1. Undulator-Based Positron Production in the Final Focus Test Beam (E-166) K.T. McDonald, J.C. Sheppard, Co-Spokespersons SLAC Experimental Program Advisory Committee November 20, 2002 The International Polarized Positron Production Collaboration

2. K.T. McDonaldNovember 20, 2002 2 E-166 Undulator-Based Production of Polarized Positrons Overview –Classical positron production schemes are marginal due to target heating and radiation damage. –Alternative: use 150 GeV e - in an undulator to produce 10 MeV  ’s, which are then converted to positrons (with less heating of target) [TESLA baseline]. –With helical undulator, get polarized  ‘s and polarized e + (Balakin&Mikhailichenko, 1979). –Physics is clear, but this scheme has never been tested. –Can produce 10 MeV  ’s via 50 GeV e - in SLAC FFTB using a 1-mm bore undulator. –FFTB available only thru 2005 (then LCLS). –E-166 collaboration proposes to demonstrate the physics and technology of undulator-based polarized positron production in the FFTB.

3. K.T. McDonaldNovember 20, 2002 3 E-166 Undulator-Based Production of Polarized Positrons –25 MeV e + from shower max of 6 GeV e - in a 4-6 radiation length target. –Heating at shower max many times that from useful positrons alone.  Need multiple targets to meet goal of 1 e + /e -. –Positrons from shower max have no memory of incident e - polarization. NLC Baseline Positron Production Scheme

4. K.T. McDonaldNovember 20, 2002 4 E-166 Undulator-Based Production of Polarized Positrons Polarized Positrons from Polarized  ’s –High energy E&M interactions are helicity conserving  Forward positrons from  -> e + e - remember the  polarization. –Only 2 charged particles in the target per positron  Less heating of target. –e + polarization is degraded by Bremsstrahlung in the target  Use target < 0.5 rad. Len. –Only upper half of positron spectrum has good polarization. -  pair (10 MeV) ~ 1/5  pair (1 GeV).  Need ~ 100  ’s per useful e +. (Olsen & Maximon, 1959)

5. K.T. McDonaldNovember 20, 2002 5 E-166 Undulator-Based Production of Polarized Positrons Polarized  ’s from a Helical Undulator -  ’s per period (with K = eB / 2  mc 2 = 1)   (1/137) -(  intensity ~ K 2, but spectrum ragged for K > 1.) -100  ’s/positron => ~ 10,000 periods. -Period ~ undulator diameter ~ 1 cm  ~ 100 m long.  First-order cutoff at ~ 10 MeV  ’s for 150 GeV e -. -Helical undulator simpler to fabricate than planar.

6. K.T. McDonaldNovember 20, 2002 6 E-166 Undulator-Based Production of Polarized Positrons TESLA, NLC, and FFTB Positron Production

7. K.T. McDonaldNovember 20, 2002 7 E-166 Undulator-Based Production of Polarized Positrons Sample Collider Polarized Positron System Layout 2 Target assemblies for redundancy Polarized e - source for system checkout (and e  e ,  running).

8. K.T. McDonaldNovember 20, 2002 8 E-166 Undulator-Based Production of Polarized Positrons Physics Motivation for Polarized Positrons –Electroweak processes e + e - -> WW, Z, ZH couple only to e - L e + R or e - R e + L (and not e - L e + L or e - R e + R ). –Slepton and squark production dominantly via e - R e + L.  Can double rate using polarized positrons (or suppress rate if both e - and e + are polarized). –Effective polarization enhanced, and error decreased, in electroweak asymmetry measurements, (N L – N R ) / (N L + N R ) = P eff A LR, P eff = (P - - P + ) / (1 – P - P + ).

9. K.T. McDonaldNovember 20, 2002 9 E-166 Undulator-Based Production of Polarized Positrons The E-166 Collaboration The E-166 Collaboration includes: – Participation from all major LC Labs (CERN, DESY, KEK, SLAC) and JLAB. – Participation from several universities. – e - polarization experts from SLD and HERMES. – e+ polarization experience via the Japanese groups.

10. K.T. McDonaldNovember 20, 2002 10 E-166 Undulator-Based Production of Polarized Positrons Scope of E-166  Make polarized photons (Stage 1):  Use a 1-m-long, short-period, pulsed helical undulator ( u = 2.4 mm, K = 0.17) in the 50-GeV Final Focus Test Beam  E  max ~ 10 MeV.  Characterize the  polarization with a transmission polarimeter.  Then make polarized positrons (Stage 2):   ’s are converted to polarized positrons in a < 0.5 radiation length target (of both Ti and W).  Characterize the positron polarization by converting them back into  ’s and using a transmission polarimeter.

11. K.T. McDonaldNovember 20, 2002 11 E-166 Undulator-Based Production of Polarized Positrons Undulator Design PULSED HELICAL UNDULATOR FOR TEST AT SLAC THE POLARIZED POSITRON PRODUCTION SCHEME. BASIC DESCRIPTION. Alexander A. Mikhailichenko CBN 02-10, LCC-106

12. K.T. McDonaldNovember 20, 2002 12 E-166 Undulator-Based Production of Polarized Positrons Double Undulator Scheme Price is factor of 2 in  rate (0.2  /e - in 50 cm). At the Collider, need better scheme: e.g., single undulator + pulsed spin rotator at ~ GeV. Or other. Sign of e - polarization can be chosen randomly on a pulse-by-pulse basis at the GaAs photoemission source. In lieu of better scheme, use 2 undulators of opposite helicity, pulsing only one.

