1. Semi-inclusive DIS at 12 GeV
H.Avakian (JLab)
For the semi-inclusive working group
JLAB PAC January 10, 2005
Semi-inclusive processes
Factorization at low energies
Large-x behavior of valence PDFs and flavor asymmetries of the sea
Transversity and transverse momentum dependent distributions
Target fragmentation and intrinsic strangeness
Summary
Note mes

2. Semi-Inclusive Deep Inelastic Scattering (SIDIS)
DIS: Major source of QCD tests and PDF studies
Probes only the sum of quarks and anti-quarks
Requires assumptions on sea
No access to transversity effect in LO
SIDIS: “Tagging” to distinguish different quark flavors.
Provide access to quark distributions with fragmentation acting as a weight factor:
Probes orbital motion of quarks.
Main focus of SIDIS studies:
orbital angular momentum of quarks
parton distributions at large x
H.Avakian, Jan 10, 2005

3. SIDIS kinematic plane and relevant variables
H.Avakian, Jan 10, 2005

4. SIDIS at leading twist Unpolarized target Longitudinally pol. target
Transversely pol. target e p
Mulders et al.
Boer
Mulders e
Sivers
transversity
Off-diagonal PDFs vanish if quarks only in s-state! In addition T-odd PDFs require FSI (Brodsky et al., Collins, Ji et al. 2002)
Factorization of kT-dependent PDFs proven at low PT of hadrons (Ji et al)
Universality of kT-dependent distribution and fragmentation functions proven (Collins,Mets…)

5. Analysis of SIDIS and DVMP are complementary
Probability to find a quark u in a nucleon P with a certain polarization in a position r and momentum k
PDFs fpu(x),…
FFs F1pu(t),F2pu(t)..
TMD PDFs: fpu(x,kT),..
d2kT
x=0,t=0 dx
Wpu(x,k,r) “Parent” Wigner distributions
d3r
(FT)
GPDs: Hpu(x,x,t), …
GPD
Measure momentum transfer to quark
kT distributions also important for exclusive studies
Measure momentum transfer to target
Exclusive meson data important in understanding of SIDIS measurements
Some PDFs same in exclusive and semi-inclusive analysis
Analysis of SIDIS and DVMP are complementary
H.Avakian, Jan 10, 2005

6. ep→e’pX: kinematic coverage at 11 GeV
Acceptance in Q2,Mx,PT gained with high luminosity and energy upgrade (at 6GeV Mx<2.5GeV, Q2<4.5GeV2, PT<1GeV)
test factorization in a wide kinematic range
study the transition between the nonperturbative and perturbative regimes of QCD
measure PDFs and study higher twists
H.Avakian, Jan 10, 2005

7. Collinear Fragmentationp
quark
Collinear Fragmentation
The only fragmentation function at leading twist for pions in eN→e’pX is D1(z)
Ee =5.7 GeV
Hall-C Ee =5.5 GeV
No significant variation observed in p+/p- ratio and in z distributions of p+ for different x ranges (0.4<z<0.7, MX>1.5)

8. A1p f-kinematic dependence for p+/-/0
CLAS 5.7 GeV
No significant z-dependence of A1 in the range 0.4<z<0.7 (p+p0)
x dependence of CLAS A1p (A2=0) consistent with HERMES data at x3 higher Q2 and with PEPSI (LUND)MC.

9. Polarized PDFs g1u=u,g1d=d….
Ee =11 GeV NH3+He3
g1u=u,g1d=d….
GeV
Asymmetry measurements with different hadrons (p+p-) and targets (p,n) allow flavor separation
H.Avakian, Jan 10, 2005

10. Flavor decomposition: polarized sea
Predictions:
Instantons (QSM):
GeV
First data from HERMES
 0
More data expected from RHIC W± production in future
H.Avakian, Jan 10, 2005

11. Longitudinally polarized Target SSA for p+
Measurement of kT dependent twist-2 distribution provides an independent test of the Collins fragmentation.
Real part of interfe-rence of wave functions with L=0 and L=1
In noncollinear single-hadron fragmentation additional FF H1(z,kT)
Efremov et al. p
Study the PT – dependence of AULsin2f
Study the possible effect of large unfavored Collins function. kT
quark

