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RCNP研究会 , 10/29-30/2007

金野正裕（筑波大学）

RHIC における陽子反陽子生成の系統的測定

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RCNP研究会 , 10/29-30/2007

RHIC Findings (1) Jet Quenching

- In central Au+Au collisions, hadrons are suppressed at high pT.- The suppression is a final state effect (parton energy loss).- Away-side jet peak disappeared in central Au+Au collisions.

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RCNP研究会 , 10/29-30/2007

0.5

1.0

Particles & Medium Effects

- Suppression/Enhancement has particle-type dependence. => Baryon/Meson difference in yields and emission patterns at intermediate pT (2-5 GeV/c).

0

Baryon enhanced B/M Splitting of v2

RHIC Findings (2)

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RCNP研究会 , 10/29-30/2007

Hadron Production in RHI Collisions

HadronizationInteractions

in the medium

Low-pT (soft)Thermal emission

Quark recombination

Thermalization

Collective flow

High-pT (hard) Jet fragmentationHard scattering

Jet quenching

- There are multiple hadronization mechanisms at intermediate pT.- The relative contributions and particle-type dependence are not yet fully understood.

Understanding Baryon/Meson difference at intermediate pT. => What is the origin?

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RCNP研究会 , 10/29-30/2007

Aerogel Cherenkov,TOF-W (PID)

EM Calorimeter (PID)

TOF-E (PID)

Drift Chamber (momentum meas.)

Pad Chambers(tracking)

PHENIX detector

- Beam Beam Counter - Zero Degree Calorimeter

Global detectors:

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RCNP研究会 , 10/29-30/2007

Particle IdentificationTime of Flight ( ~120 ps)

Aerogel Cherenkov (n=1.011)

Veto for proton ID

“New” Time of Flight ( ~90 ps)

MRPC typeScint.+PMT type

detector upgrade

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RCNP研究会 , 10/29-30/2007

Proton and Antiproton pT spectra

pT reach extended up to 6 GeV/c

for p(p) with fine centrality bins. (1) Aerogel Cherenkov (2) High statistics

NOTE: No weak decay feed-down correction applied.

Au+Au sNN = 200 GeV Cu+Cu sNN = 200 GeV

p+p sNN = 200 GeV

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RCNP研究会 , 10/29-30/2007

Freeze-out Properties

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RCNP研究会 , 10/29-30/2007

Particle Yield dN/dy at mid rapidity

- Particle yields are (roughly) scaled with Npart btw. Au+Au and Cu+Cu.- dN/dy(Cu+Cu) >~ dN/dy(Au+Au) at smaller Npart.- Statistical model describes their ratios with few parameters (T,).

Au+Au/Cu+Cu/p+p(sNN = 200 GeV)

Au+Au/Cu+Cu/p+p(sNN = 62.4 GeV)

K

p

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RCNP研究会 , 10/29-30/2007

- Clear hadron mass dependence: larger <pT> for heavier particles. => Consistent with radial flow picture.- <pT> increases with Npart. it is clearly seen for (anti)proton.

Mean Transverse Momentum

K

p

Au+Au/Cu+Cu/p+p(sNN = 200 GeV)

Au+Au/Cu+Cu/p+p(sNN = 62.4 GeV)

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RCNP研究会 , 10/29-30/2007

Blast-wave model is a parameterization within a simple boost-invariant model with transverse collective flow. pT spectra reflecting thermal freeze-out temperature and transverse flow at final state. * Ref: PRC48(1993)2462

(* Resonance decay feed-down correction not applied. Instead, tighter pT fitting range used. ; 0.6-1.2 GeV/c K; 0.4-1.4 GeV/c, p/pbar; 0.6-1.7 GeV/c)

Spectra for heavier particleshas a convex shape due to radial flow.

