PHENIX Measurement of Parity-Violating Single Spin Asymmetry in W Production in p+p Collisions at 500 GeV Stephen Pate (for the PHENIX Collaboration) New Mexico State University 12 September 20101Diffraction 2010

12 September 2010Diffraction DSSV Global Fit -- PRD80 (2009) [ ] Over many years, the worldwide program of form factor measurements, inclusive deep- inelastic and semi-inclusive deep-inelastic scattering has made possible a definitive measurement of the polarized PDFs of the u and d quarks. But the contribution of the sea and gluons to the spin of the proton remains relatively unclear.

12 September 2010Diffraction l+l+ is measuring asymmetries in W production that are sensitive to the light quark sea contribution to the proton spin.

Diffraction STAR RHIC-PHENIX 2009 run Run 09: First 500 GeV Run, March 17-April 13, 2009 Machine development in parallel with physics running to increase luminosity, polarization Integrated luminosity (with vertex cut) is  Ldt=8.6 pb -1 Polarization is =0.39±0.04 (scale uncertainty) Spin Rotators Siberian Snake pC and H-jet polarimeters Siberian Snake 12 September 2010

Polarization Measured with two polarimeters – CNI polarimeter measurements available during run – H jet polarimeter provides absolute polarization – Measured residual polarization in real time after rotation at PHENIX 12 September Polarization measured by CNI polarimeters fill-by-fill P P Diffraction 2010

6 EMC 4x4 Tower Sum Trigger ±30 cm vertex cut High energy EM Calorimeter clusters matched to charged track Loose timing cut to reduce cosmic ray bkg Loose E/p cut PHENIX Central arm: W  e  Central Arm Acceptance : |  |<0.35 in rapidity 2 arms covering  =  e Fully efficient at 12 GeV 12 September 2010

Diffraction Clusters Smooth spectrum of EMC clusters after removing bad towers Effect of Cuts 12 September 2010

Diffraction Clusters Good track match Smooth spectrum of EMC clusters after removing bad towers Have a good track pointing to an EMC cluster 12 September 2010 Effect of Cuts

Diffraction Clusters Good track match TOF cut Smooth spectrum of EMC clusters after removing bad towers Have a good track pointing to an EMC cluster Timing within start time of collision Reduces out of time backgrounds (cosmics) by ~80% 12 September 2010 Effect of Cuts

Diffraction Clusters Good track match TOF cut E/p Smooth spectrum of EMC clusters after removing bad towers Have a good track pointing to an EMC cluster Timing within start time of collision Reduces out of time backgrounds (cosmics) by ~80% E/P cut 12 September 2010 Effect of Cuts

Diffraction QCD provides the most obvious background (W. Vogelsang) Not shown here but very important Cosmics and photons (from meson decays and direct), which can have accidental matches to tracks or conversions c/b relatively small above 30 GeV, calculated at FONLL (Matteo Carciari) Z/  * background is estimated from PYTHIA (~1 count is expected in Run09). W  e is also small 12 September 2010 Signal and Background

Diffraction QCD provides the most obvious background (W. Vogelsang) Not shown here but very important Cosmics and photons (from meson decays and direct), which can have accidental matches to tracks or conversions c/b relatively small above 30 GeV, calculated at FONLL (Matteo Carciari) Z/  * background is estimated from PYTHIA (~1 count is expected in Run09). W  e is also small 12 September 2010 Hadron spectrum suppressed by poor response of EMC to hadrons Signal and Background

12 September 2010Diffraction Background subtracted spectra of positron and electron candidates Gray bands = range of background estimates. Compared to spectrum of W and Z decays from a NLO calculation [D. de Florian and W. Vogelsang, Phys. Rev. D81, (2010). P. M. Nadolsky and C. P. Yuan, Nucl. Phys. B666, 31 (2003)] These yields were used for cross section results. For the asymmetry measurement, additional cuts were applied to make the background contribution negligible.

12 September 2010Diffraction Isolation Cut – use fact that lepton from W decay is not from a jet  E < 2GeV Sum up energy in a cone around electron/positron >80% of signal is kept (blue dashed line) Factor ~4 reduction in background

12 September 2010Diffraction Signal for Asymmetry Measurement After Isolation Cut Positrons: 42 signal events (background 1.7  1.0) Electrons: 13 signal events (background 1.6  1.0) Signal region is 30 < p T < 50 GeV/c

12 September 2010Diffraction Longitudinal spin asymmetry A L Parity-violating longitudinal single spin asymmetry N + = right-handed production of W N - = left-handed production of W P = Polarization R = relative luminosities of the helicity states D = dilution by background and Z 0

12 September 2010Diffraction Parity-violating Asymmetries In W Production in PHENIX Final Results for RHIC 2009 Run [arXiv: ] Direct observation of parity-violation in W production. Likelihood function used to determine value and limits for A L (low statistics)

Muon Trigger Upgrade muID north muID south muTr north muTr south Existing Muon Arms: muID (triggering) muTr (tracking) trigger rejection ~ September Diffraction 2010

Muon Trigger Upgrade RPC1(a,b) RPC3 muID north muID south muTr north muTr south muTr-trigger Existing Muon Arms: muID (triggering) muTr (tracking) trigger rejection ~ Upgrade: 1) muTr trigger electronics: muTr 1-3  send tracking info to level-1 trigger 2) RPC stations: RPC 1+3  tracking + timing info to level-1 trigger Trigger idea: Reject low momen- tum muons Cut out-of-time beam background 12 September Diffraction 2010

RPCs: trigger level timing Timing used in Run-9 to characterize background RPC3-N installed for Run- 10 Commissioned & ready From collision or out going beam From beam background Test assembly of RPC-3 half octant support structure at UIUC 12 September Diffraction 2010

MUTRIG ready for physics in 2011 Run Minimum Ionizing Particle Good efficiency for MIPs MUTR.N installed for Run-9 MUTR.s installed for Run-10 ready to go Station 1 Station 2 Station 3 12 September Diffraction 2010

12 September 2010Diffraction Expectation for uncertainties in parity- violating asymmetries in high-p T muon production Assuming: 50 pb -1 50% polarization S/B = 0.3

12 September 2010Diffraction Expectation for uncertainties in parity- violating asymmetries in high-p T muon production Assuming: 50 pb -1 50% polarization S/B = 3.0

12 September 2010Diffraction Expectation for uncertainties in parity- violating asymmetries in high-p T muon production Assuming: 150 pb -1 50% polarization S/B = 3.0

Summary PHENIX has observed a non-zero parity- violating asymmetry in W production in pp collisions at 500 GeV Future measurements with greater precision will allow a measurement of the light quark sea contribution to the spin of the proton, using a technique complementary to that used in leptonic SIDIS 12 September 2010Diffraction

12 September 2010Diffraction

12 September 2010Diffraction Cross Sections for W Production in PHENIX Final Results for RHIC 2009 Run [arXiv: ]