Results from the PHOBOS Experiment

at RHIC - A Perspective

Russell Betts – UIC

for

The PHOBOS Collaboration

C o l l a b o r a t i o n ( O c t o b e r 2 0 0 4 )

B u r a k A l v e r , B i r g e r B a c k , M a r k B a k e r , M a a r t e n B a l l i n t i j n , D o n a l d B a r t o n , R u s s e l l B e t t s , A b i g a i l B i c k l e y , R i c h a r d B i n d e l , W i t B u s z a ( S p o k e s p e r s o n ) , A l a n C a r r o l l , Z h e n g w e i C h a i , V a s u n d h a r a C h e t l u r u ,

P a t r i c k D e c o w s k i , E d m u n d o G a r c í a , T o m a s z G b u r e k , N i g e l G e o r g e , K r i s t j a n G u l b r a n d s e n , C l i v e H a l l i w e l l , J o s h u a H a m b l e n , I a n H a r n a r i n e , C o n o r H e n d e r s o n , D a v i d H o f m a n , R i c h a r d H o l l i s ,

R o m a n H o ł y ń s k i , B u r t H o l z m a n , A n e t a I o r d a n o v a , J a y K a n e , N a z i m K h a n , P i o t r K u l i n i c h , C h i a M i n g K u o , W e i L i , W i l l i s L i n , S t e v e n M a n l y , A l i c e M i g n e r e y , G e r r i t v a n N i e u w e n h u i z e n ,

R a c h i d N o u i c e r , A n d r z e j O l s z e w s k i , R o b e r t P a k , H e i n z P e r n e g g e r , C o r e y R e e d , C h r i s t o f R o l a n d , G u n t h e r R o l a n d , J o e S a g e r e r , I o u r i S e d y k h , W o j t e k S k u l s k i , C h a d d S m i t h , M a c i e j S t a n k i e w i c z , P e t e r S t e i n b e r g , G e o r g e S t e p h a n s , A n d r e i S u k h a n o v , M a r g u e r i t e B e l t T o n j e s , A d a m T r z u p e k ,

C a r l a V a l e , S e r g e i V a u r y n o v i c h , R o b i n V e r d i e r , G á b o r V e r e s , P e t e r W a l t e r s , E d w a r d W e n g e r , F r a n k W o l f s , B a r b a r a W o s i e k , K r z y s z t o f W o ź n i a k , B o l e k W y s ł o u c h

A R G O N N E N A T I O N A L L A B O R A T O R Y B R O O K H A V E N N A T I O N A L L A B O R A T O R YI N S T I T U T E O F N U C L E A R P H Y S I C S P A N , K R A K O W M A S S A C H U S E T T S I N S T I T U T E O F T E C H N O L O G Y

N A T I O N A L C E N T R A L U N I V E R S I T Y , T A I W A N U N I V E R S I T Y O F I L L I N O I S A T C H I C A G OU N I V E R S I T Y O F M A R Y L A N D U N I V E R S I T Y O F R O C H E S T E R

Outline of the Talk

•The PHOBOS Experiment

•The Data and Analysis

•The Nature of the Matter formed in AA Collisions

•Discovery of Simple Scaling Rules

The PHOBOS Perspective on Discoveries at RHICThe PHOBOS White Paper

nucl-ex/0410022

Spectrometer

Paddle Trigger Counter

NIM A 499 (2003) 603

Octagon

TOF

SpecTrig

T0 counter

•44- Multiplicity Detector•MMagnetic Spectrometer + TOF

The PHOBOS Experiment (2004)

Multiplicity and Trigger Detectors

Octagon Paddle Trigger Counter

Ring Counter

ZDC

Counting Particles

Bulk of Particles pT<1GeV

Vertex Tracklets (I)

= 1 – 2

= 1 – 2

Tracklets are two point tracks

that are constrained by

the event vertex.

|| < 0.04 || < 0.3

Vertex Detector Event Display

Vertex Tracklets (II)

All Pairs of Hits

Hit Counting (I)

Octagon, Ring and Vertex Detectors (unrolled)

Count Hits or Deposited Energy

0 +3-3 +5.5-5.5

Discriminating BackgroundE

(“M

IP”)

20 64-2-6 -4

04

812

20 64-2-6 -4E

(“M

IP”)

04

812

Data Monte Carlo

Si

E vs. in the Octagon

From Vertex

Not from Vertex

Measure the Occupancy

!)(

NeNP

N

N=number of tracks/pad=mean number of tracks/pad

The numbers of empty and occupied padsdetermine the occupancy as a function of ,b

Method: Assume Poisson statistics

Ntra

cks/h

it pa

d

0-3%

50-55%

Octagon

Rings(central)

(peripheral)

Energy Loss Multiplicity

300 m Si

PRIM

TOTAL

i

NNiM

Measured S/N = 10 - 20 << Landau Width

Use Non-Hit pads - forCommon-Mode Noise Suppression

M = 240 ± 15 ± 5 ± CMN for one sensor (120 channels) at = 0

NoiseCommonNoiseRandomMLandauM 2i

2i

0.30 - 0.40

Energy deposited in ith pad (truncated)corrected for angle of incidence

Mean energy loss for oneparticle traversing pad RATIO OF TOTAL TRACKS

TO PRIMARY TRACKS

Elliptic Flow

(reaction plane: R)

dN/d(R ) = N0 (1 + 2V1cos (R) + 2V2cos (2(R) + ... )

View along beamline

Fourier decomposition of the Azimuthal Multiplicity Distribution:

Poskanzer and Voloshin, Phys. Rev. C58 (1998) 1671

Best estimate event plane

iii

iii

w

w

2cos

2sintan

21 1

2

V2 = < cos (2(2)) > /cos(2(2a 2

b))> )1/2

Directed flow Elliptic flow

Centrality Determination

NPart and NColl (Au+Au)

