Presentation on theme: "Open Questions: Jets and Heavy Quarks"— Presentation transcript:
1 Open Questions: Jets and Heavy Quarks (not a summary)Barbara JacakStony Brook UniversityJune 15, 2006
2 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 RHICWhere 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 plasmacharged particles arrange themselves so as to effectively shield any electrostatic fields within a distance lDlD = e0kTnee2Debye sphere = sphere with radius lDnumber electrons inside Debye sphere is typically largeND= N/VD= rVD VD= 4/3 p lD31/2in 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 couplingcoupling 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/NDfor lD = 0.3fm and e = 15 GeV/fm3VD = 4/3 p lD3 = fm3ED = 1.7 GeVto convert to number of particles, use gT or g2Tfor T ~ 2Tc and g2 = 4get ND = 1.2 – 2.5G ~ 1NB: for G ~ 1plasma is NOT fully screened – it’s strongly coupled!
7 Implications for properties & observables For incomplete screening/strongly coupled QGPrange of interaction remains significantsinteraction > spQCD 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
8 everyone gets flat RAA via radiative energy loss only A, Majumder(Quark Matter 05)Dainese, talk at PANIC05AMY
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 energyloss 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 preliminaryD = 1/3 <v> lmfp = <v>/ 3rsD collision time→ relaxation timeMoore & TeaneyPRC71, , ‘05D ~ 3/(2pT) is small!→ strong interaction of c quarkslarger D →less charm e lossfewer collisions, smaller v2
13 how important are collisions? open question #2how important are collisions?strong coupling = incomplete color screening→ interactions longer range than expected from pQCD→ transport processes complicated & importantplasma 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
14 and recallresult fromWicks, et alfor lightquarks!
15 open question #3shouldn’t we revisit the plasma density conclusions from radiative energy loss?even if collisions prove unimportant, we need to agreeon the meaning/value of qhat and “L”but perhaps perturbative radiation processes aren’tthe full/correct way to study the problem??
20 need work by experiment and theory both! sort out difference between STAR & PHENIX(factor of ~ 2)beat down the uncertaintiesbetter statistics & better control of systematicsupgrades and luminosity will provide the toolstheorycharm underprediction by pQCD is not newNLO doesn’t fix it allNNLO?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?progressfact 1: Dh “ridge” on the near sidefact 2: there is evidence for cone-like emissionfact 3: a cone-like emission pattern CAN surviveissue: going from here to physics quantities5b: 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 GeVpreliminary“jet” sloperidge slopeinclusive slope
23 evidence for a density wave in the plasma? E. Shuryakg radiates energykick particles in the plasmaaccelerate them along the jetCAN WE DO THIS?????= +/-1.23=1.91,4.37 → cs ~ 0.33(√0.33 in QGP, 0.2 in hadron gas)PHENIXdN/d(Df)p/ p p/ pDfM.Miller, QM04(1/Ntrig)dN/d(Df)STAR Preliminary
24 not an experi-mental artefact, part I PHENIX preliminarynot an experi-mental artefact, part IPHENIX preliminaryJ. Jia
25 not an experimental artefact, part II Au+Au Central 0-12% TriggeredΔ1Δ2d+AuJ. Ulery
28 immediate thermalization in flowing system U. Heinz
29 deposited energy doesn’t thermalize so fast T. RenkDfDhDh distribution +longitudinal expansiondepopulate Df = p region & shift Mach peak
30 hadrochemistry of jet-associated particles STAR preliminaryJet + RidgeSTAR preliminaryJetJ. Bielcikovajet core yields unchangedchemistry constantjet + (less) ridgev. central: baryon+mesondrops toward reco expectationA. Sicklesmeson-mesonbaryon-mesonjet & ridge similar but notidentical for Npart<50K trigger typical meson??
31 to get medium properties from jet interactions Need better data!smaller statistical & systematic uncertaintiesscan in particle type, trigger & associated pTfurther explore 3 (& more) particle correlationson theory side:combine dynamics and hadronization modelsget quantitativepre- & post-dictions of experimental observablesrelate agreement to medium propertiesfigure out implications of hadrochemistrycan they reflect correlations in the medium?
32 the open questionscan 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 conclusiona BIG thank you to the organizers of thisfascinatingstimulatingwonderfulscenicmeeting!
