1. Polarized positrons at the ILC: physics goal and source requirements EuCARD Workshop “Spin optimization at lepton accelerators” 13 February 2014 Sabine Riemann, DESY Zeuthen

2. Physics goal of future colliders Precision measurements –Higgs measurements –ee  tt Top quark mass and coupling –ee  WW Gauge couplings –Fermion pair production,  Sensitivity to deviations from SM disentangle new phenomena beyond SM discovered at LHC –new physics SUSY (?) new gauge bosons, extra dim, Dark matter … Precision and physics potential improves with polarization (e- and e+)  Status of ILC polarized e+ source S. Riemann

3. EuCARD Workshop, Mainz 2014 3 S. Riemann 3 Outline ILC positron source –e+ production –Undulator based source Helical undulator Target Optical matching device (flux concentrator) Source parameters for E cm ≥ 240GeV e+ polarization Spin flipper GigaZ Summary

4. EuCARD Workshop, Mainz 2014 4 ILC layout (TDR) 2013 Ready for construction… not to scale 31 km Requirements: Long bunch trains: ~1ms 1312 (2625) bunches per train, rep rate 5Hz 2×10 10 particles/bunch Small emittance Beam polarization P(e-) > 80% P(e+) ≥ 30% Positron source S. Riemann

5. EuCARD Workshop, Mainz 2014 5 S. Riemann 5 Production of Positrons Courtesy: K. Floettmann Problems  : Large heat load in target e-: Huge heat load in target Generation of (polarized) photons - Compton backscattering of circ. pol. laser light off an e- beam - Undulator passed by e- beam helical vs. planar undulator: -- helical undulator gives ~1.5…2 higher  yield for the parameters of interest (See also Mikhailichenko, CLNS 04/1894) -- circular polarized photons  long. Polarized e+ -- sign of polarization sign is determined by undulator, i.e. direction of the helical field

6. EuCARD Workshop, Mainz 2014 6 ILC Undulator based e+ source S. Riemann

7. EuCARD Workshop, Mainz 2014 7 S. Riemann 7 ILC Positron Source Positron source is located at the end of the electron linac required positron yield Y = 1.5 e+/e- Superconducting helical undulator – 231m maximum active length  positron beam is polarized Photon-Collimator to increase e+ pol –Removes part of photon beam with lower polarization e+ Production Target, 400m downstream the undulator Positron Capture: OMD (Optical Matching Device) –Pulsed flux concentrator 400m

8. EuCARD Workshop, Mainz 2014 8 Undulator Parameters Photon energy (cut-off first harmonic) and undulator K value Number of photons –Increase intensity of  beam by longer undulator  Y = 1.5 e+/e- ILC requirement Upper half of energy spectrum is emitted in cone  Smaller beam spot for higher E e Photon spectrum  Higher polarization with  collimation u = undulator period 12/26/2015S. Riemann

9. EuCARD Workshop, Mainz 2014 9 ILC Undulator Parameters Undulator windings: NbTi Positron yield and polarization vs. drive e- beam energy (und. Length L u =147m, w/o photon collimation)  P ≈ 30% 12/26/2015S. Riemann

10. EuCARD Workshop, Mainz 2014 10 ILC Undulator Prototype sc undulator  high peak field 4m long prototype built at Daresbury Lab (UK): –Two 1.75m long undulators (11.5mm period) –RAL team has shown that both undulators have very high field quality Field on axis 0.86 T (K=0.92) measured at 214 A ILC specification  now show stopper identified More details see J. Clarke, BAW-2 Meeting, SLAC Jan 2011, Ivanyushenko, POSIPOL2013 Workshop 12/26/2015S. Riemann

11. EuCARD Workshop, Mainz 2014 11 e+ source is located at end of the linac  polarization and yield are strongly coupled to the electron beam energy Optimum undulator parameters (K, undulator length) depend on E e With higher energies smaller beam photon beam spot size on target  high polarization is difficult to achieve for high energies (heat load on target and photon collimator) ILC TDR 12/26/2015S. Riemann