Paddle Mean

NPart

NPart and Ncoll (d+Au)

Multiplicity DistributionPseudorapidity

Glauber Calculation•Hijing 1.383

•Hulthen w.f.•41mb inelastic cross-section

•Full GEANT Simulation

HIJING Simulation

dN/d

Cou

nts

The Data

Au+Au

d+Au

Also pp at 200 GeV

Au+Au at 62.4 GeV/u

Spectra – Talk by Adam Trzupek

Particle Density near Mid-RapidityModels prior to RHIC

PRL 88, 22302 (2002)PRL 91, 052303 (2003)

PRL 85, 3100 (2000)

arXiv:nucl-ex/0405027

On the low side of

nearly all predictions

Initially released Energy per Unit Volume 5 GeV/fm3

Note: Energy Density inside Proton ≈ 0.5 GeV/fm3

11

45

1000~alld

dN

Therefore total energy released in

|| < 1 is ~2000GeV

Num

ber

of P

artic

les P

r odu

ced

at y

=0

Energy of Collision

“Relevant” Initial Volume ~ R2 ( 1 fm) 2

<E> ~ 0.7 GeV

Data from: PRL 85, 3100 (2000); PRL 88, 22302 (2002); PRL 91, 052303 (2003); arXiv:nucl-ex/0405027

dNch

/d

Energy per Unit Volume

The Energy Density is High (3-5 GeV/fm3)

and

Much Larger than Inside a Hadron

A Description in Terms of Hadronic Degrees of Freedom is

Inappropriate

Baryon Free at Mid-Rapidity

PRC 67, 021901R (2003)

Evidence from Flow

200 GeV Au+Au PHOBOS

preliminary

0 < < 1.50-55% central,

h+ + h-

PRL,89, 222301 (2002)Nucl. Phys.A715, 611c (2003)

Approaches Hydro Limit

Evidence from Low pT Particles

In a large volume, weakly interacting system we

would expect the development of particles

with long wavelength

arXiv:nucl-ex/0401006

PHOBOS PHENIX

Suppression of High-pT Particles

Au+Au

0-6%

200 GeV

PHOBOS d+Au 200 GeV

PRL 91, 072302 (2003)

The System is Strongly Interacting

atEarly Times (2fm/c)

PHOBOS

E895 E895 E8953.0 GeV Au+Au

BRAHMS

prel.NA49 NA49

3.6 GeV Au+Au

4.1 GeV Au+Au

8.8 GeV Pb+Pb

17.3 GeV Pb+Pb

200 GeV Au+Au

Plateau in Pseudorapidity Distributions is Misleading

Rapidity Distributions of Pions are Gaussian

PRL 91, 052303 (2003)

arXiv:nucl-ex/0403050

No Plateau in Rapidity Distributions

PHOBOS Preliminary v2200

PHOBOS v2130

No Boost-Invariant Plateau for v2

PRL 89, 222301 (2002)

Limiting Fragmentation in pp

beamy PRL 91, 052303 (2003)

6% central

Au+Au

dNch

/d/<

Npa

rt>/

2

Scaling – Limiting Fragmentation

PHOBOS

arXiv:nucl-ex/0403033

A Rest Frame p or d Rest Frame

Submitted to PRL

arXiv:nucl-ex 0406021

Dependence of v2 on sNN and

Limiting Fragmentation in v2

Submitted to PRL

arXiv:nucl-ex 0406021

Longitudinal Scaling

•Originally expected boost-invariance in mid-rapidity region not observed.

•Scaling in fragmentation region seen in Au+Au, d+Au and (previously) in pp and p+A

Nucl.Phys. A715 (2003) 65-74

PRL 91, 052303 (2003)

PHOBOS

Connection between Central and Fragmentation Regions

pp pX pp X

provided Mx2 is the same

Brenner et al

In pp collisions, on average, approximately half the energy goes into the

leading baryon

A.Brenner et al.Phys.Rev.D26 (1982)1497l

How to Compare Au+Au, p+p, and d+Au

arXiv:nucl-ex/0301017

arXiv:nucl-ex/0403033

Au+Au higher than

pp and d+Au.

But……..

Universal Curve for Nch vs (s)

arXiv:nucl-ex/0301017

When pp corrected

for leading baryon

Npart scaling for:

A, KA, pA, dA, AA

10 GeV to 200 GeV

Npart from 2 to 350

E178: J.E.Elias et al., Phys.Rev.D22(1980) 13

arXiv:nucl-ex/0403033

Phobos and E178 data

Preliminary

pp chosen to have the same available

energy

NPart Scaling of Total Particle Production – Independent of Nature of Collision System

Factorization of Energy and Centrality Dependence

PRC 70 (2004) 021902R

Where are the Minijets?

NNpartNNpartmid sfNfsNR 21,

arXiv:nucl-ex/0403033

Centrality Dependence of d+Au

arXiv:nucl-ex/0403033

SAME SEEN IN p+A AT ENERGIES 50-200 GeV

Factorization into Geometric and Energy Parts

arXiv:nucl-ex/0405003PHOBOS

62.4 GeV

200 GeV

Energy and Geometry Factorize – Independent of pT

Summary and Conclusions•High Energy Density Strongly Interacting Matter – Description in Terms of Simple Hadronic Degrees of Freedom Inappropriate. ( No Direct Evidence of Color Deconfinement or Partonic d.o.f)

•The Matter is Strongly Interacting – Not the Weakly Coupled QGP Originally Envisioned

•Simple Scaling Rules Unite the Data. Suggest Global Constraints or Universality

The PHOBOS Perspective on Discoveries at RHICThe PHOBOS White Paper

nucl-ex/0410022