Helen CainesThe Ohio State

University

MSU – Nucl. Seminar

June 2001

A Strange Perspective –

Preliminary Results from the STAR

Detector at RHIC

Science is a wonderful thing if one does not have to earn one's living at it

– Einstein (1879—1955)

Helen Caines

MSU - 2001

The STAR Collaboration

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: 36Collaborators: 415 Students: ~50

Spokesperson: John Harris

Helen Caines

MSU - 2001

QCD

Quarks confined within hadrons via strong force

v(r) = /r + *r

At large r -second term dominates

At small r -Coulomb-like part dominates

However function of q( mtm transfer) and -> 0 faster

than q (or 1/r) -> infinity (called asymptotic freedom)

This concept of asymptotic freedom among closely packed

coloured objects (q and g) has led to one of the most exciting

predictions of QCD !!

The formation of a new phase of matter where the colour degrees of freedom are liberated. Quarks and gluons are no longer confined within colour singlets.

The Quark-Gluon Plasma!

Helen Caines

MSU - 2001

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

MSU - 2001

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

MSU - 2001

Tfireball < Tc(170MeV) Hadron gasHard to make S 0 particles

+ N + K (Ethresh 530MeV) + K + (Ethresh 1420MeV)

Mtm phase space suppressedNeed to create 3 qq pairs

(initially there are no q) with similar momenta in a region already containing many quarks.

Tfireball >Tc(170MeV) QGP

Easy to make s quarks E=2ms ( 300MeV)Free gluons

g-g fusion - dominate ss creation faster reaction time than qq

Pauli blocking may aid creation of ss quarks

( probably not true at high T, too many states).

_

_

__

_

_

_

Why are we interested in Strangeness?

Helen Caines

MSU - 2001

Introduction

When is Strangeness Produced – Resonances

Chemical content

– Yields

Thermal Freeze-out – Radii and Inverse slopes

Flow – How much and when does it start?

Chemical Freeze-out - Ratios

Helen Caines

MSU - 2001

Previous Strangeness Highlights

WA97

Evidence of strangeness enhancement between pA and AA collisions at the SPS – Not reproducible by

models

SPS s=17GeVEnhancement > > > h

|s|

Helen Caines

MSU - 2001

Strangeness Highlights (2)

AGS and SPS > 1

Need to consider p absorption

Multi-Strange Particles appear to

freeze out at a cooler temperature/ earlier or have

less flow

SPSAGS

_

Helen Caines

MSU - 2001

The CERN announcement

Strangeness was one of the corner stones of the CERN announcement.

Have numerous pointers that there is evidence of a new state of matter even at SPS energies so why RHIC?

•Still a large baryon number so need models to understand what’s going on.

•Those models can probably be tuned to reproduce the experimental data but would require more “knobs”

•So want to go to “cleaner” system

•Less baryon number (only look at produced particles)

•Closer to the region where QCD predictions work – Definite theory not models

•Higher energies – further across phase transition boundary – In new regime for longer and more frequently

Helen Caines

MSU - 2001

Welcome to BNL- RHIC!

Helen Caines

MSU - 2001installation in 2003

Endcap Calorimeter

Year 2000,

The STAR Detector (Year-by-Year)

ZCal

Time Projection Chamber

Magnet

Coils

RICH * yr.1 SVT ladder

TPC Endcap & MWPC

ZCal

Central Trigger Barrel

FTPCs (1 + 1)

Silicon Vertex Tracker *

Vertex Position Detectors

year 2001,

+ TOF patch

Barrel EM Calorimeter

year-by-year until 2003,

Helen Caines

MSU - 2001

STAR Pertinent Facts

Field:

0.25 T (Half Nominal value)

(slightly 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” EventsL3-Real time display

Helen Caines

MSU - 2001

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

MSU - 2001

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

MSU - 2001

Particle ID Techniques - Topology

Decay vertices

Ks + + -

p + -

p + +

- + -

+ + +

+ K -

“kinks”:

K +

Vo

Helen Caines

MSU - 2001

Finding V0s

proton

pion

Primary vertex

Helen Caines

MSU - 2001

High Pt K+ & K- Identification Via “Kinks”

+/-

K+/-

Helen Caines

MSU - 2001

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

MSU - 2001

STAR STRANGENESS!

