1Jim Thomas - LBL

On the Trail of a Dense Plasma at RHIC(a detector builder’s point of view)

Jim ThomasLawrence Berkeley Laboratory

Cornell University Journal Club5/21/2004

2Jim Thomas - LBL

The Phase Diagram for Nuclear Matter

K. Rajagopol

The goal is to explore nuclear matter under extreme conditions – T > mc2 and > 10 * 0

• One of the goals of RHIC is to understand the QCD in the context of the many body problem

• Another goal is to discover and characterize the Quark Gluon Plasma

• RHIC is a place where fundamental theory and experiment can meet after many years of being apart

3Jim Thomas - LBL

This gap is a manifestation of the approximate SU(2)R x SU(2)L chiral symmetry of QCD with pions as the Nambu-Goldstone bosons

QCD is a Rich Theory with Many Features

Hadron “level” Diagram

Hadron 'level' diagram

0

500

1000

1500

0 10 20 30 40

Degeneracy

Mass (MeV)

Kfo

{W. Zajc

MFFDiL a

aˆ~

4

1We have a theory of the strong

interaction

Low(er) energy nuclear physics uses OPEP or descriptions in terms of a pion gas. These worked because QCD is a theory with a mass gap.

4Jim Thomas - LBL

Lattice QCD predictions

TC ~ 170 MeV, a very stable prediction over time & technologies(F. Karsch, hep-lat/0106019, edited by JT)

Stephan Boltzman limits for a free Quark Gluon gas

En

erg

y D

ensi

ty

Temperature

TC ~ 170 15 MeV

C ~ 0.7 GeV/fm3

0 ~ 0.16 GeV/fm3

Karsch QM2004

5Jim Thomas - LBL

Who is RHIC and What Does He Do?

RHIC

• Two independent rings

• 3.83 km in circumference

• Accelerates everything, from p to Au

s L p-p 500 1032

Au-Au 200 1026

(GeV and cm-2 s-1)

• Polarized protons

• Two Large and two small detectors have been built

h

BRAHMS

PHOBOS

PHENIX

Long Island

Long Island

STARSTAR

6Jim Thomas - LBL

RHIC Physics is Relativistic Nuclear Physics

7Jim Thomas - LBL

The Initial State is Important

• Only a few of the nucleons participate as determined by the impact parameter

• There is multiple scattering in the initial state before the hard collisions take place

– Cronin effect

• The initial state is Lorentz contracted

• Cross-sections become coherent. – The uncertainty principle allows

wee partons to interact with the front and back of the nucleus

– The interaction rate for wee partons saturates ( ρσ = 1 )

• The intial state is even time dilated– A color glass condensate

• proton • neutron • delta • pion string

8Jim Thomas - LBL

How do we ‘see’ RHIC physics?

Barrel EM Calorimeter

FTPCs

Time Projection Chamber

Silicon TrackerSVT & SSD

Endcap Calorimeter

Magnet

Coils

TPC Endcap & MWPC

Central Trigger Barrel & TOF

Beam Beam Counters

4.2 meters

Not Shown: pVPDs, ZDCs, PMD, and FPDs

A TPC lies at the heart of STAR

9Jim Thomas - LBL

TPC Gas Volume & Electrostatic Field Cage

• Gas: P10 ( Ar-CH4 90%-10% ) @ 1 atm

• Voltage : - 28 kV at the central membrane 135 V/cm over 210 cm drift path

420 CM

Self supporting Inner Field Cage: Al on Kapton using Nomex honeycomb; 0.5% rad length

10Jim Thomas - LBL

Pixel Readout of a Pad Plane Sector

A cosmic ray + deltaelectron

3 sigma threshold

11Jim Thomas - LBL

Particle ID using Topology & Combinatorics

Secondary vertex: Ks + p +

+ + K e++e-

Ks + + - K + + K -

p + - + + -

from K+ K- pairs

K+ K- pairs

m inv

m inv

same event dist.mixed event dist.

background subtracted

dn/dm

dn/dm

“kinks”

K +

12Jim Thomas - LBL

Au on Au Event at CM Energy ~ 130 GeV*A

Real-time track reconstruction

Pictures from Level 3 online display. ( < 70 mSec )

Data taken June 25, 2000.

