Gábor I. Veres

Massachusetts Institute of Technology

for the Collaboration

International Workshop on Hot and Dense Matter in Relativistic Heavy Ion Collisions

March 24-27, 2004, Budapest

Hadron pT Spectra by PHOBOS from 0.03 to 6 GeV/c

Gábor I. Veres

Collaboration (March 2004)

Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley,

Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski,

Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell,

Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman, Richard Hollis,

Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan, Piotr Kulinich,

Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej

Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Michael Ricci,

Christof Roland, Gunther Roland, Joe Sagerer, Helen Seals, Iouri Sedykh, Wojtek Skulski,

Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov,

Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Siarhei Vaurynovich, Robin Verdier, Gábor Veres, Edward

Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch

ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY

NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

Gábor I. Veres

Outline

Hadron pT-spectra: why and how to study them?

The extremes: low and high pT

Small (d+Au) to large (Au+Au) colliding systems

Flavour dependence – charged and identified hadron spectra. Particle ratios.

Importance in the details: centrality and rapidity dependence

Gábor I. Veres

Low pT

Probing long distances

Radial flow

“soft” physics (Npart?)

Longitudinal and Transverse Dynamics

High pT

Suppression, E loss, quenching…

Initial/final state effects (different systems)

“hard” physics (Ncoll?)

dN

ch/d

Gábor I. Veres

The PHOBOS Detector (2001)

137000 Silicon Pad Channels

1m

12m Be Beampipe

Spectrometer

Octagon

Vertex

Ring Counters

Paddle Trigger Counter

Čerenkov Counter

DX magnet DX MagnetZDC ZDC

NIM A 499, 603-623 (2003)

Au+Au

Gábor I. Veres

Octagon & Vertex

Spectrometer armRing

Silicon Detectors

Gábor I. Veres

dN/dpT

PID Mass +Charge

0.03 0.2 1.0 pT, GeV/c

up to 6 GeV/c

Stopping in Si dE/dx in Si ToF+Si Si

Mass

near mid-rapidity

Z

ToF

Spectra and PID in

Charge

Gábor I. Veres

Yields of (+ + ) , (K+ + K), (p + p)

pT = 30 – 200 MeV/c (depending on particle mass)

I. Hadrons in the low pT range

Negligible B field + multiple scattering:

charge sign cannot be measured

Probing long distances (truly non-perturbative QCD regime) Radial flow DCC (enhanced pion yields??) Dynamical fluctuations at phase transition??

Gábor I. Veres

PHOBOS Capability of Low pT Measurements

z

-x

10 cm

y

70 cm B=2T

Drawback:

Advantages: Sensitive detector layers close to the IP Little material between the IP and Si layers High segmentation of the Si detectors

small acceptance of the spectrometer

Gábor I. Veres

Mass measurements

(‘energy-range’ method)

Cuts on dE/dx per plane mass hypothesis

X[c

m] AB

CDE F

Z[cm]

Beam pipe

0 10 20

0

10

20

Z [cm]

Search for particles stopping in

the 5th spectrometer plane

A B C D E

dE

/dx

Ek= 8 MeV

P Ek=21 MeV

K Ek=19 MeV

Cuts on Eloss (Ek=kinetic energy)

momentum hypothesis

• Eikin=dEi+dEi+1+dEi+2…

• Mip = dE/dxi * Ei

kin m (1/2) ( m2)

Corrections acceptance, efficiency absorption, background

silicon plane

Finding very low pT particles

MC

Gábor I. Veres

Test of the method:

Reconstruction of lowmomentum MC particles

Au+Au sNN=200 GeV 15% central

MC

Measuring particle mass

p+p

K++K–

++ DATA

Gábor I. Veres

Au+Au sNN=200 GeV 15% central

-0.1< y <0.4

Invariant yields (Au+Au)

+

+

K+

+K–

p+p

nucl-ex/0401006, submitted to PRL

Momentum and energy:

From carefully calibrated MC

Yields: binned in pT and

corrected

Gábor I. Veres

Comparison to the Spectra Measured at Intermediate pT Range

PHENIX - open symbolsPHOBOS –closed symbols

Log scale!

Fit PHENIX spectra (nucl-ex/0307022)

for mT<1 GeV/c2:+1 for baryons-1 for mesons

Fits: solid curvesExtrapolations: dashed curves

Tfit: + - 0.2290.005 K + K- 0.2930.010 p + p 0.3920.015

Extrapolation of the fits to low pT agrees with our low-pT yields.Tfit increases with mass consistent with the collective transverse expansion

1/[exp(mT/Tfit)±1]

Gábor I. Veres

Model Comparisons

Event generators are not able to consistently describe low pT yields.HIJING overpredicts all yields

Gábor I. Veres

I. Low pT: Summary

No enhancement of low pT yields is observed (compared to extrapolations of intermediate pT spectra).

Spectra flatten at low pT transverse expansion Constraints for models and integrated yields.

