- Email: [email protected]
A digital hadron calorimeter with Resistive Plate Chambers
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129 www.elsevier.com/locate/nima
A digital hadron calorimeter with Resistive Plate Chambers Jose´ Repond Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Available online 22 July 2004
Abstract The concept of a digital hadron calorimeter for the Linear Collider is presented. The R&D effort to develop an active medium with the required spatial segmentation using Resistive Plate Chambers is reviewed. r 2004 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 29.49.Vj; 29.40.Mc Keywords: Resistive Plate Chambers; Calorimetry; Linear Collider
1. Calorimetry at the Linear Collider Based on worldwide consensus among the different High Energy Physics communities in America, Asia and Europe, the next major construction project will be a Linear Collider operating at center-of-mass energies in the range pffiffi of s ¼ 500–1000 GeV. The main purpose of the new machine will be to study the details the Higgs and SUSY particles which are expected to be produced at collision energies below 1 TeV. Events containing these particles will frequently involve a large number of hadronic jets. In order to take advantage of the large branching ratio of hadronic decay channels and to be able to reconstruct the underlying physics processes involving the production and decay of Higgs and SUSY particles, an E-mail address: [email protected] (J. Repond).
excellent jet energy resolution of the order of 30%/ pffiffiffiffi E is required. Up to date, the pbest ffiffiffiffi jet energy resolution, approximately 50%/ E , has been achieved by the fully compensating Uranium Scintillator sandwich calorimeter of the ZEUS experiment. To achieve a significantly better resolution new approaches have to be explored. The most promising of these is called Energy Flow Algorithm (EFA).
2. Energy flow algorithms Hadronic jets consist of four distinct components: charged particles, photons, neutral hadronic particles and neutrinos. Table 1 shows the average fraction of the jet energy taken by each of the four components.
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.014
ARTICLE IN PRESS J. Repond / Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129 Table 1 Average energy of the various components of jets Component of jet
Fraction of energy
Detector
Charged particles Photons Neutral hadrons Neutrinos
60% 20% 10% 10%
Tracker EM-calorimeter Calorimeter Lost
Traditionally, the energy of jets is measured with the calorimeter alone. The energy resolution is limited by sampling fluctuations in the (sandwich) calorimeter and by the fluctuations on the fraction of the electromagnetic and hadronic parts of the shower which are measured with different gains in non-compensating calorimeters. In contrast to the traditional method, EFAs make use of both the tracker and the calorimeter to measure the energy of jets. The tracker is used to measure the momentum of charged particles with high precision. A tracker with a large volume and high magnetic field, of the order of 4 T, is required. The energy of the photons is measured with a highresolution electromagnetic calorimeter, such as a silicon–tungsten sandwich calorimeter with thin absorber plates (p1x0 ). Finally, the energy of the neutral hadrons, i.e. the neutrons and K 0L ’s, is measured with both the electromagnetic and the hadronic calorimeters. The challenge of the EFAs is to properly identify the energy deposits in the calorimeter corresponding to the different components of the hadronic jet. To this effect, a calorimeter with extremely fine segmentation is needed. EFAs have been applied to existing colliding beam detectors, such as CDF and ZEUS, where an improvement of the energy resolution of jets has been obtained. However, these detectors have not been designed with these algorithms in mind and so the resulting improvements are somewhat modest. For the Linear Collider the detector design is being developed with the application of these algorithms in mind. To fully exploit the advantage brought by the application of EFAs, extensive Monte Carlo (MC) studies are being performed to determine the optimal set of design parameters,
127
such as the segmentation of the readout of the calorimeter. These studies rely on a realistic simulation of hadronic showers by the simulation program. A validation of the programs by comparison with detailed measurements of shower shapes with a finely segmented calorimeter is an essential condition to establish confidence in the overall design of a detector for the Linear Collider.
