Since 1995 the H1 experiment at HERA is operating a Forward Proton
Spectrometer (FPS). Scattered protons are detected in stations at 81 and
90 m behind the interaction point. The FPS has been extended with stations at 80 and
63 m starting to operate in 1997. In all stations the protons are detected with
scintillating fiber hodoscopes.
Each station consists of a moveable vacuum
section, a so called Roman Pot, in which the detector elements are mounted. Thus
the detector can be brought a few milimeters close to the proton beam.
During injection and acceleration the Roman Pots are retracted.Otherwise they
would cut into the machine aperture.
At the HERA collider facility a class of events is observed in deep inelastic
electron proton scattering and photoproduction, which is characterized by the
absence of secondary particles in a region of phase space between the outgoing
proton debris and the target jet. Deep inelastic high energy e-p interactions (DIS)
predominantly proceed by the exchange of a virtual vector boson (
, W or Z°)
between the incident electron and a constituent quark or gluon in the proton. The rest of the proton is essentially unaffected
by the interaction and continues along the flight path of the incoming proton.
In about one quarter of the neutral and charged current deep inelastic
interactions the protons remnants fragment into final states containing an
energetic forward going proton at forward angles. In general these leading
protons escape through the beam pipe and are not detected in the central HERA
detectors (take a look at the intersection of the H1 detector with the beampipe on
the H1 homepage).
With the FPS forward scattered protons at angles in the order of mrad are
detectable. These protons are recorded and their energy is measured. Further
kinematical constraints can thus be applied and additional information on deep
inelastic scattering final states is gained.
Roman Pots: These moveable vacuum sections were first used at the Intersecting Storage Rings (ISR) at CERN by a collaboration, in which a group from the Institutio Superiore di Sanità in Rome played an important role.
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