Nucleon spin structure results (from the HERMES experiment)

Nucleon spin structure resultsNucleon spin structure results(from the HERMES experiment)(from the HERMES experiment)

Michael DürenMichael Düren

Universität GießenUniversität Gießen

— XII Conference on Hadron Spectroscopy, Frascati, Oct. 10, 2007 —

EMC 1988:EMC 1988: Only 12±17% spin of Only 12±17% spin of the proton is explained by the spin of the proton is explained by the spin of the up- and down- quarksthe up- and down- quarks

momentumangular orbital

ΔLΔL

spingluonΔG

spinquark sea and valence

Δs)Δdu(

projectionspin proton

S gq21

21

z

Helicity sum Helicity sum rule:rule:

a0= (theory) (exp) (evol)

= 0.330 ± 0.011 ± 0.025 ± 0.028

MSRemember:Remember: 1/3 of the 1/3 of the proton spin comes the proton spin comes the from quark spinfrom quark spin

The Experiment:The Experiment:

* 1995* 1995 2007 2007

) plenty of beautiful data are waiting for being analyzed) plenty of beautiful data are waiting for being analyzed

**) proposed in ~1988 to solve the spin crisis) proposed in ~1988 to solve the spin crisis

M. Düren, Univ. Giessen 4

Novel techniques: longitudinally polarized high Novel techniques: longitudinally polarized high energy electron/positron beam at HERA energy electron/positron beam at HERA

for beam-spin and beam-charge asymmetries for beam-spin and beam-charge asymmetries

HERA electron beam27.5 GeV (e+/e-)

<Pb>~ 53±2.5 %


Novel techniques: The longitudinally and Novel techniques: The longitudinally and transverse polarized internal gas target for double transverse polarized internal gas target for double

spin asymmetriesspin asymmetries

Storage cell target:high polarization, no

dilution1H→ <|Pt|> ~ 85 %

2H→ <|Pt|> ~ 84

%

1H <|Pt|> ~ 74

%


• resolution: p/p~2%, <1 mrad• PID: leptons with ~98%, contam. <1%

hadrons, K,p 2<Eh<15

GeV

Novel techniques: Dual radiator RICH for Novel techniques: Dual radiator RICH for strangenessstrangeness

THE HERMES SPECTROMETER


1 Tesla superconducting solenoidPhoton Detector● 3 layers of tungsten/scintillator● PID for higher momenta● detects

Scintillating Fiber Detector● 2 barrels● 2x2 parallel and 2x2 stereo layers ● 10° stereo angle● Momentum reconstruction & PID

Novel techniques: Recoil detector for exclusive Novel techniques: Recoil detector for exclusive physicsphysics

Silicon Detector● Inside beam vacuum● 16 double-sided sensors● Momentum reconstruction & PID

Hot topics in spin physics:Hot topics in spin physics:

•Moments and QCD-fits of PDFs•Strange sea polarisation; SU(3)f

•Gluon spin•Transverse spin effects (do not appear in the helicity sum rule)•Orbital angular momentum of quarks; „3-D views“ of the proton; GPDs

Spinphysics

M. Düren, Univ. Giessen


Polarized structure function gPolarized structure function g11p,dp,d(x)(x)

[PRD 75 (2007)]HERMES data set most precise and HERMES data set most precise and

complete in valence/sea overlap regioncomplete in valence/sea overlap region

proton

deuteron


x

[Q2>1 GeV2 data only]

deuteron

a8 from hyperon beta decay

theory theoryωD=0.05±0.05

=MS

First moment First moment 11d d

deuteron

Deuteron data alone give !


x

[Q2>1 GeV2 data only]

deuteron

First moment First moment 11dd and and

a0= (theory) (exp) (evol)

= 0.330 ± 0.011 ± 0.025 ± 0.028

MS

Most precise result;Most precise result;Consistent with other Consistent with other

experimentsexperiments

a8 from hyperon beta decay

theory theoryωD=0.05±0.05

=MS

Deuteron data alone give !


q and q and G from inclusive dataG from inclusive data

•Valence quarks are well determined: uv >0, dv <0

])Q,(G)Qq(x,[e2

1

2)( g

2qq

22q

sLO1

NLO1 CxCgxg

• Gluons and sea quarks are weakly constrained by data SU(3)SU(3)ff flavor symmetry flavor symmetry

implicitly assumed!implicitly assumed!