3. Digital hadron calorimeter To this date, no complete MC study of the performance of EFAs exists which demonstrates the effect on the jet energy resolution as a function of the segmentation of the calorimeter. However, preliminary studies exist which indicate that readout cells of the order of 1 cm2 are needed. With an active area of approximately 5000 m2 the hadron calorimeter of the Linear Collider detector will count 50 106 readout channels, assuming all pads in every layer are read out individually. An analog readout for this large number of channels will be prohibitively expensive, leading to the consideration of a simple digital readout with a threshold set well below the signal given by one minimum ionizing particle traversing the active medium. Reduction of the readout to the digital information only is generally expected to have an adverse effect on the single particle energy resolution. However, MC studies of the response to K 0L in a wide range of energies indicate that, contrary to expectation, the resolution is comparable if not better in the digital case. This can be understood as a consequence of the fact, that with low threshold settings the digital readout is independent of Landau fluctuations of the energy deposited by particles traversing the active medium. With an analog readout these fluctuations are summed up and result in a slightly deteriorated energy resolution. Currently, three different choices for the active medium of a digital hadron calorimeter at the Linear Collider are being explored: Scintillator, Gas Electron Multipliers (GEMs) and Resistive Plate Chambers. The goal of these efforts is to build a 1 m3 prototype hadronic calorimeter
ARTICLE IN PRESS 128
J. Repond / Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129
section to be tested in particle beams at Fermilab or Protvino, Russia.
4. Resistive Plate Chambers Resistive Plate Chambers (RPCs) offer several advantages as active medium of a digital hadron calorimeter: their conceptual design is simple, their construction is cheap, their operation is reliable and their readout can be chosen to be very finely segmented. Despite their simple basic design, these chambers offer several design parameters which can be adjusted to the needs of a specific application. To this date our group1 built three different chambers with an area of approximately 20 20 cm2 using glass of either 0.85 or 1.1 mm thickness as resistive plates. The chambers feature two gas gaps, each of 0.64 mm thickness and are operated in avalanche mode. A mixture of freon, argon and isobutane (62:30:8) was used as default gas, but other mixtures were investigated as well. Results on induced charge, single particle detection efficiency, and noise rates as a function of high voltage have been obtained with a single pad (10 10 cm2 ) readout. The performance of the chambers was found to be more than adequate for our planned application. The chambers were also read out with 25 pads with an area of 1 cm2 each. Here the pad multiplicity and the fractional charge versus distance from the pad hit, see Fig. 1, were measured. These results indicate that pad sizes as small as 1 cm2 can be used to readout RPCs with relatively small cross-talk. The geometrical acceptance of the chambers was investigated in a cosmic ray test stand which included a tracking system. The efficiency was observed to decrease in an area close to the fishing lines which are used to ensure the uniformity of the gas gap. The effect is consistent with a complete loss of efficiency over the area covered by the lines, but full efficiency, close to 100%, everywhere else. 1
Our group consists of Argonne National Laboratory, Boston University, University of Chicago and Fermilab. A similar effort is being pursued by a group at IHEP, Protvino, Russia. See V Ammosov’s contribution to these proceedings for more details.
Fig. 1. Measurement of the fractional charge as a function of the distance from the pad traversed by the cosmic ray.
5. Design of the electronic readout system The major challenge of the 1 m3 prototype is the design of the electronic readout system. With 40 layers of 1 m2 each, the number of readout channels is 400,000. The readout system is foreseen to consist of four parts: a front-end board containing the readout pads and an ASIC connected to 64 input channels; a data concentrator reading out twelve front-end ASICs; a data collection system based on VME; and a trigger system. The analog part of the ASIC contains a preamplifier, a shaper and a discriminator. The threshold of the discriminator is common to all 64 channels and can be set externally. The conceptual design of the digital part of the ASIC is shown in Fig. 2. Two modes of operation will be possible: (a) in a trigger-less operational mode each channel number, when hit, will be stored together with a time stamp; (b) in a triggered operational mode an external trigger controls the writing of the hit information to a readout buffer (FIFO). The data concentrator boards will be located on the side of the 1 m3 calorimeter section and will be based on the use of FPGAs. The data collection system will consist of two crates containing approximately 40 VME cards.