12. EuCARD Workshop, Mainz 2014 12 Positron Target Material: Titanium alloy Ti-6%Al-4%V Thickness: 0.4 X 0 (1.4 cm) Incident photon spot size on target:  2 mm (rms) (Ee- = 150GeV) ~ 1.2 mm (Ee- = 250GeV) Power deposition in target: TDR 5-7% (~4kW) small beam size  high peak energy density  spinning wheel to avoid damage due to high energy deposition density –2000 r.p.m. (100m/s) –Diameter: 1m –Wheel is in vacuum –water-cooled Potential problems –Stress waves due to cyclic heat load  target lifetime –High peak energy deposition –Eddy currents –rotating vacuum seals to be confirmed suitable (design and prototyping is ~ongoing) S. Riemann

13. EuCARD Workshop, Mainz 2014 13 Target ‘risk’ issues and improvements ‘risk’ issues Limited lifetime of vacuum seal (2000rpm) –LLNL prototype: few weeks with vacuum spikes No further experiments due to lack of funding No tests yet with water cooling No radiation damage tests Potential improvements: –spinning target with lower speed (1000rpm instead 2000rpm) –New type of sealing –Differential pumping –better cooling –Alternative target design considerations 12/26/2015S. Riemann

14. EuCARD Workshop, Mainz 2014 14 Optical matching device –Pulsed flux concentrator 12/26/2015S. Riemann S. Riemann

15. EuCARD Workshop, Mainz 2014 15 Pulsed Flux Concentrator Pulsed flux concentrator: capture efficiency to ~25% (quarter wave transformer:  ~ 15%) - low field on target (low eddy current) - high peak field (≥3T), 1ms flat top Design: LLNL J. Gronberg 12/26/2015S. Riemann prototype test:  3.2 T peak field possible

16. EuCARD Workshop, Mainz 2014 16 Flux concentrator Full field with 1ms flat top has been demonstrated Still to be done: –full average power operation over extended period including cooling 12/26/2015S. Riemann J. Gronberg, LLNL

17. EuCARD Workshop, Mainz 2014 17 ILC e+ source parameters (TDR) e- Beam Parameters for e+ generation Ecm (GeV) 200230250350500 500 L upgrade1000 e+ per bunch at IP ( ×10 10 ) 2221.74 Number of bunches per puls 1312 26252450 Undulator period (cm) 1.15 4.3 Repetition rate (Hz) 554 Undulator strength (K value) 0.920.750.451 Beam energy (GeV) 178 253 503 Undulator length (m)147 132 e- beam bunch separation (ns) 554366 Power absorbed in e+ target (%) 75.254.4 Edep per bunch [J]0.590.31 Spot size on target (mm rms) 1.20.8 Peak power density in target (J/g) 65.667.5101.3105.4 Polarization (no collimator) (%) 3029 19 12/26/2015S. Riemann

18. EuCARD Workshop, Mainz 2014 18 E cm = 240 GeV Higgs-Boson measurements 12/26/2015S. Riemann

19. EuCARD Workshop, Mainz 2014 19 HZ ILC as Higgs Factory E cm = 240 GeV: m H = 126 GeV Higgs Strahlung (HZ) (dominating) WW Fusion S. Riemann

20. EuCARD Workshop, Mainz 2014 20 Higgs Mass and Higgs Coupling to the Z  ZH ~ g ZH 2 Select events: e+e-  ZH and Z  ,ee Peak position  Higgs mass Peak height  Model independent measurement!! Higher lumi improves precision Fit to the spectrum of recoil mass of both leptons  Higgs mass and coupling ILC RDR  m < 100 MeV S. Riemann

21. EuCARD Workshop, Mainz 2014 21 Higgs Strahlung dominates e + R e - L e + L e - R e + L e - L e + R e - R With e+ and e- polarization ‘ineffective’ processes are suppressed L (R) R (L)  ZH S. Riemann

