Quadrupole Spectrometers

  There is currently no space for Roman pots except for the dipole spectrometer pots and . The instrumentation of both the outgoing proton and anti-proton arms requires modifications to the machine lattice to create space for the detectors. The proposal here involves moving the three low beta quadrupoles on each side (, , and ) about two-thirds of a meter closer to the interaction region, in order to create two one-third meter spaces for the Roman pot stations. Roman pots would be located at either end of the electrostatic separators, which would be moved one-third meter closer to the interaction region. Figure 9 shows a sketch of the proton-side separator with Roman pots inserted. The area within the bypass is the only ``warm'' section of beam pipe in reasonable proximity to the DØ detector, and is thus the obvious choice for the location of Roman pots. The details of the modifications are discussed in Sec. 6.

  
Figure 9: A sketch of the electrostatic separator on the proton side with Roman pots inserted. The pots will be isolated from the separator by a vacuum valve on either side.

The FPD thus will consist of six Roman pot stations, the aforementioned , which has two stations, plus four stations that use the quadrupole magnets to measure the proton ( and ) or anti-proton ( and ) trajectory instead of the dipole magnets.

  
Figure 10: The ``Cross'' design of the beam pipe which allows Roman pots in the horizontal and vertical planes. There will be four detectors (``DET'') per quadrupole station. The entire beam pipe section will only be about 12 inches long.

An ideal proton detector would be an annular detector with full acceptance close to the beam. Since it is necessary to remove the detector during injection of the beam for stability and radiation considerations, such a design is impractical. Typically Roman pot stations have consisted of pairs of pots in either the horizontal or vertical plane, which generally provide adequate but not optimal acceptance. We are proposing a ``cross'' design (see Fig. 10), which maximizes the acceptance for protons and anti-protons by allowing pots in both the horizontal and vertical planes.

With this ``cross'' design there are eight independent quadrupole spectrometers, four on each side of the interaction region (two each in the x and y directions). This gives a total of 18 pots, 2 dipole pots and 16 quadrupole pots. An example of a quadrupole spectrometer is the spectrometer (first proton spectrometer) shown in Fig. 8, which has the pot located after the quadrupole about 23 m from the interaction point, and located about 31 m from z=0. A proton deflected to the left of the beam axis would be detected in this spectrometer while a proton scattered to the right would be detected in the spectrometer in pots and . There would also be and spectrometers (not shown in Fig. 8 for simplicity) for protons scattered above and below the beamline. Analogous spectrometers are located on the anti-proton side.

Although studies (discussed in Sec. 6) must be completed to show the feasibility of moving the quadrupole magnets, the implementation of this entire proposal provides a vast improvement over the dipole spectrometer alone. The gains are as follows

• Instrumenting both sides allows tagging of both protons and anti-protons and thus the unambiguous measurement of double pomeron exchange (for example, the outgoing proton could be detected in the spectrometer while the outgoing anti-proton could be detected in the spectrometer). By measuring the and |t| of the outgoing beam particles and the products of the interaction in the central detector, the event kinematics are fully determined.
• Instrumenting both sides using quadrupoles allows the detection of elastic scattering events (for example, a - combination with both particles at the beam momentum would be dominated by elastic scattering). A major concern with Roman pot detectors is alignment, and the in situ calibration afforded by elastic scattering would be invaluable. Elastic scattering events could also be used for luminosity monitoring by measuring the elastic cross section for each run.
• Diagonally opposite spectrometers can be used to reject halo background, as in-time halo tracks in one spectrometer appear as early hits in the other spectrometer (for example, a track in accompanied by an early time hit in would be a signal for a halo event).
• The combination of dipole and quadrupole spectrometers also provides many advantages. A subset of the anti-proton tracks will have hits in both the and the spectrometers. This will allow alignment of the spectrometer and provide an excellent means for checking efficiencies. The full |t| coverage of will be crucial for calculating the acceptance of the quadrupole spectrometers and avoiding uncertainties in the extrapolation to |t|=0, and also results in large acceptance for high mass events and double pomeron exchange (in conjunction with the proton quadrupole spectrometer).