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Spin and Azimuthal Asymmetries at JLAB

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Single-Spin Asymmetries Workshop, BNL June 1-3, 2005 ... Projections for Lambda polarization for 5.75 GeV (25days) with neutron target ... – PowerPoint PPT presentation

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Title: Spin and Azimuthal Asymmetries at JLAB


1
Spin and Azimuthal Asymmetries at JLAB
H. Avakian )
Jefferson Lab
Single-Spin Asymmetries Workshop, BNL June 1-3,
2005
) in collaboration with P.Bosted, V. Burkert and
L. Elouadrhiri
2
Outline
  • Introduction
  • SSA in target fragmentation
  • Transverse momentum of quarks in SIDIS
  • azimuthal asymmetries
  • Double spin asymmetries
  • SSA in SIDIS
  • beam single-spin asymmetries
  • target single-spin asymmetries
  • SSA in hard exclusive processes
  • DVCS
  • DVMP
  • Summary

3
Polarized Semi-Inclusive DIS
Cross section defined by scale variables x,y,z
Hadron-Parton transition by distribution
function f1u(x) probability to find a u-quark
with a momentum fraction x
Parton-Hadron transition by fragmentation
function Dp (z) probability for a u-quark to
produce a p with a momentum fraction z
1u
4
Single pion production in hard scattering
xFgt0 (current fragmentation)
xF - momentum in the CM frame
Wide kinematic coverage of large acceptance
detectors allows studies of hadronization both in
the target and current fragmentation regions
5
Transverse momentum of quarks
  • kT crucial for orbital momentum and spin
    structure studies
  • lead to SSA in hard scattering processes
  • kT lead to 3 dimensional description
  • lead to introduction of kT dependent/u PDFs
  • kT - important for cross section description
    (important also for exclusive production)
  • PT distributions of hadrons in DIS
  • exclusive photon production (DVCS)
  • - hard exclusive vector meson x-section
  • - pp ? p0X (E704,RHIC) x-sections

To study orbital motion of quarks in
semi-inclusive DIS measurements in a wide range
of PT, f and fS are required.
6
SIDIS (gp-gtpX) x-section at leading twist
Unpolarized target Longitudinally pol.
Transversely pol.
Studies of PDFs require three experiments
The structure functions depend on Q2, xB, z, PhT
  • 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)

7
Spin-Azimuthal Asymmetries
Spin asymmetries azimuthal dependence
new class of DIS measurements Spin-Azimuthal
Asymmetries
Significant progress made recently in studies of
Single-Spin Azimuthal Asymmetries (SSA) in
electroproduction with longitudinally polarized
target (HERMES,CLAS), transversely polarized
target (SMC,HERMES,COMPASS), and polarized beam
(CLAS,HERMES).
  • SSA are sensitive to the orbital momentum of
    quarks.
  • provide a window to the physics of partonic final
    and initial state interactions
  • model calculations indicate that SSA are not
    affected significantly by a wide
  • range of corrections.
  • Good agreement in SSAs measured in a wide energy
    range in electroproduction
  • and pp scattering.

SSAs stable observable over a wide beam energy
and Q2 range
8
The CLAS Detector
  • High luminosity, polarized CW beam
  • Wide physics acceptance, including exclusive,
    semi-inclusive processes, current and target
    fragmentation
  • Wide geometric acceptance, allowing detection of
    multi-particle final states

200 physicists 37 institutions
Q2
9
? in target fragmentation
e
e
1
p
?
2
p
L unique tool for polarization study due to
self-analyzing parity violating decay
Accessing polarized PDFs with unpolarized target!
(ud)-diquark is a spin and isospin singlet
s-quark carries whole spin of L
Ls accessible in CLAS (even at large z) are
mainly in the TFR region and can provide
information on contribution of strange sea to
proton spin
6
W.Melnitchouk and A.W.Thomas 96 J.Ellis,
D.Kharzeev, A. Kotzinian 96
10
?s in target fragmentation
e
e
1
p
?
2
p
CLAS-5.7GeV
MC
data
HERMES
  • Presence of large cosq from acceptance require
  • Detailed MC simulation OR
  • Spin asymmetry measurement to cancel the
    acceptance contribution

6
11
?s in target fragmentation
e
e
1
p
?
2
p
6
Projections for Lambda polarization for 5.75 GeV
(25days) with neutron target (predictions for
5.75 GeV from Ellis et al.)
12
Collinear Fragmentation
The only fragmentation function at leading twist
for pions in eN?epX 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 (0.4ltzlt0.7, MXgt1.5)
13
Azimuthal Asymmetries in SIDIS
  • Gluon bremsstrahlung (Georgi Politzer, Mendez
    1978) at z?1
  • Intrinsic transverse momentum of partons (Cahn
    1978)
  • Higher twists (Berger 1980, Brandenburg et al
    1995)