12. Transversely polarized target SSA
transversely polarized quarks in a transversely polarized nucleon.
AUT ~
Collins
Interpretation requires large unfavored spin dependent fragmentation.
No success so far in describing consistently Collins SSA for all 3 pions at least in cQSM!
More data needed, new observables GSI, CERN
H.Avakian, Jan 10, 2005

13. Transverse Target SSA @11 GeV
11GeV (NH3)
AUT ~
Collins p+ p0 p-
f1T┴, requires final state interactions + interference between different helicity states
Simultaneous (with pion SIDIS) measurement of, exclusive r,r+,w with a transversely polarized target important to control the background.
AUT ~
Sivers
H.Avakian, Jan 10, 2005

14. Transversity in double pion production
The angular distribution of two hadrons is sensitive to the spin of the quark h1 h2
quark RT
“Collinear” dihadron fragmentation described by two functions at leading twist: D1(z,cosqR,Mpp),H1R(z,cosqR,Mpp)
Collins et al, Ji, Jaffe et al,
Radici et al.
relative transverse momentum of the two hadrons replaces the PT in single-pion production (No transverse momentum of the pair center of mass involved )
Dihadron production provides an alternative, “background free” access to transversity
H.Avakian, Jan 10, 2005

15. SIDIS: target fragmentation
xF>0 (current fragmentation)
xF - momentum in the CM frame
xF<0 (target fragmentation, TFR)
Wide kinematic coverage at 11 GeV allows studies of hadronization in the target fragmentation region
H.Avakian, Jan 10, 2005

16. Λ in target fragmentation1 Λ p 2 π
L – unique tool for polarization study due to self-analyzing parity violating decay
(ud)-diquark is a spin and isospin singlet → s-quark carries whole spin of L
L polarization in TFR provides information on contribution of strange sea to proton spin 6
Polarized beam gives unique possibility to perform an “acceptance independent” measurement of L polarization in electroproduction.
W.Melnitchouk and A.W.Thomas ‘96
J.Ellis, D.Kharzeev, A. Kotzinian ‘96

17. JLab at 12 GeV High luminosity polarised CW beam
Wide physics acceptance
(exclusive, semi-inclusive current and target fragmentation)
Complementarity
of high luminosity and wide geometric acceptance halls
High luminosity polarised CW beam
H.Avakian, Jan 10, 2005

18. Essential for understanding the structure of the nucleon !
Summary
Semi-inclusive measurements at 12 GeV will
provide precise information on the polarized valence distributions in the nucleon
test the expected flavor asymmetry in the polarized sea distributions
determine the essentially unknown transversity and Sivers functions with high precision and in a variety of channels
sensitivity to orbital motion of quarks
improve the understanding of the fragmentation process and give access to the poorly understood target fragmentation region
Essential for understanding the structure of the nucleon !
H.Avakian, Jan 10, 2005

19. support slides…..
H.Avakian, Jan 10, 2005

20. PT-dependence of beam SSA
ssinfLU(UL) ~FLU(UL)~ 1/Q (Twist-3)
In the perturbative limit 1/PT behavior expected
Perturbative region
Nonperturbative TMD
Asymmetries from kT-odd and kT-even (g1) distribution functions are expected to have a very different behavior (flat A1p(PT) observed at 5.7 GeV).
H.Avakian, Jan 10, 2005

21. Measuring the Q2 dependence of SSA
ssinfLU(UL) ~FLU(UL)~ 1/Q (Twist-3)
For fixed x, 1/Q behavior expected
Wide kinematic coverage allows to check the higher twist nature of beam and longitudinal target SSAs
H.Avakian, Jan 10, 2005

22. Constant in perturbative limit
g1/f1: PT-dependence
Constant in perturbative limit
Asymmetries from kT-odd (f1┴, h1┴, gT..) and kT-even (g1) distribution functions are expected to have a very different behavior
H.Avakian, Jan 10, 2005

23. Transversity Sub-leading pion opposite to leading (into page)
Simple string fragmentation (Artru model)
L=1
Leading r opposite to leading p(into page)
r production may produce an opposite sign AUT r
GeV: 2 pions
p+p0(r+)
Understanding of 2 pion asymmetries will help to understand transversity mesurements
p+p-(r0)
H.Avakian, Jan 10, 2005

24. Λs in target fragmentation1 Λ p 2 π
P and p- kinematics 6