STAR Patricia Fachini QM20081 ρ 0 Production at High p T in Central Au+Au and p+p collisions at s...

Patricia Fachini QM2008 1STAR

ρ0 Production at High pT in Central Au+Au and p+p collisions at sNN =

200 GeV in STARSTAR

Patricia FachiniBrookhaven National Laboratory

for the STAR Collaboration

MotivationData Analysis

ResultsConclusions


Motivation - I

• ρ0 mass measured in STAR shifted at low pT ~30 to ~70 MeV/c2 what about high pT?

PDG value e+e- ρ0

mass in the vacuumρ0


Motivation - IIHadron production at high pT:• p+p information on pQCD, parton distribution

functions (PDF) in the proton, fragmentation functions (FF) of partons

• ρ0 FF same as pions?• Au+Au probe properties

of the strongly interacting QCD matter

J. Adams et al., Phys. Lett. B 637 (2006) 161

pT 1.7 GeV/c 2.5 GeV/c

ρ0/π- 0.82 ± 0.15 1.34 ± 0.15

ISR p+p s = 52.2 GeV

E. E. Kluge, Phys. Scrip. 19 (1979) 109

ρ0

π-ISR = 1


π and p Spectra

• Spectra compared to NLO pQCD calculations

AKK S. Albino, B. A. Kniehl and G. Kramer, Nucl. Phys. B 725 (2005) 181

KKP B. A. Kniehl, G. Kramer and B. PotterNucl. Phys. B 597 (2001) 337

DSS D. de Florian, R. Sassot, and M. Stratmann, Phys.Rev.D75 (2007) 114010 Kretzer S. Kretzer, Phys. Rev. D 62 (2000) 054001


Ratios Data and NLO π+and π-

• pQCD calculations reproduce well the pion spectra up to pT = 15 GeV/c


Ratios Data and NLO p and p

• pQCD NLO calculations need improvement for describing proton and anti-proton spectra


Particle Ratios

• Ratios extended to 15 GeV/c• The effect of the jet trigger on the ratios is negligible (comparison

between Pythia minimum bias and Pythia jet triggered data)


Particle Ratios

• Ratios extended to 15 GeV/c• The effect of the jet trigger on the ratios is negligible (comparison

between Pythia minimum bias and Pythia jet triggered data)• π-/π+ effect of valence quark• p/p significant quark jet fragmentation to baryon production


• HIJING events with a realistic simulation of detector response

• Use ω shape from HIJING to fit the data

• At high pT η and K0S

productions negligible

• NO K*0 clean particle identification

ρ0(770) π+ π-

STAR Preliminary

ρ0

sNN = 200 GeV

K0S

π+ π-

K0S

ω(782) (π+ π-) π0 and π+ π-

ω η (π+ π-) π0 and (π+ π-)

η’ (π+ π-) η and (π+ π-) ρ0η + η’K*(892)0 K π with K misidentified as πK*0 + K*0

misidentified

π+π- Invariant Mass Distribution from Monte Carlo


ρ0 at High pT p+p Jet Triggered Data

• Background parameterized to a polynomial function• Invariant mass distribution is fit before background subtraction• ρ0, f0 and f2 masses and widths fixed according PDG• ρ0/ω = 2.1 and fixed in the fit constraining the fit• f0(600)?

like-sign background


ρ0 at High pT p+p Jet Triggered Data

• ρ0 mass as a free parameter approaching ρ0 mass in the vacuum

PDG value e+e- ρ0

mass in the vacuum


ρ0 at High pT Au+AuAu+Au Minimum BiasAu+Au Central 0-12%

• Invariant mass distribution is fit after background subtraction

• ρ0, f0 and f2 masses and widths fixed according PDG

• ρ0 and ω free parameter in the fit


ρ0 Spectra

• ρ0 spectrum in p+p ρ0/π- from jet triggered data multiplied by π

yield measured in p+p minimum bias trigger effects cancel out in the ratio at first order

• ρ0 spectrum in p+p fit to a powerlaw function• ρ0 spectrum in p+p pion NLO calculations