The DIS data from EMC (1987) and Fermilab (1993)
are most consistent with intrinsic parton
transverse momentum squared, of 0.25 GeV2
14
Azimuthal Asymmetries in semi-exclusive limit
  • Higher twists (Berger 1980, Brandenburg et al
    1995) z?1 dominant contribution ue- ?e- p d

Dominant contribution to meson wave function is
the perturbative one gluon exchange and approach
is valid at factor 3 lower Q2 than in case of
hard exclusive scattering (Afanasev Carlson
1997)
15
Azimuthal asymmetries at CLAS
CLAS
Q22.23-2.66 GeV2
x0.28-0.32 z0.16-0.19 pT0.41-0.53 GeV
Preliminary
  • Unpolarized Semi-inclusive electroproduction of
    ? measured.
  • Complete 5-dimensional cross sections were
    extracted.
  • Direct separation of different structure
    functions.

16
A1p f-kinematic dependence for p/-/0
A1p
A1p
CLAS 5.7 GeV
PRELIMINARY
  • No significant z-dependence of A1 in the range
    0.4ltzlt0.7 (pp0)
  • x dependence of CLAS A1p (A20) consistent with
    HERMES data at x3 higher Q2 and with PEPSI
    (LUND)MC.

17
SIDIS factorization studies
A1pp-
GRVS
  • A1 inclusive, from pp- sum and p0 are
    consistent (in range 0.4ltzlt0.7 )
  • A1p dependence can serve an important check of
    HT effects and applicability of simple partonic
    description.
  • There is an indication that A1p of p p- is
    lower than inclusive at large z.

18
g1/f1 for p/-/0 PT-dependence
PEPSI-MC
EG2000
Data consistent with PEPSI-MC
Asymmetries from kT-odd (f1-, h1-, gT..) and
kT-even (g1) distribution functions are expected
to have a very different behavior (F.Yuan)
19
SSA PT-dependence of sinf moment
ssinfLU(UL) FLU(UL) 1/Q (Twist-3)
ALU CLAS _at_4.3 GeV
AUL (CLAS _at_5.7 GeV)
AUT HERMES _at_27.5 GeV
PRELIMINARY
Beam and target SSA for p are consistent with
increase with PT
20
ALU x-dependence
0.5ltzlt0.8
Beam SSA analyzed in terms of the Collins effect
by Schweitzer et al. using e(x) from cQSM
Knowledge of polarized fragmentation function
required to extract distribution functions.
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 and higher statistics
will allow to check the higher twist nature of
beam and longitudinal target SSAs
22
Flavor decomposition of T-odd g-
In jet SIDIS with massless quarks contributions
from H1-,E vanish
gauge link contribution
With SSA measurements for pp- and p0 on neutron
and proton (ppp-) assuming HfavHu?p
-Hu?p--Hunfav
23
SSA x-dependence
PRELIMINARY
5.7 GeV
HTSSA significant for p and p 0
(non-Collins?)
AUL (p 0) H1favoreH1unfavored
  • Study the Collins fragmentation mechanism with
    long. polarized target
  • For p - and p 0 SSA is sensitive to unfavored
    fragmentation

24
First glimpse of Twist-2 TMD h1L-
For Collins fragmentation use chirally invariant
Manohar-Georgi model (Bacchetta et al)
Distribution functions from cQSM from Efremov et
al
PRELIMINARY
CLAS-5.7GeV
Systematic error only from unknown ratio of
favored and unfavored Collins functions (R
H1d?p/H1u?p), band correspond to -2.5ltRlt0
  • More data required with p- p0
  • Exclusive 2 pion background may be important

p- and p0 SSA will also give access to h1Ld ( If
R -1 deuteron data could be crucial)
25
Future more p0 data in SIDIS
advantages
  1. SIDIS p0 production is not contaminated by
    diffractive r
  2. HT effects and exclusive p0 suppressed
  3. Simple PID by p0-mass (no kaon contamination)
  4. Provides complementary to p/- information on PDFs

disadvantages reconstruction efficiency
(requires detection of 2g)
26
CLASInner Calorimeter (IC)
424 PbWO4 ..crystals
IC
IC sE/E0.0034/E0.038/vE0.022
CLASIC
CLAS
Reconstruction efficiency of high energy p0 with
IC increases 4 times due to small angle coverage
CLAS
IC at CLAS opens new avenue for studies of spin
and azimuthal asymmetries of exclusive and
semi-inclusive g, p0,h,r
27
Longitudinally polarized target SSA using CLASIC
50 days of CLASIC
curves, cQSM from Efremov et al
Hunf-5Hfav
Hunf-1.2Hfav
Hunf0
  • Provide measurement of SSA for all 3 pions,
    extract the Mulders TMD and study Collins
    fragmentation with longitudinally polarized
    target
  • Allows also measurements of 2-pion asymmetries

