A Review of G measurements: Present & Future Abhay Deshpande RIKEN BNL Research Center...

A Review of G measurements:Present & Future

Abhay Deshpande

RIKEN BNL Research Center

[email protected]

Snowmass 2001

Working Group: Fixed Target Experiments

Riken BNL Research Center

July 3, Snowmass 2001, Delta G 2

Overview

• Fixed target polarized DIS experiments Their need to evaluate first moments of spin structure functions

Methods used to get the first moments: Old & New

New: pQCD analysis at NLO Allows to access G

Lessons…-- features and faults

Other “direct methods” in DIS –Photon Gluon Fusion-to

access G: Lessons-- Features and faults

• Polarized Gluon Measurements at Colliders RHIC spin program present and near future

Future polarized electron proton colliders: EIC/eRHIC,

Polarized HERA….THERA and their ability to get Delta G.


Polarized DIS

• To date fixed target experiments only

• Both: Probe and the Target need to be polarized

• Electron beams up to 50 GeV/c on fixed (solid and gaseous) targets were predominantly used at SLAC (EXXX series) and DESY (HERMES)

• Muon beams 100-200 GeV/c on fixed (solid) targets were used at CERN(EMC,SMC)

Compared to un-polarized DIS, the kinematic rangeis small


How far does “polarized DIS” have to go!


Need to evaluate the first moments and the methods… old/new

• Measure A1(x,Q2)• Use parameterizations of F2 and R to get g1(x,Q2)• To move data to a fixed Q0

-- OLD Method: assume A1 independent of Q -- New Method: use pQCD analysis at NLO which treat the

Q2 evolution using DGLAP equations -- Assign appropriate uncertainties• Extrapolate to unmeasured high and low x regions -- Assign appropriate uncertainties…!• Evaluate first moments of spin structure functions


A bit of theory…g1(x,Q2) and its evolution

GLAP Evolution

functionst coefficiengluon andquark are ,

)/ln(;1

2

12

2

2^)2,(1

2222

NSs

QCDi

if

qNS

gsf

qs

CC

Qten

e

GCGCnCe

Qxg

GPP

PnPt

Gdt

d

qPt

qdt

d

ggS

gq

Sqgf

sqqS

NSNS

qqS

NS

2

2

)(

2

)(

Pij s are Polarized splitting functions known to NLO


The Method

• Chose a starting scale Q2 = 1 GeV2

• Parameterize the polarized parton distributions with functional form:

• Each PD is normalized such that:

• become the first moment of the parton distributions

• First moments of singlet and gluon distributions are free parameters while the first moment of the nonsinglet is either fixed by Bjorken sum rule or in some ambitious analyses left free too.

• minimization is performed for g1 measurements from experiments and evolved value of g1 using the parton distribution functions and the DGLAP evolution equations at the measured x,Q2 of the data point.

)1()1(),,()( xaxxaNxf ff

ffff

1)1()1(),,(1

0

xaxxdxaN ff

fff

f

2

SMC PR D58 1999 (112002)


Results & Uncertainties…

• Fit to world data

• 133 data points (CERN,SLAC,DESY)

• 10 free parameters

• Chi2 = 116.1 using only statistical errors

• Experimental systematic errors handled separately

• Uncertainties of theoretical origin also handled separately

SMC PRD 1999 (112002)


PDFs and Systematic Uncertainties• Experimental Sources

-- Systematic uncertainties on A1

-- Uncertainties of F2 and R parameterizations

• Theoretical Sources

-- Functional form of initial pdf

Change that redo fits

Start at a different initial scale and

repeat fits

-- Factorization and renormalization scales

Change by a factor of 2 up/down repeat fits

-- Value of S 0.118 +/- 0.003

-- Other smaller effects due to quark mass thresholds, a_8…


Lack of low x data… consequencesQ2 = 10 GeV2

5.00 as )0(1 xxg

Regge/QCD

SMC Results


Neutron structure function… E154/SLAC

• Consequence: Unertainties in low x for

Bjorken sum rule…• Acceptable? No…

measure low x!


