Using Jet Asymmetries to Access G at

Renee Fatemi Massachusetts Institute of Technology

for the STAR Collaboration

March 23, 2006 Moriond QCD, La Thuile, Italy

1. Measurements aimed at extraction of G at STAR

2. RHIC Facility and STAR Experiment

3. STAR Jet Algorithm and Jet Characteristics

4. Jet Cross-Section and NLO comparisons

5. Double Spin Jet Asymmetries and implications for G

6 Future Improvements and Measurements

STARSTAR

A Short History of G

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JPROTON =1

2= ΔSq + ΔG + Lq + LG

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Sq

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G Indirectly accessed from DIS via Next-to-Leading order fits

I. Hirai,Kumano and Saito hep-ph/0601087

20-30% proton spin. Data from Fixed Target DIS ep scattering

Limited CM energy range results in large error on G fit.

For Q2=1 GeV2

I.

II.

Need polarized electron-proton collider or direct access to the gluon in order to continue experimental study of G €

G = 0.99 ±−0.311.17

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G = 0.50 ±1.271.27

II. SMC Collaboration Phys.Rev. D58 (1998) 112002

G=

+ +

No FF! Average over partonic kinematics

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rp

r p → jet + X

No FF! Reconstruct partonic kinematics. Statistically limited until 2006.

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rp

r p → jet + jet

Requires for partonic kinematics

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rp

r p → π + /− + X

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rp

r p → π 0 + X

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DABπ

€

qq + q q

Accessing G at STAR

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ALL =σ ++ −σ +−

σ ++ + σ +−=

ΔfAΔfB × Δσ AB →CX × DC

fA fB ×σ AB →CX × DCfA fB fC

∑

Reconstruction of partonic kinematics

Statistically limited - requires high luminosity

challenging pion background subtraction

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rp

r p → jet + γ

Inclusive measurements have contributions from several

sub-processes

Understanding of inclusive channels necessary for analysis of clean jet + photon channel

RHIC Complex + STAR Detector

Spin Rotators(transverse/longitudinal)

STAR IR

RHIC

polarimetersSiberian Snakes

Siberian Snakes

PHENIX IR

Colliding 100 GeV beams

Each bunch filled with distinct polarization state

Spin Rotators at STAR IR allow for transverse and longitudinal spin orientation

Bunch Xings every 100-200ns

CNI polarimeters + Hydrogen Jet target provide run by run & absolute polarization

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r p

r p

TPC charged tracks -1.4<<1.4

EMC neutral energy -1 < < 1 (2003-2005 only 0< < 1)

Beam Beam Counters (BBC) provide MINBIAS trigger as well as spin dependent luminosity 3.4 < < 5.0.

High Tower (HT) trigger : Requires 1 tower (0.05 x 0.05) with ET > 2.2-3.4 GeV

= -ln[tan(/2)]

What does a jet at STAR look like?What does it mean to identify a jet with ET=5 GeV at ?

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s = 200

“One can usefully define jets down to ET 5 GeV” UA1 Collaboration Nuclear Physics B309 405-425 (1988)

2-particle azimuthal correlations seen at STAR show clear evidence of correlated clusters of particles separated by also known as DIJETS! Real test is collecting these clusters into a “jet” and comparing to NLO pQCD calculations

Simple Jet Definition: A cluster of particles correlated in and that result from the fragmentation and hadronization of scattered partons.

nu

cl-e

x03

06

02

4

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φ=π

pT (GeV)A. Mischke tomorrow 10:30

STAR Jet Algorithm

i. P of TPC track, EMC tower OR particle used as seed for cluster formation

ii. Cluster P around seed inside Jet Cone Radius = 0.4

iii. Look for additional stable clusters at “midpoint” between two clusters

iv. Merge jets if Energy overlap > 50%

v. Sum of P in each stable cluster forms jet

vi. Require Jet pT > 5 GeV

vii. Same algorithm used for DATA, GEANT and PYTHIA jets

PA

RT

ON

PA

RT

ICL

ED

ET

EC

TO

R

GE

AN

T

JE

TS

DA

TA

J

ET

S

PY

TH

IA

JE

TS

Midpoint Cone Algorithm (hep-ex/0005012)

