1. A Tale of Two (Toy) Models
CERN Courier: July, 2007
Harry Lee, Dan Carman, Reinhard
Schumacher, MM
Two possible pictures of how spin
is transferred from virtual photon
to the L.
Carman, MM: photon to u-quark;
through s-sbar spin correlation to L.
Schumacher: directly from photon
to s-sbar to L.
Problem: both consistent with data!
How (else) to distinguish models?

2. Quark Pair Creation
Quark-pair creation: “kernel” of exclusive production
What field couples to the q-q current? s u K+ L d S
ss produced
From flux-tube s u d K+ L
ss produced from photon

3. Using Exclusive Production to Study Quark Pair Creation
Lund model: successful phenomenology for hadron production; e.g. in e+e- reactions
color flux-tube broken by qq production
production rate depends on constituent quark mass
: : ~ 1 : 1 : 0.2
Vector meson dominance: photon fluctuates into a virtual qq meson
production rate depends on quark charge
: : ~ 1: 0.25 : 0.25 uu dd ss uu dd ss

4. Distinguish Models Study ratios of s s : dd : uu
Measure ratios: K+L : p+n : p0p K+ L p+ n p0 p s d u s d u
: :
Ratio : exponential in quark mass

5. Distinguish Models Measure ratios: K+L : p+n : p0p L n p K+ p+ p0
: :
Ratio : proportional to (charge)2

6. Missing Mass 2-body final state (n) p+ a P (p0) a 90k 420k 540k 480k
forward f Lab Angle g backward

7. K+ Missing Mass (L) K+ a “Fake” plots 50k 150k 45k 5k mis-identified
protons
mis-identified
pions
forward f Lab Angle g backward

8. Limits of the “quark model” ?
Can the Lund model work in the limit of single qq pair production?
recent analysis of a Hall C p+(n) production experiment
Kaskulov, Gallmeister, Mosel; arXiv: v4 [hep-ph]
model was developed to explain Rosenbluth separated exclusive electro-production of p+(n)
a sL described well as t-channel exchange
a sT described well as “quark-scattering followed by string-breaking” with Lund model parameters
model also works for un-separated data

9. Exclusive np+: sum of two processes
sT: DIS via LUND model
sL: t-channel exchange d u p+ n
photon couples
to quark
photon probes
meson structure
t-channel
no form factor: q2 independent
form factor falls with q2

10. Analysis goals and plans
Model-independent goal
measure ratio of p+n : p0p : K+L cross-sections
proportional to uu : dd : ss rate
Extend Kaskulov, Gallmeister, Mosel model
extend to dd and ss production
Analysis plan
identify final states (done)
decide on binning ( high/low bins for W, q2, t )
tune PYTHIA/JETSET (fitting and MC generator)
Study single quark-pair creation

11. An interesting observation ….
For forward-going protons (qLAB < 200) :
only recoil meson is the r0 ??

12. And the winner is …. Ratio: K+L : p+n : p0p 0.23 : 0.88 : 1.12
crude accepance
correction done;
kaon decay not
corrected
Ratio: K+L : p+n : p0p
0.23 : 0.88 : 1.12
errors ~ 20% ??