Centrality determination with the Event
Plane Detector for fluctuation
measurements from STAR
Yuri Sato, for the STAR Collaboration
post QM @ Nagoya University
2019.12.22

Physics Motivation
ü Lattice QCD calculation has predicted that phasetransition around 𝜇" =0 is “smooth crossover”.
ü We search for the 1st-order phase transition and thecritical point.
ü Fluctuations of conserved quantities are considered to
be a powerful tool to search for the critical point.
Beam Energy Scan@STAR (~2014, 7.7-200 GeV)
Non-monotonic behavior of net-p 𝜅𝜎 % at low energy appeared,
which could be a signature of the critical point.

Y.Aoki et al., Nature, 443, 675 (2006)
X.Luo,PoS(CPOD2014)019
2019.12.22

post QM @ Nagoya University2

Beam Energy Scan II (BES-II)
Beam Energy Scan II (2019~)
p Lower collision energies (< 20 GeV)
p New detectors (EPD, eTOF, iTPC)
p 10-20 times larger statistics than BES-I
Event Plane Detector (EPD)ü A new scintillation detector installed in
2.1 < |η| < 5.1
ü Consist of 16 rings x 24 segments inEast and West side eachü Expected to be a centrality detector
with less autocorrelation effect2019.12.22

post QM @ Nagoya University3Autocorrelation Effectü UrQMD studies show autocorrelation effect makes fluctuations smaller.
ü Current centrality determination is based on multiplicity at mid-rapidity,excluding particles of interest.
Ø Current centrality determination maybe biased by the autocorrelation.

p Important to determine the centrality by reducing autocorrelation effect forfluctuation measurements.

X.Luo et al. , J. Phys. G: Nucl. Part. Phys. 40 105104 (2013)
T.Sugiura et al., arXiv:1903.02314(2019)
A. Chatterjee et al., arXiv:1910.08004
2019.12.22

post QM @ Nagoya University

4Performance of the EPD
Landau Fluctuation
ü The EPD measures NMip, gain calibrated energy
loss in tile, in units of Landau MPV for one MIP.
ü Considered large NMip (>NMipMax) is due toLandau fluctuation effect, and assume the NMipof the tile is NMipMax.
⇒ NMipMax=4 : If (NMip>4) NMip=4

NMipMax=4Spectators in the EPD
ü In lower energy collisions, also spectators
are measured in the EPD region.
ü Number of spectators increases inperipheral collisions.
From Prithwishʼs talk2019.12.22

post QM @ Nagoya University

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EPD-TPC Correlations in Au+Au 27 GeV
ß

A7 à

721AA

7:C74((6 .

EpdNMip(ring-by-ring)

. .
71 8

2019.12.22

1 8

post QM@ Nagoya University
0+ .A: |"| < $. &)6

Centrality Determination by the EPD
ü Positive correlation between EPD and TPC in outer rings
ü Anticorrelation in inner rings (spectator-participant correlation)
ü Summing up all rings will make the centrality resolution worse.
ü Construct a cleaner correlation between EPD and TPC
p Using only outer rings of the EPD
p Neural Network approach to recover the linearity

p Need to search for the best way to use the EPD as a centrality
detector!

2019.12.22

post QM @ Nagoya University

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Neural Network (N.N.) Approach
Artificial Neural Network is a method of Machine Learning, inspired bybiological neural network. It “learns” by updating weights and biases between
each neuron.

→ (left) non-linear correlation between EPD
and TPC because of spectators in inner rings→ (right) linear correlation between N.N. output
and TPC multiplicity
Ø Centrality Resolution can be improved!
2019.12.22

post QM @ Nagoya University

N.N.Output

TPC Multiplicity(previous centrality)

EPD NMip (All rings)

Epd NMip in each ring

8

TPC Mult.

üImpact parameter b cannot bemeasured experimentally.
üSeeing relative width of correlations,centrality resolution by the EPD can becompared.

**

RefMult : multiplicity in |η|<0.5,measured by TPC.
RefMult3 : multiplicity in |η|<1.0,measured by TPC, excluding protons
N.N.RefMult/RefMult3 :Output of N.N. trained to return
RefMult/RefMult3→ TPC Mult. distributions in acentrality class determined bythe EPD
2019.12.22

NMipMax=4

TPC Mult.

30-40%

No. of Events*EPD NMip (All rings)

Centrality Resolution of the EPD

30-40%

0-10%

post QM @ Nagoya University

TPC Mult.

0-10%

9Centrality Resolution of the EPD

σ/MeanouterInner<Npart>ü“EPD9-16” (outer 8 rings) and “EPD5-16” (outer 12 rings) have better resolution
than “EPD13-16” (outer 4 rings).
ü“N.N.RefMult” and “N.N.RefMult3” have the best centrality resolution. We canuse information by the EPD all rings weighted automatically.
ü“NMipMax=2”
has the best centrality
resolution compared to larger NMip upper10
2019.12.22
post QM @ Nagoya Universitylimit, because of reduced Landau fluctuation.

Perspective for Fluctuation Measurements
ü Ways to determine collision centrality with the EPD are discussed.
p Need to know how fluctuation results change by ways for centrality selection.
p Need to keep in mind that below 2 affect the same direction:• Worse centrality resolution makes fluctuations larger.
• Less autocorrelation makes fluctuations larger.
p Autocorrelation of the previous centrality determination really effects onfluctuations?
ReferencesØ X. Luo, PoS (CPOD2014) 019
Ø Y. Aoki et al., Nature, 443, 675 (2006)
Ø X. Luo et al., J. Phys. G: Nucl. Part. Phys. 40 105104 (2013)
Ø T. Sugiura et al., Phys. Rev. C 100, 044904(2019)2019.12.22

post QM @ Nagoya University

11

Back up2019.12.22

post QM @ Nagoya University

12Volume Fluctuation and Autocorrelation
Impact parameter
Npart

Centrality Resolution

Multiplicity
Tracking
efficiency

Landau fluctuation
Spectators

RefMult1-4
Net-proton(|η|<1.0)(|y|<0.5)
CBWC
Autocorrelation?

Centrality

2019.12.22

EpdNMip(2.1<|η|<5.1)

Volume Fluctuation

post QM @ Nagoya University

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Multiplicity/NMip Distributions
ü NMip distributions measured by theEPD have different shape compared tomultiplicity by the TPC due tospectators and Landau fluctuations.
ü Centrality can be determined for eachclass to have the same number ofevents based on these distributions.
p Need to investigate a model including
spectators and Landau fluctuations
effects for the EPD centrality ??

2019.12.22

post QM @ Nagoya University

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Energy Dependence of 𝜿𝝈𝟐• C4/C2 values at 54.4 GeV follows the energy dependence observed inother energies.
2019.12.22

post QM @ Nagoya University

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Centrality Resolution Effect• Worse centrality resolution makes fluctuations larger.

X.Luo et al. , J. Phys. G: Nucl. Part. Phys. 40 105104 (2013)
A. Chatterjee et al., arXiv:1910.08004
2019.12.22

post QM @ Nagoya University16