Soft Hadron Production at RHIC

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RIKEN Workshop (7/8/2006)M.Konno (Univ. of Tsukuba)

Soft Hadron Production at RHIC

Masahiro Konno(University of Tsukuba)

金野正裕（筑波大学）

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Outline

- A Picture of Relativistic Heavy Ion Collisions- Motivation (this talk)- RHIC-PHENIX

- Single spectra at low pT ( 2 GeV/c)

- Single spectra at Intermediate pT (2~5 GeV/c)

- Elliptic flow- Jet modification in the medium

- Summary

Res

ults

<~

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Z

Space-time evolution

1. Hard scattering

2. Thermalization & QGP

4. Cooling, Freeze-out3. Hadronization

Picture of Relativistic Heavy Ion Collisions• the created system lasts for only ~10 fm/c.

time

phenix (05)

Bjorken energy density:

€

ε(τ 0) =1

τ 0πR2

dET (τ 0)

dy

Well above ec (~1 GeV/fm3) fromlattice QCD if assuming t = 1 fm/c.

x5 !

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Motivation

Useful Observables: - Single particle spectra - Azimuthal anisotropy of particle emission - Modification of jet shape/yield in the medium

Why soft particle production important to heavy ion collisions?

=> Soft particle productionSoft particle production dominates the bulk yield. dominates the bulk yield. Through this study, we can know bulk properties of the system including collective flow. Hadrons interact strongly so it is a probe to know the evolution of the system and medium effect to hadronization.

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RHIC-PHENIX

Aerogel Cherenkov (PID)

EM Calorimeter (PID)TOF (PID)

Drift Chamber (momentum meas.)

Tracking detectors (PC1,PC2,PC3)

- Central Arm Detectors (magnetic spectrometer)- Event Characterization detectors

• Centrality and Reaction Plane

determined on an Event-by-Event basis.

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No Cut

Aerogel Veto

Clear Proton Line!Clear Proton Line!

π+

K+p

PID detectors(PID = Particle Identification)

• PID is a powerful tool to study hadron production!

Aerogel Cherenkov

Time of Flight (TOF)

p (p) ID up to 4 GeV/c-

p (p) ID up to 8 GeV/c-

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Single spectra (low pT)pT spectra for /K/p (mid rapidity)

Tkin ~ 100 MeV<vT/c> ~ 0.5

particle spectra (blast wave fit) => kinetic freeze-out properties.

PRC 69, 034909 (2004)

<pT>: <K<p (mass dependence)• consistent with radial flow picture.

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

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Single spectra (low pT)

nucl-th/0405068

particle ratios (statistical model) => chemical freeze-out properties. Tch ~160 MeV, gs ~1.0

- Hadron yields and spectra are consistent with

thermal emission from a strongly expanding

source (radial flow driven by pressure gradient).

- The observed strangeness production is

consistent with complete chemical equilibrium.

Statistical mode fit

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- High-pT suppression due to parton energy loss in the medium (jet quenching).- The suppression patterns depend on particle type. Protons are enhanced, while pions and kaons are suppressed.

Au+Au 200 GeV nucl-ex/0603010

Nuclear Modification Factor RAA

€

RAA =Yield AA/⟨Nbinary⟩AA

Yield pp

Single spectra (intermediate pT)

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PRL 91, 172301 (2003)

- p/ ratio ~1 for central Au+Au at intermediate pT (2-4 GeV/c).- Larger than expected from jet fragmentation (measured in pp, e+e-).- Baryon / Meson difference at intermediate pT. (on RAA (nuclear modification factor) , v2 (elliptic flow) etc.)

Baryon Enhancement

Elliptic flow (v2)p/ ratio

What is the origin of (anti-)proton

enhancement at intermediate pT?

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A number of models predicted a turnoverin the B/M ratio at pT just above where the available data finished…

p/

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

qq M→

qqq B→

Quark Recombination

At intermediate pT, recombination ofpartons may be a more efficient mechanismof hadron production than fragmentation.

=> pT spectra and particle ratio (Baryon/Meson) at higher pT

provide most basic tool to study the hadronization mechanism.

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pT spectra

pT reach extendedfor (anti-)protonswith fine centrality bins.

Using Aerogel Cherenkov detector

for (anti-)protons

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p/ vs. pT

* No feed-down correction.

- p/ (pbar/) ratios seem to turn over at intermediate pT, and close to the value of fragmentation at higher pT.- Indicating transition from soft to hard at intermediate pT.

p/ p/

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p/ vs. pT (centrality dependence)

p/ p/

* No feed-down correction.* p+p data (nucl-ex/0603010)

- p/ ratios look to have a peak at intermediate pT (2-4 GeV/c).- Clear peak in central events than that in peripheral.

