5. VERY FORWARD CALORIMETRY

5.1 Performance requirements

The CMS Very Forward Calorimeter has to provide the calorimetric coverage of the pseudorapidity range in order to measure the missing transverse energy and to detect so-called "tagging" jets characteristic for the processes of intermediate vector bosons fusion.

Missing transverse energy measurement will be required for detection of any heavy objects decaying into weakly interacting particles. Two natural examples of such processes are the following:

  • Standard Model heavy Higgs search in the decay channel
  • strongly interacting supersimmetry particle production.

High pt neutrinos or lightest SUSY particles will not be detected and consequently will produce high missing Et for the above processes.

The detection of processes of high energy intermediate bosons (W or Z) interaction will be attractive for at least two purposes:

  • in some cases of Higgs boson production it will allow to increase effect to background ratio;
  • it will allow to study IVB interaction in the case if the Higgs mechanism does not work.

In this case two high energy "tagging" jets caused by quarks deflected from directions of incident beams due to IVB emission should appear. The most of such jets expected to be observed in the pseudorapidity range corresponding to VF.

5.2 Design consideration

The requirements to VF energy resolution are more or less loose. However VF will be placed at that position of detector where the highest rates and of particles and the highest energy fluxes are expected. It should be able to operate reliably at the high LHC luminosity during many years. It means that VF has to provide fast response to minimize pileup of soft hadronic events, has to be extremely radiation resistant and has to be very reliable in long term operation without any maintenance.

5.3 Detector description

Two options for VFCal technologies are considered by the CMS Collaborations. The baseline solution of CMS VFC is a modular, iron/gas sampling calorimeter based on parallel plate chambers (PPC) as shown in Fig. 5.1.

A PPC is a gaseous detector, with planar metal or metallized electrodes, working in the avalanche mode. It consists of two planar electrodes kept at a fixed distance by a spacer. The volume between the electrodes is filled with gas at atmospheric pressure. By applying a high voltage between the electrodes a very high and uniform electric field is obtained.

The Russian scientists from ITEP put forward the idea of using parallel plate chambers (PPC) for the calorimetry at Very Forward region of LHC detector. Russian scientists from ITEP and PNPI are involved into PPC R&D program RD37 where several prototypes of PPC were studied and calorimeter prototype based on PPC was tested. The production of PPC elements at Russian industry was developed.

Another option of CMS VF is the quartz fiber calorimeter (QFCal) . This solution is based on the use of quartz fibers embedded in copper or iron absorber material as shown in Fig. 5.2.

This type of calorimeter offers some very specific advantages that may well prove to be crucial for VFCal design:

  • silica fibers are extremely radiation hard;
  • the detector is based on Cherenkov light as a source of the signals, therefore:

i) the calorimeter response is very fast and short in time;

ii) the lateral size of calorimeter response will be about factor of two smaller than the same for dE/dx based calorimeter;

iii) the calorimeter will be insensitive to the most of neutron induced reactions and to the most of effects of induced radioactivity;

  • Calorimeter is static without any gases, liquids, power supply and high voltage in the active volume. So it is expected to be very reliable in operation without maintenance.

5.4 Research and Development

ITEP group is involved into R&D study of quartz fiber calorimetry. In the framework of RD40 collaboration a small QFCal prototype was produced in Russia. This prototype was assembled and tested at ITEP. The response to electrons and to minimum ionizing particles as a function of the angle between QF and the beam were measured. The methods of QFCal response calibration and photodetector gain monitoring were studied. Optical properties of various types of QF were studied at CERN by French, Italian, Russian and US scientists. Several electromagnetic and two hadronic prototypes with quartz and clear plastic fibers were tested at CERN beams. The Monte-Carlo simulation program based on GEANT was developed for QF calorimeters and validated using prototype data.

Using MC simulations the design of VF with QF has been optimized and VF performance was evaluated [5.1]. The detail study of QF radiation hardness has been performed in Russia [5.2].

5.5 Plans of VF R&D

The following R&D issues are of critical importance for fine tuning the design parameters and the detector simulation programs:

  • the light collection scheme for three segment longitudinal readout,
  • measurements of the light yield and the shapes of the longitudinal and lateral profiles of high energy protons showering in a full-length prototype, with particular attention to leakage tails,
  • measurement of the calorimeter response to low-energy neutrons and gammas,
  • radiation damage studies for doses up to 10 Mgy (1 Grad), induced by gammas and neutrons. A full-length VF/QF prototype intended for these studies is under construction.

On the basis of 1995 R&D results one of two techniques (PPC or QF) will be chosen for the final design. RDMS groups interested in VF detector will then work on the chosen option.