DIS 2006

1
Proton Polarimetry at RHIC
1. Provide polarization measurements for
accelerator 2. Provide polarization measurements
for experiments

• Alekseev, A. Bravar, G. Bunce, S. Dhawan, R.
  Gill, W. Haeberli,
• H. Huang, O. Jinnouchi, K. Kurita, Y. Makdisi, A.
  Nass, H. Okada,
• N. Saito, H. Spinka, E. Stephenson, D. Svirida,
  T. Wise, J. Wood, A. Zelenski

2
Polarimetry Impact on RHIC Spin Physics
Single Spin Asymmetries
Physics Asymmetries
Double Spin Asymmetries
measurements

• measured spin asymmetries normalized by PB to
  extract Physics Spin Observables
• RHIC Spin Program requires DPbeam / Pbeam 0.05
• normalization Þ scale uncertainty
• polarimetric process with large s sizable and
  known AN
• pC elastic scattering in CNI region, AN 1 2
  large statistics gt 107 events
• (very) large - section fast measurements
• requires absolute calibration polarized gas jet
  target

3
RHIC pp accelerator complex
RHIC pC CNI polarimeters

absolute pH polarimeter
BRAHMS PP2PP
PHOBOS
RHIC
Siberian Snakes
PHENIX
STAR
Siberian Snakes
Spin Rotators
5 Snake
LINAC
BOOSTER
AGS pC CNI polarimeter
AGS
Pol. Proton Source
200 MeV polarimeter
Rf Dipoles
20 Snake
4
How It Works ?

Polarimetry Requires large F.o.M AN2 x rate
for fast measurement (not at any price however,
i.e. by increasing the rates) small AN 1
(far from ideal !) Þ requires large statistics
gt 107, for d PB few however too large rates
(i.e. thick target, detector area, etc.) Þ
occupancy and pileup Þ very difficult
operation Þ corrections to measured
asymmetries Þ larger systematic
uncertainty Absolute calibration Þ Polarized
Gas Jet target
5
Setup for pC scattering the RHIC polarimeters
inside RHIC ring _at_IP12
beam direction
Ultra thin Carbon ribbon Target (3.5mg/cm2, 10mm
wide)
1
6
5
2
Si strip detectors (ToF, EC)
3
4
Beam direction
36cm
all Si strips parallel to beam
RHIC 2 rings

• recoil carbon ions detected with Silicon strip
  detectors
• 2 72 channels read out with WFD (increased
  acceptance by 2)
• very large statistics per measurement ( 20 106
  events) allows detailed analysis
• bunch by bunch analysis
• channel by channel (each channel is an
  independent polarimeter)
• 45o detectors sensitive to vertical and radial
  components of Pbeam
• unphysical asymmetries

6
Event Selection Performance

• - very clean data, background lt 1 within
  banana cut
• - good separation of recoil carbon from a (C a
  X) and prompts
• may allow going to very high t values
• D (Tof) lt 10 ns (Þ sM 1 GeV)
• very high rate 105 ev / ch / sec

7
pC Systematics RHIC
each detector channel covers same t range 72
independent measurements of AN
width stat. error single meas.
channel by channel raw asymmetry
Fit with sine function (phase fixed)
sources of systematic uncertainties 1 D PBEAM
8.5 (normalization) PBEAM 0.386 0.029
0.016 2 energy scale 50 keV for lowest t
bin (from detector dead
layer) NB these are external factors not
intrinsic limitations
8
DAQ and WFD
Wave Form Digitizer peak sensing ADC, CFD,
common to the pC and JET DAQ system
ADC 3140 MHz
synchronized to accelerator clocks bunch -ing Þ
start TDC

• online analysis of waveform
• performed beteween consecutive
• bunch -ing
• PH, tot Q, t.o.f.
• full waveform (JET)

FPGA
Dt 2 ns DE lt 50 keV
onboard memory
DAQ PC
20 106 events in 20 seconds Þ deadtimeless DAQ
system can accept, analyze, and store 1 event /
each bunch -ing
9
AGS polarization during acceleration (ramp)
each point 50 MeV step

raw asymmetry AN PB
depolarizing resonances intrinsic Gg
imperfection Gg n
12n
36-n
36n
Gg 1.91 Ebeam
48-n
red line simulation of polarization
losses assuming constant AN
10
Bunch by Bunch
110 bunches

typical example sometime even nicer than in
textbooks
11
Bunch by Bunch Spin Patterns
high statistics 28 bunches _at_ injection

