1. Transverse Single-Spin Asymmetries Understanding the Proton: One of the fundamental building blocks of ordinary matter! Spin decomposition of proton still not understood. Expected contributions from parton spin and orbital angular momentum. Transverse spin structure cannot be deduced from longitudinal (helicity) structure Spatial rotations and Lorentz boosts don’t commute! Relationship between longitudinal and transverse structure provides information on relativistic nature of partons within proton. Measurement of the Transverse Single-Spin Asymmetry for Mid-rapidity Production of Neutral Pions in Polarized p+p Collisions at 200 GeV Center-of-Mass Energy Christine A. Aidala Columbia University Measurement of the Transverse Single-Spin Asymmetry for Mid-rapidity Neutral Pions at PHENIX Obtaining Spin-Dependent Yields, Relative Luminosity, and Polarization Handling Background Results Studying Proton Spin Structure via Spin Asymmetries Difference in spin-dependent cross sections for particle production as a fraction of the total cross section At high energies, can understand particle production in terms of factorization in perturbative QCD. Why at RHIC? The first and only polarized proton collider in the world. Various equipment to maintain and measure the polarization through acceleration and storage. Advantages of a polarized hadron collider High energy  factorization, new probes (W’s) Polarized hadrons  g+q, g+g collisions left right Versus Hard Scattering Process X q(x 1 ) g(x 2 ) Fragmentation function (FF) Partonic cross section Partonic distribution function (pdf) Left-right asymmetries up to ~30% observed in forward particle production from collisions of unpolarized with vertically polarized protons. Mechanism remains unclear, but various ideas proposed… Large asymmetries observed!  q(x) Sivers effect: k T -dependent pdf, collinear FF Collins effect: collinear pdf, k T -dependent FF Can’t be parallel  Importance of k T ! L=1 proton scattered q that fragments Transversity distribution E704 at Fermilab GeV p T = 0.5-2.0 GeV/c Transverse-momentum-dependent factorization in pQCD Collinear factorization—assumes parton momentum collinear to parent hadron. pdf’s and FF’s integrated in partonic transverse momentum, k T Keep k T dependence  several possible new pdf’s and FF’s.    Invariant mass (GeV/c 2 ) 2001-02 data—First polarized proton run at RHIC! 15% average polarization 0.15 pb -1 Measure particle production on one side of the beam for spin up vs. down OR Measure particle production to the left vs. right for a single spin direction Versus OR June 2006 Mathematically exact. Must know relative luminosity (R) of bunches with spin up vs. down. Excellent mathematical approximation for cross check. Uses equivalence of spin-up, beam-left and spin-down, beam-right. Luminosity effects cancel. Obtain polarization for each beam fill by fill from RHIC proton-carbon polarimeter Both beams polarized at RHIC. Consider spin states of one beam, average over spin states of other. 35% relative polarization uncertainty Calculate asymmetry of (signal + background) in the  0 mass integration region. Calculate asymmetry of bkg around peak as estimate of asymmetry of bkg under peak. Subtract asymmetries, weighted by fraction of background under peak (r).  0    Measure photon pairs in electromagnetic calorimeter (EMCal). Eliminate as much background as possible using EMCal cluster shower shape cut and charged track veto. Beam-beam counters as min-bias trigger and to measure R 3.0 < |  | < 3.9 2  azimuthal coverage: essential to avoid bias in transverse collisions, where azimuthal asymmetries expected! Fill index Polarization-corrected asymmetries obtained fill by fill, then averaged over all fills. AN 0AN 0 PRL 95, 202001 (2005) x F ~0.0 A N of mid-rapidity neutral pions consistent with zero within a few percent. No large asymmetry observed as for forward production. Current measurement dominated by gluon-gluon and quark-gluon scattering. Collins effect diluted/suppressed Constrains gluon Sivers pdf (Bianconi + Radici, PRD 73, 034018 (2006)) Partonic contributions to pion production left right Only for quarks!