1. Open Questions: Jets and Heavy Quarks
(not a summary)
Barbara Jacak
Stony Brook University
June 15, 2006

2. Open questions as of June 9
What are the implications of incomplete color screening?
collisional vs. radiative energy loss
transport properties of quark gluon plasma at RHIC
Where are the B mesons in single electron RAA & flow?
Need to take another look at parton densities extracted from jet quenching – impact of collisional energy loss?
Where DOES the energy lost by jets go?
density waves (Mach cones)? caught up in longitudinal flow? thermalized?

3. Screening: Debye Length
distance over which the influence of an individual charged particle is felt by the other particles in the plasma
charged particles arrange themselves so as to effectively shield any electrostatic fields within a distance lD
lD = e0kT
nee2
Debye sphere = sphere with radius lD
number electrons inside Debye sphere is typically large
ND= N/VD= rVD VD= 4/3 p lD3
1/2
in strongly coupled plasmas it’s  1

4. Debye screening in QCD: a tricky concept
in leading order QCD (O. Philipsen, hep-ph/ ) vv

5. don’t give up! ask lattice QCD
Karsch, et al.
running coupling
coupling drops off for r > 0.3 fm

6. Implications of lD ~ 0.3 fm use to estimate Coupling parameter, G
G = <PE>/<KE> but also G = 1/ND
for lD = 0.3fm and e = 15 GeV/fm3
VD = 4/3 p lD3 = fm3
ED = 1.7 GeV
to convert to number of particles, use gT or g2T
for T ~ 2Tc and g2 = 4
get ND = 1.2 – 2.5
G ~ 1
NB: for G ~ 1
plasma is NOT fully screened – it’s strongly coupled!

7. Implications for properties & observables
For incomplete screening/strongly coupled QGP
range of interaction remains significant
sinteraction > spQCD
 collisions should be important!
Transport in QGP at RHIC should be very interesting!
transport of particles → diffusion
transport of energy by particles → thermal conductivity
transport of momentum by particles → viscosity
transport of charge by particles → electrical conductivity

8. everyone gets flat RAA via radiative energy loss only
A, Majumder
(Quark Matter 05)
Dainese, talk at PANIC05
AMY

9. can another observable distinguish eloss details?
open question #1:
can another observable distinguish eloss details?
RAA vs. reaction plane & dihadron yields

10. RAA of e± from heavy flavors was a shock
Inclusion of collisional energy loss leads to better agreement with single electron data, even for dNg/dy=1000.
NB: effect of collisional energy
loss for light quarks…
Wicks, Horowitz, Djordjevic, & Gyulassy, nucl-th/

11. others say maybe collisions not needed
BUT v2 is small…

12. diffusion = transport of particles by collisions
PHENIX preliminary
D = 1/3 <v> lmfp = <v>/ 3rs
D  collision time
→ relaxation time
Moore & Teaney
PRC71, , ‘05
D ~ 3/(2pT) is small!
→ strong interaction of c quarks
larger D →less charm e loss
fewer collisions, smaller v2

13. how important are collisions?
open question #2
how important are collisions?
strong coupling = incomplete color screening
→ interactions longer range than expected from pQCD
→ transport processes complicated & important
plasma physicists study with molecular dynamics, Fokker-Planck equation, …
effect of collisions is being studied by all groups
(it’s a hard problem)
We are starting to extract transport properties
low diffusivity & viscosity

14. and recall
result from
Wicks, et al
for light
quarks!

15. open question #3
shouldn’t we revisit the plasma density conclusions from radiative energy loss?
even if collisions prove unimportant, we need to agree
on the meaning/value of qhat and “L”
but perhaps perturbative radiation processes aren’t
the full/correct way to study the problem??

16. use AdS/CFT correspondence ↔  coupling

17. WHERE are the $&#*)^% B mesons ??!!!??
open question #4
WHERE are the $&#*)^% B mesons ??!!!??
Hendrik, Greco, Rapp
nucl-th/
w.o. B meson (c flow)
w. B meson (c,b flow)

18. need better measurements!
inner trackers for PHENIX and STAR
+ RHIC II luminosity!
PHENIX
STAR

19. BTW: What IS the charm cross section?

20. need work by experiment and theory both!
sort out difference between STAR & PHENIX
(factor of ~ 2)
beat down the uncertainties
better statistics & better control of systematics
upgrades and luminosity will provide the tools
theory
charm underprediction by pQCD is not new
NLO doesn’t fix it all
NNLO?
another look at resummation of hard processes?

21. open question #5 how do we use jets to probe the medium?
5a: is there evidence that deposited energy produces density waves of some kind?
progress
fact 1: Dh “ridge” on the near side
fact 2: there is evidence for cone-like emission
fact 3: a cone-like emission pattern CAN survive
issue: going from here to physics quantities
5b: what is the fragment chemistry trying to tell us?