- Collinear and infrared safe -

DATA JETS GEANT JETS

PYTHIA JETS Corrected Jet Yield = X

Correction factor incorporates detector resolution, and energy losses due to fiducial cuts and undetected neutral energy

Jet Cross-Section Analysis 2004 DATA SAMPLE

~0.15 pb-1 sampled lum

0.8/1.4M MINB/HT events

2004 X-sec Cuts

|vertex| < 60 cm

0.2 < jet < 0.8

Trig ET > 3.5 GeV

Neutral ET / Jet ET<0.9 RA

W p

er-

eve

nt

Yie

ld

Good agreement between DATA and Simulation - PYTHIA 6.205 (CDF Tune A) + GEANT (Geisha)

EmcEt/JetEt (pT > 21.3 GeV/c)

Enhanced neutral energy in jets motivates background cut

Jet Cross-Section Results

B. Jager et.al, Phys.Rev.D70 034010

Two point overlap between HT and MINB show good agreement.

50% systematic shown in yellow band comes from uncertainty in jet energy scale. Need or gamma-jet to reduce this error.

Agreement -within systematics over 7 orders of magnitude!€

π 0

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C pT( ) =PYTHIA(pT )

GEANT(pT )

Bin migration

Detector and Jet Reconstruction inefficiencyTrigger inefficiency Undetected Neutral Energy

Bin by Bin Correction Factors

Dominant Cross-Section Systematics

* Under Study

* Under Study

* Under Study

Normalization

Pythia slope Statistics of c(pt) Background

BBC TriggerEnergy ScaleF

ract

iona

l Cha

nge

in x

-sec

tion

Fragmentation Function Systematic: Comparison between PYTHIA and HERWIG results in no significant difference in correction factors.

Jet Asymmetry Analysis 2003+2004 DATA Sample

0.3 pb-1 sampled luminosity

<PBPY> = 0.3/0.4 for 03/04

125/162k Jets after cuts 03/04

2003/2004 Cuts

|vertex| < 75/60 cm

0.2 < jet < 0.8

Neutral ET / Jet ET < 0.8/0.9€

ALL =σ ++ −σ +−

σ ++ + σ +−=

1

PB PY

N ++ − RN +−

N ++ + RN +−

bunch crossing#

B

Y

€

R =S++

S+−

S = Scaler board counts from BBC (anti) aligned (+-) ++ helicity states

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−−

Excellent Agreement between 2003/2004 ALL

2003+2004 Jet ALL

First Double Spin Asymmetry from STAR!

ALL is consistent with zero.

Results tend to disfavor the GRSV g=g scenario

Agrees well with previous DIS evaluations

Errors are statistical only

STAR systematic errors ~0.01

Polarization Systematic ~25% not included in figure

Predictions: B.Jager et.al, Phys.Rev.D70(2004) 034010

Asymmetry Systematics I

A. Single spin Asymmetries consistent with zero as expected

B. Relative Lum Systematic Error

Estimated by comparing BBC scaler results from two different timing cuts.

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δALL = 0.009

C.Random Fill Pattern Analysis 1. Randomly assign spin patterns to 56 bunches.

2. Recalculate ALL - fit with line

3. Repeat 400x and histogram fit

4. RMS of histogram is on order of ALL statistical error indicating no systematic associated with a specific bunch

AL

L

Run ID Fit Mean

p0=0.007613 ±0.0169RMS= 0.0164

Asymmetry Systematics II2004

(det)<1

bg

Jet EEMC/ETot

4. Background Asymmetry - showers from beam scraping results in jets with an excess of neutral energy. Estimate systematic effect on ALL by calculating background fraction (fBG) and asymmetry (ALL

BG).

