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Polarimetry of Proton Beams at RHIC. A.Bazilevsky Brookhaven National Laboratory Summer Students Lectures BNL, June 17, 2010. What is beam polarization?. Simple example: spin-1/2 particles (proton, electron)

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Polarimetry of Proton Beams at RHIC


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    1. Polarimetry of Proton Beams at RHIC A.Bazilevsky Brookhaven National Laboratory Summer Students Lectures BNL, June 17, 2010

    2. 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 |P|<1

    3. 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 Single spin asymmetry Double 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) • Polarimetry goals: • Measure and monitor beam polarization • Define and monitor spin direction in experimental area

    4. 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 AN – the Analyzing Power (|AN|<1) (left-right asymmetry for 100% polarized protons) Once AN is known:

    5. Polarization Measurements pC elastic scattering AN depends on the process and kinematic range of the measurements -t=2MCEkin 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

    6. Absolute Polarimeter (H jet) RHIC pC Polarimeters Siberian Snakes BRAHMS & PP2PP (p) RHIC PHENIX (p) STAR (p) Siberian Snakes Spin Rotators Solenoid Snake LINAC BOOSTER Pol. Proton Source 500 mA, 400 ms AGS Warm Snake 200 MeV Polarimeter AC Dipole AGS pC CNI Polarimeter Cold Snake RHIC and Polarimetry

    7. 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

    8. Polarized H-Jet 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 • Beam and target are both protons Forward scattered proton RHIC proton beam H-jet target recoil proton • Ptarget is provided by Breit Rabi Polarimeter

    9. HJet: Ptarget Source of normalization for polarization measurements at RHIC • Breit-Rabi Polarimeter: • Separation of particles with different spin states in the inhomogeneous magnetic field (ala Stern-Gerlach experiment) Nuclear polarization • Nuclear polarization of the atoms: • 95.8%  0.1% • After background correction: • Ptarget = 92.4%  1.8% 1 day Very stable for entire run period ! Polarization cycle (+/ 0/  ) = (500/50/500) seconds

    10. HJet: Example from Run-2006 εtarget Use the same statistics (with exactly the same experimental cuts) to measure beam and target (selecting certainspin states either for beam or for target)  Many systematic effect cancel out in the ratio εbeam t=-2MpEkin Ekin (MeV) Ekin (MeV) Provides statistical precision (P)/P~0.10 in a store (6-8 hours) HJetprovides very clean and stable polarization measurements but with limited stat. precison  Need faster polarimeter!

    11. 6 1 2 5 3 4 P-Carbon Polarimeter: 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 Carbon target Polarized proton Recoil carbon Ultra thin Carbon ribbon Target (5 mg/cm2) 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%) 18cm Si strip detectors (TOF, EC)

    12. Poarization Profile If polarization changes across the beam, the average polarization seen by Polarimeters and Experiments (in beam collision) is different H-Jet pC Collider Experiments ~1 mm 6-7 mm x=x0 P1,2(x,y) – polarization profile, I1,2(x,y) – intensity profile, for beam #1 and #2

    13. Pol. Profile and Average Polarization Scan C target across the beam In both X and Y directions Carbon Intensity I Ideal case: flat pol. profile (P=  R=0) Polarization Run-2009: Ebeam=100 GeV: R~0.1  5% correction Ebeam=250 GeV: R~0.35  15% correction P Target Position

    14. pC+HJet: Polarization vs Fill Run-2009 results (Ebeam=100 GeV) • Normalized to HJet • Corrected for polarization profile (by pC) “Blue” beam P/P < 5% • Dominant sources of syst. uncertainties: • ~3% - HJet background • ~3% - pC stability • (rate dependencies, gain drift) • ~2% - Pol. profile “Yellow” beam

    15. Absolute Polarimeter (H jet) RHIC pC Polarimeters Siberian Snakes BRAHMS & PP2PP (p) RHIC PHENIX (p) STAR (p) Siberian Snakes Spin Rotators Solenoid Snake LINAC BOOSTER Pol. Proton Source 500 mA, 400 ms AGS Warm Snake 200 MeV Polarimeter AC Dipole AGS pC CNI Polarimeter Cold Snake Need for Local Polarimeters Spin Rotators around experiments change spin direction in experimental areas  Need to monitor spin direction in experimental areas

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

    17. Monitor spin direction Asymmetry vs Measures transverse polarization PT , Separately PX and PY Longitudinal component: P – from CNI polarimeters Vertical Radial Longitudinal Vertical f ~ ±p/2 Radial f ~ 0 Longitudinal  no asymmetry -/2 0 /2

    18. 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 • 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

    19. Backups

    20. target H-jet system • Height: 3.5 m • Weight: 3000 kg • Entire system moves along x-axis 10 ~ +10 mm to adjust collision point with RHIC beam. Recoil proton RHIC proton beam IP12

    21. |1> |2> H = p++e HJet target system |1> |2> |3> |4> Hyper fine structure H2 desociater Separating Magnet (Sextuples) Atomic Beam Source RF transitions (WFT or SFT) P+ OR P Scattering chamber Holding magnet |1> |3> |2> |4> Breit-Rabi Polarimeter Separating magnet 2nd RF-transitions for calibration |1> |2> Ion gauge Ion gauge

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

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

    24. pC Analyzing Power Run04 zero hadronic spin-flip With hadronic spin-flip (E950) Phys.Rev.Lett.,89,052302(2002) unpublished Ebeam = 100 GeV pC: AN Elastic scattering: interference between electromagnetic and hadronic amplitudes in the Coulumb-Nuclear Interference (CNI) region Ebeam = 21.7GeV

    25. STAR Local Polarimeter Utilizes spin dependence of hadron production at high xF: 3.3<|h|< 5.0 (small tiles only) 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