The Linear Collider TPC: Revised Magnetic-field Requirements

A Time Projection Chamber is being developed as central tracker for a detector at the International Linear Collider. The LCTPC has to perform better and to face more complicated event topologies and higher backgrounds than TPC's at previous e+e- colliders: this puts additional requirements on the overall system design. At present the technical design of the ILC machine is considering the use of special magnetic-field elements to guide the beams as they pass along the axis of the detector. Since the B-fields from such elements affect the TPC volume, the present note revisits the requirements on the magnetic field for the LCTPC. It is proposed to replace the original requirement on the homogeneity of the B-field by one on the accuracy of the B-field map. The purpose of this study is to achieve the best possible momentum resolution for the TPC, and it is demonstrated that the goal for the Aleph B-map accuracy is also sufficient for the LCTPC. Simulations should be made to cross-check these ideas, and suggestions are made how to continue these studies.