SSA in exclusive 2 pion production

1
SSA in exclusive 2 pion production
H. Avakian, L.Elouadrhiri
Jefferson Lab
SON meeting Feb. 27

• Physics Motivation
• Exclusive 2 pion production
• PYTHIA MC
• r0 and r SSA in e16
• Summary Outlook

2
Physics Motivation
Extract the systematic error in epX SSA due to
exclusive r background Study transition from
exclusive to semi-inclusive domain
Exclusive r studies

1. Study angular distributions in exclusive 2 pion
   production
2. Study beam SSA for r0 and r

3
Large p- SSA triggered studies of r contamination
in epX sample
Sivers and Collins effect measurements at HERMES
Sivers effect
Collins effect
epX
epX
ep-X
ep-X
4
2 pion electroproduction Kinematics
5
SSA with Transversely Polarized Target

• Significant SSA asymmetry predicted both for
  exclusive r0, r

Important observable by itself with transversely
polarized target
6
PYTHIA MC

• Pythya 6 updated by HERMES to describe also the
  exclusive r production .
• Includes radiative photons.
• String fragmentation VMD

Ratio of L/T cross sections
With some tune of major exclusive channels r,w,f,
PYTHIA could become a very important tool in
exclusive and semi-inclusive studies at CLAS at
6 GeV
7
Contamination of SIDIS p from PYTHIA-MC
In the range 0.5ltzlt0.8 15-20 of p from r
8
Exclusive r at 6 GeV
PYTIA-6
2p
9
Vector mesons in GPD framework
GPDs
GPDs

• 0 and w can also proceed through a two gluon
  exchange.
• r measurements will provide additional
  information on the relative contribution.

10
Vector mesons in GPD framework
Longitudinal cross section for r, r and w
involve Different flavor combinations of GPDs
Measurements of different channels will allow to
make a flavor decomposition
11
r from e16
r measurements possible!
Fit by a simple BWpolin.background
12
Beam SSA for exclusive pp0
Missing mass
Invariant mass
Significant positive beam asymmetry in the r
region
13
Beam SSA for exclusive pp-
Simple BW fit with polin.backgroun.
14
Beam SSA for exclusive pp-
PYTHIA-6GSIMRECSIS shows reasonable agreement
with e16
15
Beam SSA for r
HERMES r80.2/-0.1
striking difference with p0,p,r
16
Beam SSA for exclusive events
Beam SSA for p0,p,r are consistent
17
Summary

• Exclusive r background to SIDIS p studied
• Contamination from r extracted using PYTHIA
• Beam asymmetries extracted for pp- and pp0
  exclusive production.

• Further tune the PYTHIA MC to describe exclusive
  production of r.
• Measure angular distributions of r0,r and
  compare with existing predictions.
• Estimate the systematic uncertainty of pion SSA
  in SIDIS due to exclusive vector meson production.

18
cosq for exclusive pp-
p fraction of r momenta
r040.62
From PYTHIA