The first 12 events were captured on tape!

13Jim Thomas - LBL

Au on Au Event at CM Energy ~ 130 GeV*A

Two-track separation 2.5 cm

Momentum Resolution < 2%

Space point resolution ~ 500 m

Rapidity coverage –1.8 < < 1.8

A Central Event

Typically 1000 to 2000 tracks per event into the TPC

14Jim Thomas - LBL

Cold nuclear matter:0 ~ 0.16 GeV/fm3

30x0

nucl-ex/0311017

PRL 87 (01) 52301

R2

dydz 0

Boost invariant hydrodynamics: Bjorken Estimate of Initial Energy Density

d

dNp

Rdy

dE

Rch

TT

2

31122

~ 0.2 - 1 fm/ctime to thermalize the

system

~ 6.5 fm

Bjorken Estimate of Initial Energy Density

3/5.4 fmGeV3/7.0 fmGeVC

15Jim Thomas - LBL

0

0 .2

0 .4

0 .6

0 .8

1

1 .2

1 .4

1 .6

-6 -4 -2 0 2 4 6

y

dy

dn

Rapidity vs xf

• xf = pz / pmax

– A natural variable to describe physics at forward scattering angles

• Rapidity is different. It is a measure of velocity but it stretches the region around v = c to avoid the relativistic scrunch

– Rapidity is relativistically invariant and cross-sections are invariant

)/(tanh 1 Epyor z

Rapidity is the natural kinematic variable for HI collisions

( y is approximately the lab angle where y = 0 at 90 degrees )

1tanh yy

z

z

pE

pEy ln

2

1

β

16Jim Thomas - LBL

Identified Particle Spectra at 200 GeV

+, -, K+, K- spectra versus centrality

PRL 92 (2004) 171801 and nucl-ex/0206008

Bose-Einstein fits

mTmAe

/mt exponential fits

K-)1/( / effTmeA

+

17Jim Thomas - LBL

Anti-Proton Spectra at 200 & 130 GeV / N

Au + Au p + X

130 GeV data

p

200 GeV data

22 2/ pAegaussian fits

p andp spectra versus centrality

PRL 92 (2004) 171801 and PRL 87 (2001) 262302

p

18Jim Thomas - LBL

• In the early universe

p / p ratio = 0.999999

• At RHIC, pair-production

increases with s

• Mid-rapidity region is not yet

baryon-free!

• Pair production is larger than

baryon transport

• 80% of protons from pair

production

• 20% from initial baryon

number transported over 5

units of rapidity

Anti-Baryon/Baryon Ratios versus sNN

In HI collisions at RHIC, more baryons are pair produced than are brought in by the

initial state

4Tr

pair

Y

Y

8.0

Transpair

pair

p

pbar

YY

Y

Y

Y

19Jim Thomas - LBL

Anti-Particle to Particle Ratios

Excellent agreement between experiments at y = 0, s = 130

STAR results on thep/p ratio p/p = 0.11 ± 0.01 @ 20 GeV p/p = 0.71 ± 0.05 @ 130 GeV p/p = 0.80 ± 0.05 @ 200 GeV

p/p ratios

K+/K- ratios

20Jim Thomas - LBL

Chemical Freeze-out – from a thermal model

Thermal model fits

Compare to QCD on Lattice:

Tc = 154 ± 8 MeV (Nf=3)

Tc = 173 ± 8 MeV (Nf=2)(ref. Karsch QM01)

MeV 629(RHIC)μ

MeV 7177(RHIC)T

B

ch

MeV 270(SPS)μ

MeV 170160(SPS)T

B

ch

• The model assumes a thermally and chemically equilibrated fireball at hadro-chemical freeze-out

• Works great, but there is not word of QCD in the analysis. Done entirely in a color neutral Hadronic basis!

input: measured particle ratios output: temperature T and baryo-chemical potential B

21Jim Thomas - LBL

• Final-state analysis suggests RHIC reaches the phase boundary

• Hadron spectra cannot probe higher temperatures

• Hadron resonance ideal gas (M. Kaneta and N. Xu,

nucl-ex/0104021 & QM02)

