What can we learn from η production in proton-proton collisions? Joe Seele MIT and University of Colorado

J. Seele - SPIN Outline Proton spin puzzle η production in (polarized) proton-proton collisions Measurements of η production at PHENIX Extraction of η fragmentation functions Constraining Δ G with η production in polarized proton-proton collisions at PHENIX

J. Seele - SPIN The Proton Spin Puzzle Fairly well measured only ~30% of spin A future challenge The proton is viewed as being a “bag” of bound quarks and gluons interacting via QCD Spins + orbital angular momentum need to give the observed spin 1/2 of proton Beginning to be measured at RHIC (and hopefully EIC)

J. Seele - SPIN Double Helicity Asymmetries and translating… Take the asymmetry of proton helicity configurations

J. Seele - SPIN Cross Section and Fragmentation Functions High p T particle production provides information about the parton distribution functions and fragmentation functions

J. Seele - SPIN Why the η ? Want a final state that: 1)Produced in large quantities 2)Theoretical Motivation PHENIX - PRC

J. Seele - SPIN Why the η ? Want a final state that: 1)Produced in large quantities 2)Theoretical Motivation Different flavor and gluon dependence of structure and fragmentation should give different sensitivities to hard subprocesses

J. Seele - SPIN RHIC Year  s [GeV]Recorded LPol [%]FOM (P 4 L) 2003 (Run 3) pb nb (Run 4) pb nb (Run 5) pb nb (Run 6) pb nb -1

J. Seele - SPIN PHENIX Central Arms: γ / π 0 / η detection –Electromagnetic Calorimeter (-0.35 < η < 0.35) –PC3 - Charge Veto Global Detectors: Relative Luminosity –Beam-Beam Counter (BBC) ( < η < 4.0) –Zero-Degree Calorimeter (ZDC) ( < η < infinity) Local Polarimetry - ZDC

J. Seele - SPIN Measuring the η at PHENIX ChannelBR (%) η->2γ η->π 0 π 0 π η->π + π - π η->π + π - γ 4.69 Choose a decay channel that couples well with the capabilities of PHENIX

J. Seele - SPIN Measuring the η at PHENIX The number of η ’s in each p T bin are determined by fitting the peak+background in the di-photon invariant mass spectrum

J. Seele - SPIN η Cross Section - Results Run3 and Run5 cross section results

J. Seele - SPIN Measuring Helicity Asymmetries What to measure : 1)N’s - Final state - the η 2)P’s - Polarization at IR 3)R - Relative Luminosity PHENIX Central Arms PHENIX ZDCs + RHIC Polarimeters PHENIX ZDCs+BBCs

J. Seele - SPIN η Asymmetry - Results Scaling uncertainty (Run5 = 9.4% and Run6 = 8.3%) from polarization not shown *p T bins identical up to p T =6. Above that they use different binning.

J. Seele - SPIN η Fragmentation Functions None previously existed, so Marco Stratmann, Christine Aidala and myself extracted the η FFs from data on e + +e - and p+p.

J. Seele - SPIN η FFs - Comparison Describes e+e- data very well over a large range in energies. Plan to incorporate BABAR, and possibly some UA1/UA2 data.

J. Seele - SPIN η Cross Section and Theory NLO+ η FF ( μ =p T ) calculation done by Marco Stratmann *In a way this is more of a self- consistency check than a prediction.

J. Seele - SPIN η and π 0 Subprocess Fractions

J. Seele - SPIN η Asymmetry and Theory Calculations by Marco Stratmann

J. Seele - SPIN Constraining Δ G with the η A series of constrained fits were done by Vogelsang and Stratmann in the GRSV model. They constrained the integral of Δ G at the input scale.

J. Seele - SPIN Constraining Δ G with the η Each set yields a different A LL Does not give much power for the negative Δ Gs

J. Seele - SPIN Constraining Δ G with the η Allows a small positive or strongly negative Δ G. It should be noted that this is only for the GRSV fits. The data needs to be incorporated into a flexible global fit.

J. Seele - SPIN Constraining Δ G with the η and π 0

J. Seele - SPIN The Polarization of the Proton’s Glue PHENIX has measured the double helicity asymmetry in η production at mid-rapidity With the new fragmentation functions it will provide a constraint on Δ G Thanks!

J. Seele - SPIN Backup

J. Seele - SPIN η FFs - Data ExperimentSystemEnergy (GeV)# Points ALEPH ’92e+e ALEPH ’00e+e ALEPH ‘02e+e L3 ‘92e+e L3 ’94e+e OPALe+e ARGUSe+e-106 CELLOe+e-354 HRSe+e-2913 JADE ’85e+e JADE ‘’90e+e MARK IIe+e-297 PHENIX 2  p+p20012 PHENIX 3  p+p2006 PHENIX ’05 prelim.p+p20019

J. Seele - SPIN Relative Luminosity at PHENIX Measured using the Beam-Beam Counter (BBC) Coincidences Systematic Studies Include : 1.Spin Independent BBC cross section 2.Effects from multiple collisions per bunch crossing Currently δ R ~ O(10 -4 ) Statistical uncertainty in δ R is negligible, but systematics could become a problem in the near future

J. Seele - SPIN Local Polarimetry at PHENIX The neutrons are measured in the PHENIX ZDCs Spin rotators are not perfect and leave a small transverse component Exploit a forward neutron single spin asymmetry in transversely polarized p+p collisions

J. Seele - SPIN Polarimetry at RHIC RHIC uses 2 types of transversely polarized elastic scattering Provides fast, high statistics measurement Gives polarization profile of beam Analyzing power empirically determined Provides self-calibrating, low statistics measurement Analyzing power unknown Provides calibration for the p+C polarimeter A tour de force measurement as this is the first time bunch-by-bunch measurements were performed at any high energy collider

J. Seele - SPIN η Asymmetry Because the eta peak sits on a background we need to subtract a possible background asymmetry MeV MeV The background asymmetry is estimated using the blue sideband regions and subtracted from the peak asymmetry