Centrality, N part & N collision at BRAHMS
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Transcript of Centrality, N part & N collision at BRAHMS
Centrality, Npart & Ncollision at BRAHMS
H. ItoUniversity of Kansas
For the BRAHMS Collaboration
Why do we want Npart?
• Comparison of P-P, P-A, and A-A?• Comparison of Different Centrality?
Can we live without Npart?•For comparison of P-P,P-A & A-A, we can compare the most central collisions.
What is the concern about using Npart?•No Direct Measurement•Is there any indirect measurement (ZDC,etc)?•If not measured, are there any model dependencies?
What Is Measured?
Ncharge in -4.7< <4.7
We can measure charged particle multiplicities.
What we have done with Npart up to now?
Using experimental centrality and comparing with the model calculation, we got Npart for our event selections.
How we calculated Npart.Measure charged
particle multiplicitiesby Silicon and Tile • Correct for missing fraction of
the cross section and background events with small multiplicity by Tile only.
• Correct anomalous background events by Silicon and Tile.
Assuming centrality and multiplicity have a linear
correlation, select events by slicing the corrected multiplicity
distribution vertically.
By knowing the centrality, use Monte Carlo simulation to get
corresponding Npart.
Event Selection for Minimum Bias
• ZDC is very close to100 % efficient.– Not efficient at the most central events.
• Beam Beam (BB) is only about 75 % efficient.– Not efficient at the peripheral events.
Although the correlated events by BB and ZDC can be used to select only real collision events, it is not used for minimum bias selection.
TPC vertex BB vertex ZDC vertex
Multiplicity and Centrality Cut
Event selection is done by slicing multiplicity distribution vertically.
5%50%
Efficiency for Minimum Bias Events
Efficiency of the Tile getting hit in minimum bias event is about 99 % in the Monte Carlo simulation.
How to get the fraction of missing cross section?
Measured multiplicity distribution by Tile
Throw out
Using Hijing with GEANT, estimate how many events we should have for very small multiplicity events.
%101.200...0
4...0
M
M
Event
Event
Centrality Selection in Monte CarloMultiplicity measured by
Silicon and Tile is use to
select centrality.
Wounded Nucleon Calculation
NN= 40mb, a = .54 fm
40-50
30-40
20-30
10-20
5-10
0-5
Npart (Hijing)
Ncollision (WN)
Npart(WN)Centrallity
346
293
228
164
114
76
352
299
235
165
114
75
873
708
516
333
206
118
Measured Neutron by ZDC
50 % CentralVery Few Neutrons ???
Correlation of Tile and Silicon multiplicity
Correlation of BB and Silicon-Tile multiplicity
Npart, Ncollision and Impact Parameter
NN= 40mb, a = .54 fm
Ncollision
Npart