# Presentation: 948121

## Spin and Azimuthal Asymmetries at JLAB

- H. Avakian *)

- Single-Spin Asymmetries Workshop, BNL June 1-3, 2005

- *) in collaboration with P.Bosted, V. Burkert and L. Elouadrhiri

- Jefferson Lab

**Notes:**
HERMES experiment in Hamburg, Germany designed to study the spin structure of the proton: “how does the proton get its spin of 1/2?” 
Brainchild of some very famous people including RGM. 
Completed its first running phase 1995-2000, lots of data, provided answers ... but also 
new questions to ask about proton spin. --> Run 2, starting with transv target

## 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**

**Notes:**
Note me

## Polarized Semi-Inclusive DIS

- Cross section defined by scale variables **x,y,z **

- Parton-Hadron transition: by fragmentation function Dπ+ (z): probability for a **u**-quark to produce a **+** with a momentum fraction **z**

- Hadron-Parton transition: by distribution function f1u(x): probability to find a **u**-quark with a momentum fraction **x**

- 1u

## Single pion production in hard scattering

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

*x**F* - momentum in the CM frame

- xF>0 (current fragmentation)

## Transverse momentum of quarks

- To study orbital motion of quarks in semi-inclusive DIS measurements in a wide  range of  **P****T****, ********and********S** are required.

- 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 → 0X (E704,RHIC) x-sections

## SIDIS (*p->X) x-section at leading twist:

- The structure functions depend on *Q**2*, x*B*, z, *P**hT*

-  Unpolarized target
-  Longitudinally pol.
-  Transversely pol.

- Studies of PDFs require _three _experiments

- 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...)

## 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 **Q****2** range** **

- Spin asymmetries + azimuthal dependence              new class of DIS measurements Spin-Azimuthal Asymmetries:

## The CLAS Detector

- The CLAS* *Detector

- ~ 200 physicists
- 37 institutions

- Q2

**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**

## Λ in target fragmentation

- Λ in target fragmentation

*p*

*e *

*e’*

- Λ

- π

- 1

- 2

- 6

-  s 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

- Accessing polarized PDFs with unpolarized target!

-  – unique tool for polarization study due to self-analyzing parity violating decay

**W.Melnitchouk and A.W.Thomas ‘96**
**J.Ellis, D.Kharzeev, A. Kotzinian ‘96**

- (ud)-diquark is a spin and isospin singlet s-quark carries whole spin of 

## Λs in target fragmentation

*p*

*e *

*e’*

- Λ

- π

- 1

- 2

- 6

- HERMES

- Presence of large cos  from acceptance require:
- Detailed MC simulation  **OR**
- Spin asymmetry measurement to cancel the acceptance contribution

- CLAS-5.7GeV

- data

- MC

## Λs in target fragmentation

*p*

*e *

*e’*

- Λ

- π

- 1

- 2

- 6

- Projections for Lambda polarization for 5.75 GeV (25days) with neutron target (**predictions for 5.75 GeV from Ellis et al**.)

## Collinear Fragmentation

- The only fragmentation function at leading twist for pions in eN→e’****X is D1(z)

- Hall-C  **E****e**** =5.5 GeV **

**E****e**** =5.7 GeV **

- No significant variation observed in  ratio and  in z distributions of + for  different **x** ranges (0.4<z<0.7, MX>1.5)

## 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

## Azimuthal Asymmetries in semi-exclusive limit

- Higher twists (Berger 1980, Brandenburg et al 1995) z→1 dominant contribution u+e- →e- + 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)

## CLAS

- Unpolarized Semi-inclusive electroproduction of + measured.
- Complete _5-dimensional cross sections_ were extracted.
- Direct separation of different structure functions.