Important remark on SU(3)Important remark on SU(3)ff and the polarization of strange quarks and the polarization of strange quarks

The violation of the Ellis-Jaffe sum rule means:The violation of the Ellis-Jaffe sum rule means:

– either the strange quark polarization either the strange quark polarization s is negatives is negative

– or SU(3)or SU(3)ff flavor symmetry is broken flavor symmetry is broken– or low-x region different than assumed in parameterizationsor low-x region different than assumed in parameterizations

Most analyses assume explicitly or implicitly SU(3)Most analyses assume explicitly or implicitly SU(3)ff flavor flavor symmetry (e.g. in parameterizations of the PDFs)symmetry (e.g. in parameterizations of the PDFs)

Only semi-inclusive data can measure directly Only semi-inclusive data can measure directly s s

Kaon asymmetries on deuterium

allow for the most directdetermination of s !s !

Final analysis isin progress.


Flavor separation Flavor separation from semi-inclusive datafrom semi-inclusive data

Results (in short):Results (in short):

u(x) u(x) is large and positiveis large and positive

d(x) d(x) is smaller and negativeis smaller and negative

s(x)s(x) is approx. zero is approx. zero

up

down

u

d

strange

[PRL92(2004), PRD71(2005)]

HERMES: Only direct 5-flavor separation of polarized PDFs


How does spin flavor separation How does spin flavor separation qqff(x) (x) work?work?

Principles: Principles:

Helicity conservation in polarized DIS: select specific quark spin Helicity conservation in polarized DIS: select specific quark spin orientationorientation

Hadron tagging: select specific quark flavorHadron tagging: select specific quark flavor

Matrix inversion brings you back from hadron asymmetries to quark Matrix inversion brings you back from hadron asymmetries to quark spin flavor distributions spin flavor distributions qqff (purity formalism) (purity formalism)

Keep beam spin constantand flip proton spin


+ + + ..q

g

q

g qg

However, other sub-processes make life hard:

How does a direct gluon spin extraction work?How does a direct gluon spin extraction work?

Principle: Principle:

Large pLarge pTT hadron pairs hadron pairs come from photon come from photon gluon fusion processesgluon fusion processes

They carry information They carry information of the gluon spinof the gluon spin


Measurement of gluon Measurement of gluon polarisationpolarisation

g/g=0.071 ± 0.034 ± 0.010+0.127-0.105

HERMES: first measurement of gluon polarisation [PRL 84 (2000)] TOPCITE 50+

Further analysis going on: •Measurement of high pT pairs •Separation of the subprocesses•Two different methods

•Significant reduction of g uncertainty •Still sign ambiguity at low x

For more details see talkFor more details see talkby Riccardo FABBRIby Riccardo FABBRI


There are two three leading-twist structure functions:There are two three leading-twist structure functions:Trans-versity

Transversity is chiral odd (i.e.does not contribute to inclusive DIS cross section!)

Quark density

Helicity distribution

Transversity

Transverse spin effectsTransverse spin effects

Interesting properties of transversity:Interesting properties of transversity: QCD-evolution independent of gluon QCD-evolution independent of gluon

distributiondistribution (to be (to be tested by experiment)tested by experiment)

11stst moment of moment of qq is is tensor chargetensor charge (pure valence (pure valence object); value predicted by lattice QCDobject); value predicted by lattice QCD


Transverse spin distribution of Transverse spin distribution of quarksquarks

The azimuthal angular distributions The azimuthal angular distributions of hadrons from a of hadrons from a transversely polarized target show two effects:transversely polarized target show two effects:

– Collins asymmetry in sin(Collins asymmetry in sin(++ss))

– Sivers asymmetry in sin(Sivers asymmetry in sin(--ss))

