Exploring Gluon Polarization in the Proton with STAR Carl Gagliardi - PowerPoint PPT Presentation

Exploring Gluon Polarization in the Proton with STAR Carl Gagliardi Texas A&M University for the Collaboration STAR STAR Outline • Introduction • Inclusive measurements • Correlation measurements

Partonic origin of the proton spin? 1 1 = = ΔΣ + Δ + 〈 〉 S G L z z 2 2 Polarized DIS: ~0.3 Poorly Constrained Three 2006 fits of equal quality: – Δ G = 0.13 ± 0.16 – Δ G ~ 0.006 – Δ G = -0.20 ± 0.41 all at Q 2 = 1 GeV 2 Leader et al, PRD 75, 074027 • Measuring the gluon polarization distribution is a primary goal of the RHIC spin program Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 2

Exploring gluon polarization at RHIC + + − + − Δ Δ σ σ f f = ∝ ˆ a b A a σ + + + σ + − LL LL f f a b Δ f : polarized parton distribution functions Partonic fractions in jet production at 200 GeV ∗ θ cos For most RHIC kinematics, gg and qg dominate, making A LL for hadrons, photons, and jets sensitive to gluon polarization . 0 10 20 30 pT(GeV) Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 3

STAR detector in two views BEMC FMS EEMC TPC FPD ZDC ZDC BBC • High precision tracking with the TPC • Electromagnetic calorimetry with the BEMC, EEMC, and FMS • Additional detectors for relative luminosity, local polarimetry, and minbias triggering Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 4

Gluon polarization measurements at STAR Inclusive measurements Correlation measurements – Features – Features • High precision measurements • Less abundant • Average over partonic • Resolve partonic kinematics kinematics on event-by-event basis • Powerful for determining the • Provide information about the scale of Δ G shape of Δ g ( x ) – Channels – Channels • Neutral pions • Charged pions opposite jets • Direct photons • γ +jet • Jets • Di-jets • Both types of measurements provide important information for global analyses • Large acceptance of STAR makes jet and di-jet measurements particularly attractive Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 5

STAR inclusive π 0 A LL at various rapidities STAR STAR | η | < 0.95 1 < η < 2 η = 3.2, 3.7 During 2006, STAR measured A LL for inclusive π 0 for three different • rapidity regions • Larger rapidity correlates to stronger dominance of qg scattering with larger x quarks and smaller x gluons Expect A LL to decrease as η increases • Status of ’09 data analysis: See poster by W. Leight Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 6

Jet cross section from 2006 data STAR STAR • Good agreement between data and simulation • Good agreement with NLO pQCD calculation after hadronization and underlying event correction is applied • Jet production is well understood at RHIC energies Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 7

Jet cross section from 2006 data STAR STAR • Good agreement between data and simulation • Good agreement with NLO pQCD calculation after hadronization and underlying event correction is applied • Jet production is well understood at RHIC energies Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 8

STAR inclusive jet A LL from 2006 STAR STAR 200 GeV -0.7 < η < 0.9 • STAR inclusive jet A LL excludes those scenarios that have a large gluon polarization within the accessible x region Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 9

STAR inclusive jet A LL from 2006 STAR STAR 200 GeV -0.7 < η < 0.9 • STAR inclusive jet A LL excludes those scenarios that have a large gluon polarization within the accessible x region Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 10

DSSV – first global analysis with polarized jets de Florian et al., PRL 101, 072001 • The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 11

DSSV – first global analysis with polarized jets de Florian et al., PRL 101, 072001 STAR • The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 12

2009 upgrades • 2009 jet patch trigger upgrades – Overlapping jet patches and lower E T threshold improve efficiency and reduce trigger bias • Net increase of 37% in jet acceptance Jet specific – Remove beam-beam counter trigger requirement: • Trigger more efficiently at high jet p T • Measure non-collision background • Improvements in jet reconstruction – Subtract 100% of track momentum from struck tower energy (2009) instead of MIP (2006) – Overall jet energy resolution improved from 23% to 18% all channels • Increased trigger rate and reduced thresholds enabled by Enhance DAQ1000 • Sampled ~ 4 times the figure-of-merit relative to 2006 Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 13

