$SU(2)$ is a Lie group, with a Lie algebra $\mathfrak{su}(2)$. I now consider the loop group 
$ LSU(2) = \{ \gamma: S^1 \to SU(2); \gamma \mathrm{ smooth} \} $. It is well-known that there exists non-trivial 2-cocycles on it, obtained from the invariant bilinear form on $\mathfrak{su}(2)$ (see Pressley and Segal for example).


The 2-cocycle on the Lie-algebra is very simple to write. However, the 2-cocycles on the group are rarely written explicitly. I know that it may be quite difficult for general groups but what about a group as "simple" as SU(2) ?


Does anybody an "explicit" expression of 2-cocycles on $LSU(2)$ such that, if I have an explicit expression of two loops $\gamma\_1$ and $\gamma\_2$ on SU(2), I can put it directly on Maple or Mathematica (or evaluate by hand) ?


EDIT: I precise my question. Consider paths in SU(2) parametrized for example by
\begin{equation}
\gamma( \theta) = 
\begin{pmatrix}
 cos \phi(\theta) e^{i\alpha(\theta)} & -\sin \phi(\theta) e^{-i\beta(\theta)} \\\\
 \sin \phi(\theta) e^{i\beta(\theta)} & cos \phi(\theta) e^{-i\alpha(\theta)} 
\end{pmatrix}
\end{equation}
where $\alpha$, $\beta(\theta)$ and $\phi(\theta)$ are smooth $2\pi$-periodic functions. Could someone write a non-trivial 2-cocycle using integral formulas over these functions. It *should* be simple but I do not manage to write it down. Maybe this parametrization is not the best one; in this case, what is the most useful one ?


Thank you in advance,
Damien.



2-cocycle on LSU(2)
I believe that [this post](https://mathoverflow.net/questions/24845/explicit-cocycle-for-the-central-extension-of-the-algebraic-loop-group-gct) contains an answer to your question.


