Phobos at RHIC

Edmundo GarciaUniversity of Illinois at Chicago

for the PHOBOS Collaboration

IV Latin American Symposium on Nuclear PhysicsMexico City

Septiembre 2001

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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

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Outline

• Introduction• The detector

• Trigger System• Multiplicity Detector• Spectrometer

• Physics Measurements• Multiplicity• Particle Ratios• Flow

• Final Notes

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atoms

particlesnucleus

qgp

energy/density

Big quark-gluon p + n low mass nuclei neutral atom star dispersion of TODAYBang plasma formation formation formation formation heavy elements

time 10-6s 10-4s 3 min 400,000 yr 109 yr >109yr 15x109yr

Copyright 1998 Contemporary Physics Education Project (CPEP)

time

Physics

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AGS: s = 4.8 GeV

SPS: s = 17 GeV

RHIC: pp, pA, AA

s : 30 - 200 GeV

Relativistic Heavy Ion Collider

LHC s = 5.2 TeV

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PHENIX STAR

PHOBOSBRAHMS

Two nuclei approach relativistically contracted

Hard collisions take place during first stages of reaction

Interactions of produced particles act at soft and hard scales

Final particles freeze out towards the detectors

RHIC collisions

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Experiment

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• One of the “small” RHIC experiments, size (6 x 6 x 3 m), and people (50 scientist)

• Designed to be able to examine and analyze a very large amount of minimum bias interactions (high trigger rate capability)

• Measurements • Multiplicity and angular distribution of charged

particles• < 5.3 over 4 coverage event by event

• Particle spectra• 0.5 < < 1.5 and 2 x 11o in (azimuthal)• Covers about 1% of particles • Capable to reconstruct low momentum particles ( 55

MeV/c )

pseudorapidityln (tan)) rapidity y = 1/2 * ln [( E + p)L/ (E - pL)]

Phobos

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Eve

nts

ZDC N

ZPAu Au

x

zPPPN

CN

CP

paddle counters

ValidCollision

cerenkov counters

zdc

goodvertex Phobos

Trigger

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Centrality. number of spectator neutrons in ZDCnumber of spectator neutrons in ZDC = f (Epaddles)Centrality Epaddles

Centrality measurement

charged spectators charged spectators

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peripheral

central

6%

Centrality selection

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Sensor Type Pad Size(width x height mm2)

Pad( rows x columns)

Spectrometer

Type 1 1 x 1 22 x 3Type 2 0.43 x 6 22 x 70

Type 3 0.67 x 7.5 8 x 64Type 4 0.67 x 15 8 x 64

Type 5 0.67 x 19 4 x 64Multiplicity

Octagon 2.6 x 9.5 30 x 40

Ring 50 64 padVertex 1 0.32 x 23.3 2 x 256

Vertex 2 0.32 x 46.6 2 x 256

Phobos silicon

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Position Sensitive Silicon DetectorsPosition Sensitive Silicon Detectors

StripStrip

DriftDrift

Pad/PixelPad/Pixel

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Multiplicity and Vertex Detector

Run 5374, Event 79495

vertex

octagon

rings

Multiplicity and vertex detectors

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SpectrometerSpectrometer

-

K-

pbar

pid

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Use energy deposited in each Si pad

Hit Counting, Analog and Tracklet methods agree to better than ±4%

Multiplicity measurement

Count hits above threshold, assume Poisson statistics

Analog information

Counting

Tracking detectors

Count three-point tracklets: two layers of Si and event vertex match

Single Layer Silicon

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Energy Systematics: 3-200 GeV

pp

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200 GeV6% central

Pseudo-Rapidity Dependence

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Centrality Dependence at =0

Kharzeev/Nardi

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Anti-particle / particle ratios

z

x

10 cm

y

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

• Determine ratio of -/+, K-/K+, p/p• Compare to lower energy (AGS/SPS) results

70 cm

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= 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

Particle Ratios at 130 GeV from PHOBOS

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Elliptic Flow

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

Elliptic flow

reaction plane

R

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Peripheral

b

Central

bNormalized Paddle Signal

(STAR : Normalized Nch )

PHOBOS Systematic error ~ 0.007

V2

Centrality dependence of V2

||<1.0sNN=130GeV preliminarypreliminary

~4 Elliptic Flow @ 130 GeV

SPS 17 GeV

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• Multiplicity• Increase in the slope of dn/d as a function of the

energy when compared to ppbar• dn/d increase with energy from 130 to 200 GeV is

modest and concentrated at mid-rapidity

• Other measurements:• Elliptic flow: Strong and -dependent• K-/K+ ratio does not follow linear extrapolation from lower

energy• Antiproton/proton ratio increasing towards unity

(approaching a baryon-free regime)• Centrality dependence of dN/d.

• No conclusive/clear signal of QGP (so far), it seems that any finding will take a systematic scan of all QGP probes at RHIC energies

Summary

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Acceptance

0 +3-3 +5.5-5.5

multiplicity detector

spectrometer

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• dN/d• Background subtraction on tracklets < ±5%• Uncertainty on due to model differences < 5%

• Total contribution due to feed-down correction < 4% (typically 1%)

• Total fraction lost due to stopping particles < 5%• Both are corrected via MC normalization

• Total uncertainty on dN/d is ±8% Npart

• Loss of trigger efficiency at low-multiplicity <10%• Uncertainty on Npart <1%

• Uncertainty in modeling paddle fluctuations • Uncertainty on Npart <6%

• ( dN/d / Npart )130 / ( dN/d / Npart )56

• Many uncertainties cancel in the ratio

Systematic Uncertainties

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Particle Ratios (Chemistry)

06.004.060.0

06.007.091.0

02.001.000.1

p

p

K

K

stat sys

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Double Metal, Single sided, AC coupled, polysilicon biased detectorsproduced by ERSO, Taiwan

AC coupled Pad (p-implant + metal 1 pad)

polisilicon bias resistor

metal 2 readout line

contact hole metal 1- metal 2

Phobos silicon

p+ Implant

n+

Polysilicon Drain Resistor

bias bussignal lines

vias

300um 5kOhm nSi

0.2um ONO1.2um ONO