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Compton Scattering and Beam Energy Measurements*
January 8th, 2001 Chicago LC Workshop Eric Torrence University of Oregon

· Beam Energy Requirements · Compton Scattering Basics · Why it doesn't work

* I thought I had a good idea ...
http://physics.uoregon.edu/~torrence/talks/ CLCW02
Eric Torrence 1/11 January 2002


Precision Electroweak Weak Mixing Angle
sin
2 eff W

E

beam

[MeV]

E

beam

[ppm]

SLD e- only Blondel e- only

0.00027 0.00005 0.00002

25 ~5 ~2

500 100 40

· 50M events · P e - = 80 %, P Blondel scheme · ~100M events · P e - = 80 %, P

e

+

= 0%

· P e / P e = 0.25 %

e

+

= 50 %

W Threshold E
beam

< 5 MeV [50 ppm]

Eric Torrence

2/11

January 2002


Beam Energy Needs Weak Mixing Angle e- only: E
beam

< 5 MeV [100 ppm] < 2 MeV [40 ppm]

Blondel: E

beam

Can use lineshape to calibrate Ebeam? WW Threshold E
beam

< 5 MeV [50 ppm]

Low beamstrahlung needed Top Mass 40 MeV in 10fb-1 [230 ppm] Higgs Mass ~ 50 MeV (Direct Reconstruction) 100-200 ppm `adequate' for HE running
Eric Torrence 3/11 January 2002


Meet the WISRD
Spectrometer Magnet Quadrupole Vertical Doublet eHorizontal Bends for Synchrotron Radiation

E

beam

l = -- B dl x



Dump Synchrotron Light Monitor

e+

SLC Parameters at 50 GeV



B dl = 3.05 Tesla meters x = 27 cm at 50 GeV

l = 15 meters

Systematic Errors per Beam
B dl : Alignment: 190 ppm Detector - IP: 135 ppm Total: 250 ppm 12.5 MeV at 50 GeV Can probably improve below 200 ppm up to 200 GeV



100 ppm

Eric Torrence

4/11

January 2002


Compton Polarimetry
532 nm Frequency Doubled YAG Laser e­ Mirror Box Pockels Cell Left or Right Circularly Polarized Photons Focusing and Steering Lens Mirror Box (preserves circular polarization) Compton Back Scattered e­ Analyzing Bend Magnet
1-93 7268A1

e+

SLD e­ Laser Beam Analyzer and Dump "Compton IP"

Cerenkov Detector Proportional Tube Detector

Compton Kinematics 4 KE y 1 + ---------2 me E K --------- x 1+y me
­1

=E

min

/E
max

2 ­1

= K / K

(Lab Frame Coordinates)
Eric Torrence 5/11 January 2002


Scattered Electrons
50 40 30 20 10 0

[mb/cm]

Unpolarized Cross Section

1.0 0.8 0.6 0.4 0.2 0.0 -0 .2 -0 .4 -0 .6 6 8 10 12 14 16 18 Transverse Distance from Neutral Beamline [cm] Scattering Asymmetry

Channel 6 Response

20

· Kinematic Endpoint · ·

E

min z e

= yE

1 ­ y2 Endpoint Asymmetry A = -------------2 1+y Asymmetry crossing x = ( 1 + y ) ­1 ,

K

= me / E

Can we use these features to measure E?
Eric Torrence 6/11 January 2002


Endpoint Position
Kinematic Endpoint e Dipole dE min -------------- = y dE Neutral Beamline

2

E

[GeV]

y 0.359 0.219 0.123 0.066

E

min [GeV]

50 100 200 400

18.0 21.9 24.6 26.2
(Assume K=2.33 eV throughout)

· Poor analyzing power · Difficult to measure absolute position · Must take out beam motion

Eric Torrence

7/11

January 2002


Endpoint to zero crossing

e-

Dipole Neutral Beamline 0

2y ----------- E Zero asymmetry E = y+1 E
[GeV]

y 0.359 0.219 0.123 0.066

E

min [GeV]

50 100 200 400

18.0 21.9 24.6 26.2

2y ----------y+1 0.528 0.359 0.219 0.123

E

0 [GeV]

26.4 35.9 43.8 49.2

1 1 2K ---------- ­ ----- = ----- 2 E min E 0 me Independent of beam energy!

Eric Torrence

8/11

January 2002


Vary photon energy
Kinematic Endpoint e Dipole Neutral Beamline Kinematic Endpoint

^ K = K/2
e Dipole Neutral Beamline ­1

2 KE ^ New Endpoint y = 1 + ---------2 me

2y = ----------y+1

Same energy as zero crossing!

^ Energy independent for any K 4K 1 1 ----------- ­ ------ = ( 1 ­ z ) 2 ^ me Em Em
where

^ K = zK

Eric Torrence

9/11

January 2002


Use scattered photons? Incoming
e

Outgoing E'
2 ­1

E

K E K --------- x 1+y me Maximum E
[GeV] K

K'

K

= me / E R ( 100 m ) 1.02 0.51 0.26 0.13
[mm]



K [µRad]

50 100 200 400

10.2 5.1 2.6 1.3

· Need 100 ppm on 1 mm radius (at 100 m) · Beam divergence will smear this out · Longer drift possible?

Eric Torrence

10/11

January 2002


Conclusions Beam Energy Requirements · 200 ppm for High Energy running · <50 ppm for precision EW

Current Proposals · WISRD (SLC-style) · BPM spectrometers (LEP-style)

Compton Scattering · Interesting kinematic properties · Potentially useful for polarization Unsuitable for Beam Energy Measurements

Other Possibilities · Bhabha/MЬller scattering · ???

Eric Torrence

11/11

January 2002