Collisional energy loss becomes probable André Peshier SUBATECH, Université de Nantes - Praha, 20 April

2 Phases of matter

3 Relativistic heavy-ion collisions initial state pre-equilibrium QGP and hydrodynamic expansion hadronization hadronic phase and freeze-out What happens???

4 A paradigm in trouble distinguished signal of QGP: jet quenching partonic energy loss: radiative  collisional transp. coeff. factor 10 too large!! Collisional energy loss would help. estimate of collisional contribution based on Bjorken, Gyulassy,... What is α ?

5 How to fix α ? consider elementary quantity: Debye screening fundamental in plasma physics: describes polarization of medium by test charge, IR cut-off for long-range interaction hot QCD: Debye mass from self-energy

6 How to fix α ? common folklore: GUESS 50% deviation (usually interpreted as higher order effects)

7 Back to the roots Calculate  (instead of guess) running coupling from renormalized loop corrections, at running coupling from renormalized loop corrections, at appropriate momentum scale Which loop corrections? recall: Debye mass is property of POTENTIAL NO...

8 Heavy quark potential (T=0) pQCD “confinement” ~ string potential

9 Heavy quark potential (T=0) define coupling from force string parametrization: string parametrization inadequate: NO asymptotic freedom!

10 Heavy quark potential (T=0) perturbation theory for small r (large momenta) running coupling from renormalized loop corrections = running coupling bare coupling

11 Heavy quark potential (T=0) legitimate question: Where does pQCD fail? single parameter: Λ = 0.2GeV QCD pQCD good for α ~1 asympt. freedom

12 QCD Debye mass mended in-medium pQCD potential = α (t) T-part of selfenergy T self-consistent definition

13 QCD Debye mass parameter-free “prediction”! [AP, hep-ph/060119] pQCD works near T c

14 Didn't he want to talk energy loss? YES... it's coming NOW

15 QCD collisional e-loss, textbook version consider mean energy loss [Bjorken'82] IR cut-off “by hand” Gyulassy, Braaten, Thoma calculate with hard thermal loop formalism Conceptional inconsistency: BGBT resum thermal fluctuations, but “forget” vacuum contributions ► value of coupling unspecified in

16 QCD collisional e-loss mended the connection to the potential for s  t resummation renormalization  ► consequential modification of AP, hep-ph/

17 QCD collisional e-loss mended Theory changed! instead of theory now predictive, 1 parameter: 

18 Collisional quenching weights consider probability for energy transfer  AP, hep-ph/

19 Collisional quenching weights mean free path length implicit definition mean free path ~ particle distance strongly coupled QGP

20 Probabilistic quenching initialize jets - r φ ► “near-side” & “far-side” +

21 Probabilistic quenching Monte-Carlo dynamics: Bjorken boost local T-dependent quenching weights

22 Probabilistic quenching

23 Probabilistic quenching AP, hep-ph/ for reasonable geometry/dynamics: sizable quenching from collisions (parameter-free)

24 Summary predictions require thorough renormalization 1 parameter  pQCD “applicable” in large-coupling regime  =  0.2GeV adjusted to lattice QCD correct theory of collisional energy loss now with asymptotic freedom changed paradigm collisions important for quenching collisions important for jet quenching

25 Physics of the QGP Length scales : particle distance, thermodynamics : particle distance, thermodynamics : quasi-particle masses, Debye screening : quasi-particle masses, Debye screening : small angle scattering, interaction rates : small angle scattering, interaction rates : large angle scattering, transport coefficients : large angle scattering, transport coefficients for  1, clear hierarchy: