Polarimetry of Proton Beams at RHIC

A.Bazilevsky

Summer Students LecturesJune 17, 2010

What is beam polarization? Simple example: spin-1/2 particles (proton, electron)

Can have only two spin states relative to certain axis Z: Sz=+1/2 and Sz =-1/2

2/12/1

2/12/1

ZZ

ZZ

SS

SS

NN

NNP

022

22

P

5.013

13

P

104

04

P

|P|<1

Why polarized protons?See lecture by A.Vossen

Longitudinally polarized protonsProton helicity structure: (anti-)quark and gluon polarization inside proton

Transversely polarized protons(Anti-)quark transversityParton orbital momentum

Polarimetry goals: Measure and monitor beam polarizationDefine and monitor spin direction in experimental area

BPA

1Single

21Double

1

BB PPA

Double spin asymmetrySingle spin asymmetry

“+” spin aligned with beam direction (long. spin) or spin up (transverse spin)“” spin anti-aligned with beam direction (long. spin) or spin down (transverse spin)

p

p

How to measure proton beam polarizationThere are several established physics processes sensitive to the spin direction of the transversely polarized protons

Scattering to the right

Scattering to the left

AN – the Analyzing Power (|AN|<1)(left-right asymmetry for 100% polarized protons)

NAP

Once AN is known:

PANN

NNN

RightLeft

RightLeft

Polarization MeasurementsRightLeft

RightLeft

NN NN

NN

AAP

1

AN depends on the process and kinematic range of the measurements

pC elastic scattering

NAP

N

11)( Precision of the

measurements N=NLeft+NRight

For (P)=0.01 and AN~0.01 N~108 !

Requirements: Large AN or/and high rate (N)Good control of kinematic range

-t=2MCEkin

RHIC and Polarimetry

BRAHMS & PP2PP (p)

STAR (p)PHENIX (p)

AGS

LINAC BOOSTER

Pol. Proton Source500 A, 400 s

Spin RotatorsSolenoid Snake

Siberian Snakes

200 MeV Polarimeter

AGS pC CNI PolarimeterAC Dipole

RHIC pC Polarimeters

Absolute Polarimeter (H jet)

RHIC

Siberian Snakes

Cold Snake

Warm Snake

6

RHIC PolarimetryPolarized hydrogen Jet Polarimeter (HJet)

Source of absolute polarization (normalization to other polarimeters)Slow (low rates needs lo-o-ong time to get precise measurements)

Proton-Carbon Polarimeter (pC)Very fast main polarization monitoring toolMeasures polarization profile (polarization is higher in beam center)Needs to be normalized to HJet

Local Polarimeters (in PHENIX and STAR experiments)Defines spin direction in experimental areaNeeds to be normalized to HJet

All of these systems are necessary for the proton beam polarization measurements and monitoring

Beam and target are both protons

beam

beam

target

target

PPtAN

RHIC proton beam

Forward scatteredproton

H-jet target

recoil proton

target

target beam

beam PP

02 inout ppt

Ptarget is provided by Breit Rabi Polarimeter

Left-right asymmetry in elastic scattering: due to spin-orbit interaction: interaction between (electric or strong) field of one proton and magnetic moment associated with the spin of the other proton

8

PNN

NN

PA N

RL

RLN

1

Polarized H-Jet Polarimeter

1 day

Breit-Rabi Polarimeter:

Separation of particles with different spin states in the inhomogeneous magnetic field (ala Stern-Gerlach experiment)

Nuclear polarization

Very stable for entire run period !Polarization cycle (+/ 0/ ) = (500/50/500) seconds

HJet: PtargetSource of normalization for polarization measurements at RHIC

Nuclear polarization of the atoms:

95.8% 0.1%

After background correction:

Ptarget = 92.4% 1.8%

HJet:

Provides statistical precision (P)/P~0.10 in a store (6-8 hours)

target

target beam

beam PP Example from Run-2006

εbeam

εtarget

t=-2MpEkin

Use the same statistics (with exactly the same experimental cuts) to measure beam and target

(selecting proper spin states either for beam or for target)

Many systematic effect cancel out in the ratio Ekin (MeV)Ekin (MeV)

target

beam

HJet Provides very clean and stable polarization measurements but with limited stat. precison

Need faster polarimeter!

