overview of results from phobos experiment at rhic

Andrzej Olszewski/INPStrange Quarks in Matter, Frankfurt 2001 -1-

Overview of Resultsfrom PHOBOS experiment at RHIC

Andrzej OlszewskiInstitute of Nuclear Physics, Kraków, Poland

for the

PHOBOS Collaboration


PHOBOS at RHIC


PHOBOS Collaboration

ARGONNE NATIONAL LABORATORY

BROOKHAVEN NATIONAL LABORATORY

INSTITUTE OF NUCLEAR PHYSICS, KRAKOW

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

NATIONAL CENTRAL UNIVERSITY, TAIWAN

UNIVERSITY OF ROCHESTER

UNIVERSITY OF ILLINOIS AT CHICAGO

UNIVERSITY OF MARYLAND

Birger Back, Nigel George, Alan Wuosmaa

Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Stephen Gushue, Dale Hicks, Burt Holzman,Robert Pak, Marc Rafelski, Louis Remsberg, Peter Steinberg, Andrei Sukhanov

Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak

Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Michel Rbeiz, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch

Chia Ming Kuo, Willis Lin, Jaw-Luen Tang

Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly,Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs

Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer

Richard Bindel, Alice Mignerey


The PHOBOS Detector (2001)

Ring Counters

Time of Flight

Spectrometer

• 4 Multiplicity Array

- Octagon, Vertex & Ring Counters• Mid-rapidity Spectrometer• TOF wall for high-momentum PID• Triggering

- Scintillator Paddles Counters- Zero Degree Calorimeter (ZDC)

Vertex

Octagon

ZDC

z

yx

Paddle Trigger Counter

Cerenkov

137000 silicon pad readout channels

1m


Central Part of the Detector

(not to scale)

0.5m


PHOBOS in PHOTOS

Octagon Detector

Ring Counter

Silicon pad sizes Octagon Detector: 2.7 x 8.8 mm2

Vertex Detector: 0.5 x (12-24) mm2

Ring Counter: (5x5) - (10x10) mm2

Spectrometer: (1x1) - (0.5x19) mm2

~ 25

cm

Spectrometer

Vertex Detector


PHOBOS Running Summary

• Commissioning: (May-July)• Part of silicon installed

• Au+Au collisions at

sNN = 56 GeV and 130 GeV

First published results on dNch/d|||<1, sNN=56 and 130 GeV

• Physics run: (July-August)• 1 spectrometer arm setup

• Au+Au collisions at sNN =

130 GeV: ~3.5 M collisions total

Year 2000 running

Essentially flawless performance of PHOBOS detector

• Commissioning: (mid-July)• Add 2nd spectrometer arm

• Au+Au collisionssNN = 130 GeV and 200 GeV

First published result on dNch/d|||<1, sNN= 200 GeV

• Physics run: (mid-August)• 2 spectrometer arm setup

• Au+Au collisions at sNN =

200 GeV: ~3.5 M collisions by end of August

Year 2001 running


Charged particle density

•Versus energyCentral collisions, 0:sNN = 56 and 130 GeV PRL 85 (2000) 3100sNN = 200 GeV, submitted to PRL

•Versus centrality:sNN = 130 GeV, 0 submitted to PRC

•Versus angle and centralitysNN = 130 GeV, ||<5.4 PRL 97 (2001) 102303

Elliptic flow

•Versus angle and centralitysNN= 130 GeV, ||<5.3

QM2001, to be submitted soon

Results to Date

Particle ratios p/p, K-/K+, -/+

Central collisions: sNN = 130 GeV

PRL 97 (2001) 102301

_


t (ns)

Eve

nts

Triggering on Collisions

Negative

Paddles

Positive Paddles

ZDC N

ZDC PAu Au

PPPN Paddle Counter

• Coincidence between Paddle counters at t = 0 defines a valid collision

• Paddle + ZDC timing reject background

• Sensitive to 97% of inelastic cross section for Au+Au at sNN = 130/200 GeV

ValidCollision

ZDCCounter


Selecting Collision Centrality

Paddle signal (a.u.)

Data

Co

un

ts

Larger signal = more central collision.

