Gluons’ polarization Gluons’ polarization in the nucleonin the nucleon

Summary of results and ideasSummary of results and ideaswhere we are where we are

and and where we gowhere we go

Ewa Rondio, Ewa Rondio, A. Soltan Institute for Nuclear StudiesA. Soltan Institute for Nuclear Studies

Warsaw, PolandWarsaw, Poland

planplan

IntroductionIntroduction Early information from QCD fitsEarly information from QCD fits Need for „direct measurements”Need for „direct measurements” Present results from DISPresent results from DIS Important contribution from ppImportant contribution from pp Fit in NLO including DIS and pp data Fit in NLO including DIS and pp data Complementarity of Complementarity of g/g resultsg/g results

introductionintroduction What spinsin the nucleon?

It all started 20 years ago (1988/89)from this measurement

EMC measurement of cross sectionasymmetries on polarized protonextended towards low xand showed that:Quark contribution is

not enough to explain spin of the proton

What are the other candidates?• gluons• orbital angular momentum

of quarks and gluons

Remarkable experimental progress Remarkable experimental progress in QCD spin physics in the last 20 in QCD spin physics in the last 20 yearsyears

Inclusive spin-dependent DISInclusive spin-dependent DIS – EMC, SMC, COMPASSEMC, SMC, COMPASS– E142,E143,E154,E15E142,E143,E154,E1555– HERMESHERMES– Jlab-Hall A, B(CLAS)Jlab-Hall A, B(CLAS)

Semi-inclusive DISSemi-inclusive DIS– SMC, COMPASSSMC, COMPASS– HERMESHERMES

Polarized pp collisionsPolarized pp collisions– RHICRHIC

PHENIX & STARPHENIX & STAR

QCD analysis done by many QCD analysis done by many groups groups (experimental and (experimental and theoretical)theoretical)input: DIS inclusive data input: DIS inclusive data g g11(x,Q(x,Q22) for p, d, ) for p, d, 33HeHe

QCD fits to all inclusive measurementsQCD fits to all inclusive measurements information on information on g(x) from evolution (indirect)g(x) from evolution (indirect) as gluons do not couple to photon, but influence Qas gluons do not couple to photon, but influence Q22 evolution evolution method:method: assume functional form for parton distributions at selected Qassume functional form for parton distributions at selected Q00

22

calculate expected values of gcalculate expected values of g11 at every (x,Q at every (x,Q22) point where measured ) point where measured usingusing

look for parameters giving best description of the datalook for parameters giving best description of the data first only inclusive DIS , more inputs first only inclusive DIS , more inputs later later

g(x,Q02) is parametrized, important is the selected Q0

2 functional form and also first moment depend on it, flexibility is an issue

Early phase of analysisEarly phase of analysis

Many efforts in the past Many efforts in the past have been made have been made - - Ball, Forte, Ridolfi (1994)Ball, Forte, Ridolfi (1994)– Gluck, Reya, Stratmann, Gluck, Reya, Stratmann,

Vogelsang (2001)Vogelsang (2001)– Blumlein and Bottcher Blumlein and Bottcher

(2003)(2003)– Leader, Sidorov, Leader, Sidorov,

Stamenov (2006)Stamenov (2006)– Hirai, Kumano, Saito Hirai, Kumano, Saito

(2006)(2006)– ……....

Weak constraints from the data, only very simple gluon functionswere possible, Q0

2 of the fit is importantfunctional form and also first moment depend on it

Gluon is a natural candidateGluon is a natural candidateto carry spin of the nucleon to carry spin of the nucleon it carries about 50% of the proton it carries about 50% of the proton momentummomentumit can contribute also to spinit can contribute also to spin fits with different inputsfits with different inputs

(changing with time)(changing with time) more precisemore precise assumptions assumptions

functional form flexiblefunctional form flexiblepositivepositivenegativenegativewith sign changewith sign change

higher twistshigher twists……………………

Spread getting smaller with Spread getting smaller with time, but still largetime, but still large

Let’s look at the latest Let’s look at the latest resultsresults

Polarized gluon distributions from QCD fits all presently available (also historical)

Gluon polarization form Gluon polarization form fits (DIS data)fits (DIS data)

LSS

2three solutions with very similar

| | 0.4G

comparison ofLSS and Compass fit (same input data)

Differences:

functional formhigher twistspositivityconstraints

more data did notreduce uncertainty

it showed that functions for g(x,Q2

0) were

too simple !!!|G| 0.2 - 0.3

LSS – higher twists and influence of data LSS – higher twists and influence of data setssets

ACC - with additional data and DIS only

Small effect on g(x) but big reductionof the uncertainty

Status from QCD fitsStatus from QCD fitsto DIS inclusive datato DIS inclusive data