- x=0.28-0.32
- z=0.16-0.19
- pT=0.41-0.53 GeV

- Q2=2.23-2.66 GeV2

- Preliminary

**CLAS**

- Azimuthal asymmetries at CLAS

## A1p f-kinematic dependence for +/-/0

- A1p f-kinematic dependence for +/-/0

- CLAS 5.7 GeV

- A1p

- A1p

-  No significant z-dependence of A1 in the range 0.4<z<0.7 ()
**x **dependence of CLAS  **A****1****p** (A2=0) consistent with HERMES data at x3 higher Q2 and with PEPSI (LUND)MC.

- PRELIMINARY

## SIDIS: factorization studies

-   **A****1** inclusive, from  sum and  are consistent (in range 0.4<z<0.7 )
-   **A****1****p**  dependence can serve an important check of HT effects and applicability of simple partonic description.
- There is an indication that **A****1****p**  of ********+************is lower than inclusive at large z.

- GRVS

- A1+-

## g1/f1 for +/-/0 : PT-dependence

- Data consistent with PEPSI-MC

- EG2000

- 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)

## SSA: PT-dependence of sin moment 

- SSA: PT-dependence of sin moment

- sinLU(UL) ~FLU(UL)~ 1/Q (Twist-3)

**A****LU**** **CLAS @4.3 GeV

- Beam and target SSA for + are consistent with increase with PT

**A****UL**** **(CLAS @5.7 GeV)

**A****UT**** **HERMES @27.5 GeV

- PRELIMINARY

## ALU:  x-dependence

- ALU:  x-dependence

- 0.5<z<0.8

- Knowledge of polarized fragmentation function required to extract distribution functions.

**Beam SSA analyzed  in terms of the Collins effect by Schweitzer et al. using e(x) from ********QSM**

## Measuring the Q2 dependence of SSA

******sin********LU(UL)**** ~F****LU(UL)****~ 1/Q (Twist-3)**

- Wide kinematic coverage and higher statistics will allow to check the higher twist nature of beam and longitudinal target SSAs

- For fixed x,  1/Q behavior expected

## Flavor decomposition of T-odd g┴

- With SSA measurements for and  on neutron and proton () assuming Hfav=Hu→+ ≈ -Hu→-=-Hunfav

- In jet SIDIS  with massless quarks contributions from H1┴,E vanish

- gauge link contribution

## SSA: x-dependence

- Study the Collins fragmentation mechanism with long. polarized target
-  For   - and   0 SSA is sensitive to unfavored fragmentation

- HT–SSA significant for
-   + and   0 (non-Collins?)

- AUL ( 0) ~ H1favore+H1unfavored

- PRELIMINARY

- 5.7 GeV

## First glimpse of Twist-2 TMD h1L┴

- Systematic error only from unknown ratio of favored and unfavored Collins functions (R= H1d→/H1u→), band correspond to -2.5<R<0

- - and 0 SSA will also give access to h1Ld ( If R ≈ -1 deuteron data could be crucial)

- CLAS-5.7GeV

- First glimpse of Twist-2 TMD h1L┴

- PRELIMINARY

- More data required with - & 0
- Exclusive 2 pion background may be important

**Distribution functions from********QSM from Efremov et al**

## Future: more 0 data in SIDIS

**SIDIS********0**** production is not contaminated by diffractive******
**HT effects and exclusive ********0**** suppressed**
**Simple PID by ********0****-mass (no kaon contamination)**
**Provides complementary to ********+/-**** information on PDFs**

**disadvantages: **
**	****reconstruction efficiency (requires detection of 2********)**

**advantages:**

## CLAS+Inner Calorimeter (IC)

- IC at CLAS opens new avenue for studies of spin and azimuthal asymmetries of exclusive and semi-inclusive    ****************

- CLAS

- CLAS+IC

- CLAS

**IC**

- 424 PbWO4    ........crystals

**Reconstruction efficiency of high energy ********0**** with IC increases ~ 4 times due to small angle coverage**

**IC ********E****/E=0.0034/E****+0.038/√E+0.022**

## Longitudinally polarized target SSA using CLAS+IC

**Longitudinally polarized target SSA using CLAS+IC**

**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**

- H**unf**=-1.2H**fav**

- Hunf=-5Hfav

- H**unf**=0

- 50 days of CLAS+IC

**curves, ********QSM  from Efremov et al**

## Transverse Target SSA at CLAS @5.7GeV

**Expected precision of  the A****UT**** with transversely polarized target**

- AUT ~

- Collins

- AUT ~

- Sivers

**Collins effect at CLAS kinematically suppressed compared to the Sivers effect.**

- Simultaneous measurement of SIDIS, exclusive  and DVCS asymmetries with a transversely polarized target.

