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\title{Plans for the HERMES Experiment at HERA II}
\author{E.C. Aschenauer $^a$ \\ on behalf of the HERMES Collaboration\\
$^a$DESY Zeuthen, Platanenallee 6, 15738 Zeuthen, Germany}

\maketitle

%-------ABSTRACT----------------------------------------------
\begin{abstract}

The HERMES experiment \cite{hermes} has been operational at the HERA 
accelerator of DESY since 1995. 

In HERMES Run-1 the focus was on a first accurate determination of
the flavour separated quark helicity distributions \cite{deltaq}. This was 
done on the basis of a large data set on both a longitudinally polarized 
hydrogen and a longitudinally polarized deuteron target. For the deuteron 
data HERMES
could substantially improve the accuracy because of a RICH detector 
\cite{rich}, which was installed in 1998. This enabled the identification of
pions and kaons over most of the momentum range. For the earlier hydrogen
data only pion identification was possible using the previously installed
threshold Cherenkov detector.  The valence polarizations are
well determined, but the much smaller sea polarizations are essentially
compatible with 0. The accuracy of these results could substantially be
improved by taking an additional 4 Mio DIS events with a longitudinally
polarized hydrogen target and the RICH detector. 
This improvement in statistical accuracy would be particularly important
for the sea polarizations.  With the present statistics no such asymmetry
is evident. Various theoretical calculations exist (again largely prompted
by the data taking of HERMES) that predict a non-zero asymmetry. The
additional 4 Mio DIS events would improve the statistics enough (red data
points) to allow verification of several models.

In semi-inclusive reactions, when not only the scattered positron but also
some of the produced hadrons are detected, a lot of interest has been generated
by the appearance of single-spin asymmetries. These offer the best road
towards a measurement of the unknown transversity distribution of quarks.
HERMES has started Run-2, where a first determination of the transversity 
through the Collins mechanism is the prime objective for the next years. 
It was partly driven by the fact that for this the beam does not need to 
be polarized. 
Up to February 2003 about 900'000 events have been recorded with a transversely
polarized hydrogen target. From estimates reported in e.g. the HERMES
Long Range Plan it is known that about 7 Mio DIS events are needed for a
statistically satisfactory measurement. This is necessary to
discriminate between different theoretical models for the transversity
distribution. In fact, the results published by HERMES on Run-1 \cite{trans} 
with a {\em longitudinally} polarized $H$ and $D$ targets, and the fact that
HERMES now runs with a transversely polarized target have led to a spate of
new calculations by several theory groups around the globe.

As is well known a measurement of exclusive reactions gives access to
the newly introduced Generalized Parton Distributions (GPD), which contain more
detailed information on the internal quark structure of the nucleon than
the standard Parton Distribution Functions. The cleanest way to arrive
at the GPDs is through the Deeply Virtual Compton Scattering (DVCS)
process, which was already measured and published in HERMES Run-1 \cite{excl}. 
However, it is obvious that HERMES in its present configuration has 
insufficient
energy resolution to allow an event-by-event separation of exclusive
reactions from non-ground state transitions. The easiest way to improve on
this situation is by the installation of a recoil detector around the
target to detect the recoiling target proton. Such a recoil detector
is presently under construction for HERMES and will be ready for
installation for the last two years of HERA operation.

Another essential improvement brought by the recoil detector is the much
better resolution in momentum transfer $-t$. 
A full programme of DVCS measurements with the recoil detector needs about
2 years of beam time. Since the installation of the recoil detector
necessitates the removal of the present polarized target system, the most
efficient planning requires these 2 years to be the last 2 years of data
taking with HERMES. An additional condition is that these 2 years of
data should consist of roughly equal data sets with opposite beam charge.
HERA is the only facility worldwide where beam-charge asymmetries in the
interference of DVCS and Bethe-Heitler can be exploited to get to the
DVCS amplitudes (rather than cross sections). 1 year of data with electrons
and 1 year of data with positrons (both polarized!) will yield unprecedented
accuracy for this asymmetry.

The physics case for the next phases in the HERMES running will be presented
and an overview of the challenges ahead for the experiment will be given.
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\end{abstract}
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\begin{thebibliography}{99}
%
\bibitem{hermes}
K.~Ackerstaff et al., Nucl.~Instr.~and Meth.~A \textbf{417} (1998) 230
%
\bibitem{deltaq}
K.~Ackerstaff et al., Phys.~Lett.~B \textbf{464} (1999) 123 \\
A.~Airapetian et al., Phys. Rev. Lett. ~\textbf{92} (2004) 012005
%
\bibitem{rich}
N. Akopov et al. Nucl.~Instr.~and Meth.~A~\textbf{479} (2002) 511
%
\bibitem{trans}
A.~Airapetian et al., Phys.~Rev.~Lett.~\textbf{84} (2000) 4047\\
A.~Airapetian et al., Phys.~Rev.~D \textbf{64} (2001) 097101\\
A.~Airapetian et al., Phys.~Lett.~B \textbf{562} (2003) 182
%
\bibitem{excl}
A.~Airapetian et al., Phys.~Lett.~B \textbf{535} (2002) 85\\
A.~Airapetian et al., Phys.~Rev.~Lett.~\textbf{87} (2001) 182001 \\
F. Ellinghaus et al., hep-ex/0212019 \\
F. Ellinghaus et al., hep-ex/0207029 
\end{thebibliography}

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