1 measurement of single target-spin asymmetry in semi-inclusive n (e,eπˉ) reaction on a...

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Measurement of Single Target-Spin Asymmetry in Semi-Inclusive n↑(e,e

′πˉ) Reaction on a Transversely Polarized 3He Target

PR-06-010 (E-03-004)

PAC-29, January 2006

Jen-Chieh Peng

University of Illinois at Urbana-Champaign

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Hall A Collaboration Experiment

The Institutions

California State Univ., Duke Univ., Florida International. Univ., Univ. Illinois, JLab, Univ. Kentucky, Univ. Maryland, Univ. Massachusetts, MIT, Old Dominion Univ., Rutgers Univ., Temple Univ., Penn State Univ., Univ. Virginia, College of William & Mary, Univ. Sciences & Tech, China Inst. Of Atomic Energy, Beijing Univ., Seoul National Univ., Univ. Glasgow,

INFN Roma and Univ. Bari, Univ. of Ljubljana, St. Mary’s Univ., Tel Aviv Univ.

A. Afanasev, K. Allada, J. Annand, T. Averett, F. Benmokhtar, W. Bertozzi, F. Butaru, G. Cates, C. Chang, J.-P. Chen (Co-SP), W. Chen, S. Choi, C. Chudakov, E. Cisbani, E.

Cusanno, R. De Leo, A. Deur, C. Dutta, D. Dutta, R. Feuerbach, S. Frullani, L. Gamberg, H. Gao, F. Garibaldi, S. Gilad, R. Gilman, C. Glashausser, J. Gomez, M. Grosse-Perdekamp, D. Higinbotham, T. Holmstrom, D. Howell, M. Iodice, D. Ireland, J. Jansen, C. de Jager,

X. Jiang (Co-SP), Y. Jiang, M. Jones, R. Kaiser, A. Kalyan, A. Kelleher, J. Kellie, J. Kelly, A. Kolarkar, W. Korsch, K. Kramer, E. Kuchina, G. Kumbartzki, L. Lagamba, J. LeRose, R.

Lindgren, K. Livingston, N. Liyanage, H. Lu, B. Ma, M. Magliozzi, N. Makins, P. Markowitz, Y. Mao, S. Marrone, W. Melnitchouk, Z.-E. Meziani, R. Michaels, P. Monaghan, S. Nanda, E. Nappi, A. Nathan, V. Nelyubin, B. Norum, K. Paschke, J. C. Peng (Co-SP), E. Piasetzky, M. Potokar, D. Protopopescu, X. Qian, Y. Qiang, B. Reitz, R. Ransome, G. Rosner, A. Saha, A. Sarty, B. Sawatzky, E. Schulte, S. Sirca, K. Slifer, P. Solvignon, V. Sulkosky, P. Ulmer, G.

Urciuoli, K. Wang, D. Watts, L. Weinstein, B. Wojtsekhowski, H. Yao, H. Ye, Q. Ye, Y. Ye, J. Yuan, X. Zhan, X. Zheng, S. Zhou, X. Zong,

Collaboration members (103 members)

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• Remaining frontier of kT – independent structure functions

• Connections to many other kT – dependent distribution and fragmentation functions

• Major experimental efforts to measure transversity using lepton and hadron beams

Physics Motivation: Transversity

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Transversity• Some characteristics of transversity:

– δq(x) = Δq(x) for non-relativistic quarks– δq and gluons do not mix → Q2-evolution for δq and Δq are

different– Chiral-odd → not accessible in inclusive DIS

Chiral-quark soliton model Quark – diquark model (solid) and pQCD-based model (dashed)

Similar to helicity distributionsB. –Q. Ma, I. Schmidt and J. –J. Yang,

PRD 65, 034010 (2002)hep-ph/0101300

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How to measure transversity?

• Transversely Polarized Drell-Yan

• Semi-Inclusive DIS– Single-hadron (Collins fragmentation function,

H1┴(z))

– Two hadrons (Interference fragmentation function)

– Vector meson polarization– Λ - polarization

• Chiral-odd → not accessible in DIS

• Require another chiral-odd object

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Leading-Twist Quark Distributions

Three have no kT

dependence

The other five are transverse momentum (kT)

dependent (TMD)

( A total of eight distributions)

Semi-inclusive DIS can access all leading-twist quark distributions

Transversity

Sivers function

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Observation of Single-Spin Azimuthal Asymmetry

Longitudinally polarized target

ep → e’πx HERMES

<ST> ~ 0.15

Collins effect: Correlation between the quark’s transverse spin with pion’s pT in the fragmentation process δq(x) • H1

┴(z).

Sivers effect: Correlation between the transverse spin of the proton with the quark’s transverse momentum f1T

┴(x) • D(z).

Other higher twist effects could also contribute.

Origins of the azimuthal asymmetry ?

