Jet Production in Polarized pp Collisions at RHIC

Jet Production in Polarized pp Collisions at RHIC Carl A. Gagliardi Texas A&M University for the Collaboration STAR Outline • • Carl Gagliardi – DIS 2008 – Jets in pp at RHIC Introduction 2005 results 2006 results Looking ahead 1

Origin of the proton spin? Polarized DIS: 0. 2~0. 3 Poorly Constrained Three recent fits of equality: – ΔG = 0. 13 ± 0. 16 – ΔG ~ 0. 006 – ΔG = -0. 20 ± 0. 41 all at Q 2 = 1 Ge. V 2 Leader et al, PRD 75, 074027 • A primary goal of the RHIC Spin program is to determine the gluon polarization distribution Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 2

Polarized pp collisions at RHIC f: polarized parton distribution functions Partonic fractions in jet production at 200 Ge. V 0 10 20 30 p. T(Ge. V) Carl Gagliardi – DIS 2008 – Jets in pp at RHIC For most RHIC kinematics, gg and qg dominate, making ALL for jets sensitive to gluon polarization. 3

RHIC: the world’s first polarized hadron collider RHIC p. C Polarimeters Absolute Polarimeter (H jet) BRAHMS PHOBOS Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) - Pol. H Source Spin flipper Spin Rotators (longitudinal polarization) Solenoid Partial Siberian Snake LINAC Helical Partial Siberian Snake BOOSTER AGS 200 Me. V Polarimeter AGS Internal Polarimeter AGS p. C Polarimeters Rf Dipole Strong Helical AGS Snake • Spin varies from rf bucket to rf bucket (9. 4 MHz) • Spin pattern changes from fill to fill • Spin rotators provide choice of spin orientation • “Billions” of spin reversals during a fill with little if any depolarization “Thousands of millions” Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 4

STAR detector E-M Calorimeter Projection Chamber Time of Flight Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 5

Jet reconstruction in STAR GEANT PYTHIA Particle Detector Data jets MC jets Midpoint cone algorithm (Adapted from Tevatron II - hep-ex/0005012) • Seed energy = 0. 5 Ge. V • Cone radius in - • R=0. 4 with 0. 2 < < 0. 8 (2005) • R=0. 7 with -0. 7 < < 0. 9 (2006) • Splitting/merging fraction f=0. 5 Use PYTHIA + GEANT to quantify detector response Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 6

Jet production at RHIC STAR PRL 97, 252001 • Jet structure at 200 Ge. V is well understood • Mid-rapidity jet cross section is well described by p. QCD over 7 orders of magnitude Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 7

2005 inclusive jets ALL systematics (x 10 -3) Reconstruction + Trigger Bias 2 -5 (p. T dep) Non-longitudinal Polarization 0. 1 -0. 8 (p. T dep) Relative Luminosity 0. 94 Backgrounds 0. 70 p. T systematic [-5. 4%, +6. 7%] Neutral Energy Fraction • Reconstruction bias: combination of steeply falling spectrum and ~25% jet ET resolution • Trigger bias: non-uniform sampling of gg, qg, and qq processes • Will illustrate impact on ALL with 2006 data Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 8

2005 inclusive jets ALL STAR hep-ex ar. Xiv: 0710. 2048 Model calcs from: Jager et al, PRD 70, 034010 200 Ge. V 0. 2 < η < 0. 8 • Comparison to predictions within the GRSV framework with various input values of ΔG demonstrate sensitivity Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 9

Limits on ΔG from 2005 jet results STAR hep-ex ar. Xiv: 0710. 2048 GRSV DIS • Significant new constraints on ΔG when compared to predictions derived from one global fit to DIS data Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 10

Other global analyses xΔg(x) at Q 2 = 10 Ge. V 2 • There are many other global analyses of the polarized DIS data Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 11

Additional ALL predictions for 2005 • Many predictions for ALL vs. p. T • Is the sensitivity unique to GRSV? Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 12

Comparison to other global analyses STAR • The STAR data exclude a broad range of models that have ΔG larger than that in GRSV-std • The counterexample is GS-C, which is negative at large x, has a node near x ~ 0. 1, and has a large first moment at small x Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 13