22. EuCARD Workshop, Mainz 2014 22 Higgs Strahlung eff. luminosity e + R e - L  ZH e + L e - R  ZH e + L e - L  ZH e + R e - R  ZH P(e-,e+) = (±80%; ∓30%)  24% lumi gain   ZH reduced by 11% WW Fusion e + R e - L  H e + L e - R  H e + L e - L  H e + R e - R  H L (R) R (L) L R for P(e+)=+1  (e + R e - L  H) enhanced by factor 2  = 0 for 100% polarized beams With e+ and e- pol. suppression of ‘ineffective’ processes S. Riemann

23. EuCARD Workshop, Mainz 2014 23 ILC as Higgs factory E cm = 240 GeV –For E e < 150 GeV e+ yield decreases below 1.5e+/e- (low  energy  only ‘small’ shower0  TDR: “10 Hz scheme” 1.Alternating with e- beam for physics (Ee~120GeV) an e- beam with E e =150GeV passes undulator generate  for e+ production 2.use almost full length of undulator and optimize system see: next talk, and A. Ushakov, LC-REP-2013-019 10Hz scheme: e- beam (150GeV) to dump 12/26/2015S. Riemann

24. EuCARD Workshop, Mainz 2014 24 e- Beam Parameters for e+ generation Ecm (GeV) 240350500 500 L upgrade1000 e+ per bunch at IP ( ×10 10 )22221.74 Number of bunches per pulse 1312 26252450 Undulator period (cm)1.15 4.3 Nominal 5Hz mode 4Hz Undulator strength (K value)0.840.920.750.451 Beam energy (GeV)120 178 253 503 Undulator length (m)192147 132 10Hz alternate pulse mode Undulator strength (K value)0.92 Beam energy for e+ prod.(GeV)150 Undulator length (m)147 Beam energy for lumi (GeV) 101117127 e- beam bunch separation (ns) 554366 Power absorbed in e+ target (%) 9.2 75.254.4 Spot size on target (mm rms) 1.20.8 Peak power density in target (J/g) 44 65.667.5101.3105.4 Polarization (no collimator) (%)31 3029 19 ILC undulator @ low electron beam energies (120GeV) use almost full length of undulator and optimize system (Ushakov, LC-REP-2013-019 )  see next talk by Andriy Ushakov 12/26/2015S. Riemann

25. EuCARD Workshop, Mainz 2014 25 E cm = 350 GeV Top-quark measurements  High positron polarization desired 12/26/2015S. Riemann

26. EuCARD Workshop, Mainz 2014 26 Top quark physics @ LC heaviest quark (as heavy as gold atom), pointlike Extremely unstable (  ~ 4×10 -25 s) –Decay of top-quark before hadronization –Top-polarization gets preserved to decay (similar to  -lepton)  The ‘pure’ top quark can be studied –Top mass and coupling are important for quantum effects affecting many observables –Top quark coupling as test of SM and physics beyond  top quark coupling, spin, spin correlations are observable by measuring polarization and LR asymmetries with high precision –Need high degree of polarization –e+ polarization highly desired (see i.e., Grote, Koerner, arXiv:1112.0908) –Need precise measurement of polarization S. Riemann

27. EuCARD Workshop, Mainz 2014 27 59 60.4 Photon collimator parameters for polarization upgrade 60% e+ polarization at 350GeV  ~60% of photon beam power absorbed in collimator  high load on the collimator materials ILC TDR 350 S. Riemann

28. EuCARD Workshop, Mainz 2014 28 Photon beam collimation Increase of e+ polarization using photon collimator –Details see talk of Friedrich Staufenbiel –Collimator parameters depend on energy  Multistage collimator (3 stages with each pyr. C, Ti, Fe)  flexible design for energies 120 GeV ≤ Ee- ≤ 250GeV 12/26/2015S. Riemann E e- ≤150 GeV P e+ = 50% E e- = 175 GeV, E cm = 350 GeV P e+ ≈ 60%

29. EuCARD Workshop, Mainz 2014 29 E cm ≥ 500 GeV Full spectrum of physics processes Best flexibility with polarized e+ and polarized e- beam -- Polarization as high as possible and reasonable e+ polarization improves substantially identification of models in case of deviations from SM -- Physics with transversely polarized beams; only possible if both beams are polarized S. Riemann

