1. Comparison of energy loss formalisms Marco van Leeuwen, UU TECHQM meeting LBNL, 15-16 Dec 2008

2. 2 TECHQM `Brick Problem’ Idea: define a few ‘model systems’ to compare different calculational approaches Fixed-length homogeneous ‘brick’ of QGP: L = 2 fm, L = 5 fm Explore parton energy dependence: E = 10 GeV, E = 100 GeV (quarks) Compare models at ‘comparable phenomenology’, i.e. fixed  E/E (0.05, 0.1, 0.2, 0.4) Calculate P(  E) and P(N gluon ) (partial) results available for: WHDG/GLV, ASW/BDMPS, AMY, Higher Twist, YaJEM Note: not all models calculate P(  E)

3. 3 Comparing P(  E) Note importance of P(0) Continuous part of P(  E) more sharply peaked for WHDG-GLV ASW-GLV and WHDG-GLV are different Due to treatment of  E/E →1? Are these differences significant?

4. 4 L-dependence Increase L at constant : continuous part increases; peak at small values Difference WHDG-ASW persists

5. 5 Energy-dependence Increase energy: reduces P(0), importance of  E/E → 1 Difference WHDG and ASW reduced at 100 GeV?

6. 6 Simple folding (only ‘loss term’): Artefact in KKP parametrisation Towards phenomenology: fragmentation functions E = 10 GeV, WHDG steeper than ASW (reflects different P(  E) E = 100 GeV: shapes similar, normalisation difference?

7. 7 Next steps Compare to other formalisms (Higher twist, AMY, YaJEM) Compare at same medium density? Calculate R AA, I AA,  -jet Use more realistic geometry Immediate future: Strategy towards future: and/or To discuss in TECHQM