for the Collaboration

Low-pT Spectra of Identified Charged Particles

in sNN = 200 GeV Au+Au Collisions

from PHOBOS Experiment at RHIC

Adam Trzupek

Institute of Nuclear Physics, Kraków, Poland

International Europhysics Conference on High Energy Physics

EPS (July 17th-23rd 2003) in Aachen, Germany

Adam Trzupek

Au+Au sNN=200 GeV

PHOBOS preliminary

RHIC experiments measured particle spectra at pT > 0.2 GeV/c => jet quenching

PHOBOS has a unique capability to measure particles down to very small transverse momenta << 0.2GeV/c

Particle pT distributions at RHIC

Adam Trzupek

3

2 Th

foeff

mTT

for pT << mh

Enhanced production of low pT particles is expected due to the presence of NEW long wave-length phenomena, e.g. large volume of the system at freeze-out should lead to an increase of low pT particle yields

Why production of particles with very low pT is interesting?

Enhanced yield of pions with pT < 0.2 GeV/c with respect to the B-E distribution may occur when disoriented chiral condensates (DCC) are formed

Mass-dependent suppression at low pT due to the collective transverse expansion of the system is expected

Adam Trzupek

• pT > 0.2 GeV/c track curvature in B field => MOM dE/dx in Si, ToF => PID

• pT = 0.03 - 0.2 GeV/c low pT particles stop in silicon wafers => PID, MOM

B field negligible ( layers A-F )

0 10 20 Z [cm]

X[c

m] A

BC

DEF

Be pipe

. .

pT and PID measurement in PHOBOS spectrometer

z

-x

10 cm

y

70 cm PHOBOS Spectrometer (2001) • two arms at mid rapidity• dipole magnetic field of 2T at maximum• 16 layers of silicon wafers• fine/optimal pixelization, precise dE measurement

(++-)

=> (K++ K-)

(p + p)

Adam Trzupek

Eloss = dEi , i = A, ... ,EMp = < Eloss dE/dx > mh

( mh2) (1/2) Bethe-Bloch function

A B C D E

Ek= 8 MeV

P Ek=21 MeV

K Ek=19 MeV

silicon layer

Candidates should have dE/dx values within dE/dx bands for (+

+-), (K++K-) and (p+p)

Particle identification at very low pT

d

E/d

x [M

eV]

Eloss [MeV]

(+

+-)

(K++K-)

(p+p)

MC

Mp

Correction for energy loss in layer E was applied under the assumption that particle stops in this layer. This correction procedure and energy lossfluctuations in beam pipe and Si lead to a correlation Mp vs Eloss

1 bands

Adam Trzupek

Experimental data sample

Eloss [MeV]

(++-)

(K+

+K-)

(p+p)

DATA

Mp

Sample used for this analysis:

15% most central Au+Au events of sNN = 200 GeV

Number of events used: 400,000 out of 2M availablefor final results

Preliminary

Adam Trzupek

Momentum distributions:

(++-)

(K++K-)

(p+p)

PHOBOS preliminary

pT [GeV/c]

dydpp

Nd

2

1

TT

2

Acceptance & efficiency corrections embedding single MC tracks into experimental events

Background corrections: feeddown, secondaries, misID, ghosts;

(p +p) 41±8%

(K++K-) 16±11%

(++-) 39±3% based on track vertex distributions (DCA) and rescaled HIJING with detector (Geant) simulations

Current systematic errors 20%() ~40%(K) ~50%(p)

Centrality 0-15%sNN=200 GeV Au+Auy= -0.1 — 0.4

Adam Trzupek

Comparison to other RHIC experimentsT. Ullrich Nucl.Phys.A 715,p.399c(2003)

• Yields at lowest pT at RHIC are measured with the PHOBOS spectrometer • Uncertainties in fit extrapolations to pT =0 are reduced• Flattening of proton spectra indicates strong collective radial expansion

Adam Trzupek

Pion low pT yields are well described by Bose - Einstein function extrapolated from higher pT (>0.2GeV/c)

Excess in low pT pion production below 0.2 GeV/c predicted by DCC models is not observed J. Randrup Nucl.Phys. A681 (2001) 100c

B-E fit, pT = 0.2 - 0.7 GeV/c

Pion mT distributions:

12

1)/exp(1

21 BET

TT

TmAdydm

Ndm

Bose - Einstein distribution:

mT = pT2+mh

2

Adam Trzupek

Models constraints

P. Kolb and R. Rapp; PRC 67 (2003) 044903

H

1/(

2m

T)

d2N

/dyd

mT /<

Np

art/2

>HIJING RQMD

HYDRO+THERMALW.Broniowski,W.Florkowski(PRL87,2001,272302)

(p+p)PHOBOS preliminary

Model predictions differ from data by factors 2-6 at loo pT

Hydrodynamic simulation with (initial) transverse boost describes data well from the very low pT up to 1.5 GeV/c

Adam Trzupek

Summary

Algorithm was developed to determine yields of (++-), (K++ K-) and (p + p) at very low pT ( 0.03 - 0.2 GeV/c ) using the PHOBOS

spectrometer No enhancement in low pT yields for pions is observed

Flattening of (p+p) spectra down to very low pT indicates

strong radial flow in the system

Models should account for the dynamics ( rescattering,

expansion ) which has largest effect on low pT (p + p) yields

Adam Trzupek

Collaboration (May 2003)

Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Becker, Russell

Betts,

Abigail Bickley, Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan

Carroll,

Patrick Decowski, Edmundo Garcia, Tomasz Gburek, Nigel George, Kristjan

Gulbrandsen,

Stephen Gushue, Clive Halliwell, Joshua Hamblen, Adam Harrington, Conor

Henderson,

David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Erik

Johnson,

Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo, Jang Woo Lee, Willis Lin, Steven

Manly,

Alice Mignerey, Gerrit van Nieuwenhuizen, Aaron Noell, Rachid Nouicer, Andrzej

Olszewski,

Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg, Christof

Roland,

Gunther Roland, Joe Sagerer, Pradeep Sarin, Pawel Sawicki, Iouri Sedykh, Wojtek

Skulski,

Chadd Smith, Peter Steinberg, George Stephans, Andrei Sukhanov, Ray Teng,

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

Bernard Wadsworth, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan

Wuosmaa, Bolek Wysłouch, Jinlong Zhang

ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGYNATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO

UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

Adam Trzupek

Backup slides

Adam Trzupek

1m

PHOBOS Detector (2001) • Triggering

-Scintillator Paddles- Zero Degree Calorimeter

• 4 Multiplicity Array

- Octagon, Vertex & Ring Si Counters• Two Mid-rapidity Spectrometers• TOF wall for High-Momentum PID

Adam Trzupek

1/(2pT)d2N/dydpT

acceptance & efficiency correctionsembedding single MC tracks into experimental events in phase space cells:

pT, y, Zver ,

(- + +)

Adam Trzupek

PHOBOS preliminary

(++) (K+

+K–) (p+p)

mT – m0

10-2 10-1 1

HIJING RQMD

HYDRO+THERMAL W.Broniowski,W.Florkowski

(PRL87,2001,272302;PRC65,2002,064905)

PHENIX 130 GeV

1/(

2m

T)

d2N

/dyd

mT /<

Np

art/2

>

(Au+Au yields )/<Npart/2>

1

10-1

10

PHOBOS preliminary PHOBOS preliminary

10 10

1 1

10-1 10-1

10-2 10-1 1 10-2 10-1 1

Model Constraints

014.0183.002.061.002.080.0 expexpexp

pHIJKHIJHIJ Y

Y

Y

Y

Y

Y