Precise determination of quark polarizationPrecise determination of quark polarization Improvements in flavour decomposition with inclusion Improvements in flavour decomposition with inclusion

of semi-inclusive data in the fitof semi-inclusive data in the fit No clear answer concerning higher twists in proton and No clear answer concerning higher twists in proton and

neutron gneutron g11

Gluon not well constrainedGluon not well constrained– With better data more freedom in functional form resulted in With better data more freedom in functional form resulted in

much bigger spread of possible solutionsmuch bigger spread of possible solutions– Positive, negative and changing sign forms all give acceptable Positive, negative and changing sign forms all give acceptable

solutions in the fitssolutions in the fits Additional information needed to resolve this ambiguityAdditional information needed to resolve this ambiguity Options:Options:

– Measurement of Measurement of processes directly sensitive to processes directly sensitive to gluonsgluons

being done in lepton-nucleon and proton-protonbeing done in lepton-nucleon and proton-proton– Much bigger QMuch bigger Q22 range (lepton-proton collider) range (lepton-proton collider) far future far future

How gluon can be How gluon can be accessedaccessed

in DIS in DIS in pp colissions in pp colissions

DIS

Signal of gluon polarization Signal of gluon polarization D meson from PGFD meson from PGF

πKD0 ss0* ππKπDD

nD0 = 37398

nD* = 8675

Compass data:

Thick target, no D0 vertex reconstructionselection: decay angle, momentum fraction z(D0) & RICH PID

SBSS

G

G

1

D* tagging : cut on 3body invariant mass

weighting with S/B depending on event kinematics improves precision

No D* tagging

•Asymmetries for signal from D0+D* in z,pT bins available

G/G from Compass (LO)G/G from Compass (LO)

from Neural Network

trained on MC (AROMA):

input variables : Q2, xbj, y, pT, zD

Correlation ~80%

/ 0.49 0.27( .) 0.11( )G G stat syst 0.110.050.11gx

2 213GeV

In LO approximation from Compass data

bkgPGF

PGFLLPGFTB AR

g

gaRfPPA )1(exp

Work on NLO interpretation of this result in progress

Where NLO has to be taken into account?Where NLO has to be taken into account?Calculation of aCalculation of aLLLL

Definition of S/(S+B)Definition of S/(S+B) because in NLO light quark can emmit gluon because in NLO light quark can emmit gluon and contribute to PGFand contribute to PGF (but not bring information about gluon polarization)(but not bring information about gluon polarization)

PGF with light quarksPGF with light quarkshigh phigh pTT hadrons or hadron pairs hadrons or hadron pairs

R.D.Carlitz, J.C.Collins and A.H.Mueller, Phys.Lett.B 214, 229 (1988)

Revisited by A.Bravar,D.von Harrach and A.Kotzinian, Phys.Lett.B 421, 349 (1998)

Applied by SMC, HERMES and COMPASS First results (published):First results (published):

– Hermes Hermes PRL 84(2000)2584PRL 84(2000)2584 g/g= 0.42+/-0.18+/-0.03 at g/g= 0.42+/-0.18+/-0.03 at

<x>=0.17<x>=0.17 in photoproduction regionin photoproduction region– SMC SMC PRD 70(2004)012002PRD 70(2004)012002 g/g= -0.20+/-0.28+/-0.10 at g/g= -0.20+/-0.28+/-0.10 at

<x>=0.08<x>=0.08 in DIS region (Qin DIS region (Q22>1GeV>1GeV22))– Compass Compass PLB 633 (2006) 25-32 g/g=0.024+/-0.089+/-0.057g/g=0.024+/-0.089+/-0.057 QQ22<1 GeV<1 GeV22 (nonperturbative (nonperturbative

region)region)

Now more precise information Now more precise information from Hermes and Compassfrom Hermes and Compass

high pT is more likely withtwo partons in the final stateselect PGF and QCD Comptonsuppresses diminant process of photon absorption

Compass – high pCompass – high pTT pairspairs

3 basic processes, PGF probes gluons in the nucleon Q2>1 GeV2 gives perturbative scale, resolved photon small

aLL and R from MC (using NN)for every event

ALL2h and A1 from measurements

in Compass

3 processescontributeto both A1

and ALL

2h

but withdifferentfractions

Evaluation of gluon polarizationEvaluation of gluon polarization

The analysis are done in LO approximation – NLO effects are partially taken into account via parton shower concept in MC.Dominant systematic error is from MC (data description, PS, parameters..)