2006 data Improved figure of merit Ø Luminosity : Ø Polarization: Ø Barrel EMC coverage: 2 4. 7 pb-1 50% 60% (online polarization) [0, 1] [-1, 1] In addition Ø Jet cone radius: 0. 4 0. 7 Ø -0. 7 < jet axis < 0. 9 Ø Neutral energy fraction < 0. 85 Ø Increased trigger thresholds Ø Inclusion of Endcap EMC towers Ø Improved tracking at large | | Ø Shielding from backgrounds Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 14

Jet reconstruction and trigger bias for 2006 data Step 1: Use PYTHIA+GEANT to estimate the p. T shift from detector jets to particle jets Step 2: Simulate the difference in ALL between particle and detector jets for various gluon polarization scenarios Particle jets Shifted detector jets Detector jets • • Maximum deviation determines ALL systematic Confidence level calculations take account of any residual Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 15

2006 inclusive jets ALL STAR Model curves calculated with cone radius 0. 7 and -0. 7 < < 0. 9 200 Ge. V -0. 7 < < 0. 9 ALL systematics (x 10 -3) Reconstruction + Trigger Bias [-1, +3] (p. T dep) Non-longitudinal Polarization ~ 0. 03 (p. T dep) Relative Luminosity 0. 94 Backgrounds 1 st bin ~ 0. 5 else ~ 0. 1 p. T systematic 6. 7% • Statistical uncertainties are 3 -4 times smaller than in ’ 05 data for p. T > 13 Ge. V/c Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 16

Limits on ΔG from 2006 jet results STAR GRSV DIS • Within the GRSV framework: – GRSV-std excluded with 99% CL – ΔG < -0. 7 excluded with 90% CL Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 17

Looking beyond inclusive ALL measurements • Inclusive ALL measurements at fixed p. T average over a broad x range. • Need a global analysis to determine the implications • Can hide considerable structure if Δg(x) has a node Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 18

DSSV – first global analysis with polarized jets de Florian et al. , ar. Xiv: 0804. 0422 [hep-ph] • The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing • Finds a node in the gluon distribution near x ~ 0. 1, but with the opposite phase from GS-C Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 19

The next few years: di-jets and Δg(x) STAR Mass 2005 preliminary di-jet distributions Rapidity |cos( *)| Ratio • Di-jets provide direct access to parton kinematics at LO Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 20

Projected di-jet sensitivity for the 2009 run Rel FOM ~ 70% x 1 ~ 0. 33 x 2 ~ 0. 044 GS-C x 1 ~ 0. 45 x 2 ~ 0. 16 x 1 ~ 0. 27 x 2 ~ 0. 27 • Assumes 50 pb-1 of 200 Ge. V pp collisions with 60% polarization Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 21

Conclusions • STAR inclusive jet ALL measurements have excluded extreme gluon polarization scenarios • Over the next few years, STAR di-jet measurements will provide direct information about Δg(x) • STAR will also obtain complementary information about Δg(x) from γ + jet • Stay tuned! Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 22

Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 23

Double longitudinal spin asymmetry, ALL N : Spin dependent yields (# of reconstructed jets) P : Beam polarization (measured by RHIC Polarimeter) R : Relative luminosities between different spin states Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 24

STAR detectors for jet measurements § Time Projection Chamber | |<1. 4 tracking § Barrel EM Calorimeter | |<1 triggering & calorimetry § Endcap EM Calorimeter 1. 09< <2 triggering & calorimetry § Beam-Beam Counters 3. 4<| |<5 triggering, luminosity, local polarimetry Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 25

Other global analyses xΔg(x) at Q 2 = 10 Ge. V 2 Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 26

Additional ALL predictions for 2005 ALL Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 27

Estimating the non-jet background Data Monte Carlo Charged Energy Fraction Data Carl Gagliardi – DIS 2008 – Jets. Charged Energy Fraction in pp at RHIC Charged Energy Fraction • Non-jet backgrounds stand out in the data as single-jet events that have a small charged energy fraction • Isolate the background in the ratio of mono-jet to di-jet events vs. charged energy fraction 28

ALL vs. run index for 2006 data STAR Preliminary Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 29

False asymmetries in 2006 data STAR Preliminary • Longitudinal single-spin (parity-violating) asymmetries are consistent with zero, as expected at this precision Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 30

Inclusive jet ALL for fixed values of ΔG • Calculations by Stratmann and Vogelsang within the GRSV framework Carl Gagliardi – DIS 2008 – Jets in pp at RHIC 31