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Introduction (What, who) Motivation (Why) Experiment and Polarimetry (How) Outlook

Polarized Positrons at a Linear Collider and FFTB (SLAC E-166). Achim W. Weidemann University of South Carolina, Columbia (@SLAC). Introduction (What, who) Motivation (Why) Experiment and Polarimetry (How) Outlook. E-166 Experiment.

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Introduction (What, who) Motivation (Why) Experiment and Polarimetry (How) Outlook

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  1. Polarized Positrons at a Linear Collider and FFTB (SLAC E-166) Achim W. Weidemann University of South Carolina, Columbia (@SLAC) • Introduction (What, who) • Motivation (Why) • Experiment and Polarimetry (How) • Outlook 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  2. E-166 Experiment E-166 is a demonstration of undulator-based polarized positron production for linear colliders • E-166 uses the 50 GeV SLAC beam in conjunction with 1 m-long, helical undulator to make polarized photons in the FFTB. • These photons are converted in a ~0.5 rad. len. thick target into polarized positrons (and electrons). • The polarization of the positrons and photons will be measured. 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  3. E-166 Collaborators 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  4. Physics Motivation for Polarized Positrons Polarized e+ in addition to polarized e- is recognized as a highly desirable option by the WW LC community (studies in Asia, Europe, and the US) Having polarized e+ offers (next slides): • Higher effective polarization -> enhancement of effective luminosity for many SM and non-SM processes • Ability to selectively enhance (reduce) contribution from SM processes (better sensitivity to non-SM processes) • Access to many non-SM couplings (larger reach for non-SM physics searches) • Access to physics using transversely polarized beams (only works if both beams are polarized) • Improved accuracy in measuring polarization. 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  5. Physics Motivation for Polarized Positrons • Electroweak processes e+e- -> WW, Z, ZH couple only to e-Le+R or e-Re+L (and not e-Le+L or e-Re+R). • Can double or suppress rate using polarized positrons (in addition to pol. e-). • Effective polarization enhanced, and error decreased, in electroweak asymmetry measurements, (NL – NR) / (NL + NR) = Peff ALR, Peff = (P- - P+) / (1 – P-P+). - Improved accuracy in polarization measurement (Blondel scheme) ►Must have both e+ and e- polarization for Giga-Z project (sin2θW ) 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  6. (SUSY)Physics Motivation for Polarized Positrons • Slepton and squark produced dominantly via • (and not or ). • Separation of the (LL, LR) selectron pair • with longitudinally polarized beams to test association of chiral quantum numbers to scalar fermions in SUSY : • With P(e-)= -80% and: • P(e+)= 0% => no separation! • P(e+)= -40% => 163fb vs 66 fb • Can’t do without positron polarization! 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  7. Physics Motivation for Polarized Positrons • Transverse polarization of both beams • ..allows separation of new physics, e.g. extra dimensions • More examples in JLC, TESLA TDRs, Reviews, e.g. by G. Moortgat-Pick, (POWER [Polarization at Work in Energetic Reactions ] collaboration http://www.ippp.dur.ac.uk/~gudrid/power/)… • Next question: How to make polarized positrons? 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  8. Polarized Positrons at LC 2 Target assemblies for redundancy (+ polarized e- source) 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  9. Polarized Positrons at FFTB • 50 GeV, low emittance electron beam • 2.4 mm period, K=0.17 helical undulator • 10 MeV polarized photons • 0.5 r.l. converter target • 51%-54% positron polarization 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  10. E-166 vs LC E-166 is a demonstration of undulator-based production of polarized positrons for linear colliders (next slide): • Photons are produced in the same energy range and polarization characteristics as in LC • Same target thickness and material • Polarization of the produced positrons is in the same range as at LC • Simulation tools, diagnostics: same as those being used for LC polarized positron source • But: the intensity per pulse is low by a factor of 2000. 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  11. LC / E-166 Parameter Comparison 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  12. Helical Undulator l=2.4 mm, K=0.17 Energy Polarization Alexander A. Mikhailichenko, Pulsed Helical Undulator….CBN 02-10, LCC-106 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  13. Circ. γ -> long. e+polarization N(e+) P(e+) P(e+) Olsen & Maximon, 1959 0.5 r.l. Ti Alloy target; 0.5% yield, P(e+)=54% averaged over full spectrum 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  14. Polarimeter Overview 4 x 109 4 x 107  1 x 1010 e-  4 x 109 4 x 109  2 x 107 e+ 2 x 107 e+  4 x 105 e+ 4 x 105 e+  1 x 103 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  15. Photon Transmission Polarimetry M. Goldhaber et al. Phys. Rev. 106 (1957) 826. For photons of undulator spectrum, use number- or energy-weighted integral. 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  16. Expected Photon Polarimeter Performance Si-W Calorimeter Expected measured energy asymmetry δ = (E+-E-)/(E++E-) and energy-weighted analyzing power byanalytic integration and, with good agreement, from polarized GEANT simulation: Energy-weighted Mean Aerogel Cerenkov will measure P for E > 5 MeV; 1% stat. measurements very fast (~ minutes), main syst. error of ΔP /P ~ 0.05 from Pe 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  17. Polarimetry of Positrons 2-step Process: • re-convert e+   via brems/annihilation process • polarization transfer from e+ to  well-known • measure polarization of re-converted photons with photon transmission • infer P(e+) from measured photon polarization Experimental Challenges: • large angular distribution of the positrons at production target: • e+ collection & transport efficiency; - background rejection issues • angular distribution of the re-converted photons • detected signal includes large fraction of Compton scattered photons • requires simulations to determine effective Analyzing Power 14-20% Formal Procedure: Stat. Error (~108 photons /15 minutes) δ(P) ~ 2 – 4 % Expected systematic Error of δ(P)/P ~5% dominated by eff. Magnetization of iron 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  18. Polarimetry Summary • Transmission polarimetry is well-suited for photon and positron beam measurements in E166 • Analyzing power determined from simulations is sufficiently large and robust • Measurements will be very fast with negligible statistical errors • Expect systematic errors of ΔP/P ~ 0.05 from magnetization of iron 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  19. E-166 Outlook • Experiment approved mid-June 2003; • …with proviso: should study backgrounds first; • Installation under way now (Aug.2004) • Will run Oct.2004, Jan 2005 (….before end of 2005, after which FFTB will become LCLS) • Hope to blaze the way for polarised positrons at a future LC! For References, details see: http://www.slac.stanford.edu/exp/e166 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  20. Backup Slides 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  21. Positron PolarimeterLayout 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  22. Photon Detectors For Photons: Si-W Calorimeter Threshold Cerenkov (AeroGel) 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  23. Positron Transport System e+ transmission (%) through spectrometer photon background fraction reaching CsI-detector 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  24. CsI Calorimeter Detector Crystals: from BaBar Experiment Number of crystals: 4 x 4 = 16 Typical front face of one crystal: 4.7 cm x 4.7 cm Typical backface of one crystal: 6 cm x 6 cm Typical length: 30 cm Density: 4.53 g/cm³ Rad. Length 8.39 g/cm² = 1.85 cm Mean free path (5 MeV): 27.6 g/cm² = 6.1 cm No. of interaction lengths (5 MeV): 4.92 Long. Leakage (5 MeV): 0.73 % Photodiode Readout (2 per crystal): Hamamatsu S2744-08 with preamps 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

  25. Expected Positron Polarimeter Performance Expected systematic Error of δ(P)/P ~5% dominated by eff. Magnetization of iron 5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann

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