manuel ca lderón de la barca sánchez indiana university

1

Hot quarks! : What do we see in Relativistic Heavy Ion

collisions

“How to cook the primordial soup”

Manuel Calderón de la Barca Sánchez

Indiana University

How does the strong force, QCD, behave at high energy?

How can we study it?

What tools do we use?

What have we learned?

2Manuel Calderón de la Barca, P408/508 Seminar

Disclaimermany of the concepts I will present are

“works in very active progress”

an unavoidable consequence of exciting, cutting edge science


Heavy Ions: How does nuclear matter look at high temperature?

~ 1-3 GeV/fm3

High Density QCD Matter in Laboratory

Determine its properties

QCD Prediction: Phase Transitions

Deconfinement to Q-G Plasma

Chiral symmetry restoration

Relevance to other research areas?

Quark-hadron phase transition in early Universe

Cores of dense stars

High density QCD


The phase diagram of water

Analogous graphs• superfluids• superconductors• metal/insulator• …


Generating a deconfined state

Nuclear Matter(confined)

Hadronic Matter(confined)

Quark Gluon Plasmadeconfined !

Present understanding of Quantum Chromodynamics (QCD)• heating• compression deconfined color matter !

Mike Lisa

Would be very cool to “reverse” the animation on the heating/compression slide, showing cooling/decompression.The point would be that even if we went into a QGP, the system would rapidly rehadronize (a process we don’t understand, but must, if we are to understand the strong force) into a hadronic system– hadrons in / hadrons out we will not “see” the QGP, but must infer it from hadronic patterns in the final state


Expectations from Lattice QCD

/T4 ~ # degrees of freedom

confined:few d.o.f.

deconfined:many d.o.f.

TC ≈ 173 MeV ≈ 21012 K ≈ 130,000T[Sun’s core]


How can we study it?


Imagine…

• You know that ice exists…

• Your theory friends with huge computers tell you that there is something called water…

• You don’t have a way to heat ice…

• So you put millions of ice cubes in an ice-accelerator

• Send them at 99.995% of the speed of light to collide

• Generating thousands of ice-cube+ice-cube collisions per second…

• And you watch it all from the vicinity of Mars!


Key Idea: Bulk Matter

• We must create/compress/heat a bulk (geometrically large) system

– freeze/melt a single H20 molecule?

– fundamental distinction from particle physics

• Only achievable through collisions of the heaviest nuclei (Au, Pb) at the highest available energy– at Relativistic Heavy Ion Collider (RHIC)

1000’s ofparticles producedin each collision


What tools do we use?

How fast? How massive? How long?

Detectors and accelerators are our “bread-and-butter”.


“RHIC is big”

as seen by the Landsat-4 satellite…

• big facility• big detectors• big collaborations• “big” collisions


RHIC BRAHMSPHOBOS

PHENIXSTAR

AGS

TANDEMS

Relativistic Heavy Ion Collider (RHIC)

1 km

v = 0.99995c = 186,000 miles/sec


RHIC BRAHMSPHOBOS

PHENIXSTAR

AGS

TANDEMS


PHENIX~ 500 collaborators


RHIC BRAHMSPHOBOS

PHENIXSTAR

AGS

TANDEMS


STAR ~500 Collaborators


The STAR ExperimentSTAR: Solenoidal Tracker at RHIC

multipurpose detector system for hadronic measurements

• large coverage (geometrical acceptance)• tracking of charged particles in high multiplicity environment• measure correlations of observables• study of hard processes (jet physics)

Solenoidal Tracker At RHIC

goal: track “all” charged hadrons (bags of quarks) emitted in each collision


Operation of a Time Projection Chamber

SCA/ADCDAQ

Copper pads ~ 1cm2

Amplifying and digitizing electronics connected to each pad

Anode wires with +HVsitting ~5 mm above pads

Charged particle fliesthrough TPC gas…

..generating a clusterof liberated electrons

Electricfield

“Avalanche” as electronsapproach anode wire...

..capacitively inducinga signal on nearby pads... ..which is amplified, digitized, and recorded for later analysis

V

t AD

Cbucket #


One collision seen by STAR

TPCMomentum determined by trackcurvature in magnetic field…

…and by direction relative to beam


Particle momentum from tracking…

… how to get particle space-timeinformation??