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1

, , 1 1

1

Product of ( ) ( ) is non-zero

A surprising flavor dependence : / 1

Extraction of ( ) requires an independent measurement of

Collins function ( )

unfavored favored

q x H z

H H

q x

H z

AUTsin() from transv. pol. H target

Simultaneous fit to sin( + s) and sin( - s)

„Collins“ moments

hep-ex/0507013

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Collins functions from Belle

• Significant non-zero asymmetries

• Rising behaviour vs. z

• First direct measurement of the Collins functionz1

z2

hep-ex/0507063

(cos 2 correlation between pions)e e x

1 1 1 1 1 2 1 2Asymmetry ( ( ) / ( )) ( ( ) / ( ))H z D z H z D z

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Extraction of Collins functions from the Collins asymmetry measurements

Fits to the Hermes data “Prediction” of the Compass data

, ,1 1 1 1Assuming ( ) (1 ) ( ); ( ) (1 ) (

0.29 0.04, 0.33 0.

)

04

fav fav u

fav unfa

nfav favfav unfav

v

H z C z z D z H z C z z D

C C

z

( Vogelsang and Yuan, hep-ph/0507266 )

( , )p e e ( , )d h

, ,1 1/ 1unfavored favoredH H

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Sivers moments from transversity experiments

“Sivers“ moments

AUTsin(-s) from Hermes transv. pol. H target

First measurement of Sivers asymmetry

Sivers function nonzero orbital angular momentum of quarks

hep-ex/0507013

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Extraction of Sivers functions from the Sivers moment measurements

Fits to the Hermes data “Prediction” of the Compass data

, ,1 1Assuming ( ) (1 ) ( ); ( ) (1 ) ( )

0.81 0.07, 1.86 0.28

u dT u T d

u dS

f x S x x u x f x S x x u x

S

( Vogelsang and Yuan, hep-ph/0507266 )

Striking flavor dependence of the Sivers function

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Opportunities at JLab for transversity experiments

• High-intensity CW electron beam• High-density polarized 3He target which could be

polarized transversely• Probe valence-quark region similar to HERMES

kinematics, providing complimentary information on transversely polarized neutron

• An independent test of the striking flavor structures of Collins and Sivers functions observed at HERMES/COMPASS

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3He↑(e,e’π‾)x at Hall-A

• Beam– 6 GeV, 15 μA e- beam

• Target– Optically pumped Rb-K spin-exchange 3He target, 50 mg/cm2, ~42%

polarization, transversely polarized with tunable direction• Electron detection

– BigBite spectrometer, Solid angle = 60 msr, θLab = 300

• Charged pion detection– HRS spectrometer, θLab = -160

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Kinematic coverage of the electron arm

• BigBite spectrometer set at θ=30° at beam-right detecting electrons with 0.5 < E’ < 2.2 GeV.

• The coverage in Bjorken-x is 0.135 < x < 0.405, corresponding to valence-quark region.

• For the four x bins, the range of mean-Q2 is 1.3 < <Q2> < 3.1 (GeV/c)2.

• The coverage in W, the invariant mass of the hadronic system, is 2.33 < W < 3.05 GeV, well above the resonances region.

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Kinematic coverage of the hadron arm

• HRSL situated at θ = -16° will measure charged hadrons with mean momentum p = 2.4 GeV/c.

• The fraction of the virtual photon energy carried by the hadron, z = Eh/ν, is z ≈ 0.5 to detect leading pion in the current fragmentation region.

• A cut of W ’ > 1.5 GeV is required to stay away from the delta resonance production region.

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Hall-A polarized 3He target

• 40-cm long Rb-K spin-exchange hybrid cell at 10 atm with beam current of 15 μA

• 42% target polarization with spin-flip frequency of 20 minutes

• A third set of Helmholtz coils will be added, together with the laser optics, to allow for vertical polarization of the 3He target

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Coverage of the Collins angle

90 (re0 (black 270 (pur), pl180 (blue )),d), e

l lColl

lS

l

ins h S

l lS SS

<x>=0.135

<x>=0.405

<x>=0.225

<x>=0.315

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Coverage of the Sivers angle

90 (red0 (black), 180 (blue), 270 (purpl )), e

l lSiver

llS

lS

s h S

lS S

<x>=0.135

<x>=0.405

<x>=0.225

<x>=0.315

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Beam time request and count rate estimate

Time (Hour)

Production on 3He 528

Reference cell runs 16

Target spin rotation and polarization measurement

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Total 576 (24 days)

<x> Rate (Hz) Nπ‾(103) δ AUT (n) %

0.135 0.17 317.5 1.47

0.225 0.12 231.1 1.82

0.315 0.07 139.7 2.46

0.405 0.05 95.8 3.03

Beam time request Count rate estimate

• Quark distribution function from CTEQ5M

• Pion fragmentation functions from KKP parameterization

• Gaussian pion PT distribution with <PT2> = 0.26 (GeV/c)2

• Effective neutron polarization of 86.5% in 3He and a dilution factor, f ≈ 0.3, are used to relate measured 3He asymmetry to deduced neutron asymmetry

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Projected sensitivities of Collins and Sivers asymmetries

