20120321john chin-hao chen1 quark gluon plasma: the hottest matter on earth john chin-hao chen (...

20120321 John Chin-Hao Chen 1

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


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?


Outline

• What is quark gluon plasma (QGP)?

• Some properties of QGP


Part I: What is QGP


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”?


The Idea of Quark-Gluon Plasma

• Typical nucleon energy density (energy inside the nucleon) is about 0.13 GeV/fm3.

• Higher temperature → higher energy density → create more new particles (by E = mc2)

• When the energy density exceeds 1GeV/fm3, 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 ”


How do we melt the nucleon?

But how hot do we need?


How hot do we need?

• At critical temperature, TC, the energy density increases rapidly due to the increase of degrees of freedom. (d -> dQGP)

• TC ~ 175MeV. The energy density ~1GeV/fm3.

• TC ~ Trillion (1012) K

• Temperature of the core of Sun: T~107 K Temperature

En

erg

y d

en

sity

/ T

4

Hadrons

Plasma

Gas

Phase transition


Where in the universe can we achieve the extreme condition?

• T~1012 K: 1 micro second after the big bang?

• Super high pressure: maybe inside the neutron star?

• Where else?


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!


What happens in heavy ion collisions

• The beams travel at 99.995% the speed of light.• The two ions look flat as a pancake due to Relativity. (~106 at

full energy collision @ RHIC).• The two ions collide and smash through each other for 10-23 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.


Two General Purpose Detectors

STARspecialty: large acceptancemeasurement of hadrons

PHENIXspecialty: rare probes, leptons,

and photons


Events viewed by detectors


PHENIX: Pioneering High Energy Nuclear Interaction eXperiment


A even hotter matter at LHC

• The Large Heavy Ion Collider• Three experiments: ATLAS, CMS, ALICE (dedicated HI

experiment)• Collides Pb+Pb @ √sNN = 2.76/5.5 TeV, and p+p at √s =

2.76 TeV for reference


Part II: Some properties of QGP


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


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


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)

• Npart: Number of nucleons participating the collision

• Ncoll : Number of binary collisions

• pT : transverse momentum


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 radiati

on from QGP!!– T ~ 220 MeV – Tc ~175MeV– Or 4 trillion degrees Celsiu

s– Or 250000 times hotter tha

n the center of the Sun

enhancement

pp

AuAu

Nu

mb

er o

f p

ho

ton

s


Low pT Direct Photon vs different system

• Low pT direct photon ratio in various collision systems are measured in PHENIX

• large enhancement above pQCD calculations (lines) in AA

r = (# of direct ) / (# of inclusive )


What is the initial temperature?

• Various theory calculations to describe the data

• Tini ~ 300-600 MeV (initial state dependent)

• All above Tc (~170 MeV from lattice QCD)

Tc ~170 MeV


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.


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+(2vncos n(-n))

• vodd is possible, which is due to shape fluctuations


vn(n) vs pT

• All vn increases with pT

• v3 is independent from centrality


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”


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


v2 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, 172301 (2007)


vn vs theory

• All theory predicts v2 well

• v3 adds in additional discrimination power

• Data favors Glauber + /s = 1/4


Other perfect fluid?

• Electrons in graphene (2010 Nobel Physics)

• T ~ room temperature

• Cold atomic gas • T ~ 10-9 K


How do the particle flow?

• In higher pT, the v2 is saturated

• v2 is particle type dependent

• The matter is strongly coupled!

• v2,M(pT)~2v2,q(pT/2)v2,B(pT)~3v2,q(pT/3)

• The quarks have collective motion.


Beam energy dependence of vn

• Various beam energy: 39, 62, 200 GeV

• vn does not have significant beam energy dependence

• Hydro dynamical behavior down to 39 GeV


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


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


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.


Jets in the plasma

Au+Au @ 200 GeV

p-p di-jet Event STARSTARWhat happened to the jets in the medium ?

Where is the jet?

p+p @ 200 GeV


How do we study jets?

• Best way:– Direct jet measurement (difficult at RHIC)– -jet (also difficult)

• We can also study– High pT single particles (coming from jet

fragmentations)

– Two particle correlations (leading high pT particles from jet fragmentations, correlated with another lower pT particle)


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

centralN

coll = 975 94


High pT hadrons are suppressed!

Phys. Rev. Lett. 91, 072301 (2003)

ppAuAubinary

AuAuAA Yield N

YieldR

•RAA : nuclear modification factor•If no “effects”:

•RAA = 1 at high-pT where hard scattering dominates

• Suppression: •RAA < 1 at high-pT

The matter is DENSE.


Summary of particle suppression @ RHIC

• Hadrons (made of quarks) lose energy in the medium• Photons don’t feel strong interactions, so loss no energ

y in the plasma


RAA @ LHC

• Similar suppression trend till 20 GeV/c• RAA increases with pT when pT > 20 GeV/c• No suppression in photon, Z

arXiv:1202.2554

http://arxiv.org/abs/arXiv:1202.2554


Awayside jet is suppressed in HI collision!

Phys. Rev. Lett. 91, 072304 (2003)

Same side, no modification

away side, jet is absorbed in the medium

same

away


Jet suppression @ LHC

• Jets are “visible”• Di-jet suppression!• Consistent with RHIC

Pb+Pb @ √sNN = 2.76 TeV


Dijet energy imbalance

• AJ=(pT1-pT2)/(pT1+pT2)AJ = 0 means no suppression

• Significant suppression in central Pb+Pb collisions


-jet (-hadron correlation)

• Use direct photon to tag jet energy to study the energy loss• Jet energy is lost in AA collisions!

zT = pTa/pTt

T ~ -ln(zT)

High zTLow zT


The shape of the Correlation

• Jet shape depends on particle energy

• Jet shape is modified by the plasma

Phys. Rev. C 78 014901 (2008)


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, 232302 (2007)

D


shoulder ridge

rad

Peripheral Au+Au Central Au+Au

Both near and away sideare modified!

Two particle correlations


v3, reason for ridge and shoulder?

• Ridge sits at ~ 0, shoulder sits at ~2/3, 4/3– A 3-peak structure!

• v3 (Fourier Coefficient of the cos3term) gives a natural 3-peak structure

• Is v3 the explanation?


Jet shape with higher vn modulated background subtraction

• When v3 modulation is included in the background subtraction, the double peak structure in away-side disappears.

200GeV Au+Au0-20%, inc. -had.


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


Why QGP is so interesting?

• highest energy density frontier

• It was last seen when the universe is 0.000001 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…


Connection with other area of physics

QGP is highly active!!

QGP

Nuclearphysics

Particlephysics

Theoreticalphysics

Atomicphysics

Condensedmatter

Cosmology

Stringtheory


Summary

• A hot dense matter, quark gluon plasma, is created in heavy ion collisions

• It has the lowest /s ~ 1/4which 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!)

20120321john chin-hao chen1 quark gluon plasma: the hottest matter on earth john chin-hao chen (...

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