 4], leading to a product of many 6j symbols, although on this route one looses certain
symmetries, and that causes difficulties later on. The gain, however, is that it is more easy
to work with 6j symbols than with 12j or 18j symbols. Since important sign factors have
been omitted in refs.[3, 4] and only final result has been reported there, we feel it necessary
to give a detailed derivation below.
In d = 3 dimensions all plaquettes are shared by two adjacent cubes, therefore, it is
natural to divide all cubes of the lattice into two classes which we shall call ‘even’ and ‘odd’,
and to attribute plaquettes only to even cubes. We shall call the cube even if its left-lower-
forward corner is a lattice site with even coordinates, ( 1)x+y+z = +1. It will be called
−
odd in the opposite case. The even and odd cubes form a 3-dimensional checker board, as
illustrated in Fig.1, where only even cubes are drawn explicitly. The even cubes touch each
other through a common edge or link, as do the odd ones among themselves. The even and
odd cubes have common faces or plaquettes. All plaquettes will be attributed to even cubes
only: that is the reason for the division of cubes into two classes.
Let us consider an even cube shown in Fig.2.
A,B,C,D,E,F denote its 6 faces, numbers from 1 to 12 denote its links or edges,
a,b,c,d,e,f,g,h denote its 8 vertices or sites. Correspondingly, we shall denote plaque-
tte angular momenta by J , link variables by U , and the ‘magnetic’ numbers of the
A−F 1−12
D-functions will carry indices a h referring to the sites the D-functions are connecting.
−
One can write the traces of products of four D-functions over plaquettes in various ways.
To be systematic we shall adhere to the following rule: Link variables in the plaquette are
5
h
g
7
8 11
F
e 6
D
5
E 10
d
f
12 C
3
4 B c
A 9
2
a
1
b
Figure 2: Elementary even cube
taken in the anti-clock-wise order, as viewed from the center of the even cube to which the
given plaquette belongs. If the link goes in the positive direction of the x,y,z axes we ascribe
the U variable to it; otherwise we ascribe the U† variable to it.
With these rules the six plaquettes of the elementary cube shown in Fig.2 bring in the
following six traces of the D-function products:
Cube = DJA (U )DJA (U )DJA (U†)DJA (U†) DJB (U†)DJB (U )DJB (U )DJB (U†)
iaib 1 ibic 2 icid 3 idia 4 jbja 1 jaje 12 jejf 5 jfjb 9
h i h i
DJC (U†)DJC (U )DJC (U )DJC (U† ) DJD(U )DJD(U )DJD (U†)DJD (U† )
kckb 2 kbkf 9 kfkg 6 kgkc 10 ldlc 3 lclg 10 lglh 7 lhld 11
h i h i
DJE (U† )DJE (U )DJE (U )DJE (U†) DJF (U†)DJF (U )DJF (U )DJF (U†) .
mema 12 mamd 4 mdmh 11 mhme 8 nfne 5 nenh 8 nhng 7 ngnf 6
h i h (15i)
Each link variable U appears in this product twice: once as U, the other time as
1−12
U†. For D(U†) we use eq. (77) of the Appendix A to write it in terms of D(U). After
that we can apply the decomposition rule (82) of that Appendix to write down pairs of D-
functions interms ofone D-functionandtwo 3jmsymbols. The new D-functionscorrespond
to the links and carry angular momenta which we denote by j’s. The 3jm symbols have
‘magnetic’ indices which get contracted when all indices related to a given corner of the cube
are assembled together. Though this exercise is straightforward it is rather lengthy, and we
relegate it to Appendix B. As a result we get the following expression which is identically
equal to (15):
Cube = (2j +1)...(2j +1)
1 12
j1X...j12
Dj1 (U )Dj2 (U†)Dj3 (U†)Dj4 (U )Dj5 (U†)Dj6 (U )
oaob 1 −pc,−pb 2 −qc,−qd 3 rard 4 −sf,−se 5 tftg 6
Dj7 (U )Dj8 (U†)Dj9 (U†)Dj10 (U )Dj11 (U† )Dj12 (U )
uhug 7 −vh,−ve 8 −wf,−wb 9 xcxg 10 −yh,−yd 11 zaze 12
j j j j j j j j j j j j
1 4 12 1 9 2 2 3 10 4 3 11
o r z o w p p q x r q y
a a a ! b b b ! c c c ! d d d !