2 map

€

dN

mT dmT

∝ rdrmTK1

mT coshρ

Tfo

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟I00

R

∫ pT sinhρ

Tfo

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

€

ρ =tanh−1βT

€

βT = β s

r

R

⎛

⎝ ⎜

⎞

⎠ ⎟n

Blast-wave Model Fit

Tfo ~120 MeV, βT ~0.7

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RCNP研究会 , 10/29-30/2007

- <βT>: increasing with Npart.- Npart scaling of <βT> between Au+Au and Cu+Cu.

- Almost same <βT> at √sNN = 62.4, 200 GeV.

<βT> ~0.5

Transverse Flow Velocity

Au+Au/Cu+Cu/p+p(sNN = 200 GeV)

Au+Au/Cu+Cu/p+p(sNN = 62.4 GeV)

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RCNP研究会 , 10/29-30/2007

- Tfo: decreasing with Npart.- Npart scaling of Tfo between Au+Au and Cu+Cu.

- Almost same Tfo at √sNN = 62.4, 200 GeV.

Tfo ~120 MeV

Kinetic Freeze-out Temperature

Au+Au/Cu+Cu/p+p(sNN = 200 GeV)

Au+Au/Cu+Cu/p+p(sNN = 62.4 GeV)

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RCNP研究会 , 10/29-30/2007

Summary - Freeze-out properties

Characterizing bulk properties: - Chemical freeze-out - Kinetic freeze-out

=> Hadron production at low pT : “Thermal emission + Radial flow”

Scaling properties between different systems: - Chemical/kinetic freeze-out properties show similarities between different collision systems.

- Npart scaling of freeze-out properties (Au+Au, Cu+Cu), * even though the overlapped region has a different shape. => System volume Npart is a control parameter. * Particle yield: (Cu+Cu) > (Au+Au) at smaller Npart

- Similarity at sNN = 200 and 62.4 GeV.

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Baryon Enhancement

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RCNP研究会 , 10/29-30/2007

- (Anti-)proton enhancement observed/confirmed in 200 GeV Au+Au/Cu+Cu.- Larger than expected from jet fragmentation (measured in pp, e+e-).- p/ (p/) ratios turn over at 2~3 GeV/c ,and fall towards the ratio in p+p.

Baryon enhancement at sNN = 200 GeVp/

p/

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Au+Au sNN = 200 GeV/K0s

STAR, nucl-ex/0601042

Strange Baryon enhancement

Baryon enhancement seen in strange.

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RCNP研究会 , 10/29-30/2007

Baryon enhancement at sNN = 62.4 GeVp/

p/

- (Anti-)proton enhancement observed/confirmed in 62.4 GeV Au+Au/Cu+Cu.- Similar pT dependence as at 200 GeV.

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RCNP研究会 , 10/29-30/2007

Cu+Cu vs. Au+Au (200 GeV)

- Npart scaling of p/ (p/) at same √sNN.- The ratios are controlled by the initial overlap size of colliding nuclei, even though overlap region has a different geometrical shape.

p/ ratio vs. Npart1/3

Cu+Cu vs. Au+Au (62.4 GeV)

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RCNP研究会 , 10/29-30/2007

- Comparison with p+p spectra (reference) in binary collision scaling.- Proton, antiproton are enhanced at 1.5 - 4 GeV/c for all centralities. - Suppression is seen for , K.

Nuclear Modification Factor

RAA

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RCNP研究会 , 10/29-30/2007

- Proton is enhanced for all centralities, while /K are suppressed.

Comparison of RAA in Au+Au/Cu+Cu

Pion RAA (pT=2.25 GeV/c) Proton RAA (pT=2.25 GeV/c)

RAA (Cu+Cu) > RAA (Au+Au)

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RCNP研究会 , 10/29-30/2007

(Cu+Cu: b=0.0 fm, Au+Au: b=8.6 fm)

<Npart> ~117

Comparison of Au+Au and Cu+Cu

<Npart> ~100

Cu+Cu: good resolution at smaller Npart

Glauber model calculation

Even though Ncoll-Npart relation is almost same between Au+Au and Cu+Cu, the geometrical overlap shape is different.