Impact parameter & Reaction plane

Impact parameter vector b :

beam direction

connects centers of colliding nuclei

b = 0 “central collision”many particles produced

“peripheral collision”fewer particles produced

b


Impact parameter & Reaction plane

Impact parameter vector b :

beam direction

connects centers of colliding nuclei

b = 0 “central collision”many particles produced

“peripheral collision”fewer particles produced

b

b

Reaction plane:

spanned by beam direction and b

Mike Lisa

Could point out that the vector b cannot be "controlled" by, like, aiming the beam or something like that. We measure all collisions, and collision-by-collision we estimate magnitude and direction of b.


Collision Geometry and Centrality

x

z

y

Non-central Collisions

Reaction plane

sNN = 200 GeV

Multiplicity

centralperipheral

5% Central

STAR

Multiplicity (arb. units)

ZD

C

Paddles/BBCZDC ZDC

Au Au

Paddles/BBC Central

Multiplicity Detectors


What sorts of things have we learned?

• We hoped to create bulk matter,

– do we see evidence for collective behaviour?

• We hoped to create high density matter,

– do we see evidence for dissipative behaviour?


How do semi-central collisions evolve?

b

1) Superposition of independent p+p:momenta pointed at randomrelative to reaction plane



b

1) Superposition of independent p+p:

2) Evolution as a bulk system

momenta pointed at randomrelative to reaction plane

highdensity / pressure

at center

“zero” pressurein surrounding vacuum

Pressure gradients (larger in-plane) push bulk “out” “flow”

more, faster particles seen in-plane

Mike Lisa

in practice with Uli and Mercedes, took 1/2 hour to get to this slide



1) Superposition of independent p+p:

2) Evolution as a bulk system

momenta pointed at randomrelative to reaction plane

Pressure gradients (larger in-plane) push bulk “out” “flow”

more, faster particles seen in-plane

N

-RP (rad)0 /2 /4 3/4

N

-RP (rad)0 /2 /4 3/4

Mike Lisa

in practice with Uli and Mercedes, took 1/2 hour to get to this slide


Azimuthal distributions at RHICSTAR, PRL90 032301 (2003)

b ≈ 4 fm

“central” collisions

b ≈ 6.5 fm

midcentral collisions


Azimuthal distributions at RHICSTAR, PRL90 032301 (2003)

b ≈ 4 fmb ≈ 6.5 fmb ≈ 10 fm

peripheral collisions


Elliptic flow – collectivity& sensitivity to early system

STAR, PRL90 032301 (2003)

“v2”

“Elliptic flow”

• evidence ofcollective motion

• quantified by v2

• geometrical anisotropy momentum anisotropy

• sensitive to early pressure

• evidence for• early thermalization

• QGP in early stage

Hydrodynamiccalculation ofsystem evolution


A more direct handle?• elliptic flow (v2) and other measurements (in a few slides)

evidence towards QGP at RHIC– indirect connection to geometry

• Are there more direct handles on the space-time geometry of collisions?

– yes ! Even at the 10-15 m / 10-23 s scale !

• What can they tell us about the QGP and system evolution?

– Let’s put Quantum Mechanics to good use!


222111 p)xr(i22

p)xr(i11T e)p,x(Ue)p,x(U

probing source geometry through interferometry

12 ppq

2

21

2121 )q(~1

)p(P)p(P)p,p(P

)p,p(C

C (Q

inv)

Qinv (GeV/c)

1

2

0.05 0.10

Width ~ 1/R

Measurable! F.T. of pion source

)xx(iq2

*21

*1T

*T

21e1UUUU

Creation probability (x,p) = U*U5 fm

1 m source

(x)

r1

r2

x1

x2

{2

1

}e)p,x(Ue)p,x(U 212121 p)xr(i21

p)xr(i12

p1

p2

The Bottom line…if a pion is emitted, it is more likely to emit another

pion with very similar momentum if the source is small

experimentally measuring this enhanced probability: quite challenging


Correlation functions for different colliding systems

C2(

Qin

v)

Qinv (GeV/c)

STAR preliminary p+pR ~ 1 fm

d+AuR ~ 2 fm

Au+AuR ~ 6 fm

Different colliding systems studied at RHIC

Interferometry probes extremely small scales!


beam directi

on

More detailed geometryRelative momentum between pions is a vector can extract 3D shape information

1 2q p p

p2

p1

q1 2K p p

R long

RsideRout

Rlong – along beam direction

Rout – along “line of sight”

Rside – “line of sight”


Timescales

• Evolution of source shape

– suggests system lifetime is shorter than otherwise-successful theory predicts

• Is there a more direct handle on timescales?


