[PPT] - Challenges for Polarimetry at the ILC Moritz Beckmann, Anthony PowerPoint Presentation

SLIDE 1

Challenges for Polarimetry at the ILC

Moritz Beckmann, Anthony Hartin, Jenny List

DESY - FLC

EUCARD Workshop “Spin optimization at Lepton accelerators” Mainz, Germany February 12, 2014

SLIDE 2

Outline

Introduction
Polarimetry in the ILC beam delivery system
Spin transport
Results
Beamline simulation
Collision effects
Polarization measurement at the disrupted beam
Beamline design in view of the polarization measurement
Impact of the laser spot size at the downstream polarimeter

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 2 / 26

SLIDE 3

ILC Beam Delivery System (BDS)

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 3 / 26

SLIDE 4

Laser Compton Polarimeter (Principle)

dipole magnets polarized laser scattered e± e± beam detector

Laser: Compton scattering

e± γ → e± γ

Scattering cross section depends on Pz
Magnet chicane separates scattered e± from beam
Pz is determined from scattered electrons

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 4 / 26

SLIDE 5

Polarimetry at the ILC

150 m ~1 650 m upstream polarimeter IP downstream polarimeter

e± beamlines

Two Compton polarimeters per beam to measure Pz
Upstream polarimeter undisturbed by collision effects
Downstream polarimeter assesses collision effects
0.25 % systematic uncertainty (goal)
What do these measurements tell us about the

longitudinal polarization at the IP? → spin transport simulation

Aim to understand spin transport to 0.1 %

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 5 / 26

SLIDE 6

Spin Precession

Spin precession in electromagnetic fields:

T-BMT equation

For

B⊥ only: ϑspin = b(E) · ϑorbit b(E) = aγ + 1 = g−2

2

· E

m + 1

≈ 568 for 250 GeV-electrons

Dipole magnets, no beam energy spread:

spin vectors precess uniformly, | P| conserved

B

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 6 / 26

SLIDE 7

Spin Fan-Out in Quadrupole Magnets

P P' P quadrupole magnets

For illustration purposes, the second quadrupole is stronger. Two-dimensional betatron oscillations are not taken into account here.

Different precession angles after first quadrupole

⇒ polarization | P| decreases

|

P| recovered by second quadrupole

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 7 / 26

SLIDE 8

Beam-Beam Collision Effects

e e Pairs

+ _

Beamstrahlung

A. Vogel

Bunches focus each other by their electromagnetic fields:

Spin fan-out (like in quadrupole magnets)
Spin flip by emission of beamstrahlung

(Sokolov-Ternov effect)

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 8 / 26

SLIDE 9

Spin Transport Simulation Framework

UP IP DP beam-beam collision Guinea-Pig++ data analysis polarimeter measurement polarimeter measurement beamline layout particle / spin transport along the BDS Bmad beam parameters

Developed a beamline simulation (based on Bmad)
Simulate 40 000 (macro)particles per bunch, generated from

beam parameters at the beginning of the BDS

Interfaced directly to the simulation of the collisions

(Guinea-Pig++)

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 9 / 26

SLIDE 10

Results

√s = 500 GeV
Beam parameters according to Reference Design Report

(RDR, 2007)

Collision effects also for beam parameters according to

Technical Design Report (TDR, 2013)

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 10 / 26

SLIDE 11

Spin Transport in the BDS: Basic Configuration

distance s along BDS [m]

500 1000 1500 2000 2500 3000 3500

polarization

0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8

UP IP DP

e

z

P | P | UP/DP: up-/downstream polarimeter Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 11 / 26

SLIDE 12

Spin Transport in the BDS: Basic Configuration

distance s along BDS [m]

3400 3450 3500 3550

polarization

0.799 0.7995 0.8

]

3

relative change [10

1
0.5

IP DP

e

z

P | P | extraction line quadrupoles dipole chicanes

DP: downstream-polarimeter

Quadrupoles cause spin fan-out
Changes in Pz well below 0.1 % without collisions

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 12 / 26

SLIDE 13

Factors affecting the spin transport (without collisions)

contribution uncertainty [10−3] Beam and polarization alignment 0.72 (∆ϑbunch = 50 µrad, ∆ϑpol = 25 mrad) Random misalignments (10 µm) 0.43 Variation in beam parameters (few %) 0.03 Bunch rotation (crab cavities) < 0.01 Detector solenoid 0.01 Synchrotron radiation 0.005 Total (quadratic sum) 0.85 Now: e+e− beam collisions

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 13 / 26

SLIDE 14

Spin Transport after Collision

distance s along BDS [m]

3400 3450 3500 3550

z

long. polarization P

0.76 0.77 0.78 0.79 0.8

]

3

relative change [10

40
20

IP DP

e

no collision lumi-weighted after collision measurable no collision lumi-weighted after collision measurable