28
Transverse Target SSA at CLAS _at_5.7GeV
Expected precision of the AUT with transversely
polarized target
Collins
AUT
Collins effect at CLAS kinematically suppressed
compared to the Sivers effect.
Sivers
AUT
Simultaneous measurement of SIDIS, exclusive
r,r,w and DVCS asymmetries with a transversely
polarized target.
more details in X.Jiangs talk
29
Deeply Virtual Compton Scattering ep-gtepg
DVCS
BH
GPD
TBH given by elastic form factors TDVCS
determined by GPDs
Polarized beam, unpolarized target

DVCS
DsLU sinfImF1H x(F1F2)H kF2E
Kinematically suppressed
BH
Unpolarized beam, longitudinal target

DsUL sinfImF1Hx(F1F2)(H ..
x xB/(2-xB ),k t/4M2
Kinematically suppressed
GPD combinations accessible as azimuthal moments
of the total cross section.
30
CLAS/DVCS (ep ? epX) at 5.75 GeV
VGG
VGG
PRELIMINARY (not for circulation)
PRELIMINARY (not for circulation)
0.15 lt xBlt 0.4 1.50 lt Q2 lt 4.5 GeV2 -t lt 0.5 GeV2

DsLU sinfImF1H x(F1F2)H kF2E
Kinematic dependences of DVCS beam SSA consistent
with GPD based predictions (same pdfs)
31
JLab dedicated DVCS experiments in 2004 - 2005
JLab/Hall A
JLab/CLAS
Calorimeter and superconducting magnet within
CLAS torus
  • Dedicated, high statistics, DVCS experiments
  • ? Detection of 3 particles e, p and ? in final
    state
  • ? Firmly establish scaling laws (up to Q2 5
    GeV2),
  • if observed, or deviations thereof
    understood,
  • first significant measurement of
    GPDs.
  • ? Large kinematical coverage in xB and t
  • leads to femto-tomography of the
    nucleon
  • ? Opens the way for an ambitious program
  • with JLab_at_12GeV (CLAS12 and other)

p
e
e
?
details in J-P.Chens talk
32
DVCS with a polarized target in CLAS
  • Detect all 3 particles in the final state
    (e,p,?) to eliminate contribution from N
  • p0 asymmetry measured

5.7 GeV run with NH3 longitudinally polarized
target, Q2 up to 4.5 GeV2
VGG

PRELIMINARY (not for circulation)
DsUL sinfImF1Hx(F1F2)(H ..
Projections for 50 days with IC
Kinematical dependences of DVCS target SSA
consistent with GPD based predictions
33
Meson production in GPD framework
Only longitudinal photons
GPDs
GPDs
  • Different final state mesons filter out different
    combinations of unpolarized (H,E) and polarized
    (H,E) GPDs.
  • 2. Studies needed to define on how far is the
    asymptotic regime and guide theory in describing
    HT.



34
Exclusive ? meson production ep ? ep?
CLAS (4.2 GeV)
CLAS (5.75 GeV)
Regge (JML)
GPD formalism (beyond leading order) describes
approximately data for xBlt0.4, Q2 gt1.5 GeV2
GPD (MG-MVdh)
Analysis in progress
C. Hadjidakis et al., PLB 605
Two-pion invariant mass spectra
Decent description in pQCD framework already at
moderate Q2
35
pion SSA from r(pp-/pp0)
(CLAS _at_5.7GeV)
PYTHIA at 5.7 GeV
r0
Larger fraction of p from r at low x and large z
p SSA at large z may also have a significant
(20) contribution from r
Exclusive r (higher twist for SIDIS) crucial for
pX and ppX studies
36
Summary
  • Spin and azimuthal asymmetries measured at 5.7
    GeV with polarized beam and longitudinally
    polarized target at JLab.
  • SIDIS multiplicities and double spin asymmetries
    of pions are consistent with factorization and
    partonic picture may be used in future NLO QCD
    fits.
  • sinf and sin2f beam and target SSA measured,
    providing access to the twist-2 and twist-3 TMD
    distributions and testing the Collins
    fragmentation function
  • Studies of hard exclusive photon and vector
    meson production underway, allowing access to
    GPDs and also providing important info for the
    interpretation of SIDIS SSAs.
  • Ongoing and future measurements with CLAS at
    6GeV, will greatly improve p0//-, r, and DVCS
    data, allowing extraction of underlying TMDs and
    GPDs.
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