Disintegration timescaleRelative momentum between pions is a vector can extract 3D shape information

1 2q p p

p2

p1

q1 2K p p

Rout




Rside

increases with emission timescale

OUT

SIDE

R

R


Disintegration timescale - expectation

3D 1-fluid Hydrodynamics Rischke & Gyulassy, NPA 608, 479 (1996)

withtransition

withtransition

“” “”

Long-standing favorite signature of QGP:

• increase in , ROUT/RSIDE due to deconfinement confinement transition

• expected to “turn on” as QGP energy threshold is reached


Disintegration timescale - observation

4

6

8

4

6

8

1.0

1.25

1.5R

O (

fm)

RS (

fm)

RO

/ R

S

increasing collision energy

RHIC

• no threshold effect seen

• RO/RS ~ 1




• RO/RS ~ 1

• toy model calculations suggest very short timescales


What have we learned (part I)?

• There is evidence that the system behaves collectively!

• Space-momentum correlations consistent with

– a very rapidly expanding source

– developed in extremely short timescales ~1fm

(so short that no models can account for it)

Do we know that the system is at high densisty?


How can you know it’s high density?

• High energy collisions: access to high-momentum transfer processes

– High pT particle production

– High Mass particle production (Heavy Flavors!)

• Name of the game:

– Theorists can calculate this for p+p collisions

– Check how it is modified in Au+Au collisions


Jets in high energy collisions

nucleon nucleon

parton

jetFree quarks and gluons not observed: high transverse energy partons fragment into collimated sprays (jets) of hadrons

• fundamental expectation of QCD • occurs in all high energy collisions: e++e-, p+pbar, Au+Au,…

Hard scattering of partons: quarks or gluons drawn from wavefunction of colliding projectiles


Partonic energy loss in dense matter

Multiple soft interactions:

Strong dependence of energy loss on gluon density gluemeasure color charge density at early hot, dense phase

Gluon bremsstrahlung

Opacity expansion:

glueSmediumT

SR

kq

LqC

E

2

2

ˆ

ˆ4

L

ELogrdCCE jet

glueSaA 23 2

,

Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,…


Jets at RHIC

nucleon nucleonparton

jet

p+p jet+jet (STAR@RHIC)

Find this …

Au+Au ??? (STAR@RHIC)

in this !!!


Partonic energy loss via leading hadrons

-

Energy loss softening of fragmentation suppression of leading hadron yield

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

Binary collision scaling p+p reference


Leading hadrons at lower energy

Multiple scattering in initial state(“Cronin effect”)

p+A collisions:

Central Pb+Pb collisions at CERN SPS (s=20 GeV)

SPS: any parton energy loss effects buried by initial state multiple scattering, transverse radial flow,…

tppA ppA


Au+Au and p+p: inclusive charged hadrons

p+p reference spectrum measured at RHIC

PRL 89, 202301

nucl-ex/0305015, PRL in press


Suppresion of inclusive hadron yield

• central Au+Au collisions: factor ~4-5 suppression • pT>5 GeV/c: suppression ~ independent of pT

nucl-ex/0305015

Au+Au relative to p+p Au+Au central/peripheralRAA RCP


PHENIX observes a similar effect (0s)

nucl-ex/0304022

Factor ~5 suppression for central Au+Au collisions

lower energy Pb+Pb

lower energy


PHOBOS ( ~ 0.8) and BRAHMS ( ~ 2.0)

PHOBOS: nucl-ex/0302015BRAHMS: nucl-ex/0307003

Also have = 0


High-pt Evidence

• The yield of high pt particles is significantly suppressed in central Au+Au collisions.


Jets and two-particle azimuthal distributions

p+p dijet • trigger: highest pT track, pT>4 GeV/c

• distribution: 2 GeV/c<pT<pTtrigger

• normalize to number of triggers

trigger

Phys Rev Lett 90, 082302


Azimuthal distributions in Au+Au

Au+Au peripheral Au+Au central

Near-side: peripheral and central Au+Au similar to p+p

Strong suppression of back-to-back correlations in central Au+Au

pedestal and flow subtracted



High-pt Evidence

• The yield of high pt particles is significantly suppressed in central Au+Au collisions.

• We see an away side jet in p+p, but not in central Au+Au collisions.


What might all this mean?

?

Conjecture: core of reaction volume is opaque to jets

surface emission

Consequences: near-side corrleations unchanged suppression of back-to-back correlations suppression of inclusive rates azimuthal correlations with reaction plane

Compelling picture, but is it right?