Central Collision: Large Npart

Peripheral Collision: Small number of participating nucleons

“side” view of colliding nuclei “side” view of colliding nuclei

3<||<4.5

PN PP

b


Multiplicity in Paddles

Np

art

% (Paddle mult.) % (Npart)

HIJING + GEANT

(3<||<4.5)

% Error on Npart

Analysis is limited to events with Npart > 70

Centrality Determination

Npart


Charged Particle Density

Measurement

• Energy Dependence • System Size • Angular Dependence

Context

Study

•Energy/entropy density production

•Response to properties of nuclear/partonic medium

•Saturation•Jet quenching

• Importance of hard and soft processes

•Re-scattering effects•Long-range particle correlations

•Memory of the initial geometry in the final state

• Charged Particle Density• Event Anisotropy - Flow


Tracklets

• Tracklet: Two-hit combination + vertex position

• >300 tracklets/central event in Vertex, >100 in Spectrometer

Vertexdetector


Analog and Digital Hit-Counting

0 +3-3 +5.5-5.5

DigitalCount hits above energy threshold,assume Poisson-statistics in thedistribution of hits among the pads

AnalogUse deposited energy (dE/dx) in each pad to estimate number of particles that crossed the pad

Hits in Octagon, Ring and Vertex

for single event


Charged Particle Density

Four counting methods

• Tracking detectors - measurements

• Tracklets in Spectrometer• Tracklets in Vertex detector

• Single layer detectors - 4 measurements

• Use deposited energy (dE/dx) in each Si-pad• Count hits above threshold, assume Poisson-statistics

All four measurements corrected for: secondary particles, feed-down from weak decay, stopping particlesSystematic uncertainty: from 4.5% (Tracklets in Spectrometer) to 10% (Hit counting)

All four methods deliver consistent results - final results averaged


Charged Particle Density at 0

PHOBOS first measurements

- charged particle density

- in mid-rapidity- for 6% of the most central events

dNch/d|||<1(56 GeV) = 408 12(stat) 30(syst)

dNch/d|||<1(130 GeV) = 555 12(stat) 35(syst)

dNch/d|||<1(200 GeV) = 650 35(syst)


dNch/d|||<1 vs Energy

AGS

PHOBOS 56

RHIC 130

SPS

p+p_

Preliminary

PHOBOS 200

nucl-ex/0108009Submitted to PRL


PHOBOS Measurement

90% confidence

dNch/d|||<1: Ratio 200/130 GeV

R200/130 =

1.14 +/- 0.05(sys)

New results for 200 GeV

nucl-ex/0108009

dNch/d|||<1 =

650 35

dNch/d|||<1/0.5Npart

= 3.78 0.25


Scaled multiplicity increases

with Npart

Similar to Kharzeev/Nardi

dNch/d = a·Npart + b·Ncoll

Stronger than in EKRT

Less steep than in HIJING

Evolution with Npart

Multiplicity at =0 vs Centrality

sNN =130GeV

nucl-ex/0105011


Multiplicity in 4 - Centrality Dependence

The width of the distribution changes with centrality

sNN = 130GeVPRL 97 (2001) 102303


3% most central collisions

<Nch> = 4200 470

(dNch/d)/(0.5Npart) central(0-6%)

peripheral(35-45%)

Multiplicity in 4 - Centrality Dependence

central(0-6%)

Total Nch(|| 5.4)

Additional particle

production near =0

PRL 97 (2001) 102303


Change in dN/d with Energy

200 GeV - 6%

130 GeV - 6%

UA5 200 GeV(NSD)

•First attempt to compare dN/d shape for Au+Auat 130 and 200 GeV

•‘Limiting fragmentation’ check by plotting dN/d with - Ybeam

•Agreement for AA in thefragmentation region

•Different slope when compared to pp

Systematic errors not shown


Azimuthal Angular Distributions

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

V2 determines to what extent the initial state spatial/momentum anisotropy is preserved in the final state.

R reaction plane

“head on” view of colliding nuclei

b


• Anisotropy increases for peripheral collisions

• Large V2 signal compared to lower energy

Centrality Dependence of V2

V2

Normalized Paddle Signal

SPS

(STAR : Normalized Nch )

|| < 1.0 sNN=130GeV

PHOBOS Systematic error ~ 0.007

Peripheral Collisionsb

Central Collisionsb

17 GeV

Preliminary


V2 (elliptical flow) vs

• Averaged over centrality• V2 drops for || > 1.5

V2PHOBOS Preliminary

STAR (PRL)

PHOBOS Systematic error ~ 0.007

sNN= 130 GeV

All Charged


• Microscopic viewpoint:

Antiproton/proton ratio determined by: • Baryon stopping;

• Pair production;

• Absorption in nuclear medium

Why Measure Antiparticle/Particle Ratios?