30. EuCARD Workshop, Mainz 2014 30 52.3 70.1 50 59 Photon collimator parameters for polarization upgrade 60% e+ polarization at 500GeV  collimator absorbs ~70% of photon beam power  50% e+ polarization should be ‘sufficient’ ILC TDR 500 S. Riemann

31. EuCARD Workshop, Mainz 2014 31 Photon beam collimation Increase of e+ polarization using photon collimator –Details see talk of Friedrich Staufenbiel –Collimator parameters depend on energy  Multistage collimator (3 stages with each pyr. C, Ti, Fe) 12/26/2015S. Riemann E e- = 250 GeV P e+ ≈ 50%

32. EuCARD Workshop, Mainz 2014 32 W. Gai, W. Liu OMD = QWT u (cm) TeV upgrade scenarios 12/26/2015S. Riemann Goal A reasonable scheme for the 1 TeV option without major impact on the ILC configuration. Assumptions Drive beam energy: 500 GeV Target: 0.4 X 0 Ti Drift from end of undulator to target: 400m OMD: FC 1 st approach (Gai, Liu) –Longer undulator period,  u = 4.3cm –K = 1 (B = 0.25T) P = 20% –Polarization upgrade requires small collimator iris (≤0.85mm) 2 nd approach (see next talk by Andriy) –  u = 4.3cm – K ≈2, P ≈ 50%

33. EuCARD Workshop, Mainz 2014 33 Helicity reversal – rapid or slow? S. Riemann

34. EuCARD Workshop, Mainz 2014 34 One precision key observable: Left-right polarization asymmetry for measurements with equal luminosities for (- +) and (+ -) helicity Effective polarization –P eff is larger than e- polarization –error propagation   P eff is substantially smaller than the uncertainty of e- beam polarization,  P Higher effective luminosity  Smaller statistical error =1/P eff Precision Measurements with P e+ > 0 S. Riemann

35. EuCARD Workshop, Mainz 2014 35 Helicity reversal Net polarization depends on direction of undulator windings  Need facility to reverse e+ helicity It has to be synchronous with reversal of e- polarization to achieve: –enhanced luminosity –Cancellation of time-dependent effects  small systematic errors S. Riemann

36. EuCARD Workshop, Mainz 2014 36 Left-Right Asymmetry A LR for equal luminosities L LR = L RL Essential for A LR measurement: –luminosity and polarization have to be helicity-symmetric: L LR = L RL P L = -P R –Achieved by rapid helicity reversal (SLC: bunch-to-bunch) –small differences have to be known and corrected A L = asymmetry in luminosity for LR and RL A Peff = asymmetry in LR and RL polarization A LR  1  A L is more important than A Peff Remember SLC: A L,  A L ~ 10 -4 A P,  A P ~ few 10 -3  Small A L,  A L, A Peff,  A Peff required also for ILC Slow helicity reversal: Despite of very precise monitoring of relative intensities and polarizations for LR and RL states, systematic uncertainties will be larger for most observables ( could be even larger than without e+ polarization) S. Riemann

37. EuCARD Workshop, Mainz 2014 37 Spin flipper Net polarization depends on direction of undulator windings Reversal of e+ helicity necessary It has to be synchronous with reversal of e- polarization to achieve –enhanced luminosity –Cancellation of time-dependent effects  small systematic errors Helicity reversal requires spin flipper (5Hz)  Spin rotation + flipper (2 parallel lines) S. Riemann

38. EuCARD Workshop, Mainz 2014 38 Spin flipper (see Gudi’s talk, and L. Malysheva, …..) beam is kicked into one of two identical parallel transport lines to rotate the spin Horizontal bends rotate the spin by 3 × 90° from the longitudinal to the transverse horizontal direction. In each of the two symmetric branches a 5m long solenoid with an integrated field of 26.2Tm aligns the spins parallel or anti-parallel to the B field in the damping ring. Both lines are merged using horizontal bends and matched to the PLTR lattice. The length of the splitter/flipper section section ~26m; horizontal offset of 0.54m for each branch 12/26/2015S. Riemann

