Institut für Theoretische Physik, Universität Giessen,
D-35392 Giessen, Germany
Carsten Greiner
Institut für Theoretische Physik, Universität Giessen,
D-35392 Giessen, Germany
Langevin dynamics
- Brownian motion with colored noise: nonmarkovian Langevin dynamics
- Effective semi-classical action for the soft modes in the 
-theory
- Stochastic interpretation of Kadanoff-Baym equations
In my talk I first gave a brief review of some ideas behind the use
of a semi-classical Langevin treatment of the low momentum
modes with 
in thermal quantum field theory.
This was based on a work [1],
which concerns a principle strategy within the influence functional
technique for obtaining an effective real-time action
for the soft modes by integrating out the hard thermal modes.
From another perspective I have also discussed how one can
give the famous (non-equilibrium) Kadanoff-Baym equations
a stochastic interpretation by means of (quantum) Langevin dynamics
[2].
Stochastic DCC formation
- Langevin description of the DCC formation within the linear 
model
- Dissipation and fluctuation
- Pion number distribution in various scenarios
- Cumulant expansion of the pion number distribution
In the second part I adressed recent work [3, 4], where
by applying a microscopically motivated Langevin description
of the linear sigma model, we investigated for various different scenarios
the stochastic evolution of a disoriented chiral
condensate (DCC) in a rapidly expanding system.
I stressed the important role of (understanding) dissipation and
fluctuations during the evolution of the DCC order parameter.
By choosing some idealized global parameters for the expansion,
our findings show that an experimentally feasible DCC, if it does exist
in nature, has to be a rare event with some finite probability.
The statistical distribution 
of final emitted pion number
out of domains shows a striking nonpoissonian and nontrivial
behaviour.
Unusual events out of sample contain a multiple in the
number of pions compared to the average.
One should indeed interpret those
particular events as semi-classical `pion bursts' similar to
the mystique Centauro candidates.
The further analysis of this unusual
distribution by means of the cumulant expansion shows that the reduced
higher order factorial cumulants 
for 
exhibit
an abnormal, exponentially increasing tendency.
We advocate that an analysis by
means of the higher order cumulants serves as a new and powerful signature.
In conclusion, the occurence of a rapid chiral phase transition
(and thus DCCs) might then probably only be identified
experimentally by inspecting
higher order facorial cumulants 
()
for taken distributions of low momentum pions.