John Chin-Hao Chen1 Quark Gluon Plasma: the Hottest Matter on Earth John Chin-Hao Chen ( 陳勁豪 ) RIKEN Brookhaven Research Center Brookhaven National Laboratory 03/21/2012

John Chin-Hao Chen2 Once upon a time… The universe startsfrom a big bang As time goes by, big fireball starts to cool off 13 billion years later, it is where we are now What is the very beginning of the universe looks like? Quark Soup?

John Chin-Hao Chen3 Outline What is quark gluon plasma (QGP)? Some properties of QGP

John Chin-Hao Chen4 Part I: What is QGP

John Chin-Hao Chen5 Quarks are confined Quarks are confined in protons and neutrons The further quarks apart, the stronger the force; the closer the quarks, the weaker the interaction What will happen if we increase the energy “high enough”?

John Chin-Hao Chen6 The Idea of Quark-Gluon Plasma Typical nucleon energy density (energy inside the nucleon) is about 0.13 GeV/fm 3. Higher temperature → higher energy density → create more new particles (by E = mc 2 ) When the energy density exceeds 1GeV/fm 3, many new particles are made → packed close together matter will exist not as hadrons (protons, neutrons…), but as independent quarks and gluons. In this medium, the quarks and gluons are deconfined. It is called “Quark–Gluon Plasma ”

John Chin-Hao Chen7 How do we melt the nucleon? But how hot do we need?

John Chin-Hao Chen8 How hot do we need? At critical temperature, T C, the energy density increases rapidly due to the increase of degrees of freedom. (d  -> d QGP ) T C ~ 175MeV. The energy density  ~1GeV/fm 3. T C ~ Trillion (10 12 ) K Temperature of the core of Sun: T~10 7 K Temperature Energy density / T 4 Hadrons Plasma Gas Phase transition

John Chin-Hao Chen9 Where in the universe can we achieve the extreme condition? T~10 12 K: 1 micro second after the big bang? Super high pressure: maybe inside the neutron star? Where else?

John Chin-Hao Chen10 Collides p+p, Au+Au and other species at various energies!! Maximum energy: Au+Au at  s = 200 GeV per nucleon pair The Hottest Matter is on Long Island!

John Chin-Hao Chen11 What happens in heavy ion collisions The beams travel at % the speed of light. The two ions look flat as a pancake due to Relativity. (  ~106 at full energy RHIC). The two ions collide and smash through each other for s The collision “melts” protons and neutrons, and liberates the quark and gluons. Thousands of particles are created and fly out from the collision area; plasma cools off.

John Chin-Hao Chen12 Two General Purpose Detectors STAR specialty: large acceptance measurement of hadrons PHENIX specialty: rare probes, leptons, and photons

John Chin-Hao Chen13 Events viewed by detectors

John Chin-Hao Chen14 PHENIX: Pioneering High Energy Nuclear Interaction eXperiment

John Chin-Hao Chen15 A even hotter matter at LHC The Large Heavy Ion Collider Three experiments: ATLAS, CMS, ALICE (dedicated HI experiment) Collides √s NN = 2.76/5.5 TeV, and p+p at √s = 2.76 TeV for reference

John Chin-Hao Chen16 Part II: Some properties of QGP

John Chin-Hao Chen17 Some properties of QGP will be discussed Is it hot enough? –Temperature How does the bulk behave? –Collective flow How do we probe the QGP –Hard probes

John Chin-Hao Chen18 How do we understand the properties of QGP? We collide heavy ions (Au/Pb) which creates QGP We have most simple system p+p as a baseline measurement We compare what we see in pp (no QGP) and AuAu (with QGP), see what happened

John Chin-Hao Chen19 Some useful terminology Central collision: the two nuclei collide “head on” (0-10% centrality) Peripheral collision: the two nuclei touch by edge (70-92% centrality) N part : Number of nucleons participating the collision N coll : Number of binary collisions p T : transverse momentum

