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
Patricia Fachini QM2008 2STAR
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
Patricia Fachini QM2008 3STAR
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
Patricia Fachini QM2008 4STAR
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
Patricia Fachini QM2008 5STAR
Ratios Data and NLO π+and π-
• pQCD calculations reproduce well the pion spectra up to pT = 15 GeV/c
Patricia Fachini QM2008 6STAR
Ratios Data and NLO p and p
• pQCD NLO calculations need improvement for describing proton and anti-proton spectra
Patricia Fachini QM2008 7STAR
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)
Patricia Fachini QM2008 8STAR
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
Patricia Fachini QM2008 9STAR
• 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
Patricia Fachini QM2008 10STAR
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
Patricia Fachini QM2008 11STAR
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
Patricia Fachini QM2008 12STAR
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
Patricia Fachini QM2008 13STAR
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
Patricia Fachini QM2008 14STAR
Ratios Data and NLO ρ0 and π
• pQCD calculations for π similar to ρ0 spectra
Patricia Fachini QM2008 15STAR
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
Patricia Fachini QM2008 16STAR
Nuclear Modification Factor
• π more suppressed than protons opposite to color charge effect• ρ0 nuclear modification factor comparable to pions RAA
Patricia Fachini QM2008 17STAR
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
Patricia Fachini QM2008 18STAR
Back-up Slides
Patricia Fachini QM2008 19STAR
STAR Detector
• Using jet triggered data for p+p TPC + EMC minimum amount of energy requested in the EMC
Patricia Fachini QM2008 20STAR
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
Patricia Fachini QM2008 21STAR
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
Patricia Fachini QM2008 22STAR
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
Patricia Fachini QM2008 23STAR
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
Patricia Fachini QM2008 24STAR
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
Patricia Fachini QM2008 25STAR
Pythia