1. PHENIX measurement on direct photon production
Jiamin Jin
Journal Club
07/12/2006

2. Outline motivation direct photon in pp & AuAu (RUN2)
-- hep-ex/ , nucl-ex/
direct photon in AuAu (RUN4)
extend to higher pT
two analyses to improve on mid pT range
using stachastic cuts (David et al)
photon internal conversion (Y. Akiba)
-- various ANs
2018/12/3
J. Jin - JClub

3. What are direct photons?
@LO level, 3 channels:
Compton scatting g+qg+q
Annihilation q+qbarg+g
The latter is suppressed for p+p, because qbar is less probable than g in the proton
The parton-parton scattering with the scattered quark or gluon fragmenting to a g
@NLO level:
Bremstrahlung emission of g from the quarks undergoing hard scattering
2018/12/3
J. Jin - JClub

4. photons not from hadronic decays
Experimental challenge to differentiate gdirect & gdecay
cocktail subtraction method
p0 tagging method
Isolation cuts
Direct photons != isolated photons
Fragmentation and bremstralung processes will produce photons with jets
2018/12/3
J. Jin - JClub

5. Why direct photons? p+p A+A Test on QCD
explore PDF of the gluon inside the proton
Reduce uncertainty on pQCD photons in AA
A+A
Photons don’t strong interact with medium
Hard photons
Allow test of Ncoll scaling for hard processes
Interpret high-pt hadron suppression at RHIC
Thermal photons
Carry information of early stage of collisions
Detect QGP via thermal photon radiation
2018/12/3
J. Jin - JClub

6. Direct photons in p+p 1st step: determine direct-photon-candidate hits
reject a EMCal cluster if:
form an invariant mass in the p0 or h range with other clusters
asymmetry cut
combinatorial background studied by embedding (small effect in p+p)
a factor of 2 rejection achieved for cluster pT>5GeV/c
2018/12/3
J. Jin - JClub

7. Formula of differential cross section
N: # of direct-photon-candidates in a DpT&Dy bin
Creco: acceptance and efficiency correction (MC sim.)
Cconv: correction for photon conversions (<7.4%)
Closs: correction for the loss of true direct photons due to combinatorial bkgd (<2%)
f = 0.75(+/-0.02): MinBias trigger efficiency
L: integrated luminosity (~41nb-1 for RUN2 p+p)
2018/12/3
J. Jin - JClub

8. 2nd step: deal with the remaining hadronic decay photons
mostly from missing the partner photon
also from other decay sources (w,h’,…)
plot from data
feed p0 spectrum into MC, simulate
get double gamma ratio Rg
mT scaling for other decay sources
2018/12/3
J. Jin - JClub

9. Results
Ratio of direct-photon-candidates to the p0 spectrum (data & MC)
Direct g cross section
small but significant signal observed
agree with NLO pQCD
2018/12/3
J. Jin - JClub

10. Other methods I
p0 tagging method:
2018/12/3
J. Jin - JClub

11. Other methods II Isolation cuts Emphasize the Compton process
photon energy < 10% of cone energy
cone size: 0.5 around the candidate photon
cone energy: sum of track momentum in DC & EMCal cluster energy
correct for limited PHENIX acceptance, which causes fake isolated photons
Emphasize the Compton process
direct access to gluon distribution
2018/12/3
J. Jin - JClub

12. Results (RUN3 p+p) direct photon production
spectrum up to 16GeV/c
well described by NLO pQCD for pT>5GeV/c
ratio of isolated photons to all direct photons
large systematics in low pT
agree with pQCD(lines) for pT>7GeV/c
2018/12/3
J. Jin - JClub

13. Direct photons in Au+Au
What’s new in Au+Au?
centrality determination by BBC&ZDC
high pT hadron suppression in central Au+Au, measure Rg directly:
Rg > 1, indicates direct photon signal
Direct photon spectra extracted as:
2018/12/3
J. Jin - JClub

14. Results double gamma ratio plot advantage: many systematics cancel
an excess over unity is observed at high pT
magnitude increases with centrality due to hadron suppression
expectation for Ncoll scaling of direct photons holds for all centrality bins
2018/12/3
J. Jin - JClub

15. spectra shown 9 centrality bins and MB comparison to NLO
at low to mid-pT where additional production of thermal photons appears, we have large error bars
2018/12/3
J. Jin - JClub

16. Improvement on mid-pT thermal photon region
2018/12/3
J. Jin - JClub

17. goal: improve upon mid-pT region systematic errors
using most stable subset of RUN4 data
improving various corrections
p0 peak extraction
energy scale/linearity
energy smearing
fitting function to p0 spectrum
using stochastic cuts to extract photons
2018/12/3
J. Jin - JClub

18. Going to low pT: No apparent excess
2018/12/3
J. Jin - JClub

19. Excess above the scaled pQCD?
new points agree with, but consistently higher than the old ones ~4GeV/c
2018/12/3
J. Jin - JClub

20. A new approach: very low-mass di-lepton pairs
2018/12/3
J. Jin - JClub

21. p0 g e+ e- g*
The idea: Dalitz decay
mass distribution of e+e- pairs (Knoll-Wada formula):
1) pure QED part, universal to all Dalitz decays
2) phase factor part, depends on M of parent hadron
3) hadronic form factor
at low mass, both the second and third part become 1
at high mass, suppressed by phase factor
2018/12/3
J. Jin - JClub

22. Now direct photons: ÷ g*
Compton scattering
Now direct photons: e- g* q g q
Any source of real g produces virtual g with very low mass
Rate and mass distribution given by same formula
No phase space factor for mee<< pT photon
MeV
0-30
Rdata ÷
We measure Rdata:
N(90<Mee<300)/ N(0<Mee<30)
We can calculate:
Rp0, Rh, Rdirect
2018/12/3
J. Jin - JClub

23. Rdata excess over calculated expected ratio from hadronic decays:
True in very
low mass window!
Rdata excess over calculated expected ratio from hadronic decays:
virtual & real direct photon signal
2018/12/3
J. Jin - JClub

24. Results: Rdata
0-20 %
2018/12/3
J. Jin - JClub

25. g*direct/g*inclusive
0-20 %
Significant 10% excess of very-low-mass virtual direct photons
2018/12/3
J. Jin - JClub

26. Indication for centrality dependence
more peripheral
Indication for centrality dependence
2018/12/3
J. Jin - JClub

27. Comparison to conventional results
( + 1 )
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J. Jin - JClub

28. gdirect
2018/12/3
J. Jin - JClub

29. The Spectrum Compare to the published Run2 results (black arrow)
2018/12/3
J. Jin - JClub

30. The Spectrum Compare to NLO pQCD excess above pQCD 2018/12/3
J. Jin - JClub

31. The Spectrum Compare to NLO pQCD excess above pQCD
Compare to thermal model
data above thermal at high pT
2018/12/3
J. Jin - JClub

32. The Spectrum Compare to NLO pQCD excess above pQCD
Compare to thermal model
data above thermal at high pT
Compare to thermal + pQCD
data consistent with thermal + pQCD
2018/12/3
J. Jin - JClub

33. Backup
2018/12/3
J. Jin - JClub

34. Schematic Photon Spectrum in Au+Au
Decay photons
thermal:
hard:
2018/12/3
J. Jin - JClub