Centrality dependence of the low pT yields

Negatively charged particle yields

Attempt to measure BE correlations at very low mT

Future (high statistics Au+Au run):

Gábor I. Veres

II. High-pT spectra. Tracking

x10 cm

1

2

By

z

Bea

m

1) find straight tracks in the field-free region

2) curved tracks found in B field by clustering in (1/p, ) space

3) Pieces matched

4) Momentum fit using the full track, and detailed field map

5) Quality cuts, DCA cuts

Very clean track sample with high efficiency

Gábor I. Veres

High-pT spectra: acceptance

Acceptance Momentum resolution

2001 Au+Au run (200 GeV): Data Sample:

7.8 M minimum bias Au+Au events (2004: over 200 M)

32 M reconstructed particles

Gábor I. Veres

Data+MC

Npart

Triggering on Collisions & Centrality

HIJING + GEANT Model of paddle trigger

Data

Centrality Determination

% (dNch signal) % (Npart, Ncoll, b)

3% uncertainty in TOT (trigger efficiency)

less than 10% uncertainty in Npart for Npart>100

Paddle Signal (a.u.)

Gábor I. Veres

“Participant” Scaling

Ncoll= # of NN collisions: ~A4/3

L~A1/3

Npart/2 ~ A

“Collision” Scaling

Why Centrality Matters?

Ncoll

Npart

b [fm]

Gábor I. Veres

PHOBOS-Spectra @ 200GeV Au+Au

Spectra corrected for

Acceptance/efficiency

Ghost tracks

Momentum resolution

Variable bin width

Secondaries, feed-down

At 200 GeV, min. bias p+p reference data exists (UA1)

(Ge

V/c

)-2

0.2<y<1.4

x 10-1

x 10-2

x 10-3

x 10-4

x 10-5

0.2<y<1.4

(h++h-)/2

Phys.Lett. B 578 (2004) 297

Gábor I. Veres

Scaled Au+Au Spectra / p+p-Fit

344 ± 11

276 ± 9

200 ± 8

138 ± 6

93 ± 5

65 ± 4

<Npart>

0-6%

6-15%

15-25%

25-35%

35-45%

45-50%

Centrality

Centrality range:

<b> from 10 to 3 fm

<> from 3 to 6

Phys.Lett. B 578 (2004) 297

Gábor I. Veres

Evolution with Centrality (Au+Au)

gradual change of shape

peak develops at 1.5 GeV/c

Phys.Lett. B 578 (2004) 297

Spectra normalized to a fit to the pT spectrum at Npart = 65 (most peripheral bin)

Low and high pT: approximate scaling with Npart

Gábor I. Veres

Is Suppression an Initial or Final State Effect?

Strong suppression of hadron yields at high pT!

high density strongly interacting matter (final state)? ORmulti-partonic effects in the nuclear wave-function

(initial state)?

Turn off final state to discriminate between the two scenarios d+Au collisions

Gábor I. Veres

Predictions for d+Au

Vitev, Phys.Lett.B 562 (2003) 36Vitev and M.Gyulassy, Phys.Rev.Lett. 89 (2002)

Kharzeev, Levin, McLerran, Phys.Lett.B 561 (2003) 93

“~30%suppression of high pT particles”(central vs peripheral) 16% increase central vs peripheral

Parton Saturation(initial state)

pQCD (final state)

Gábor I. Veres

PHOBOS Results from d+Au

Centrality <Npart> <Ncoll>

70-100% 3.30.7 2.20.6 40-70% 6.70.9 5.40.8 20-40% 10.90.9 9.70.8 0-20% 15.51.0 14.60.9

Cronin Effect in d+Au vs. Centrality

6% mostcentral Au+AuPhys.Rev.Lett. 91, 072302 (2003)

peripheral

central

PRL 91, 072302 (2003)

Gábor I. Veres

No high-pT suppression at y~0 in d+Au

Initial state effects may show up at HIGHER rapidities?!

(small-x region of the Au nucleus is probed)

Cronin Effect as a Function of (d+Au)- all centrality bins together -

Cronin Effect as a Function of (d+Au)- all centrality bins together -

Evolution of RdAu with

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II. Inclusive pT spectra: Summary

Charged hadron spectra measured in d+Au and Au+Au collisions vs. pT and centrality

High-pT suppression in Au+Au observed (compared to Ncoll scaling)

Control experiment: d+Au spectra

Suppression is not an initial state effect (strongly interacting quark-gluon liquid?)

Latest findings show suppression at high rapidities in d+Au!

Gábor I. Veres

Antiparticle/particle ratios as a function of Npart and pT

Identified particle spectra

III. Identified Hadrons

Flavour dependence of the effects shown Baryon transport in small and large systems Properties of the system at chemical freezeout Scaling features of different species (mT)

Important to compare more elementary (d+Au) and heavy ion (Au+Au) collisions

Motivation:

Gábor I. Veres

pT (GeV/c)0.05 0.5 5.0

Stoppingparticles dE/dx TOF

Particle ID from low to high pT

PHOBOS PID Capabilities

1 2 3 4 50p (GeV/c)

30

40

50

60

70

1/v

(ps/

cm)

0 5 10 15 20 25ETOT (MeV)

0

1

MP (

10-3

Ge

V2/c

m)

Gábor I. Veres

z

-x

10 cm

y

70 cmReversible 2T magnetic field

Two symmetric spectrometer arms

Independent measurements Acceptance & efficiency corrections cancel

B=2T

Antiparticle to particle ratios in Au+Au

Careful corrections for feed-down, absorption in the material, secondaries

Gábor I. Veres

<–>/<+> = 1.025 ± 0.006(stat.) ± 0.018(syst.)<K–>/<K+> = 0.95 ± 0.03(stat.) ± 0.03(syst.) <p>/<p> = 0.73 ± 0.02(stat.) ± 0.03(syst.)

