Towards Precision Polarimetry at the ILC: Concepts

Introduction The Overall Polarimetry Concept Simulation Studies Testbeam Summary & Outlook

Towards Precision Polarimetry at the ILC:

Concepts, Simulations, Testbeam Results

C. Bartels, C. Helebrant, Dr. D. Kafer, Dr. J. List

DESY Hamburg

NSS, Dresden, October 23, 2008

IntroductionThe Overall Polarimetry ConceptSimulation StudiesTestbeamSummary & Outlook

Towards Precision Polarimetry at the ILC Dr. J. List 1


The International Linear Colliderthe goals:

I Unveil the nature of physics beyond the Standard Model

I precision measurements of known and new particles

the tools:

I Electron - positron collisions at√

s = 90 GeV up to 1 TeV

I Polarisation: Pe− = 80-90%, Pe+ = 30-60%

the challenge:

I determine luminosity weighted average polarisation at thecollision point to δP/P = 0.1%

I ... and in some cases even to δP/P = 0.01%



Compton Polarimetry at the ILCI Compton scattering off laser beam:

I hit O(103) e± per bunch of 1010

I P propotional to energy asymmetryI scattered e± colimated within 10 µradI ⇒ spectrometer magnets: energy → position

I achieved (SLD): δP/P = 0.5% , ILC: δP/P = 0.25% (syst.)

I not possible at e+e− IP, but upstream and downstream

I typical timescales: few bunches / trains



Polarimetry with Annihilation Data

e+e− → W +W−

I from total cross-section or dσd cos θ

I contribution of new physics?⇒ common determination with triple gauge couplings

I longterm (O (years)) absolute scale to δP/P = 0.1%

c.f. LC-PHSM-2001-022, update underway

Blondel Scheme

I needs Pe+ 6= 0 and all four e± helicity combinations

I determines Peff =|Pe− |+|Pe+ |

1+|Pe− | |Pe+ | to δPeff/Peff = 0.01%

c.f. K. Monig, LCWS S2004



Complementarity of Polarimeters and Annihilation Data

Tasks

I tune spin rotators, monitor timedependence and correlations

I determine spin transport effects

I depolarisation due to collisions

I analysis of first years’ data

I direct access to luminosityweighted average polarisation

I ultimate calibration of absolutepolarisation scale

I cross check, cross check, crosscheck!

Tools

I fast → polarimeters

I 2 locations → polarimeters

I non-colliding → polarimeters

I”fast“ → polarimeters

I annihilation data

I annihilation data

I polarimeters andannihilation data



Complementarity of Up- and Downstream Polarimetry

Upstream Polarimeter

I 1.8 km upstream of IP

+e /e

+e /e IP

Dipole 18.1m

Dipole 2

8.1m

Dipole 4

Dipole 3

16.1m

8 m

16.1m

CherenkovDetector

total length: 74.6 m

125 GeV

25 GeV

50 GeV

Magnetic Chicane

laserwiredetector

250 GeV24

cm

45.6 GeV

inout

LaserIP

?

Downstream Polarimeter

I 140 m downstream of IP

Combination

I without collisions: spin transport in Beam Delivery System

I with collisions: depolarisation at IP

I cross check each other!1

1c.f.”Spin Dance“ Exp., Phys. Rev. ST Accel. Beams 7 042802 (2004)



Complementarity of Up- and Downstream Polarimetry

Upstream Polarimeter

I 1.8 km upstream of IP

I clean environment

I stat. error 1% after 6 µs

I machine tuning (upstreamof tune-up dump)

Downstream Polarimeter

I 140 m downstream of IP

I high backgrounds

I stat. error 1% after ' 1 min

I access to depolarisationat IP

Combination

I without collisions: spin transport in Beam Delivery System

I with collisions: depolarisation at IP

I cross check each other!1

1c.f.”Spin Dance“ Exp., Phys. Rev. ST Accel. Beams 7 042802 (2004)



Design of the Upstream Polarimeter ChicaneWhy a 4-Dipole-Chicane?

I Compton edge position (least energetic e±) at detectorindependent of Ebeam if B-field constant

I price to pay: Compton IP moves laterally with Ebeam

+e /e

+e /e IP

Dipole 18.1m

Dipole 2

8.1m

Dipole 4

Dipole 3

16.1m

8 m

16.1m

CherenkovDetector


125 GeV

25 GeV

50 GeV

Magnetic Chicane

laserwiredetector

250 GeV

24 c

m

45.6 GeV

inout

LaserIP

?



Design of the Upstream Polarimeter Chicane

Scaled field operation?