110 bunches _at_ flattop 1011 p / bunch
110 bunches _at_ flattop with messed spin pattern
12
Beam Polarization Profiles
to avoid target location dependencies, sweep the
target through the beam gt average bunch
polarization
2 mm

intensity profile
13
Beam Emittance
by sweeping the carbon target through the beam
1 sec. measurement

14
The Road to Pbeam with the JET target

• Requires several independent measurements
• 0 JET target polarization Ptarget (Breit-Rabi
  polarimeter)
• 1 AN for elastic pp in CNI region AN 1 /
  Ptarget eN
• 2 Pbeam 1 / AN eN
• 1 2 can be combined in a single measurement
  Pbeam / Ptarget eN / eN
• self calibration works for elastic scattering
  only
• 3 CALIBRATION ANpC for pC CNI polarimeter in
  covered kinematical range
• ANpC 1 / Pbeam eN
• (1 ) 2 3 measured simultaneously with several
  insertions of carbon target
• 4 BEAM POLARIZATION Pbeam 1 / ANpC eN to
  experiments
• at each step pick-up some measurement errors

expected precision
transfer calibration measurement
15
JET Target
ANbeam (t ) ANtarget (t ) for elastic
scattering only! Pbeam Ptarget . eNbeam /
eNtarget

16
JET target polarization performance

• the JET ran with an average intensity of 11017
  atoms / sec
• the JET thickness of 1 1012 atoms/cm2
  record intensity
• target polarization cycle
• /0/- 500 / 50 / 500 sec
• polarization to be scaled down due to a 3 H2
  background
• Ptarget 0.924 0.018
• (current understanding)
• no depolarization from beam
• wake fields observed !

17
pp elastic data collected
ToF vs EREC correlation Tkin ½ MR(dist/ToF)2
recoil protons elastic pp ?pp scattering
JET Profile measured selecting pp elastic events
FWHM 6 mm as designed
D ToF lt 8 ns
background 118 cts. subtracted
Number of elastic pp events
T Kin MeV
CNI peak AN 1 lt E REC lt 2 MeV
? source calibration
Hor. pos. of Jet 10000 cts. 2.5 mm
prompt events and beam-gas

• recoil protons unambiguously identified !
• 100 GeV 1.8 106 events for 1.5 103 lt -t lt
  1.0 102 GeV2
• similar statistics for 1.0 102 lt -t lt 3.0
  102 GeV2
• 24 GeV 300 k events

18
Energy - Position correlations
Tkin µ q2 (i.e. position2)

number of events (a. u.)
Energy
pp elastic events clearly identified !
Position
19
Event selections Backgrounds
Strip distribution for energy interval 1250
1750 keV
signal
background
Si 2
Si 6
implement the energy / angle correlation in
selecting elastic pp events typically, for each
energy bin, select 3 to 4 strips per
detector Background only from selected channels
not from whole detector (4 5 smaller !) Total
Backgrounds lt 8 a source lt 4, interaction
backgrounds lt 4
20
PBEAM
self calibrating Target eT target
asymmetry average over beam polarization Beam
eB beam asymmetry average over
target polarization
ratio eB / eT
reduced c2 1
tot sys 0.016

• PBEAM 0.392 0.021 (stat) 0.008 (D PTARGET)
  0.014 (sys) 0.392 0.026
• 2004 ERROR D PBEAM / PBEAM 6.6
• 2005, 2006 in progress D PBEAM / PBEAM lt 5 !

21
Summary

• fast measurements of Pbeam in 30 sec.
• provides also information on beam properties
• steady progress in understanding and addressing
  systematic issues
• online precision 10 (relative)
• absolute normalization with a polarized gas JET
• during 2004 run with Jet target precision on beam
  polarization
• D PBEAM / PBEAM 6.6
• 2005 2006 (in progress) D PBEAM / PBEAM lt 5
  !