Polarimetry at NLC How Precise?

e-e- 99 Workshop, UC Santa Cruz Dec. 10-12, 1999

Mike WoodsSLAC

Standard Model asymmetries in e+e- and e-e-

• testing for physics beyond SM• polarimetry from SM asymmetries• running at Z resonance

Other considerations for precision polarimetry• background suppression of W pairs in e+e-

• depolarization in beam-beam interaction• design of extraction line and beam losses

Assumptions for Machine Performance

Parameter e+e- e-e-

500 GeV 500 GeV 80 fb-1 25 fb-1

P1 0 90%P2 90% 90%

s

Ldt

SM Asymmetries in e+e-

From Snowmass ‘96 study,

Consider,

%90

0

99.0cos

80

5001

P

P

fbLdt

GeVs

Final State #events ALR

W+W - 560K 100% q q 250K 45% 0.005l+l - 120K 10% 0.032

LR

statLR

A

A

4107

22

2

22

21

statLR

systLRLR

LRLRLR

RL

RLLR

AAA

NP

PAA

P

PA

NNNNA

SM Asymmetries in e+e-

From Snowmass ‘96 study,

Consider,

%90

0

99.0cos

80

5001

P

P

fbLdt

GeVs

Final State #events ALR

W+W - 560K 100% q q 250K 45% 0.005l+l - 120K 10% 0.032

LR

statLR

A

A

4107

22

2

22

21

statLR

systLRLR

LRLRLR

RL

RLLR

AAA

NP

PAA

P

PA

NNNNA

SM Asymmetries in e+e- (cont.)

Notes:1. Better than 1% polarimetry is needed to fully exploit

these measurements for SM tests.

2. Can we use asymmetry in forward W pairs as a polarimeter?Yes, if can achieve backgrounds below 1%.(This level of backgrounds is achieved for LEP200 W mass measurements, if require one W to decay to ee or .)

• advantage wrt Compton polarimetry is that any depolarization in beam-beam interaction is properly accounted for

• disadvantage wrt Compton polarimetry is Compton can achieve 1% accuracy in a few minutes

e-

e+

W-

W+

From F. Cuypers and P. Gambino,Phys. Lett. B388: 211-218, 1996,

Consider,

%90

995.0cos

25

500

21

1

PP

fbLdt

GeVs

Measure 3 asymmetries:

RLLR

RLLR

LRRR

LRRR

RRLL

RRLL

NN

NNA

NN

NNA

NN

NNA

3

2

1

to determine: 212 ,,sin PPeff

W

009.0

00026.0sin

2

2

1

1

2

P

P

P

P

effW

For comparison,

i) SLD has achieved ii) E158 at SLAC will achieve (at Q2=0.02 GeV2)

00026.0sin2 effW

0008.0sin2 effW

SM Asymmetries in e-e- e-e-

SM Asymmetries in e-e- (cont.)

Notes:1. Achieves better than 1% polarimetry using a SM

physics asymmetry. Again, has advantage wrtCompton polarimetry that it properly takes into account any depolarization due to beam-beam effects.But disadvantage is that Compton can achieve 1%accuracy in a few minutes.

The Linear Collider Z-factory option

Some anomalies remain from the LEP/SLC era(sin2W

eff, Ab, N)

May be very desirable to accumulate a large Z sample (>>10M) with polarized beam(s)(ex. Monig and Hawkings, DESY-99-157)

Ideally the positron beam has P+=0.6,and can then use Blondel scheme for polarimetryfrom the measured physics asymmetries in the detector.

However, if positron beam is unpolarized thenwill want a very precise Compton polarimeter,better than the 0.5% accuracy achieved with SLD’s Compton. And will want the Compton tomeasure any beam-beam polarization effects.

Other Considerations for Precision Polarimetry

Background suppression of W pairs in e+e-

• most important is to achieve high polarization;increasing P from 80% to 90% allows for afactor 2 further background reduction

• need more precise polarimetry as P increases

An example P =90%Observe 400 events -- after analysis cuts, but no polarization cutObserve 40 events -- after additional requirement on polarization state

An excess of 20 events is observed above the expected W pair background.Would like 1% polarimetry in order to achieve a 4signal.

P

PstatAmeas

LR

)(024.080.0

measLRA

P

P

0 2.4%1% 2.6%2% 3.1%

Depolarization in beam-beam interaction

• need Compton polarimeter in extraction line to measure polarization with and without collisions, or polarization measured from a physics asymmetry

• need to emphasize that depolarization should be included in parameter tables for the Interaction Region

• need to encourage the simulation programs Guinea-Pig and CAIN to include polarization effects

Design of Extraction Line; effect of beam losses

• ideally, want to have a large number of diagnostic devices for measuring and optimizing luminosity, polarization and energy measurements• in practice, need to balance this with cleanly transporting the beams to the dumps. Want to minimize beam losses and backgrounds for the detector.• ZDR approach allowed for a Compton polarimeter, a wire scanner and other devices

Increased disruption effects in higher luminosity schemes or e-e- option, may lead to elimination of some extraction line diagnostics

• important to point out how this may limit the physics capability• important to still try to incorporate polarization and energy diagnostics in the extraction line

But

Dis

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beam

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xist

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extr

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incr

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s fr

om 0

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to 3

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Summary

Standard Model asymmetries- better than 1% polarimetry is needed for testing SM and

probing for new physics- SM asymmetries in e-e- e-e- and in e+e- W+W- should

achieve better than 1% polarimetry (very good detector coverage and capability needed for forward angles)

Other considerations for precision polarimetry- should have a Compton polarimeter in the extraction line- depolarization effects should be calculated in beam-beam simulations

and tabulated in IR paramater tables- high luminosity scenarios and e-e- option significantly complicate

the design for a Compton polarimeter in the extraction line,and could make it impractical