Net

Bary

on

Nu

mb

er

y

• Thermodynamic viewpoint: Particle ratios can be used to estimate hadro-chemical potentials


Anti-particle / particle Ratios

p

K+

+

K-

p

-

70 c

m

•Tracking in the spectrometer•Alternate 2T magnetic fields•Energy loss and momentum


= 1.00 ±0.01 (stat) ± 0.02 (syst)

K/K+ = 0.91 ± 0.07 (stat) ± 0.06 (syst)

p/p = 0.60 ± 0.04 (stat) ± 0.06 (syst)

K-/K+ vs Energy p/p vs Energy

Results for Ratios

Higher values of K-/K+ and p/p than at lower energies

PRL 97 (2001) 102301


Results consistent withB=45±5 MeV, which ismuch lower than that observedat SPS (~240-270 MeV)

Assumes freezeout temp ~170 MeV in statistical model of Redlich (QM01)

Results for RatiosPRL 97 (2001) 102301


Charged Particle Densities(Entropy)

• dNch/d at 0 per participant• First look at Au+Au at 200 GeV

- increase in density by 14% compared to 130GeV• Logarithmic increase with energy from AGS to RHIC

• Npart evolution stronger then linear, indicates increasing contributions from hard processes

• dNch/d in 4• Additional particle production concentrated

near 0 for central events• Decreasing width with increasing centrality• On average 4200 particles in central collisions at 130GeV

Summary 1


Summary 2

• Elliptic flow• Increase of elliptic flow (V2) for more peripherial events

• Increase of flow effect with increasing energy• V2 at mid-rapidity up to 0.06

• V2 drops for || > 1.5

• Particle ratios• K-/K+ and p/p significantly higher than at AGS or SPS

~ 45 MeV vs 270 MeV at SPS• p/p between HIJING and RQMD predictions• Central region closer to baryon free state


Outlook: Year 2001

• 100x statistics• Physics:

• low-pT physics

• Spectra• HBT

• Resonances (at low pT

• Event-by-Event physics

• Energy systematics• Species systematics


The End


Silicon Signals 200 vs 130 GeV

Signal shapes are (almost) identical:

Methods developed for 130 GeV good also at 200 GeV


Tracklet counting method (2 Si layers)

Tracklets three-point tracks two-hit combinations+measured event vertex

x ~ 450 my ~ 200 mz ~ 200 m

Spectrometer Vertex Detector

D = (2 + 2) 1/2 < 0.015 || < 0.04 , || < 0.3

The measurements in the two different Si pad detectors(different location, granularity, acceptance, systematics)

dNch/d at mid-rapidity: ||<1


)N,Z(N

ddN

hitsvtx

Datatrackletsch

(Zvtx, Nhits) = MCprimaries

MCtracklets N/N

TOTAL systematic errors: 4.5% (Spectrometer) 7.5% (Vertex)

FINAL RESULTSCombined Vertex and Spectrometer measurements

(weighted by the inverse of their total systematic error)

Tracklet analysis


•Count hits in (,Npart) bins

•Evaluate number of particles per hit pad * Assume Poisson statistics P(N)=Ne-/N! - determined by the measured ratio (p) of occupied to empty pads: = ln(1+p) * Perform a multi-Landau fits to E (,Npart) spectra (convoluted with gaussian)

•Calculate acceptance

•Fold in a final background correction (MC) FBkg(,Npart) =(dNch/d)MC

Truth/(dNch/d)MCReconstructedBkg

hit/trhitsch FA

NN

d

dN

Nhits(,Npart)

Ntr/

hit(,Npart)

A(,Zvtx)

FBkg(,Npart)

Hit Counting method


•Get energy deposited in each Si pad E(,Npart)

•Divide this energy by the average energy per track <E()>

•Correct for the fraction of primaries fprim()

<E> and fprim are obtained from HIJING+GEANT simulations

primch f

EE

d

dN

Hit Counting and Analog methods agree to within 5%(systematic errors in each method are 10%)

Analog method


• Secondary particles (+2%)• Little material between interaction point and sensitive

volume

• Antiproton absorption in detector (+8%)• From GEANT simulations

• Feed-down from weak decays (-2%)• Reduced by tracking within 10cm of vertex• Further limit by distance-of-closest-approach cut on tracks

Corrections to the Raw Numbers

overview of results from phobos experiment at rhic

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