Experiments at RHIC: Exciting Discoveries & Future Plans
outline l introduction plasmas and strong coupling l collective effects hydrodynamics and viscosity l transmission of probes by the quark-gluon plasma l heavy quark probes diffusion color screening l Color Glass Condensate (discussed by Dima) l Plasma Physics of the Quark Gluon Plasma at RHIC II
required conditions (per lattice) T/T c Karsch, Laermann, Peikert ‘99 /T 4 T c ~ 170 ± 10 MeV (10 12 °K) ~ 3 GeV/fm 3 ~15% from ideal gas of weakly interacting quarks & gluons We now know this leaves room for a big deviation from weak coupling
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: 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 can 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! other strongly coupled plasmas behave as liquids, even crystals for ≥ 150 dusty plasmas, cold atoms+ions, warm dense matter
to study experimentally: 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
RHIC at Brookhaven National Laboratory Collide Au + Au ions for maximum volume s = 200 GeV/nucleon pair, p+p and d+A to compare
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 (electromagnetic) forces l magnetic fields generated by moving charges give rise to magnetic interactions
search for collectivity in QGP use “internal” probes – emitted particles dN/d ~ 1 + 2 v 2 (p T ) cos (2 ) + … “elliptic flow” Almond shape overlap region in coordinate space x y z momentum space
v 2 is large & reproduced by hydrodynamics large pressure buildup anisotropy happens fast fast equilibration! Kolb, et al Hydrodynamics reproduces elliptic flow of q-q and 3q states Mass dependence requires softer than hadronic EOS!! NB: these calculations have viscosity ~ 0 “perfect” liquid (D. Teaney, PRC68, 2003)
Elliptic flow scales with number of quarks implication: quarks are the relevant degrees of freedom when the pressure is built up. Hadronization: quark coalescence Greco, Ko, Levai: PRC 68 (2003)034904
D. Morrison, SQM’06 at high p T v 2 reflects opacity of medium approximately expected level from jet quenching
hadrons q q leading particle leading particle schematic view of jet production for QGP: fast g and quarks probes must carry color charge transmission of probes which interact with plasma EM plasma: x-ray transmission
nuclear modification factor l photons escape plasma l pions and other hadrons: strong interaction, absorbed
flat R AA via radiative energy loss only (Quark Matter 05) Dainese, talk at PANIC05 AMY A, Majumder
R AA wrt reaction plane – more discriminating Energy loss depends on the path-length, expansion, collisions(?)
dihadrons: away side suppressed (low p T ) Pedestal&flow subtracted
some away side particles “reappear” at higher p T STAR nucl-ex/0604018 p T trigger > 8 GeV/c
away side yield: some jets escape, some eaten STAR nucl-ex/0604018 Note similarity of away side jet fragmentation. Only yield changes
how does plasma respond to deposited energy? 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 in jet direction: a sound wave
not an experi- mental artefact! PHENIX preliminary J. Jia
generally a phenomenon in crystals but not liquids
3 particle correlations support cone-like structure Au+Au Central 0-12% Triggered Δ1Δ1 Δ2Δ2 d+Au Δ1Δ1 J. Ulery, HP06
to further test interaction: heavy quarks ~ same E loss as u,d quarks energy loss not all radiative need collisions! charm also flows thermalization with the light quarks? not so easy!
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…
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
aim to measure the screening length J/ bound state of c and cbar quarks) Tests screening & confinement: do bound c + c survive the medium? or does QGP screening kill them? Look at R AA for J/ different bound states probe different lengths
At RHIC: CuCu 200 GeV/c AuAu 200 GeV/c dAu 200 GeV/c J/ muon arm 1.2 < |y| < 2.2 measured/expected
At RHIC: CuCu 200 GeV/c AuAu 200 GeV/c dAu 200 GeV/c AuAu ee 200 GeV/c CuCu ee 200 GeV/c J/ muon arm 1.2 < |y| < 2.2 J/ ee Central arm -0.35 < y < 0.35
At RHIC: CuCu 200 GeV/c AuAu 200 GeV/c dAu 200 GeV/c AuAu ee 200 GeV/c CuCu 62 GeV/c J/ muon arm 1.2 < |y| < 2.2 J/ ee Central arm -0.35 < y < 0.35 Factor ~3 suppression in central events CuCu ee 200 GeV/c
R AA vs N part : PHENIX and NA50 NA50 data normalized at NA50 p+p point. Suppression similar in the two experiments, although the collision energy is 10 times higher (200GeV in PHENIX & 17GeV in NA50)
What suppression should we expect? Models that were successful in describing SPS data fail to describe data at RHIC - but lattice QCD says bound states until ~2T c -
regeneratio n direct Recombined only Regeneration Narrowing of p T and y? p T broadening in between Thews direct & in-medium formation: some regeneration Recombination → narrower rapidity distribution with increasing N part BUT: From p+p to central Au+Au : no significant change in y distribution. No Recombination Thews et al. slide from T. Ullrich, HP06
Karsch, Kharzeev, Satz, hep-ph/0512239 Probe experimentally: onium spectroscopy 40% of J/ from and ’ decays they are screened but direct J/ not?
