Systematics of identified particle spectra

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Systematics of identified particle spectra
Levente 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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Identified particle spectra in AuAu at 62.4GeV
STAR Preliminary
STAR Preliminary
STAR Preliminary

• Particle spectra are measured at midrapidity
  y lt 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) )

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Particle - antiparticle ratios
STAR Preliminary
STAR Preliminary

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

BRAHMS, nucl-ex/0410020

• ? / ?- 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.

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Unlike particle ratios vs centrality
STAR Preliminary

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

6
Average transverse momenta
STAR Preliminary

• ltpTgt follows the
• same trend in Nch
• lt pT gtK,p is extracted
• from Blast-Wave fit.
• lt pT gtp is extracted
• from Bose-Einstein fit.

Nch
STAR 200GeV AuAu Phys. Rev. Lett. 92 (2004)
11230

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

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Freeze-out parameters I.
STAR Preliminary
STAR Preliminary
ltßgt

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

STAR Preliminary
8
Freeze-out parameters II.
STAR Preliminary
STAR Preliminary
Nch
?
Becattini, hep-ph/0202071

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

STAR Preliminary
Nch
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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
STAR Preliminary
Tkin90MeV b0.6 n0.8

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

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KMinus Spectra
STAR Preliminary
Tkin90MeV b0.6 n0.8
STAR Preliminary
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Proton Spectra
STAR Preliminary
Tkin90MeV b0.6 n0.8
All Protons Thermal protons Lambda Delta Xi Sigma
Omega Sigma1385 Lambda1520
STAR Preliminary

• Main contribution from ?, ?, ?,

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Fit I.
STAR Preliminary
STAR Preliminary
Tkin90MeV b0.6 n0.8
pT (GeV/c)
pT (GeV/c)
?2/ndf1.547

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

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Fit II.
STAR Preliminary
STAR Preliminary
?2/ndf1.396
Tkin90MeV b0.64 n0.8

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

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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.
STAR Preliminary
STAR Preliminary
Tkin90MeV b0.64 n0.8
?2/ndf0.305

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

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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 r0-4.
• In the following calculations regular flow is
  used ??S (r/R)n0.8