Patricia Fachini for the STAR collaboration

STAR QM2009, Knoxville, March 30 - April 4 Patricia Fachini 1

Patricia Fachini

for the STAR collaboration

Motivation

Measurements

Results

Conclusions

ρ0 Production in Cu+Cu Collisions at √sNN = 200 and 62.4 GeV in STAR


Motivation - I

• In-medium modification of mass and/or width Chiral Symmetry Restoration?

• Leptonic decay channel probes all stages of the collision.• Hadronic decay channel probes only late stages of the collision.

ρ0 ρ0π-

π+

π+

π-

ρ0 c = 1.3 fm

ρ0

ρ0

+

+

-

-

π- π+

ρ0

-

+

π- π+

ρ0

π+

π-


• Hadrons scale number constituent quarks v2/n• ρ0 v2 scale number constituent quarks?• ρ0(770) production mechanism scale NCQ v2/n

– ππ ρ0 n = 4 – qq ρ0 n = 2

• Difference between two limiting cases 30%

Motivation - II

C. Nonaka et al., Phys.Rev. C69 (2004) 031902

Hadron Gas

QGP

Hadron Gas + QGP


Motivation - III

• If ρ0 decays before kinetic freeze-out not reconstructed due to rescattering of daughters

• Chemical freeze-out elastic interactions π+π- ρ0 π+π- regenerate ρ0 until kinetic freeze-out

• Is the regeneration and rescattering driven by the hadron cross-sections?

Chemical freeze-out

Kinetic freeze-out

ρ0 measured

ρ0

π+π-

π+ π-ρ0

measured

ρ0

ρ0 lostρ0

π-

π+

ρ0

π-

π+

π-

π+


Motivation - IV

• ρ0 regeneration σ(ππ)

• ρ0 daughter rescattering σ(πp) and σ(Kπ)

• σ(ππ) >> σ(Kπ)

• σ(πp) > σ(ππ)

Probe dynamics between chemical and kinetic freeze-outs

Chemical freeze-out Kinetic freeze-out

K*π

K

Δ++p

π

πρ

π

π

πρ

π

π

σ(ππ)

K*

π

K

π

σ(Kπ)

π p

π

π p

π

σ(πp)

Δ++

Δ++

π

π

σ(ππ)

π

ρ


• HIJING events with a realistic simulation of detector response

• Use ω and K*0 shape from HIJING to fit the data

• K*0 signal is fixed using STAR measurement

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


Invariant Mass Distribution - Cocktail

• Background subtracted like-sign technique.• ρ0 and f0 widths fixed according PDG.


Phase Space

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 Note, March 2000, unpublished; E.V. Shuryak and G.E. Brown, Nucl.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 = Temperature

• 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 =

BW(M) = Γ(M)

(M2 – M2)2 + M

2 Γ(M)2Γ(M) = Γρ

M2 – 4mπ2

Mρ2 – 4mπ

2

32 Mρ

M


Mass

• Mass shift ~45 MeV/c2 observed possible explanations– Medium modifications

– Bose-Einstein correlations

R. Rapp, Nucl.Phys. A725, 254 (2003), E.V. Shuryak and G.E. Brown, Nucl. Phys. A 717 (2003) 322

G.D. Lafferty, Z. Phys. C 60, 659 (1993); R. Rapp, Nucl.Phys. A725 (2003) 254-268

S. Pratt et al., Phys.Rev. C68 (2003) 064905

Statistical errors only Statistical errors only

J. Adams et al., Phys. Rev. Lett. 92 (2004) 92301


Spectra

• ρ0 measured in four different centralities in Cu+Cu.• Statistical and systematic errors added in quadrature.• Small signal to background ratio prevents the measurement in

central collisions (~1/1000).• Solid lines exponential fits.


Mean pT

• Statistical and systematic errors added in quadrature.

• ρ0 mean pT slightly increases with Npart.

• ρ0 mean pT comparable with proton mean pT.

B. I. Abelev et al., Phys. Rev. C 78 (2008) 44906J. Adams et al., Phys. Rev. C 71 (2005) 64902J. Adams et al., Phys. Rev. Lett. 92 (2004) 92301


M. Bleicher et al. J. Phys. G 25 (1999) 1859

Particle Ratios

σ(Kπ)

σ(ππ)

σ(πp)• Cross-section regeneration or rescattering. • K* rescattering of the daughters.

• ρ0 regeneration compensating rescattering of

the daughters.


M. Bleicher et al. J. Phys. G 25 (1999) 1859

Particle Ratios

σ(Kπ)

σ(ππ)

σ(πp)• Cross-section regeneration or rescattering. • K* rescattering of the daughters.• ρ0 regeneration compensating rescattering of

the daughters.


Particle Ratios – Transport Model M. Bleicher et al. J. Phys. G 25 (1999) 1859

σ(Kπ)

σ(ππ)

σ(πp)

• UrQMD long expansion time π+π- scattering will not have enough energy to produce ρ0.

STAR

• Significant ρ0 v2 measured pT > 1.2 GeV/c v2 ~ 13 ± 4%.

Elliptic Flow

15QM2009, Knoxville, March 30 - April 4 Patricia Fachini


Elliptic Flow

• Resonance v2 ρ0(770) production mechanism scale NCQ v2/nππ ρ0 n = 4 or qq ρ0 n = 2

a, b, c, and d constants extracted using KS0 and Λ v2 ρ0 v2 n= 4.7 ± 2.9

pT range covered not sufficient for conclusive statement on the ρ0 production mechanism.

v2(pT,n) = - dn1 + exp[-(pT/n – b)/c]

anX. Dong et al., Phys.Lett. B597 (2004) 328

n=2 n=4

STAR

• Significant ρ0 production measured in Cu+Cu collisions at √sNN= 200 and 62.4 GeV.

• Production measured in 20-60% of hadronic cross-section.• Mass shift ~45 MeV/c2 observed

– Medium modifications.– Bose-Einstein correlations.

• Regeneration and rescattering driven by hadron cross-sections.

• K* rescattering of the daughters.

• ρ0 regeneration compensating rescattering of the daughters.

• First measurement of the ρ0 v2 significant ρ0 v2 measured.

• pT range covered not sufficient for conclusive statement on the ρ0 production mechanism.

See the poster by Prabhat Pujahari for details on the analysis of the results presented in this talk.

QM2009, Knoxville, March 30 - April 4 Patricia Fachini 17

Conclusions


Elliptic Flow


• Significant ρ0 v2 measured pT > 1.2 GeV/c v2 ~ 13 ± 4%.

Elliptic Flow


• 1. Dynamical effect ρ0

interactions at late stages of the collision– nucleons, hyperons and

baryon resonances– pions, kaons and ρ0-mesons– t-channel exchanges

Distortion of the ρ0 spectral shape mass shift and/or broadening

-23 MeV/c2 shift in the ρ0 mass

-38 MeV/c2 shift in the ρ0 mass due to t-channel attraction

R. Rapp, hep-ph/0305011

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

Possible Explanations for Mass Shift - I


• 2. Interference between various π+π- scattering channels can effectively distort the line shape of resonances R. Longacre, nucl-exp/0305015

• 3. Bose-Einstein correlations between ρ0 decay daughters and pions in the surrounding matter can also distort the resonance shape G.D. Lafferty, Z. Phys. C 60, 659 (1993); R. Rapp, hep-ph/0305011; S. Pratt et al., nucl-th/0308087.

Possible Explanations for Mass Shift - II

ρ0

π-

π+

Dπ-

π+

Aρ0

π-

π+

A Interference

Patricia Fachini for the STAR collaboration

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