l moments of the distribution function of particles f(x,v) 0 th moment → particle density (n) higher moments are & temperature, pressure tensor, heat flux tensor opacity/transmission is a probe of choice l Transport properties (e.g. diffusion, viscosity) l Screening l Collective Effects hydrodynamic expansion, shock propagation, plasma waves → density correlations inside plasma l Radiation bremsstrahlung, blackbody, collisional and recombination l Plasma oscillations, instabilities Plasma Physics of the QGP with RHIC II plasma diagnostics:
l moments of the distribution function of particles f(x,v) 0 th moment → particle density (n) higher moments are & temperature, pressure tensor, heat flux tensor opacity/transmission is the first probe l Transport properties (e.g. diffusion, viscosity) l Screening l Collective Effects hydrodynamic expansion velocity, shock propagation → density correlations inside plasma l Radiation bremsstrahlung, blackbody, collisional and recombination l Plasma oscillations, instabilities Plasma properties & “diagnostics”
next step in transmission study: jet tomography 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 x10 for: statistics for clean -jet & multi-hadron correlations system scan in a finite time l tool of choice to study medium response/conductivity small , low rate
measure D & B decays; onium spectroscopy inner trackers for PHENIX and STAR STAR PHENIX + RHIC II luminosity!
need high luminosity to scan energy & system l pin down viscosity (and the collision dynamics) sort out via 3D hydro + measure v2 vs. v3, v4 c, flows to separate late stage dissipation from early viscous effects l thermalization plasma temperature via radiated * c, flows can we identify signals of early plasma instability? l probe 2q correlations via baryon production? direct search for density correlations poses a challenge to experiment & theory both!
conclusion - discoveries l The matter created shows collective flows developed early, with quarks/gluons the likely d.o.f. magnitude implies very low viscosity QGP behaves as a liquid similar to other strongly coupled plasmas! l Very opaque to color charged probes even charm quarks lose energy and flow! another result of strong coupling l J/ suppressed, but only partially perhaps screening + recombination from thermal bath? or sequential melting of and ` l Evidence of CGC initial state l The matter behaves as expected for a plasma!
conclusion – future plans l figure out the plasma physics of this new kind of matter: temperature transport properties collective excitations expansion dynamics density waves instabilities? screening length l need detector upgrades (planning, construction underway) high luminosity of RHIC II (x10 via electron cooling) low probes, scan properties with system & energy
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
conclusions l the matter formed at RHIC is a “perfect” fluid shows collective flows with small viscosity huge interaction cross sections, very opaque multiple collisions affect even heavy charm quarks l color is partially, but not completely, screened l this is like other strongly coupled plasmas as it should be → it is a plasma! neutrality scale > interparticle distance l How does this super high energy density plasma work? □ map properties of the new stuff at RHIC how does the plasma transport the “lost” energy? radiation rate? initial temperature achieved? (theory says ~380 MeV) □ collide Pb+Pb at the LHC for higher T initial reach ~ 800 MeV: is coupling strong or weak?
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
not an experi- mental artefact, part I PHENIX preliminary J. Jia
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
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
can get better agreement with data if add formation of “extra” J/ by coalescence of c and anti-c from the plasma caveat: not necessarily unique or correct explanation!
baryon puzzle… baryons enhanced for pT < 5 GeV/c R AA
NA50 and NA60 show suppression in Pb+Pb & In+In suppression follows system size Normal nuclear absorption from p+A data: = 4.18±0.35 mb At CERN (√s = 17 GeV) :
Is the energy density high enough? 5.5 GeV/fm 3 (200 GeV Au+Au) well above predicted transition! PRL87, 052301 (2001) R2R2 2c Colliding system expands: Energy to beam direction per unit velocity || to beam value is lower limit: longitudinal expansion rate, formation time overestimated
Saturation of gluons in initial state (colored glass condensate) Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse. Saturation at higher x at RHIC vs. HERA due to nuclear size suppressed jet cross section; no back-back pairs r/ gg g Mueller, McLerran, Kharzeev, … d + Au collisions cent/periph. (~R AA )
Why no energy loss for charm quarks? l “dead cone” predicted by Kharzeev and Dokshitzer, Phys. Lett. B519, 199 (1991) l Gluon bremsstrahlung: k T 2 = 2 t form / transverse momentum of radiated gluon p T in single scatt. mean free path ~ k T / gluon energy But radiation is suppressed below angles 0 = M q /E q soft gluon distribution is dP = s C F / d / k T 2 dk T 2 /(k T 2 + 2 0 2 ) 2 not small for heavy quarks! causes a dead cone
FONLL Predictions l Mateo Cacciari provided a prediction using the Fixed Order Next Leading Logarithm pQCD approach l His calculation agrees perfectly with our “poor man’s” HVQLIB+PYTHIA predictions l Data exceed the central theory curve by a factor of 2-3 l Possible explanations: NNLO contribution Fragmentation mechanisms need to be studied in more details
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
can get better agreement with data if add formation of “extra” J/ by coalescence of c and anti-c from the plasma caveat: not necessarily unique or correct explanation!
screening and thermal masses 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 significant not very gauge-dependent, but DOES grow w/ lattice size (long range is important) Nakamura, Saito & Sakai, hep-lat/0311024
use to get & compare to molecular dynamics 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 →
RHIC’s “perfect fluid” l one that exhibits ideal, non-dissipative hydrodynamics “not-quite-ideal” = can support a shear stress, ≠ 0 Viscosity is ~ / ~ √kT/ so viscosity increases with T decreases with Ideal hydrodynamics ( ~ 0) reproduces data at RHIC does this make sense to QCD? → ≥ h S : conjectured quantum mechanical limit “A Viscosity Bound Conjecture”, P. Kovtun, D.T. Son, A.O. Starinets, hep-th/0405231 P. KovtunD.T. SonA.O. Starinetshep-th/0405231 entropy density
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.
Color glass condensate? Hadron Punch Through Better agreement with BRAHMS data “normal” shadowing cannot explain (R. Vogt hep-ph/0405060) …sign of CGC? Kharzeev, hep-ph/0405045 Centrality, p T dependence ~ correct
Compare with BRAHMS Overall consistent. nucl-ex/0411054
deposited energy doesn’t thermalize so fast T. Renk distribution + longitudinal expansion depopulate region & shift Mach peak