Invited talk given at the Topical Conference on Spontaneous Symmetry. Breaking, Trieste, April 1976. Research supported in part by the U.S.. Energy Research ...
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May 1976
Weyl Symmetry
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DESY Bibliothek 2 Hamburg 52 Notkestieg 1 Germany
Broken Weyl Symmetry
+
by
G. Domokos Deutsches Elektronen-Synchrotron DESY, Hamburg and Department of Physics, Johns Hopkins University •
Balt~more,
+
MD 21218, USA
++
Invited talk given at the Topical Conference on Spontaneous Symmetry Breaking, Trieste, April 1976. Research supported in part by the U.S. Energy Research and Development Administration, under Contract No. E(11-1) 3285. Report No. C00-3285-29.
++
Permanent address.
-
I -
Abstract
We argue that conformal symmetry can be properly understood in the framework of field theories in curved space. In such theories, invariance is required under general coordinate transformations and conformal re-
scalings. We examine a gauge model coupled to a Higgs field. In the tree approximation, the vacuum solution exhibits two Higgs phenomena: both the phase (Goldstone boson) and the coordinate dependent part of the radial component of the scalar field can be removed by a Riggs-Kibble transformation. The resulting vacuum solution corresponds to a space of constant
curvature and constant vacuum expectation value of the scalar field.
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I, Introduction
Theories exhibiting exact conformal symmetry are attractive from the point of view of a theorist; they are outrageous from the point of view of almost
all the experimental facts known today.
ffi1y are they attractive?
Because they are more restrictive than theories based on the Poincare group alone. This is not surprising: any enlargement of the invariance group of a
theory is likely to lead to further restrictions on its structure. This statement seems to be true at whatever level one approaches a theory.
i) At a Lagrangian level (considering fields of low spin, i.e. 0, 1/2, 1) conforQal symmetry severely restricts the possible couplings between fields. For instance, a minimal coupling between a Dirac field and a gauge field
("' e
+x''A,.., '\' )
( "' 't
~
ku-
F I'""
is allowed, but Pauli-type couplings
'f' )
are not.
ii) At a more phenomenological level, conformal invariance is known to impose severe (and physically attractive) restrictions on the short distance expansions of operator products or of
n -point functions. Hence, the pre-
sence of the "larger" symmetry may help in understanding short distance phenomena (deep inelastic scattering of leptons on hadrons, for instance) in terms of fewer parameters than Poincare invariance alone would give.
iii) There is a reasonable hope that conforQally invariant theories are "better behaved" at short distances, see e.g. remarks at the end of this
paper.
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Why are conformally invariant theories outrageous from an experimental point of view?
The answer is extremely simple. Conformally invariant theories are devoid of any fixed mass (or length) scale. Experimentally, however, we do seem to have rather well-defined energy scales (-v 1 GeV for strong interactions, a length scale of
rr
or~~"-10
-16
em for weak interactions, etc.). Thus
conformal symmetry is necessarily broken.
Now, experience with gauge theories has taught us that the best way of breaking a symmetry while retaining most of the benefits reaped from its presence, is to break the symmetry spontaneously, i.e. to retain the symmetry of the operator structure of the theory, but to look for a ground state of lower symmetry instead.
In the case of conformal symmetry this means in particular that the ground state (the "vacuum") supplies the energy scale of excitations. This intriguing pass ibi li ty h.as been particularly emphasized 1n recent works of Fubini et al. even though it is implicit in earlier works on the subject, see e.g. Salam and Strathdee Z)
In this talk I cannot do justice to all the beautiful work done on conformally invariant quantum field theories. Rather, in order to give an idea about the physics involved, I deliberately take the simplistic point of view that a tree approximation to quantum field theories gives a rough but not entirely unreliable guide to what we may expect to happen.
1 )
- 4 -
As far as the geometrical aspects of the problem are concerned, I want to advertise a somewhat unconventional (for a particle physicist, at least ... ) point of view. I wish to claim that the proper frame,;ork in which conformal symmetry is to be investigated 1s a field theory in curved space, i.e. gravity cannot be neglected, except perhaps at some later stage of the calculations. (I shall try to justify this claim by exhibiting a- somewhat bizarre - form of a Higgs phenomenon which can be understood only if one considers theories in curved space.)
2. The geometrical aspects of conformal symmetry
I adopt Weyl's original point of view 3 ) (which was also the starting point of Kastrup in his works
4
) ) about conformal transformations. A con-
formal transformation is a local rescaling of the units of length (or, equivalently, of the units of energy, since we use natural units,~~ C= I.).
If at any point, P , of space one 1s given an infinitesimal vector cl~ ~CP) (representing, classically, a "measuring rod" of laboratory size), then
we change the units locally, by
( s
(In what follows, the point P
real)
1s characterized by its coordinates, so
that I write s(x), instead of s(P), etc.)
Hence, the length squared of
4>< f'l
becomes
(I)
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Now, since we generally deal with functions of points (fields, currents, etc.) and not with functions of infinitesimal displacements, one reinterprets
the transformation- via eq. (I) - as a rescaling of the metric tensor, rather than that of the infinitesimal vector.
( stx)
In infinitesimal form
~
1 -t-
/\Cx>) J
(2)
from which
Sq~ll ex)
-=- 2 N.x) g ~"-lfc)(),
8-f-9 · = 4 t\Cx) f-g
(3)
follow immediately.