22. the ridge J. Putschke Au+Au 0-10% preliminary 3<pt,trigger<4 GeV
pt,assoc.>2 GeV
preliminary
“jet” slope
ridge slope
inclusive slope

23. evidence for a density wave in the plasma?
E. Shuryak
g radiates energy
kick particles in the plasma
accelerate them along the jet
CAN WE DO THIS?????
= +/-1.23=1.91,4.37 → cs ~ 0.33
(√0.33 in QGP, 0.2 in hadron gas)
PHENIX
dN/d(Df)
p/ p p/ p Df
M.Miller, QM04
(1/Ntrig)dN/d(Df)
STAR Preliminary

24. not an experi-mental artefact, part I
PHENIX preliminary
not an experi-mental artefact, part I
PHENIX preliminary
J. Jia

25. not an experimental artefact, part II
Au+Au Central 0-12% Triggered
Δ1
Δ2
d+Au
J. Ulery

26. an experimental artefact

27. generally
a phenomenon
in crystals but
not liquids

28. immediate thermalization in flowing system
U. Heinz

29. deposited energy doesn’t thermalize so fast
T. Renk Df Dh
Dh distribution +
longitudinal expansion
depopulate Df = p region & shift Mach peak

30. hadrochemistry of jet-associated particles
STAR preliminary
Jet + Ridge
STAR preliminary
Jet
J. Bielcikova
jet core yields unchanged
chemistry constant
jet + (less) ridge
v. central: baryon+meson
drops toward reco expectation
A. Sickles
meson-meson
baryon-
meson
jet & ridge similar but not
identical for Npart<50
K trigger typical meson??

31. to get medium properties from jet interactions
Need better data!
smaller statistical & systematic uncertainties
scan in particle type, trigger & associated pT
further explore 3 (& more) particle correlations
on theory side:
combine dynamics and hadronization models
get quantitative
pre- & post-dictions of experimental observables
relate agreement to medium properties
figure out implications of hadrochemistry
can they reflect correlations in the medium?

32. the open questions
can an observable beyond RAA distinguish eloss details?
how important are collisions?
shouldn’t we revisit the plasma density conclusions from radiative energy loss?
where are the B mesons ?
how do we use jets to probe the medium?

33. a BIG thank you to the organizers of this fascinating stimulating
conclusion
a BIG thank you to the organizers of this
fascinating
stimulating
wonderful
scenic
meeting!

34. Jet tomography at RHIC II to go beyond <r>
jet quenching vs. system size, energy
→ parton & energy density for EOS
→ vary pT to probe medium coupling,
early development of system
golden channel: g-jet correlations
g fixes jet energy
flavor-tagged jets to sort out g vs. q energy loss
need detector upgrades (calorimeter coverage, DAQ)
must have RHIC II’s increased luminosity for:
statistics for clean g-jet & multi-hadron correlations
system scan in a finite time
cross section is small, so rate is low

35. radiation vs. collisions? consider leptons in matter
electrons stop in matter
g (bremsstrahlung) radiation
muons have long range
radiation is suppressed by the large mass
dominant energy loss mechanism is via collisions
implication
use heavy quarks as second kind of probe
collisions should be important for c, b quarks
is light quark energy loss radiation dominated?
EM plasmas → no
radiation: blackbody, bremsstrahlung, collisional, recombination

36. collective effects
a basic feature distinguishing plasmas from ordinary matter
simultaneous interaction of each charged particle with a considerable number of others
due to long range of the forces
EM plasma: charge-charge & charge-neutral interactions
charge-neutral dominates in weakly ionized plasmas
neutrals interact via distortion of e cloud by charges
very sensitive to coupling, viscosity…
magnetic fields generated by moving charges give rise to magnetic interactions

37. strong elliptic flow; scales w/ number of quarks

38. minimum h at phase boundary?
seen in strongly coupled dusty plasma
MD: solve the
equations of motion
for massive particles
subject to (screened)
interaction potential
follow evolution of
particle distribution
function (&correlations)
solve coupled diff.eq’s
over nearby space
density-density
correlations → h
B. Liu and J. Goree, cond-mat/
minimum arises because kinetic part of h decreases with G & potential part increases

39. challenge: can a jet excite a density wave in the plasma?
g radiates energy
kick particles in the plasma
accelerate them along the jet
non-equilibrium process
M.Miller, QM04
(1/Ntrig)dN/d(Df)
STAR Preliminary
PHENIX
dN/d(Df)
p/ p p/ p Df

40. generally
a phenomenon
in crystals but
not liquids

41. Energy density of matter
high energy density:
e > 1011 J/m3
P > 1 Mbar
I > 3 X 1015W/cm2
Fields > 500 Tesla
QGP energy density
> 1 GeV/fm3
i.e. > 1030 J/cm3

