harut avakian (jlab)

CLAS12 workshop, Feb 261

Harut Avakian (JLab)Harut Avakian (JLab)

CLAS12 European Workshop February 25-28, 2009- Genova, Italy

Study of Polarized Quark Fragmentation with CLAS and CLAS12


Outline

IntroductionFragmentation of transversely polarized quarks and

Collins effect Unpolarized targetLongitudinally polarized targetTransversely polarized target

Projections for 12 GeVSummary

Transverse structure of the nucleon and correlations between transverse spin and momentum.


z

sin2moment of the cross section for unpolarized beam and longitudinal target

U unpolarizedL long.polarizedT trans.polarized

SIDIS kinematical plane and observables

Transverse spin effects are observable as correlations of transverse spin and transverse momentum of quarks.


Single hadron production in hard scattering

h

Target fragmentation Current fragmentation

Fracture Functions

xF

M

0-1 1

h

h

PDF GPD

kT-dependent PDFs Generalized PDFs

Wide kinematic coverage of large acceptance detectors allows studies of hadronization both in the target and current fragmentation regions

xF - momentum

in the CM frame

xF>0 (current fragmentation)

xF<0 (target fragmentation)

h


Transverse Momentum Dependent (TMD) Distributions

Quark polarization

Nucleon polarization

Real and imaginary parts of the L≠0 interference contributions

•kT – leads to 3D description with 8PDFs

Factorization of kT-dependent PDFs proven at low PT of hadrons (Ji et al)

Twist-2

Twist-3


Collins Effect: azimuthal modulation of the fragmentation function

D(z,PT)=D1(z,PT)+H1┴(z,PT) sin(hS’)

spin of quark flips wrt y-axisS’ = -S sin(hS)

C

S

STy

x

h

PT sT

S’

C

FUT∞h1H1┴ sT(q×PT)↔ H1┴

x

PT

hS=h

y

y

x

S h

PTS = +h

x

PT

h

S=y

hadronizing quark

Intial quark polarization


Collins SSA measurements

+(u,d) K+ (u,s)

• K+ and + asymmetries consistent within error bars •K- and - asymmetries may have opposite sign

e+e-→hhX

Observed SSA show strong dependence on the final state hadron

BRAHMS

p↑p→hX


Collins effect

Simple string fragmentation (Artru model)

Leading pion out of page ( - direction )

Collins analyzing power may be indeed much bigger for unfavored fragmentation

L

z

kicked in the opposite to the leading pion(into

the page)

Sub-leading pion opposite to leading (double kick into the

page)

L


Collins effect

Simple string fragmentation for pions (Artru model)

leading pion out of page

production may produce an opposite

sign AUT

Leading opposite to leading (into page)

hep-ph/9606390 Fraction of in eX

% left from eX asm

20%

40%

~75%

~50%

Fraction of direct kaons may be significantly higher than the fraction of direct pions.

LUND-MC

L

z

L

z


The Collins function

First calculation of the Collins functionBacchetta et al, Phys.Lett.B659:234-243,2008

The Kaon Collins effect may be significant!

Kaon Pion

HERMES/COMPASS/Belle

spectator model


Pros1. Small field (∫Bdl~0.005-0.05Tm)2. Small dilution (fraction of events from polarized material)3. Less radiation length4. Less nuclear background (no nuclear attenuation)5. Wider acceptance

much better FOM, especially for deuteron Cons

1. HD target is highly complex and there is a need for redundancy due to the very long polarizing times (months).

2. Need to demonstrate that the target can remain polarized for long periods with an electron beam with currents of order of 1-2 nA

3. Additional shielding of Moller electrons necessary (use minitorus)

CLAS transversely polarized HD-Ice target

Heat extraction is accomplished with thin aluminum wires running through the target (can operate at T~500-750mK)

HD-Ice target at ~2nA ~ NH3 at 5 nA

HD-Ice target vs std nuclear targets


Collins SSA at CLAS @5.7GeV

CLAS with a transversely polarized target will allow simultaneous measurement of SIDIS asymmetries in current and target fragmentation regions and exclusive and asymmetries

UT ~Collins

sin(C)=sin(hS)

S’ = -S

25 days with HD-Ice


CLAS12LTCC

FTOF

PCAL

ECHTCC

Lumi = 1035cm-2s-1

High beam polarization 80%High target polarization 85%NH3 (30 days) ND3 (50 days)