DP: downstream-polarimeter

Luminosity-weighted (•): Pz of the colliding particles
Larger angular divergence / energy spread after collision
Large spin fan-out in extraction line quadrupoles

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 14 / 26

SLIDE 15

Spin Transport after Collision

distance s along BDS [m]

3400 3450 3500 3550

z

long. polarization P

0.76 0.77 0.78 0.79 0.8

]

3

relative change [10

40
20

IP DP

e

no collision lumi-weighted after collision measurable no collision lumi-weighted after collision measurable

DP: downstream-polarimeter

Extraction line design: restore luminosity-weighted Pz

(•) at the downstream polarimeter

Employ spin fan-out: focus beam at downstream

polarimeter with half divergence angle w. r. t. the IP

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 15 / 26

SLIDE 16

Spin Transport after Collision

distance s along BDS [m]

3400 3450 3500 3550

z

long. polarization P

0.76 0.77 0.78 0.79 0.8

]

3

relative change [10

40
20

IP DP

e

no collision lumi-weighted after collision measurable no collision lumi-weighted after collision measurable

DP: downstream-polarimeter

θx ≫ θy ⇒ ∆Pz ∝ θx

2

∆Plum

z

≈ 1

4∆Pz ∝

θx 2 2 Idea: |R22(IP → DP)| = 0.5 ⇒ Plum

z

= PDP

z

Further reading: SLAC-PUB-4692, SLAC-PUB-8397 Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 16 / 26

SLIDE 17

Laser and Particle Bunch at the Downstream Polarimeter

after collision without collision

laser spot e± beam

~mm ~cm 0.04mm ~cm

Without collision: entire beam exposed to laser
After collision: center of beam exposed to laser

sample of scattered electrons representative?

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 17 / 26

SLIDE 18

Downstream Measurement

Downstream polarimeter located in magnet chicane

⇒ particle position correlated with energy (dispersion)

0.4
0.3
0.2
0.1
0.02
0.015
0.01
0.005

0.005 0.01 0.015 1 10

2

10

3

10

4

10

5

10

6

10

particle energy [GeV]

160 180 200 220 240

vert. particle position y [mm]
20
10

10

Laser spot size

Laser spot size at Compton-IP only ∼ 0.1 - 1 mm

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 18 / 26

SLIDE 19

Downstream Measurement

Beamstrahlung correlates energy and Pz

⇒ Pz correlated with particle position ⇒ Selective measurement, measurement bias

0.015
0.01
0.005

z

long. polarization P

0.75 0.76 0.77 0.78 0.79 0.8

vert. particle position y [mm]
15
10
5

laser spot size

Measurable longitudinal polarization := average Pz of

particles within a given (laser spot) radius

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 19 / 26

SLIDE 20

Spin Transport for Different Beam Parameters

20 40 60

z

long. polarisation P

0.79 0.795 0.8

]

3

relative change [10

15
10
5

no energy spread/loss RDR TDR TDR* RDR TDR TDR*

before collision lumi-weighted after collision DP DP measurable (r=0.1/0.2/0.5/1.0 mm)

DP: downstream polarimeter

No energy spread/loss: no discrepancy between measurement

() and average Pz () at downstream polarimeter

RDR → TDR: stronger focussing ⇒ higher collision intensity

⇒ larger spin fan-out in collision and afterwards

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 20 / 26

SLIDE 21

Spin Transport for Different Beam Parameters

20 40 60

z

long. polarisation P

0.79 0.795 0.8

]

3

relative change [10

15
10
5

no energy spread/loss RDR TDR TDR* RDR TDR TDR*

before collision lumi-weighted after collision DP DP measurable (r=0.1/0.2/0.5/1.0 mm)

DP: downstream polarimeter

Extraction line design: restore Plum

z

(•) at downstream pol. ()

Design (|R22| = 0.5) assumes Dx ≪ 1

DRDR

x

= 0.17 DTDR

x

= 0.3

More beamstrahlung (not accounted for by design)

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 21 / 26

SLIDE 22

Spin Transport for Different Spin Configurations

DP TDR*

R22=-0.5

P' P P P'

IP TDR

after collision

T-BMT T-BMT

assuming θspin= (aγ+1)·θorbit at the beginning assuming θspin= (aγ+1)·θorbit at the beginning

For illustration only. All angles exaggerated. Beamstrahlung effects neglected.

