1. Third TECHQM Collaboration Meeting, CERN July 6-10, 2009 Xin-Nian Wang Lawrence Berkeley National Laboratory The Brick Problem in High-Twist Approximation in collaboration with Wei-tian Deng and Abhijit Majumder

2. High-Twist Approach e-e- e-e- Production & propagation in the same framework However, a general proof of factorization is not done Extension to Hevay-ion collisions: Generalized Collinear Approx. in pQCD

3. High-Twist approach to multiple scattering Description of medium as collection of quasi-particle states e-e-

4. DGLAP Evolution in vacuum Splitting function q p k1k1 k2k2 p q perturbative region: no matter how large is initial

5. Induced gluon emission in twist expansion q Ap xp Ap q x 1 p+k T transverse momentum of medium gluon

6. Gauge Invariance Expansion in k T One should also consider Final matrix elements should contain: k p (not all models have this)

7. Induced gluon emission in twist expansion Collinear expansion: Double scattering Eikonal contribution to vacuum brems.

8. Modified Fragmentation Modified splitting functions Guo & XNW’00 Two-parton correlation:

9. Validity of collinear expansion Collinear expansion: One has to re-sum higher-twist terms (Or model the behavior of small l T behavior) Need to include all: Region of validity:pQCD If one is bold and goes beyond no one has gone before:

10. Comparison with GLV quasi-particle density

11. 11

12. Modified DGLAP Evolution Modified splitting functions

13. Parton-gluon scattering Quark-gluon Compton scattering Schafer, XNW & Zhang 07

14. The Brick Problem Initial conditions:

15. Shower parton distr. in a gluon jet g--> g g --> q

16. Shower parton distr. in a quark jet q --> g q--> qbar

17. 17 Valence quark energy loss

18. 18 Valence quark distribution

19. 19 Beyond the Brick Problem Jet transport parameter Jet transport parameter & phases of dense matter Constrain “implementations” with DIS data (talk by Wei-tian Deng)

21. Theoretical improvements Elastic vs radiative: for finite E & L Qin, et al ‘’08 Recoil in radiative process: Quark-annihilation Flavor changing process q Ap q NLO corrections to LO collinear factorized contribution Mass correction for heavy quarks

22. Different cut-diagrams + …..