3. HADRON CALORIMETRY


Overview and requirements

The combined CMS hadron calorimeter system has to measure particle jet energies and directions to provide information on quark, gluon and neutrino directions and energies as well as magnitude and direction of missing transverse energy flow. The determination of missing energy will be crucial for to the signatures for new particles and phenomena, such as will be encountered in the searches for the supersymmetric partners of quarks and gluons. The hadron calorimeter will also help in the identification of electrons, photons and muons in conjunction with the electromagnetic calorimeter and the muon system. Thus the Hadron Calorimeter is the essential subsystem of the CMS detector which provides a significant contribution to the identification of potential new physics signals.

The CMS Hadron Calorimeter (HCAL) surrounds the crystal electromagnetic calorimeter of CMS. The combined response of the electromagnetic and hadron calorimeters provide the raw data for the reconstruction of particle jets and missing transverse energy. The central pseudorapidity range (||<3.0) is covered by the barrel and endcap calorimeter system consisting of a hermetic crystal electromagnetic calorimeter (ECAL) followed by the hadron calorimeter barrel (HB) and endcap (HE) as show in Fig.3.1.

The very forward region (3.0<||<5.0) is covered by the very forward calorimeter system (HF) physically separated from the central calorimeter by several meters.


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3.1. ENDCAP HADRON CALORIMETER

The endcap and barrel calorimeters sit inside the 4 Tesla field of the CMS solenoid and hence are necessarily fashioned out of non-magnetic material (copper and stainless steel). The active elements of the entire central calorimeter are plastic scintillator tiles with wavelength shifting (WLS) fiber readout. The Endcap Hadron Calorimeter (HE) will cover a rapidity region between 1.3 and 3.0 with good hermiticity, good transverse granularity, moderate energy resolution and a sufficient depth. Perspective view of the endcap hadron calorimeter is shown in Fig. 3.2.

A lateral granularity (eta*phi) was chosen ~0.087*0.087. The hadron calorimeter granularity must match the EM granularity to simplify trigger.

The Endcap Hadron Calorimeter design and construction are under full responsibility of the RDMS Collaboration. Note that according to CMS installation plans the Endcap Hadron Calorimeter must be ready at the middle of 2001 year as the first CMS system. Now the first version of the Technical Design Report (also the CMS subsystem TDR) is ready.

Test beam results

To test the influence of magnetic field on the calorimeter response and to optimize the sampling, a 12 tons "hanging file" calorimeter was designed and constructed. During 1994-96 years several runs were carried out inside superconducting magnet and without magnetic field. The main results are the following:

  1. There is a strong dependence of a sandwich calorimeter response on the magnetic field orientation. For transverse magnetic field the response for electrons and hadrons is rising linearly and does not depend on particle energy. For longitudinal magnetic field the response does not depend on particle species and the response of the calorimeter is the same as for radioactive source. This fundamental result drastically changes the approach to calorimeter calibration. Paper will be published in the Nucl. Inst. and Meth. To check this results the special setup with superconducting magnet was built in Dubna. Measurements with different radioactive sources were confirmed completely test beam results. The paper will be published in Nucl. Inst. and Meth. also.

    Fig3.3:The relative change of the light yield of the hadron calorimeter vs. magnetic field with transverse(a) and longitudinal (b) orientation

  2. To study influence of a dead material(which could be introduced by electron beam welding) on calorimeter performance we measured the calorimeter energy resolution in dependence on the distance from a spacer 1 cm wide. The measurements shown that the energy resolution drastically degradates near the dead material zone. These measurements provide strong argument against the electron beam welding usage.

  3. Study of the energy resolution dependence on the sampling shown that optimal thickness of the copper plate is 8 cm. Results are shown in Fig. 3.4.

  4. Measurements with intense proton beam during 1996 year did not confirm a statement that hadron beams make more damage to scintillator (about 2 times) than radioactive source.


Design and construction

Each endcap is an 18 sided polyhedron that covers and closes one end of the barrel calorimeter. The endcap is constructed of plates, separated by staggered spacers, that are perpendicular to the beam axis. Scintillating plates are mounted on aluminum plates forming trapezoidal tray (or "pizza pan") structures which are installed in the gaps of the endcap absorber. The construction allows easy access to the "pizza pans" and provides a rigid structure.