John Chin-Hao Chen20 How do we measure the temperature of QGP? We measured the direct photons from pp and AuAu collisions We see enhancement of photon yield in AuAu –Similar to black body radiation from QGP!! –T ~ 220 MeV –T c ~175MeV –Or 4 trillion degrees Celsius –Or times hotter than the center of the Sun enhancement pp AuAu Number of photons

John Chin-Hao Chen21 Low p T Direct Photon vs different system Low p T direct photon ratio in various collision systems are measured in PHENIX large enhancement above pQCD calculations (lines) in AA r  = (# of direct  ) / (# of inclusive  )

John Chin-Hao Chen22 What is the initial temperature? Various theory calculations to describe the data T ini ~ MeV (initial state dependent) All above T c (~170 MeV from lattice QCD) T c ~170 MeV

John Chin-Hao Chen23 How do the particles move? Collision area has “almond” shape due to overlap geometry of the nuclei. Almond shape leads to un- uniform momentum distribution. Pressure gradient pushes the “almond” harder in the short direction. This is a “hydrodynamic” effect.

John Chin-Hao Chen24 We can also measure the shape fluctuations The nuclear is not perfect in shape Nucleon distribution is not smooth Azimuthal symmetry of the colliding area no longer available dN/d   1+  (2v n cos n(  -  n )) v odd is possible, which is due to shape fluctuations

John Chin-Hao Chen25 v n (  n ) vs p T All v n increases with p T v 3 is independent from centrality

John Chin-Hao Chen26 What is viscosity Viscosity is the resistivity of the fluid Low viscosity: milk High viscosity: honey Low viscosity means the energy can transfer through the fluid very fast no viscosity = “ideal fluid”

John Chin-Hao Chen27 New Tool to Calculate the QGP Properties: String Theory Through Ads/CFT correspondence, some strongly coupled 4 dimension quantum field theory situation can be transformed in to a black hole in 5 dimensions By solving the relatively “easy” black hole properties in 5-dim space, we can calculate many properties for strongly coupled system, such as QGP QGP AdS/CFT space

John Chin-Hao Chen28 v 2 and Viscosity String theory predicts there is a universal minimum on viscosity for strongly coupled system (  /s = 1/4  ) QGP: tiny viscosity per particle (  /s ~0.08 in hydrodynamics and under Glauber initial state conditions) The most perfect fluid in the world!! Phys. Rev. Lett 99, (2007)

John Chin-Hao Chen29 v n vs theory All theory predicts v 2 well v 3 adds in additional discrimination power Data favors Glauber +  /s = 1/4 

John Chin-Hao Chen30 Other perfect fluid? Electrons in graphene (2010 Nobel Physics) T ~ room temperature Cold atomic gas T ~ K

John Chin-Hao Chen31 How do the particle flow? In higher p T, the v 2 is saturated v 2 is particle type dependent The matter is strongly coupled! v 2,M (p T )~2v 2,q (p T /2) v 2,B (p T )~3v 2,q (p T /3) The quarks have collective motion.

John Chin-Hao Chen32 Beam energy dependence of v n Various beam energy: 39, 62, 200 GeV v n does not have significant beam energy dependence Hydro dynamical behavior down to 39 GeV

John Chin-Hao Chen33 Hard probes on QGP Hard probes –Jets, –Heavy flavor (c, b) –Quarkoniums (J/  ) –direct photons, –Z/W (LHC) Usually produced at the beginning to the collision Probes live through out the whole formation stages

John Chin-Hao Chen34 What is jet? When 2 quarks collide, back-to-back jets are produced Jet is a narrow cone of hadrons and other particles resulting from a fast quark or gluon In heavy ion collision, the colliding area has high energy and particle density, which will modify the jet passing through this hot and dense medium. Also called Jet Tomography

John Chin-Hao Chen35 Jet tomography in daily life– P. E. T. PET - Position Emission Tomography Physiologic images based on the detection of radiation from the emission of positron- electron annihilation. PET scan of human brain. Picture from Wikipedia.

