Polarimetry of Proton Beams at RHIC A.Bazilevsky Summer Students Lectures June 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: S z =+1/2 and S z =-1/2 |P|<1

Why polarized protons? See lecture by A.Vossen Longitudinally polarized protons Proton helicity structure: (anti-)quark and gluon polarization inside proton Transversely polarized protons (Anti-)quark transversity Parton orbital momentum Polarimetry goals: Measure and monitor beam polarization Define and monitor spin direction in experimental area 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)

How to measure proton beam polarization There are several established physics processes sensitive to the spin direction of the transversely polarized protons Scattering to the right Scattering to the left A N – the Analyzing Power (|A N |<1) (left-right asymmetry for 100% polarized protons) Once A N is known:

Polarization Measurements A N depends on the process and kinematic range of the measurements pC elastic scattering Precision of the measurements N=N Left +N Right For  (P)=0.01 and A N ~0.01  N~10 8 ! Requirements: Large A N or/and high rate (N) Good control of kinematic range -t=2M C  E kin

RHIC and Polarimetry 6

RHIC Polarimetry Polarized 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 tool Measures 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 area Needs 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 RHIC proton beam Forward scattered proton H-jet target recoil proton P target 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 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: P target Source of normalization for polarization measurements at RHIC Nuclear polarization of the atoms: 95.8%  0.1% After background correction: P target = 92.4%  1.8%

HJet: Provides statistical precision  (P)/P~0.10 in a store (6-8 hours) Example from Run-2006 ε beam ε target t=-2M p  E kin 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 E kin (MeV) 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/cm 2 ) Si strip detectors (TOF, E C ) 18cm Polarized proton Recoil carbon Carbon target 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 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 ~1 mm 6-7 mm pC Collider Experiments P 1,2 (x,y) – polarization profile, I 1,2 (x,y) – intensity profile, for beam #1 and #2 x=x0x=x0 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 beam In both X and Y directions Target Position Intensity Polarization II PP Run-2009: E beam =100 GeV: R~0.1  5% correction E beam =250 GeV: R~0.35  15% correction Ideal case: flat pol. profile (  P =   R=0)

pC+HJet: Polarization vs Fill Run-2009 results ( E beam =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 Spin Rotators around experiments may change spin direction in experimental areas  Need to monitor spin direction in experimental areas

Local Polarimeter: PHENIX Utilizes spin dependence of very forward neutron production discovered in RHIC Run-2002 (PLB650, 325) neutron charged particles Zero Degree Calorimeter Quite unexpected asymmetry Theory can not yet explain it But already can be used for polarimetry!

Monitor spin direction Vertical   ~ ±  /2 Radial   ~ 0 Longitudinal  no asymmetry Measures transverse polarization P T, Separately P X and P Y Longitudinal component: P – from CNI polarimeters 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 fill Polarization 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 target

RF transitions (WFT or SFT) |1> |2> |3> |4> Separating Magnet (Sextuples) H 2 desociater Holding magnet 2 nd 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> Ion gauge Hyper fine structure HJet target system

Stern-Gerlach Experiment Separation of spin states in the inhomogeneous magnetic field

HJet: Identification of Elastic Events proton beam Forward scattered proton proton target recoil proton Array of Si detectors measures T R & ToF of recoil proton. Channel # corresponds to recoil angle  R. Correlations (T R & ToF ) and (T R &  R )  the elastic process 23 BLUE mode YELLOW mode Energy vs Channel # ToF vs Energy

pC: A N zero hadronic spin-flip With hadronic spin-flip (E950) Phys.Rev.Lett.,89,052302(2002) pC Analyzing Power E beam = 21.7GeV E beam = 100 GeV unpublished Run04 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 x F : Bunch-by-bunch (relative) polarization Monitors spin direction in STAR collision region Capable to precisely monitor polarization vs time in a fill, and bunch-by-bunch