1 / 12

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:

glenda
Télécharger la présentation

Parton energy loss

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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)

More Related