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 22 / 26

SLIDE 23

Spin Transport for Different Spin Configurations

20 40 60

z

long. polarisation P

0.79 0.795 0.8

]

3

relative change [10

15
10
5

no energy spread/loss RDR TDR TDR* RDR TDR TDR*

before collision lumi-weighted after collision DP DP measurable (r=0.1/0.2/0.5/1.0 mm)

DP: downstream polarimeter

TDR* with respect to TDR:

All spin vectors parallel before collision, bunch focussed

(45 µrad divergence angle)

Mostly same behaviour in collision (, •, ), but different

value at downstream polarimeter ()

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 23 / 26

SLIDE 24

Spin Transport for Different Beam Parameters

20 40 60

z

long. polarisation P

0.79 0.795 0.8

]

3

relative change [10

15
10
5

no energy spread/loss RDR TDR TDR* RDR TDR TDR*

before collision lumi-weighted after collision DP DP measurable (r=0.1/0.2/0.5/1.0 mm)

DP: downstream polarimeter

Polarization varies by several % along the extraction line
Discrepancies between Plum

z

and Pz at the downstream pol. (•, , ) in the range 0.1 − 0.4 %; discrepancies cancel partially, but only coincidentally

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 24 / 26

SLIDE 25

Conclusions

Cross-calibration (without collisions) to precision of < 0.1 %
Polarization vector alone not sufficient anymore to describe

spin configuration of beam:

Spin fan-out becomes relevant due to higher measurement

precision, higher energy and more intensive collisions

How well do we know the initial spin configuration?

→ “cradle-to-grave” simulation

Extraction line design (restore Plum

z

at downstream pol.):

Works as foreseen for low-intensity collisions
TDR beam parameters: higher intensity → larger discrepancies
Beamstrahlung not taken into account; Dx no longer ≪ 1
Disrupted beam lets knowledge of the laser spot size/position

at the downstream polarimeter become crucial for the measurement precision

Larger laser-spot? Drawbacks: required laser power,

low-energy tail undesired in polarimeter

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 25 / 26

SLIDE 26

Thanks for your attention!

Backup Slides

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 27 / 26

SLIDE 28

Differences RDR - TDR

Parameter symbol RDR TDR Bunches per train 2 625 1 312 Horizontal bunch size σx [nm] 639 474 Vertical bunch size σy [nm] 5.7 5.9 Beam energy spread (e−/e+) σE/E [10−3] 1.4/1.0 1.24/0.7 e+e− luminosity L [1038 m−2 s−1] 2 1.47

incl. waist shift

1.8 Beamstrahlung parameter Υglobal 0.048 0.062

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 28 / 26

SLIDE 29

Thomas-Bargmann-Michel-Telegdi (T-BMT) Equation

d dt

S =
ΩB +

ΩE

×

S

ΩB = − q

mγ

(1 + aγ)

B − a p · B (γ + 1) m2c2 p

= − q

mγ

(1 + aγ)

B⊥ + (1 + a) B

ΩE =

q mγ · 1 mc2

a +

1 1 + γ

p ×

E

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 29 / 26

SLIDE 30

Compton Scattering

50 100 150 200 250 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

σ Compton [mbarn / GeV]

Compton−scattered Electron Energy [GeV] λ Pe = +1

(same)

λ Pe = −1

(opposite)

100 200 300 400 500 5 10 15 20 25 30

Compton Endpoint Energy [GeV] Beam Energy [GeV]

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 30 / 26

SLIDE 31

Polarimeter Chicane (upstream)

Constant magnetic field
Dispersion (depending on beam energy): 1-11 cm
Scattering for every bunch per bunch train
Energy spectrum is polarization-dependent
Energy distribution → spatial distribution
Cherenkov gas detector counts electrons per channel

+ e /e + e /e IP

16.1m 8 m 16.1m

Cherenkov Detector

125 GeV 25 GeV 50 GeV

Magnetic Chicane

250 GeV

24 cm

45.6 GeV in

ut

Laser IP

8.1m

Dipole 2 Dipole 3

8.1m

Dipole 4 Dipole 1

P11 P10 P12 P1 P2 P3 P5 P4 P6 P7 P8 P9

total length: 74.6 m Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 31 / 26

SLIDE 32

Polarimeter Chicane (downstream)

Constant magnetic field
Dispersion (depending on beam energy): 1-11 cm
Scattering for 3 bunches per bunch train
Energy spectrum is polarization-dependent
Energy distribution → spatial distribution
Cherenkov gas detector counts electrons per channel

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 32 / 26

SLIDE 33

Polarimeter Detector

e PM LED (calibration) Cherenkov photons

2

10 cm 10 x 10 mm cross section: 15 cm

Al−tubes beam aluminum tubes x y z photodetectors LEDs

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 33 / 26

SLIDE 34

Compton Polarimeters: Systematic Errors

Goal: relative systematic error on measurement < 0.25 % (SLD polarimeter: 0.5 %)

Detector linearity: contribution of ∼ 0.1 − 0.2 % (goal)
Laser polarization: ∼ 0.1 %
Analyzing power: ∼ 0.1 % (UP: , DP: ?)
Detector alignment: can be determined from data ()

