STUDY OF HIGH-PT DIRECT PHOTONSIN SMALL SYSTEMS AT PHENIX
Zhandong Sun for the PHENIX collaboration
University of Debrecen, Hungary

Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

1Outline■ Motivation■ Extraction of π0■ 2D-matrix Method■ Uncertainty study■ Decay photons■ Corrected π0 and raw direct photons■ SummaryAug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

2

MOTIVATION

Nuclear modification factor for π0in p/d+Au collisions
Reading the plot…

𝑑𝑁𝐴𝐴
𝑑𝑝𝑇𝑅𝐴𝐴 𝑝𝑡 =𝑑𝑁𝑝𝑝
𝑁𝑐𝑜𝑙𝑙
𝑑𝑝𝑇• Both p+Au and d+Au show large centralitydependence.
• In high pT we observe suppression in central eventsand enhancement in peripheral events.
Conclusion:
Something interesting is behind the centrality dependence !
Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

3

MOTIVATION(possible) Mis-binning of centrality?
𝑑𝑁𝐴𝐴(𝑁𝑐ℎ)𝑑𝑝𝑇
𝑅𝐴𝐴 𝑝𝑡 =𝑑𝑁𝑝𝑝
𝑁𝑐𝑜𝑙𝑙
𝑑𝑝𝑇

Ncoll: Average number of binary collisions in a type of event
according to Glauber Model.

This works well when twolarge ions collide■ Ncharge↔ 𝑵𝒄𝒐𝒍𝒍Phys. Rev. Lett. 123.022301
Aug. 23, 2021

Annu. Rev. Nucl. Part. Phys. Vol. 57:205-243
The 20th Lomonosov Conference on Elementary Particle Physics

4

MOTIVATION(possible) Mis-binning of centrality?
𝑑𝑁𝐴𝐴(𝑁𝑐ℎ)𝑑𝑝𝑇
𝑅𝐴𝐴 𝑝𝑡 =
𝑑𝑁𝑝𝑝
𝑁𝑐𝑜𝑙𝑙
𝑑𝑝𝑇

Ncoll: Average number of binary collisions in a type of event
according to Glauber Model.

However, in small systems…

■ Ncharge↔ 𝑵𝒄𝒐𝒍𝒍
Direct photons
Phys. Rev. Lett. 123.022301
Aug. 23, 2021

Annu. Rev. Nucl. Part. Phys. Vol. 57:205-243
The 20th Lomonosov Conference on Elementary Particle Physics5MOTIVATION

Au+Au

Suppression in π0 and non-suppression in direct 𝛾 RAA

Phys. Rev. L 101 (2008) 232301

Aug. 23, 2021

Phys. Rev. L 109 (2012) 152302

The 20th Lomonosov Conference on Elementary Particle Physics

6MOTIVATIONThe centrality dependence that we see in π0spectrum is because of true physics effects.
And it should not affect direct photon.
Corrected direct
photon spectrum
Corrected Pi0Spectrum

The centrality dependence that we see inπ0 spectrum is because of mis-binning ofcentrality. This should affect direct photonequally.
Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

7

NEUTRAL PION EXTRACTION

𝑚𝛾𝛾 =2𝐸1 𝐸2 1 − cos ∅Mixed event method at low-pT

π0Aug. 23, 2021

Average bkg. subtraction method at high-pT
Real with Mixed

RealReal-Mixed

Real-average

Real-Mixed-pol2Fitting[0,05,0,25]

The 20th Lomonosov Conference on Elementary Particle Physics

π0Real-averaged[0,05,0,25]

8CORRECTION FOR ACCEPTANCE etc.

Acceptance:• Coverage of rapidity |η|<0.35• Dead/hot towers in EMCal• Cluster selection geometrically for analysis
Efficiency:• PID efficiency• Merging effect at high pT• Cluster break-up at high pT

Measured pT

What should be corrected ?

Smearing:• The pT of the reconstructed π0 might be different fromthe true pT
Migration takes place between pT bins.
True pT is steep , so migration is mostly to lower pT.
Inverting this response matrix is problematic due to thesmall, fluctuating off-diagonal elements.
Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

Original pT9

CORRECTION USING 2D-RESPONSE-MATRIX

Simulated response matrix -- true -> measured

nn is the number of π0 generated at pT n
xmn is the number of π0 generated at pT n but
reconstructed as pT m

Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

10

CORRECTION USING 2D-RESPONSE-MATRIXHow to use the response matrix without inverting it

Trial yield

Measured yield if trial
yield is the input

Compare to raw
yield from data

Disagree

Agree

Final trial function isthe corrected yield

Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

11

CORRECTION USING 2D-RESPONSE-MATRIXpAu

Measured pT

Final trial π0 spectrum

measured π0 from final trial π0
raw π0 from data

raw π0 from datameasured π0 from final trial π0

Original pT
The blue curve is the raw spectrum extracted from data.
The magenta is the trial spectrum after 4 iterations.

Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

12

ERROR STUDY

Propagation of statistical uncertainty
1) 3000 sample spectra are generated using thevalue and original uncertainty at each pT.
2) Correct all sample spectra by 2-D responsematrix.
3) Plot all corrected spectra together and fill
histograms for each pT.
4) A Gaussian fit tells us about the stat. uncertaintyof the corrected spectra.
Randomizing from raw spectra in pT binsfor 3000 sample spectra

Fit randomized corrected spectra in pT bins, mean andsigma go to the final corrected spectrum
Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

13

DECAY PHOTONS

Inclusive photons can’t by corrected by one single matrix.

Decay photons

Direct photons

π0(20GeV)->γγpT [GeV/c]pT(γ) [GeV/c]

Decay γKinematically, the 2γ distribution should be flat, butwhen actually measured in the detector, it isn't flat.

Inclusive photons

Direct photons don’t have the merging
effect, nor do they exhibit wrong
energy sharing of separate, but closeby clusters as seen in decay photons.

π0->γγDecay γπ0(8GeV)->γγ

pT [GeV/c]Aug. 23, 2021

pT(π0) [GeV/c]

The 20th Lomonosov Conference on Elementary Particle Physics

14

HADRON DECAY PHOTONSOther sources of decay photons : η, η’, ω, etc.

Decay γFrom cocktail ratio calculation using mT scaling ,Usually η/π0 ratio is considered as asymptotically constant, but according
to a recent study arXiv:2102.05220, we see the decreasing of η/π0 withincreasing pT.
Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

arXiv:2102.05220

15

HADRON DECAY PHOTONS

Decay photons are calculated using 2-D response matrix for π0 decay
and then scaled by cocktail ratio.

corrected π0π0 decay γ

hadron decay γ

Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics16

RAW DIRECT PHOTONS

Only the stat. uncertainty is shown.
Currently, we are working on the Sys. uncertainty.

Aug. 23, 2021

First spectra of raw direct photons.
There is a significant contamination by hadrons in theEMCal at lower energies, which is currently under study.

The 20th Lomonosov Conference on Elementary Particle Physics

17

SUMMARYSummary■ Centrality dependence in nuclear modification factor of π0 in p/d+Au collisions■ 2-D response matrix method is introduced for corrections■ Statistical uncertainty propagation in the yield of π0■ Invariant yield of pion and raw direct photon spectra■ The ratio direct γ/π0 is the ultimate goalAug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

18

THANKYOU FORYOURATTENTION!

Aug. 23, 2021

The 20th Lomonosov Conference on Elementary Particle Physics

19