Polarized positrons at the ILC: physics goal and source requirements EuCARD Workshop “Spin...

EuCARD Workshop, Mainz 2014 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


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 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 Ecm ≥ 240GeV

e+ polarization• Spin flipper • GigaZ

• Summary


ILC layout (TDR) 2013

Ready for construction…

not to scale

31 kmRequirements: • Long bunch trains: ~1ms 1312 (2625) bunches per train, rep rate 5Hz 2×1010 particles/bunch

• Small emittance• Beam polarization P(e-) > 80% P(e+) ≥ 30%

Positron source

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EuCARD Workshop, Mainz 2014 5S. 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


ILC Undulator based e+ source

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EuCARD Workshop, Mainz 2014 7S. 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


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 Ee

• Photon spectrumHigher polarization with collimation

-e / m / photons 2

u

ph Kmm1

6.30

dL

dN

2

u

2e

1 K1mm1

GeV50EMeV7.23E

2K1

mm1T1

B0934.0K u0

u = undulator period

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ILC Undulator

Parameters

Undulator windings: NbTi

Positron yield and polarization vs.drive e- beam energy

(und. Length Lu=147m,w/o photon collimation) P ≈ 30%

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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

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• 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 Ee

• 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

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Positron Target• Material: Titanium alloy Ti-6%Al-4%V Thickness: 0.4 X0 (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)

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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

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Optical matching device– Pulsed flux concentrator

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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

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prototype test:3.2 T peak field possible


Flux concentrator• Full field with 1ms flat top has been demonstrated

• Still to be done:– full average power operation over extended period including

cooling

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J. Gronberg, LLNL


ILC e+ source parameters (TDR)

e- Beam Parameters for e+ generation

Ecm (GeV)

200 230 250 350 500500 L

upgrade 1000

e+ per bunch at IP ( ×1010) 2 2 2 1.74

Number of bunches per puls 1312 2625 2450

Undulator period (cm) 1.15 1.15 4.3

Repetition rate (Hz) 5 5 4

Undulator strength (K value)

0.92 0.75 0.45 1

Beam energy (GeV) 178 253 253 503

Undulator length (m) 147 147 132

e- beam bunch separation (ns) 554 366 366

Power absorbed in e+ target (%) 7 5.2 5 4.4

Edep per bunch [J] 0.59 0.31 0.31

Spot size on target (mm rms) 1.2 0.8 0.8

Peak power density in target (J/g) 65.6 67.5 101.3 105.4

Polarization (no collimator) (%) 30 29 29 1904/21/23S. Riemann


Ecm = 240 GeV

Higgs-Boson measurements

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HZ

ILC as Higgs Factory

Ecm = 240 GeV:mH = 126 GeV Higgs Strahlung (HZ) (dominating)

WW Fusion

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Higgs Mass and Higgs Coupling to the Z

ZH ~ gZH2

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 couplingILC RDR

m < 100 MeV

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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 ZH

ZH ZH

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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

eeeff PP1L

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

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ILC as Higgs factory

Ecm = 240 GeV

– For Ee < 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 Ee=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

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Ecm (GeV)

240 350 500500 L

upgrade 1000

e+ per bunch at IP ( ×1010) 2 2 2 2 1.74

Number of bunches per pulse 1312 2625 2450


Nominal 5Hz mode

4Hz

Undulator strength (K value) 0.84 0.92 0.75 0.45 1

Beam energy (GeV) 120 178 253 253 503

Undulator length (m) 192 147 147 132

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) 101 117 127


Power absorbed in e+ target (%) 9.2 7 5.2 5 4.4

Spot size on target (mm rms) 1.2 0.8 0.8

Peak power density in target (J/g) 44 65.6 67.5 101.3 105.4

Polarization (no collimator) (%) 31 30 29 29 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

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Ecm = 350 GeV

Top-quark measurements High positron polarization desired

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Top quark physics @ LC

• heaviest quark (as heavy as gold atom), pointlike• Extremely unstable ( ~ 4×10-25s)

– 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

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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

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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

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Ee- ≤150 GeVPe+ = 50%

Ee- = 175 GeV, Ecm = 350 GeVPe+ ≈ 60%


Ecm ≥ 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

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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

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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)

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Ee- = 250 GeVPe+ ≈ 50%


0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10l u (cm)

Posi

tron

yiel

d

0.00

0.05

0.10

0.15

0.20

0.25

Pola

rizat

ion

YieldPolarization

W. Gai, W. Liu

OMD = QWT

u(cm)

TeV upgrade scenarios

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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 X0 Ti• Drift from end of undulator to target: 400m • OMD: FC

1st approach (Gai, Liu)– Longer undulator period, u = 4.3cm– K = 1 (B = 0.25T) P = 20%– Polarization upgrade requires small collimator iris (≤0.85mm)

2nd approach (see next talk by Andriy) – u = 4.3cm– K ≈2, P ≈ 50%


Helicity reversal – rapid or slow?