We shall call these transformations Weyl transformations or conformal rescalings. W.eyl transformations do not affect the coordinates.
It is intuitively evident that if Riemann space, then so is
s'£>0
.e(~)
lS a massless complex Higgs field,
Al'
(8)
lS the vector potential
lS the massless, gauge-invar iant Lagrangian density of - say -
quarks and leptons. The "charge" carried by the Higgs field lS f.
The generalizat ion of this action to curved space is straight-for ward.
Basically, the recipe is:
~
r!J
ot4,x
r,;t
~
~ ~ 1.1" CX)
> ~~r-g ~
Yf
(covariant derivative) .
One finds then that a piece of the action (8), namely
-
W'::. ~ J.4x v::-~
10 -
ti +~~~q, ~y.vAr-Av -~ (~q,)
- ~ t ~*~r- cP ~~"' Av - ~
+ .L (~)1 ~s
2
Fy.v Fpa- a(F ~vcr-
automatic ally Weyl invariant , see Zumino loc. cit. and the previous
Section. One can even add coupling terms between
o)
~ ().-r.:: ,.,
)\)'(I ,
1-1
. (It is to be noted, however, that in
these coordinates do not cover the entire space.)
There is some potential trouble with this interpretation. One easily convinces oneself that all the mass parameters arising in such a theory are proportional to Planck mass,rv 10
19
'X = ~1i G Y''"L-
, which is of the order of the
GeV. In other words, how can one get down to mass scales
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. . . 1 1) govern1ng, say, strong 1nteract1ons?
(As far as I can understand matters, Fubini et al. face an equally serious problem. They want to interpret K as a "strong interaction scale", say
K rJ 1 GeV
2
In the geometrical language used here, their trouble - as
emphasized by De Alfaro and Furlan at this Conference - stems from the fact that De Sitter space is not asymptotically flat. Hence, one does not get momentum conservation unless one resorts to additional physical
postulates
1 ) .)
In our interpretation, at any rate, there is no translation invariance and there should not be- because the "vacuum is curved".
(Of course,
De Sitter space has a tO-parameter group, which, however, contracts to
the Poincare group in the limit K ~ O,I.f one assumes that
rx
by the ground state - and thus it is fixed - then K~ 0 means
is furnished ~...,. 0
).
The morale is, I think, that as long as one breaks Heyl symmetry spontaneously, one is given just one length (or mass) scale. Therefore, whatever point of view is taken about the physical meaning of the results, one is either facing the problem of "getting down" from the Planck mass to - say or one has to explain why do the angles of a triangle of sides
~
1 GeV,
1 em
(or 1 ~ngstrom) add up so accurately to 180 degrees.
A final remark is in order. Hhile we worked in the tree approximation throughout, there are indications that a fully quantized version of such a theory -with unbroken Weyl symmetry - is quite well-behaved and may even be renormalizable
12
)
It remains to be seen whether spontaneously broken
-
Weyl synnnetry retains the
11
19 -
good 11 short distance properties, while
bringing at the same time the model closer to physical reality.
Acknowledgemen ts
Most of the new material presented in this talk arose from collaboration and discussions with M.M. Janson and S. KOvesi-Dornokos; this applies, in
particular to the suggestion of a "second Higgs mechanism" I)). I wish to thank Profs. S. Fubini and G. Furlan for discussions on their point of view concerning the breaking of conformal invariance, and to Prof. K. Symanzik
for his critical remarks.
Hy thanks are due to Prof. H. Joos for the hospitality extended to me at DESY where this work was completed. Financial support by the Alexander von Humboldt Stiftung- in the form of a senior U.S. scientist award- is gratefully acknowledged.
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Footnotes and References
I) S. Fubini, CERN preprint, TH-2129 (1976).
V. De Alfaro, S. Fubini and G. Furlan, CERN preprint, TH 2125 (1976) 2) A. Salam and J. Strathdee, Phys. Rev. 184, 1760 (1969) 3) H. Heyl, Raum-Zeit-Mate rie. 6th edition, Springer, Berlin (1970). References to Weyl's original papers are found in this book. 4) H.A. Kastrup, Ann. der Physik,
z,
388 (1962)
5) B. Zumino, in Brandeis University Summer Institute (Lectures on Elementary Particles and Quantum Field Theory, Ed. S. Deser et al.),
I•
438. M.I.T. press, Cambridge, MA. (1970) 6) H. Weyl, lac. cit. and T. Fulton, F. Rohrlich and L. Witten, Rev. Mod. Phys. 34, 442 (1962) 7) F. Giirsey, Ann. Phys. (N.Y.) 24, 211 (1963) 8) R. Penrose, Proc. Roy. Soc. A284, 204 (1965) 9) G. Domokos, M.M. Janson and S. Kovesi-Domokos, Nature 257, 203 (1975) 10) Ya. B. Zel'dovich, Sov. Phys. Uspekhi, quoted therein
~.
209 (1968) and references
II) There have been, however, speculations that in a unified theory of weak,
electromagnetic and strong interactions one indeed needs mass scales of the order of the Planck mass; see H. Georgi, H. Quinn and S. Weinberg, Phys. Rev. Lett. 1_2, 451 (1974) 12) R. Kallosh, Phys. Lett. 55B, 173 (1975) F. Englert, C. Truffin and R. Gastmans, Bruxelles preprint (1976) 13) G. Domokos and M.M. Janson, in preparation.