- RAA (Cu+Cu) > RAA (Au+Au) - Geometrical shape : Au+Au more deformed - No. of N-N scatterings per N : narrow peak in Cu+Cu

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Beam energy dependence

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RCNP研究会 , 10/29-30/2007

Beam energy dependence of enhancement

- p/+ ratio : decreasing as a function of sNN.

- p/- ratio : increasing as a function sNN.

- Antiproton is a good probe to study the baryon enhancement.

* No weak decay feed-down correction applied.

p/

p/

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RCNP研究会 , 10/29-30/2007

- No Npart scaling of p/ (p/) in Au+Au between 62.4 and 200 GeV.- Transverse energy density dET/d scaling of p/ is favored.- dET/d is a connection key between different √sNN.

p/ ratio vs. (dET/d)1/3

Proton production at 62.4 GeV is partly from baryon number transport, not only proton-antiproton pair production.

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RCNP研究会 , 10/29-30/2007

Energy loss per nucleon: 73±6 GeV

Net proton distribution

it drastically changeswith beam energy.

BRAHMS, PRL 93 (2004) 102301

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RCNP研究会 , 10/29-30/2007

Au+Au/Cu+Cu/p+p(sNN = 200 GeV)

Au+Au/Cu+Cu/p+p(sNN = 62.4 GeV)

Chemical Potential

- q (200 GeV) : ~8 MeV, independent of Npart

- q (62.4 GeV) : increasing with Npart => more baryon stopping at central

€

p

p= exp(−6μq /Tch )

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RCNP研究会 , 10/29-30/2007

Summary - Baryon enhancement

Baryon enhancement: - Proton and antiproton enhancement confirmed at intermediate pT (2-5 GeV/c) in Au+Au/Cu+Cu. A turnover of p/ ratio seen at pT = 2-3 GeV/c. - In terms of binary collision scaling, (anti)protons are enhanced while pions/kaons are suppressed.

Low energy 62.4 GeV data: - At lower energy 62.4 GeV, proton production seems to be more affected by baryon number transport process. => Antiproton is a good indicator of the baryon enhancement.

Scaling properties between different systems: - Npart scaling of p/ (p/) - dET/d scaling of p/

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Two-component model(Soft+Hard)

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RCNP研究会 , 10/29-30/2007

high-pT

particleslow-pT

particles

Particle production in expanding matter

z-axis

time

x-axis

time

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RCNP研究会 , 10/29-30/2007

Soft component : Thermal emission + Radial flow - Described by Blast-wave model - Npart scaling seen - Thermal distribution extrapolated up to high pT

Hard component : Jet fragmentation + Jet suppression - Measured p+p spectra - Ncoll scaling - Constant suppression factor (power-law distribution & fractional energy loss)

Two-component Model (Soft+Hard)

€

dN tot

pT dpT

=dNsoft

pT dpT

+dNhard

pT dpT

€

dNsoft

pT dpT

= A rdrmTK1

mT coshρ

Tfo

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟I00

R

∫ pT sinhρ

Tfo

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

€

dNhard

pT dpT

= RAA × Ncoll ×dN p + p

pT dpT

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- Hard component (in p+p) at high pT depends on s.- In Au+Au, suppression effect should be taken into account.

Hard component in p+p and Au+Aup+p sNN = 200 GeV

200 GeV

62.4 GeV

Au+Au 200 GeV

(pi0: diamond, h+h-: circle)

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Pion pT spectra Au+Au 200 GeV

+

-

Soft Line Hard Line

Reproduce the measured pion pT spectra.

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RCNP研究会 , 10/29-30/2007

Pion fraction vs. pTAu+Au 200 GeV

+

-

Soft HardResidual

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Proton pT spectra Au+Au 200 GeV

p

p

Soft Line Hard Line

Reproduce the measured proton pT spectra.

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RCNP研究会 , 10/29-30/2007

Proton fraction vs. pTAu+Au 200 GeV

p

p

Soft HardResidual

RAA vs. Npart

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Proton pT spectra Au+Au 200 GeV

p

p

Soft Line Hard Line

Using pion’s RAA for suppression factor.