– TCH = 175 ± 10 MeV

– B = 40 ± 10 MeV

• <E>/N ~ 1 GeV(J. Cleymans and K. Redlich, PRL 81, p. 5284, 1998 )

Lattice results

Neutron STAR

Putting RHIC on the Phase Diagram

We know where we are on the phase diagram but now

we want to know what other features are on the diagram

22Jim Thomas - LBL

Chemical and Kinetic Freeze-out

• Chemical freeze-out (first)– End of inelastic interactions

– Number of each particle species is frozen

• Useful data– Particle ratios

• Kinetic freeze-out (later)– End of elastic interactions

– Particle momenta are frozen

• Useful data– Transverse momentum distributions

– and Effective temperatures

space

tim

e

inelasticinteractions

Chemicalfreeze-out

elasticinteractions

Kineticfreeze-out

blue beam yellow beam

23Jim Thomas - LBL

Transverse Flow

The transverse radial expansion of the source (flow) adds kinetic energy to the particle distribution. So the classical expression for ETot

suggests a linear relationship

-

K -

p

Au+Au at 200 GeV

STAR Preliminary

2KFOObs massTT

Slopes decrease with mass. <pT> and the effective temperature increase with mass.

T ≈ 575 MeV

T ≈ 310 MeV

T ≈ 215 MeV

24Jim Thomas - LBL

Kinetic Freezeout from Transverse Flow

<ßr> (RHIC) = 0.55 ± 0.1 cTKFO (RHIC) = 100 ± 10 MeV

Explosive Transverse Expansion at RHIC High Pressure

Tth

[GeV

]<

r>

[c]

STA

RPH

EN

IX

Thermal freeze-out determinations are done with the blast-wave model to find <pT>

STAR Preliminary

25Jim Thomas - LBL

Anisotropic (Elliptic) Transverse Flow

• The overlap region in peripheral collisions is not symmetric in coordinate space

– Almond shaped overlap region• Easier for particles to emerge in the

direction of x-z plane• Larger area shines to the side

– Spatial anisotropy Momentum anisotropy• Interactions among constituents

generates a pressure gradient which transforms the initial spatial anisotropy into the observed momentum anisotropy

• Perform a Fourier decomposition of the momentum space particle distributions in the x-y plane

– v2 is the 2nd harmonic Fourier coefficient of the distribution of particles with respect to the reaction plane

2cos2 vx

y

p

patan

Peripheral Collisions

Anisotropic Flow

x

yz

px

py

...)2cos(2)cos(21 21 vv

26Jim Thomas - LBL

v2 vs. Centrality

• v2 is large– 6% in peripheral

collisions

– Smaller for central collisions

• Hydro calculations are in reasonable agreement with the data

– In contrast to lower collision energies where hydro over-predicts anisotropic flow

• Anisotropic flow is developed by rescattering

– Data suggests early time history

– Quenched at later times

Anisotropic transverse flow is large at RHIC

PRL 86, (2001) 402

more central

Hydro predictions

27Jim Thomas - LBL

v2 vs. pT and Particle Mass

• The mass dependence is reproduced by hydrodynamic models

– Hydro assumes local thermal equilibrium

– At early times

– Followed by hydrodynamic expansion

D. Teaney et al., QM2001 Proc.P. Huovinen et al., nucl-th/0104020

Hydro does a surprisingly good job!

PRL 86, 402 (2001) & nucl-ex/0107003

28Jim Thomas - LBL

V2 at high Pt shows meson / baryon differences

Asym. pQCD Jet Quenching

Bulk PQCD Hydro

qn Coalescence

29Jim Thomas - LBL

The Recombination Model ( Fries et al. PRL 90 (2003) 202303 )

The flow pattern in v2(pT) for hadrons

is predicted to be simple if flow is developed at the quark level

pT → pT /n

v2 → v2 / n ,

n = (2, 3) for (meson, baryon)

30Jim Thomas - LBL

What does this mean?

• Hadrons are created by the recombination of quarks and this appears be the dominant mechanism for hadron formation at intermediate PT

• Baryons and Mesons are produced with equal abundance at intermediate PT

• The collective flow pattern of the hadrons appears to reflect the collective flow of the constituent quarks.