P-Carbon Polarimeter:

Ultra thin Carbon ribbon Target(5 g/cm2)

11

3344

55

66

22

Si strip detectorsSi strip detectors(TOF, E(TOF, ECC))

18cm18cm

Polarized proton

Recoil carbon

Carbon target

LL NN or

RR NN or

or

Left-right asymmetry in elastic scattering: due to spin-orbit interaction: interaction between (electric or strong) field of Carbon and magnetic moment associated with the spin of the proton

Pbeam NAN

pC

N NL NR

NL NR

Target Scan mode (20-30 sec per measurement)

Stat. precision 2-3%

Polarization profile, both vertical and horizontal

Normalized to H-Jet measurements over many fills (with precision <3%)

Poarization Profile

H-Jet

p

~1 mm

6-7 mm

pC ColliderExperiments

P1,2(x,y) – polarization profile, I1,2(x,y) – intensity profile, for beam #1 and #2

x=x0

),(),( 01011 yxIyxPP ),(),(),( 2111 yxIyxIyxPP ),(),( 111 yxIyxPP

If polarization changes across the beam, the average polarization seen by Polarimeters and Experiments (in beam collision) is different

Pol. Profile and Average Polarization

Carbon

Scan C target across the beamIn both X and Y directions

Target Position

Inte

nsity

Pola

rizati

on

I

P

2

2

P

IR

YX

YX

YX

HJet

Exp RR

RR

RR

P

P

4

11

21

121

1

11

Run-2009:

Ebeam=100 GeV: R~0.1 5% correction

Ebeam=250 GeV: R~0.35 15% correction

Ideal case: flat pol. profile (P= R=0)

pC+HJet: Polarization vs FillRun-2009 results (Ebeam=100 GeV)

Normalized to HJet Corrected for polarization profile (by pC)

P/P < 5%

Dominant sources of syst. uncertainties:

~3% - HJet background

~3% - pC stability (rate dependencies, gain drift)

~2% - Pol. profile“Yellow” beam

“Blue” beam

Need for Local Polarimeters

BRAHMS & PP2PP (p)

STAR (p)PHENIX (p)

AGS

LINAC BOOSTER

Pol. Proton Source500 A, 400 s

Spin RotatorsSolenoid Snake

Siberian Snakes

200 MeV Polarimeter

AGS pC CNI PolarimeterAC Dipole

RHIC pC Polarimeters

Absolute Polarimeter (H jet)

RHIC

Siberian Snakes

Cold Snake

Warm SnakeSpin Rotators around experiments may change spin direction in experimental areas

Need to monitor spin direction in experimental areas

Local Polarimeter: PHENIXUtilizes spin dependence of very forward neutron production discovered in RHIC Run-2002 (PLB650, 325)

neutronchargedparticles

Zero Degree Calorimeter

Quite unexpected asymmetryTheory can not yet explain itBut already can be used for polarimetry! PAN

Monitor spin direction

Vertical ~ ±/2Radial ~ 0Longitudinal no asymmetry

Measures transverse polarization PT , Separately PX and PY

Longitudinal component:P – from CNI polarimeters

22TL PPP

Vertical

Radial

Longitudinal

-/2 0 /2

Asymmetry vs

Summary Polarimetry is a crucial tool in RHIC Spin Program

Provides precise RHIC beam polarization measurements and monitoring

Provides crucial information for RHIC pol. beam setup, tune and development

RHIC Polarimetry consists of several independent subsystems, each of them playing their own crucial role (and based on different physics processes)

HJet: Absolute polarization measurements

pC: Polarization monitoring vs bunch and vs time in a fillPolarization profile

PHENIX and STAR Local Polarimeters: Monitor spin direction (through trans. spin component) at collision

Backups

H-jet system

RHIC proton beam

Recoil proton

• Height: 3.5 m

• Weight: 3000 kg

• Entire system moves along x-axis 10 ~ +10 mm to adjust collision point with RHIC beam.

IP12

zyx

target

　 RF transitions (WFT or SFT)

|1> |2> |3> |4>

Separating Magnet (Sextuples)

H2 desociater

Holding magnet

2nd RF-transitions for calibration

P+ OR P

H = p+ + e

Atomic Beam Source

Scattering chamber

Breit-Rabi Polarimeter

Separating magnet

Ion gauge

|1> |3> |2> |4>

|1> |2>

|1> |2>

Ion gauge

Hyper fine structure

HJet target system

Stern-Gerlach ExperimentSeparation of spin states in the inhomogeneous magnetic field

HJet: Identification of Elastic Events

proton beam

Forward scatteredproton

proton target recoil proton

Array of Si detectors measures TR & ToF of recoil proton. Channel # corresponds to recoil angle R.Correlations (TR & ToF ) and (TR & R ) the elastic process

23

BLUE mode

YELLOW mode

Energy vs Channel #

ToF vs Energy

pC: AN

zero hadronic spin-flip

With hadronic spin-flip (E950)

Phys.Rev.Lett.,89,052302(2002)

pC Analyzing Power

Ebeam = 21.7GeVEbeam = 21.7GeV Ebeam = 100 GeVEbeam = 100 GeV

unpublished

Run04

hadflip

emflipnon

hadflipnon

emflipN CCA *

2

*

1

Elastic scattering: interference between electromagnetic and hadronic amplitudes in the Coulumb-Nuclear Interference (CNI) region

STAR Local Polarimeter

3.3<||<　5.0　(small　tiles　only)　

Utilizes spin dependence of hadron production at high xF:

Bunch-by-bunch (relative) polarization

Monitors spin direction in STAR collision regionCapable to precisely monitor polarization vs time in a fill, and bunch-by-bunch