1. ILC Baseline Design: Physics with Polarized Positrons
Sabine Riemann (DESY)
24 May 2007
Posipol, Orsay

2. S. Riemann: ILC Baseline Positron Polarization
Outline
Baseline Design  Low positron polarization
Physics Case ?
Utilization of Pe+ ≈ 30%
helicity reversal
requirements
Summary and outlook
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

3. S. Riemann: ILC Baseline Positron Polarization
Physics case
Refer to previous talk(s) given by Gudi and others:
e+ polarization
 improves accuracy of SM measurements
 increases sensitivity to physics beyond SM
 decisively to find out what the underlying physics is
With e+ polarization
processes can be enhanced or suppressed;
clean initial states with known helicities
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

4. S. Riemann: ILC Baseline Positron Polarization
Baseline - Physics
Physics between 200 GeV and 500 GeV
Luminosity:
Running year zero for commissioning
Year 1-4: Lint = 500 fb-1:
1. year 10%  Lint ≈ 50 fb-1
2. year 30%  Lint ≈ 150 fb-1
3. Year 60%  Lint ≈ 300 fb-1
4. year 100%  Lint ≈ 500 fb-1
 expected statistics:
few 104 eeHZ at 350 GeV (mH≈120 GeV)
ee tt at 350 GeV
5·105 (1·105) ee  qq (mm) at 500 GeV
ee  WW at 500 GeV
 statistical cross section uncertainties at per-mille level !!
 e+ polarization will help (beginning of LC studies: Lint ~ 50 fb-1)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

5. ILC Baseline design: e+ Polarization ?
RDR: helical undulator (60% e+ = update value)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

6. ILC Baseline design: e+ Polarization
RDR: helical undulator  ~30% e+ polarization
e+ spectrum with g collimator 3.4mrad
photon beam: distance undulator center target ~ 500m
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

7. W. Gai
Yield and Pol.
The yield will drop from ~1.37 down to ~1.29 when length of drift increased up to 500m from 50m.

8. S. Riemann: ILC Baseline Positron Polarization
Utilization of P=~30%
30% e+ polarization for physics?
remember:
first LC studies were done also with a
(60%, 40%) option !!
(60%; 40%)  Peff=0.81
(80%; 40%)  Peff=0.91
(80%; 30%)  Peff=0.88
30%  test of facilities during the first years
of operation
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

9. S. Riemann: ILC Baseline Positron Polarization
D Peff
Decrease of error on Peff=(Pe- + Pe+)/(1+Pe- Pe+)
see also Gudi’s talk for the advantages
30%: Improvement by factor 2 (1.5)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

10. Draft RDR: Positron Source
Linac to Damping Ring Beam Line:
spin rotation line  need also spin flip for (+) AND (–) helicity of positrons
If no polarization is needed  we have to destroy the 30%!
(Few turns in DR without spin rotation before DR are not sufficient;
see studies of L. Malysheva)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

11. S. Riemann: ILC Baseline Positron Polarization
e+ Helicity Reversal
e+ helicity flip less frequent than e- helicity reversal
e- trains
e+ trains
50% spent to ‘wrong’ helicity pairing 
gain due to xs enhancement for J=1 processes by e+ pol is lost
improvement of DPeff remains for quite frequent reversal – and if systematic errors are small enough
asymmetries can be measured, systematic effects are largely cancelled out
If the e+ helicity will be switched quite frequently this
scheme corresponds to a ‘slow’ Blondel scheme with
luminosity ratio 1/1/1/1 for s++ / s+- / s-+ / s--
Can use annihilation data for polarization measurement
(see POWER report and work done by K. Moenig)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

12. Helicity reversal: Blondel Scheme
Perform 4 independent measurements (s-channel vector exch.)
Can determine Pe+ and Pe- simultaneously (ALR≠0)
need polarimeters at IP for measuring polarization differences d|P+|±, d|P-|± between + and – states  DP (dP±)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

13. S. Riemann: ILC Baseline Positron Polarization
Helicity reversal
Blondel Scheme technique measures directly lumi-weighted polarizations
depolarization effect properly taken into account
Polarization differences have to be measured with high accuracy
Disadvantage of Blondel Scheme with high energy data: new physics in s-channel
Estimated accuracy needed for the first 4 years:
 dPeff/Peff ≤0.3% (0.5%)
Long-term intensity stability  correction and additional syst. error
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

14. S. Riemann: ILC Baseline Positron Polarization
Helicity reversal
Frequency of e+ helicity reversal:
+ and – helicity with equal ratio  No reversal during the first year(s) is not an option at all! (…E166…)
No reversal 
Advantage of reduced error DALR on Peff is lost!!
Low reversal frequencies (days):
each measurement is done separately  large luminosity/intensity corrections
 Need accurate measured lumi and intensities etc.
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

15. S. Riemann: ILC Baseline Positron Polarization
Helicity reversal
Tolerances:
Intensity asymmetry: desired O(‰) (?)
at the beginning 1% is more realistic
polarization asymmetry: <1%
Need to understand relative detector efficiency for ‘+ -’ and ‘- +’ modes at level of few 10-3, later 10-4
Need to measure polarization difference, |Pe+|+ - |Pe+|- at level of <10-2 later 10-3 ( IP)
To reach the high accuracy will be difficult unless we can measure the (+)
and (-) modes simultaneously, i.e. to switch positron polarization randomly
train-to-train
Note: even if positrons are nominally unpolarized, we have tor verify this
 desired (at least for the ~60% e+ polarization): train-by-train
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

16. S. Riemann: ILC Baseline Positron Polarization
space for spin
rotators must be
foreseen
K. Moffeit et al.,
SLAC-TN
Layout of positron damping ring system showing the parallel spin rotation beam lines for randomly selecting positron polarization direction. A pair of kicker magnets is turned on between pulse-trains to deflect the beam to the spin rotation solenoids with negative B-field.
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

17. S. Riemann: ILC Baseline Positron Polarization
(90%, 0%)  (80%, 30%) ?
similar size of effective polarization, Peff ~0.9
BUT: D Peff (90%; 0%) = 2…1.4 ·D Peff (80%; 30%)
(uncor…correlated)
Suppression of undesired helicity states for some processes with (80%, 30%)
Is (90%,30%) an alternative to (80%, 60%) ?
No - due to less significant physics goals
(no transverse polarization  see Gudi’s talk)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization

18. S. Riemann: ILC Baseline Positron Polarization
Summary & Conclusion
We will have a polarized machine from the beginning!
Already 30% e+ polarization is benefit for physics
Low P(e+) allows test of operation with both beams polarized
Utilization of low e+ polarization needs
- Positron polarization measurement
- Spin rotation
- Spin flip
• frequency? Desired: train-by-train
• proposed scheme exists: spin rotators before (LTR) and
after the DR (RTL) are needed (see SLAC-TN ,
EUROTeV-Report )
• other solutions for helicity reversal?
• no reversal is perhaps worse than no polarization!
Further design & simulation work has to be done and should include the ~30% option (depolarisation, polarimeter, spin-flip-frequency etc.)
24 May Posipol, Orsay
S. Riemann: ILC Baseline Positron Polarization