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

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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz ILC e+ source parameters (TDR) e- Beam Parameters for e+ generation Ecm (GeV) L upgrade1000 e+ per bunch at IP ( ×10 10 ) Number of bunches per puls Undulator period (cm) Repetition rate (Hz) 554 Undulator strength (K value) Beam energy (GeV) Undulator length (m) e- beam bunch separation (ns) Power absorbed in e+ target (%) Edep per bunch [J] Spot size on target (mm rms) Peak power density in target (J/g) Polarization (no collimator) (%) /26/2015S. Riemann

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

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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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 Hz scheme: e- beam (150GeV) to dump 12/26/2015S. Riemann

EuCARD Workshop, Mainz e- Beam Parameters for e+ generation Ecm (GeV) L upgrade1000 e+ per bunch at IP ( ×10 10 ) Number of bunches per pulse Undulator period (cm) Nominal 5Hz mode 4Hz Undulator strength (K value) Beam energy (GeV) Undulator length (m) Hz 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) e- beam bunch separation (ns) Power absorbed in e+ target (%) Spot size on target (mm rms) Peak power density in target (J/g) Polarization (no collimator) (%) ILC low electron beam energies (120GeV) use almost full length of undulator and optimize system (Ushakov, LC-REP )  see next talk by Andriy Ushakov 12/26/2015S. Riemann

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

EuCARD Workshop, Mainz Top quark LC heaviest quark (as heavy as gold atom), pointlike Extremely unstable (  ~ 4× 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: ) –Need precise measurement of polarization S. Riemann

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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%

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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%

EuCARD Workshop, Mainz 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%

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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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 ~ A P,  A P ~ few  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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz Backup S. Riemann

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz 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

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

EuCARD Workshop, Mainz 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

EuCARD Workshop, Mainz e- Beam Parameters for e+ generation Ecm (GeV) L upgrade1000 e+ per bunch at IP ( ×10 10 ) Number of bunches per pulse Undulator period (cm) Nominal 5Hz mode 4Hz Undulator strength (K value) Beam energy (GeV) Undulator length (m) Hz 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) e- beam bunch separation (ns) Power absorbed in e+ target (%) Spot size on target (mm rms) Peak power density in target (J/g) Polarization (no collimator) (%) /26/2015S. Riemann