- more details in X.Jiang’s talk

## Deeply Virtual Compton Scattering ep->e’p’

- GPD combinations accessible as  **azimuthal moments** of the total cross section.

- DVCS

- BH

******LU******~ sin****Im{F1H + (F1+F2)H +kF2E}

**~**

- Polarized beam, unpolarized target:

- Unpolarized beam, longitudinal target:

******UL**~ sin****Im{F1H+(F1+F2)(H +..  }

**~**

-  = xB/(2-xB ),k = t/4M2

- Kinematically suppressed

- Kinematically suppressed

- DVCS

- BH

- TBH : given by elastic form factors
- TDVCS: determined by GPDs

**GPD**

## ~

- 0.15 < *x*B< 0.4
- 1.50 < *Q**2 *< 4.5 GeV2
*-t *< 0.5 GeV2

- PRELIMINARY
- (not for circulation)

- VGG

***CLAS/DVCS (ep ******→ epX)****** at 5.75 GeV***

- PRELIMINARY
- (not for circulation)

******LU******~ sin****Im{F1H + (F1+F2)H +kF2E}

- Kinematic dependences of DVCS beam SSA consistent with GPD based predictions (same pdfs)

- VGG

**~**

## JLab dedicated DVCS experiments in 2004 - 2005

**JLab/Hall A**

**JLab/CLAS**

*Calorim**eter and superconducting magnet within CLAS torus*

**e**

**e’**

**p**

**γ**

_**Dedicated, high statistics, DVCS experiments**_
- → Detection of 3 particles e, p and γ  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@12GeV (CLAS12 and other)

***JLab dedicated DVCS experiments in 2004 - 2005***

- details in J-P.Chen’s talk

**Notes:**
Intro

## ~

- 5.7 GeV run with **NH****3** longitudinally polarized target, Q2 up to 4.5 GeV2

***DVCS with a polarized target in CLAS***

**Detect all 3 particles** in the final state     (e,p,γ) to eliminate contribution from N
- * π0 asymmetry measured

- PRELIMINARY
- (not for circulation)

- Projections for 50 days with IC

******UL**~ sin****Im{F1H+(F1+F2)(H +..  }

**~**

- VGG

- Kinematical dependences of DVCS target SSA consistent with GPD based predictions

## Meson production in GPD framework

- 2. Studies needed to define on how far is the asymptotic regime and guide theory in describing HT.

- GPDs

- GPDs

- ~

- ~

- Only longitudinal photons

## Exclusive ρ meson production: ep → epρ

***Exclusive ******ρ****** meson production: ep ******→****** ep******ρ***

- CLAS (4.2 GeV)

- Regge (JML)

- C. Hadjidakis et al., PLB 605

- GPD formalism (beyond leading order) describes approximately data for xB<0.4, Q2 >1.5 GeV2

- GPD
- (MG-MVdh)

- CLAS (5.75 GeV)

- Analysis in progress

- Two-pion invariant mass spectra

- Decent description in pQCD framework already at moderate Q2

## pion SSA from 

- + SSA at large z may also have a significant (~20%) contribution from 

- 

- PYTHIA at 5.7 GeV

- (CLAS @5.7GeV)

- Larger fraction of + from at low **x** and large **z**

- Exclusive higher twist for SIDIS) crucial for X and X studies

## 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.**
** ****sin********and********sin2******** 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 ********0/+/-****,**** ******** and DVCS data, allowing extraction of underlying TMDs and GPDs.**** **