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The forward region of the future ILC detector
Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341 www.elsevier.com/locate/npbps
The forward region of the future ILC detector A. Rosca (on behalf of the FCAL Collaboration)a a
Physics Faculty, West University of Timisoara, Bd. V. Parvan nr. 4, 300223 Timisoara, Romania In the very forward region of the future detector at the International Linear Collider (ILC) the following subsystems will be considered: a luminosity detector (LumiCal) for precise measurement of the Bhabha event rate; a beam calorimeter (BeamCal) and a beamstrahlung photon monitor (GamCal) for providing a fast feed-back in tuning the luminosity. BeamCal and GamCal will support also the determination of beam parameters. Both LumiCal and BeamCal will extend the angular coverage of the electromagnetic calorimeter at small polar angles. Detailed simulations are currently done to optimize the design of these detectors. Both LumiCal and BeamCal are planned as compact and highly segmented sandwich calorimeters. Tungsten disks of one X0 thickness are interspersed with sensor planes. For LumiCal, silicon pad sensors will be used. BeamCal will work in an extreme radiation environment, hence radiation hard sensors will be used. The sensors have to withstand a dose of up to several MGy per year. Several options for possible sensor materials have been investigated: polycrystalline and single crystal CVD diamond, as well as GaAs. Since the occupancy of LumiCal is relatively large and BeamCal must be readout after each bunch crossing, a fast readout electronics is necessary. The uncertainty of the luminosity measurement must be smaller than 10−3 for the running at high energy and about 10−4 in the GigaZ program focused to precision measurements of the Z boson. Systematic effects originating from hardware design as the accuracy of the sensor position, the calorimeter position and the dynamic range of the readout electronics, but also from physics processes have been studied. This work presents the current status of the research carried out to instrument this difficult region of the future ILC detector.
1. INTRODUCTION Although the Standard Model of elementary particles provides an extremely successful description of the fundamental constituents of matter and the forces among them, we know that it does have limitations. Gravitational interactions are not included; neutrinos are assumed to be massless but there is growing evidence that they have finite masses; the model contains many arbitrary parameters − masses, mixing angles, coupling constants, and one has to understand the origin of those parameters. Understanding the abundance of the dark matter and the asymmetry between matter and antimatter requires new physics, beyond the Standard Model. Such questions are addressed currently through precision measurements at present and future accelerators. There are strong theoretical reasons which point to the TeV scale as the arena for new phenomena. While the LHC proton − proton collider 0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.10.098
is the ideal instrument for exploring new physics phenomena in this new energy domain, an electron − positron collider at the TeV scale will have the capability to extend the discovery potential through high precision measurements. The International Linear Collider (ILC) is one of the two potential future electron − positron linear accelerators presently under development within a world-wide study group. The ILC is based on super-conducting accelerator technology and has been designed for the energy range 0.5 to 1 TeV. Other basic parameters needed for the planned physics program are the following: • A total integrated luminosity of 500 fb−1 within the first 4 years. • A polarization degree of better than 80% for the electron beam is mandatory. For the positron beam a degree of 50% polarization is useful, which should be reachable with the undulator positron source in the present
338
A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
ILC design. • Beam energy and polarization have to be stable and measurable with extremely high accuracy. Beam energy must be measured to the 100 ppm level to achieve the desired precision in the threshold energy scans and mass measurements. Polarization must be known to the 1000 ppm level for precision measurements at the Z and the 2000 ppm level for measurements at higher energies. • In addition to the standard e+ e− running, the ILC offers several options that can be realized with reasonable modifications if required by physics. For cost reasons only one IR is foreseen and two detectors will be pulled in and out of the beam lines. 