Au+Au sNN = 200 GeV, 12% most central

High precision measurements

Corrections to the measured ratios : +3.7% absorption +0.7% secondary negligible -1.2% feed-down

p/p K-/K+

-/+

Result: ratios at sNN = 200 GeV Au+Au

Phys.Rev.C 67, 021901R, 2003

Gábor I. Veres

d+Au: =Ncoll

/Npart

d

Particle Ratios Using dE/dx PID

Submitted to Phys.Rev.C, nucl-ex/0309013

PHOBOS 200 GeV

Mean number of collisions per projectile nucleon <>

...p/p Compared to Models (p+p, d+Au)

200 GeV

PHOBOS Preliminary

Mean number of collisions per projectile nucleon <>

Gábor I. Veres

Particle Ratios Using dE/dx PID

PHOBOS 200 GeV

Subm. to Phys.Rev.C nucl-ex/0309013

Au+Au: =Ncoll

/(Npart

/2)d+Au: =N

coll/N

partd

Mean number of collisions per projectile nucleon <>

Gábor I. Veres

Identified spectra in d+AuOnly ToF wall can identify above 1 GeV momentum in PHOBOS

Many experimental challenges to solve

1.) high pT-reach desired

2.) high collision rate (10-100 kHz)

and low multiplicity in d+Au, p+p

x150 – x500

improving TOF RESOLUTION:

new start time detector

increased distance from interaction point

improving STATISTICS:

new high-pT trigger system (x15 – x50)

DAQ upgrade (x10)

Gábor I. Veres

SPECTRIG

T0

T0

mini-pCal

pCal

Moved TOF walls far (5 m) from IP New, on-line high pT Spectrometer Trigger

New start-time (T0) Čerenkov detectors On-line vertexing and ToF start time

Forward proton calorimeters on Au and d sides DAQ upgrade (x10 higher rate!)

Response to Importance of High

PT Studies

Upgrades in PHOBOS for the d+Au run (2003)

Au+Au

d+Au, p+pTOFTOF

Gábor I. Veres

Trigger detectors (d+Au)

Segmented scintillator detectors at 45 and 90 degrees from beam line

Combined with the ToF walls:

selects events with particle hitting ToF and SpecTrig walls enhances high-pT (straight) tracks: “online” tracking decision-making in 50 ns

SpecTrig

ToF

rejectedaccepte

d

Gábor I. Veres

High statistics d+Au track sample

1 2 3 4 50

p (GeV/c)

30

40

50

60

70

1/v

(ps/

cm) p

K

positives, 1.6<p<1.8 GeV/c

p

K

Particle/Antiparticle Ratios using the TOFd+Au

T

per projectile nucleon <>

Identified pT -spectra in d+Au

Not feed-down corrected

Scale uncertainty: 15%

Particle Composition in d+Au

Not feed-down corrected

Comparison: Low Energy d+Au

Cronin, PRD 11, 3105 (1975)-0.1< <0.2y

y0.2< <1.2-0.5< <-0.2y

lab=3.26

Not feed-down corrected

Identified mT-spectra in d+Au

Scale uncertainty: 15%m

T=m +p

T

2 2 2

Not feed-down corrected

Identified mT-spectra at 200 GeV

Subm. to Phys.Rev.Lett.nucl-ex/0401006

d+Au

Scale uncertainty: 15%

Not feed-down corrected

Au+Au

Spectra normalized at 2 GeV/c

200 GeV

2

PHOBOS Preliminary(no feed-down corrections)

Identified mT-spectra at 200 GeV

d+Au

Scale uncertainty: 15%

Not feed-down corrected

Au+Au

Spectra normalized at 2 GeV/c

200 GeV

Subm. to Phys.Rev.Lett.nucl-ex/0401006

Gábor I. Veres

III. Identified Hadrons: Summary

PHOBOS has PID coverage from 0.03 to 3.5 GeV/c in pT

First PID ratios and spectra shown from the PHOBOS TOF

Surprisingly small centrality dependence of p/p ratios

Approximate mT scaling in d+Au as opposed to Au+Au

Particle composition in d+Au: similar pT-dependence to lower energy data (but different overall proton fraction)

Gábor I. Veres

OutlookFuture of the PHOBOS hadron spectra program:

Further RAA measurements

Energy scan (63 GeV, …?)

Species scan (Cu+Cu ? Si+Si ?)

Identified spectra in p+p and Au+Au at 200GeV

Charged spectra at very low pT with charge separation

…and many other observables besides spectra!

Gábor I. Veres