EPAC June 2003 Undulator-Based Production of Polarized Positrons A proposal for the 50 GeV Beam in the FFTB E-166 Undulator-Based Production of Polarized Positrons A proposal for the 50 GeV Beam in the FFTB Thursday, June 12, 2003 K-P. Sch ü ler and J. C. Sheppard

EPAC June Undulator-Based Production of Polarized Positrons E-166 Collaboration (45 Collaborators)

EPAC June Undulator-Based Production of Polarized Positrons E-166 Collaborating Institutions (15 Institutions)

EPAC June 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.

EPAC June The Need for a Demonstration Experiment Production of polarized positrons depends on the fundamental process of polarization transfer in an electromagnetic cascade. While the basic cross sections for the QED processes of polarization transfer were derived in the 1950’s, experimental verification is still missing

EPAC June The Need for a Demonstration Experiment Each approximation in the modeling is well justified in itself. However,the complexity of the polarization transfer makes the comparison with experiment important so that the decision to build a linear collider w/ or w/o a polarized positron source is based on solid ground. Polarimetry precision of 10% is sufficient to prove the principle of undulator based polarized positron production for linear colliders

EPAC June 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: 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.

EPAC June Separation of the selectron pair in with longitudinally polarized beams to test association of chiral quantum numbers to scalar fermions in SUSY transformations Physics Motivation: An Example

EPAC June NLC/USLCSG Polarized Positron System Layout 2 Target assembles for redundancy

EPAC June TESLA, NLC/USLCSG, and E-166 Positron Production

EPAC June E-166 Vis-à-vis a Linear Collider Source E-166 is a demonstration of undulator-based production of polarized positrons for linear colliders: Photons are produced in the same energy range and polarization characteristics as for a linear collider; The same target thickness and material are used as in the linear collider; The polarization of the produced positrons is expected to be in the same range as in a linear collider. The simulation tools are the same as those being used to design the polarized positron system for a linear collider. However, the intensity per pulse is low by a factor of 2000.

EPAC June E-166 Beamline Schematic 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

EPAC June E-166 Helical Undulator Design, =2.4 mm, K=0.17 PULSED HELICAL UNDULATOR FOR TEST AT SLAC THE POLARIZED POSITRON PRODUCTION SCHEME. BASIC DESCRIPTION. Alexander A. Mikhailichenko CBN 02-10, LCC-106

EPAC June Helical Undulator Radiation Circularly Polarized Photons

EPAC June Photon Int., Angular Dist., Number Spctr., Polarization

EPAC June Polarized Positrons from Polarized  ’s (Olsen & Maximon, 1959) Circular polarization of photon transfers to the longitudinal polarization of the positron. Positron polarization varies with the energy transferred to the positron.

EPAC June Polarized Positron Production in the FFTB Polarized photons pair produce polarized positrons in a 0.5 r.l. thick target of Ti-alloy with a yield of about 0.5%. Longitudinal polarization of the positrons is 54%, averaged over the full spectrum Note: for 0.5 r.l. W converter, the yield is about 1% and the average polarization is 51%.

EPAC June Polarimetry K-Peter Schüler Presentation

EPAC June 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 

EPAC June Transmission Polarimetry of (monochromatic) Photons M. Goldhaber et al. Phys. Rev. 106 (1957) 826. all unpolarized contributions cancel in the transmission asymmetry  (monochromatic case)

EPAC June Transmission Polarimetry of Photons Monochromatic Case But, undulator photons are not monochromatic:  Must use number or energy weighted integrals  Analyzing Power:

EPAC June Transmission Polarimetry of Positrons 2-step Process: re-convert e+   via brems/annihilation process –polarization transfer from e+ to  proceeds in well-known manner measure polarization of re-converted photons with the photon transmission methods –infer the polarization of the parent positrons from the measured photon polarization Experimental Challenges: large angular distribution of the positrons at the production target: –e+ spectrometer 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 the effective Analyzing Power Formal Procedure: Fronsdahl & Überall; Olson & Maximon; Page; McMaster

EPAC June Spin-Dependent Compton Scattering Simulation with modified GEANT3 (implemented by V. Gharibyan) standard GEANT is unpolarized ad-hoc solution: - substitute unpolarized Compton subroutines with two spin-dependent versions (+1 and -1) and run these in sequence for the same same beam statistics - then determine analyzing power from this data

EPAC June Analyzer Magnets g‘ =  for pure iron, Scott (1962) Error in e- polarization is dominated by knowledge in effective magnetization M along the photon trajectory: active volume Photon Analyzer Magnet: 50 mm dia. x 150 mm long Positron Analyzer Magnet: 50 mm dia. x 75 mm long