2 (1)1 1

2 (1)1 1

2 21 1

21 1

( ) ( )(1 )

(1 / 2) (

( ) ( )

(

(

)

)

( )

)

q qqqCollin

q qq TqSivers

UT q qq

sUT

q

q

q q

q

e h x H zyA

y y e f x D

e f x D zA

e f x z

z

D

22

Predictions of Collins asymmetry on neutron

1 14n unfav favUTA d H u H

1 1 1 1/ /unfav fav unfav favH H D D

1 1/ 1unfav favH H

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Predictions of Sivers asymmetry on neutron(1) (1)

1 1 1 14n d unfav u favUT T TA f D f D

(11 1

) (1)1 1/ 3

4

31/ n d uu

Unfav fav

T TA fD fD

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Progress since the approval of E-03-004• Improvement on the polarized 3He target (K-Rb hybrid,

new laser optical fiber system, etc.)• Commissioning of the BigBite spectrometer for the SRC

and GnE experiments. Background rate test run in April

2005• Operation of the Lumi detectors as luminosity monitor• Optimization of the experimental configuration and

detailed simulation of the background• First SSA SIDIS data on transversely polarized targets

from HERMES and COMPASS• Many theoretical progress including the proof of

factorization in SIDIS• First SIDIS data from Hall-C and CLAS

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Is SIDIS applicable at 6 GeV?

-

Preliminary results from Hall-C E00

( , ) and ( ,

-1

) at 0.3

08

p e e p e e x

Data are well described by SIDIS calculations for 0.4 < z < 0.7

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Summary• The physics of transversity and kT-dependent

quark distribution and fragmentation functions is an exciting frontier in nucleon structure.

• High-luminosity JLab beam together with the transversely polarized 3He target and the spectrometers at Hall-A provide a unique opportunity to test the intriguing flavor dependence observed in recent SSA experiments.

• Various recent progress both at JLab and around the world has further strengthened the physics case and shown the urgency of the proposed measurements.

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Backup Slides

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Is SIDIS applicable at 6 GeV?Preliminary results from Hall-C E00

( , ) and ( ,

-

) at 0.3

108

p e e d e e x

Data are well described by SIDIS calculations for 0.4 < z < 0.7

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Disentangling Collins from Sivers asymmetries

simulation taking into account of the finite acceptance of the spectrometer

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Disentangling Collins from Sivers asymmetries

simulation taking into account of the finite acceptance of the spectrometer, and the 3Φh-Φs term

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Systematic errors

• Nuclear effects in 3He– Proton carries ~ 2.8 % of the polarization and can be well

corrected for, using the asymmetry data from HERMES

• Target polarization drift– Only contributes to the relative uncertainty of the measured AUT at

a level of 4 %

• Decays from exclusive ρ-meson production– Negligible at z=0.5, based on the simulation of Hall-C E00-108

• Other terms in SSA– Monte-Carlo simulations indicate very small effect

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Why not wait for 12 GeV?

• The measurements can already be done at 6 GeV, and the impact of this first measurement on our current knowledge on SSA should be huge.

• It will provide extremely valuable inputs for optimizing a future program of transversity (and semi-inlcive DIS, in general) at 12 GeV.

• JLab will continue to play an important role in the global effort to understand the spin structure of the nucleons.

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π- versus π+, which do we prefer?

• If both π- and π+ data are obtained, one can make an independent extraction of the Sivers functions based on Jlab data alone (and compare them with Hermes data).

• π- and π+ data will provide two independent tests of the current results on Sivers and Collins function obtained at Hermes and Compass.

• If only one charged pion data will be measured, then one can make a single test of the results on Sivers and Collins function. In this case, there is no difference which charged state one selects.

• Under severe beam-time constraints, a measurement for both pions with somewhat reduced statistics might be considered.

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All Eight Quark Distributions Are Probed in Semi-Inclusive DIS

4

26 4

Q

sxd

),()(])1(1{[ 211

,

22 h

qq

qqq PzDxfey

),()()sin()1(||

),()()2sin(4

)1(||

),()()2cos(4

)1(

2

,11

2

2

,1

)1(1

22

2

2

,1

)1(1

22

2

hqq

qqq

lS

lh

h

hT

hqq

qqLq

lh

hN

hL

hqq

qqq

lh

hN

h

PzHxhezM

PyS

PzHxheMMz

PyS

PzHxheMMz

Py

)},()()cos()2

11(||

),()()2

11(||

),()()3sin(6

)1(||

),()()sin()2

11(||

21

)1(1

,

2

21

,1

2

,

21

)2(1

223

3

21

)1(1

,

22

hqq

Tqq

qlS

lh

N

hTe

hq

qq

qqLe

qqh

qqTq

lS

lh

hN

hT

hqq

Tqq

qlS

lh

N

hT

PzDxgezM

PyyS

PzDxgeyyS

PzHxheMMz

PyS

PzDxfezM

PyyS

Unpolarized

Polarized target

Polarized beam and

target

SL and ST: Target Polarizations; λe: Beam Polarization

Sivers

Transversity