In the CMS Technical Proposal (1994 year) the Electron Beam Welding (EBW) to construct the mechanical structure of the endcap calorimeter sampling 5 and 10 cm was considered. But in this case dead zones will appear between the sectors. The width of the dead zone is in range 1-2 cm. The influence of the dead zone on the energy resolution was measured with "hanging file” prototype calorimeter. As one can see even a 1 cm wide spacers affects the energy resolution in a way that can't be neglected. To suppress this effect there was proposed by RDMS team a bolted structure instead of EBW. In this case there are no dead zones besides the mechanical structure and assembly are simplified. For such design it is more convenient to have the absorber plates with the same thickness or the same sampling. Then it would be possible to have only one longitudinal division which is difficult to make with different sampling. As a compromise 8 cm sampling was considered. For this sampling MC simulations and measurements were carried out, the results show that energy resolution for 8 cm sampling is practically the same as for 5 and 10 cm sampling. Also a good linearity is observed in all measured range(20-300 GeV). MC calculations confirmed that the linearity is in the range 10-500 GeV (10.3 without ECAL). No tail catcher is needed in this case.

The following design was performed during 1996 year:


  1. Basic design of the calorimeter absorber.

  2. Design of the preproduction prototype sector. The general view of the preproduction prototype sector is shown in Fig. 3.5.

  3. Design of the scintillator tray.

  4. Design of the interface HE/HF.

  5. Design of the laser control system.

  6. Design of the decoder box.

  7. Design of the splicing device for joining WLS fiber and clear fiber.

  8. Design of the scanning device to test the performance of the scintillator tray.

Working drawings for preproduction prototype absorber and calorimeter itself are practically ready.

It was defined the interconnection RDMS scheme (JINR - IHEP - INR - ENTEK (Moscow) - NCPHPE (Minsk)) for production of the full scale prototype, endcaps and interface systems as shown in Fig. 3.6. Production plant (Minsk) was defined also.

Drawings for interface system is under production. A first scintillator tray is under construction to test the basic ideas such as: rigidity of the structure, light collection from the scintillator, engineering solutions etc.

The prototype laser control system including electronics for computer control of the system was produced and tested during 1996 year.

A mock up of the decoder box was made to optimize the box diminution and fiber rooting. A splicing device was constructed and tested. Production of the scanning device is started and will be completed during this year.


Plans

Practically all materials for active elements have been purchased (scintillator, fibers, Tyvek etc.) for preproduction prototype. Production of scintillators and machining of scintillators and fibers are started. Half of the copper for preproduction prototype is purchased and the machining of the absorber will begin in April. To the end of this summer the preproduction prototype equipped with control system is planned to be ready for test. The main goals of the preproduction prototype are to understand the performance of the calorimeter, to check the engineering solutions, to develop tooling and technology for mass production and to chose the best vendors for copper, scintillator etc.


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3.2. FORWARD CALORIMETER


Beam tests results and performance simulations

The performance of quartz fiber calorimeter prototypes was studied with pions and electrons of energies from 10 to 350 GeV. The energy resolutions, lateral and longitudinal response profiles, were studied. The time structure of prototype response to pions and electrons was also measured. Fig. 3.7 shows the measured energy resolutions for electrons and pions which found to be adequate for very forward region where high energy hadron jets should be detected. Jet energy resolution was estimated using these experimental data.

Circles in Fig. 3.8 shows the expected energy resolution for jets in the case of longitudinally non-segmented calorimeter. Longitudinal segmentation will allow to improve the resolution (triangles in fig. 3.8) and will provide jet energy resolution of about 11% at 1 TeV energy of jet. This in turn will led to resolution of transversal energy of jet of few GeV. The measured narrow lateral profile of quartz fiber calorimeter response and its very fast signal will allow to minimize background for CMS detector operation at the high luminosity of the LHC machine.

The basic parameters of CMS forward calorimeter were optimized using detail simulations of its performance. The optimal sizes of read-out cells were determined. The HF shielding was optimized by the joint efforts of RDMS CMS group and CERN CMS radiation group. As a result there was found the shielding lay-out, which will allow to decrease the neutron fluxes at the vicinity of photodetectors below 105 n/cm2/sec, will provide an easy access for photodetector replacement and will minimize the lengths of quartz fibers.

Mechanical design and preparation to absorber module production

The mechanical design of forward calorimeter based on the concept of modular structure of copper absorber blocks with holes for quartz fibers was elaborated. The technology of absorber block manufacturing was evaluated by ITEP group together with VNIITF engineers. The absorber block of small size made using diffusion welding technique was manufactured. The preparation to manufacturing at VNIITF of the full size preproduction prototype is underway. The technical design report for the CMS forward calorimeter is preparing now and it will be submitted for CMS internal review by May 1.


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