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RCNP研究会 , 10/29-30/2007

Proton fraction vs. pTAu+Au 200 GeV

p

p

SoftHard

ResidualNeed 3rd component ?

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RCNP研究会 , 10/29-30/2007

Fraction of soft and hard components

€

hard

soft + hard

€

soft

soft + hard

+

-

p

p

- Both soft and hard components are necessary to reproduce the hadron spectra at intermediate pT (2-5 GeV/c).- Soft component is extended to higher pT in central.- Intermediate pT: Hard pions vs. Soft protons

- Cross point (S=H) vs. pT -

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Summary - Two-component model

Two-component model: - Reproduce the measured pT spectra for pions and protons with a consistent way. - Identify crossover region from soft to hard hadron production at intermediate pT (2-5 GeV/c).

Baryon/Meson difference: - Intermediate pT: “Hard” pions vs. “Soft” protons - Origin of baryon enhancement is radial flow. It pushes heavier particles to higher pT. Baryon/Meson difference is trivial?

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Quark Flow vs. Hadron Flow

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RCNP研究会 , 10/29-30/2007

Quark recombination- One of the hadronization mechanisms.- Recombination of thermal quarks in local phase space: qq Meson, qqq Baryon- At intermediate pT, (recombination) > (fragmentation) because quark distribution is thermal: ~exp(-mT/T).- At high pT, fragmentation (power-law shape) would be dominant.

€

Ed3NM

d3 p= CM w2(pT /2)

€

Ed3NB

d3 p= CBw3(pT /3)

€

vM2(pT ) = 2v q

2(pT /2)

€

vB2(pT ) = 3v q

2(pT /3)

Fries, R et al PRC 68 (2003) 044902Greco, V et al PRL 90 (2003) 202302Hwa, R et al PRC 70(2004) 024905

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p/ vs. pT

- Baryon enhancement & quark number scaling of v2

explained by “Quark recombination”- v2 at quark level => Collective flow at quark level

Applicability of quark recombination model

- In a simple recombination picture, radial flow cannot be distinguished between hadron and quark phases. => Can we separate hadron flow and quark flow ?

v2/n vs. KET/n

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RCNP研究会 , 10/29-30/2007

- Ideal gas: P=(1/3)- Entropy conservation- Longitudinal expansion & Transverse expansion

z

x

y

1+1D Adiabatic Expansion

bj vs. Np

- cooling curves -

tfo fixed at 10 fm/cat most central

T scaled with (bj)1/4

at t = 1 fm/c Cooling stopped at Tfo

€

bj (τ ) =1

τπR2

dET (τ )

dy

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RCNP研究会 , 10/29-30/2007

- More central collisions freeze-out later at lower temperature.- Consistent with freeze-out condition: (t)=R(t)- Even if quark phase is created before hadronization, hadronic scattering should be taken into account.

Freeze-out Time & TemperatureFreeze-out time vs. Np

- As expected, Tfo is lower than Tch. Different centrality dependences.- Tfo dropping is consistent with 1+1D adiabatic expansion.

Freeze-out temperature vs. Np

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Conclusions- Systematic measurement of proton and antiproton pT spectra (Au+Au, Cu+Cu, p+p at sNN = 200/62.4 GeV)

- Proton and antiproton enhancement confirmed at intermediate pT (2-5 GeV/c).

- Antiproton is a good indicator for study of the baryon enhancement.

- p/ ratio & freeze-out properties show Npart scaling between Au+Au and Cu+Cu at same sNN. The Initial volume (~Npart) of colliding nuclei is a control parameter.

- Baryon enhancement is caused by transverse radial flow - pT and centrality dependences are described by two-component model. - Intermediate pT (2-5 GeV/c): hard pions vs. soft protons - Chemical/Kinetic Freeze-out temperatures provide a hint for further expansion at hadronic stage.