Is suppression initial or final state effect?

Initial state?

Final state?

partonic energy loss in dense medium generated in collision

strong modification of Au wavefunction initial jet production rates suppressed for heavy nuclei (e.g. gluon saturation at low xBjorken)


nucl-ex/0305015RCP

Inclusive suppression: theory vs. data

pQCD-I: Wang, nucl-th/0305010pQCD-II: Vitev and Gyulassy, PRL 89, 252301Saturation: KLM, Phys Lett B561, 93

pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching

Final state

Initial state


Is suppression initial or final state effect?

Initial state?

Final state?gluon

saturation

partonic energy loss

How to discriminate? Turn off final state d+Au collisions


d+Au vs. p+p: Theoretical expectations

All effects strongest in central d+Au collisions

pQCD: no suppression, small broadening due to Cronin effect

0 /2

(radians)

0

High pT hadron pairs

saturation models: suppression due to mono-jet contribution?

suppression?

broadening?

If Au+Au suppression is initial state(KLM saturation: 0.75)

1.1-1.5

pT

RA

B

1

Inclusive spectra

If Au+Au suppression is final state

~2-4 GeV/c


d+Au inclusive yields relative to binary-scaled p+p


STARSTARddpdT

ddpdNR

Tpp

AB

TAB

AB /

/

• d+Au : enhancement

• Au+Au: strong suppression

Suppression of the inclusive yield in central Au+Au is a final-state effect

pT


PHENIX, PHOBOS, BRAHMS find similar results

Phys Rev Lett 91, 072302/3/5 (2003)

PHENIX

BRAHMS

PHOBOS


Azimuthal distributions

pedestal and flow subtracted

Near-side: p+p, d+Au, Au+Au similarBack-to-back: Au+Au strongly suppressed relative to p+p and d+Au

Suppression of the back-to-back correlation in central Au+Au is a final-state effect



Summary of STAR high pT

measurements hadrons at pT>~3 GeV/c are jet fragments central Au+Au:

strong suppression of inclusive yield at pT>5 GeV/c suppression factor ~ constant for 5<pT<12 GeV/cstrong suppression of back-to-back hadron pairs

Possible interpretation:Hard scattered partons (or their fragments) interact strongly with mediumObserved fragments are emitted from the surface of the hot & dense zone created in the collision

?


Experiment: the strong suppression of the inclusive yield and back-to-back correlations at high pT observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.

Theory: pQCD models which reproduce the inclusive suppression in central Au+Au collisions require color charge densities a factor 30-50 greater than that of cold nuclear matter

RHIC is generating very high energy density matter

What are the main lessons so far…


Have we found the Quark Gluon Plasma at RHIC?

We now know that Au+Au collisions generate a medium that

• is dense (pQCD theory: many times cold nuclear matter density)

• is dissipative

• exhibits strong collective behavior :we do create bulk matter

Not yet, there is still work to do

We have yet to show that:

• dissipation and collective behavior both occur at the partonic stage

• the system is deconfined and thermalized

• a transition occurs: can we turn the effects off ?

This represents significant progress in our understanding of strongly

interacting matter


Extra Slides


• “observe” the source from all angles relative to the reaction plane

• expect oscillations in radii for non-round sources

big RSIDE

small RSIDE

Source shapeR

SID

E

-RP (rad)0 /2 /4 3/4

reaction plane


STAR, PRL93 012301 (2004)

Measured final source shape

centralcollisions

mid-centralcollisions

peripheralcollisions

Expected evolution:


“Big” and “Small” – in meters

100 106 1012 1018 102410-610-1210-18

1,000,000,000,000,000,000,000,000 m

0.000000000000000001 m


“Short” and “long” – in seconds

Today’s lecture

100 106 1012 1018 102410-610-1210-1810-24

as many yoctoseconds (10-24 s ~ 3 fm/c) in a secondas seconds in 10 thousand trillion years


How can we check if it’s deconfined?