Jet EEMC/ETot>0.9(det)<1

AbgLL 2004

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ALL

meas pT( ) =ALL pT( ) + f BG pT( ) × ALL

BG pT( )1+ f BG pT( )

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f BG = 0.05

ALLBG ≈ 0.06 ± 0.03

δbgALL< 0.003

5. Trigger Bias - The HT trigger can alter the “natural” weight of contributions from gg,qg,qq. Use GRSV std + CTEQ5L to simulate ALL and estimate systematic due to A) subprocess changes B) detector bias C) bin migration

δTRGALL< 0.006 PYTHIA jets

High Tower jets

ALL

• PYTHIA - HT

Simulations

Future Results

I. Use of new Jet Patch trigger in 2005 increased jet reconstruction efficiency, reduced trigger bias and reduced the size of trigger efficiency corrections for Jet cross-section measurements

II. Longer dedicated runs will increase statistics

Increased Statistics will allow STAR to distinguish between gluon senarios.

2005 Projections from data on tape

2005 HT1

2005 JP1

C (

pT)

PYTHIA Simulations

Summary First Jet Results from STAR reflect several years of work on

Detector Design, Trigger Algorithms, Jet Algorithms and Luminosity Monitoring tools.

Jet X-seci. Understand Detector/Trigger enough to provide good agreement

between Simulation/Dataii. Agreement within systematics for NLO pQCD calculationsiii. Future possibilities - access to high x PDFs + jet shape analysisiv. Motivates application of Jet Algorithm in this energy regime to jet

asymmetry measurements. Jet Asymmetry

i. First double spin asymmetry results are consistent with zero.ii. This measurement is in full agreement with previous DIS evaluations of

the gluon polarizationiii. Initial measurements are statistics limited.

2003+2004 results reflect statistics from limited preliminary runs. Inclusive predictions from 2005+2006 statistics indicate the ability to discriminate between gluon scenarios.

BACKUP SLIDES

pp Run 2002 2003 2004 2005

>2006

CM Energy 200 GeV 200 GeV 500 GeV

<Pb> and direction at

STAR0.15 T

0.30 T/L 0.40 L 0.45 L/T 0.7 L/T 0.7 L/T

Lmax [ 1030 s-1cm-2 ] 2 6 6 16 80 200

Lint [pb-1 ] at STAR 0.3 0.5 / 0.4 0.4 5 320 800

Stable polarization direction - transverseLongitudinal polarization at STAR/Phenix

AGS Heclical Partial Snake

Number of bunches: 55 - 1122x1011 protons/bunch (max)

PHOBOS

Jet Cross-Section Results

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1

dΩ

dσ

dpT

=1

2π ⋅0.6⋅

1

ΔpT

⋅1

L ⋅dt∫⋅

1

c pT( )⋅

dN

dpT

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1

L ⋅dt∫=

σ BBC ⋅εvertMB

NMB−eventsaccepted

hep-ph/0404057

Due to size of systematic errors connected with small c(pT) at low pT bins only use HT data > 7 GeV

Two point overlap between HT and MINB show good agreement.

50% systematic shown in yellow band comes from uncertainty in jet energy scale. Need or gamma-jet to reduce this error.

Agreement (within systematics) over 7 orders of magnitude!€

π 0

p+12C CNI elastic scattering: Recoil Carbon Detection

– Carbon filament target (5mg/cm2 10 m)

in the RHIC beam.– Measure recoil carbon ions at Lab90º

having energies, 100 keV < Ecarbon< 1 MeV – Measure E and TOF to identify recoil

carbon ions, determine 4-momentum (-t) and determine left-right/up-down asymmetries.

– All data analysis performed on wave-form digitizer board to allow high counting rates (~0.5 MHz) via scaler measurement δ ~ 310-4 in ~1 minute.

Coulomb Nuclear Interference (CNI) Polarimeter at RHIC

Beam’s View

Si #1

Si #2 Si #4

Si #3

left right

down

up

ADC values

Arr

ival

tim

e (n

s)

CarbonE950

PRL 89 (2002) 052302

O. Jinnouchi (RIKEN) and I. Akekseev (ITEP) SPIN 2002