# Open Questions: Jets and Heavy Quarks

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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 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?

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

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

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

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!

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

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

can another observable distinguish eloss details?
open question #1: 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. NB: effect of collisional energy loss for light quarks… Wicks, Horowitz, Djordjevic, & Gyulassy, nucl-th/

others say maybe collisions not needed
BUT v2 is small…

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

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

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

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??

use AdS/CFT correspondence ↔  coupling

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)

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

BTW: What IS the charm cross section?

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?

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?

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

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

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

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

an experimental artefact

generally a phenomenon in crystals but not liquids

immediate thermalization in flowing system
U. Heinz

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

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??

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?

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?

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!

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

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

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

strong elliptic flow; scales w/ number of quarks

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

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

generally a phenomenon in crystals but not liquids

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

backup slides

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

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)

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)

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

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

PHENIX preliminary 0-5% PHENIX preliminary

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/

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

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.

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

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!

away side jets are strongly modified by the medium

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

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

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

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.