EPAC June Photon Polarimeter Detectors Si-W Calorimeter Threshold Cerenkov (Aerogel) E-144 Designs:

EPAC June Positron Polarimeter Layout

EPAC June Positron Transport System e+ transmission (%) through spectrometer photon background fraction reaching CsI-detector

EPAC June 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

EPAC June % 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 determined through analytic integration and, with good agreement, through special polarized GEANT simulation will measure P  for E  > 5 MeV (see Table 12) Aerogel Cerenkov

EPAC June Expected Positron Polarimeter Performance I Simulation based on modified GEANT code, which correctly describes the spin-dependence of the Compton process Photon Spectrum & Angular Distr. Number- & Energy-Weighted Analyzing Power vs. Energy 10 Million simulated e + per point & polarity on the re-conversion target

EPAC June Expected Positron Polarimeter Performance II Analyzing Power vs. Target Thickness Analyzing Power vs. Energy Spread

EPAC June Expected Positron Polarimeter Performance III Table 13

EPAC June 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

EPAC June Beam Request J. C. Sheppard Presentation II

EPAC June E-166 Beam Request 6 weeks of activity in the SLAC FFTB: 2 weeks of installation and check-out 1 week of check-out with beam 3 weeks of data taking: roughly 1/3 of time on photon measurements, 2/3 of time on positron measurements.

EPAC June E-166 Beam Measurements Photon flux and polarization as a function of K. Positron flux and polarization for K=0.17, 0.5 r.l. of Ti vs. energy. Positron flux and polarization for 0.1 r.l. and 0.25 r.l. Ti and 0.1, 0.25, and 0.5 r.l. W targets. Each measurement is expected to take about 20 minutes. A relative polarization measurement of 10% is sufficient to validate the polarized positron production processes

EPAC June E-166 Institutional Responsibilities Electron BeamlineSLAC UndulatorCornell Positron BeamlinePrinceton/SLAC Photon BeamlineSLAC Polarimetry: OverallDESY Magnetized Fe AbsorbersDESY Cerenkov DetectorsPrinceton Si-W CalorimeterTenn./ S. Carolina CsI CalorimeterDESY/Humboldt DAQHumboldt/Tenn./S. Car.

EPAC June 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. –The hardware and software expertise developed for E-166 form a basis for the implementation of polarized positrons at a linear collider.

EPAC June E-166 Costs Sub-system EFD Labor SLAC Labor SLAC M&S Coll. Contr. Exist. FFTB Exist. non- FFTB Total Elec. BT Gam. BT Posi. BT Gen/Infrstr Grand Total (All entries in k$) Experiment E-166_attach xls (J. Weisend, E-166 Impact Report)

EPAC June E-166 Institutional Responsibilities (All entries in k$) SLACBeamline, infrastructure207 CornellUndulator, pulser85 PrincetonSpctr. Magnets, Aerogel cntr60 DESYFe absorber magnets20 U. Tenn/U. S. CarolinaSi-W cal., DAQ15 Humboldt U.CsI cal, DAQ15 SimulationsAll50 ErroneousHSB PS14 Experiment E-166_attach xls (J. Weisend, E-166 Impact Report)

EPAC June Undulator Photon Beam I Undulator basics (1st harmonic shown only) E166 undulator parameters (polarimetry extra)

EPAC June Undulator Photon Beam II photon spectrum, angular distribution and polarization (polarimetry extra)

EPAC June Positron Beam Simulation distributions behind the converter target (0.5 r.l. Ti) based on polarized EGS shower simulations by K. Flöttmann (polarimetry extra)

EPAC June Low-Energy Polarimetry Candidate Processes Photons: Compton Scattering on polarized electrons –forward scattering (e.g. Schopper et al.) –backward scattering –transmission method (e.g. Goldhaber et al.) Positrons: all on ferromagnetic = polarized e - targets –Annihilation polarimetry (e + e -   ) (e.g. Corriveau et al.) –Bhabha scattering (e + e -  e + e - ) (e.g. Ullmann et al.) –brems/annihilation (e +   ) plus  -transmission (Compton) polarimetry (polarimetry extra)

EPAC June Trade-offs Principal difficulties of e + polarimetry: –huge multiple-scattering at low energies even in thin targets –cannot employ double-arm coincidence techniques or single-event counting due to poor machine duty cycle –low energies below 10 MeV, vulnerable to backgrounds All of the candidate processes have been explored by us:  the transmission method is the most suitable (polarimetry extra)

EPAC June Compton Cross Section (polarimetry extra)

EPAC June e+ polarimeter: typical GEA NT output (example) I (polarimetry extra)

EPAC June e+ polarimeter: typical GEANT output (example) II * * Assuming 2 x 10 5 e + per pulse (1% e + spectrometer transmission) (polarimetry extra)