0.5 mm precision sufficient

Alignment of magnets negligible compared to detector

Field inhomogeneities? to be investigated

Disrupted electron beam at downstream polarimeter:
Dependence on laser-spot size and position: ??
Beam energy spread no concern for small laser-spot sizes

thanks to dispersion

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 34 / 26

SLIDE 35

Polarization Measurement at the IP

1

Luminosity fb

200 400 600 800 1000 1200

% error polarization

0.2 0.4 0.6 0.8 1

pol

pol 80% e

+

30% e positron Blondel electron Blondel positron Fit electron Fit

1

Luminosity fb

200 400 600 800 1000 1200

% error polarization

0.2 0.4 0.6 0.8 1

pol

pol 80% e

+

60% e positron Blondel electron Blondel positron Fit electron Fit

I. Marchesini

Blondel scheme:

|Plumi

z

(e±)| =

(σ−+ + σ+− − σ−− − σ++)(±σ−+ ∓ σ+− + σ−− − σ++)

(σ−+ + σ+− + σ−− + σ++)(±σ−+ ∓ σ+− − σ−− + σ++)

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 35 / 26

SLIDE 36

Polarization Measurement at the IP

I. Marchesini

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 36 / 26

SLIDE 37

Polarization Measurement at the IP

I. Marchesini

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 37 / 26

SLIDE 38

Quadrupole Magnet

S N S N S N S N

Black arrows: magnetic field lines Blue arrows: forces on an incoming electron beam

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 38 / 26

SLIDE 39

Bunch Rotation at the IP

Collision under crossing angle of 14 mrad
Maximize luminosity: rotate bunches using crab cavities
Time-dependent transverse deflection of particles

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 39 / 26

SLIDE 40

Downstream Pol.: Dispersion w/o Collision

0.008
0.006
0.004
0.002

0.002 0.004

0.25
0.2
0.15
0.1
0.05

0.05 0.1 0.15 0.2

3

10 × 1 10

2

10

3

10

4

10

5

10

particle energy [GeV]

248 248.5 249 249.5 250 250.5 251

vert. particle position y [mm]
0.2
0.1

0.1 0.2 Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 40 / 26

SLIDE 41

Downstream Pol.: Dispersion after Collision

0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0.02
0.015
0.01
0.005

0.005 0.01 1 10

2

10

3

10

4

10

5

10

6

10

particle energy [GeV]

160 180 200 220 240

vert. particle position y [mm]
20
10

10

0.05
0.04
0.03
0.02
0.01
0.4
0.2

0.2 0.4

3

10 × 1 10

2

10

3

10

4

10

5

10

particle energy [GeV]

238 240 242 244 246 248 250

vert. particle position y [mm]
0.5

0.5 Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 41 / 26

SLIDE 42

Downstream Measurement

Longitudinal polarization vs. energy at the downstream polarimeter, after collision

0.3
0.2
0.1
long. polarization

0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8

particle energy [GeV]

160 170 180 190 200 210 220 230 240 250

(E)

z

P

z

average P

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 42 / 26

SLIDE 43

50

z

long. polarization P

0.79 0.795 0.8

]

3

relative change [10

15
10
5

UP before collision lumi-weighted after collision DP DP measurable (r=0.1/0.2/0.5/1.0 mm)

e

n

C

C

n

C

C

L n

B

S , S R n

S

R r e f . W S

W S

L

T D R

ϒ

20 40 60 80

| P entire polarization |

0.79 0.795 0.8

]

3

relative change [10

15
10
5
e

n

C

C

n

C

C

L n

B

S , S R n

S

R r e f . W S

W S

L

T D R

ϒ

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 43 / 26

SLIDE 44

50

z

long. polarization P

0.296 0.297 0.298 0.299 0.3

]

3

relative change [10

15
10
5

UP before collision lumi-weighted after collision DP DP measurable (r=0.1/0.2/0.5/1.0 mm)

+

e

n

C

C

n

C

C

L n

B

S , S R n

S

R r e f . W S

W S

L

T D R

ϒ

20 40 60 80

| P entire polarization |

0.296 0.297 0.298 0.299 0.3

]

3

relative change [10

15
10
5

+

e

n

C

C

n

C

C

L n

B

S , S R n

S

R r e f . W S

W S

L

T D R

ϒ

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 44 / 26

SLIDE 45

Why Polarization?

Electroweak processes: cross sections depend on Pz

e. g. W +W − pair production
I. Marchesini

Polarized beams

provide new observables
can be used to enhance/suppress processes

Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 45 / 26

SLIDE 46

The International Linear Collider (ILC)

e+e− collider as complement to LHC
√s ≤ 500 GeV, upgradable to 1 TeV
Longitudinally polarized beams: |Pz(e−)| = 80 %

|Pz(e+)| = 30 to 60 %

Moritz Beckmann

DESY-FLC EUCARD Workshop Mainz 2014 46 / 26