2. THE DESIGN OF THE ILC DETECTORS Physics drives the performance criteria for the ILC detectors. The detector performance goals set by ILC physics include an unprecedented jet energy resolution of σE /E = 30%/ (E) , where E is the jet energy in GeV, a momentum resolution σ(1/pT ) = 5 × 10−5 , where pT is the momentum perpendicular to the beam axis measured in GeV/c and an impact parameter resolution of σrφ ≈ σz ≈ 5+10/(p sin3/2 θ) μm, where p is the momentum of the charged track in GeV/c and θ is the polar angle with respect to the beam axis. The jet energy resolution, which is a factor of 2 better than SLC and LEP calorimeters, is required in order to clearly separate Z and W bosons in their hadronic decays. The momentum resolution required for the tracker is a factor of 10 better than LEP experiments or a factor of 3 better than at LHC. The impact parameter resolution, which is a factor of 3 better than what SLD achieved, allows the flavor of a jet to be tagged with high efficiency and a clean discrimination between charm, bottom, and the light quarks. In addition, the detector must have the capability to isolate the bunch crossing, in which the recorded collision event has occurred. In the ILC nominal
beam parameters set, there are five trains of 2820 bunches, separated by 307 ns, every second. An ILC detector represents an enhancement of the classical e+ e− detectors. The main components starting from the IP are the vertex detector, a large main tracker, calorimeters based on the Particle Flow Algorithm (PFA) concept which need to be embedded inside a large superconducting solenoid coil and outside a muon system. The detector will be equipped with an instrumented forward region [1] defined as the area covering the polar angles between about 4 to 90 mrad. The physics mission in the forward region is a precise measurement of the luminosity using forward Bhabha pairs with a dedicated electromagnetic calorimeter covering the polar angle between about 40 to 90 mrad. A second goal is to extend the calorimeters hermeticity into the forward region for physics searches. For that purpose another device will be placed at polar angles between 4 to 40 mrad. Beamdiagnostics can be done using this calorimeter, assisted by an additional system to exploit the beamstrahlung photons. 3. THE CHALLENGES IN THE FORWARD REGION The integrated luminosity will be determined counting the total number of Bhabha events produced in the acceptance region of the luminosity calorimeter, and the corresponding cross section. The cross section of the Bhabha process is precisely calculated in QED and it is sufficiently large to deliver high statistics for a measurement with the required precision, smaller than 10−3 for the running at high energy and about 10−4 in the GigaZ program. Requirements on the detector precision and alignment have been studied [2] and it was found that the inner radius has to be controlled to better than 10 μm, the distance between the calorimeters to better than 600 μm and the accuracy of the radial beam position should be greater than 1 mm. The luminosity calorimeter in the current design [3] is a sandwich calorimeter made of 30 layers of W and Si sensors. It will be positioned at 2.27 m from the IP, around the beam pipe,
A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
covering larger polar angles, outside the reach of the beamstrahlung pairs. Si sensors will be positioned with micron accuracy. Electrons and positrons originating from beamstrahlung photon conversions deposit energy in BeamCal, about few TeV per BX. The total energy and the distribution of the energy can be measured and used to assist in tuning the beams on a bunch-to-bunch basis [4]. The GamCal, measuring the energy of the Beamstrahlung photons can help to improve the instantaneous luminosity monitoring. It has been found that the ratio between the Eγ /Ee+ e− is proportional to the luminosity. However, the price to be paid is that the material of the sensors for these devices must be very radiation hard since doses of 10 MGY per year are expected. The measurement of the e+ e− → sleptons in the presence of the two photon background gives the criteria for the detector performance for hermiticity. A measurement of the local depositions from a single high energy electron to the lowest possible angle is desirable to veto two-photon processes. This can be done by exploiting the longitudinal profile of the electromagnetic shower. In the current design the BeamCal will be a sandwich calorimeter made of 30 layers of W and radiation hard sensors. Fine granularity and small radiation radius of about 1 cm is necessary to identify localized depositions from high energy electrons on top of the broad energy spread from beamstrahlung pairs. Therefore each layer will be divided into 16 rings and 8 sectors making about 3000 pads in total which need to be readout one by one. 4. SENSOR R&D We investigated several candidate materials for the sensors of the beam calorimeter, such as pCVD and sCVD diamonds, GaAs and radiation hard silicon. Diamond is a very attractive material and its radiation hardness has been investigated extensively also for the LHC beam monitors. We studied sensor performance as a function of the absorbed dose in a 10 MeV electron beam at the S-Dalinac linear accelerator in Darmstadt. The
339
Figure 1. Experimental set-up for the sensor irradiation.
