Lecture 1 Heavy Ion Collisions: geometry, timescales, and collective behavior
1 Barbara Jacak Stony Brook University January 10, 2012 x y z
Outline(s)
TODAY Introduction and motivation to study the QCD plasma Stages and timescales in heavy ion collisions Geometry of the collisions Collective flow, hydrodynamics, and “the perfect liquid” Effects of fluctuations 2 Lectures on heavy ion collisions and quark gluon plasma Geometry and collective behavior in heavy ion collisions Energy loss and opacity in the quark gluon plasma Properties of strongly coupled plasmas and what the string theorists tell us Electromagnetic probes, screening, and QGP outlook
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Goal: Create the hottest matter on earth Heat to T > 1012 K last seen: ~ 1 second after the Big Bang! What is it? How does it work? What can we learn about the transition?
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Inside a nucleon Q2: momentum transfer in a collision x: momentum fraction carried by parton Gluon number is not fixed Virtual q-q pairs abundant Scatter electrons off a p slogQ2)n terms sln(1/x))n terms Theorists’ view Experimenter’s view
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Hot, dense QCD matter Gluons carry color interact among themselves theory is non-abelian Curious property at large distance: confinement of quarks in hadrons + +… asymptotic freedom At high temperature/density screening by produced colored particles Expect phase transition to deconfined quark gluon plasma Lattice QCD Tc ~ 170 MeV
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From the practical point of view What we see in the lab: All the particles which come out at the end of the collision
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RHIC at Brookhaven: heat matter up Collide Au + Au ions for maximum volume s = 200 GeV/nucleon pair, p+p and d+A to compare
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The Tools STAR specialty: large acceptance measurement of hadrons PHENIX specialty: rare probes, leptons, and photons
At the LHC 10 Pb+Pb at 2.76 TeV per NN Also, ATLAS and CMS take data with Pb+Pb
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heavy ion collision & diagnostics p, K, p, n, f, L, D, X, W, d, Hadrons qqq baryons & qq mesons reflect (thermal) properties when inelastic collisions stop not possible to measure as a function of time nature integrates over the entire collision history thermal radiation (g, g* e+e-, m+m-) Hot QGP e, pressure builds up Hard scattered q,g (short wavelength) probes of plasma formed time
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Questions Can we create quark gluon plasma in the lab? thermodynamic properties (equilibrium) T, P, r Equation Of State (relation btwn T, P, V, energy density) vsound, static screening length transport properties (non-equilibrium)* particle number, energy, momentum, charge diffusion sound viscosity conductivity In plasma: interactions among charges of multiple particles charge is spread, screened in characteristic (Debye) length, lD also the case for strong, rather than EM force *measuring these is new for nuclear/particle physics!
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study plasma with radiated & “probe” particles as a function of transverse momentum 90° is where the action is (max T, r) pL between the two beams: midrapidity pT < 1.5 GeV/c “thermal” particles radiated from bulk medium “internal” plasma probes pT > 3 GeV/c large Etot (high pT or M) set scale other than T(plasma) autogenerated “external” probe describe by perturbative QCD control probe: photons EM, not strong interaction produced in Au+Au by QCD Compton scattering
Geometry matters
14 ameter = b Use Glauber model of nucleons in the nucleus calculate # of participant nucleons Npart # of binary NN collisions Ncoll
Glauber model: calculate probabilities
15 b bB bA zB zA Nucleus B Nucleus A volume element in B: dbBdzB volume element in A: dbAdzA Probability of finding a nucleon in volume element B = rB(bB,zB) dbBdzB (rB is nuclear density * nucleons in B)
OK, so what happens when two nuclei collide? NB: b ≠ 0!
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Does the matter exhibit collectivity? Look for collective flow via velocity boosts Is the expansion hydrodynamical? Model expansion of the system with fluid dynamics
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Collective motion & elliptic flow (v2) dN/df ~ 1 + 2 v2(pT) cos (2f) + … hydrodynamics works! Almond shape overlap region in coordinate space x y z momentum space
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Surprise: matter flows like a liquid huge pressure buildup large anisotropy it all happens fast efficient equilibration mechanism?? only works with low viscosity/entropy “perfect” liquid (D. Teaney, PRC68, 2003) Kolb, et al Hydrodynamics reproduces elliptic flow of q-q and 3q states Mass dependence requires soft EOS, NOT gas of hadrons
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