13. K.T. McDonaldNovember 20, 2002 13 E-166 Undulator-Based Production of Polarized Positrons Polarized Positron Yield at the FFTB e + /  = 0.005 in 0.5 r.l. Ti target. N + = N - (  /e - ) (e + /  ) = (2e10)(0.2)(0.005) = 2e7 / pulse. Longitudinal polarization, P ave, of the positrons is 54%, averaged over the full spectrum (For 0.5 r.l. W converter, yield is double and P ave is 51%.)

14. K.T. McDonaldNovember 20, 2002 14 E-166 Undulator-Based Production of Polarized Positrons Layout of E-166 in the FFTB 50 GeV, low emittance e - beam. 2.4 mm period, K = 0.17, helical undulator. 10 MeV polarized  ’s. 0.5 r.l. converter target. 51%-54% e + polarization. Moffeit/Woods

15. K.T. McDonaldNovember 20, 2002 15 E-166 Undulator-Based Production of Polarized Positrons  Polarimetry via Transmission Thru an Iron Block -Measure only  ’s transmitted thru a block of magnetized iron. -  Compton depends on both P  and P e. -  = (  ++ -  +- ) / (  ++ +  +- ) = 0.05 for 10 MeV  ’s in 15 cm Fe. -Transmission polarimetry is a “1-Compton” polarimetry.

16. K.T. McDonaldNovember 20, 2002 16 E-166 Undulator-Based Production of Polarized Positrons Transmission Polarimeter +  Detector Moffeit/Woods To avoid soft backgrounds, convert  ’s to e + e - and detect Cerenkov light. Deconvolve  spectrum via use of several Cerenkov radiators with graded energy thresholds.

17. K.T. McDonaldNovember 20, 2002 17 E-166 Undulator-Based Production of Polarized Positrons e + Polarimeter Using Transmission Polarimetry  Convert the e + back to  ’s.  54% e + pol.  8.4%  pol.   Compton ~ 2.5% asymmetry.  Will be statistics limited.  Can it work close to  beam?

18. K.T. McDonaldNovember 20, 2002 18 E-166 Undulator-Based Production of Polarized Positrons Compton/Annihilation Polarimeter for  ’s and e + Annihilation polarimetry for e + : e + e -   in a thin magnetized iron foil. Asymmetry  = P e+ P foil A A, P foi ~ 1/13, A A ~ 1 at 90  in CM frame,   max ~ 0.08 P e+. Problem is low rate (high background?). (If use e+e- scattering, A ~ 7/9.) For  ’s, can use Compton scattering in the polarized foil (+ sweep magnet SM): Asymmetry  = P  P foil A C, A C (  ) up to 0.7   max ~ 0.05 P  Scattering polarimeter is “noninvasive”.

19. K.T. McDonaldNovember 20, 2002 19 E-166 Undulator-Based Production of Polarized Positrons E-166 as Linear Collider R&D –E-166 is a proof-of-principle demonstration of undulator based production of polarized positrons for a linear collider. This technique is much less demanding on target performance than conventional positron production with e - on a thick target. A helical undulator is simpler than a planar one -- and provides polarized positrons. –The pulsed helical undulator is a scale model, 1% in length, ~ 20% in diameter, of that appropriate for a collider. –The  and e + polarimeters are prototypes of those appropriate for low-energy diagnostics at a collider. (High energy polarimetry is also needed at a collider, but is well demonstrated at present e - and e + e - facilities.) –The hardware and software expertise developed for E-166 will be the basis for implementation of polarized positrons at a linear collider.

20. K.T. McDonaldNovember 20, 2002 20 E-166 Undulator-Based Production of Polarized Positrons Endorsement by the NLC Machine Advisory Committee Well, I hope that we have an endorsement to mention here!

21. K.T. McDonaldNovember 20, 2002 21 E-166 Undulator-Based Production of Polarized Positrons E-166 Beam Request –E-166 is to be performed in the FFTB, with the undulator just upstream of the e- dump magnets, and polarimeters downstream of these. –E-166 needs initial background studies (parasitic T- experiment) –followed by Stage 1 running with the  polarimeter(s), –and Stage 2 running with the e + polarimeter(s) –all before the FFTB is dedicated to the LCLS (~ 2005).

22. K.T. McDonaldNovember 20, 2002 22 E-166 Undulator-Based Production of Polarized Positrons Time Line –We plan to start construction of equipment in FY03, finish construction and installation in the first 6 month of FY04, and do the experiments in the second half. –In case background problems require additional measures, or we find unexplained fact, we will request additional time in FY05.

23. K.T. McDonaldNovember 20, 2002 23 E-166 Undulator-Based Production of Polarized Positrons E-166 Budget Estimate (1)

24. K.T. McDonaldNovember 20, 2002 24 E-166 Undulator-Based Production of Polarized Positrons E-166 Budget Estimate (2)