For each event we get (from NN) probability for3 processes

Comparing results with true probability from MCGives confidence in the NN classification

aLL is a ratio of partonic spin dependent and spinIndependent cross sections for sub-processes

Hermes – Hermes – hadron production at hadron production at high phigh pTT

looking at tagged, antitagged samples (with e,without e), h+, h-, pairslooking at tagged, antitagged samples (with e,without e), h+, h-, pairs

Asymmetries compared with prediction from model assumptions on gluons

Hermes – Hermes – g/gg/g

Rsig and Rbg

taken from Pythia MCAbg

model

h+,h- antitagged: 4 points between1.05<pT<2.5 GeVh+,h- tagged: 1 point for pT>1 GeVPairs: 1 point for

2 22Tp GeV

Consistency between: samples, targets, charges

Dominating sampleis from untagged hon deutron

Combining h+ and h-

Hermes – Hermes – g/g g/g method method IIII

exp)()(2 010.0034.0071.0),(g

Δg sysstatx +0.127-0.105 (sys-model)

data

Assumes functional form for g/g(x)only small range in pT

average x of measurement

|| ||MC measA AFit: find g(x)/g(x) such that

Difference between functions is a systematic uncertainty<2>=1.35 GeV2

<x>=0.22

g/g results from g/g results from lepton-nucleon lepton-nucleon scatteringscattering

Value smallValue small Possibly =0Possibly =0 at least at x~0.1at least at x~0.1

Compass high-pT

Open charm

Hermeshigh-pT

New(not published)

Compasslow Q2, updated

Extracting Extracting G/G from G/G from pppp scattering of composite objects,scattering of composite objects, accessing gluons through kinematic accessing gluons through kinematic

selectionsselections Very many nice measurements, Very many nice measurements,

appology that appology that only few will be shown only few will be shown herehere

(selection is for illustration, not choosing (selection is for illustration, not choosing most important)most important)

Double longitudinal spin asymmetry

Combined effect of several processes

How we access gluons in pp scattering?Simplified picture at leading order

gluons are probed in gluon-gluon andgluon-quark scatteringquark-quark is a background

contribution of processes depends on the event characteristics for example it is a function of jet pT

Collider allows wide range of CM energy scalesBut it is not easy to extract signal/scale fromcomplicated event structure (even more in NLO)

Sensitivity to gluon polarization depends on the analyzing power aLL, changing with event kinematics

competing requirements: sensitivity to gluons, hard scale and analyzing power

1 2 ˆ ˆ ˆgg qg qqLL LL LL LL

g g g q q qA dx dx a a a

q qg g qg

underlying underlying processes,processes, many contributionsmany contributions

Partonic kinematics determination from final states

p p X

0p p X

p p jet X

p p jet jet

Requires knowledge of fragmentation function

only average partonic kinematics

allows reconstruction ofpartonic kinematics…. But statisticaly limitedp p jet

p p X additional difficulty is background from p0

( )p p c c b b heavy flavourproduction- tagged gluon-gluon

Steps towards Steps towards gluon polarizationgluon polarization Check consistency of the measured Check consistency of the measured

cross-sections, correlations and cross-sections, correlations and fragmentation funct. with fragmentation funct. with assumptionsassumptions

Get estimates for effects of Get estimates for effects of approximations and correctionsapproximations and corrections

Extract asymmetries for different Extract asymmetries for different processes from the dataprocesses from the data

Use them with tested assumptions to Use them with tested assumptions to get preferable get preferable g(x) (parametrization, g(x) (parametrization, limitations)limitations)

What can be expected: from gluon distributions to asymmetries

pT is related to the xg

Interpretation in pQCDInterpretation in pQCD* * first step first step show that this is a good show that this is a good way to describe the processes in way to describe the processes in questionquestion PHENIX (PHENIX (, , ))

good good description of description of over many orders over many orders of magnitude, NLO of magnitude, NLO importantimportant

STAR (jets)STAR (jets)

Phys.Rev.D76,051105

Direct photonsRun5, preliminary

PRL 97,252001

NLO describes NLO describes high phigh pTT processes processes in many in many reactionsreactions

safe to study underlying partonic kinematics

but it is not easy for 0..Uncertainty can arise also from the fragmentation functions

here it is larger than scale uncertaintyand reflects in the predictions for ALL

What is the data telling What is the data telling us:us:00 asymmetries asymmetries

both experiments very consistentlymeasure asymmetries consistent with zero

•this conclusion holds for both measured energies •the range of probed xg shifts

•Extention towards low xg can be achieved with more forward 0 or with higher energy (difficult)

Photons – „golden Photons – „golden chanel”chanel”

clean signallinear inG

two contributing processes, q-g dominates in ppSelection by photon „isolation”Background from 0

Next to Leading Order Next to Leading Order vs.vs.

Next to Leading Logarithms Next to Leading Logarithms

scaleuncertainty

Possible way to estimate role ofmissing terms, factor 2 inscale is arbitrary

An alternative way is to compare with another approximation:

how does it affect the asymmetries?