Partonic Collectivity

31Jim Thomas - LBL

Lets look at some collision systems in detail …

Final stateInitial state

Au + Au

d + Au

p + p

32Jim Thomas - LBL

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

33Jim Thomas - LBL

Au+Au and p+p: inclusive charged hadrons

p+p reference spectrum measured at RHIC

PRL 89, 202301 nucl-ex/0305015, PRL in press

34Jim Thomas - LBL

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/peripheral

RAA RCP

35Jim Thomas - LBL

PHENIX observes a similar effect (0s)

nucl-ex/0304022

Factor ~5 suppression for central Au+Au collisions

lower energy Pb+Pb

lower energy

36Jim Thomas - LBL

The Suppression occurs in Au-Au but not d-Au

37Jim Thomas - LBL

Jet Physics … it is easier to find one in e+e-

Jet event in eecollision STAR Au+Au collision

38Jim Thomas - LBL

Identifying jets on a statistical basis in Au-Au

STAR DataAu+Au @ 200 GeV/c

0-5% most central4 < pT(trig) < 6 GeV/c

2 < pT(assoc.) < pT(trig)

• Identify jets on a statistical basis in Au-Au

• Given a trigger particle with pT > pT (trigger), associate particles with pT > pT (associated)

),(11

),(2 NEfficiencyN

CTRIGGER

• You can see the jets in p-p data at RHIC

39Jim Thomas - LBL

Angular Distribution: Peripheral Au+Au data vs. pp+flow

C2(Au Au) C2(p p) A *(1 2v22 cos(2))

Ansatz: A high pT triggered Au+Au event is a superposition of a high pT triggered p+p event plus anisotropic transverse flow

v2 from reaction plane analysis

“A” is fit in non-jet region (0.75<||<2.24)

40Jim Thomas - LBL

Angular Distribution: Central Au+Au data vs. pp+flow

C2(Au Au) C2(p p) A *(1 2v22 cos(2))

41Jim Thomas - LBL

correlations vs the reaction plane

pTtrigger=4-6 GeV/c, 2<pT

associated<pTtrigger, ||<1

y

x

in-plane

Out-of-plane

Effect of path length on suppression is experimentally accessible

-/4

/43/4

-3/4

Back-to-back suppression out-of-plane stronger than in-plane

42Jim Thomas - LBL

What does it mean?

• The backward going jet is missing in central Au-Au collisions when compared to p-p data + flow

• The backward going jet is not suppressed in d-Au collisions

• These data suggest opaque nuclear matter and surface emission of jets

Surface emission

Suppression of back-to-back correlations in central Au+Au collisions

43Jim Thomas - LBL

Exp: Conclusions About Nuclear Matter at RHIC

• Its hot– Chemical freeze out at 175 MeV

– Thermal freeze out at 100 MeV

– The universal freeze out temperatures are surprisingly flat as a function of s

• Its fast– Transverse expansion with an average velocity greater than 0.55 c

– Large amounts of anisotropic flow (v2) suggest hydrodynamic expansion and high pressure at early times in the collision history

• Its opaque– Saturation of v2 at high pT

– Suppression of high pT particle yields relative to p-p

– Suppression of the away side jet

• And its not inconsistent with thermal equilibrium– Excellent fits to particle ratio data with equilibrium thermal models

– Excellent fits to flow data with hydrodynamic models that assume equilibrated systems

44Jim Thomas - LBL

‘They say …’

• ‘Some theorists say’ we have discovered the Quark Gluon Plasma at RHIC

– Evidence for PQCD via vn bulk collective flow of 104 , K,p,– Evidence for pQCD jet quenching in Au+Au at RHIC

– Evidence jet un-quenching in D+Au = Null Control

• The experimental community is somewhat more cautious because, in part, its not clear what the words mean

– Web definition for plasma: A low-density gas in which the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.

– In the collision of two heavy ions, we have found a high density plasma containing quarks and gluons.

So whats the problem with the experimentalists?

45Jim Thomas - LBL

Are the theories a unique explanation for the data?