− − −
j j j j j j j j j j j j
12 8 5 6 5 9 6 10 7 7 11 8
z v s t s w t x u u y v
e e e ! f f f ! g g g ! h h h !
− − −
6
h
g
7
811
e I
6
5 f
10
d
12
III 3
4 9 c
15
a
2
1
14
13 17
IV b
16
18 VI
V
II
Figure 3: Several cubes combine to produce 6j symbols composed of link momenta j. These
are the same cubes as in Fig.1.
j j j j j j j j j j j j
7 11 8 6 10 7 6 5 9 12 8 5
×( JE JF JD )( JD JF JC )( JB JC JF )( JF JB JE )
j j j j j j j j j j j j
4 3 11 2 3 10 1 9 2 1 4 12 . (16)
J J J J J J J J J J J J
( D E A )( D C A )( C A B )( E B A )
Here j are the angular momenta attached to the links of the cube, (...) are 3jm- and
1−12
... are 6j-symbols. We see that there is a 6j symbol attached to each corner of the even
{ }
cube; its arguments are three plaquette momenta J and three link momenta j intersecting
in a given corner. The 3jm symbols involve only link variables j.
We have, thus, rewritten all twelve pairs of DJ-functions entering a cube as a product of
single Dj-functions, where j’s are the new momenta associated with links. It is understood
that this procedure should be applied to all even cubes of the lattice. After that, one has
only two Dj-functions of the same link variable U, for all links of the lattice. It becomes,
therefore, straightforward to integrate over link variables, using eq. (13).
Itisconvenient tointegratesimultaneously over sixlinks entering onelatticesite, because
in that way one gets a full contraction over all ‘magnetic’ numbers. The derivation is, again,
straightforward but lengthy: the details are given in Appendix C. The result is that the 3jm
factors in eq. (16) are contracted with analogous 3jm symbols arising from neighbouring
even cubes, and produce 6j symbols attached to every lattice site and composed of the six
link momenta j intersecting at a given lattice site. In notations of Fig.3 we get for the
vertices a,b:
j j j j j j
“a” = 1 4 12 , “b” = 1 9 2 , (17)
j j j j j j
( 15 14 13 ) ( 17 18 16 )
and similarly for other vertices. A sign factor ( 1)2j should be attributed to every link of the
−
lattice. As shown in Appendix C it is actually equivalent to a sign factor ( 1)2J attributed
−
to every lattice plaquette.
7
3 Lattice partition function as a product of 6j symbols
We summarize here the recipe derived in the previous section. One first divides all 3-cubes
into two classes, even and odd ones. They form a 3-dimensional checker board depicted in
Fig.1. All even cubes are characterized by their plaquette momenta J. The edges of even
cubes have link momenta j; each link is shared by two even cubes.
To each of the eight corners of an even cube one attributes a 6j symbol of the type
j j j
1 2 3 (18)
J J J
( A B C )
where J’s are plaquette and j’s are link momenta intersecting in a given corner of a cube.
The rule is that link 1 is perpendicular to plaquette A, link 2 is perpendicular to plaquette
B and link 3 is perpendicular to plaquette C. Four triades, (j J J ),(j J J ),(j J J )
1 B C 2 A C 3 A B
and (j j j ) satisfy triangle inequalities.
1 2 3
To each lattice site one attributes a 6j symbol of the type
j j j
1 2 3 (19)
j j j
( 4 5 6 )
where j’s are the six link momenta entering a given lattice site. The rule is that link 4 is
a continuation of link 1 lying in the same direction, link 5 is a continuation of link 2 and
link 6 is a continuation of link 3. Four triades, (j j j ),(j j j ),(j j j ) and (j j j ) satisfy
1 2 3 1 5 6 2 4 6 3 4 5
triangle inequalities.
Actually, each lattice site has five 6j symbols ascribed to it: four are originating from
the corners of the even cubes adjacent to the site and are of the type (18), and one is of the
type (19).
The lattice partition function (1) or (12) can be identically rewritten as a product of the
6j symbols described above. Independent summation over all possible plaquette momenta J
and all possible link momenta j is understood. We write the partition function in a symbolic
form:
#of plaquettes
2
= I (β) (2J +1) T (β) ( 1)2JP (2j +1)
Z "β 1 # P JP − l
JXP,jl plaqYuettes lYinks
j j j j j j
. (20)
× ( J J J ) ( j j j )
evencubescorners latticesites
Y Y
The plaquette weights T (β) are given by eq. (11). Apart from the sign factor essentially the
J
same expression was given in refs. [3, 4] 1. The sign factor is equal to 1 if the total number
±
of half-integer plaquettes J’s is even (odd). Since plaquettes with half-integer momenta form
closed surfaces it may seem that the sign factor can be omitted. In a general case, however,
when one consideres vacuum averages of operators this is not so, therefore, it is preferable
to keep the sign factor.