K0s

K+

(Preliminary)

Helen Caines

MSU - 2001

Triggering/Centrality

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

~30K Events |Zvtx| < 200 cm

• “Central”

CTB threshold set to upper 15%REQUIRE:Coincidence ZDC East and West

REQUIRE:

Min. Bias + CTB over threshold

Helen Caines

MSU - 2001

The Collisions

The End Product

Helen Caines

MSU - 2001

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

MSU - 2001

p/p Ratio_

Phys. Rev. Lett March 2001

Still finite baryon number

Ratio is flat as function of pt and y

Slight fall with centrality

Ratio = 0.65 ±0.03(stat) ±0.03(sys)

Helen Caines

MSU - 2001

Strange Baryon Ratios

Ratio = 0.73 ± 0.03 (stat)

~0.84 /ev, ~ 0.61/ev

Reconstruct: Reconstruct:_

STAR Preliminary

Ratio = 0.82 ± 0.08 (stat)

_ ~0.006 /ev, ~0.005/ev

Helen Caines

MSU - 2001

Preliminary /Ratio

= 0.73 0.03 (stat)_

Ratio is flat as a function of pt and y

Central events

|y|<0.5

Helen Caines

MSU - 2001

and from mixed event Studies

Good cross-check with

standard V0 analysis.

Low pt measurement

where there is no V0 analysis

High efficiency (yields are ~10X V0 analysis yields)

Background determined by mixed event

STAR preliminary

The ratio is in agreement with “standard” analysis

= 0.77

0.07 (stat)

_

Helen Caines

MSU - 2001

¯______

_

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

MSU - 2001

Particle Freeze-out Conditionsti

me

3. freeze-out

1. formation

Chemical Freeze out: inelastic scattering stops

2. hot / denseKinetic Freeze out: elastic scattering stops

Helen Caines

MSU - 2001

K+/K- Ratio - Nch

dE/dx Kinks

•K+/K-= 1.08±0.01(stat.)± 0.06(sys.) (dE/dx). (The kink method is systematically higher.)

STAR preliminary

STAR preliminary

K+/K- constant over measured centrality

Helen Caines

MSU - 2001

K-/- Ratios

K-/ ratio is enhanced by almost a factor of 2 in central collisions when compared to peripheral collisions

STAR preliminary

SPS

Helen Caines

MSU - 2001

K0* and K0* Identification

Short lifetime (c =4fm) – sensitive to the evolution of the system?

_

First measurement in heavy ion collisions

Helen Caines

MSU - 2001

K0*/h-

Strangeness Enhancement?

Represents a 50% increase

compared to K0*/ measured

in pp at the ISR.

Also look at K*/K

From spin counting

K*/K = vector meson/meson

= V/(V+P)

=0.75

e+e-(LEP)K*/K = 0.32 ±0.02

pp (ISR)K*/K = 0.6 ± .09 ± .03Au-Au (STAR)= 0.42

Helen Caines

MSU - 2001

Comparing to SPS

K+/K-(kink) = 1.2 ±

K+/K-(dE/dx) = 1.08 ±0.01 (stat.)± 0.06 (sys.)

K-/= 0.15 ± 0.02 (stat.)

K*/h- = 0.06 ± 0.006

(stat.)± 0.01 (sys.)

K*/h- = 0.058 ± 0.006

(stat.)± 0.01 (sys.)

p/p = 0.6 0.02 (stat.) 0.06 (sys.)

¯/ = 0.73 ± 0.03 (stat.)

± 0.08 (stat.)

¯

¯

¯

Helen Caines

MSU - 2001

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

sdu D=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

MSU - 2001

s1

qs

2qs

2sq

2s

2s

4q

s2q

1s

2q

T/

uds

dss

uds

sdu

e

Particle Ratios and Chemical Content

j= Quark Chemical Potential

T = Temperature

Ej – Energy required to add quark

j– Saturation factor Use ratios of particles to determine Tch and

saturation factor

ij

i ejNT

j

Helen Caines

MSU - 2001

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

s (RHIC) < 0.004 GeV

s (SPS)

Helen Caines

MSU - 2001

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

MSU - 2001

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

MSU - 2001

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

MSU - 2001

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

MSU - 2001

No evidence of mass modification

Identification

STAR Preliminary

Helen Caines

MSU - 2001

Inverse slope for Hyperons

T=352±6(stat) MeV

15% Most Central

As / ratio is flat as a function of pt can infer that the slope is the same – backed up by fitting to corrected spectrum

Some evidence that a single exponential fit is not the best fit to the data

e(-mt/

T)

Same slope as

Helen Caines

MSU - 2001

Radial Flow and Strange Particles

Do not follow “radial flow systematics” early kinetic freeze out?