beam current was set between 10-50 nA corresponding to a rate of 60-300 kGY/h. The experimental setup is presented in Fig. 1. The electron beam comes from the left side and crosses the sensor placed in a box, after it was shaped by a copper collimator situated in front of the sensor box. The beam is then stopped on a Faraday cup made of copper. Measuring the current in the Faraday cup we can calculate the absorbed dose by the sensor. Every hour the Charge Collection Distance (CCD) of the sensor has been measured. It is shown in Fig. 2 as a function of the absorbed dose. Both pCVD diamond samples show an initial increase of the CCD by a factor of 2 due to pumping. For doses of 5−6 MGy we find a drop of the CCD value of about 50% while the leakage current is stable [5]. After the irradiation the two samples have been illuminated with UV light. This way the trapped charge carriers are released and the samples go back to their depumped state. It can be seen in Fig. 2 that the value of the CCD has decreased with respect to the value in the initial state.
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A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
Figure 2. Results from diamond irradiation: CCD as a function of the absorbed dose for two pCVD diamond samples.
5. FRONT END ELECTRONICS IN THE FORWARD REGION The requirements of the FEE depends on the accelerator beam structure. ILC will deliver bunches separated by 307 ns in trains of about 3000 bunches. 5 such trains will collide per second. High occupancy in the forward calorimeters impose a readout of the pads after each BX (or few BX). The feedback signal for the beam tuning should have low latency, of the order of μs. The solution for LumiCal [6] is presented in Fig. 3. This is an ASIC made in AMS 0.35 μm technology. 1 FE ASIC will contain 32-64 channels. It will shape, amplify and sample-and-hold the amplitudes of the signal. A 10 bit digitization is enough for the luminosity measurement. For the BeamCal readout data need to be readout at 10 bits for physics purposes. A low latency output signal for beam diagnostics is obtained by summing of the channels per chip and reading at 8 bit after each BX. It is foreseen to be done in TSMS CMOS 0.18 μm technology.
Figure 3. Block diagram of the LumiCal electronics.
6. SUMMARY The FCAL Collaboration develops calorimeters in the very forward region of the ILC detectors. These devices will provide precise luminosity measurement (LumiCal), identification of single high energy electrons to the lowest possible angle relevant for new physics searches (BeamCal), beam diagnostics (BeamCal) and luminosity monitoring (BeamCal, GamCal). The LumiCal must be positioned extremely precisely. Radiation hard sensors are essential for the BeamCal. Electronics for all detectors should be fast (about 100 ns), low power and radiation hard.
ACKNOWLEDGEMENTS The author would like to thank the organizers for the invitation to give this presentation and their hospitality, the FCAL Collaboration for a collaborative spirit and the DESY laboratory for support.
A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
REFERENCES 1. H. Abramowicz et al., Instrumentation of the very forward region of a linear collider detector, IEEE Trans. Nucl. Sci. vol. 51, (2004) 2983. 2. Achim Stahl, Luminosity measurement via Bhabha scattering: precision requirements for the luminosity calorimeter, LC-DET-2005004, 2005. 3. H. Abramowicz, R. Ingbir, S. Kananov and A. Levy, A luminosity detector for the International Linear Collider, LC-DET-2007-006, 2007. 4. C. Grah and A. Sapronov, Fast luminosity measurement and beam parameter determination, EUROTeV-Report-2007-006, 2007. 5. C. Grah et al., Polycrystalline CVD Diamonds for the Beam Calorimeter of the ILC, Proc. of IEEE NSS/MIC (2007), Honolulu, Hawai, 2007. 6. M. Idzik et al., Status of VFCAL, EUDETMemo-2008-01, 2008.
