gluons’ polarization in the nucleon
DESCRIPTION
Gluons’ polarization in the nucleon. Summary of results and ideas where we are and where we go Ewa Rondio, A. Soltan Institute for Nuclear Studies Warsaw, Poland. plan. Introduction Early information from QCD fits Need for „direct measurements” Present results from DIS - PowerPoint PPT PresentationTRANSCRIPT
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