42. backup slides

43. plasma ionized gas which is macroscopically neutral
exhibits collective effects
interactions among charges of multiple particles
spreads charge out into characteristic (Debye) length, lD
multiple particles inside this length
they screen each other
plasma size > lD
“normal” plasmas are electromagnetic (e + ions)
quark-gluon plasma interacts via strong interaction
color forces rather than EM
exchanged particles: g instead of g

44. screening masses from gluon propagator
Screening mass, mD, defines inverse length scale
Inside this distance, an equilibrated plasma is sensitive to insertion of a static source
Outside it’s not.
Nakamura, Saito & Sakai, hep-lat/
T dependence of electric &
magnetic screening masses
Quenched lattice study
of gluon propagator
figure shows:
mD,m= 3Tc, mD,e= 6Tc at 2Tc
lD ~ 0.4 & 0.2 fm
magnetic screening mass is non-zero
not very gauge-dependent, but DOES
grow w/ lattice size (long range is important)

45. data + hydrodynamics → very low viscosity
Ideal hydrodynamics (h/S =0)
enough to conclude viscosity=0?
Deviations → viscous effects?
Kolb, et al
sort out via 3D hydro +
measure v2 vs. v3, v4
scan in system size & energy
c, W, X, f flows to separate late stage dissipation from early viscous effects
 RHIC II luminosity
note: softer than hadronic EOS!!
RHIC viscosity has drawn great interest from other fields
including string theorists,
who conjecture a lower bound h/S ≥ (h/4p)

46. plasma properties known, so far
Extract from models, constrained by data
Energy loss <dE/dz> (GeV/fm)
7-10
0.5 in cold matter
Energy density (GeV/fm3)
14-20
>5.5 from ET data
above hadronic E density!
dN(gluon)/dy
~1000
From energy loss, hydro huge!
T (MeV)
Experimentally unknown as yet
Equilibration time t0 (fm/c)
0.6
From hydro initial condition; cascade agrees very fast!
NB: plasma folks have same problem & use same technique
Opacity (L/mean free path)
3.5
Based on energy loss theory

47. baryon puzzle…
baryons enhanced for pT < 5 GeV/c
RAA

48. PHENIX preliminary
0-5%
PHENIX preliminary

49. use this technique to measure viscosity
melt crystal with laser light
induce a shear flow (laminar)
image the dust to get velocity
study:
spatial profiles vx(y)
moments, fluctuations → T(x,y)
curvature of velocity profile
→ drag forces
viscous transport of drag in
 direction from laser
compare to viscous hydro.
extract h/r
shear viscosity/mass density
PE vs. KE competition governs
coupling & phase of matter
Csernai,Kapusta,McLerran nucl-th/

50. look at radiated & “probe” particles
as a function of transverse momentum
pT = p sin q (with respect to beam direction)
90° is where the action is (max T, r)
midway between the two beams!
pT < 1.5 GeV/c
“thermal” particles
radiated from bulk of the medium
internal plasma probes
pT > 3 GeV/c
jets (hard scattered q or g)
heavy quarks, direct photons
produced early→“external” probe

51. Fast equilibration, high opacity (even for charm): how?
Molnar
multiple collisions using free q,g scattering cross sections doesn’t work!
need s x50 in the medium
Lattice QCD shows qq
resonant states at T > Tc, also implying high interaction cross sections
Hatsuda, et al.

53. Plasma Coulomb coupling parameter G
ratio of mean potential energy to mean kinetic energy
a = interparticle distance
e = charge
T = temperature
typically a small number in a normal, fully shielded plasma
G = 1/(number particles in Debye sphere)
when G > 1 have a strongly coupled, or non-Debye plasma
many-body spatial correlations exist
behave like liquids, or even crystals when G > 150
lD < a

54. estimate G using this use l=0.2 fm from electric screening mass
e=15 GeV/fm3 from hydro initial conditions constrained by v2
density from dE/dx constrained by RAA
put them together: get 0.5 GeV inside Debye sphere
FEW particles! ~1
→ G ~ 1
 quark gluon plasma should be a strongly coupled plasma
As in warm, dense plasma at lower (but still high) T
dusty plasmas, cold atom systems
such EM plasmas are known to behave as liquids!

55. away side jets are strongly modified by the medium

56. but it’s not very sensitive to DE distribution
T. Renk

57. v2 becomes smaller at large pT
D. Morrison, SQM’06

58. Collisional energy loss
Radiative energy loss
Collisional energy loss
Radiative energy loss comes from the processes which there are more outgoing than incoming particles:
Collisional energy loss comes from the processes which have the same number of incoming and outgoing particles:
0th order
0th order
1st order
M. Djordjevic

59. Collisional v.s. medium induced radiative energy loss
M.D., nucl-th/
Collisional and radiative energy losses are comparable!
Complementary approach by A. Adil et al., nucl-th/ : consistent results obtained.