Ratios Data and NLO ρ0 and π

• pQCD calculations for π similar to ρ0 spectra


Particle Ratios ρ0/π±

5 < pT < 10GeV/c p+p Au+Au minbias Au+Au central

ρ0/π± 1.78 ± 0.12 ±0.27 1.60 ± 0.08 ±0.24 1.47± 0.05 ±0.22

π± =π++ π-

2

RHIC s = 200 GeVT

ISR p+p s = 52.2 GeVpT 1.7 GeV/c 2.5 GeV/c

ρ0/π- 0.82 ± 0.15 1.34 ± 0.15E. E. Kluge, Phys. Scrip. 19 (1979) 109


Nuclear Modification Factor

• π more suppressed than protons opposite to color charge effect• ρ0 nuclear modification factor comparable to pions RAA


Conclusions • p+p spectra for pions and protons extended to pT = 15

GeV/c π data reasonably reproduced by NLO pQCD calculations

• RAA central collisions pion more suppressed than protons up to 12 GeV/c

• First measurement of ρ0 at 5 < pT < 10 GeV/c

• ρ0 mass in p+p approaching ρ0 mass in the vacuum

• ρ0 p+p spectrum similar to NLO pQCD calculations for π and ρ0 RAA similar to π RAA


Back-up Slides


STAR Detector

• Using jet triggered data for p+p TPC + EMC minimum amount of energy requested in the EMC


P. Braun-Munzinger et.al., CERES Int. Note, March 2000, unpublished; E.V. Shuryak and G.E. Brown, Nucl. Phys. A 717 (2003) 322; P.K. Kolb and M. Prakash, nucl-th/0301007; H.W. Barz et al., Phys. Lett. B 265, 219 (1991); R. Rapp, hep-ph/0305011.

ρ0

π-

π+

π-

π+

• M = Invariant Mass; pT = transverse momentum; T = Inverse Slope

• pp particle composition reasonably reproduced by statistical model T = 160 MeV also dAu

F. Becattini, Nucl. Phys. A 702, 336 (2002); Z. Phys. C 69, 485 (1996); F. Becattini and U. Heinz, Z. Phys. C 76, 269 (1997)

• Au+Au between chemical and kinetic freeze-out resonances formed until particles too far apart resonances emitted T = 120 MeV

E.V. Shuryak and G.E. Brown, Nucl. Phys. A 717 (2003) 322

e- M2 + pT

2

TM

M2 + pT2

Phase Space =

Γ(M) = Γρ

M2 – 4mπ2

Mρ2 – 4mπ

2

32 Mρ

MBW(M) =

Γ(M)

(M2 – Mρ2)2 + Mρ

2 Γ(M)2

Phase Space


Transport Model - UrQMD

• UrQMD Only imaginary part No medium modification

• Central Au+Au ρ0 mass shifted ~30 MeV at low pT

• ρ0 shape reproduced by p-wave Breit-Wigner × Phase Space

– Mρ= 765.6 MeV for 0.2 ≤ pT < 0.4 GeV/c

– Mρ = 761.2 MeV for 1.2 ≤ pT < 1.4 GeV/c

– Mρ = 769 MeV/c2 input

Г = 150 MeV

0 b ≤ 3 fm

√sNN = 200 GeV1.2 ≤ pT < 1.4 GeV/c

|y| ≤ 0.5

Au+Au

BW × PS

Mρ = 769 MeV/c2

b ≤ 3 fm Au+Au

ρ0 0.2 ≤ pT < 0.4 GeV/c

|y| ≤ 0.5


Particle Identification

• Relativistic rise of dE/dx • π particle identification with

a Nσ cut• Nσ energy loss

expressed in the units of standard deviation of a Gaussian formed by the logarithm of truncated energy loss (top 30%)

π+K+p

Only π within ~3% purity


π Spectra

• π spectra in p+p obtained from jet triggered data using Pythia to correct the spectra

• Pythia reproduces well both triggered and minimum bias data


Particle Ratios

ρ±

ω

• e+e- OPAL ρ±/ω > 1• Au+Au minimum bias and central STAR ρ0/ω ~ 1.67• p+p STAR ρ0/ω ~ 2.1

K. Ackerstaff et al. Eur. Phys. J. C 5, (1998) 411

ρ0

ω


Pythia

STAR Patricia Fachini QM20081 ρ 0 Production at High p T in Central Au+Au and p+p collisions at s...

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