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Physics goals |
In recent years, diffractive physics at high energy has received considerable interest, as well from the experimental side as from theory. In strong interactions, diffraction is intimately related to elastic scattering and thus to the total cross section. Its detailed understanding is of fundamental importance in quantum chromodynamics (QCD), in particular the interpretation in terms of quarks and gluons of the exchanged object, the pomeron.
Due to the results obtained by the ZEUS and H1 experiments, considerable progress has been achieved in the partonic interpretations of the diffractive processes. In particular the observed transition between the "soft" and "hard" behaviour of diffraction, the confirmation of the gluon dominance in the pomeron and the relevance of a QCD evolution of parton distributions in the pomeron. However, the small cross sections involved and the difficulty in selecting clean diffractive event samples have left many basic QCD predictions unanswered. Therefore further progress in this field will rely on collecting large statistics in various inclusive, semi-inclusive and exclusive diffractive channels, in particular those in which a hard scale is present (large exchanged boson virtuality Q2, large four-momentum transfer squared, t, to the scattered proton, large transverse momentum of jets, large mass of participating quarks).
Most diffractive studies performed up to now at HERA have been based on the the characteristic presence of a rapidity gap, devoid of energy, in the diffractive final state. However the only precise and unambiguous way of studying diffraction is by tagging the diffracted proton and measuring its four momentum by means of a proton spectrometer. Such devices have been installed by the H1 and ZEUS and have delivered interesting results, but their acceptances are small, with the result that the collected statistics are limited. To fully profit from the luminosity upgrade in the study of diffraction after the year 2000, a proton spectrometer which identifies and measures the momentum of the diffracted proton with a very good acceptance is thus essential.
The new proton spectrometer, called Very Forward Proton Spectrometer, will be installed at 220 m downstream of the H1 main detector.during the winter 2002-2003. At this location, due to the strong horizontal beam bend, the spectrometer acceptance is close to 100% for the full range in t for protons having lost more than 0.5 % of the incident proton energy. Large statistics can thus be accumulated and uncertainties related to extrapolations in t avoided. With the measurement of two impact point positions, the momentum transfer t, the momentum loss and the azimuthal angle of the scattered proton can be measured. The anticipated acceptances and resolutions are given in the PRC proposal note.
The installation of the high acceptance proton spectrometer will thus provide outstanding improvements in the study of diffraction: genuine elastic events not contaminated by proton dissociation can be selected; large statistics will be accumulated, in particular, in the crucial diffractive final states with a hard scale i.e. channels with a high transverse momentum jet, charm or beauty; measurements of basic importance can be performed: determination of the longitudinal cross section via the measurement of the azimuthal angle of the scattered proton and measurement of the fully differential F2D4 structure function, including the measurement of the t dependence. More details can be found in this proposal addendum.
Proton spectrometer layout |
The VFPS is a set of two "Roman pots".
Each pot consists of an insert into the beam pipe,
allowing two tracking detectors equipped with scintillating fibres
to be moved very close to the proton beam.
The insert (plunger vessel) and detectors can
be moved horizontally towards the beam, by means of mechanical gears; the
detectors are operated at atmospheric pressure.
The Roman pots are retracted during injection and beam dump;
during stable beam conditions they are moved to a position as close
as possible to the beam
whilst ensuring that the particle rate in the detectors is not too high
and the Roman pots do not limit the beam lifetime.
The Roman pots, positioned at 3.7 m
from each other, will be installed in a 6.2 m long drift section of the
proton beam line located at about 220 m downstream of the ep
interaction region.
This drift section is in the "cold" part of the HERA proton ring,
which is equipped with supraconducting magnets.
However, in order to access the beam pipe with Roman pot detectors, the proton
beam line has to be at room temperature.
This implies that the beam pipe has to be separated in this area
from the cold elements of the drift tube,
and a bypass has to be installed to transport horizontally the
helium lines to the next cold section.
Many aspects of the design of the Roman pots, including the
stainless plunger vessel and the scintillating fibre detectors,
are adaptations of the existing proton spectrometer (FPS), installed and
operational in H1 since 1994.
However, the experience accumulated in various
experiments (H1, HERA-B) as well as technical progress regarding
scintillating fibres and photomultipliers have resulted in
several modifications and improvements.
Both detectors of each Roman pot consists of two planes of
scintillating fibres perpendicular to the beam line direction and
oriented at +- 45o from the horizontal direction.
Each detector thus provides the reconstruction of the position of one
impact point of the scattered proton trajectory.
A sub-detector, measuring a single coordinate, is composed of 5 layers of
120 scintillating fibres.
The coordinates of the impact points of the
scattered proton will be measured with a precision of about 100
micrometers.
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