Carl Gagliardi – DIS2008 – Jets in pp at RHIC 1

Jet Production in Polarized pp Collisions at RHIC

Carl A. GagliardiTexas A&M University

for the Collaboration

Outline• Introduction• 2005 results• 2006 results• Looking ahead

STARSTAR

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 2

Origin of the proton spin?Polarized DIS: 0.2~0.3 Poorly Constrained

gz

qz

pz LLGS

2

1

2

1

• A primary goal of the RHIC Spin program is to determine the gluon polarization distribution

Three recent fits of equal quality:– ΔG = 0.13 ± 0.16– ΔG ~ 0.006– ΔG = -0.20 ± 0.41all at Q2 = 1 GeV2

Leader et al, PRD 75, 074027

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 3

Polarized pp collisions at RHIC

LLba

baLL a

ff

ffA ˆ

f: polarized parton distribution functions

cos

For most RHIC kinematics, gg and qg dominate, making ALL for jets sensitive to gluon polarization. 10 20 30 pT(GeV)

Partonic fractions in jet production at 200 GeV

0

0.25

0.5

0.75

1

0 0.2 0.4 0.6 0.8 1

|cos (theta*)|

qg

->

qg

FO

M

0

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 4

RHIC: the world’s first polarized hadron collider

“Thousands of millions”

• 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

BRAHMS

PHENIX

AGS

BOOSTER

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV PolarimeterAGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC Polarimeters

Strong Helical AGS Snake

Helical Partial Siberian Snake

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

STAR

PHOBOS

Pol. H- SourceLINAC

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 5

E-M Calorimeter

Time of    Flight

Projection           Chamber

STAR detector

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 6

Midpoint cone algorithm(Adapted from Tevatron II - hep-ex/0005012)

• Seed energy = 0.5 GeV

• 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

Jet reconstruction in STARD

etec

tor

etcp

e

,,

,

gq,

GE

AN

TP

YT

HIA

Data jets MC jets

Par

ticl

e

Use PYTHIA + GEANT to quantify detector response

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 7

Jet production at RHIC

STARSTAR PRL 97, 252001

• Jet structure at 200 GeV is well understood

• Mid-rapidity jet cross section is well described by pQCD over 7 orders of magnitude

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 8

• 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

Neutral Energy Fraction

2005 inclusive jets ALL systematics

ALL systematics (x 10 -3)

Reconstruction + Trigger Bias

2-5 (pT dep)

Non-longitudinal Polarization

0.1-0.8 (pT dep)

Relative Luminosity 0.94

Backgrounds 0.70

pT systematic [-5.4%,+6.7%]

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 9

2005 inclusive jets ALL

• Comparison to predictions within the GRSV framework with various input values of ΔG demonstrate sensitivity

STARSTAR hep-ex arXiv:0710.2048

Model calcs from:Jager et al, PRD 70, 034010

200 GeV0.2 < η < 0.8

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 10

Limits on ΔG from 2005 jet results

• Significant new constraints on ΔG when compared to predictions derived from one global fit to DIS data

GRSV DIS

STARSTAR hep-ex arXiv:0710.2048

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 11

Other global analyses

• There are many other global analyses of the polarized DIS data

xΔg(x) atQ2 = 10 GeV2

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 12

Additional ALL predictions for 2005

• Many predictions for ALL vs. pT

• Is the sensitivity unique to GRSV?

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 13

Comparison to other global analyses

• 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

STARSTAR

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 14

2006 data

Improved figure of merit Luminosity : 2 4.7 pb-1

Polarization: 50% 60% (online polarization)

Barrel EMC coverage: [0,1] [-1,1]

In addition Jet cone radius: 0.4 0.7 -0.7 < jet axis < 0.9 Neutral energy fraction < 0.85

Increased trigger thresholds

Inclusion of Endcap EMC towers

Improved tracking at large ||

Shielding from backgrounds

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 15

Jet reconstruction and trigger bias for 2006 data

Step 1: Use PYTHIA+GEANT to estimate the pT 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 jetsShifted detector jetsDetector jets

• Maximum deviation determines ALL systematic

• Confidence level calculations take account of any residual

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 16

2006 inclusive jets ALL

• Statistical uncertainties are 3-4 times smaller than in ’05 data for pT > 13 GeV/c

ALL systematics (x 10 -3)

Reconstruction + Trigger Bias

[-1,+3]

(pT dep)

Non-longitudinal Polarization

~ 0.03

(pT dep)

Relative Luminosity

0.94

Backgrounds 1st bin ~ 0.5

else ~ 0.1

pT systematic 6.7%

Model curves calculated with cone radius 0.7 and -0.7 < < 0.9

200 GeV-0.7 < < 0.9

STARSTAR

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 17

Limits on ΔG from 2006 jet results

• Within the GRSV framework:– GRSV-std excluded with 99% CL– ΔG < -0.7 excluded with 90% CL

GRSV DIS

STARSTAR

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 18

Looking beyond inclusive ALL measurements

• Inclusive ALL measurements at fixed pT 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 – DIS2008 – Jets in pp at RHIC 19

DSSV – first global analysis with polarized jets

• 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

de Florian et al., arXiv:0804.0422 [hep-ph]

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 20

The next few years: di-jets and Δg(x)

• Di-jets provide direct access to parton kinematics at LO

Mass Rapidity |cos(*)|

Ratio

2005 preliminary di-jet distributions

2

|ηη|tanh|cosθ|

2

ηη

x

xln

2

1y

sxxM

epeps

1x

epeps

1x

43*

43

2

1

21

η4

η32

η4

η31

43

43

STARSTAR

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 21

Projected di-jet sensitivity for the 2009 run

• Assumes 50 pb-1 of 200 GeV pp collisions with 60% polarization

GS-C

x1 ~ 0.45x2 ~ 0.16

x1 ~ 0.27x2 ~ 0.27

Rel FOM ~ 70%

x1 ~ 0.33x2 ~ 0.044

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 22

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 – DIS2008 – Jets in pp at RHIC 23

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 24

ALL

1

P1P2

N RN

N RN

N : Spin dependent yields (# of reconstructed jets)

P : Beam polarization (measured by RHIC Polarimeter)

R : Relative luminosities between different spin states

Double longitudinal spin asymmetry, ALL

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 25

STAR detectors for jet measurements

Time Projection Chamber ||<1.4 tracking

Barrel EM Calorimeter||<1 triggering & calorimetry

Endcap EM Calorimeter1.09<<2 triggering & calorimetry

Beam-Beam Counters3.4<||<5 triggering, luminosity, local polarimetry

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 26

Other global analyses

xΔg(x) atQ2 = 10 GeV2

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 27

Additional ALL predictions for 2005

ALL

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 28

Charged Energy Fraction

Estimating the non-jet background

Data Monte Carlo

• 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

Data

Charged Energy Fraction

Charged Energy Fraction

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 29

ALL vs. run index for 2006 data

STAR Preliminary

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 30

False asymmetries in 2006 data

STAR PreliminarySTAR Preliminary

• Longitudinal single-spin (parity-violating) asymmetries are consistent with zero, as expected at this precision

Carl Gagliardi – DIS2008 – Jets in pp at RHIC 31

Inclusive jet ALL for fixed values of ΔG

• Calculations by Stratmann and Vogelsang within the GRSV framework