1. Parton energy loss Marco van Leeuwen

2. 2 Hard probes of QCD matter Use ‘quasi-free’ partons from hard scatterings to probe ‘quasi-thermal’ QCD matter Interactions between parton and medium: -Radiative energy loss -Collisional energy loss -Hadronisation: fragmentation and coalescence Sensitive to medium density, transport properties Calculable with pQCD Quasi-thermal matter: dominated by soft (few 100 MeV) partons

3. 3 pQCD illustrated CDF, PRD75, 092006 jet spectrum ~ parton spectrum fragmentation Factorisation: hadron spectrum is a convolution-product of parton spectrum and fragmentation function ‘Analytical approach’

4. 4 Note: difference p+p, e + +e - p+p: steeply falling parton spectrum Hadron spectrum convolution of jet spectrum with fragmentation e + + e - QCD events: all partons have p=1/2 √s dN/dz, z = p h / E parton Directly measure frag function

5. 5 Parton energy loss and R AA modeling Qualitatively: `known’ from e + e - known pQCDxPDF extract Parton spectrum Fragmentation (function) Energy loss distribution This is the medium effect Modelling: perform convolution and compare to data Convolution in words: 1.Generate parton spectrum dN/dE parton 2.Apply energy loss E parton → E parton -  E 3.Fragmentation p hadron = z E parton

6. 6 Energy loss distribution Brick L = 2 fm,  E/E = 0.2 E = 10 GeV Typical examples with fixed L  E/E> = 0.2 R 8 ~ R AA = 0.2 Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to  E/E = 1)

7. 7 One more thing: geometry Medium properties ‘seen’ by the parton depend on trajectory Need to average over production points, directions … another convolution

8. 8 Geometry II: ‘surface bias’ Short path length small E-loss Likely to ‘survive’ Away-side large L Measurements with parton pairs sample geometry in a specific way; different from single hadrons Detected hadrons biased towards small  E, small L Expect: (due to interference effects)

9. 9 Heavy quark fragmentation Light quarks Heavy quarks Heavy quark fragmentation: leading heavy meson carries large momentum fraction Less gluon radiation than for light quarks, due to ‘dead cone’

10. 10 Dead cone effect Radiated wave front cannot out-run source quark Heavy quark:  < 1 Result: minimum angle for radiation  Mass regulates collinear divergence

11. 11 How to picture a QCD event MC event generators (PYTHIA, HIJING, HERWIG) use this picture Initial hard scattering high virtuality Q 2 generates high-p T partons Followed by angle-ordered gluon emissions: fragmentation At hadronic scale: hadronisation prescription (e.g. clustering in HERWIG) Medium-induced gluon radiation (energy loss) takes place at this point

12. 12 Research project Model heavy quark energy loss –Parton spectrum –Energy loss (including average over geometry) –Fragmentation Compare to measurements Compare to light hadron results Explore potential for future measurements (e.g. back- to-back pairs)