Open Questions:Jets and Heavy Quarks

(not a summary)

Barbara Jacak

Stony Brook University

June 15, 2006

Open questions as of June 9

What are the implications of incomplete color screening?collisional vs. radiative energy losstransport 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?

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 D

D = 0kT

-------

nee2

Debye sphere = sphere with radius D

number electrons inside Debye sphere is typically largeND= N/VD= VD VD= 4/3 D

3

1/2

in strongly coupled plasmas it’s 1

Debye screening in QCD: a tricky concept

in leading order QCD (O. Philipsen, hep-ph/0010327)

vv

don’t give up! ask lattice QCD

run

nin

g co

up

ling

coupling drops off for r > 0.3 fm

Karsch, et al.

Implications of D ~ 0.3 fm

use to estimate Coupling parameter,

= <PE>/<KE> but also = 1/ND

for D = 0.3fm and = 15 GeV/fm3

VD = 4/3 D3 = 0.113 fm3

ED = 1.7 GeVto convert to number of particles, use gT or g2T

for T ~ 2Tc and g2 = 4

get ND = 1.2 – 2.5 ~ 1

NB: for ~ 1plasma is NOT fully screened – it’s strongly coupled!

Implications for properties & observables

For incomplete screening/strongly coupled QGPrange of interaction remains significantinteraction > pQCD

collisions should be important!

Transport in QGP at RHIC should be very interesting!transport of particles → diffusiontransport of energy by particles → thermal conductivitytransport of momentum by particles → viscositytransport of charge by particles → electrical

conductivity

everyone gets flat RAA via radiative energy loss only

(Quark Matter 05)

Dainese, talk at PANIC05AMY

A, Majumder

open question #1:

can another observable distinguish eloss details?can another observable distinguish eloss details?

RAA vs. reaction plane & dihadron yields

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.

Wicks, Horowitz, Djordjevic, & Gyulassy, nucl-th/0512076

NB: effect of collisional energyloss for light quarks…

others say maybe collisions not needed

BUT v2 is small…

diffusion = transport of particles by collisions

PHENIX preliminary

Moore & TeaneyPRC71, 064904, ‘05

D ~ 3/(2T) is small! → strong interaction of c quarks

larger D →less charm e loss fewer collisions, smaller v2

D = 1/3 <v> mfp = <v>/ 3D collision time → relaxation time

open question #2

how important are collisions?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 propertieslow diffusivity & viscosity

and recallresult fromWicks, et alfor light quarks!

open question #3

shouldn’t we revisit the plasma density shouldn’t we revisit the plasma density conclusions from radiative energy loss?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??

use AdS/CFT correspondence ↔ coupling

open question #4

WHERE are the $&#*)^% B mesons ??!!!??WHERE are the $&#*)^% B mesons ??!!!??

Hendrik, Greco, Rappnucl-th/0508055

w.o. B meson (c flow)w. B meson (c,b flow)

need better measurements!

inner trackers for PHENIX and STAR

STAR

PHENIX

+ RHIC II luminosity!

BTW: What IS the charm cross section?

need work by experiment and theory both!

experimentsort out difference between STAR & PHENIX

(factor of ~ 2)beat down the uncertainties

better statistics & better control of systematicsupgrades and luminosity will provide the tools

theorycharm underprediction by pQCD is not newNLO doesn’t fix it all

NNLO? another look at resummation of hard processes?

open question #5

how do we use jets to probe the medium?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: “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?

the ridge

Au+Au 0-10%preliminary

3<pt,trigger<4 GeV

pt,assoc.>2 GeV

J. Putschke

preliminary“jet” sloperidge slopeinclusive slope

evidence for a density wave in the plasma?

M.Miller, QM04

(1/N

trig)d

N/d

()

STAR Preliminary

cGeVp

cGevpassocT

trigT

/42.0

/64

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(

)

E. Shuryakg radiates energykick particles in the plasmaaccelerate them along the jet

not an experi-mental

artefact,part I

PHENIX preliminary

PHENIX preliminary

J. Jia

not an experimental artefact, part IIAu+Au Central 0-12% Triggered

Δ1

Δ2

d+Au

Δ1

J. Ulery

an experimental artefact

generallya phenomenonin crystals butnot liquids

immediate thermalization in flowing systemU. Heinz

deposited energy doesn’t thermalize so fast

T. Renk

distribution +longitudinal expansion depopulate region & shift Mach peak

STAR preliminary

Jet + Ridge

STAR preliminary

Jethadrochemistry of jet-associated particles

jet & ridge similar but not identical for Npart<50K trigger typical meson??

J. Bielcikova

jet core yields unchangedchemistry constant

jet + (less) ridgev. central: baryon+meson drops toward reco expectation

A. Sickles

meson-meson

baryon-meson

to get medium properties from jet interactions

Need better data!smaller statistical & systematic uncertaintiesscan in particle type, trigger & associated pT

further explore 3 (& more) particle correlations

on theory side:combine dynamics and hadronization modelsget quantitative

pre- & post-dictions of experimental observablesrelate agreement to medium properties

figure out implications of hadrochemistrycan they reflect correlations in the medium?

the open questions

1)1) can an observable beyond Rcan an observable beyond RAAAA distinguish eloss distinguish eloss

details?details?

2)2) how important are collisions?how important are collisions?

3)3) shouldn’t we revisit the plasma density shouldn’t we revisit the plasma density conclusions from radiative energy loss?conclusions from radiative energy loss?

4)4) where are the B mesons ?where are the B mesons ?

5)5) how do we use jets to probe the medium?how do we use jets to probe the medium?

conclusion

a BIG thank you to the organizers of this

fascinating

stimulating

wonderful

scenic

meeting!