Wide detector and physics acceptance (current/target fragmentation)

Replace 2 sectors of LTCC with a proximity RICH detector to identify Kaons


CLAS12: Kinematical coverage

Large Q2 accessible with CLAS12 are important for separation of HT contributions

Q2>1GeV2

W2>4 GeV2(10)y<0.85MX>2GeV

SIDIS kinematics

eX


Non-perturbative TMD Perturbative region

Unpolarized target: Boer-Mulders Effect

•BM cos2moment: the only leading twist azimuthal moment for unpolarized target•PT-dependence of BM asymmetry allows studies of transition from non-perturbative to perturbative description (Unified theory by Ji et al).

CLAS12

2000h @ 11 GeV with 1035sec-1cm-2)

-


Collins fragmentation: Longitudinally polarized target

UL ~KM

curves, QSM from Efremov et al

Kotzinian-Mulders Asymmetry

•KM sin2moment, sensitive to spin-orbit correlations: the only leading twist azimuthal moment for longitudinally polarized target•More info will be available from SIDIS (COMPASS,EIC) and DY (RHIC,GSI)

Transversely polarized quarks in the long. polarized nucleon


Collins Effect

UT ~Collins

Study the Collins fragmentation for all 3 pions with a transversely polarized target and measure the transversity distribution function.


CLAS12: Transversity projections

AUT ~Collins

Simultaneous measurement of, exclusive with a transversely polarized target

Collins function required to extract transversity from transverse target SSA measurements


Pretzelosity @ CLAS12:

•CLAS12 will provide first pretzelosity measurement in the valence region for Kaons and pions.

B. Pasquini et al. arXiv:0806.2298

Exciting relation:(in bag & spectator model)

g1q (x) h1

q (x)h1Tq (x)

helicity - transversity = ‘measure’ of relativistic effects

http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+%22Pasquini%2C%20B%2E%22

http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+%22Pasquini%2C%20B%2E%22


Collins Effect: from asymmetries to distributions

Combined analysis of Collins fragmentation asymmetries from proton and deuteron may provide independent to e+e- (BELLE)Information on the underlying Collins function.

need


Sivers effect in the target fragmentation

A.Kotzinian

High statistics of CLAS12 will allow studies of kinematic dependences of the Sivers effect in target fragmentation region


polarization in the target fragmentation

xF - momentum

in the CM frame

Wide kinematical coverage of CLAS12 allows studies of hadronization in the target fragmentation region

polarization in TFR provides information on contribution of

strange sea to proton spin

Study polarized diquark fracture functions sensitive to the correlations between struck quark transverse momentun and the diquark spin (Sivers)


Summary

Probe the Collins polarized fragmentation function of pions and kaons for unpolarized, longitudinally polarized and transversely polarized targets

Provide precision measurements of the leading twist chiral-odd transverse spin distributions of valence quarks.

Study higher twist effects and test factorization in a wide range of Q2

Measure polarization effects in the target fragmentation region.

Large kinematical acceptance of CLAS12@ 11 GeV with L=1035cm-2sec-1 would allow us to :

Experiments confirm that Collins fragmentation function is large both for favored and unfavored hadron production.


Support slides….


Collins fragmentation: Longitudinally polarized target

•Study the Collins function of kaons•Provides independent information on the RSMT TMD

Kotzinian-Mulders Asymmetry

proton deuteron

Pasquini et al.


Collins Effect: azimuthal modulation of the fragmentation function

D(z,PT)=D1(z,PT)+H1┴(z,PT) sin(hS’)

spin of quark flips wrt y-axisS’ = -S sin(hS)

C

S

STy

x

h

PT sT

S’

C

FUT∞h1H1┴

S

y

x

h

PT

sT(p×kT)↔ h1┴

FUU∞h1 ┴ H1┴

S = +h

sT(q×PT)↔ H1┴

x

sin(2h)

PT

h

S=h

y

FUL∞h1L H1┴┴

(sTkT)(pSL)↔ h1L┴sin(h+S)

cos(2h)


Low energy electromagnetic processes, especially Møller scattering of beam electrons off atomic electrons are the main contributor to the background load in an open large acceptance spectrometer such as CLAS12.

The full event and background load has been measured with CLAS, e.g. for DVCS process at 5.7 GeV. The GEANT simulation reproduces hit occupancy on tracking chambers.

