Systematics of identified particle spectra
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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