WWND XXI., Feb 2005 Levente Molnar, Purdue University 1

Systematics of identified particle spectraLevente Molnar, Purdue University

For the STAR Collaboration

Outline:

• Physics motivation• Particle spectra and properties at 62.4GeV and beyond …• Resonance effect on extracted freeze-out parameters• Summary, outlook, …

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Motivation

• New data set: 62.4GeV Au-Au collisions,• first step in the energy scanning program at RHIC.

• Bulk particles are the ‘final’ dynamic thermometers of the collision system:

• Identified particle spectra at low pT

• Study particle production vs. psNN , centrality, …

• Particle ratios → chemical freeze-out properties

• Shape of the spectra → kinetic freeze-out temperature and transverse radial flow: dynamics of the collision.

• Study of resonance decay effect on the extracted freeze-out parameters.

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• Particle spectra are measured at midrapidity: |y| < 0.1 in the STAR-TPC.• Spectra evolution with centrality is similar to that observed at 200GeV.• Particle spectra are fitted with Blast-Wave model (thermal source + flow )( E. Schnedermann et al. PRC48, 2462, (1993) )

p K

STAR PreliminarySTAR PreliminarySTAR Preliminary

Identified particle spectra in AuAu at 62.4GeV

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Particle - antiparticle ratios

• / - ¼ 1.• K- / K+ ratio is smaller at 62.4GeV.• Significant drop in antiproton to proton ratio! • Statistical models are very successful describing ratios from AGS-RHIC.

STAR Preliminary

Errors shown:•200GeV systematic errors•62.4GeV stat. errors

BRAHMS, nucl-ex/0410020

STAR Preliminary

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Unlike particle ratios vs centrality

Errors shown:•200GeV systematic errors•62.4GeV stat. errors

effect?baryon Net

?centrality with /in drop pronounced More

200GeVat thosetoevolution centrality

similar show Ratios

p

STAR Preliminary

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Average transverse momenta

NchSTAR: 200GeV AuAu: Phys. Rev. Lett. 92 (2004) 11230

• <pT> follows the same trend in Nch

•< pT >K,p is extractedfrom Blast-Wave fit.

• < pT >π is extractedfrom Bose-Einstein fit.

Errors shown:•200GeV systematic errors•62.4GeV stat. errors

STAR Preliminary

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Freeze-out parameters I.

√sNN (GeV)√sNN (GeV)

Tki

n (

GeV

)

<β

> (

c)

AGS SPS RHIC AGS SPS RHIC

Tkin and <β> in centralheavy ion collisions

Nu Xu, Nucl.Phys. A698 (2002) 306

Errors shown:• 200GeV systematic errors• 62.4GeV stat. errors

STAR Preliminary STAR Preliminary

STAR Preliminary

<β

>

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Freeze-out parameters II.

Errors shown:• 200GeV systematic errors• 62.4GeV stat. errors

Nch

Nch

STAR Preliminary

STAR Preliminary

STAR Preliminary

Becattini, hep-ph/0202071

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Estimate of Resonances and Method• The blast wave model study shown so far treated particles as if they were primordial.

Systematic errors include resonance effects studied by a stand alone MC.

• A more complete study of resonance effects based on code from ref.: U.A.Wiedemann, U.Heinz, Phys.Rev. C56 (1997) 3265-3286.

• Improvements:– Increase the number of resonances included: , , ’, , K*, KS, , 1520, ,

1385, , .– Implementation of two freeze-out temperatures

• Chemical model fit provides Tchem, B, S and • Calculate primordial particle yields• Blast wave model; shape of particle spectra including resonances. (Ref.: E.

Schnedermann et al. PRC48, 2462, (1993) , … )• Resonance decay channels, ref. see above• Addition of decay channels ! Inclusive spectra.

• The inclusive simulated spectra then are fitted to the measured , K, p spectra.

• Extract Tkin and for which the 2/ndf is minimum.

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PiMinus Spectra

• Note: pion spectra (data) are corrected for weak decays, no and K0Scontributions..

STAR Preliminary

Tkin=90MeV0.6n=0.8

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KMinus Spectra

STAR Preliminary

Tkin=90MeV0.6n=0.8

STAR Preliminary

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• Main contribution from , , , …

Proton Spectra

Tkin=90MeV0.6n=0.8

STAR Preliminary

All Protons

Thermal protons

Lambda

Delta

Xi

Sigma

Omega

Sigma1385

Lambda1520

STAR Preliminary

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Fit I.

• Pion spectra can be reproduced, kaons and protons are less well described .

STAR Preliminary STAR Preliminary

/ndf=1.547

Tkin=90MeV0.6n=0.8

pT (GeV/c) pT (GeV/c)

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Fit II.

• Kaons and protons agree well, pions are less well described.• Short lived resonances, eg.:

STAR Preliminary STAR Preliminary

Tkin=90MeV0.64n=0.8

/ndf=1.396

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Rho contribution

• It is an open question what flow velocity and temperature should be assigned to short-lived resonances such as , , …

• They decay and are regenerated constantly during the system evolution:– assume decay pions and other pions are in equilibrium and

behave similarly ( similar temperature and flow velocity ). • the does not gain stronger flow due to its large mass during

its short lifetime.

• Assume does not contribute to spectral shape change, i.e. we take primordial pion shape for the decay pions as well. decays are still included but their contribution is small.

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Fit III.

• With the contribution as described before, blast wave describe all spectra well. The parameters agree with published values within syst. errors.

STAR Preliminary STAR Preliminary

Tkin=90MeV0.64n=0.8

/ndf=0.305

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• We have measured π, K, p spectra in 62.4GeV AuAu collisions.• Spectra and particle ratios show similar evolution with centrality as at

200GeV:– K– / π – ratio indicates similar strangeness production.– p / p ratio decreases from 200GeV → 62.4GeV due to the net baryon density, and

also decreases with increasing centrality.– Particle production can be described with statistical model over a wide range of

collision energies.

• The system shows similar freeze-out properties at 62.4GeV as at 200GeV:– Tch is ~ 160MeV ( ~Tc) and independent of centrality.– Tkin decreases and <β> increases from peripheral to central collisions.– Freeze-out parameters seem to follow similar trend in Nch.

• Detailed resonance study is performed:– Particle spectra are reproduced in simulation including resonance particles at

temperatures and flow velocities within the systematic errors. – Pion spectrum and short lived resonances.

• To do: – Further study of resonances, automatic fit to obtain final temperature and flow

velocity, 62.4GeV data, …

Summary and Outlook

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PiMinus Spectra No Rho

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Paper plots

• Pion spectra – with the decay channels – as in paper can be reproduced.• Note: linear transverse flow rapidity profile is used in paper: t= f r, where r=0-4.

• In the following calculations “regular” flow is used =S (r/R)n=0.8