"LC-TPC: Large Collaborative Projects"

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Executive Summary

A Time Projection Chamber (TPC) has been chosen as the central tracking device for two of the current detector concepts at the International Linear Collider. The LC-TPC group is carrying out a comprehensive R&D program to develop the technology and prove the feasibility of a high-performance TPC required for this application. The new Micro-Pattern Gas Detector (MPGD) technologies, Gas Electron Multiplier(GEM) or Micromegas(MM), are attractive candidates for the gas-amplification because better precision and granularity may be achieved than in past TPCs. Extensive testing using GEM and MM is being pursued with the results being compared with each other and with the multi-wire proportional chamber (MWPC) technology used in TPCs up to the present. In addition, the proof-of-principle of CMOS readout techniques is being studied with both GEM and MM; if successful this will be a candidate for a final TPC. In addition to optimization of the gas-amplification, other issues including minimizing endplate material alignment and calibration most be understood before a TPC can meet the goals of the ILC. The R&D work is proceeding in three phases: 1) demonstration phase using small prototypes; 2) consolidation phase consisting of the building of a Large Prototype TPC with GEM and MM using both standard and CMOS readout techniques; 3) design phase which will profit from the experience gained in the first two phases. Presently (2005) phase 1) is under way and phase 2) is starting.

This project involves groups from institutions in all regions as listed below. To meet these goals the institutes listed on this research statement are working together, sharing information and experience in the process of developing a TPC for the linear collider, and of providing common infrastructure and tools to facilitate these studies. The distribution of efforts among these institutions is given in the text.

America Canada: Carleton, Montreal, Victoria. USA: Cornell, Indiana, LBNL, MIT, Purdue, Yale.

Asia China: Tsinghua. Japan: Chiba, Hiroshima, KEK, Kinki U Osaka, Saga, Kogakuin U Tokyo, Tokyo UAT, Tokyo ICEPP, NRICP Tokyo, Tsukuba. Philippines: Minadamo SU-IIT.

Europe France: LAL Orsay, IPN Orsay, CEA Saclay. Germany: RWTH Aachen, DESY Hamburg, Freiburg, Hamburg, Karlsruhe, MPI-Munich, Rostock. Netherlands: NIKHEF. Poland: UMM Krakow. Russia: BINP Novosibirsk, PNPI St.Petersburg. Sweden: Lund. Switzerland: CERN.

Goals and addressing the needs of the detector concepts

The goal of the LC-TPC group is to develop a TPC to serve as the tracking device of a detector meeting the demands of the ILC physics program. A detector at the International Linear Collider (ILC) will combine a tracking system of high precision with a calorimeter system of very high granularity. This detector will measure charged tracks with excellent accuracy, typically surpassing the precision of previously built detectors at LEP, the Tevatron, HERA or the LHC by a factor of 10. At the same time this detector must be optimized for the reconstruction of multi-jet final states stressing the jet energy resolution and the reconstruction of individual particles in jets. For the latter, the efficiency and reliability in reconstructing charged tracks are more important than precision. A TPC has been chosen as the central tracking device for two of the current detector concepts: the Global LC Detector concept (GLD) and the Large Detector Concept (LDC). These concepts each have a tracking system consisting of a large TPC combined with silicon detectors for vertexing, intermediate and external tracking. The GLD and LDC concepts differ mainly in their calorimetry. Arguments for a TPC as main tracker are:

• The tracks can be measured with a large number of (r/phi,z) space points, so that the tracking is continuous and the efficiency remains close to 100% for high multiplicity jets and in presence of large backgrounds.
• It presents a minimum of material to particles crossing it. This is important for getting the best possible performance from the electromagnetic calorimeter, and to minimize the effects from the ~10^3 beamstrahlung photons per bunch crossing which traverse the detector.
• The comparatively moderate point and double-hit resolution are compensated by the continuous tracking and the large volume which can be filled with fine granularity.
• The timing is precise to about 1-2 ns (corresponding to 50 micrometer/ns drift speed of tracks hooked up to the Si detector with 25 micrometer strip-pitch), so that tracks from different bunch crossings can readily be distinguished via time stamping.
• It is well suited for a large magnetic field since the electrons drift parallel to the B-field, which in turn improves the two-hit resolution by compressing the transverse diffusion of the drifting electrons.
• Non-pointing tracks, e.g. from the decays of neutral particles, are an important addition to the particle-flow measurement and help in the reconstruction of physics signatures in many scenarios beyond the standard model.
• The TPC gives good particle identification via the specific energy loss, dE/dx, which is important for many physics analyses, electron-identification and particle-flow applications.
• A TPC is easy to maintain because, when designed appropriately, an endplate readout chamber can readily be accessed or exchanged if it is having problems.

Several issues must be addressed in developing a TPC to meet the requirements of the ILC physics program. TPCs have been used in a number of large collider experiments in the past and have performed well. However, these TPCs were read out using multi-wire proportional chambers (MWPCs). The thrust of our present R&D is to develop a TPC based on novel micro-pattern gas detectors (MPGDs), which promise better point and two-track resolution than possible with the MWPC readout and to be more robust in high backgrounds.

To obtain good momentum resolution and to surpress backgrounds near the vertex, the TPC must operate in a strong magnetic field. This magnetic field must be mapped and understood to O(10^-5) to minimize corrections for the distortion of drifting electrons.

