Oana Catu, Yale University for the STAR Collaboration Quark Matter 2008, February 4-10, Jaipur, India System size dependence of dihadron correlations and

Oana Catu, Yale University

for the STAR Collaboration

Quark Matter 2008, February 4-10, Jaipur, India

System size dependence of dihadron correlations and fragmentation functions

in Cu+Cu and Au+Au at √s=200 GeV

Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 2

MOTIVATIONMOTIVATION Study medium-induced modification of fragmentation function due to energy

loss Without full jet reconstruction,

jet/parton energy not measurable

→ z not measured (z=phadron/pparton )

Di-hadron fragmentation function = ratio of di-hadron jet-like correlated yield to single hadron yield

zT=pTassoc/pTtrig

(for experimental data d-Au data used as substitute of p-p)

H.Zhang, J.F. Owens, E. Wang, X.N. Wang - Phys. Rev. Lett. 98: 212301 (2007)


MOTIVATION (II)• Back-to-back di-hadron spectra - more sensitive to initial gluon density than single spectra

• smaller surface bias

• Test model prediction in different size system – Cu-Cu → constrain path length dependence

• Comparing STAR di-hadron and single hadron spectra with model calculations - extract the transport coefficient

y (f

m)

x (fm)T. Renk and Eskola- Phys. Rev. C75: 054910 (2007)

H. Zhang, J.F. Owens, E. Wang, X.N. Wang –

Phys. Rev. Lett. 98: 212301 (2007)

2.8 ± 0.3 GeV2/fmqq ˆ~00

(IAA)2

y (f

m)

x (fm)

Distribution of points of origin of di-hadron pairs and single hadrons, respectively


ANALYSIS TECHNIQUE – NEAR ANALYSIS TECHNIQUE – NEAR SIDE JETSIDE JET

- histograms of particles

correlated with trigger particle

mixed events technique to correct for

detector effects

– non-uniform efficiency in

– limited acceptance in → pair

acceptance in (“triangle”)

In Au-Au near-side ( 0 )

correlations of particles with large

separation observed – “ridge”

pTtrig=3-6 GeV/c, 1.5 GeV/c <pT

assoc< pTtrig

Au+Au central @ 200 GeV

jetridge

STAR


Ridge + flow

Ridge +

flow

jet

Jet+Ridge+Flow Ridge+

Flow

1

2

2

1 2

2 methods: projecting onto ,

1) PROJECTION ON subtract ridge - construct

correlations for particles with || < 0.7 and subtract correlations with 0.7< || < 1.7.

2) PROJECTION ON Project jet range (| |<0.78) assuming ridge flat in -

subtract flat background

ANALYSIS TECHNIQUE – NEAR ANALYSIS TECHNIQUE – NEAR SIDE JET (II)SIDE JET (II)


NEAR-SIDE JET – NEAR-SIDE JET – ΔΔΦΦ METHOD METHOD

CuCu 0-10%CuCu 0-10%

● normalize large Δη correlations to match the small Δη correlations in the away-side region

4< pT trig < 6 GeV , pT assoc > 3 GeV

Background subtracted(black minus red)

• || < 0.7

• 0.7<|| < 1.7

●method subtracts uncorrelated background pairs, ridge and elliptic flow – assumed constant in Δη● add content of bins in ||<0.78 range of background-subtracted correlation plots → jet yield


● Used projection in Δη to extract yield ( also a consistency check)

● Subtract flat background (uncorrelated background correlations, “ridge” )

● Get jet yield by bin-counting in |Δη| < 0.7

CuCu 0-10% 4< pT trig < 6 GeV , pT assoc > 3 GeV

NEAR-SIDE JET - η METHOD


NEAR-SIDE JET YIELD DEPENDENCE ON Npart

4< pT trig < 6 GeV 6< pT trig < 10 GeVpT assoc >3 GeV

● jet yield consistent in the two methods

● Near-side jet yield consistent in Cu-Cu and Au-Au for same Npart

● Jet yield independent of Npart - unmodified vacuum fragmentation?