I fixed Compton IP position

I facilitates energy collimation, emittance diagnostics

+e /e

+e /e IP

16.1m

Dipole 3

16.1m 8.1m

Dipole 4

Dipole 2

Dipole 18.1m

125 GeV

25 GeV

50 GeV

24 c

mtotal length: 77.6 m

MPSCollimator

CherenkovDetector

laserwiredetector

?

LaserIP

out inMagnetic Chicane 3 + 8 m

250 GeV

45.6 GeV



Scaled vs Fixed Field Operation

I detector acceptance varies with Ebeam

⇒ inhomogeneous quality ofpolarisation measurement

I calibration of polarimeter: Comptonedge position w.r.t. main beam

I simulation study for 1cm channels:I fixed field:δP/P = 0.1% ⇔ δx 0.4 mm

I scaled field:δP/P = 0.1% ⇔ δx 0.2 mm

I ⇒ systematic deviations for largescale factors

I not compatible with extreme precisionrequirements c.f. ILC-NOTE-2008-047

(GeV)beam E0 50 100 150 200 250 300 350 400 450 500

det

ecto

r ac

cept

ance

(m)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18 fixed field Compton edge

fixed dispersion Compton edge

zero crossing

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 0.2 0.4 0.6 0.8 1 1.2

1.873 / 4P1 -0.2243 0.8276E-01P2 0.2250 0.8986E-01

B-Field scale factor

P mea

s - P

true



The Cherenkov Detector of the SLD Polarimeter

LED & DESY Testbeam

I Cherenkov gas C4F10, n = 1.0014, 10 MeV threshold

I 3 GeV single e− at DESY II

PM

PM

PM

PM

PMPM

PM

PM

PM

control + readout

comparison / analysis

DAQ:

Cherenkov gas tubes9 channels of Al−coated

and photomultipliers

LEDs +optical fibers

photo−diode

ADC

Reference Side

preampADC 1ADC 2

QDC

Test Side



Single Electron ResponseChannel 5, Data & Simulation

normal5Entries 1000434

/ ndf 2χ 448.3 / 449PhElectr 0.013± 7.082 Area 1± 1.104e+04 GSigma 0.000± 2.489

Charge [pC]0 10 20 30 40 500

500

1000

1500

2000

2500





SLD Chan 5

sim_0_5Entries 50000

/ ndf 2χ 714.4 / 717PhElectr 0.000± 6.048 Area 43.6± 873.8 GSigma 0.360± 2.971

Charge [pC]0 10 20 30 40 500

50

100

150

200

250

300

sim_0_5Entries 50000


Simulation SLD Chan 5 sim_0_5Entries 50000


Channel 7, 0◦ & 90◦normal7

Entries 200000Mean 891.7RMS 447.2

/ ndf 2χ 516.7 / 345PhElectr 0.000± 4.133 Area 8.0± 2338 GSigma 0.000± 2.108

Ladung [pC]0 20 40 60 80 1000

100

200

300

400

500

600

700

normal7Entries 200000Mean 891.7RMS 447.2

/ ndf 2χ 516.7 / 345PhElectr 0.000± 4.133 Area 8.0± 2338 GSigma 0.000± 2.108



0◦ / 90◦ ratio vs channel numberI channels’

”middle“ sections longer ⇒ more light yield

I length of middle sections scales with channel numberI less reflections to PMT for 90◦ orientationI goal: determine reflectivity, tune simulation (red line:

R = 0.94%)



Results: Crosstalk & Channel Geometry

I observation: neighboringchannels on the outside ofthe first bend observe partof signal





I explanation: cross-talk if e−

traverses neighboringchannel close to mirror!





I explanation: cross-talk if e−

traverses neighboringchannel close to mirror!

I ILC solution: use U-shapedchannels, bend in 3rddimension!

e

20 identical channels diameter: 10 mm

10 c

m

PM

15 cm

Al−

tube

s

Cherenkov photons

Laser / LED (calibration)



Spatial Distribution of Light in Channel

I SLD: inhomogenous lightyield due to widening ofchannels

I avoid in ILC design!



Summary

I precision goals of ILC require combination of upstream anddownstream polarimeters as well as annihilation data

I best design for upstream polarimeter is a four-magnet chicanewith fixed field operation at all beam energies

I polarimters should improve by factor of 2 w.r.t. SLD

I Cherenkov detector of SLD has been operated in testbeam

I good agreement with simulation

I several improvements for ILC design identified



Outlook

I ILC-like prototype under construction

I various photodetectors under test (c.f. poster session)

I testbeam measurements with multiple electron events atELSA in spring 2009



BACKUP



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Dipole 18.1m 8.1m

Dipole 416.1m 16.1m

D3D2

8 m

a =0.837 mrad

CherenkovDetector

24 c

m

2mm

?


250 GeV

Synchrotron Radiation Geometry

45.6 GeVIP

Laser


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Towards Precision Polarimetry at the ILC: Concepts