39. EuCARD Workshop, Mainz 2014 39 GigaZ Running again at the Z resonance (LEP, SLC) High statistics: 10 9 Z decays/few months With polarized e+ and e- beams the left-right asymmetry A LR and the effective polarization P eff can be determined simultaneously with highest precision  relative precision of less than 5x10 -5 can be achieved for the effective weak mixing angle, sin 2  W eff, 10x better than LEP/SLD  Together with other LC precision measurements at higher energies (i.e. top-quark mass) theoretical predictions can be tested and physics models beyond the Standard Model distinguished S. Riemann

40. EuCARD Workshop, Mainz 2014 40 Blondel scheme Can perform 4 independent measurements (s-channel) determination of P e+ and P e-,  u and A LR simultaneously (A LR ≠0); for P e (+) = P e (-):  need polarimeters at IP for measuring polarization differences between + and – helicity states  Have to understand correlation between P e (+) = P e (-) =0 (SM) if both beams 100% polarized S. Riemann

41. EuCARD Workshop, Mainz 2014 41 Summary Weak interaction is parity violating  Polarized beam(s) are mandatory for future precision physics at high energy e+e- colliders ILC: helical undulator  P(e+) ≥ 30%; useful for physics at all energies –Higher effective luminosity, enhancement of interesting processes –new physics signals can be fixed and interpreted with substantially higher precision –No problem to achieve P(e+) ~60% at Ecm≈350GeV. But e+ source parameters and P(e+) depend on energy –High e+ polarization allows polarization measurement using annihilation data no design show stoppers identified for a polarized e+ source; however, R&D and prototyping still necessary S. Riemann

42. EuCARD Workshop, Mainz 2014 42 Backup S. Riemann

43. EuCARD Workshop, Mainz 2014 43 Target design improvements Lower rotation speed 2000rpm  1000rpm? Friedrich Staufenbiel (POSIPOL13, LCWS13): –Simulation of dynamic response to cyclic heat load on target (ANSYS)  no shock waves –Inertia and torque calculation and simulation –Torque |  | = m r 2  2 –Lower  increases energy deposition density in target  Heat load with lower  ? Reaction force Beam induced deformation reaction 12/26/2015S. Riemann

44. EuCARD Workshop, Mainz 2014 44 Ti-alloy fatigue stress limit (to be checked and verified) Dynamical stress of the Ti-wheel F. Staufenbiel, LCWS2013  Reduction to ~1000rpm seems possible 12/26/2015S. Riemann

45. EuCARD Workshop, Mainz 2014 45 Bullet target system W. Gai (ANL) Target system alternative

46. EuCARD Workshop, Mainz 2014 46 Rail gun target Braking –Eddy current braking –Magnetic braking with or without external power source Cooling –Conduction cooling in the recycling line –Turnarounds of bullets  ~60s cooling time –Details require simulation taking into account non- uniform energy deposition –Estimate: using a 10 o C cooling agent outside on bottom of recycling line, 200 o C can be cooled to 25 o C within 46s Further studies needed but it seems feasible 12/26/2015S. Riemann W. Gai

47. EuCARD Workshop, Mainz 2014 47 e- Beam Parameters for e+ generation Ecm (GeV) 200230250350500 500 L upgrade1000 e+ per bunch at IP ( ×10 10 )22221.74 Number of bunches per pulse 1312 26252450 Undulator period (cm)1.15 4.3 Nominal 5Hz mode 4Hz Undulator strength (K value) 0.920.750.451 Beam energy (GeV) 178 253 503 Undulator length (m)147 132 10Hz alternate pulse mode Undulator strength (K value)0.92 Beam energy for e+ prod.(GeV)150 Undulator length (m)147 Beam energy for lumi (GeV) 101117127 e- beam bunch separation (ns) 554366 Power absorbed in e+ target (%) 7 75.254.4 Spot size on target (mm rms)1.41.20.8 Peak power density in target (J/g) 51.7 65.667.5101.3105.4 Polarization (no collimator) (%)31 3029 19 12/26/2015S. Riemann