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

5 th 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.

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

5 th 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.

5 th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann 5 –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 ).  Can double or suppress rate using polarized positrons (in addition to pol. e-). –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 + ). - Improved accuracy in polarization measurement (Blondel scheme) ►Must have both e + and e - polarization for Giga-Z project (sin 2 θ W ) Physics Motivation for Polarized Positrons

5 th 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!

5 th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann 7 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 )… Next question: How to make polarized positrons? Physics Motivation for Polarized Positrons

5 th 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)

5 th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann 9 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 Polarized Positrons at FFTB

5 th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann 10 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 E-166 vs LC

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

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

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

5 th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann 14 Polarimeter Overview 1 x e -  4 x 10 9  4 x 10 9   2 x 10 7 e + 2 x 10 7 e +  4 x 10 5 e + 4 x 10 5 e +  1 x 10 3  4 x 10 9   4 x 10 7 

5 th 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.

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

5 th 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 (~10 8 photons /15 minutes) δ(P) ~ 2 – 4 % Expected systematic Error of δ(P)/P ~5% dominated by eff. Magnetization of iron

5 th 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

5 th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann 19 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! E-166 Outlook For References, details see:

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

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

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

5 th 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

5 th 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 S with preamps

5 th 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