46Jim Thomas - LBL

Au+Au Datasets

This year: extremely successful run, all goals met Greater than order of magnitude increase in dataset

Au Events

0

20

40

60

80

100

120

2000 2001 2002 2004

Year

Mil

lio

n E

ven

ts d+Au

AuAu 62 GeV

AuAu 130 GeV

AuAu 200 GeV

47Jim Thomas - LBL

Rcp ratios

At At =0 the =0 the central events central events have the ratio have the ratio systematically systematically above that of above that of semi-central semi-central events. We see events. We see a reversal of a reversal of behavior as we behavior as we study events at study events at =3.2=3.2

Rcp

1/<Ncoll central> NABcentral(pT,

1/<Ncoll periph> NAB

periph(pT,)

48Jim Thomas - LBL

RdAu ratios

CroninCronin like like enhancement enhancement at at =0.=0.

Clear Clear suppression suppression as as changes changes up to 3.2up to 3.2

Same ratio Same ratio made with made with dn/ddn/d follows the follows the low plow pTT R RdAudAu

RdA d2Nd+Au/dpTd

<Ncoll> d2Nppinel/dpTd

where < < Ncoll> = 7.2> = 7.2±0.3

49Jim Thomas - LBL

Current State of Affairs

• A theory is something nobody believes, except the person who made it.

• An experiment is something everybody believes, except the person who made it.

– attributed to Albert Einstein

50Jim Thomas - LBL

Recombination TestedThe complicated observed flow pattern in v2(pT)

d2n/dpTd ~ 1 + 2 v2(pT) cos (2 )

is predicted to be simple at the quark level under pT → pT / n , v2 → v2 / n , n = 2,3 for meson,baryon

if the flow pattern is established at the quark level

Compilation courtesy of H. Huang

51Jim Thomas - LBL

At low pt: mass ordering

hydrodynamics

At larger pt: Baryons – mesons

quark coalescence

We need v2 of and 0

Quark Coalescence

S.A. Voloshin, Nucl. Phys. A715, 379 (2003).

Z. Lin et al., Phys. Rev. Lett., 89, 202302 (2002).

R. Fries et al., nucl-th/0306027.

D. Molnar and S.A. Voloshin, PRL 91, 092301(2003).

52Jim Thomas - LBL

Multi-Strange Baryons v2

Multi-strange baryons flow !

Partonic Collectivity !Partonic Collectivity !

53Jim Thomas - LBL

Outer and Inner Sectors of the Pad Plane

60 cm

190 cmOuter sector

6.2 × 19.5 mm pads3940 pads

Inner sector2.85 × 11.5 mm pads

1750 pads

• 24 sectors (12 on a side)

• Large pads good dE/dx resolution in the Outer sector

• Small pads for good two track resolution in the inner sector

54Jim Thomas - LBL

Sector Operation for 20:1 signal to noise

Sector gas gain anodevoltage

inner 3000 1150outer 1100 1380

TPC Sector Detail

• Gating Grid

• Ground Plane of Wires

• Anodes– No field shaping wires

• Simple and reliable

– Individually terminated anode wires limit cross-talk

– Low gain

• Pad Plane

55Jim Thomas - LBL

Sector Readout of Pad Planes

Readout arranged like the face of a clock - 5,690 pixels per sector

56Jim Thomas - LBL

Particle Identification by dE/dx

dE/dx PID range:

~ 0.7 GeV/c for K/

~ 1.0 GeV/c for K/p

Anti - 3He

57Jim Thomas - LBL

Summary of Performance Achieved to date

• Features of the STAR TPC– 4 meters in diameter, 210 cm drift

– No field wires in the anode planes

– Pad readout, Low gain on anodes

– Low drift field

– Very compact FEE electronics

– Analog Delay with SCA then onboard ADC

– Data delivered via optic fiber

– Uniform E and B fields

– ‘ExB’ and most Electrostatic distortions correctable to 50 – 100 m level

• Position resolution– 500 m in the real world with calibration

errors

– Space point resolution ~ 100 m for select laser events, 250 - 350 m for select tracks

– Function of dip angle and crossing angle

• Good particle separation using dE/dx– 6.5% dE/dx resolution @ 100 cm – -proton separation : > 1 GeV/c

• 2-Track resolution– 2.5 cm for HBT pairs – 1.5 cm for laser tracks– limited by 3 pad response function

and desire for fast algorithms

• Momentum resolution– 2% minimum at 0.25 Tesla (half field)– for pT > 1.5 GeV in 0.25 T field

• dk/k = 0.016 pT + 0.012 (central)• dk/k = 0.011 pT + 0.013 (peripheral)• 2.9% 3.3% peripheral/central

@ 1.5 GeV

STAR performance is excellent and meets essentially all design

specifications!

