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)
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 3
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 4
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 5
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)
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 6
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 7
● 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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 8
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?
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 9
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 10
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 11
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 12
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)
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 13
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 14
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 16
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
Oana Catu Quark Matter 2008, February 4-10,Jaipur, India 17
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