One of the outstanding problems in high energy physics is the origin
of CP violation, a phenomena discovered already 30 years
ago in decays of
neutral kaons.
The most promising laboratory for CP violation studies are
decays of neutral B mesons, where CP violating effects are
expected to be large.
Decay channels which can exhibit CP asymmetries are extremely rare,
typically suppressed by 4 to 5 orders of magnitude. Cuts to select the events
and to identify the b flavour reduces the useful rates further.
Therefore a measurement of CP violation requires a large number of B
produced mesons, i.e. a machine acting as a B factory.
HERA-B uses the HERA protons to generate B mesons in 820 GeV
proton-nucleus interactions on a fixed target.
Here several tenths B mesons per second are rather easily produced, but
the events contain a large number of particles besides the decay products
of the B mesons.
In addition
the 
production cross section at HERA energy
is six orders of magnitudes smaller than the total inelastic cross section.
The ambitious challenge of the experiment are the detectors which will
be operated in a very high rate environment
and the triggers which have to provide a background reduction by six
orders of magnitude.
The main goal of HERA-B is the observation of CP violation in the
decay mode (cp. Fig. 1)
Figure 1: The "gold platted" decay
with some kinematical quantities at HERA-B .
by measuring the asymmetry:
where 
is the mixing parameter, 
the lifetime
of the B meson and 
the term measuring CP violation.
Considering the cross section, the branching ratios, the trigger and the
reconstruction efficiency of the HERA-B detector one ends up with a
total efficiency of approximately 
.
A first significant CP measurements requires 
events
and therefore 
interactions. This means
one year ( 
sec) running at a rate of 40 MHz.
Regarding the HERA bunch frequency of 8 MHz, this leads to
5 simultaneous interactions per bunch crossing (bx).
HERA-B uses a set of 8 ribbons which are positioned around the beam at a distance of 4 - 6 r.m.s. beam widths, i.e. inside the beam halo or close to the beam core but outside the core. The main idea is to absorb protons, which leaves the beam core and would get lost anyhow, and bring them to interaction in the target (cp. Fig. 2).
Figure 2: Basic idea of a halo target: protons which are drifting
outwards interact on the wires before hitting any aperture limitation.
Such a wire target is mechanically stable, easy to operate and it gives well localized and separated main vertices. The operation of the target has to ensure that neither the beam quality is affected nor the e-p luminosity is reduced or the data taking of the other HERA experiments is disturbed by background. To achieve routinously the anticipated rate of 40 MHz it is essential that at least 50% of the halo protons are absorbed in the target before they get lost on any aperture limitation.
In this article the basic properties of the HERA-B target are reviewed. Main emphasis lies on the interference with HERA beam operation. After a brief description of the HERA machine and the experimental setup the main requirements and the basic functionality are summarized. The impacts on the target efficiency are considered and the performance is discussed by a few selected measurements.