Exclusive Vector Meson Electroproduction

at 12 GeV

Paul Stoler Rensselaer Polytechnic Institute

What do we want to do at 12 GeV?

(a) Approach to the small–size regime

(b) Learn about short–range nucleon and meson structure

Is 12 GeV the right energy?

high xB

valence quarks

low xB

sea quarks

From valence quarks to sea quarks and gluons

From gluon dressed to bare quarks.

(Craig Roberts et al.)

                   M N =M Ng +M Nq

M μ+,μ+Ni ∝ ea

a∑ Ca dx∫ Hμλ,μλ

i

λ∑ H i(x,ξ,t)

Hμλ ,μλi = dτ db  ∫ αS  Ψ̂V  Fμλ,μλ

i   e−S⎡⎣

⎤⎦

GPD’s

Hi =H ,E,H , %E          HT ,ET ,HT ,%ET    

γL* p → pρ 0

VGGGK

VGG+D

JML

Example of world data

GPD: n=3

γL* p → p φ

s

Example of CLAS 6 GeV date

s

Q2=1-3 GeV2, W=2-3

Jlab 6 GeVJlab 12 GeV

f - ds/dt’

γL* p → p ρ 0 γL

* p → p φ

u =(pi − pp )2 smallt =(pi − pp )2 large

J.P Landsberg, B. Pire, L. Szymanowski

Elastic scattering from proton core in a dressed nucleon.

Backward angle (high -t) electroproduction

Kinematic covered. W ~ 2 4.5 GeV➞

Q2 ~ 1 13 GeV➞ 2

−t’ ~ 0 to > 15 GeV2

Explore the transition from soft physics to the dominance of QCD quarks and gluons

CLAS12 Proposal

γ * +  p →  p + ρ°               γ * +  p →  p + φ

γ * +  p → n + ρ +               γ * +  p →  p + ω

Exclusive Vector Meson Electroproduction with CLAS12

γ *  p → φ  p          φ → K +K −     detect epK +  and/or epK +K −

φ→ KS0KL

0        KS0 → π +π −   detect pπ +π −  

γ *  p → ρ 0 p        ρ 0 → π +π −   detect ′e pπ +  and ′e π −π +

γ *  p → ρ +n        ρ + →  π + π 0 → π +γ  γ   

γ *  p → ω   p        ω → π +π −π 0     detect epπ +π −

Meson Decay Modes

Goals of the experiment

dσdt*dφ

=σT +εσL +εσTT cos(2φ* )+ 2ε(ε +1)σLT cosφ

* +h 2ε(ε −1)σL ′Tsinφ*

Goal-obtain information about all 5 structure functions available with polarized beam and unpolarized target

σT + εσ L( )   σ TT   σ LT    obtained from cos(f) and cos(2f) with h = 0

σT σ L     –angular distribution of meson decay products.

Beam spin cross section differences

Δσ  = dσuru

dt*dφ − 

dσs uu

dt*dφ  

σL ′T     beam spin asymmetries

Structure Functions:

W (cosθHS) = 38

1−r0004

( ) + 3r0004 −1( )cos

2θHS⎡⎣

⎤⎦

R = σ L

σ L

= 1ε

r0004

1−r0004.

L/T separation

H

Helicity Frame

Simulations

Extensive simulations were carried out for all 4 mesons channels.

In general, statistics are quite good due to large acceptance, high luminosity and efficient PID

Compared with previously successful CLAS 6 GeV r0 r+ f and w experiments, statistics will increase by factor ~102

r0 channel simulations

γ * +  p → ρ 0  +  p        ρ 0 → π + + π −

MM ′e pπ +( ) ⇒  π −       IM π +π −

( )  ⇒  ρ 0            

MM ′e π +π +( )⇒   p IM π +π +

( )⇒ ρ 0 ← large - t

0.1 xB

Q2

1 GeV2

13 GeV2

0.8

0

ds/d

t ( m

b/G

eV2 )

-t (GeV2)10 20

f simulations

γ *  p → φp        φ→ K + K −

1. MM ′e pK +( ) ⇒  K −    

2.MM ′e K + K −( ) ⇒   p  

3.Detect ′e K + K − p( )  

⎧

⎨

⎪⎪

⎩

⎪⎪

⎫

⎬

⎪⎪

⎭

⎪⎪

     IM K + K −( )  ⇒  φ

γ *  p → φp        φ→ KS0 KL

0       

KS0 → π +π −    MM pKS

0( ) → KL

0    

Acceptance - e p K+

6 backup slides follow

Statistics•Average acceptance ≃ 10 to 50%. (depending on kinematics)

SystematicsBackground subtraction~ few % (sharp state)Acceptance ~5-10 %

Net: s ~7-12%

Errors on cross sections

Statistics•⇒Statistics: very high•Average acceptance ≃ 10 to 30%. (depending on kinematics)

Systematics Background subtraction: systematics 10%

Net: s ~15%