6. MC simulations The response of the 1 m3 prototype section with Scintillator, RPCs and GEMs as active medium
ARTICLE IN PRESS J. Repond / Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129
129
content and higher density than in a small gap filled with gas. These results point to a distinct advantage of gas detectors for the application of EFAs. Narrower showers will enhance the ability of the detector to disentangle the energy deposits of the various components of a hadronic jet.
7. Conclusions and outlook
Fig. 2. Conceptual design of the digital part of the front-end ASIC.
has been studied in MC simulations. A comparison of the lateral extend of electromagnetic showers shows that the energy deposits in gas detectors are more collimated than in scintillator. This can be understood as due to the insensitivity of gas detectors to Compton scattering of lowenergy-photons which may have traveled far from the core of the shower before converting. Scintillator due to its higher density has a higher probability of detecting these photons. Recently two independent studies showed that hadronic showers as well appear to be more collimated in a calorimeter read out with gas detectors. The reason for this effect is related to the higher probability of detecting neutrons, which again may have traveled far from the shower core, in scintillator with a higher hydrogen
In p order ffiffiffiffi to achieve a jet energy resolution of 30%/ E at the Linear Collider a new approach, called EFA, is proposed. The application of these algorithms requires a large tracking volume with high magnetic field and a calorimeter with very fine segmentation, of the order of 1 cm2 laterally and layer-by-layer in depth. The resulting large number of readout channels of the hadron calorimeter precludes an analog readout. MC simulations of single particle responses show that the energy resolution obtained with digital readout is comparable to the one obtained with analog readout. RPCs offer distinct advantages as active medium of such a hadron calorimeter. Various tests with prototype chambers read out with either a single pad or multiple pads, each with an area of 1 cm2 , have been performed. The chambers are found to satisfy all requirements on the active medium of a hadron calorimeter for the Linear Collider. An intermediate goal of building a 1 m3 prototype section to be tested in particle beams has been identified. A conceptual design of the electronic readout system for the test section has been developed.
Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129 www.elsevier.com/locate/nima
A digital hadron calorimeter with Resistive Plate Chambers Jose´ Repond Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Available online 22 July 2004
Abstract The concept of a digital hadron calorimeter for the Linear Collider is presented. The R&D effort to develop an active medium with the required spatial segmentation using Resistive Plate Chambers is reviewed. r 2004 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 29.49.Vj; 29.40.Mc Keywords: Resistive Plate Chambers; Calorimetry; Linear Collider
1. Calorimetry at the Linear Collider Based on worldwide consensus among the different High Energy Physics communities in America, Asia and Europe, the next major construction project will be a Linear Collider operating at center-of-mass energies in the range pffiffi of s ¼ 500–1000 GeV. The main purpose of the new machine will be to study the details the Higgs and SUSY particles which are expected to be produced at collision energies below 1 TeV. Events containing these particles will frequently involve a large number of hadronic jets. In order to take advantage of the large branching ratio of hadronic decay channels and to be able to reconstruct the underlying physics processes involving the production and decay of Higgs and SUSY particles, an E-mail address: [email protected] (J. Repond).
excellent jet energy resolution of the order of 30%/ pffiffiffiffi E is required. Up to date, the pbest ffiffiffiffi jet energy resolution, approximately 50%/ E , has been achieved by the fully compensating Uranium Scintillator sandwich calorimeter of the ZEUS experiment. To achieve a significantly better resolution new approaches have to be explored. The most promising of these is called Energy Flow Algorithm (EFA).
2. Energy flow algorithms Hadronic jets consist of four distinct components: charged particles, photons, neutral hadronic particles and neutrinos. Table 1 shows the average fraction of the jet energy taken by each of the four components.