STAR Preliminary

Neither the or the proton are corrected for feed-down.

Correction would drive the p slope up.

What about p absorption/annihilation?

Lower momentum more collisions more absorption/annihilation.

_

Helen Caines

MSU - 2001

C (Q

inv)

Qinv (GeV/c)

1

2

0.05 0.10

Width ~ 1/R

1D: overallrough “size”

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

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

Helen Caines

MSU - 2001

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

MSU - 2001

What have we “learnt” so far

• 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

Strange Particles The and do not seem to flow with the other particles.

Reduced rescattering for the kaons from decay and/or

feel less flow

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

Helen Caines

MSU - 2001

This Year – RICH,TOF Patch,SVT,FTPC

RICH and TOF:

Increase K identification in pt over a limited geometric acceptance

Centered at mid-rapidity they provide complimentary pt coverage

TOF patch 0.3< pt <1.5 GeV/c

RICH 1.1 < pt < 3.0 GeV/c

Overlaps with the TPC kink and dE/dx measurement

kink pt < 5 GeV, dE/dx pt < 0.8 GeV

SVT: Increased efficiency for all strange particles and resonances due to improved tracking

Should measure spectra for all particles this year.

HBT with strange particles

Exotica

FTPC: Strange particles at high y

Helen Caines

MSU - 2001

The Silicon Vertex Tracker

Radii – 6,10 15 cm

Length ±12.4 cm

± 18.6 cm

± 21.7cm

(-1 < < 1)

Helen Caines

MSU - 2001

SVT

STAR detector gets new silicon heart – CERN Courier

SVT installed and operational April 2001!!

91% live (out of 103,680 channels)

Helen Caines

MSU - 2001

SVT being Assembled

Radiation Length 1.5%/layer (including Electronics+Cooling)

216 wafers on 36 Ladders

0.7m2 Silicon

Half assembled.

Fully assembled

Helen Caines

MSU - 2001

Principle of Operation

SDD gives unique position in X-Y

* 6.3 cm x 6.3 cm area

* 280 m thick n-type Si wafer

* 20 m position resolution

Ionizing particle

X-position from drift time

Electron cloud

X

SDD

Y-position from readout anode number

Helen Caines

MSU - 2001

SVT Performance

1ch=2mV

Noise

Hits from Au-Au Event

Cosmic Ray Event–L3 TriggerThreshold at 4mV 6% live

Helen Caines

MSU - 2001

Conclusion

Lots doneLots still to be done

The future looks bright and exciting

Helen Caines

MSU - 2001

Comparison of AGS, SPS, RHIC

AGS SPS RHIC

Energy density 1GeV/fm3 5.3GeV/fm3 17GeV/fm3

Multiplicity 1,000 3,000 10,000

Baryon chemical potential b 520MeV 167MeV 47MeV

Freeze-out temperature (T) 120MeV 130 MeV 160MeV

Helen Caines

MSU - 2001

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

MSU - 2001

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

MSU - 2001

Calibration – Cosmic Rays

Determine momentum resolution

p/p < 2% for most tracks

Helen Caines

MSU - 2001

K+/K- vs pt

Helen Caines

MSU - 2001

K- Inverse Slope Results

KinkdE/dx

h- mid rapidity dN/d

Incre

asin

g

cen

trality

Helen Caines

MSU - 2001

Helen Caines

MSU - 2001

Large h- multiplicityNearly Boost invariant

Helen Caines

MSU - 2001

The central rapidity region

ExcitedVacuum

hadrons

hadrons

• Almost net-baryon free dNnet-B/dy ~ 30 • Large particle multiplicity dN/d ~ 800

C. Bernard et al, PRD 55, 6861 (1997)

Helen Caines

MSU - 2001

Helen Caines

MSU - 2001

In case you thought it was easy…

BeforeAfter