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A.Bazilevsky and W.Fischer BNL The 20 th International Symposium on Spin Physics (SPIN2012)

Impact of 3D Polarization Profiles on Spin-Dependent Measurements in Colliding Beam Experiments . [1] W.Fischer and A.Bazilevsky , Phys. Rev. Spec. Topics - Accel . and Beams, 15, 041001 (2012 ) . A.Bazilevsky and W.Fischer BNL

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A.Bazilevsky and W.Fischer BNL The 20 th International Symposium on Spin Physics (SPIN2012)

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  1. Impact of 3D Polarization Profiles on Spin-Dependent Measurements in Colliding Beam Experiments [1] W.Fischer and A.Bazilevsky, Phys. Rev. Spec. Topics - Accel. and Beams, 15, 041001 (2012) A.Bazilevsky and W.Fischer BNL The 20th International Symposium on Spin Physics (SPIN2012) JINR, Dubna, Russia September 17-22, 2012

  2. Polarization profile – variation of polarization vs phase space coordinates RHIC experience • More than a decade of experience with RHIC as the only polarized proton collider in the world • Polarization Profile evolves during beam acceleration and storage • Polarization Profile as a primary reason for reduction of the observed polarization • Because of Polarization Profile, polarization measured by polarimeters is different from polarization observed by experiments • Importance of Polarization Profile measurements in all three dimensions The goal: quantify the effect of 3D polarization profile on the measured and observed polarizations

  3. From RHIC Polarimetry group Trans. Pol. Profile Measurements Scan thin C target across the beam in both trans. directions 1. Directly measure I and P : Carbon Intensity I I 2. Obtain R directly from the P(I) fit:    P Polarization P Precise target positioning is not necessary Target Position

  4. Trans. Pol. Profile Measurements From RHIC Polarimetry group R, fill by fill, in Run11 250 GeV beams P(I) in one fill R=0.290.07 I/Imax R~ 0.1−0.4 for 250 GeV beams R~ 0.05−0.15 for 100 GeV beams

  5. From RHIC Polarimetry group Long. Pol. Profile Measurements C-target ToFvsEkin for scattered C (in the polarimeter based on pC elastic scattering) σ σ Slicing “banana” shape alongside allows us to measure longitudinal profiles

  6. From RHIC Polarimetry group Long. Pol. Profile Measurements Fill 10532, yellow • Example from Run11 • 250 GeV beam: • No indication on sizable longitudinal polarization profile: • R ~ 0 ± ? … <0.1 • To better quantify – more analysis is needed to correct for: • Background • Energy and ToF smearing Intensity Polarization ToF, ns

  7. y Beam Profile Parameterization x Intensity I … Extending to two other dimensions and the corresponding momentum (angle) space: Polarization P P0 - polarization at 0 coordinate in both position and momentum (angle) space Position across the beam

  8. Polarizations P and I are functions of (x,x’,y,y’,s,s’); L ~ IB IY Average bunch polarization (as measured by polarimeter) Y Average polarization observed in a collision (used in single spin asymmetry measurements) Average two-beam polarization product in a collision (used in double spin asymmetry measurements) B Also Figures of Merit (FOM): For single spin: For double spin:

  9. Polarizations Polarization measured by polarimeter: Y Polarization moments in a collision: B

  10. Polarizations: simplified case • No hourglass effect, or short bunches (σs<<β*x,y): h(tx,ty)=1 • No long. pol. profile (Rs=0): h(tx,mn,ty,mn)=1 • Equal trans. pol. profiles (Rx, Ry) in both beams • Round beam (σx=σy) of the same size in both beams Polarization moments in a collision: Polarization measured by polarimeter:

  11. Polarization: polarimetervs coll. experiment Ppol/ Pexp • Collision experiments see higher polarization than polarimeters • Due to higher weight of beam center in a collision Rx=Ry=R

  12. Double Spin Measurements • With sharp polarization profiles, polarimetry should provide not only individual beam polarizations, but also a product of two beam polarizations • <PB><PY> underestimates <PBPY> Rx=Ry=R

  13. Polarization: FOM Single Spin Double Spin Rx=Ry=R Polarimetry should also provide polarization second moments and underestimates underestimates

  14. Effect of longitudinal profile Ppol / Pexp FOMpol / FOMexp Single Single Double Double • Due to long. pol. profile, polarimeter would underestimate the polarizations observed in collision experiments • Due to higher weight of beam center in a collision

  15. Polarization and FOM losses Assuming that polarization losses are due to pol. profiles Single Spin Double Spin Pol FoM

  16. Summary Polarization profiles have a strong impact on both the average beam polarizations and FoM Due to pol. profiles, polarizations measured by polarimeters need to be corrected to be used in coll. experiments Polarimetry has to provide coll. spin experiments not only with single beam polarizations <PB> and <PY>, but also with <P2B>, <P2Y>, <PBPY> and <P2BP2Y> Polarization profile measurements and monitoring in all three dimensions are necessary for precision spin experiments • Not discussed in this talk: • Do polarization profiles change from the point it is measured (polarimeter location in a ring) to experimental coll. points, even if they are separated by Siberian snakes and spin rotators? – A study is underway

  17. Backup

  18. Coordinates

  19. Luminosity

  20. Polarization Moments

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