Moving beyond inclusive probesMoving beyond inclusive probes

Here ALL predictions are LO, would be interesting to compare with NLO (work in progress)

It is also important to understand It is also important to understand more fine structure of eventsmore fine structure of events(to check description used)(to check description used)

Jets with high Jets with high pTpT

fraction of energyIn the cone for a jet

Jet selection algorithm optimal cone size dependent on pT

effects smearing jet pT needed to becorrected for

max

min

( )x

x

g x dx

1

0

( )G g x dx

how to quantify the how to quantify the conclusions?conclusions?

Many results were presented in terms of (CL) as a funct. of

2

or(better)

But by now we know that GRSV does not describe DIS data, so less bias way is

better to use all PDF’s and computeCL for asymmetries comparison with measurements

and the bestoptionjust becomeavailable…

NLO fitNLO fit by de Florian, Sassot, Stratmann and by de Florian, Sassot, Stratmann and Vogelsang (hep-ph/0804.0422) Vogelsang (hep-ph/0804.0422)

in which pp collision jet data are includedin which pp collision jet data are included for the first for the first timetime. .

(Technically(Technically challenging!)challenging!)

Input data:

NLO fitNLO fit DDSSV SSV what it says about gluon polarization?what it says about gluon polarization?

Gluons are treated in a special way:-single truncated moment is dominated by x around xmin

-Low x is very badly constrained by data split calculations in 3 egions:

0.001 – 0.05 – small x 0.05 – 0.2 - „RHIC” region 0.2 – 1.0 - large x

in the region coveredby RHIC data gluon polarization

is small, crosses zero? for Q0

2 at 1 GeV2

DSSV PDFDSSV PDF – – gluongluonwhat constrains it?what constrains it?

Future Future prospects:prospects:– Precision for jets and Precision for jets and

pionspions– Dijets asymmetriesDijets asymmetries– Direct photonsDirect photons– Inclusion of open Inclusion of open

charm asymmetriescharm asymmetries– W production asymm. W production asymm.

(RHIC@500GeV)(RHIC@500GeV)– QQ22 range with EIC range with EIC

Present precision and effect fromInclusion of specific data sets

G is close to zero, but value ofof about +/-0.2 not excluded

1

0

( )G g x dx

Plans,scenarios?Plans,scenarios? how to use data to get how to use data to get g/g?g/g? Global analysis vs. extraction from single measurementGlobal analysis vs. extraction from single measurement even with much simplified assumptionseven with much simplified assumptions

– each of them is neededeach of them is needed– this is a cross check of our understanding – not competitionthis is a cross check of our understanding – not competition

Global analysis should have as much input as possibleGlobal analysis should have as much input as possible– uniform treatmentuniform treatment– most up-to date theoretical achievementsmost up-to date theoretical achievements

Experimentalists should be encouraged to go as far as Experimentalists should be encouraged to go as far as they can with interpretation of the datathey can with interpretation of the data– best understanding of corrections, systematicsbest understanding of corrections, systematics– pushes toward improvements of experimental techniques and pushes toward improvements of experimental techniques and of data analysisof data analysis– consistency checks allowing better control of systematic effectsconsistency checks allowing better control of systematic effects– comparison of several results gives measure of precisioncomparison of several results gives measure of precision– more channels can be used (without full theoretical treatment)more channels can be used (without full theoretical treatment)

We can see such We can see such complementarity in complementarity in SS determination determination important for important for a check of systematic effects a check of systematic effects

Lesson from this example for analysis of gluon polarization:

For experimentalists-Measure more and try interpretation (even if simplified)

For theorists-Include as much as possible to the combine fits-Introduce alternative approach to the existing one

Summary and conclusions Summary and conclusions – decomposition of nucleon – decomposition of nucleon spin?spin?

Quarks give about 1/3 of what is needed

Results on G point to a rather small gluon contribution

… but still two scenarios remain possible:

1. Gluons give about 0.2-0.3 (enough to make nucleon spin)

2. Gluons are unpolarized additional contribution from orbital momenta (likely of gluons if quarks do not contribute, as lattice results suggest)

With small G, as observed, anomalous contribution to axial charge a0 is small and cannot explain „spin crisis”

Thank you!Thank you!

Many thanks to all people who Many thanks to all people who contributed to the selection of contributed to the selection of results results

I was using results from paralell I was using results from paralell sessions presentad by sessions presentad by – Hermes and Compass collaboratios on Hermes and Compass collaboratios on

DISDIS– PHENIX and STAR on ppPHENIX and STAR on pp– LSS, AAC and DSSV grops on QCD fitsLSS, AAC and DSSV grops on QCD fits