There are two features of a TPC which must be compensated by proper design work as part of our R&D program. First, the readout endplanes and electronics present a significant amount of material to the interaction products in the forward direction. The goal is to keep this below 30% X_0. Second, the 50 microsecond memory time of the the readout (due to the TPC drift length) integrates over background and signal events from 160 ILC bunch crossings at 500 GeV. The latter is being compensated by designing for the finest possible granularity: the sensitive volume is envisaged to consist of at least 1.5 x 10^6 pads and 10^3 time buckets per pad, giving more 1.5 x 10^9 3D-electronic readout voxels (two orders of magnitude better than at LEP). In the case that CMOS techniques are ripe, the granularity could be one to four orders of magnitude larger, depending on the design. The occupancy of the TPC predicted by present simulations is about 0.3% due to backgrounds from beam-beam effects and gamma-gamma interactions based. The TPC will be designed to cope with a factor ~50 higher backgrounds

Results to date

Systems under study at the moment are Micromegas meshes (MM) and Gas Electron Multiplier (GEM) foils. Both operate in a gaseous atmosphere and are based on the avalanche amplification of the primary produced electrons. The gas amplification occurs in the large electric fields in the MPGD microscopic structures with sizes of the order of 50 micrometers. MPGDs lend themselves naturally to the intra-train un-gated operation at the ILC, because, with proper voltage settings, they display a significant suppression of the number of back-drifting ions.

Small test TPC have been operated using GEM and MM gas-amplification with results presented at the international and regional ILC workshops.

• The Aachen group has studied GEM gas-amplification and suppression of back-drifting ions. In addition, the Aachen group is studying techniques for reducing the material in the field cage.
• The LBNL, Orsay amd Sacly groups have studied MM gas-amplification in a magnetic field.
• The Carleton, Orsay and Saclay groups have studied GEM and MM gas-amplification with a resistive coating on the pad structure to disperse the signal to the optimum size.
• The Cornell and Purdue groups have studied MWPC and GEM gas-amplification, with plans to include MM, in zero magnetic field. There are plans to make comparative measurements of ion back-drift.
• The DESY group…
• The NIKHEF group is studying CMOS readout techniques using GEM and MM gas-amplification.
• The MPI and Asian groups are carrying out a systematic comparison of MPWC, GEM and MM gas-amplification in a ?T magnetic field.
• The Victoria group has studied GEM gas-amplification is a ?T magnetic field.
• others
• others
• and others
• including every institution listed

Plans

The TPC R&D work is taking place in three phases:

(1) Small prototying

Subgroups have been performing studies using small TPC prototypes, with sensitive pad sizes ~ 10-20 mm^2 and pad planes ~ 100 cm^2, in magnetic fields up to 5T, for a few years. These studies will continue for another year or so. During these studies much experience has been gained in the GEM and MM technologies. Most of the work has been done with cosmic rays using Star or Aleph electronics. At this time, serious test-beam studies have started. There are several plans for this phase listed below.

• Operate MWPC and MPGDs in small test TPCs in magnetic fields and compare to prove that MPGDs can be used reliably in the final LC TPC.
• Investigate the charge transfer properties in MPGD structures and understand the resulting ion backflow.
• Study the behaviour of GEM and MM with and without magnetic fields.
• Study the achievable resolution of a MPGD-TPC for different gas mixtures and carry out ageing tests.

(2) Large prototype

The large prototype study, currently in the design stage, will continue for another four years. During this phase, the subgroups will consolidate to build a TPC prototype with a diameter of 80 cm to be operated in a B-field of 1T. We will use the large prototype will test the manufacture of large chambers with GEM and MM technologies and the feasibility of CMOS readout techniques. Envisaged is also the manufacture of prototype electronics better matched to these new technologies. This phase will gain experience for building a final TPC and will allow the final choice as to which technology is appropriate for the LC. Studies of using the large prototype will take advantage of a detector R&D facility to be built in Desy, which will be used by vertex, TPC and calorimeter groups. Other plans during this phase are listed below.

• Study ways to reduce the area occupied per channel of the readout electronics by a factor of at least 5 compared to the Alice experiment.
• Study alternatives for minimizing the endplate mechanical thickness, such as the use of power switching of the electronics to minimize power and cooling.
• Investigate the possibility of using Si-readout techniques (CMOS) or other new ideas for handling the large number of channels.
• Investigate ways of building a thin field cage which will meet the requirements at the ILC.
• Devise strategies for robust performance.
• Pursue software and simulation developments needed for understanding prototype performance.

Distribution of responsibilities for the components of the large prototype is being organized at present (2005).

(3) Design and construction phase.

Based on the experience gained, the the TPC for an ILC experiment will be designed, built and commissioned. This will take another four years, so that altogether the final TPC will be ready for installation in the ILC detector by 2015.

For more details of the R&D program see LC Note LC-TPC-2001 at http ://www-flc.desy.de/lcnotes/ .

Critical Items and funding

Timely construction of the EUDET test facility at DESY is a critical item for the large prototype phase of this project. Beyond that, all groups are in need of increased funding for all phases. This is particularly true of the large prototype phase. It is planned that EUDET will provide the field cage as well as the magnet and other infrastructure. However, other critical, costly components, such as the endplates and and read-out modules must be provided by the contributing institutions.

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Please address the following questions in your statement.

• What are the goals of this R&D project. How does this R&D project address the needs of one or more of the detector concepts?

• If there are multiple institutions participating in this project, please describe the distribution of responsibilities.

• Are there significant recent results?

• What are the plans for the near future(about 1 year)? What are the plans on a time scale of 2 to 3 years?

• Are there critical items that must be addressed before significant results can be obtained from this project?

• Is the support for this project sufficient? Are there significant improvements that could be made with additional support?

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Topic revision: 22 Dec 2005, DanPeterson

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