1
We are grateful to P.Pobylitsa who has independently derived eq. (20).
8
= 2
4 Simple example: d Yang–Mills
In a simple exactly soluble case of the 2-dimensional SU(2) theory every link is shared by
only two plaquettes. Therefore, the link integration is of the type given by eq. (13): it
requires that all plaquettes on the lattice have identical momenta J. The partition function
thus becomes a single sum over the common J:
#of plaquettes
2
= I (β) [T (β)]#of plaquettes, (21)
1 J
Z "β #
J
X
the number of plaquettes being equal to V/a2 where V is the full lattice volume (full area
in this case) and a is the lattice spacing.
A slightly less trivial exercise is to compute the average of the Wilson loop. Let the
Wilson loop be in the representation j . It means that one inserts Djs(U) for all links
s
along the loop. One gets therefore integrals of two D-functions outside and inside the loop,
and integrals of three D-functions for links along the loop. The first integral says that all
plaquettes outside the loop are equal to a common J. The second integral says that all
plaquettes inside the loop are equal to a common J′. Integrals along the loop require that
J,J′ and j satisfy the triangle inequality. We have thus for the average of the Wilson loop
s
of area S:
W (S) = J [TJ(β)]aV2 JJ′+=j|sJ−js|[TJ′(β)/TJ(β)]aS2 . (22)
h js i P P J [TJ(β)]aV2
This is an exact expression for the lattice WPilson loop, however we wish to explore its
continuum limit. It implies that V/a2 , S/a2 but S V; β , a 0 but
→ ∞ → ∞ ≪ → ∞ →
βa2 = 4/g2 fixed, where g2 is the physical coupling constant having the dimension of mass2,
2 2
see eq. (4).
We take V/a2 first of all, which requires that only the J = 0 term contributes to
→ ∞
the sum, with T (β) 1; consequently all momenta inside the loop are that of the source,
0
≡
J′ = j . Taking into account the asymptotics (11) of T (β) at large β we obtain
s J
g2
hWjs(S)i = [Tjs(β)]aS2 = exp"− 22js(js +1)S# (23)
which is, of course, the well-known area behaviour of the Wilson loop with the string tension
proportional to the Casimir eigenvalue.
5 Dual lattice: tetrahedra and octahedra
We now turn to the construction of the dual lattice.
Each 6j symbol of the exact partition function (20) encodes four triangle inequalities
between the plaquette J’s and the link j’s. It is therefore natural to represent all 6j symbols
by tetrahedra whose six edges have lengths equal to the six momenta of a given 6j symbol.
Four faces of a tetrahedron form four triangles, so that the triangle inequalities for the
momenta are satisfied automatically.
9
A
E
B
C
4
A
1 B
12
2
9
a
1
b
Figure 4: Tetrahedra corresponding to the 6j symbols sitting at vertices a and b.
h F
8 g
7
11 D
F
e 6
D E
5
10
E
d f
12 C 4 O
3
4 B A9 c 12 C
a 2
B 2
1
1
A
b
Figure 5: Octahedron dual to the even cube.
Let us first consider the eight 6j symbols corresponding to the eight corners of an even
cube. These eight 6j symbols are given explicitly in eq. (16) with notations shown in Fig.2.
Let us represent all of them by tetrahedra of appropriate edge lengths. For example, the
tetrahedra corresponding to the corners a and b are shown in Fig.4. We denote the plaquette
momenta J ,... just by their Latin labels A,B,... and the link momenta j ,j ,... by their
A 1 2
numerical indices 1,2,... We notice immediately that the two tetrahedra have a pair of equal
faces, in this case it is the triangle (A,B,1). Therefore, we can glue the two tetrahedra
together so that this triangle becomes their common face. The gluing can be done in two
ways. To be systematic we shall always glue tetrahedra so that their volumes do not overlap.
In the same way we glue together other tetrahedra. Being glued together the eight
tetrahedra of the cube form an octahedron shown in Fig.5. Its center point O is connected
with six lines to the vertices denoted as A F; the lengths of these lines are equal to the
−
corresponding plaquette momenta J . The external twelve edges of the octahedron have
A−F
10