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PHENIX measurement on direct photon production
Jiamin JinJournal Club07/12/2006
2023.04.19 J. Jin - JClub 2
Outline
motivation direct photon in pp & AuAu (RUN2)
-- hep-ex/0502006, nucl-ex/0503003 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
2023.04.19 J. Jin - JClub 3
What are direct photons? @LO level, 3 channels:
Compton scatting g+q+q Annihilation q+qbar+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
@NLO level: Bremstrahlung emission of g from the
quarks undergoing hard scattering
2023.04.19 J. Jin - JClub 4
photons not from hadronic decays Experimental challenge to differentiate
direct & decay
cocktail subtraction method 0 tagging method Isolation cuts
Direct photons != isolated photons Fragmentation and bremstralung
processes will produce photons with jets
2023.04.19 J. Jin - JClub 5
Why direct photons? p+p
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
2023.04.19 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 0 or 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
2023.04.19 J. Jin - JClub 7
Formula of differential cross section
N: # of direct-photon-candidates in a pT&y 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 c
ombinatorial bkgd (<2%) f = 0.75(+/-0.02): MinBias trigger efficiency L: integrated luminosity (~41nb-1 for RUN2 p+p)
2023.04.19 J. Jin - JClub 8
2nd step: deal with the remaining hadronic decay photons mostly from missing the partner photon also from other decay sources (,’,…) plot from data feed p0 spectrum into MC, simulate get double gamma ratio R
mT scaling for other decay sources
2023.04.19 J. Jin - JClub 9
Results
Ratio of direct-photon-candidates to the 0 spectrum (data & MC)
Direct cross section small but significant sig
nal observed agree with NLO pQCD
2023.04.19 J. Jin - JClub 10
Other methods I
0 tagging method:
2023.04.19 J. Jin - JClub 11
Other methods II
Isolation cuts photon energy < 10% of cone energy cone size: 0.5 around the candidate photon cone energy: sum of track momentum in DC & E
MCal cluster energy correct for limited PHENIX acceptance, which c
auses fake isolated photons Emphasize the Compton process
direct access to gluon distribution
2023.04.19 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
2023.04.19 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+A
u, measure R directly:
R > 1, indicates direct photon signal Direct photon spectra extracted as:
2023.04.19 J. Jin - JClub 14
Results
double gamma ratio plot advantage: many systema
tics cancel an excess over unity is obs
erved at high pT
magnitude increases with centrality due to hadron suppression
expectation for Ncoll scaling of direct photons holds for all centrality bins
2023.04.19 J. Jin - JClub 15
spectra shown 9 centrality bins and M
B comparison to NLO at low to mid-pT where
additional production of thermal photons appears, we have large error bars
2023.04.19 J. Jin - JClub 16
Improvement on mid-pT thermal photon region
2023.04.19 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
2023.04.19 J. Jin - JClub 18
Going to low pT: No apparent excess
2023.04.19 J. Jin - JClub 19
Excess above the scaled pQCD?
new points agree with, but consistently higher than the old ones ~4GeV/c
2023.04.19 J. Jin - JClub 20
A new approach:very low-mass di-lepton pairs
2023.04.19 J. Jin - JClub 21
The idea: Dalitz decay
mass distribution of e+e- pairs (Knoll-Wada formula):
0
0
e+
e-
1) pure QED part, universal to all Dalitz decays2) phase factor part, depends on M of parent hadron
3) hadronic form factor
at low mass, both the second and third part become 1at high mass, suppressed by phase factor
2023.04.19 J. Jin - JClub 22
Now direct photons:
Any source of real produces virtual with very low mass
Rate and mass distribution given by same formula No phase space factor for mee<< pT
photon
Compton scattering
q
g q
e+
e-
90-3
00
MeV
0-3
0
Rdata
÷
We measure Rdata:N(90<Mee<300)/ N(0<Mee<30)
We can calculate:R0, R, Rdirect
2023.04.19 J. Jin - JClub 23
Rdata excess over calculated expected ratio from hadronic decays: virtual & real direct photon signal
N
Nd irect
a ll
0 30
0 30
True in very low mass window!
2023.04.19 J. Jin - JClub 24
0-20 %
Results: Rdata
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
2023.04.19 J. Jin - JClub 25
*direct/*inclusive
Significant 10% excess of very-low-mass virtual direct photons
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
0-20 %
2023.04.19 J. Jin - JClub 26
Centrality Dependence
Indication for centrality dependence
more peripheral
2023.04.19 J. Jin - JClub 27
Comparison to conventional results
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
( + 1 )
2023.04.19 J. Jin - JClub 28
direct
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
2023.04.19 J. Jin - JClub 29
The Spectrum
Compare to the published Run2 results (black arrow)
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
2023.04.19 J. Jin - JClub 30
The Spectrum
• excess above pQCDCompare to NLO pQCD
2023.04.19 J. Jin - JClub 31
• excess above pQCDCompare to NLO pQCD
Compare to thermal model
• data above thermal at high pT
The Spectrum
2023.04.19 J. Jin - JClub 32
Compare to thermal + pQCD
• data consistent with thermal + pQCD
• excess above pQCDCompare to NLO pQCD
Compare to thermal model
• data above thermal at high pT
The Spectrum
2023.04.19 J. Jin - JClub 33
Backup
2023.04.19 J. Jin - JClub 34
Schematic Photon Spectrum in Au+Au
Decay photons
nT
1
phard:
/ E Tethermal: