crossing a new threshold first results from the relativistic heavy ion collider

STAR

Helen CainesThe Ohio State

University

March 2001

Crossing a New Threshold

First Results from the Relativistic Heavy Ion Collider

Science is a wonderful thing if one does not have to earn one's living at it – Einstein (1879—1955)

Helen Caines

OSU – March 2001STAR

Motivation

Why Relativistic Heavy Ion Collisions?

To study a hadronic matter at high energy density

Early universe

Center of stars

To study the deconfined state of QCD

Where is the phase transition?

What order is it?

To study the Vacuum – Chiral symmetry restoration

Origin of (hadronic) mass

Helen Caines


The Phase Space Diagram

TWO different phase transitions at work!

– Particles roam freely over a large volume

– Masses change

Calculations show that these occur at approximately the same point

Two sets of conditions:

High Temperature

High Baryon Density

Lattice QCD calc. Predict:

Tc ~ 150-170 MeV

c ~ 0.5-0.7 GeV/fm

Deconfinement transition

Chiral transition

Helen Caines


most dangerous event in

human history: - ABC

News –Sept ‘99

Don’t Panic!!!

"Big Bang machine could

destroy Earth" -The

Sunday Times – July ‘99

the risk of such a catastrophe is essentially zero. – B.N.L. – Oct ‘99

- New Scientist

Will Brookhaven

Destroy the Universe? –

NY Times – Aug ‘99

No… the experiment will not tear our region of space to subatomic shreds.

- Washington Post – Sept ‘99

Apocalypse2 – ABC News – S

ept

‘99

Helen Caines


Welcome to BNL- RHIC!

Helen Caines


The Collisions

The End Product

Helen Caines


The STAR Detector (Year-by-Year)

• Year 2000, year 2001, year-by-year until 2003, installation in 2003

ZCal

Silicon Vertex Tracker *

Central Trigger Barrel+ TOF patch

FTPCs (1 + 1)

Time Projection Chamber

Vertex Position Detectors

Magnet

Coils

RICH * yr.1 SVT ladder

Barrel EM Calorimeter

TPC Endcap & MWPC

Endcap Calorimeter

ZCal

Helen Caines


How a TPC works

420 CM

• Tracking volume is an empty volume of gas surrounded by a field cage

• Drift gas: Ar-CH4 (90%-10%)

• Pad electronics: 140000 amplifier channels with 512 time samples – Provides 70 mega pixel, 3D image

Helen Caines


Needle in the Hay-Stack!

How do you do tracking in this regime?

Solution: Build a detector so you can zoom in close and “see” individual tracks

Good tracking efficiency

Clearly identify individual tracks

high resolution

Pt (GeV/c)

Helen Caines


Spectators – Definitely going down the beam line

Participants – Definitely created moving away from beamline

Triggering/Centrality

ImpactParameter

Spectators

Spectators

Zero-Degree Calorimeter

Participants

Several meters

• “Minimum Bias”ZDC East and West thresholds set to lower edge of single neutron peak.

REQUIRE:Coincidence ZDC East and West

• “Central”CTB threshold set to upper 15%

REQUIRE: Min. Bias + CTB over threshold

~30K Events |Zvtx| < 200 cm

Helen Caines


Au-Au Event at 130 A-GeV

Peripheral EventFrom real-time Level 3 display.

Helen Caines


Au- Au Event 130 A-GeV

Mid-Central EventFrom real-time Level 3 display.

Helen Caines


Au -Au Event 130 A-GeV

Central EventFrom real-time Level 3 display.

Helen Caines


STAR Pertinent Facts

Field:

0.25 T (Half Nominal value)

worse resolution at higher p

lower pt acceptance

TPC:

Inner Radius – 50cm

(pt>75 MeV/c)

Length – ± 200cm

( -1.5 1.5)

Events:

~300,000 “Central” Events –top 8% multiplicity

~160,000 “Min-bias” Events

Helen Caines


Particle ID Techniques - dE/dx

dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p

12

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

12

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

Kp

d

edE

/dx

(keV

/cm

)

0

8

4

dE/dx

6.7%Design

7.5%With calibration

9 %No calibration

Resolution:

Even identified anti-3He !

Helen Caines


Particle ID Techniques - Topology

Decay vertices

Ks + + -

p + -

p + +

- + -

+ + +

+ K -

“kinks”:

K +

Vo

Helen Caines


STAR STRANGENESS!

K0s

K+

(Preliminary)

Helen Caines


Physics Measurements

•dN/dfor h- (||<= ~1.5) particle density, entropy

•Flow early dynamics, pressure

•p/p, / stopping

•Particle spectra temperature, radial flow

•Particle ratioschemistry

•Particle correlations geometry, collective flow

•High Pt jet quenching

__

•Neutral particle decays ,K0s, strangeness production

Helen Caines


The Serious Predictions

>factor 2 variation in yields Radii increase from SPS

R0/Rs >= 1.6 (long lifetime)

Little Stopping –

Net proton yield = 4 – 20

Transverse flow –

Same a SPS - much higher

Heavier particles not see flow

Helen Caines


Negative Hadrons: Distribution and Multiplicity

h-

Full efficiency corrections

h-

Increased particle production per participant pair:43% compared to Pb+Pb @ 17.2 GeV30% compared to pp @ 200 GeV

dN(h-)/d = 264 1 18 (extrap. to all pt)

At low end of predictions – Kills many models

More than just pp happening

Helen Caines


Transverse Energy

PHENIX Preliminary

Phenix Electromagnetic Calorimeter measures transverse energy in collisions

Central Events:

Lattice predicts transition at

~ 5.0 GeV/fm3

critical ~ 0.5-0.7 GeV/fm3

Have the Energy Density!!

dydE

RBjt

02 2

11

Helen Caines


Is there Thermalization?

Almond shape overlap region in coordinate space

y2 x2 y2 x2

2cos2 v

x

y

p

patan

Origin: spatial anisotropy of the system when created and rescattering of evolving system

Look at “Elliptic” Flow

Helen Caines


Hydro Calculation of Elliptic Flow

P. Kolb, J. Sollfrank, and U. Heinz

Equal energy density lines

• Elliptic flow observable sensitive to early evolution of system

• Large v2 is an indication of early

thermalizationFirst time in Heavy-Ion Collisions a system created which approaches hydrodynamic model predictions

Flow:

A pressure build up -> Explosion with azimuthal asymmetry

•zero for central events

Hydrodynamics:

Assumes continuum matter with local equilibrium

•Locally equilibrated or “thermalized”.

|| < 1.3

0.1 < pt < 2.0

Hydro Calculations

STAR

PRL 86 (2001) 402

Helen Caines


OK

•Have a high enough energy density to cause transition

•Have a source that is consistent with being thermalized and has a large elliptic flow

But what did we create?

Helen Caines


Baryon Stopping/Transport

Anti-baryons - all from pair production

Baryons - pair production + transported

B/B ratio =1 - Transparent collision

B/B ratio ~ 0 - Full stopping, little pair production

Measure p/p, / , K-/K+

(uud/uud) (uds/uds) (us/us)

_

_

_ _

- - - - - - - -

Helen Caines


p/p Ratio_

Phys. Rev. Lett March 2001

Ratio = 0.65 ±0.03(stat) ±0.03(sys)

Ratio is flat as function of pt and y

Slight fall with centrality

Helen Caines


Strange Baryon Ratios

Ratio = 0.73 ± 0.03 (stat)

~0.84 /ev, ~ 0.61/ev

Reconstruct: Reconstruct:_

STAR Preliminary

~0.006 /ev, ~0.005/ev

Ratio = 0.82 ± 0.08 (stat)

Helen Caines


¯______

_

Anti-baryon/Baryon Ratios versus s

STAR preliminary

Baryon-pair production

increases dramatically with

s – still not baryon free

65.0

Trpair

pair

p

pbar

YY

Y

Y

Y

2Tr

pair

Y

Y

2/3 of protons from pair production , yet pt dist. the same

– Another indication of thermalization

Pair production is larger than baryon transport

Helen Caines


Simple Model

Assume fireball passes through a deconfined state can estimate particle ratios by simple quark-counting models

*Duds

sdu*

s

s

u

u

uss

ssu

p

p*D

uud

duu

p

p*

s

s

u

u

uds

sduD=1.12

D=1.12

No free quarks so all quarks have to end up confined within a hadron

Predict

Predict

D=1.08± 0.08

su

su

K

K

s

s

u

uD

Measure

System consistent with having a de-confined phase

Helen Caines


Kinetic Freeze-out and Radial Flow

If there is transverse flow

Look at mt = (pt2 + m2 )

distributionA thermal distribution gives a linear distribution

dN/dmt e-(mt/T)

mt

1/m

t d2N

/dyd

mt

Slope = 1/T

Slope = 1/Tmeas

~ 1/(Tfo+ 0.5mo<vt>2)

Want to look at how energy distributed in system.

Look in transverse direction so not confused by longitudinal expansion

Helen Caines


T = 190 MeV

T = 300 MeV

Tp = 565 MeV

mid-rapidity

mt slopes vs. Centrality

• Increase with collision centrality

consistent with radial flow.

Helen Caines


Radial Flow: mt - slopes versus mass

Naïve: T = Tfreeze-out + m r 2 where r = averaged flow velocity

Increased radial flow at RHICßr (RHIC) ßr (SPS/AGS) = 0.6c = 0.4 - 0.5cTfo (RHIC) Tfo (SPS/AGS) = 0.1-0.12 GeV = 0.12-0.14 GeV

Helen Caines


Particle Ratios and Chemical Content

j= Quark Chemical Potential

T = Temperature

Ej – Energy of quark

j– Saturation factor

Use ratios of particles to determine Tch and saturation factor

ij

i ejNT

jjE

)(

Helen Caines


Chemical Fit Results

Not a 4-yields fit!

s 1

2 1.4

Thermal fit to preliminary data:

Tch (RHIC) = 0.19 GeV

Tch (SPS) = 0.17 GeV

q (RHIC) = 0.015 GeV

<< q (SPS) = 0.12-0.14 GeV

Helen Caines


P. Braun-Munzinger, nucl-ex/0007021

Chemical Freeze-out

Baryonic Potential B [MeV]

Chem

ical Tem

pera

ture

Tch

[M

eV

]

0

200

250

150

100

50

0 200 400 600 800 1000 1200

AGS

SIS

LEP

/ SppS

SPS

RHIC quark-gluon plasma

hadron gas

neutron stars

early universe

thermal freeze-out

deconfinementchiral restauration

Lattice QCD

atomic nuclei

Helen Caines


OK (2)

Shown that the collision region:

•Some evidence that source is thermalized•Particles kinetically freeze-out with common T•Large transverse flow -

common to all species•Particles chemically freeze out earlier (higher T)•Near y axis on phase diagram•Relative particle production consitant with having

had free quarks

Helen Caines


K

RoutRside

Measuring the Source “Size” (HBT)

222111 xyipxyip ee~

~5 fm

x1

x2

y1

y2 ~1 m 122211 xyipxyip ee

)xpcos(1~)p,p(P *21

C (Q

inv)

Qinv (GeV/c)

1

2

0.05 0.10

Width ~ 1/R

1D: overallrough “size”

3D decomposition of relative momentum provides handle on shape and time as well as size

Helen Caines


HBT and the Phase Transition

withouttransition

“”

withtransition

c

Rischke & GyulassyNPA 608, 479 (1996)

Generic prediction of 3D hydrodynamic models

Primary HBT “signature” of QGP

~ emission

timescale

Phase transition longer lifetime; Rout/Rside ~ 1 + ()/Rside

Helen Caines


Two-particle interferometry (HBT)

• Correlation function for identical bosons:

• 1d projections of 3d Bertsch-Pratt• 12% most central out of 170k

events• Coulomb corrected• |y| < 1, 0.125 < pt < 0.225

qout

STAR preliminary

STAR preliminary

qlong

fmR

fmR

fmR

Long

Side

Out

)21.012.007.7(

)16.009.047.5(

)23.011.086.5(

03.001.050.0

Helen Caines


Radii dependence on centrality and kt

•Radii increase with multiplicity - Just geometry (?)

•Radii decrease with kt – Evidence of flow (?)

low kT central collisions

“multiplicity”

STAR preliminary

x (fm)

y (f

m)

Helen Caines


Pion HBT Excitation Function

• Central AuAu (PbPb)

• Decreasing parameter

• Decreased correlation

strength

• More baryon resonances ?

• Saturation in radii

• Geometric or dynamic

(thermal/flow) saturation

• No jump in effective lifetime

• No significant rise in size of the emitting source

• Lower energy running needed!

STAR Preliminary

Compilation of world 3D -HBT parameters as a function of s

Helen Caines


2/)( 21TTT ppK

STAR

Preliminary

Tomášik, Heinz nucl-th/9805016

=0.0

=0.5

opaqueness

The ROut/RSide Ratio

Emission duration for transparent sources:

TSideOut RR 22

Small radii + short emission time + opaqueness short freeze-out

Helen Caines


K0s-K0

s Correlations

= 0.7 ±0.5

R = 6.5 ± 2.3

•No coulomb repulsion

•No 2 track resolution

•Few distortions from resonances

•K0s is not a strangeness eigenstate -

unique interference term that provides additional space-time information

K0s Correlation will

become statistically meaningful once we have ~10M events

Helen Caines


Hard Probes in Heavy-Ion Collisions

a) formation phaseparton scattering

b) hot and dense phaseQuark Gluon PlasmaHadron Gas

c) freeze-outemission of hadrons

• “hard” probes: cc, bb and jets

– during formation phase parton scattering processes with large Q2

– create high mass or high momentum objects

– penetrate hot and dense matter– sensitive to state of hot and dense

matter

color screening:

J/suppression dE/dx

jet quenchingQGP

vacuum

Helen Caines


Negative Hadrons: pt - distributions

Power Law

A (1 + pt /p0) - n

p0 = 2.74 ± 0.11 GeV/c

n = 13.65 ± 0.42

STAR

<pt> = 0.514 ± 0.012 GeV/c

NA49

<pt> = 0.414 ± 0.004 GeV/c

UA1

<pt> = 0.392 ± 0.003 GeV/cSTAR preliminary

Mean pt higher than SPS and pp

Helen Caines


Au+Au/pp: Compare pt - distributions

• “Hard” Scaling

• Nuclear Overlap Integral

• TAA = 26 mb-1 for 5% most central

• NAA / Npp= Nbin coll = 1050

• “Soft” Scaling

• NAA / Npp= ( 344 / 2 )

Jet Quenching:First hint for QGP formation at RHIC ?

STAR preliminary

Helen Caines


Conclusions

• Mapping out “Soft Physics” Regime

Net-baryon 0 at mid-rapidity! ( y = y0-ybeam ~ 5 )

Chemical parameters

Chemical freeze-out appears to occur at same ~T as SPS

Strangeness saturation similar to SPS Kinetic parameters

Higher radial flow than at SPS

Thermal freeze out same as at SPS

Unexpected: small HBT radii Strong elliptic flow Pion phase-space density at freeze-out seems to be universal

• Promising results from “Hard Physics” pt spectra from central collisions show clear deviation from p-p

extrapolation high-pt data are consistent with “jet quenching” predictions !

More than we ever hoped for after the first run !!!

Helen Caines


Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino

U.S. Labs: Argonne, Berkeley, Brookhaven National Labs

U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue,Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan

England: University of Birmingham

France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany: Max Planck Institute – Munich University of Frankfurt

Poland: Warsaw University, Warsaw University of Technology

Institutions: 36 Collaborators: 415

The GroupProfs: PostDocs: Students:T.Humanic Me S.BekeleM.Lisa B.Neilson M.Lopez-

NoriegaE.Sugarbaker R.Wells

R.Wilson

The STAR Collaboration

Helen Caines


0 0.1 0.2 0.3 0.4 0.5 0.6

pt

0

0.05

0.1

0.15

0.2

0.25

0.3

<f>

Pion Phase Space Density

NA49

STAR PreliminarySTAR

Radius Fits

fBE;no flowT0=99.5 MeVT0=94.3 MeVT0=89.7 MeV

fBE;flowT0=94.3 MeVT0=89.7 MeV

The Phase Space Density

• “Universal” phase space density observed at SPS appears to hold at RHIC as well

• Consistent with thermal distribution (T94MeV) and strong collective flow ( 0.58)

• Fundamental phase space saturation may relate increases in geometry, temperature, multiplicity

pion occupation of cell in coordinatemomentum space:

LSO

1/2

T

2

Tπ

3

T RRR

π) (λ

my

N

mE2

)()m(

dd

dcf

Helen Caines


Calibration – Cosmic Rays

Determine momentum resolution

p/p < 2% for most tracks

Helen Caines


Calibration - Lasers

Using a system of lasers and mirrors illuminate the TPC

Produces a series of

>500 straight lines criss-crossing the TPC volumeDetermines:

• Drift velocity

• Timing offsets

• Alignment

Helen Caines


QGP prediction: Enhancement > > > h

Evidence for Strangeness Enhancement

WA97

Helen Caines


What about the Chemical Freeze-out?

Yields of hadrons characterised by a few simple parameters

T, V, q (or expq/T), S

Absolute abundances require more sophisticated descriptions including such details as flow effects and the fact that the fire-ball

isn’t at rest.

Perform a least-squared fit to the data with T, V, q /T and S as free parameters

Made simpler by taking particle ratios.

Helen Caines


Energy Density Estimate

What is the energy density reached?

Is it high enough to cause phase transition?

Is there thermalization?Bjorken formula for thermalized energy density dy

dE

RBjt

02 2

11

R2

2c0

Measure Et at y=0

Assume 0 = 0.5 fm/cAssume full overlap

Helen Caines


Elliptic Flow of Pions and Protons

• Hydro calculations: P. Huovinen, P. Kolb and U. Heinz

Mass dependence of v2(pt) shows a

behavior in agreement with hydro calculations

Helen Caines


Elliptic Flow Excitation Function

STAR, PRL 86 (2001) 402

Helen Caines


v2(pt) for high pt particles

M. Gyulassy, I. Vitev and X.N. Wang, nucl-th/00012092

Helen Caines


BeforeAfter

In case you thought it was easy…

Helen Caines


Particle ID Techniques Combinatorics

Ks + + - K+ + K-

p + - p + +

Combinatorics

from K+ K- pairs

K+ K- pairs

m inv

m inv

same event dist.mixed event dist.

background subtracted

dn/dm

dn/dm Breit-Wigner fit

Mass & width

consistent w. PDG

K* combine all K+ and -

pairs (x 10-5)

m inv (GeV)

Helen Caines


Charged particle anisotropy 0< pt< 4.5 GeV/c

Around pt > 2

GeV/c the data starts to deviate from hydro.

However, v2 stays

large.

Only statistical errors

Systematic error 10% - 20% for pt = 2 – 4.5 GeV/c

crossing a new threshold first results from the relativistic heavy ion collider

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