E-166: Polarized Positron Source Studies at SLAC

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A beam of polarized positrons would be of great importance for future linear electron-positron colliders and would allow increased precision in many important measurements. However, there has not yet been an experimental demonstration of a scheme to produce polarized positrons of possible use for high-energy linear colliders.

A concept to generate polarized positrons from polarized undulator-generated photons had been proposed in the 1980ies by Balakin and Mikhailichenko. Twenty years later, Rainer Pitthan and John Sheppard proposed that an experiment should be done at SLAC's FFTB beamline, to demonstrate the feasibility of this concept in view of its application to a future linear collider, ILC. Subsequently, a collaboration ('E-166'), was formed with spokespersons Kirk McDonald of Princeton University, and John Sheppard of SLAC, to perform this experiment. In this experiment the 50 GeV beam of the FFTB (Final Focus Test Beam), passes through a ~ 1m-long, ~1mm-period helical undulator to produce ~10 MeV polarized gammas. The polarized gammas are used in turn on a "thin' (=<0.5 radlength) Ti target to make polarized positrons by pair-production.

In this experiment, one measures the circular polarization of the photons generated by the undulator (by 'transmission Compton polarimetry')and then, the polarization of the generated positrons. (by converting the positrons back to photons, and measuring their polarization as above, by transmission Compton polarimetry. (In 'transmission Compton polarimetry' of photons, the photon beam is sent through a block of iron, which is magnetised either up or down; this aligns the spins of up to 2 of each iron atoms's electron; then measuring the small difference in the number of photons transmitted for each case allows to deduce the photon polarization, as the Compton cross section depends on the relative alignment of the electron and photon spins).

The proposal was approved after the EPAC meeting in June 2003.
Backgrounds (stray particles leaving a trace in our detectors) are of particular concern for this experiment, and studies of such backgrounds in the FFTB beam line had already been performed during the 2003 and 2004 ('parasitically,' that is while other experiments are running). Before the experiment proper could be done, much more work was necessary, such as further detailed design, simulation of all aspects of it, acquiring detectors and their electronics, stringing cables between the accelerator tunnel and the 'counting house,' design and programming of the data acquisition system. The original plan had the experiment performed in two 'runs' of data-taking in October 2004 and January 2005; however, these two runs were postponed, but did eventually happen, in June 2005 and September 2005.

The data acquired show some of the desired asymmetry, but a more detailed analysis was required; this was done and published in Physical Review Letters (Vol. 100, p 210801 (2008)). A longer paper with more detail on the instrumentation and analysis methods had been completed in May, 2009, and is being published as a NIM A article. The publication of this article completes the work of the E-166 collaboration. However, some of us and others will continue to work on making polarized positrons possible at future linear colliders, as the ILC (see also: ILC at SLAC; ILC-Positron Source Links at SLAC )
(Note: The 'internal' web pages directed at collaborators are an internal record of this work [Please don't quote from them].)



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