Observation: Features and Faults

• Method reliable… (refer to unpolarized NLO analysis of F2), uncertainties estimates rather straightforward.. Although tedious.

• Largest uncertainties come from the unmeasured low x region.

• pQCD analysis needs large Q2 arms which are absent in available data

• Need a collider experiment e-N with sufficiently large CM energy to cross the low x barrier at the same time have large enough values of Q2 so that pQCD methods can be reliably be used to get at a the values of Delta G.


Other methods to get at G: Photon-Gluon Fusion

HERMES Collaboration, PRL 84 (2000)

Signal

Background


High pT Hadron:PGF HERMES Results

)???()(03.0)(18.041.0/ theorysyststatGG

No estimate of theoretical Uncertainty


Experimental & Theoretical Difficulties at low Scales

Fraction of VDM and its uncertainty? Scale dependence significant?

A high energy polarized collider will overcome these!

W. Vogelsang, SPIN2000HERMES Collaboration


RHIC: Polarized Proton Collider

BRAHMS & PP2PP (p)

STAR (p)

PHENIX (p)

AGS

LINACBOOSTER

Pol. Proton Source500 A, 300 s

GeVs

L

50050

onPolarizati%70

cms102 2132max

Spin Rotators

Partial Siberian Snake

Siberian Snakes

200 MeV Polarimeter AGS Internal PolarimeterRf Dipoles

RHIC pC Polarimeters

Absolute Polarimeter (H jet)

2 1011 Pol. Protons / Bunch = 20 mm mrad


RHIC Spin Physics Program

Production ),( 0 XgqggALL

Heavy Flavors ),( XbbccggALL

Direct Photon )( XgqALL

Jet Photon )( XJetgqALL

STA

R +

P

HEN

IX

Jet Jet )( XJetJetgqALL

Spin Transversed

d

d

d

u

u

u

u , , ,

W Production

G :onPolarizati Gluon

)( lL lWduA

STAR +PHENIX

XppAT

),(

,

ionFragmentat ceInterferen

STAR +PHENIX+PHOBOS

: ty Transversi 1h

:sAsymmetrie Pion Single

BRAHMS


New Experiments for MeasurementsG

Polarized pp Polarized DIS

RHIC/BNL HERA/DESY, EIC@BNL and CERN/SPS

Production ),( 0 XgqggALL

Heavy Flavors ),( XbbccggALL

Direct Photon )( XgqALL

Jet Photon )( XJetgqALL

STA

R

PH

EN

IX

pQCD at low x

Photo production of charm and high hadron pairs

)( qqgALL

),( 21 QxA

Pola

rized

HER

A &

EIC

HER

MES

CO

MP

AS

S

Jet Jet )( XJetJetgqALL

Tp

Single and Di-Jets DIS &Photoproduction

))(( XJetJetgALL

EIC: Electron Ion Collider 3-10 GeV e 30-250 GeV polarized protons


from Prompt Photon ProductionG

gggg

qqqq

qqg

qqqq

gqq

)()(

)()(

)(

)(

)(

)()(

211

1

,, 22

,, 22

1

1

xAxg

xgqqga

xfe

xfe

xg

xgqqgaA

LL

sdui ii

sdui ii

LLLL

Double Spin Asymmetry

g

q

g

q

q

q

Gluon Compton (85% of ) Annihilation (15% of )


Prompt Photon Production

• Direct access to both in PHENIX and STAR

Jet for x-gluon reconstructionObservables: Double spin asymmetries

G


Kinematic range RHIC vs. OthersG from Heavy Flavors

GeVs 200 :RHIC

prompt photon

Xecc

Xebb

a

AQQgg

LL

XQQpp

LL

xG

xG

xG

xG

xx

)(

2

2

1

1

21

)(

)(

)(

)(

)(

),(

1p

2p


for leading Pions: Year 2 LLA

Use high in order to tag Jet Tp

Model Calculation using PYTHIA and polarized PDFs from Gehrmann, Sterling 10% of design Luminosity

GeV 200s

Pion Asymmetries

Gluon A

Gluon B

Gluon C


The Electron Ion Collider (EIC) w.r.t. Other Experimental Facilities

• New kinematic region to be explored

• EIC = eRHIC + EPIC

• Kinematic Reach for DIS:

• High Luminosity!