Target spin




– Collins asymmetry in sin(Collins asymmetry in sin(++ss)) Product of the chiral-odd Product of the chiral-odd transversity distribution transversity distribution hh11(x)(x) and the chiral-odd and the chiral-odd fragmentation function fragmentation function HH11

(z)(z) related to the transverse related to the transverse spin distribution of quarksspin distribution of quarks

– Sivers asymmetry in sin(Sivers asymmetry in sin(--ss))

Target spin

First results from Belle




– Collins asymmetry in sin(Collins asymmetry in sin(++ss)) Product of the chiral-odd Product of the chiral-odd transversity distribution transversity distribution hh11(x)(x) and the chiral-odd and the chiral-odd fragmentation function fragmentation function HH11

(z)(z) related to the transverse related to the transverse spin distribution of quarksspin distribution of quarks

– Sivers-Asymmetrie in sin(Sivers-Asymmetrie in sin(--ss)) Product of the T-odd Product of the T-odd distribution function distribution function ff1T1T

(x)(x) and the ordinary and the ordinary fragmentation function fragmentation function DD11(z)(z) related to the orbital related to the orbital angular momentum of quarksangular momentum of quarks

Target spin

First results from Belle


Sivers function is related to Sivers function is related to orbital angular momentum of quarksorbital angular momentum of quarks

Side viewSide view

proton

quarks

Proton with spin out of this plane

left

photon

Right-left symmetric

many goright

photon

some goleft

photon

Right-left SSA

right

photon

attractive finalstate interaction

(M. Burkardt)


)()()( 1T1Ssiv zDxfA )()()( 1T1Ssiv zDxfA )()(q)( 1Scoll zHxA )()(q)( 1Scoll zHxA

First measurement of naïve T-odd distr. fkt. in DIS 0L0A:π q

sinφUT

•Significant non zero asymmetries

•A+>0, A-<0

•Evidence of

fav1,disf1, HH

SSA for pions SSA for pions [PRL94(2005)][PRL94(2005)] Collins Sivers


Asiv(K+) > Asiv(+)

Sea quarks may provide important

contribution to Sivers function

)()()( 1T1Ssiv zDxfA )()()( 1T1Ssiv zDxfA )()(q)( 1Scoll zHxA )()(q)( 1Scoll zHxA

SSA for pions and kaonsSSA for pions and kaonsCollins Sivers

For more details see talkFor more details see talkby Luciano PAPPALARDOby Luciano PAPPALARDO


GPD

General Parton DistributionsGeneral Parton DistributionsQuantum phase-space „tomography“ of the Quantum phase-space „tomography“ of the

nucleonnucleon Wigner introduced the first phase-space distribution in Wigner introduced the first phase-space distribution in

quantum mechanics (1932)quantum mechanics (1932)

The Wigner function contains the

most complete (one-body) info about a quantum system.

A Wigner A Wigner operatoroperator can be defined that describes can be defined that describes quarksquarks in the in the nucleon; nucleon; The reduced Wigner The reduced Wigner distribution is related todistribution is related to GPDGPDss

GPDs contain a more complete info than

form factors or parton distributions


Quarks in quantum mechanical phase-Quarks in quantum mechanical phase-spacespace

Elastic form factorsElastic form factors charge distribution (space coordinates) charge distribution (space coordinates)

Parton distributionsParton distributions momentum distribution of quark momentum distribution of quark (momentum space) (momentum space)

Generalized parton distributions (GPDs) are reduced Wigner Generalized parton distributions (GPDs) are reduced Wigner functionsfunctions correlation in phase-space correlation in phase-space e.g. the orbital e.g. the orbital momentum of quarks: momentum of quarks:

Angular momentum of quarks can be extracted from GPDs: Angular momentum of quarks can be extracted from GPDs:

Ji sum rule:Ji sum rule:

GPDs provide a GPDs provide a unified theoretical frameworkunified theoretical framework for many for many experimental processesexperimental processes