The Original Idea …Matsui & Satz (PLB 178 (1986) 416J/ suppression by QGPColor screening of static potential between heavy quarks:

4/)( ,

3

4 )( 2 rgr

rrV qq


The Original Idea …Matsui & Satz (PLB 178 (1986) 416J/ suppression by QGPColor screening of static potential between heavy quarks:

4/)( ,

3

4 )( 2 rgr

rrV qq

D. Kharzeev and H. Satz, Phys. Lett. B477 (2000): Hard gluons needed for breaking J/ not available in hadron gas

S. Digal et al., Phys. Rev. D 64, 094015 (2001)C.-Y. Wong, Phys. Rev. C 65, 034902 (2002)Calculations using lattice potentials: sequential suppression’, c dissolves below TC

J/ dissolves at 1.2 TC


Heavy Quarks in the Lattice• Profile of the action density between static quarks at

distance 1.5 fm, Phys. Rev. D51 (1995) 5165


Static free energy in full QCD

3F: Improved staggered Asqtad action, lattices

K. Petrov, P.P, PRD 70 (04) 054503

String breaking

screening

Vacuum physics

2F: Improved staggered p4 action lattices,

O. Kaczmarek et al, Progr. Theor. Phys. Suppl 153 (04) 287

P.Petrevczky


Taking into account the entropy and the internal energy

Kaczmarek, Karsch, P.P., Zantow, hep-lat/0309121see talk by Kaczmarek

Potential energyIn the Lattice shows screening!


… and its Confirmation …CERN/SPS s 17 GeV• mid-rapidity• large statistics (beyond RHIC)


Electron PID with MRPC TOF/TPC and EMCEMC

1. use TPC for p and dE/dx2. use Tower E p/E3. use SMD shape to reject hadrons4. e/h discrimination power ~ 105

5. works for pT > 1.5 GeV/c

ToF1. use TPC and ToF PID

2. works for pT < 3 GeV/c


Quarkonium Trigger Development

• Use calorimeter for triggering on 2 high towers with large invariant mass

• J/ Trigger for pp and d+Au

Trigger even for Au+Au


Azimuthal anisotropy in non-central collisions

xy z

Initial spatial anisotropy

py

px

Final momentum anisotropy

x

y

p

patan

y 2 x 2

y 2 x 2

v2 px

2 py2

px2 py

2

Reaction plane defined by “soft” (low pT) particles

Elliptic term

)](2cos[21 2 Rvd

dN

Reaction plane azimuthal angle


Elliptic flow at high pT: theory

Snellings; Gyulassy, Vitev and Wang (nucl-th/00012092)

Jet propagation through anisotropic matter (non-central collisions)

Finite v2: high pT hadron correlated with reaction plane defined by the “soft” part of event (pT<2 GeV/c)

jet

jet


Elliptic flow at high pT: data

STARSTAR

PRL 90, 032301

• pT<2 GeV: detailed agreement with hydrodynamics • pT >4 GeV: finite v2 in qualitative agreement with jet quenching expectations

STAR preliminary

STARSTAR


More informationRelative momentum between pions is a vector can extract 3D shape information

1 2q p p

p2

p1 1 2K p p

Rout




Rside

study as K grows…


Why do the radii fallwith increasing momentum ??


Why do the radii fallwith increasing momentum ??

It’s collective flow !!

Direct geometrical/dynamical evidencefor bulk behaviour!!!

Amount of flow consistent with p-space



PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

N()

Heinz & Kolb, hep-ph/0204061


• RO/RS ~ 1

• toy model calculations suggest very short timescales• rapid, explosive evolution• too explosive for “real” models

which explain all other data

An important space-time“puzzle” at RHIC

- actively under study


Forces and structures in Nature

1) Gravity• one “charge” (mass)• force decreases with distance

m1 m2

2) Electric (& Magnetic)• two “charges” (+/-)• force decreases with distance

+ -

+ +

Atom


Atomic nuclei and the “nuclear” force

Nuclei composed of:• protons (+ electric charge)• neutrons (no electric charge)

Does not blow up!? “nuclear force”• overcomes electrical repulsion

• determines nuclear reactions(stellar burning, fusion…)

• arises from fundamental strong force (#3)

• acts on color charge of quarks

proton

neutron

quark


Strong color fieldEnergy grows with separation !!!E=mc2 !

An analogy… and a difference!to study structure of an atom…

“white” proton

…separate constituents

Imagine our understanding of atoms or QED if we could not isolate charged objects!!

nucleus

electron

quark

quark-antiquark paircreated from vacuum

“white” proton(confined quarks)

“white” 0

(confined quarks)

Confinement: fundamental & crucial (but not understood!) feature of strong force- colored objects (quarks) have energy in normal vacuum

neutral atom

To understand the strong force and the phenomenon of confinement:Create and study a system of deconfined colored quarks (and gluons)

manuel ca lderón de la barca sánchez indiana university

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