58Jim Thomas - LBL

Something Funny Happens at T > mc2

An exponentially increasing density of hadronic states suggests– A “limiting temperature” TH

– A phase transition(?) in hadronic matter

This was noticed before quarks were identified as the constituents of matter– ( Hagedorn, Nuovo Cimento Supp., 3 (147) 1965 )

Fit this form with TH = 163 MeVDensity of States .vs. Energy

HTmaemdm

dnm /~)(

dmem

dmem

TTm

a

Tm

H

)11

(

/

~

)(

Which requires T < TH

Thermal equilibrium suggests

59Jim Thomas - LBL

Meson – baryon Effect ?

Exp. data consistent with quark

recombination/coalescence scenario

S.A. Voloshin, Nucl. Phys. A715, 379 (2003).Z. Lin et al., Phys. Rev. Lett., 89, 202302 (2002).R. Fries et al., nucl-th/0306027.D. Molnar and S.A. Voloshin, PRL 91, 092301(2003).

Further tests: , 0, K*, …

v2: Scaling

60Jim Thomas - LBL

single point

fluctuation excesstp

statisticalreference

data

2D scale inversion

pt fluctuation inversion

minijet dissipation & velocity/temperature structure:• elongation on

• necking on

200 GeV Au-Au data

fluctuation scale dependence two-particle correlations

STAR preliminary

soft partons asextended objects

peripheral

central

<pT> Fluctuations → Correlations

61Jim Thomas - LBL

Baryon Enhancement

Simple fragmentation picture fails for pT less than ~6 GeV/c

p+pbar/h enhancement in Au + Au not fully explained by Cronin effect

Strong baryon/meson modification in Au + Au also in /K0s ratio

S.S. Adler et al., nucl-ex/0305036.

62Jim Thomas - LBL

RCP of Strange Hadrons

Two groups (2<pt<6GeV/c):

- K0s, K, K*, mesons

- , , baryons

dependence on number of

valence quarks

limited to pt<6GeV/c ?

hadron production from

quark recombination/

coalescence ?

63Jim Thomas - LBL

Jet Quenching

Au + Au, RAA << 1; d+Au, RdAu > 1

p+p, away side jet exists Au + Au, away-side correlation suppressed d+Au, away-side correlation exists

jet quenching ?J. Adams et al., Phys. Rev. Lett. 91, 072304 (2003).

64Jim Thomas - LBL

Hadron suppression @62 GeV

• Significant suppression @62, 130, 200 GeV

• Smooth evolution of suppression with energy

65Jim Thomas - LBL

Triggered Correlation Studies

blue band: <pt> in away-side

correlation, decreases with centrality

solid line: <pt> of bulk, increases with

centrality transverse radial flow

Au+Au: Away-side

suppression is larger in the out-

of-plane direction compared to

in-plane

jet quenching ?

STAR PreliminarySTAR Preliminary

pTrigger = 4 -6GeV/c

66Jim Thomas - LBL

0

0.2

0.4

0.6

0.8

1

-3 -2 -1 0 1 2 3

y

antiprotonto proton

ratio

BRAHMS

STAR

PHOBOS

PHENIX

Points reflected about y = 0

s = 130 Results

Pbar/ P | Y=0.0 = 0.64

Pbar/ P | Y=0.7 = 0.66

Pbar/ P | Y=2.0 = 0.41

BRAHMS PRL 87 No. 11 (2001).

Evidence for the development of a (nearly) baryon-free central region

Anti-Proton to Proton Yields vs. Y

67Jim Thomas - LBL

Part I: full jet reconstruction in p+p, d+Au

ET

ET

Including EMCal information

A first look…