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.014
ARTICLE IN PRESS J. Repond / Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129 Table 1 Average energy of the various components of jets Component of jet
Fraction of energy
Detector
Charged particles Photons Neutral hadrons Neutrinos
60% 20% 10% 10%
Tracker EM-calorimeter Calorimeter Lost
Traditionally, the energy of jets is measured with the calorimeter alone. The energy resolution is limited by sampling fluctuations in the (sandwich) calorimeter and by the fluctuations on the fraction of the electromagnetic and hadronic parts of the shower which are measured with different gains in non-compensating calorimeters. In contrast to the traditional method, EFAs make use of both the tracker and the calorimeter to measure the energy of jets. The tracker is used to measure the momentum of charged particles with high precision. A tracker with a large volume and high magnetic field, of the order of 4 T, is required. The energy of the photons is measured with a highresolution electromagnetic calorimeter, such as a silicon–tungsten sandwich calorimeter with thin absorber plates (p1x0 ). Finally, the energy of the neutral hadrons, i.e. the neutrons and K 0L ’s, is measured with both the electromagnetic and the hadronic calorimeters. The challenge of the EFAs is to properly identify the energy deposits in the calorimeter corresponding to the different components of the hadronic jet. To this effect, a calorimeter with extremely fine segmentation is needed. EFAs have been applied to existing colliding beam detectors, such as CDF and ZEUS, where an improvement of the energy resolution of jets has been obtained. However, these detectors have not been designed with these algorithms in mind and so the resulting improvements are somewhat modest. For the Linear Collider the detector design is being developed with the application of these algorithms in mind. To fully exploit the advantage brought by the application of EFAs, extensive Monte Carlo (MC) studies are being performed to determine the optimal set of design parameters,
127
such as the segmentation of the readout of the calorimeter. These studies rely on a realistic simulation of hadronic showers by the simulation program. A validation of the programs by comparison with detailed measurements of shower shapes with a finely segmented calorimeter is an essential condition to establish confidence in the overall design of a detector for the Linear Collider.
3. Digital hadron calorimeter To this date, no complete MC study of the performance of EFAs exists which demonstrates the effect on the jet energy resolution as a function of the segmentation of the calorimeter. However, preliminary studies exist which indicate that readout cells of the order of 1 cm2 are needed. With an active area of approximately 5000 m2 the hadron calorimeter of the Linear Collider detector will count 50 106 readout channels, assuming all pads in every layer are read out individually. An analog readout for this large number of channels will be prohibitively expensive, leading to the consideration of a simple digital readout with a threshold set well below the signal given by one minimum ionizing particle traversing the active medium. Reduction of the readout to the digital information only is generally expected to have an adverse effect on the single particle energy resolution. However, MC studies of the response to K 0L in a wide range of energies indicate that, contrary to expectation, the resolution is comparable if not better in the digital case. This can be understood as a consequence of the fact, that with low threshold settings the digital readout is independent of Landau fluctuations of the energy deposited by particles traversing the active medium. With an analog readout these fluctuations are summed up and result in a slightly deteriorated energy resolution. Currently, three different choices for the active medium of a digital hadron calorimeter at the Linear Collider are being explored: Scintillator, Gas Electron Multipliers (GEMs) and Resistive Plate Chambers. The goal of these efforts is to build a 1 m3 prototype hadronic calorimeter
ARTICLE IN PRESS 128
J. Repond / Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129
section to be tested in particle beams at Fermilab or Protvino, Russia.