341
The forward region of the future ILC detector A. Rosca (on behalf of the FCAL Collaboration)a a
Physics Faculty, West University of Timisoara, Bd. V. Parvan nr. 4, 300223 Timisoara, Romania In the very forward region of the future detector at the International Linear Collider (ILC) the following subsystems will be considered: a luminosity detector (LumiCal) for precise measurement of the Bhabha event rate; a beam calorimeter (BeamCal) and a beamstrahlung photon monitor (GamCal) for providing a fast feed-back in tuning the luminosity. BeamCal and GamCal will support also the determination of beam parameters. Both LumiCal and BeamCal will extend the angular coverage of the electromagnetic calorimeter at small polar angles. Detailed simulations are currently done to optimize the design of these detectors. Both LumiCal and BeamCal are planned as compact and highly segmented sandwich calorimeters. Tungsten disks of one X0 thickness are interspersed with sensor planes. For LumiCal, silicon pad sensors will be used. BeamCal will work in an extreme radiation environment, hence radiation hard sensors will be used. The sensors have to withstand a dose of up to several MGy per year. Several options for possible sensor materials have been investigated: polycrystalline and single crystal CVD diamond, as well as GaAs. Since the occupancy of LumiCal is relatively large and BeamCal must be readout after each bunch crossing, a fast readout electronics is necessary. The uncertainty of the luminosity measurement must be smaller than 10−3 for the running at high energy and about 10−4 in the GigaZ program focused to precision measurements of the Z boson. Systematic effects originating from hardware design as the accuracy of the sensor position, the calorimeter position and the dynamic range of the readout electronics, but also from physics processes have been studied. This work presents the current status of the research carried out to instrument this difficult region of the future ILC detector.
1. INTRODUCTION Although the Standard Model of elementary particles provides an extremely successful description of the fundamental constituents of matter and the forces among them, we know that it does have limitations. Gravitational interactions are not included; neutrinos are assumed to be massless but there is growing evidence that they have finite masses; the model contains many arbitrary parameters − masses, mixing angles, coupling constants, and one has to understand the origin of those parameters. Understanding the abundance of the dark matter and the asymmetry between matter and antimatter requires new physics, beyond the Standard Model. Such questions are addressed currently through precision measurements at present and future accelerators. There are strong theoretical reasons which point to the TeV scale as the arena for new phenomena. While the LHC proton − proton collider 0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.10.098
is the ideal instrument for exploring new physics phenomena in this new energy domain, an electron − positron collider at the TeV scale will have the capability to extend the discovery potential through high precision measurements. The International Linear Collider (ILC) is one of the two potential future electron − positron linear accelerators presently under development within a world-wide study group. The ILC is based on super-conducting accelerator technology and has been designed for the energy range 0.5 to 1 TeV. Other basic parameters needed for the planned physics program are the following: • A total integrated luminosity of 500 fb−1 within the first 4 years. • A polarization degree of better than 80% for the electron beam is mandatory. For the positron beam a degree of 50% polarization is useful, which should be reachable with the undulator positron source in the present
338
A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
ILC design. • Beam energy and polarization have to be stable and measurable with extremely high accuracy. Beam energy must be measured to the 100 ppm level to achieve the desired precision in the threshold energy scans and mass measurements. Polarization must be known to the 1000 ppm level for precision measurements at the Z and the 2000 ppm level for measurements at higher energies. • In addition to the standard e+ e− running, the ILC offers several options that can be realized with reasonable modifications if required by physics. For cost reasons only one IR is foreseen and two detectors will be pulled in and out of the beam lines. 