1

121 )0,,()0,,( xExHxdxJ qqq

prL


Q2

t

Q2 large, t small

• high Q2 hard regime

• high luminosity ~1/Q4, 1/Q6

• high resolution exclusivity

Quantum number of final state selects different GPDs: Vector mesons ( H EH E

Pseudoscalar mesons (: H E H E

DVCS () depends on H, E, H, EH, E, H, E~~~~

~~~~

polarization provides new observables sensitive

to different (combinations of) GPDs

Exclusive processes: The handbag Exclusive processes: The handbag diagramdiagram


Deeply Virtual Compton Scattering Deeply Virtual Compton Scattering (DVCS)(DVCS)

sin sin

γpepe ''d

Beam Spin Asymmetry

[PRL87(2001)] +100 top cite

only at HERA

Beam Charge Asymmetry

γpepe ''/-

cos cos

[PRD 75 (2007)]


GPDs predictions:• Vanderhaeghen, Guichon & Guidal [PRD 60 (1999)] (LO and LO+ power corrections)

Model calculation:• Regge formalism, Laget [PRD 70 (2004)]

• LO predictions + power correction to space like pion form factor in agreement with magnitude of data• Regge formalism for long. and transv. part of the cross section provides good description of dependence of data

Hard exclusive meson productionHard exclusive meson productione p e p e’ n e’ n ++

Information on pion form factor at high Q2 and on polarised GPDs

[hep-ex/0405078,

arXiv 07070222 PLB submitted]


Quark total angular momentumQuark total angular momentum

first model dependent extraction of first model dependent extraction of JJuu & & JJdd possible possible

VGG-Code: GPD-model: LO/Regge/D-term=0VGG-Code: GPD-model: LO/Regge/D-term=0

AAUTUT most sensitive observable to access J most sensitive observable to access Jqq via via GPDsGPDs

HERMES data 2002-04:

•U: unpolarized beam

•T: transv. pol. Target

• ~50% of total stat.

]Im[F 12 EFΗ

GPD model by: [Goeke et al. (2001)] [Ellinghaus et al. (2005)]

hep-ex/0606061

0709.0450[nucl-ex]

arXiv:0705.4295[hep-lat]


The HERMES recoil detector (2006-07)The HERMES recoil detector (2006-07)

Installed Dec 05, commissioned Installed Dec 05, commissioned and successfully operatedand successfully operated

Dedicated to exclusive Dedicated to exclusive processes:processes:– Recoil proton detectionRecoil proton detection– High luminosity:High luminosity:2006: 7 M DIS events e-2006: 7 M DIS events e-

20 M DIS events e+20 M DIS events e+2007: 20 M DIS events e+2007: 20 M DIS events e+

PID (first data)

missing mass(from MC)

semi-incl.

exclusive

For more details see talkFor more details see talkby Tibor KERIby Tibor KERI


Goodbye

ConclusionsConclusions 1/31/3 of the proton spin comes from of the proton spin comes from quark spinquark spin Direct measurements of the Direct measurements of the strange quark seastrange quark sea could not could not

verify a negative strange polarization. This indicates a verify a negative strange polarization. This indicates a SU(3)SU(3)ff flavor symmetry breaking flavor symmetry breaking or large contributions from low x.or large contributions from low x.

Analysis of Analysis of gluon spingluon spin from inclusive DIS is difficult and from inclusive DIS is difficult and ongoingongoing

Transversity and Sivers functionsTransversity and Sivers functions are non-zero. Sivers are non-zero. Sivers asymmetry for kaon data indicates an asymmetry for kaon data indicates an orbital angular orbital angular momentum contribution from sea quarks.momentum contribution from sea quarks.

DVCS beam charge and beam spin asymmetriesDVCS beam charge and beam spin asymmetries have been measured. First data of orbital angular have been measured. First data of orbital angular momentum (OAM) fits to GPD functions indicate momentum (OAM) fits to GPD functions indicate non-zero OAM of up-quarksnon-zero OAM of up-quarks

HERMESHERMES has the potential of further discoveries has the potential of further discoveries in spin physics with the in spin physics with the new abundant recoil data!new abundant recoil data!

Documents

Nucleon spin structure results (from the HERMES experiment)