John Chin-Hao Chen36 Jets in the plasma 200 GeV p-p di-jet Event STAR What happened to the jets in the medium ? Where is the jet? 200 GeV

John Chin-Hao Chen37 How do we study jets? Best way: –Direct jet measurement (difficult at RHIC) –  -jet (also difficult) We can also study –High p T single particles (coming from jet fragmentations) –Two particle correlations (leading high p T particles from jet fragmentations, correlated with another lower p T particle)

John Chin-Hao Chen38  0 spectra in Au+Au vs. p+p in central Au+Au collision, less  0 is produced than expected yield from scaled p+p The yield in Au+Au is suppressed central N coll = 975  94

John Chin-Hao Chen39 High p T hadrons are suppressed! Phys. Rev. Lett. 91, (2003) R AA : nuclear modification factor If no “ effects ” : R AA = 1 at high-p T where hard scattering dominates Suppression: R AA < 1 at high-p T The matter is DENSE.

John Chin-Hao Chen40 Summary of particle RHIC Hadrons (made of quarks) lose energy in the medium Photons don’t feel strong interactions, so loss no energy in the plasma

John Chin-Hao Chen41 R LHC Similar suppression trend till 20 GeV/c R AA increases with p T when p T > 20 GeV/c No suppression in photon, Z arXiv:

John Chin-Hao Chen42 Awayside jet is suppressed in HI collision! Phys. Rev. Lett. 91, (2003) Same side, no modification away side, jet is absorbed in the medium same away

John Chin-Hao Chen43 Jet LHC Jets are “visible” Di-jet suppression! Consistent with RHIC √s NN = 2.76 TeV

John Chin-Hao Chen44 Dijet energy imbalance A J =(p T1 -p T2 )/(p T1 +p T2 ) A J = 0 means no suppression Significant suppression in central Pb+Pb collisions

John Chin-Hao Chen45  -jet (  -hadron correlation) Use direct photon to tag jet energy to study the energy loss Jet energy is lost in AA collisions! z T = p Ta /p Tt  T ~ -ln(z T ) High z T Low z T

John Chin-Hao Chen46 The shape of the Correlation Jet shape depends on particle energy Jet shape is modified by the plasma Phys. Rev. C (2008)

John Chin-Hao Chen47 Do we see the Mach cone in QGP? The far side peak moved to ~   in central collisions! “Mach Cone” like shape Sound wave propagate through QGP? If so, it can be used to estimate the speed of sound in QGP Phys. Rev. Lett. 98, (2007) D

John Chin-Hao Chen48 shoulder ridge   rad  Peripheral Au+Au Central Au+Au Both near and away side are modified! Two particle  correlations

John Chin-Hao Chen49 v 3, reason for ridge and shoulder? Ridge sits at  ~ 0, shoulder sits at  ~2  /3, 4  /3 –A 3-peak structure! v 3 (Fourier Coefficient of the cos3  term) gives a natural 3-peak structure Is v 3 the explanation?

John Chin-Hao Chen50 Jet shape with higher v n modulated background subtraction When v 3 modulation is included in the background subtraction, the double peak structure in away-side disappears. 200GeV Au+Au 0-20%, inc.  -had.

John Chin-Hao Chen51 Some properties of QGP will be discussed Temperature: it is hot! Collective flow: it flows like a perfect fluid! Jet quenching: it is dense, and opaque to fast quarks

John Chin-Hao Chen52 Why QGP is so interesting? highest energy density frontier It was last seen when the universe is second old A test ground for string theory It is a hottest perfect fluid, what about the coldest end, the atomic gas? In solids? A lot more…

John Chin-Hao Chen53 Connection with other area of physics QGP is highly active!! QGP Nuclear physics Particle physics Theoretical physics Atomic physics Condensed matter Cosmology String theory

John Chin-Hao Chen54 Summary A hot dense matter, quark gluon plasma, is created in heavy ion collisions It has the lowest  /s ~ 1/4  which is the most perfect fluid in the world Partons lose energy in the medium A highly active field, many questions need to be answered! (and to be found!)