2. THE DESIGN OF THE ILC DETECTORS Physics drives the performance criteria for the ILC detectors. The detector performance goals set by ILC physics include an unprecedented jet energy resolution of σE /E = 30%/ (E) , where E is the jet energy in GeV, a momentum resolution σ(1/pT ) = 5 × 10−5 , where pT is the momentum perpendicular to the beam axis measured in GeV/c and an impact parameter resolution of σrφ ≈ σz ≈ 5+10/(p sin3/2 θ) μm, where p is the momentum of the charged track in GeV/c and θ is the polar angle with respect to the beam axis. The jet energy resolution, which is a factor of 2 better than SLC and LEP calorimeters, is required in order to clearly separate Z and W bosons in their hadronic decays. The momentum resolution required for the tracker is a factor of 10 better than LEP experiments or a factor of 3 better than at LHC. The impact parameter resolution, which is a factor of 3 better than what SLD achieved, allows the flavor of a jet to be tagged with high efficiency and a clean discrimination between charm, bottom, and the light quarks. In addition, the detector must have the capability to isolate the bunch crossing, in which the recorded collision event has occurred. In the ILC nominal
beam parameters set, there are five trains of 2820 bunches, separated by 307 ns, every second. An ILC detector represents an enhancement of the classical e+ e− detectors. The main components starting from the IP are the vertex detector, a large main tracker, calorimeters based on the Particle Flow Algorithm (PFA) concept which need to be embedded inside a large superconducting solenoid coil and outside a muon system. The detector will be equipped with an instrumented forward region [1] defined as the area covering the polar angles between about 4 to 90 mrad. The physics mission in the forward region is a precise measurement of the luminosity using forward Bhabha pairs with a dedicated electromagnetic calorimeter covering the polar angle between about 40 to 90 mrad. A second goal is to extend the calorimeters hermeticity into the forward region for physics searches. For that purpose another device will be placed at polar angles between 4 to 40 mrad. Beamdiagnostics can be done using this calorimeter, assisted by an additional system to exploit the beamstrahlung photons. 3. THE CHALLENGES IN THE FORWARD REGION The integrated luminosity will be determined counting the total number of Bhabha events produced in the acceptance region of the luminosity calorimeter, and the corresponding cross section. The cross section of the Bhabha process is precisely calculated in QED and it is sufficiently large to deliver high statistics for a measurement with the required precision, smaller than 10−3 for the running at high energy and about 10−4 in the GigaZ program. Requirements on the detector precision and alignment have been studied [2] and it was found that the inner radius has to be controlled to better than 10 μm, the distance between the calorimeters to better than 600 μm and the accuracy of the radial beam position should be greater than 1 mm. The luminosity calorimeter in the current design [3] is a sandwich calorimeter made of 30 layers of W and Si sensors. It will be positioned at 2.27 m from the IP, around the beam pipe,
A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
covering larger polar angles, outside the reach of the beamstrahlung pairs. Si sensors will be positioned with micron accuracy. Electrons and positrons originating from beamstrahlung photon conversions deposit energy in BeamCal, about few TeV per BX. The total energy and the distribution of the energy can be measured and used to assist in tuning the beams on a bunch-to-bunch basis [4]. The GamCal, measuring the energy of the Beamstrahlung photons can help to improve the instantaneous luminosity monitoring. It has been found that the ratio between the Eγ /Ee+ e− is proportional to the luminosity. However, the price to be paid is that the material of the sensors for these devices must be very radiation hard since doses of 10 MGY per year are expected. The measurement of the e+ e− → sleptons in the presence of the two photon background gives the criteria for the detector performance for hermiticity. A measurement of the local depositions from a single high energy electron to the lowest possible angle is desirable to veto two-photon processes. This can be done by exploiting the longitudinal profile of the electromagnetic shower. In the current design the BeamCal will be a sandwich calorimeter made of 30 layers of W and radiation hard sensors. Fine granularity and small radiation radius of about 1 cm is necessary to identify localized depositions from high energy electrons on top of the broad energy spread from beamstrahlung pairs. Therefore each layer will be divided into 16 rings and 8 sectors making about 3000 pads in total which need to be readout one by one. 4. SENSOR R&D We investigated several candidate materials for the sensors of the beam calorimeter, such as pCVD and sCVD diamonds, GaAs and radiation hard silicon. Diamond is a very attractive material and its radiation hardness has been investigated extensively also for the LHC beam monitors. We studied sensor performance as a function of the absorbed dose in a 10 MeV electron beam at the S-Dalinac linear accelerator in Darmstadt. The
339
Figure 1. Experimental set-up for the sensor irradiation.
