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L. L. p. p. p. p. Baryonic Resonance. Studies with. Why resonances and why S * ? How do we search for them ? What did we learn so far? What else can we do in the near future?. S *. p. Sevil Salur Yale University STAR Collaboration.

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L

L

p

p

p

p

Baryonic Resonance

Studies with

  • Why resonances and why S* ?

  • How do we search for them ?

  • What did we learn so far?

  • What else can we do in the near future?

S*

p

Sevil Salur Yale University STAR Collaboration


Why do we study resonances in heavy-ion collisions?

Due to the very short lifetime ( t<Δt) of resonances:

Δt

  • Large fraction of the decays occur inside the reaction zone

S*

  • Possible change in the physical properties:

  • width broadening

  • mass shift

  • change in pT spectra

L

S*

S* measured

S* lost

L

S*

S*

  • Determination of the hadronic expansion time between chemical and thermal freeze-out

L

L

L

S* measured

Thermal freeze-out

Chemical freeze-out

  • Information about strangeness production due the strange quark content and high mass of S*(1385)

time

S(1385)L+p M:1385 MeV/c2

Г:35 MeV/c2 I(JP) =1(3/2+)


Particle identification
Particle Identification

NENTRIES

A  candidate is combined with a  to get a *(1385). The background is formed by mixing  mesonsfrom one event with  candidates from another event.

sNN=200 GeV p+p

Minv GeV/c2

NENTRIES

S*(1385)

X

X  L + p- and S* ±  L + p±


S*Invariant Mass Spectra

sNN=200 GeV @ p+p

The masses and widths of X are in agreement with the PDG and the results with other topological reconstruction techniques.

NENTRIES

S*±

X-

( S-(1385) M = 1383 MeV , =36 MeV

S+(1385) M = 1387 MeV , =39 MeV )

sNN=200 GeV @ d+Au

NENTRIES

sNN=200 GeV @ Au+Au 0-5%

S*±

X-


S corrected p t spectra
S*Corrected pT Spectra

|y|<0.5

Exponential Fit Function :

<pT>=1.02±0.02±0.07 GeV/c T inv slope= 319±9±16 MeV

Similar <pT> is measured.

|y|<0.5

|y|<0.5

<pT>=1.28 ± 0.15 ± 0.09 GeV/c T inv slope= 456 ± 54 ± 23 MeV

<pT>=1.14 ± 0.05 ± 0.08 GeV/c T inv slope= 386 ± 15 ± 27 MeV


pT vs Particle Mass

Parameterization is from ISR data at √s=26 GeV (Not correct for heavy particles. )

pTvalues merge for Au+Au and p+p for heavier particles.

  • Are heavier particles produced predominately in more violent p+p collisions?

  • Do heavier particles flow less in Au+Au with respect to p?


S* in p+p and Pythia Comparisons

Leading Order pQCD model

K factor represents the factorization of next-to-leading order (NLO) processes.

A large K factor Large NLO contributions

Version 6.3


Pythia Comparisons: K=3 is also needed to describe strange baryons

S* in p+p and Pythia Comparisons

K=3 too hard for the light mesons.

S(1385)

Version 6.3

K=3 ok for strange baryons


Thermal Model Predictions

Particle ratios are represented except L*/L .

B and Q = 2, S = 0

T is same for pp and AuAu

Particle ratios are well described except the L*/L


Resonance Ratios in p+p, d+Au and Au+Au

K*/K and L*/L exhibits a slight suppression  the re-scattering

Regeneration σ(K*) > σ(L*)

S*/L  regeneration

If there is re-scattering then regeneration is needed !

t= 2 ± 1 fm/c from K*/K

t= 10 ± 6 fm/c from L*/L


Nuclear modification factor r dau

d+Au

p+p

Nuclear Modification Factor RdAu

BARYONS

√sNN=200 GeV

MESONS

Cronin Effect might explain R dAu above 1

Less so for mesons than baryons !

S(1385) follow h++h-


Conclusions
Conclusions

  • The S(1385) resonance can be clearly identified via combinatorial techniques in all collision environments. No strong increase of pT ppAuAu

  • Different production mechanisms (jets in p+p)?Pythia with K=3 factor.

  • No suppression or enhancement in the ratios of S*/L in p+p, d+Au and 0-5% Central Au+Au collisions within the uncertainties of the measurement.

  • Regeneration is needed for the re-scattering picture if the time between chemical and thermal freeze-out is non-zero. Sequential freeze-outs instead.

  • T is similar for pp and AuAu collisions.

  • Nuclear modification factor (RdAu) for S(1385) follows the same trend as p.

  • Species or mass dependence can be further investigated with RAA measurement from Run 4

  • More data is available from Run 4 !!! Better centrality measurement for S*. Au+Au at √sNN=200 GeV, 50 Million Events taken.

More to Come. Keep Tuned !


First Resonance Measurement (Y*)

Resonances are strongly decaying, extremely short lived particles. [~fm/c]

L+p

  • Two possible explanations:

  • Resonant States (Energies at which the cross section is a maximum)

  • Resonance Particles (Real Particles)

Invariant Mass Distribution of Y* =S*(1385)

Nobel Prize 1968

"for his decisive contributions to elementary particle physics, in particular the discovery of a large number of resonance states, made possible through his development of the technique of using hydrogen bubble chamber and data analysis"

Alvarez

M. Alston, Phys. Rev. Lett. 5, 520 (1960).


Thermal Model Predictions

J. Cleymans hep-ph 0212335

  • s is higher in AuAu

  • An enhancement in the K/p ratios ~ 50%

Particle ratios are represented except L*/L .

B and Q = 2, S = 0

The relative strangeness production for Pb+Pb at SPS similar to p+p at RHIC .

T is same for pp and AuAu

Particle ratios are well described except the L*/L


Thermal Model Predictions

Particle ratios are represented except L*/L .

Small B No incoming baryon number

Particle ratios are well described except the L*/L


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