Direct photon cross section and A LL from 2009 • Cross section at 200 GeV is consistent with NLO pQCD calculations • A LL for direct photons has very clean theoretical interpretation, but – Cross section is very small For details: – Background is very large See poster by – Very difficult measurement! M. Betancourt • Need far more statistics Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 14

A LL for inclusive jets: 2006 to 2009 STAR STAR • 2009 STAR inclusive jet A LL measurements are a factor of 3 (high-p T ) to >4 (low-p T ) more precise than 2006 • Results fall between predictions from DSSV and GRSV-STD • Precision sufficient to merit finer binning in pseudorapidity Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 15

2009 STAR inclusive jet A LL STAR STAR Mid rapidity Forward rapidity • A LL separated into two pseudorapidity ranges • Forward jets involve: – A larger fraction of quark-gluon scattering with: • Higher x quarks that are more polarized • Lower x gluons that are less polarized ∧ – Larger |cos( θ * )|, which reduces a LL • A LL falls between the predictions from DSSV and GRSV-STD Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 16

Expected future inclusive jet A LL precision Inclusive Jet A_LL for |eta|<1 Assumes 190 pb -1 with 50% polarization 200 GeV 0.015 Run 9 Preliminary Run 9 Run 12 Proj Stat GRSV-Std (500) 0.010 DSSV (500) GRSV-Std (200) A_LL DSSV (200) 0.005 500 GeV 0.000 projection -0.005 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 x_T • STAR will measure inclusive jet A LL in 500 GeV collisions during the 2012 and 2013 RHIC runs – Higher beam energy provides sensitivity to smaller x g – Expect ~ 90 pb -1 during 2012; much more during 2013 Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 17

Expected future inclusive jet A LL precision Inclusive Jet A_LL for |eta|<1 200 GeV Run 9 Preliminary 0.015 Run 12 Proj Stat Run 9 GRSV-Std (500) 0.010 DSSV (500) 200 GeV GRSV-Std (200) future DSSV (200) A_LL 0.005 Fut 200 GeV 500 GeV 0.000 projection -0.005 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 x_T • STAR will measure inclusive jet A LL in 500 GeV collisions during the 2012 and 2013 RHIC runs – Higher beam energy provides sensitivity to smaller x g – Expect ~ 90 pb -1 during 2012; much more during 2013 • STAR also anticipates significant future reductions in the uncertainties for 200 GeV collisions relative to the 2009 results Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 18

Beyond inclusive A LL measurements STAR STAR PRL 100, 232003 • Inclusive A LL measurements at fixed p T average over a broad x range . • Can hide considerable structure if Δ g ( x ) has a node • Correlation measurements can constrain the shape of Δ g ( x ) Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 19

Charged pions opposite jets STAR STAR A LL A LL • Trigger and reconstruct a jet, then look for a charged pion on the opposite side Events with high- z π + emphasize gluon scattering • off highly-polarized u quarks • Significantly increases the sensitivity of A LL ( π + ) Status of ’09 data analysis: See poster by J. Hays-Wehle Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 20

2006 di-jet cross section STAR STAR ( ) 1 = η + η x p e p e 3 4 1 T,3 T,4 s ( ) 1 − − η η = + x p e p e 3 4 2 T,3 T,4 s = M x x s 1 2 + 1 x η η = = 1 3 4 y ln 2 x 2 2 − | η η | * = 3 4 | cos θ | tanh 2 • Di-jets permit event-by-event calculations of x 1 and x 2 at LO • Di-jet cross section is well-described by NLO pQCD with corrections for hadronization and underlying event Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 21

2006 di-jet A LL STAR STAR • 2006 di-jet A LL provides a start at constraining the shape of Δ g ( x ) Carl Gagliardi – Exploring Gluon Polarization in the Proton with STAR 22