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Parton energy loss

Parton energy loss. Marco van Leeuwen. Hard probes of QCD matter. Use ‘quasi-free’ partons from hard scatterings. Calculable with pQCD. to probe ‘quasi-thermal’ QCD matter. Quasi-thermal matter: dominated by soft (few 100 MeV) partons . Interactions between parton and medium:

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Parton energy loss

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  1. Parton energy loss Marco van Leeuwen

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

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

  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 Directly measure frag function dN/dz, z = ph / Eparton

  5. Parton energy loss and RAA modeling Qualitatively: Parton spectrum Energy loss distribution Fragmentation (function) known pQCDxPDF extract `known’ from e+e- This is the medium effect Modelling: perform convolutionand compare to data • Convolution in words: • Generate parton spectrum dN/dEparton • Apply energy loss Eparton→ Eparton - DE • Fragmentation phadron = z Eparton

  6. Energy loss distribution Typical examples with fixed L <DE/E> = 0.2 R8 ~ RAA = 0.2 Brick L = 2 fm, DE/E = 0.2 E = 10 GeV Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to DE/E = 1)

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

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

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

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

  11. How to picture a QCD event Initial hard scattering high virtuality Q2generates high-pT partons Followed by angle-ordered gluonemissions: fragmentation Medium-induced gluon radiation (energy loss) takes place at this point At hadronic scale: hadronisation prescription (e.g. clustering in HERWIG) MC event generators (PYTHIA, HIJING, HERWIG) use this picture

  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)

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