Jet tomography at RHIC II to go beyond <>

jet quenching vs. system size, energy→ parton & energy density for EOS→ vary pT to probe medium coupling,

early development of system golden channel: -jet correlations

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 -jet & multi-hadron correlationssystem scan in a finite time

cross section is small, so rate is low

radiation vs. collisions? consider leptons in matter

electrons stop in matter (bremsstrahlung) radiation

muons have long range radiation is suppressed by the large massdominant 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 → noradiation: blackbody, bremsstrahlung, collisional, recombination

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 forcesEM plasma: charge-charge & charge-neutral interactions

charge-neutral dominates in weakly ionized plasmasneutrals interact via distortion of e cloud by charges

very sensitive to coupling, viscosity…

magnetic fields generated by moving charges give rise to magnetic interactions

strong elliptic flow; scales w/ number of quarks

minimum at phase boundary?

B. Liu and J. Goree, cond-mat/0502009

minimum arises because kinetic part of decreases with & potential part increases

MD: solve theequations of motionfor massive particlessubject to (screened)interaction potential

follow evolution ofparticle distributionfunction (&correlations)

solve coupled diff.eq’sover nearby space

density-densitycorrelations →

seen in strongly coupled dusty plasma

challenge: can a jet excite a density wave in the plasma?

M.Miller, QM04

(1/N

trig)d

N/d

()

STAR Preliminary

cGeVp

cGevpassocT

trigT

/42.0

/64

PHENIX

d

N/d

()

g radiates energykick particles in the plasmaaccelerate them along the jetnon-equilibrium process

generallya phenomenonin crystals butnot liquids

Energy density of matter

high energy density: > 1011 J/m3

P > 1 MbarI > 3 X 1015W/cm2 Fields > 500 Tesla

QGP energy density > 1 GeV/fm3

i.e. > 1030 J/cm3

backup slides

plasma

ionized gas which is macroscopically neutralexhibits collective effects

interactions among charges of multiple particlesspreads charge out into characteristic (Debye) length, D

multiple particles inside this lengththey screen each other

plasma size > D

“normal” plasmas are electromagnetic (e + ions)quark-gluon plasma interacts via strong interaction

color forces rather than EMexchanged particles: g instead of

screening masses from gluon propagator

Screening mass, mD, defines inverse length scaleInside this distance, an equilibrated plasma is sensitive to

insertion of a static sourceOutside it’s not.

T dependence of electric &magnetic screening massesQuenched lattice studyof gluon propagator

figure shows: mD,m= 3Tc, mD,e= 6Tc at 2Tc D ~ 0.4 & 0.2 fm

magnetic screening mass is non-zeronot very gauge-dependent, but DOESgrow w/ lattice size (long range is important)

Nakamura, Saito & Sakai, hep-lat/0311024

data + hydrodynamics → very low viscosity

Kolb, et al

RHIC viscosity has drawn great interest from other fieldsincluding string theorists,

who conjecture a lower bound /S ≥ (h/4)

note: softer than hadronic EOS!!

sort out via 3D hydro +measure v2 vs. v3, v4

scan in system size & energyc, flows to separate late stage dissipation from early viscous effects RHIC II luminosity

Ideal hydrodynamics (/S =0) enough to conclude viscosity=0?Deviations → viscous effects?

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) 380-400

Experimentally unknown as yet

Equilibration time0 (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

baryon puzzle…

baryons enhanced for pT < 5 GeV/c

RAA

0-5%

PHENIX preliminary

PHENIX preliminary

use this technique to measure viscosity

melt crystal with laser lightinduce a shear flow (laminar)image the dust to get velocitystudy: spatial profiles vx(y) moments, fluctuations → T(x,y) curvature of velocity profile → drag forces viscous transport of drag in direction from lasercompare to viscous hydro. extract shear viscosity/mass densityPE vs. KE competition governs coupling & phase of matterCsernai,Kapusta,McLerran nucl-th/0604032

look at radiated & “probe” particles

as a function of transverse momentumpT = p sin with respect to beam direction)

90° is where the action is (max T, )midway between the two beams!

pT < 1.5 GeV/c

“thermal” particles radiated from bulk of the mediuminternal plasma probes

pT > 3 GeV/c

jets (hard scattered q or g)heavy quarks, direct photons produced early→“external” probe

Fast equilibration, high opacity (even for charm): how?

multiple collisions using free q,g scattering cross sections doesn’t work! need x50 in the medium

Molnar

Lattice QCD shows qqresonant states at T > Tc, also implying high interaction cross sections

Hatsuda, et al.

Plasma Coulomb coupling parameter

ratio of mean potential energy to mean kinetic energy

a = interparticle distancee = chargeT = temperature

typically a small number in a normal, fully shielded plasma = 1/(number particles in Debye sphere)

when > 1 have a strongly coupled, or non-Debye plasmamany-body spatial correlations existbehave like liquids, or even crystals when > 150 D < a

estimate using this

use =0.2 fm from electric screening mass

=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

→ ~ 1

quark gluon plasma should be a strongly coupled plasmaAs in warm, dense plasma at lower (but still high) Tdusty plasmas, cold atom systems

such EM plasmas are known to behave as liquids!

away side jets are strongly modified by the medium

but it’s not very sensitive to E distribution

T. Renk

v2 becomes smaller at large pT

D. Morrison, SQM’06

Radiative energy loss Collisional energy loss

Collisional energy loss comes from the processes which have the same number of incoming and outgoing particles:

Radiative energy loss comes from the processes which there are more outgoing than incoming particles:

0th order

1st order

0th order

M. Djordjevic

Collisional v.s. medium induced radiative energy loss

Collisional and radiative energy losses are comparable!

M.D., nucl-th/0603066

Complementary approach by A. Adil et al., nucl-th/0606010: consistent results obtained.