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Future DG Measurements at STAR
Renee Fatemi University of Kentucky
RHIC Spin Meeting, LBNL, November 21st 2009
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” & DG in DSSV Framework
DeFlorian, Sassot, Stratmann and Vogelsang, Phys.Rev.Lett. 101:072001, 2008
DeFlorian, Sassot, Stratmann and Vogelsang, Phys.Rev.D 80:034030, 2009
Q2 =10 GeV
RHIC Spin Meeting, LBNL, November 21st 2009
STAR DG DATA
Inclusive Jets
Inclusive neutral pions
Inclusive charged pions
DG data covers only one decade in X compared to the 3 decades for .DS
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
DG 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
RHIC Spin Meeting, LBNL, November 21st 2009
X Mapping Techniques
Increased sensitivity to XBJ in Run 9 via Di-jets @ 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+WBEMC
WBEMC+EEMC
EBEMC+WBEMC
2exp 43
21
ηη
s
Mx,x
RHIC Spin Meeting, LBNL, November 21st 2009
Framework 2009 Dijet ALL 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
Photon-jets @ 200 GeV in Run9
Very clean and sensitive to partonic XBJ.
Relatively rare and difficult to measure.
LOTS of development on algorithms and simulations. Plot shows STAR sensitivity for :
pT,γ > 10 GeV/c 1.09 < ηγ < 2 (EEMC) √s = 200 GeV 50 pb-1 @ 60% pol.
DOES NOT INCLUDE: Efficiency and background effects
RHIC Spin Meeting, LBNL, November 21st 2009
Charged and Neutral pion channels @ 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
RHIC Spin Meeting, LBNL, November 21st 2009
Run 6 Inclusive Jets @ 200 GeV
Leading systematics, such as trigger + reconstruction bias, are reduced as constraints on DG increase.
The stats from
requested Run 9 @ 200 GeV
The stats from
received Run9 200
GeV
xT = 2 pT / √s
RHIC Spin Meeting, LBNL, November 21st 2009
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 DG at low x.
The sub-process fractions are stable WRT to XT and the low “x” region is still dominated by gluon-gluon scattering.
DG 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 300 pb-1 @ 500 GeV
Lower x reach Brings smaller ALL
Assumes 30 pb-1 at 70% polarization
EB+EB
EB+EE
WB+WB
WB+EE
EB+WB
Inclusive Jet Projections for 100 pb-1 @ 500 GeV
RHIC Spin Meeting, LBNL, November 21st 2009
What does this xT range correspond to? Very small ALL and errors! DSSV error ~0.0025 in this region so error bars bigger than this loose their utility quickly….
Stat Uncerts
only
RHIC Spin Meeting, LBNL, November 21st 2009
How far should we go?
EMC Collaboration Nucl. Phys. B328, 1 (1989)
1) Current situation very different compared to DS. Apparent 2 decades ago from the EMC data that g1
p was saturating.
2) Measuring a small DG gives little indication about rest of integral.
3) What mechanisms limit sea quark polarization at low/high x? These mechanisms must also limit gluon polarization at low/high x.
4) How do you construct a reasonable functional form from the mechanisms in 3)?
… as far as we can!… until integral saturates?… range covered by DS?
RHIC Spin Meeting, LBNL, November 21st 2009
How do we sensible draw conclusions about the unmeasured part of the integral?
RHIC Spin Meeting, LBNL, November 21st 2009
Summary 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 roots2 play the eta game – using polarized q to access
low x gluons.
RHIC Spin Meeting, LBNL, November 21st 2009
Dijet Kinematics
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 <ps> = 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)