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

– Cu+Cu 22.5 GeV central data reaches the p+p values.– Cu+Cu 62.4 GeV central data is higher than that in 22.5 GeV.

Beam energy dependence in Cu+Cu

Suggesting a significant contribution of incoming protons(not produced protons) in lower energies Cu+Cu.

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Beam energy dependence of net protons

nucl-ex/0313023 nucl-ex/0410003

- The shape of net proton distribution change dramatically with beam energy.

SPS AGS

- pbar/ ratio could be a good indicator of thermalization.

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Pions suppressed by a factor of ~5 with respect to protons

RdAu vs. RAuAu

- In d+Au Particle type dependence: , K < p (recombination in d+Au?)

- Cronin effect (in d+Au) cannot account for the huge gap between protons and pions in central Au+Au collisions.

Proton Cronin effect larger by ~30%

preliminary preliminary

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React

ion

plan

e

X

Z

Y

Px

Py Pz

- Overlap region is like ellipsoid at the beginning of collision.

- Spatial anisotropy of the system followed by multiple scattering of particles (pressure gradient) in the evolving system

- Spatial anisotropy => momentum anisotropy

v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane

Elliptic Flow

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• KET ~ mT–m0 at y ~ 0

M. Issah, A. Taranenko, nucl-ex/0604011

Elliptic Flow

- Large elliptic flow observed.- Mass ordering seen at low pT (<1.5 GeV/c).- KET scaling (for hadronic flow) vanish this mass dependence but give clear splitting of meson/baryon v2.

PHENIX preliminary

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PHENIX preliminary

v2q(pT) = v2

h(pT/n)/n, v2q(KET) = v2

h(KET/n)/n

Elliptic Flow

- Species dependence of v2 well accounted for by scaling v2 and pT (KET) with # of quarks.- Evidence of partonic flow! v2 is developed before hadrons form.

M. Issah, A. Taranenko, nucl-ex/0604011

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meson -- A test of recombination( -> KK)

- RAA looks like 0 rather than

proton even if mass() ~ mass(p).- Suggest that it’s not mass effect (radial flow).

- Mass ordering at pT <2 GeV/c.- For pT >2 GeV/c, v2 favors quark composition rather than mass

- KET and Nq scaling works for .

- Thermal s recombined to form .

H.M

asui

CIP

AN

P06

centrality: 20-60%

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Jet modification in the medium- In heavy ion collisions, we expect: (1) Scattered partons travel through the medium. (2) Partons loose their energy because of a large gluon density. (3) Suppression of high-pT leading particles.

Observables: - Single spectra RAA -- to look at yield suppression - Angular correlation -- to look at jet modification

Jet partons interact with the medium via the strong force.So jet can be a probe of the medium.

Interests: - Mapping of momentum of trigger/partner. - Shape of away-side jet - Study the composition of the jets <= Focus on this.

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Method of Jet study

- Using two-particle correlation function.- Should treat large combinatorial back ground (flow, decay) in heavy ion collisions.- Use mixed events for correction of pair acceptance.

€

1

N trig

dN pairs

dΔφ∝C Δφ( )

€

∝dNreal /dΔφ

dNmix /dΔφ

C()

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PHENIX Preliminary

Jets at intermediate pT

Meson vs. Baryon associated partner (for fixed Hadron trigger)

- Particle species dependence of jet modification- BTW, how can hadrons at intermediate pT show jet-like structure? => pickup of soft quarks by jets?

W.Holzmann HardProbes06

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Meson vs. Baryon associated partner (for fixed Hadron trigger)

Jet Associated Identified Conditional Yield

- Different pT trends of associated meson and baryon yields.

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- Near-side: like p+p, Away-side: Larger B/M ratio.

- Baryon/meson ratio can be an indicator for the degree

of thermalization (density of comoving constituents) in a jet.

Jet Associated Baryon to Meson RatioMeson vs. Baryon associated partner (for fixed Hadron trigger)

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Summary- The medium created in heavy ion collisions: (1) has bulk properties like collective flow, thermalization (2) induce the jet modification.

- Recombination models seem to be matched to the experimental observations (B/M difference) : (1) particle ratios (p/ etc.) (2) Consistent quark scaling of v2 etc.

- p/ ratio indicates a transition from soft to hard production at intermediate pT.

- Soft particle production (flow, recombination, etc.) observed at low ~ intermediate pT ( 5 GeV/c). <~

Systematic study (especially for colliding species, √s)

with enhanced PID capabilities.

Soft Hadron Production at RHIC

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