4. Resistive Plate Chambers Resistive Plate Chambers (RPCs) offer several advantages as active medium of a digital hadron calorimeter: their conceptual design is simple, their construction is cheap, their operation is reliable and their readout can be chosen to be very finely segmented. Despite their simple basic design, these chambers offer several design parameters which can be adjusted to the needs of a specific application. To this date our group1 built three different chambers with an area of approximately 20 20 cm2 using glass of either 0.85 or 1.1 mm thickness as resistive plates. The chambers feature two gas gaps, each of 0.64 mm thickness and are operated in avalanche mode. A mixture of freon, argon and isobutane (62:30:8) was used as default gas, but other mixtures were investigated as well. Results on induced charge, single particle detection efficiency, and noise rates as a function of high voltage have been obtained with a single pad (10 10 cm2 ) readout. The performance of the chambers was found to be more than adequate for our planned application. The chambers were also read out with 25 pads with an area of 1 cm2 each. Here the pad multiplicity and the fractional charge versus distance from the pad hit, see Fig. 1, were measured. These results indicate that pad sizes as small as 1 cm2 can be used to readout RPCs with relatively small cross-talk. The geometrical acceptance of the chambers was investigated in a cosmic ray test stand which included a tracking system. The efficiency was observed to decrease in an area close to the fishing lines which are used to ensure the uniformity of the gas gap. The effect is consistent with a complete loss of efficiency over the area covered by the lines, but full efficiency, close to 100%, everywhere else. 1
Our group consists of Argonne National Laboratory, Boston University, University of Chicago and Fermilab. A similar effort is being pursued by a group at IHEP, Protvino, Russia. See V Ammosov’s contribution to these proceedings for more details.
Fig. 1. Measurement of the fractional charge as a function of the distance from the pad traversed by the cosmic ray.
5. Design of the electronic readout system The major challenge of the 1 m3 prototype is the design of the electronic readout system. With 40 layers of 1 m2 each, the number of readout channels is 400,000. The readout system is foreseen to consist of four parts: a front-end board containing the readout pads and an ASIC connected to 64 input channels; a data concentrator reading out twelve front-end ASICs; a data collection system based on VME; and a trigger system. The analog part of the ASIC contains a preamplifier, a shaper and a discriminator. The threshold of the discriminator is common to all 64 channels and can be set externally. The conceptual design of the digital part of the ASIC is shown in Fig. 2. Two modes of operation will be possible: (a) in a trigger-less operational mode each channel number, when hit, will be stored together with a time stamp; (b) in a triggered operational mode an external trigger controls the writing of the hit information to a readout buffer (FIFO). The data concentrator boards will be located on the side of the 1 m3 calorimeter section and will be based on the use of FPGAs. The data collection system will consist of two crates containing approximately 40 VME cards.
6. MC simulations The response of the 1 m3 prototype section with Scintillator, RPCs and GEMs as active medium
ARTICLE IN PRESS J. Repond / Nuclear Instruments and Methods in Physics Research A 533 (2004) 126–129
129
content and higher density than in a small gap filled with gas. These results point to a distinct advantage of gas detectors for the application of EFAs. Narrower showers will enhance the ability of the detector to disentangle the energy deposits of the various components of a hadronic jet.
7. Conclusions and outlook
Fig. 2. Conceptual design of the digital part of the front-end ASIC.
has been studied in MC simulations. A comparison of the lateral extend of electromagnetic showers shows that the energy deposits in gas detectors are more collimated than in scintillator. This can be understood as due to the insensitivity of gas detectors to Compton scattering of lowenergy-photons which may have traveled far from the core of the shower before converting. Scintillator due to its higher density has a higher probability of detecting these photons. Recently two independent studies showed that hadronic showers as well appear to be more collimated in a calorimeter read out with gas detectors. The reason for this effect is related to the higher probability of detecting neutrons, which again may have traveled far from the shower core, in scintillator with a higher hydrogen
In p order ffiffiffiffi to achieve a jet energy resolution of 30%/ E at the Linear Collider a new approach, called EFA, is proposed. The application of these algorithms requires a large tracking volume with high magnetic field and a calorimeter with very fine segmentation, of the order of 1 cm2 laterally and layer-by-layer in depth. The resulting large number of readout channels of the hadron calorimeter precludes an analog readout. MC simulations of single particle responses show that the energy resolution obtained with digital readout is comparable to the one obtained with analog readout. RPCs offer distinct advantages as active medium of such a hadron calorimeter. Various tests with prototype chambers read out with either a single pad or multiple pads, each with an area of 1 cm2 , have been performed. The chambers are found to satisfy all requirements on the active medium of a hadron calorimeter for the Linear Collider. An intermediate goal of building a 1 m3 prototype section to be tested in particle beams has been identified. A conceptual design of the electronic readout system for the test section has been developed.