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One precision key observable: Left-right polarization asymmetry

for measurements with equal luminosities for (- +) and (+ -) helicity • Effective polarization

– Peff is larger than e- polarization

– error propagation Peff is substantially smaller than the uncertainty of e- beam polarization, P

• Higher effective luminosity

Smaller statistical error

effRLLR

RLLR

ee

ee

RLLR

RLLRLR P

1

NN

NN

PP

PP1A

=1/Peff

Precision Measurements with Pe+ > 0

eeeff PP1L

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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

eeeff PP1L

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Left-Right Asymmetry ALR

for equal luminosities

LLR = LRL

• Essential for ALR measurement: – luminosity and polarization have to be helicity-symmetric: LLR = LRL

PL = -PR – Achieved by rapid helicity reversal (SLC: bunch-to-bunch)– small differences have to be known and corrected

AL = asymmetry in luminosity for LR and RL

APeff = asymmetry in LR and RL polarization

ALR 1 AL is more important than APeff

Remember SLC: AL, AL ~ 10-4 AP , AP ~ few 10-3

Small AL, AL , APeff , APeff 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)

...AAAAAfP

1

P

AA LPeff

2measLRbgr

measLRbgr

effeff

measLR

LR

effRLLR

RLLR

effRLLR

RLLR

eff

measLR

LR P

1

NN

NN

P

1

P

AA

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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)

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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

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GigaZ• Running again at the Z resonance (LEP, SLC)• High statistics: 109 Z decays/few months• With polarized e+ and e- beams the left-right

asymmetry ALR and the effective polarization Peff can be determined simultaneously with highest precision

relative precision of less than 5x10-5 can be achieved for the effective weak mixing angle, sin2W

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

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Blondel scheme• Can perform 4 independent measurements (s-channel)

• determination of Pe+ and Pe-, u and ALR simultaneously (ALR≠0); for Pe(+) = Pe(-):

need polarimeters at IP for measuring polarization differences between + and – helicity states

Have to understand correlation between Pe(+) = Pe(-)

=0 (SM) if both beams 100% polarized

eeLReeu

eeLReeu

PPAPP

PPAPP

1

1

41

41

21

eP

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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

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• Backup

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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 r2 2

– Lower increases energy deposition density in target

Heat load with lower?Reaction force

Beam induced deformation

reaction

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0,0 0,2 0,4 0,6 0,8 1,0 1,20

50

100

150

200

250

300

350

400

450

500

= 2000, 1000, 500 rpm

max.

equivale

nt s

tress

[MPa]

time [s]

max. stress, 250 GeV, = 2000 rpm max. stress, 250 GeV, = 1000 rpm max. stress, 175 GeV, = 1000 rpm max. stress, 175 GeV, = 500 rpm max. stress, 125 GeV, = 1000 rpm max. stress, 125 GeV, = 500 rpm

Ti-alloy fatigue stress limit

(to be checked and verified)

Dynamical stress of the Ti-wheel

F. Staufenbiel, LCWS2013

Reduction to ~1000rpm seems possible04/21/23S. Riemann


Bullet target systemW. Gai (ANL)

Target system alternative


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 10oC cooling agent outside on

bottom of recycling line, 200oC can be cooled to 25oC within 46s

Further studies needed but it seems feasible

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W. Gai



Ecm (GeV)

200 230 250 350 500500 L

upgrade 1000

e+ per bunch at IP ( ×1010) 2 2 2 2 1.74

Number of bunches per pulse 1312 2625 2450


Nominal 5Hz mode 4Hz

Undulator strength (K value)

0.92 0.75 0.45 1

Beam energy (GeV) 178 253 253 503

Undulator length (m) 147 147 132

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) 101 117 127


Power absorbed in e+ target (%) 7 7 5.2 5 4.4

Spot size on target (mm rms) 1.4 1.2 0.8 0.8

Peak power density in target (J/g) 51.7 65.6 67.5 101.3 105.4

Polarization (no collimator) (%) 31 30 29 29 19

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Polarized positrons at the ILC: physics goal and source requirements EuCARD Workshop “Spin...

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Transcript of Polarized positrons at the ILC: physics goal and source requirements EuCARD Workshop “Spin...