NEAR SIDE DI-HADRON FRAGMENTATION FUNCTIONS

Near-side di-hadron fragmentation functions in Au-Au and Cu-Cu – similar to d-Au→ consistent with parton fragmenting in vacuum

(it still allows energy loss prior to fragmentation)

4< pT trig < 6 GeV 6< pT trig < 10 GeV


ANALYSIS TECHNIQUE – AWAY SIDE

Project - histograms on

summing range ||<1.7

- For consistency with previous studies - no

(triangle) correction

determine background level at ||~1

(ZYA1)

Construct flow modulated background

jet yield – in region 1.7-4.4 - add bin content and subtract area under red

curve (best estimate v2 )

get syst errors - subtract area under green and blue curves (higher and lower

limit on v2)

AuAu 40-80%,4< pT trig < 6 GeV , pT ass > 3 GeV


AWAY-SIDE SUPPRESSION - DEPENDENCE ON Npart

4< pT trig < 6 GeV pT assoc > 3 GeV

PQM: C. Loizides, Eur. Phys. J. C 49, 339-345 (2007) Modified frag: nucl-th/0701045 - H.Zhang, J.F. Owens, E. Wang, X.N. Wang

6 < pT trig < 10 GeV

values are given for 0-10% AuAu and CuCu 2.8 ± 0.3 GeV2/fmqq ˆ~00

● modified fragmentation model – describes Cu-Cu vs Au-Au but slightly misses shape ● PQM – does not describe relationship between Cu-Cu and Au-Au yields with the chosen values; misses shape

● Away side jet yield consistent in Cu-Cu and Au-Au for the same Npart

● models fail to describe magnitude of suppression for the lower pT trig

6< pT trig < 10 GeV pT assoc >3 GeV


AWAY SIDE DI-HADRON FRAGMENTATION FUNCTIONS

6< pT trig < 10 GeV

• di-hadron frag functions suppressed but shape not modified in the considered zT

range– consistent with vacuum fragmentation after energy loss• comparison to modified fragmentation model - use parameters fitted to previous Au-Au data to predict Cu-Cu – match data

• less suppressed in Cu-Cu than central Au-Au – Npart dependence?

COMPARE TO MODIFIED FRAGMENTATION MODEL

2.8 ± 0.3 GeV2/fmqq ˆ~00Theory - X.N.Wang private communicationH.Zhang, J.F. Owens, E. Wang, X.N. Wang - Phys. Rev. Lett. 98: 212301 (2007)


AWAY SIDE DIHADRON FRAGMENTATION FUNCTIONS (II)

• Investigate Npart dependence of di-hadron fragmentation function

• consistent in Au-Au and Cu-Cu for similar Npart

• trends observed in the away-side yields – verified in di-hadron fragmentation functions

• Average path length of all partons (geometric path length) – quite similar in the two systems for same Npart

• path length dependence of surviving partons

- surface bias – different ?

- model dependent

Npart ≈ 100

Npart ≈ 20Npart ≈ 100

Npart ≈ 20

6< pT trig < 10 GeV

Loizides – private comunicationLoizides – private comunication


CONCLUSIONSCONCLUSIONS Systematic study of jet-like di-hadron correlations in 3 systems Near-side

• jet yield same in heavy ion collisions as in d-Au

– consistent with vacuum frag in the near side

• same behavior in Cu-Cu and Au-Au for same Npart

Away side suppression

• increases with Npart

• same behavior in Cu-Cu and Au-Au for same Npart

– not expected in PQM

– modified frag funct model – describes relationship but misses shape

• Di-hadron frag funct shape not modified for considered zT range – consistent with vacuum fragmentation after energy loss

– agreement with theory for higher pT triggers

Available data would allow to quantitatively constrain the models



PQMPQM

qhat = product of parton-medium cross-section * color-charge density BDMPS quenching weights – only coherent radiative energy loss via gluon

bremsstrahlung Realistic geometry Static Avg qhat -> time-dependent density = scheme dependent\ No initial state multiple scattering No modified nuclear parton distribution functions


MODIFIED FRAG FUNCTIONMODIFIED FRAG FUNCTION

Longitudinally expanding medium Hard spheres PDFs factorized into nucleon ones and HIJING parameterization of the impact

parameter dependent nuclear modification factor radiative gluon energy loss – incorporated in effective medium modified FF


GLVGLV

Realistic geometry Bjorken expanding medium Radiative energy loss – gluon brehmsstralung to all orders in opacity Calculate a priori (w/o en loss) the average path length to the edge – use it to calculate

partonic energy loss

Documents

Oana Catu, Yale University for the STAR Collaboration Quark Matter 2008, February 4-10, Jaipur, India System size dependence of dihadron correlations and