1. Quark Pair Creation What’s a quark? … what’s a quark-pair? ….. what current does it couple to? Why aren’t quarks ever seen alone? … what is the nature of the vacuum? ….. what is the nature of the strong force-field? Are quarks useful ? … as effective degrees of freedom?

2. The Quark Model Protons (and neutrons) have sub-structure - evidence from electron scattering - charged constituents (quarks) - strong (neutral) force-field Show evidence: elastic form-factor vs. qsq inelastic “scaling”

3. Effective Degrees of Freedom Toy top (spinning) - atomic level: forces between atoms (complicated !!) - small sub-volumes with constraints (complicated !!) - one unit: - center of mass, moment of inertia - angular momentum around single axis - torques act on angular momentum vector Structural details not important for motion !!

4. Effective Degrees of Freedom (2) Superconductivity - electron wave-function in crystal lattice - electron-electron scattering - electron-lattice scattering - very complicated dynamics - two electrons with an exchanged phonon ! - this (new) unit has ~zero interactions - dynamics is much simplified

5. Effective Degrees of Freedom (3) Planetary Motion in Solar System - Earth at center - sun, moon, planes move in epicycles - major cycle: daily cycle of sun around earth - minor cycles: planets around sun - minor, minor cycles …. - Sun at center - all planets move in ellipses - simple, independent dynamics

6. Effective Degrees of Freedom (4) Hadronic Sub-Structure Currents (carry momentum, interact with fields) - hadrons themselves ! - interaction: one hadron > two hadrons - many hadrons > many interactions - interactions depend on spectra - complicated ! (but true ?) View currents as quarks themselves !

7. Quark Pair Creation Quark model with flux-tube – what is inside of baryons and mesons – characteristics of the strong force-field Breaking the flux-tube: quark pair creation – a simple quantum mechanical picture Studying quark pair creation with exclusive production – electron scattering, CLAS detector, analysis, results Phenomenology – tunneling probability, angular momentum state

8. 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  Carman, MM: photon to u-quark; through s-sbar spin correlation to . Schumacher: directly from photon to s-sbar to . Problem: both consistent with data! How (else) to distinguish models?

9. Quark Pair Creation Quark-pair creation: “kernel” of exclusive production What field couples to the q-q current? susu sudsud K+K+  susu K+K+  udSudS ss produced From flux-tube ss produced from photon

10. 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 uudd ss uudd ss

11. Distinguish Models Study ratios of s s : dd : uu Measure ratios: K +  :  + n :  0 p K+K+  ++ n 00 p s s u u d d 0.2 : 1 : 1 Ratio : exponential in quark mass

12. Distinguish Models Measure ratios: K +  :  + n :  0 p K+K+  ++ n 00 p s d 1 : 1 : 4 Ratio : proportional to (charge) 2 s d u u

13. ~0 84k 350k 910k P (  0 )  (n)  +  90k 420k 540k 480k forward  Lab Angle  backward Missing Mass  2-body final state

14. forward  Lab Angle  backward (  )  +  50k 150k 45k 5k “Fake” plots mis-identified protons mis-identified pions K + Missing Mass

15. Limits of the “quark model” ? Can the Lund model work in the limit of single qq pair production? – recent analysis of a Hall C  + (n) production experiment Kaskulov, Gallmeister, Mosel; arXiv:0804.1834v4 [hep-ph] – model was developed to explain Rosenbluth separated exclusive electro-production of  + (n)    L described well as t-channel exchange    T described well as “quark-scattering followed by string- breaking” with Lund model parameters – model also works for un-separated data

16. Exclusive n  + : sum of two processes t-channel form factor falls with q 2 dudu ++ n uddudd photon couples to quark photon probes meson structure no form factor: q 2 independent  T : DIS via LUND model  L : t-channel exchange

17. Analysis goals and plans Model-independent goal – measure ratio of  + n :  0 p : K +  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, q 2, t ) – tune PYTHIA/JETSET (fitting and MC generator) Study single quark-pair creation

18. An interesting observation …. For forward-going protons (  LAB < 20 0 ) : only recoil meson is the   

19. And the winner is …. Ratio: K +  :  + n :  0 p 0.23 : 0.88 : 1.12 errors ~ 20% ?? crude accepance correction done; kaon decay not corrected