beam current was set between 10-50 nA corresponding to a rate of 60-300 kGY/h. The experimental setup is presented in Fig. 1. The electron beam comes from the left side and crosses the sensor placed in a box, after it was shaped by a copper collimator situated in front of the sensor box. The beam is then stopped on a Faraday cup made of copper. Measuring the current in the Faraday cup we can calculate the absorbed dose by the sensor. Every hour the Charge Collection Distance (CCD) of the sensor has been measured. It is shown in Fig. 2 as a function of the absorbed dose. Both pCVD diamond samples show an initial increase of the CCD by a factor of 2 due to pumping. For doses of 5−6 MGy we find a drop of the CCD value of about 50% while the leakage current is stable [5]. After the irradiation the two samples have been illuminated with UV light. This way the trapped charge carriers are released and the samples go back to their depumped state. It can be seen in Fig. 2 that the value of the CCD has decreased with respect to the value in the initial state.
340
A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
Figure 2. Results from diamond irradiation: CCD as a function of the absorbed dose for two pCVD diamond samples.
5. FRONT END ELECTRONICS IN THE FORWARD REGION The requirements of the FEE depends on the accelerator beam structure. ILC will deliver bunches separated by 307 ns in trains of about 3000 bunches. 5 such trains will collide per second. High occupancy in the forward calorimeters impose a readout of the pads after each BX (or few BX). The feedback signal for the beam tuning should have low latency, of the order of μs. The solution for LumiCal [6] is presented in Fig. 3. This is an ASIC made in AMS 0.35 μm technology. 1 FE ASIC will contain 32-64 channels. It will shape, amplify and sample-and-hold the amplitudes of the signal. A 10 bit digitization is enough for the luminosity measurement. For the BeamCal readout data need to be readout at 10 bits for physics purposes. A low latency output signal for beam diagnostics is obtained by summing of the channels per chip and reading at 8 bit after each BX. It is foreseen to be done in TSMS CMOS 0.18 μm technology.
Figure 3. Block diagram of the LumiCal electronics.
6. SUMMARY The FCAL Collaboration develops calorimeters in the very forward region of the ILC detectors. These devices will provide precise luminosity measurement (LumiCal), identification of single high energy electrons to the lowest possible angle relevant for new physics searches (BeamCal), beam diagnostics (BeamCal) and luminosity monitoring (BeamCal, GamCal). The LumiCal must be positioned extremely precisely. Radiation hard sensors are essential for the BeamCal. Electronics for all detectors should be fast (about 100 ns), low power and radiation hard.
ACKNOWLEDGEMENTS The author would like to thank the organizers for the invitation to give this presentation and their hospitality, the FCAL Collaboration for a collaborative spirit and the DESY laboratory for support.
A. Rosca / Nuclear Physics B (Proc. Suppl.) 197 (2009) 337–341
REFERENCES 1. H. Abramowicz et al., Instrumentation of the very forward region of a linear collider detector, IEEE Trans. Nucl. Sci. vol. 51, (2004) 2983. 2. Achim Stahl, Luminosity measurement via Bhabha scattering: precision requirements for the luminosity calorimeter, LC-DET-2005004, 2005. 3. H. Abramowicz, R. Ingbir, S. Kananov and A. Levy, A luminosity detector for the International Linear Collider, LC-DET-2007-006, 2007. 4. C. Grah and A. Sapronov, Fast luminosity measurement and beam parameter determination, EUROTeV-Report-2007-006, 2007. 5. C. Grah et al., Polycrystalline CVD Diamonds for the Beam Calorimeter of the ILC, Proc. of IEEE NSS/MIC (2007), Honolulu, Hawai, 2007. 6. M. Idzik et al., Status of VFCAL, EUDETMemo-2008-01, 2008.
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