EPAC June 2003 The EPAC June 2003 Questions 1. Clarify the Motivation for the Proposal. 2. How to ensure the e+ polarimeter works right away? 3. What is the Analyzing Power and its Uncertainty from Simulation? 4. What Precision in Polarimetry is Required? 5. What are the Benefits and Drawbacks of Two Shorter Runs vs. One Single Run of the Same Total duration? 1

EPAC June Clarify the Motivation for the Proposal (I) The reason for doing E-166 is to demonstrate that an undulator-based polarized positron source will produce a positron beam with sufficient polarization to justify the performance expectations and cost (of the polarized positron source) for the linear collider. Performance expectations are for positron polarization of about 60% and full positron intensity. The incremental cost of building a polarized positron source for the NLC/JLC or CLIC is estimated to be approximately 100M$ (it is significantly less to upgrade the TESLA baseline design). 2

EPAC June Clarify the Motivation for the Proposal (II) The concepts of the polarized positron source are all well understood but have never been brought together in a system test. E-166 puts the essential features of the polarized source together: an undulator to produce polarized photons, a conversion target, and polarimetry sufficient to characterize the positron polarization to 5%. A successful E-166 demonstration of polarized production is necessary and sufficient to move forward the acceptance of this concept within the accelerator community. 3

EPAC June How to ensure the e+ polarimeter works right away? There are 3 major components of the polarimeter: –(a) spectrometer –(b) analyzer magnet –(c) CsI calorimeter The spectrometer and the analyzer magnet will be built and delivered to SLAC in the summer of They will then be subjected to functional tests for about two months before installation and system integration in FFTB. 4

EPAC June How to ensure the e+ polarimeter works right away? The Detector components exist already. We will check the parts carefully on the component level in general. In particular: – for the CsI crystals, build on the extensive expertise and work of the Babar Collaboration for a dynamic calibration of the CsI crystals between a few MeV and the GeV range. –For the SiW calorimeter build on the extensive expertise and work of the Tennessee group (E-144, SLD) and their collaborators. Parasitic work in the FFTB, already started, on observing and controlling beam produced shower background will continue. 5

EPAC June What is the Analyzing Power and its Uncertainty from Simulation? (I) The analyzing power of the e + polarimeter is shown as a function of e + beam energy in Fig. 17 on page 23, and in Table 13 on page 50 in the E-166 proposal. It ranges from –(18.6  0.9)% at 3 MeV to –(13.9  0.6)% at 10 MeV. The indicated errors correspond to the finite simulation statistics of 10 Million events for each of the two spin configurations, or roughly 30 minutes of computing time. This statistical error of the simulation can be made negligible with extra computing time. 6

EPAC June What is the Analyzing Power and its Uncertainty from Simulation? (II) There will of course also be systematic errors associated with certain simplifications and approximations of the physical processes and the description of the polarimeter hardware. However, these should all be far smaller than the presumed dominating systematic error of DP/P = 0.05 associated with the uncertainty in the magnetization of the iron. 7

EPAC June What is the Analyzing Power and its Uncertainty from Simulation? (III) 8

EPAC June What is the Analyzing Power and its Uncertainty from Simulation? (IV) 9

EPAC June What Precision in Polarimetry is Required? The E-166 positron polarization is expected to be in the 40-70%. A precision of 5% is sufficient to make a meaningful measurement of this level of polarization. While a measurement with a relative accuracy of 10% is arguably adequate, 5 % matches the expected systematic error from the iron magnetization. 10

EPAC June What are the Benefits and Drawbacks of Two Shorter Runs vs. One Single Run of the Same Total duration (I)? Benefits: First run reveals problems which potentially take time to remedy (poorly understood result, inadequate diagnostics, broken equipment, insufficient shielding, …). Time between runs is used to make adjustments to better ensure ultimate success; success defined as a well understood measurement result that is in full agreement with modeling/simulations. Drawbacks: The time to tune-up the beam, reduce backgrounds, and commission equipment can use all available time in a short run. Beam will be tuned during the initial 3 weeks of installation, checkout and preliminary operations. A short run has the risk of being dominated by the tuning and start up, leaving little or no time to take measurements. 11

EPAC June 2003 Benefits and Drawbacks (II) A less defensible issue is a possible question of limited travel funds available within the collaboration for multiple trips to SLAC. This can be negotiated within the collaboration. There is the issue of possible interference with other experiments. This is not the primary responsibility of E-166 and which can be ameliorated in the scheduling process. 12