Presentation on theme: "Open Questions: Jets and Heavy Quarks (not a summary) Barbara Jacak Stony Brook University June 15, 2006."— Presentation transcript:
Open Questions: Jets and Heavy Quarks (not a summary) Barbara Jacak Stony Brook University June 15, 2006
Open questions as of June 9 l What are the implications of incomplete color screening? collisional vs. radiative energy loss transport properties of quark gluon plasma at RHIC l Where are the B mesons in single electron R AA & flow? l Need to take another look at parton densities extracted from jet quenching – impact of collisional energy loss? l Where DOES the energy lost by jets go? density waves (Mach cones)? caught up in longitudinal flow? thermalized?
Screening: Debye Length l 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 = 0 kT ------- n e e 2 Debye sphere = sphere with radius D l number electrons inside Debye sphere is typically large N D = N/V D = V D V D = 4/3 D 3 1/2 in strongly coupled plasmas it’s 1
Debye screening in QCD: a tricky concept l in leading order QCD (O. Philipsen, hep-ph/0010327) l vv
don’t give up! ask lattice QCD running coupling coupling drops off for r > 0.3 fm Karsch, et al.
Implications of D ~ 0.3 fm use to estimate Coupling parameter, = / but also = 1/N D for D = 0.3fm and = 15 GeV/fm 3 V D = 4/3 D 3 = 0.113 fm 3 E D = 1.7 GeV to convert to number of particles, use gT or g 2 T for T ~ 2T c and g 2 = 4 get N D = 1.2 – 2.5 ~ 1 NB: for ~ 1 plasma is NOT fully screened – it’s strongly coupled!
Implications for properties & observables l For incomplete screening/strongly coupled QGP range of interaction remains significant interaction > pQCD collisions should be important! l 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 (Quark Matter 05) Dainese, talk at PANIC05 AMY A, Majumder
open question #1: can another observable distinguish eloss details? R AA vs. reaction plane & dihadron yields
R AA of e± from heavy flavors was a shock Inclusion of collisional energy loss leads to better agreement with single electron data, even for dN g /dy=1000. Wicks, Horowitz, Djordjevic, & Gyulassy, nucl-th/0512076 NB: effect of collisional energy loss for light quarks…
others say maybe collisions not needed BUT v 2 is small…
diffusion = transport of particles by collisions PHENIX preliminary Moore & Teaney PRC71, 064904, ‘05 D ~ 3/(2 T) is small! → strong interaction of c quarks larger D →less charm e loss fewer collisions, smaller v 2 D = 1/3 mfp = / 3 D collision time → relaxation time
open question #2 how important are collisions? l 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, … l effect of collisions is being studied by all groups (it’s a hard problem) l 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? l even if collisions prove unimportant, we need to agree on the meaning/value of qhat and “L” l but perhaps perturbative radiation processes aren’t the full/correct way to study the problem??
need work by experiment and theory both! l experiment 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 l 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? l 5a: is there evidence that deposited energy produces density waves of some kind? progress fact 1: “ridge” on the near side l fact 2: there is evidence for cone-like emission l fact 3: a cone-like emission pattern CAN survive issue: going from here to physics quantities l 5b: what is the fragment chemistry trying to tell us?
