[email protected] 1 baryonic resonance why resonances and why * ? how do we search for them ?...
Post on 20-Dec-2015
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Baryonic Resonance
•Why resonances and why* ?
•How do we search for them ?
•What did we learn so far?
•What else can we do in the near future?
Sevil Salur Yale University STAR Collaboration
p
p
Studies with
Chemical freeze-out
Thermal freeze-out
*
*
measured
*
lost
*
measured
time
Why do we study resonances in heavy-ion collisions?
Due to the very short lifetime ( <Δt ) of resonances:
• Large fraction of the decays occur inside the reaction zone
• Possible change in the physical properties: width broadening mass shift change in pT
spectra • Determination of the hadronic expansion time between chemical and thermal freeze-out
• Information about strangeness production due the strange quark content and high mass of *(1385)
Δt
MeV/c2
Г:35 MeV/c2 I(JP) =1(3/2+)
Particle Identification
Minv GeV/c2
*(1385)
NE
NT
RIE
S
sNN=200 GeV p+p
- and ±
±
NE
NT
RIE
S
A candidate is combined with a to get a *(1385). The background is formed by mixing mesons from one event with candidates from another event.
** Invariant Mass SpectraInvariant Mass Spectra
*±
The masses and widths of are in agreement with the PDG and the results with other topological reconstruction techniques.
M = 1387 MeV , 39 MeV )
( M = 1383 MeV , 36 MeV
*±
sNN=200 GeV @ d+Au
sNN=200 GeV @ p+p
sNN=200 GeV @ Au+Au 0-5%
NE
NT
RIE
SN
EN
TR
IES
** Corrected pCorrected pT T SpectraSpectra
T
)mm(
0T
2
T
0T
e)Tm( T 2
dydN
dydm
Nd
m 2
1 −−
+=
Exponential Fit Function :
|y|<0.5
<pT>=1.02±0.02±0.07 GeV/c T inv slope= 319±9±16 MeV
|y|<0.5
<pT>=1.14 ± 0.05 ± 0.08 GeV/c T
inv slope= 386 ± 15 ± 27 MeV
|y|<0.5
<pT>=1.28 ± 0.15 ± 0.09 GeV/c T inv slope= 456 ± 54 ± 23 MeV
Similar <pT> is measured.
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 ?
Version 6.3
Leading Order pQCD model
K factor represents the factorization of next-to-leading order (NLO) processes.
A large K factor Large NLO
contributions
* in p+p and Pythia * in p+p and Pythia Comparisons Comparisons
Version 6.3
* in p+p and Pythia * in p+p and Pythia Comparisons Comparisons
K=3 too hard for the light mesons.
Pythia Comparisons: Pythia Comparisons: K=3 is also needed to describe strange baryons
(1385)
K=3 ok for strange baryons
T 171 ± 9 MeV
s 0.53 ± 0.04
r 3.49 ± 0.97 fm
B and Q = 2, S = 0 T 168 ± 6 MeV
s
0.92 ± 0.06
r 15 ± 10 fm
B (4.5 ± 1.0) X 10-2 GeV
S (2.2 ± 0.7) X 10-2 GeV
Q (-2.1 ± 0.8) X 10-2 GeV
Particle ratios are represented except .
Particle ratios are well described except the */T is same for pp and AuAu
Thermal Model Predictions
Resonance Ratios in p+p, d+Au and Resonance Ratios in p+p, d+Au and Au+AuAu+Au
If there is re-scattering then regeneration is
needed !
/( ) ( ) tkinetic chemical
resonance resonancee
stable stable−Δ= × Δt= 2 ± 1 fm/c from K*/K
Δt= 10 ± 6 fm/c from */
K*/K and */ exhibits a slight suppression the re-scattering
Regeneration σ(K*) > σ(*)
*/regeneration
Nuclear Modification Factor RdAu
(1385) follow h++h-
Cronin Effect might explain R dAu above 1
Less so for mesons than baryons !
d+Au
p+p
√sNN=200 GeV MESONS
BARYONS
Conclusions
• The(1385) resonance can be clearly identified via combinatorial techniques in all collision environments. No strong increase of pT ppAuAu
Different production mechanisms (jets in p+p)? Pythia with K=3 factor.
•No suppression or enhancement in the ratios of */ 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 (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 *. Au+Au at √sNN=200 GeV, 50 Million Events taken.
More to Come. Keep Tuned !
First Resonance Measurement (Y*)
Alvarez
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"
Resonances are strongly decaying, extremely short lived particles. [~fm/c]
Invariant Mass Distribution of Y* =*(1385)
Two possible explanations:
• Resonant States (Energies at which the cross section is a maximum)
• Resonance Particles (Real Particles)
M. Alston, Phys. Rev. Lett. 5, 520 (1960).
T 171 ± 9 MeV
s 0.53 ± 0.04
r 3.49 ± 0.97 fm
B and Q = 2, S = 0 T 168 ± 6 MeV
s
0.92 ± 0.06
r 15 ± 10 fm
B (4.5 ± 1.0) X 10-2 GeV
S (2.2 ± 0.7) X 10-2 GeV
Q (-2.1 ± 0.8) X 10-2 GeV
Particle ratios are represented except .
Particle ratios are well described except the */T is same for pp and AuAu
J. Cleymans hep-ph 0212335
The relative strangeness production for Pb+Pb at SPS similar to p+p at RHIC .
s is higher in AuAu
An enhancement in the K/ ratios ~ 50%
Thermal Model Predictions
Thermal Model Predictions
T 168 ± 6 MeV
s
0.92 ± 0.06
r 15 ± 10 fm
B (4.5 ± 1.0) X 10-2 GeV
S (2.2 ± 0.7) X 10-2 GeV
Q (-2.1 ± 0.8) X 10-2 GeV
Particle ratios are represented except .
Particle ratios are well described except the */
Small B No incoming baryon number