[Review] DDF states
Posted by Urs Schreiber
The recent discussion about attempts to quantize the (super-)string by studying its algebra of classical invariants (i.e. of phase space functions that Poisson-commute with the (super-)Virasoro constraints) has shown that perhaps one should not forget about the special role played by the so-called DDF operators in the worldsheet (S)CFT.
These operators, named after DelGuidice, DiVecchia and Fubini, have the special property that they commute with the entire (super-)Virasoro algebra. Applying them to any physical state hence yields another physical state. Indeed, they are even complete in that they generate the spectrum generating algebra, i.e. all physical states of the string can be obtained by acting with DDF operators on a massless (or tachyonic) state.
The DDF operators can furthermore be chosen so as to satisfy the usual (super-)oscillator algebra and hence they neatly encode all the information about the string except for that contained in the worldsheet ground states themselves. It is straightforward to go from the quantum DDF operators to the corresponding classical invariant observables on the string’s phase space, and these DDF invariants should hence be an alternative to the Pohlmeyer charges. (But I have to admit that so far all I know about the theory of Pohlmeyer charges is what is summarized in Thiemann’s paper.)
Before having a closer look at the relation between Pohlmeyer charges and DDF invariants I would like to review the construction of the most general DDF operators in (S)CFT here.
In the standard textbook literature one can find
- in Green, Schwarz & Witten (using non-CFT language) the construction of
- transversal bosonic (section 2.3.2)
- transversal supersymmetric (section 4.3.2)
- longitudinal bosonic (p. 11)
and in Polchinski (using CFT language) the construction of
- transversal bosonic (eq. (8.2.29))
DDF states.
Here I give a summary and derivation (in CFT language) of all
- transversal and longitudinal, bosonic and fermionic
DDF states (for a free supersymmetric worldsheet theory).
This is also summarized in the following notes
Urs Schreiber, DDF-like classical invariants of (super)string.
Using the usual normalization
for the bosonic and fermionic worldsheet fields, the (super-)Virasoro currents read
The DDF operators are defined as a set of operators that commute with all modes of and (are ‘gauge invariant observables’) and satisfy an algebra that mimics that of creation/annihilation operators.
First of all one needs to single out two linearly independent lightlike Killing vectors and on target space, and we choose to normalize them so that . The span of and is called the longitudinal space and its orthogonal complement is called the tranverse space.
For
a primary field of weight we shall refer to the OPE
as the tensor law for the sake of conciseness of some of the following formulas.
The modes of and are denoted by and as usual.
The crucial idea behind the construction of DDF states is to make use of the fact that the 0-modes of primary fields commute with all Virasoro modes . In canonical language is nothing but the fact that the integral over a unit weight density is reparametrization invariant.
Therefore the task of finding DDF states is reduced to that of finding linearly independent fields that have the desired commutation relations and, in the case of the superstring, are closed with respect to (see below).
bosonic string:
For the bosonic string the DDF operators are defined by
Let’s check that this is really an invariant:
First consider the transverse DDF operators. For a transverse target space vector the operator is manifestly the 0-mode of an primary field (the exponential factor has due to ) and hence invariant.
Furthermore also obviously commutes with the .
The only subtlety arises for the longitudinal . Here, the non-tensor behaviour of
is precisely canceled by the curious logarithmic correction term
Namely because of
one has
which hence makes the entire integrand of transform as an primary, as desired.
superstring:
The analogous construction for the superstring has to ensure in addition that the DDF operators commute with the supercharges . This is simply achieved by ‘closing’ the integral over a given primary field to obtain the operator
Here and in the following the brackets denote supercommutators.
The resulting operator is manifestly the zero mode of a weight tensor and hence commutes with all . Furthermore it commutes with because of
where we have used the tensor law
in the last step
Since and generate the entire algebra the ‘closed’ operator indeed commutes with all , .
It is therefore clear that the superstring DDF operators, which we can define as
commute with the super-Virasoro generators, since the second arguments of the supercommutators are integrals over weight 1/2 tensors. The nature and purpose of the logarithmic correction term in the second line is just as discussed for the bosonic theory above: It cancels the non-tensor-law term in
Evaluating the supercommutators in the definition of the superstring DDF operators yields the explicit form for and :
It is straightforward to check that the transversal DDF operators have the usual oscillator (super-)commutators when appropriate normalization factors are included.