GeVs

GeVEGeVE pe

10020

25030,103

42

422

101

6.010,1

Q

xGeVQ

23433 sec//1010~ cmL

EVERY THING I SAY ABOUT EIC FROM NOWIN TERMS OF ITS PHYSICS CAPABILITIES HOLDS

ALSO FOR POLARIZED HERA COLLIDER


The EIC w.r.t. Other Experimental Facilities

Large luminosity and high CM Energy makes EIC unique!

Variable CM energy enhancesits versatility!


If the EIC is built at RHIC “eRHIC”I. Ben-Zvi et al./S. Peggs et al.

• Use the existing infrastructure and resources of the RHIC at BNL

RHIC: Polarized proton beams 50 GeV 250 GeV ?325 GeV

• Exists an unused Experimental Hall: The one at 12 o’clock position of the present RHIC reserved for “future major detector”

• Add a electron LINAC beam energy variable: 3 GeV12 GeV

RHIC at BNLRing-Linac Design

Blue ringYellow ring

ElectronLinac


Spin Structure Function g1 at low xA. Deshpande et al.

~5-7 days of data

A Unique Measurement! No present/future approved experiment will measure this.

3 years of data


• pQCD analysis of g1 structure function at NLO gives the first moment of the polarized gluon distribution. Present value and uncertainty is: (at Q2 = 1 GeV2)

1.0 (stat) (exp.sys.) (theory/low-x)

• Major source of uncertainty from low x unmeasured region: Theory completely unconstrained in this region.

• If EIC data is obtained and the analysis is repeated, the theoretical uncertainties improve by factors of 3-5; the statistical uncertainty improves by even bigger factors.

+1.0 + 0.4 +1.4 - 0.4 - 0.2 - 0.5

Complimentary determination of G to that fromRHIC Spin

First Moment of the G(x).A. Deshpande et al.


Result of Di-Jet analysis at NLOG. Radel et al./A.Deshpande et al.

• Easy to differentiate between different scenarios of G: Improves G by factor of ~3• Combined analysis: Di-Jet + pQCD analysis of g1:G constrained by these two

together further improves the uncertainties by additonal factor of ~3 Effectively factors of 10 improvement in G can be expected!

Statistical accuracy shown for EIC for 2 luminosities

Detector smearing effects studied

NLO analysis for Di-Jet Included


Polarized Parton Distribution of the Photon

• Photoproduction studies with single and di-jet and one and 2 high pT opposite charged hadrons.

• At high enough energies the photon can resolve itself into its parton content

• With polarized protons asymmetries related to the spin structure of the photon can be extracted! A UNIQUE measurement!

• Asymmetries sensitive to the gluon structure as well!

Resolved PhotonDirect Photon


Spin structure of polarized photon!M. Stratmann & W. Vogelsang

• Statistical uncertainty with 1 inv.fb.

• ~2wks running for EIC

• Single and double jet asymmetries

• ZEUS Acceptance cuts

• Will resolve the photon spin structure easily!

Direct Photon Resolved Photon


Summary of G Measurements

• Polarized gluon density remains to be measured. Ample evidence for it being non-zero & positive.

• Attempts to measure them have been serious but have had limited success due to various things:

a) Beam facilities of the past + Detectors

b) Kinematics covered by data make the interpretation difficult.

• Future dedicated experiments at colliders will do better:

High energy colliders … better detector designs … will cover larger kinematic regions which have little theoretical issues

• Starting with RHIC Spin leading to possible EIC at BNL or Polarized HERA at DESY the prospects of accessing G and chances of uncovering other surprises remain very good!


References for Material Related to Polarized e-p colliders

EIC, Polarized HERA, THERA

http://www.phenix.bnl.gov/WWW/publish/abhay/Home_of_EIC/

http://www.bnl.gov/eic/

Will soon become…

Everything you ever wanted to know about future polarized & Unpolarized lepton-hadron colliders but were afraid to ask….

A Review of G measurements: Present & Future Abhay Deshpande RIKEN BNL Research Center...

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