Future  G Measurements at STAR Renee Fatemi University of Kentucky RHIC Spin Meeting, LBNL, November 21st 2009

Discussion Outline Where we started. Where we are now. Where should we go next? When have we gone far enough? RHIC Spin Meeting, LBNL, November 21st 2009

“Pre-RHIC” and “Mid-RHIC”  &  G in DSSV Framework RHIC Spin Meeting, LBNL, November 21st 2009 DeFlorian, Sassot, Stratmann and Vogelsang, Phys.Rev.Lett. 101:072001, 2008 DeFlorian, Sassot, Stratmann and Vogelsang, Phys.Rev.D 80:034030, 2009 Q 2 =10 GeV

STAR  G DATA Inclusive Jets Inclusive neutral pions Inclusive charged pions RHIC Spin Meeting, LBNL, November 21st 2009   G data covers only one decade in X compared to the 3 decades for   Need to lower/raise X reach.  Quark-gluon scattering in forward regions provide access to low x gluons.  Move to higher center of mass scattering energies while measuring same kinematic range   G data (to date) integrates over X range while DIS/SIDIS accesses X directly.  Need to reduce integration bins in X.  Correlation data – di-jets and photon-jets. RANGE X mapping

X Mapping Techniques RHIC Spin Meeting, LBNL, November 21st 2009

Increased sensitivity to X BJ in Run 9 via 200 GeV  LO and NLO track fairly closely  West BEMC + EEMC provides access to lowest x values!  EBEMC+EEMC harder to measure due to smaller partonic aLL. But can access same kinematics in E+W BEMC. RHIC Spin Meeting, LBNL, November 21st 2009 EBEMC+EBEMC EBEMC+EEMC WBEMC+WB EMC WBEMC +EEMC EBEMC+WBEMC

Framework 2009 Dijet A LL prepared  Dijet X-sec from 2005  New preliminary result shown at DNP in October  Corresponds with West-West BEMC x range  Good agreement with NLO RHIC Spin Meeting, LBNL, November 21st 2009

200 GeV in Run9  Very clean and sensitive to partonic X BJ.  Relatively rare and difficult to measure.  LOTS of development on algorithms and simulations.  Plot shows STAR sensitivity for : p T, γ > 10 GeV/c 1.09 < η γ < 2 (EEMC) √s = 200 GeV 50 pb 60% pol. DOES NOT INCLUDE: Efficiency and background effects

RHIC Spin Meeting, LBNL, November 21st 2009 Charged and Neutral pion 200 GeV Continue to look at inclusive channels with increased precision.

While moving toward jet-hadron correlation measurements.  NLO calculations show a strong correlation between the real x and z values and LO estimates from jet – hadron measurements. de Florian Phys. Rev. D79:114014, 2009

Run 6 Inclusive 200 GeV RHIC Spin Meeting, LBNL, November 21st 2009  Leading systematics, such as trigger + reconstruction bias, are reduced as constraints on  G increase.

The stats from requested Run 200 GeV The stats from received Run9 200 GeV x T = 2 p T / √s

Extending X reach RHIC Spin Meeting, LBNL, November 21st 2009

Moving to higher CM Energies  Higher √s WILL provide access to lower x gluons BUT this does NOT automatically translate into increased sensitivity to  G at low x.  The sub-process fractions are stable WRT to X T and the low “x” region is still dominated by gluon-gluon scattering.   G is not large so we must enhance our sensitivity by using the larger valence quark analyzing power.  Optimize for qG scattering! Gluon goes forward and quark stays at mid-rapidity.

Di-jet Projections for GeV  Lower x reach  Brings smaller A LL  Assumes 30 pb -1 at 70% polarization EB+EB EB+EE WB+WB WB+EE EB+WB

Inclusive Jet Projections for 100 pb 500 GeV RHIC Spin Meeting, LBNL, November 21st 2009 What does this xT range correspond to? Very small A LL and errors! DSSV error ~ in this region so error bars bigger than this loose their utility quickly…. Stat Uncerts only

How far should we go? RHIC Spin Meeting, LBNL, November 21st 2009 EMC Collaboration Nucl. Phys. B328, 1 (1989) 1)Current situation very different compared to . Apparent 2 decades ago from the EMC data that g 1 p was saturating. 1)Measuring a small  G gives little indication about rest of integral. 2)What mechanisms limit sea quark polarization at low/high x? These mechanisms must also limit gluon polarization at low/high x. 3)How do you construct a reasonable functional form from the mechanisms in 3)? … as far as we can! … until integral saturates? … range covered by  ?

RHIC Spin Meeting, LBNL, November 21st 2009 How do we sensible draw conclusions about the unmeasured part of the integral?

Summary RHIC Spin Meeting, LBNL, November 21st 2009  STAR’s inclusive measurements have had a strong impact on the determination DG.  If our goal is to measure DG as precisely as we have measured DS then we have to change our tactics.  Get a better grasp on functional form via correlation measurements. Dijets, jet-hadron correlations and photon- jets will allow more direct map from pT to x.  Expand x range by 1 higher precision inclusive measurements at higher roots 2 play the eta game – using polarized q to access low x gluons.

Dijet Kinematics RHIC Spin Meeting, LBNL, November 21st 2009

Inclusive jet projection for 500 GeV Run 11  Assume 100 pb -1 sampled at an average polarization of 50%  JP2+AJP take-all threshold at 13 GeV (same as Run 9)  ~250 Hz at C-AD projected max possible lumi  ~120 Hz at C-AD projected final average lumi  Above figure assumes a JP1 trigger at 7.3 GeV with = 30  ~100 Hz at C-AD projected final average lumi  If an additional ~100 Hz is tolerable, a better choice would be:  ~100 Hz with JP thr ~ 5.5 GeV (i.e., Run 9 pp200 JP1 thr)  ~100 Hz with JP thr ~ 9 GeV (split diff with take-all level)