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 systemgolden channel: g-jet correlationsg fixes jet energyflavor-tagged jets to sort out g vs. q energy lossneed detector upgrades (calorimeter coverage, DAQ)must have RHIC II’s increased luminosity for:statistics for clean g-jet & multi-hadron correlationssystem scan in a finite timecross section is small, so rate is low
35 radiation vs. collisions? consider leptons in matter electrons stop in matterg (bremsstrahlung) radiationmuons have long rangeradiation is suppressed by the large massdominant energy loss mechanism is via collisionsimplicationuse heavy quarks as second kind of probecollisions should be important for c, b quarksis light quark energy loss radiation dominated?EM plasmas → noradiation: blackbody, bremsstrahlung, collisional, recombination
36 collective effectsa basic feature distinguishing plasmas from ordinary mattersimultaneous interaction of each charged particle with a considerable number of othersdue to long range of the forcesEM plasma: charge-charge & charge-neutral interactionscharge-neutral dominates in weakly ionized plasmasneutrals interact via distortion of e cloud by chargesvery 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 plasmaMD: solve theequations of motionfor massive particlessubject to (screened)interaction potentialfollow evolution ofparticle distributionfunction (&correlations)solve coupled diff.eq’sover nearby spacedensity-densitycorrelations → hB. 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 energykick particles in the plasmaaccelerate them along the jetnon-equilibrium processM.Miller, QM04(1/Ntrig)dN/d(Df)STAR PreliminaryPHENIXdN/d(Df)p/ p p/ pDf
43 plasma ionized gas which is macroscopically neutral exhibits collective effectsinteractions among charges of multiple particlesspreads charge out into characteristic (Debye) length, lDmultiple particles inside this lengththey screen each otherplasma size > lD“normal” plasmas are electromagnetic (e + ions)quark-gluon plasma interacts via strong interactioncolor forces rather than EMexchanged particles: g instead of g
44 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.Nakamura, Saito & Sakai, hep-lat/T dependence of electric &magnetic screening massesQuenched lattice studyof gluon propagatorfigure shows:mD,m= 3Tc, mD,e= 6Tc at 2TclD ~ 0.4 & 0.2 fmmagnetic screening mass is non-zeronot very gauge-dependent, but DOESgrow 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 alsort out via 3D hydro +measure v2 vs. v3, v4scan in system size & energyc, W, X, f flows to separate late stage dissipation from early viscous effects RHIC II luminositynote: softer than hadronic EOS!!RHIC viscosity has drawn great interest from other fieldsincluding string theorists,who conjecture a lower bound h/S ≥ (h/4p)
46 plasma properties known, so far Extract from models, constrained by dataEnergy loss <dE/dz> (GeV/fm)7-100.5 in cold matterEnergy density (GeV/fm3)14-20>5.5 from ET dataabove hadronic E density!dN(gluon)/dy~1000From energy loss, hydro huge!T (MeV)Experimentally unknown as yetEquilibration time t0 (fm/c)0.6From hydro initial condition; cascade agrees very fast!NB: plasma folks have same problem & use same techniqueOpacity (L/mean free path)3.5Based on energy loss theory
47 baryon puzzle…baryons enhanced for pT < 5 GeV/cRAA
49 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 forcesviscous transport of drag in direction from lasercompare to viscous hydro.extract h/rshear viscosity/mass densityPE vs. KE competition governscoupling & phase of matterCsernai,Kapusta,McLerran nucl-th/
50 look at radiated & “probe” particles as a function of transverse momentumpT = 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” particlesradiated from bulk of the mediuminternal plasma probespT > 3 GeV/cjets (hard scattered q or g)heavy quarks, direct photonsproduced early→“external” probe
51 Fast equilibration, high opacity (even for charm): how? Molnarmultiple collisions using free q,g scattering cross sections doesn’t work!need s x50 in the mediumLattice QCD shows qqresonant states at T > Tc, also implying high interaction cross sectionsHatsuda, et al.
53 Plasma Coulomb coupling parameter G ratio of mean potential energy to mean kinetic energya = interparticle distancee = chargeT = temperaturetypically a small number in a normal, fully shielded plasmaG = 1/(number particles in Debye sphere)when G > 1 have a strongly coupled, or non-Debye plasmamany-body spatial correlations existbehave like liquids, or even crystals when G > 150lD < 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 v2density from dE/dx constrained by RAAput them together: get 0.5 GeV inside Debye sphereFEW particles! ~1→ G ~ 1 quark gluon plasma should be a strongly coupled plasmaAs in warm, dense plasma at lower (but still high) Tdusty plasmas, cold atom systemssuch 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 lossCollisional energy lossRadiative 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 order0th order1st orderM. 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.