Letter of Intent to Submit 2 Proposals to Test Production of Polarized Positrons with the SLAC 50 GeV Beam in the FFTB

Transcription

1 SLAC-PROPOSAL-E16x Sept. X, 2002 Letter of Intent to Submit 2 Proposals to Test Production of Polarized Positrons with the SLAC 50 GeV Beam in the FFTB Proposal I: Production and Polarimetry of Polarizied γ s with a Helical Undulator in SLAC s FFTB Proposal II: Production of Polarized e + with the Polarized γ s from Proposal I, and their Polarimetry P. Anthony SL, R. Arnold MA, A. Airapet DE, D. Amidei MU, S. Berridge UT, P. Bosted MA, B. Bugg UT, G. Chudakov JL, J. Clendenin SL, Y. Efremenko UT, T. Fieguth SL, C. Field SL, K. Finley FL, K. Floettmann DE, M. Fukuda TO, A. Jackson LB, V. Gharibyan DE, T. Handler UT, T. Hirose TO, I. Kamychkov UT, A. Kulikov SL, W. Lorenzon DE, T. Maruyama SL, K. McDonald PR, A. Mikhailichenko CO, K. Moffeit SL, T. Omori KE G. Petratos SL, R. Pitthan SL, M. Purohit SC, L. Rinolfi CE, S. Rock MA, P. Schuler DE, J. Sheppard SL, S. Spanier UT, D. Walz SL, A. Weidemann SC, M. Woods SL, B. Zihlmann DE CE CERN, CH-1211 Geneva 23, Switzerland CO Cornell University, Ithaca, NY DE DESY, D Hamburg, Germany KE KEK, Tsukuba-shi, Ibaraki, Japan FL Fermi National Accelerator Laboratory, Batavia, IL LB Lawrence Berkeley National Laboratory, Berkeley, CA MA University of Massachusetts, Amherst, MA MU University of Michigan, Ann Arbor MI NI Northern Illinois University, DeKalb, IL SL Stanford Linear Accelerator Center, Stanford, CA TO Tokyo Metropolitan University, Hachioji-shi, Tokyo, Japan WI University of Wisconsin, Madison, WI

2 Abstract 2

3 Contents 1 Introduction - Why Positron Polarization at All? 4 2 Conceptual Ways of Producing Polarized Gammas 5 3 Conceptual Ways of Producing Polarized Positrons 5 4 The Actual Choice of Wiggler 5 5 Conceptual Methods of Measuring Polarization for γ s and Positrons 5 6 The Actual Choice of Polarimetry for γ s 5 7 The Actual Choice of Polarimetry for Positrons 5 8 Request to SLAC 5 3

4 1 Introduction - Why Positron Polarization at All? A beam of polarized positrons will be of great importance for future linear electron-positron colliders and will allow increased precision in many important measurements. The most direct explanation is that a sizeable positron polarization is equivalent to an increase in the (effective) electron polarization [1], as shown in Figure 1. Already at the SLC/SLD the systematic error in the polarization measurement was the dominating error. With polarized positrons measured asymmetries of properties which are proportional to the polarization increase, and their errors decrease, allowing measurements with smaller systematic errors. 1 Effective Polarization vs. Positron Polarization P(e ) = 0.9 P effective P(e ) = e + Polarization Figure 1: The desirability of a polarization of the positron is visible in this Figure: the effective polarization goes up steeply even with modest positron polarization However, there has not yet been an experimental demonstration of a as of yet viable scheme to produce, and measure the polarization of γs and positrons for possible use at a high-energy Linear Collider. In the following we will briefly discuss several proposed schemes for polarized γs and positron production, and reason why the production with undulators seem to be the most realistic. We will also give the reasons why an effort to understand the polarimetry of gammas and positrons at the 10 s of MeV range, a terra incognita, should be understood now. But even without the polarization angle, there is good reason to look into positron production with externally produced γs. Traditionally positrons have been produced by impinging electrons on a heavy Z target, typically WRe alloys of many radiation lengths thickness. The electrons first produce bremsstrahlung γs in the target material, which then produce electron-positron pairs. The demands for several positrons per picosecond 4

5 pulse has pushed the material strength of the target to the limit. The SLC target has experienced failure; going this route for the NLC requires several targets in parallel and is all but impossible for the requirements of TESLA. If the γs needed are produced externally, be it by undulators or Compton back scattered lasers, and not in the target, the stress and radiation demands on the target are smaller by an order of magnitude. To boot, alight material like Ti or Carbon can be used. Historically the foundation of the theory of polarized positrons (and electrons) and how conceptually to measure their polarization has been laid in the 50 s and 60 s by The concepts of polarimetry has been laid in the early 60 by Schopper [2] and Ullmann et al. [3]. The basic concepts of positron production by existing γ s has been laid by Balakin and Mikhailichenko [4] and Amaldi and Pelligrini [5]. 2 Conceptual Ways of Producing Polarized Gammas 3 Conceptual Ways of Producing Polarized Positrons 4 The Actual Choice of Wiggler 5 Conceptual Methods of Measuring Polarization for γ s and Positrons 6 The Actual Choice of Polarimetry for γ s 7 The Actual Choice of Polarimetry for Positrons 8 Request to SLAC We request N weeks of checkout at 10HZ repetition rate at electrons, and M weeks of running at a linac repetition rate of 30 Hz. This assumes that the run takes place between PEP-II fills with an average data collection efficiency of 50%. The collaboration will provide many of the target and beam line components, such as... We request that SLAC acquire equipment needed to run in FFTB, such as... Acknowledgments We thank..., H. Wiedemann,..., and many others for useful discussions and other assistance. 5

6 References [1] American Linear Collider Working Group, Linear Collider Physics Resource Book for Snowmass 2001, Chapter 12, Report SLAC-R-570, 2001; [2] H.Schopper, Measurement of Circular Polarization of γ-rays, Nucl. Instr. Meth. 3 (1960) [3] J.D. Ullmann, H. Frauenfelder, H.J. Lipkin, and A. Rossi, Determination of Electron and Positron Helicity with Møller and Bhabha Scattering, Phys. Rev. 122 (1961) 536. [4] V.E. Balakin and A.A. Mikhailichenko, The Conversion System for Obtaining High Polarized Electrons and Positrons, Budker Institute of Nuclear Physics, Novosibirsk (1979). [5] U. Amaldi and C. Pellegrini, Energy Recovery in Superconducting Colliding Linacs Without Particle Recovery, Proc. 2 nd ICFA Workshop on Possibilities and Limitations of Accelerators and Detectors, Les Diablerets (1979). 6