evidence for a density wave in the plasma? M.Miller, QM04 (1/N trig )dN/d( ) STAR Preliminary CAN WE DO THIS????? +/- 1.23=1.91,4.37 → c s ~ 0.33 (√0.33 in QGP, 0.2 in hadron gas) PHENIX dN/d( ) E. Shuryak g radiates energy kick particles in the plasma accelerate them along the jet
not an experi- mental artefact, part I PHENIX preliminary J. Jia
not an experimental artefact, part II Au+Au Central 0-12% Triggered Δ1Δ1 Δ2Δ2 d+Au Δ1Δ1 J. Ulery
generally a phenomenon in crystals but not liquids
immediate thermalization in flowing system U. Heinz
deposited energy doesn’t thermalize so fast T. Renk distribution + longitudinal expansion depopulate region & shift Mach peak
STAR preliminary Jet + Ridge STAR preliminary Jet hadrochemistry of jet-associated particles jet & ridge similar but not identical for Npart<50 K trigger typical meson?? 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
to get medium properties from jet interactions l Need better data! smaller statistical & systematic uncertainties scan in particle type, trigger & associated p T further explore 3 (& more) particle correlations l 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 1) can an observable beyond R AA distinguish eloss details? 2) how important are collisions? 3) shouldn’t we revisit the plasma density conclusions from radiative energy loss? 4) where are the B mesons ? 5) 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 l jet quenching vs. system size, energy → parton & energy density for EOS → vary p T to probe medium coupling, early development of system golden channel: -jet correlations fixes jet energy l flavor-tagged jets to sort out g vs. q energy loss l need detector upgrades (calorimeter coverage, DAQ) l must have RHIC II’s increased luminosity for: statistics for clean -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 l electrons stop in matter (bremsstrahlung) radiation l muons have long range radiation is suppressed by the large mass dominant energy loss mechanism is via collisions l 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 l simultaneous interaction of each charged particle with a considerable number of others l 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 l very sensitive to coupling, viscosity… l 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 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 → seen in strongly coupled dusty plasma
challenge: can a jet excite a density wave in the plasma? M.Miller, QM04 (1/N trig )dN/d( ) STAR Preliminary PHENIX dN/d( ) g radiates energy kick particles in the plasma accelerate them along the jet non-equilibrium process
generally a phenomenon in crystals but not liquids
Energy density of matter high energy density: > 10 11 J/m 3 P > 1 Mbar I > 3 X 10 15 W/cm 2 Fields > 500 Tesla QGP energy density > 1 GeV/fm 3 i.e. > 10 30 J/cm 3
plasma l ionized gas which is macroscopically neutral exhibits collective effects l interactions among charges of multiple particles spreads charge out into characteristic (Debye) length, D multiple particles inside this length they screen each other plasma size > D l “normal” plasmas are electromagnetic (e + ions) quark-gluon plasma interacts via strong interaction color forces rather than EM exchanged particles: g instead of
screening masses from gluon propagator Screening mass, m D, defines inverse length scale Inside this distance, an equilibrated plasma is sensitive to insertion of a static source Outside it’s not. T dependence of electric & magnetic screening masses Quenched lattice study of gluon propagator figure shows: m D,m = 3Tc, m D,e = 6Tc at 2Tc D ~ 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) Nakamura, Saito & Sakai, hep-lat/0311024
data + hydrodynamics → very low viscosity Kolb, et al RHIC viscosity has drawn great interest from other fields including string theorists, who conjecture a lower bound /S ≥ (h/4 ) note: softer than hadronic EOS!! sort out via 3D hydro + measure v 2 vs. v 3, v 4 scan in system size & energy c, 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 (GeV/fm)7-100.5 in cold matter Energy density (GeV/fm 3 )14-20>5.5 from E T data above hadronic E density! dN(gluon)/dy~1000From energy loss, hydro huge! T (MeV)380- 400 Experimentally unknown as yet Equilibration time 0 (fm/c) 0.6From hydro initial condition; cascade agrees very fast! NB: plasma folks have same problem & use same technique Opacity (L/mean free path)3.5Based on energy loss theory
baryon puzzle… baryons enhanced for pT < 5 GeV/c R AA
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 v x (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 shear viscosity/mass density PE vs. KE competition governs coupling & phase of matter Csernai,Kapusta,McLerran nucl-th/0604032
look at radiated & “probe” particles l as a function of transverse momentum p T = p sin with respect to beam direction) 90° is where the action is (max T, ) midway between the two beams! l p T < 1.5 GeV/c “thermal” particles radiated from bulk of the medium internal plasma probes l p T > 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 qq resonant states at T > Tc, also implying high interaction cross sections Hatsuda, et al.
Plasma Coulomb coupling parameter l ratio of mean potential energy to mean kinetic energy a = interparticle distance e = charge T = temperature l 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 plasma many-body spatial correlations exist behave like liquids, or even crystals when > 150 D < a
estimate using this use =0.2 fm from electric screening mass =15 GeV/fm 3 from hydro initial conditions constrained by v 2 density from dE/dx constrained by R AA put them together: get 0.5 GeV inside Debye sphere FEW particles! ~1 → ~ 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 E distribution T. Renk
v 2 becomes smaller at large p T D. Morrison, SQM’06
Radiative energy lossCollisional 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: 0 th order 1 st order 0 th 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.