We used the calibrated simulation code to extrapolate to 11 GeV and simulate the same process at higher luminosity for CLAS12 situation.

This background was also studied in a full Geant4 simulation.

CLAS12 - Electromagnetic Background


Q2>1GeV2

W2>4 GeV2(10)y<0.85MX>2GeV

SIDIS kinematics

Kaon distributions in ep e’KX

High energy kaons are at small angles (<30o)

LUND-MC

More kaons at small x

forward


5 T Magnetic Field and Shielding

Photons

One Event

Electrons

Photons

One Event

Shielding

Background at L=1035cm-2s-1, T = 150ns

CLAS12


Rpd-

Both ratios agree with PDF models for z<0.7 (Mx>1.4 GeV)


Unfavored to favored ratio consistent with HERMES and EMC for z=0.55

D-/D+ from Deuteron + to - ratio


multiplicities in SIDIS ep→e’X

+/- multiplicities at large z diverge from SIDIS predictions0 multiplicities less affected by higher twists0.4<z<0.7 kinematical range, where higher twists are expected to be small

DSS (Q2=2.5GeV2)

DSS (Q2=25GeV2)

M.AghasyanHall-C



Combined analysis of Collins fragmentation asymmetries from proton and deuteron may provide independent to e+e- (BELLE)Information on the underlying Collins function.

need


Higher Twist SSAs and Quark-Gluon Correlations

Target sin SSA (Bacchetta et al. 0405154)

Discussed as main sources of SSA due to the Collins fragmentation

In jet SIDIS only contributions ~ D1 (Sivers type)

With H1┴ (0)≈0 (or measured) Target and Beam SSA can be a valuable source of info on HT T-odd distribution functions

Transversely polarized quarks


SSA with unpolarized target

quark polarization


SSA with long. polarized target

quark polarization


CLAS: Fraction from baryonic decays in SIDIS

Significant fraction from target fragmentation at pion momenta below 2 GeV


Dilution factor in SIDIS

Multiple scattering and attenuation in nuclear environment introduces

additional PT-dependence for hadrons

Fraction of events from polarized hydrogen in NH3

Nu,Np -total counts from NH3 and carbon normalized by lumi

u, p -total areal thickness of hydrogen (in NH3), and carbon target

Cn=Nitr/Carbon ratio (~0.98)Diff. symbols for diff x-bins

-


CLAS12: Acceptance deformation due to incomplete azimuthal coverage

No significant effect seen from limited coverage by RICH

Lab


Inbendin/outbending configurations

Different polarities increase the acceptance of positive and negative hadrons.


Critical for separation moment range 2<PK<5 and <25 degree

Kinematic dependence of K/ ratios


Q2>1GeV2

W2>4 GeV2(10)y<0.85MX>2GeV

SIDIS kinematics

Kaon distributions in ep e’KX

High energy kaons are at small angles


A Rich detector for CLAS12MC simulation: 3 cm thick C5F12 radiator 80 cm CH4 proximity gap 1 cm pixel pad size 5 o-30 o radiator polar angle

4 K- separation at 5 GeV/c 80 % kaon eff. with 1:1000 rejection 95 % kaon eff. with 1:100 rejection

Contalbrigo Marco PAC34 27 January 2009

Already with 2 sectors gain of factor~3 in the relevant z region of interest


Transverse Momentum Dependent (TMD) Distributions

Quark polarization

Nucleon polarization

Real and imaginary parts of the L=0 and L=1 interference contribution

Related to transversity by Lorentz Invariant relations.

In constituent quark model (Pasquini et al).

•kT – leads to 3D description with 8PDFs


Compare SIDIS experiments

COMPASS/HERMES/CLAS

cover different Q2 for the

same x-range

x=0.3 → Q2=~2 GeV2 (CLAS),

~7 GeV2 (HERMES)

~30 GeV2 (COMPASS)


HERMES: Diffractive corrections to DIS



Combined analysis of Collins fragmentation asymmetries from SIDIS and e+e- (BELLE) would allow separation of transverse spin distributions (Anselmino et al., arXiv:0707.1197 )

need

Brodsky & Yuan (2006)

CLAS12


JLab@12GeV: Inclusive DIS

BBS/LSS no OAM

PDF measurements at large x provide additional information on OAM

BBS/LSS with OAM


The Large-Nc Behavior of the PDFs

Use the large Nc limit of QCD to study TMD PDFs

qg interaction constant

In color singlet Feynman diagrams every vertex

loop

Introduced by ‘t Hooft in1974

isospin

Nc-

power

P.Pobylitsa hep-ph/0301236


The Large-Nc limit of QCD to study TMD PDFs

3 – isospin Pauli matrice

t1,t2 – isospin projection of

quark fields

(3)uu =1, (3)dd=-1

Change sign from + (SIDIS) to – (DY)

Nucleon mass M=O(Nc)

Large-Nc approach predicts signs and relative Nc power of TMDs, used in phenomenology.

3 – nucleon isospin

P.Pobylitsa hep-ph/0301236

Do not change sign (isoscalar)

All others change sign u→d (isovector)

Introduced by ‘t Hooft in1974

qg interaction constantIn color singlet Feynman diagrams every vertex

loop


GSIM12

Events for exclusive + production on proton (ep→e’+n)

Typical event


SIDIS (*p→X) cross section at leading twist (Ji et al.)

structure functions = pdf × fragm × hard × soft (all universal)

eUnpolarized target

Longitudinally pol. target

Transversely pol. target e

e

p

pBoer-Mulders

1998

Kotzinian-Mulders1996

Collins-1993

To observe the transverse polarization of quarks in SIDIS spin dependent fragmentation is required!

Do we understand well the helicity distributions?


Azimuthal Asymmetries in SIDIS

Due to color coherence the configuration with gluon inside the quark cone is more probable

Why <cos> < 0 ? Chay,Ellis,Stirling-1991

x

=180

=0


HT and Semi-Exclusive Pion Production E. Berger, S. Brodsky 1979 (DY), E.Berger 1980,A.Brandenburg, V. Khoze, D. Muller 1995

A.Afanasev, C.Carlson, C. Wahlquist Phys.Lett.B398:393-399,1997

+

Fragmentation +

0

• Azimuthal asymmetries with opposite sign from HT effects

• Effect may be suppressed for semi-exclusive 0 compared to +/-


Flavor Decomposition

Use double spin asymmetries for different targets and final state particles to extract the helicity distributions for different flavors

Sum over quark flavors

Extraction of kT-dependent distributions q+ (f1+g1) and q- (f1-g1) will require

unfolding of spin independent and spin dependent contributions

BBS/LSS no OAM

BBS/LSS with OAM

H.A.,S.Brodsky,A.Deur,F.Yuan (2007)


HT and Semi-Exclusive Pion Production E. Berger, S. Brodsky 1979 (DY), E.Berger 1980,A.Brandenburg, V. Khoze, D. Muller 1995

A.Afanasev, C.Carlson, C. Wahlquist Phys.Lett.B398:393-399,1997

+

Fragmentation +

0

• Azimuthal asymmetries with opposite sign from HT effects

• Effect may be suppressed for semi-exclusive 0 compared to +/-


3D structure of the nucleon

Wide kinematic coverage of large acceptance detectors allows studies of exclusive (GPDs) and semi-inclusive (TMDs) processes providing complementary information on transverse structure of nucleon

h

TMDs

Semi-Inclusive processes and transverse momentum distributions

,h

Hard exclusive processes and spatial distributions of partons

GPDs


Flavor DecompositionSum over quark flavors

Extraction of kT-dependent distributions q+ (f1+g1) and q- (f1-g1) will require

unfolding of spin independent and spin dependent contributions

BBS/LSS

BBS/LSS +OAM H.A.,S.Brodsky,A.Deur,F.Yuan (2007)

Large contribution from orbital motion


Non-perturbative TMD Perturbative region

Boer-Mulders Asymmetry: PT-dependence

In the perturbative limit 1/PT

2 behavior

expected (F.Yuan)

Missing: predictions for K-, dedicated predictions for K+

CLAS12

2<Q2<5 (2000h @ 11 GeV with 1035sec-1cm-2)


SSA in ep→e’X

contribution to SSA (~20%) may be responsible for the difference in andbeam SSA at large z

Larger fraction of from at low x and large z

PYTHIA at 5.7 GeV


Target SSA in exclusive production

Large positive 0 target SSA in the exclusive limit confirmed by CLAS at 5.7 GeV

HERMES 27.5 GeV

CLAS @5.7GeV

ep->e’p


Space and size of RICH sector (LTCC option)

harut avakian (jlab)

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