Measurements of direct photons in Au + Au collisions with PHENIX - - PowerPoint PPT Presentation
Measurements of direct photons in Au + Au collisions with PHENIX Hard Probes 2013 Benjamin Bannier for the PHENIX collaboration Stony Brook University November 5, 2013 1 / 19 Outline Low momentum direct photons: 0 . 4 GeV / c < p T < 5 .
Hard Probes 2013 Benjamin Bannier for the PHENIX collaboration
Stony Brook University
November 5, 2013
1 / 19
Low momentum direct photons: 0.4 GeV/c < pT < 5.0 GeV/c How are low momentum real photons measured in PHENIX? Spectra and centrality dependence of the low momentum real photons from RHIC
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◮ long mean free path, escape heavy ion collision with almost
no final state interaction
◮ produced at all stages of the collision in scatterings of
constituents of each other or the medium
◮ probe complete temperature and flow evolution of the collision ◮ experimentally characterized by momentum-dependent yields
and angular correlations with event planes
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(a) PRL 104, 132301 (2010)
2 4 6 8 10 12
0.05 0.1 0.15 0.2 0.25
(b)
)
BBC 2Φ (
2v
dir.γ 2 4 6 8 10 12
0.05 0.1 0.15 0.2 0.25
(a)
)
BBC 2Φ (
2v π )
BBC 2Φ (
2v
inc.γ 2 4 6 8 10 12
0.05 0.1 0.15 0.2 0.25
(d)
0.6 fm/c 0.4 fm/c 2 4 6 8 10 12
0.05 0.1 0.15 0.2 0.25
(c)
0~20 [%] 2 4 6 8 10 12
0.05 0.1 0.15 0.2 0.25
(f)
2 4 6 8 10 12
0.05 0.1 0.15 0.2 0.25
(e)
20~40 [%]
2
v
dir.
γ ,
inc.
γ , π [GeV/c]
T
p
(b) PRL 109, 122302 (2012)
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Rγ = Y γ
incl.
Y γ
hadron
= εf Nγ
incl.
Nγ
π0
Y γ
hadron
Y γ
π0
Requirements:
◮ clean photon sample ◮ high π0-tagging efficiency εf
Photon sample
◮ measurement of low momentum photons in electromagnetic
calorimeters is difficult due to e.g. MIPs
◮ PHENIX has good electron reconstruction capability down to
pT = 200 MeV/c
◮ reconstruct real photons down to 400 MeV/c from e+e− pairs
→ no hadron contamination
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[GeV]
cgl
M
0.005 0.01 0.015 0.02 0.025 0.03
[GeV]
atm
M
0.005 0.01 0.015 0.02 0.025 0.03
100 200 300 400 500 600
Pairs from Data
+
FG e
◮ momentum can be reconstructed assuming production at the
nominal event vertex or a defined radius
◮ conversion pairs can be selected through their invariant mass
under hypotheses for production radius Nγ
incl.pconvae+e−εe+e−
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[GeV]
cgl
M
0.005 0.01 0.015 0.02 0.025 0.03
[GeV]
atm
M
0.005 0.01 0.015 0.02 0.025 0.03
100 200 300 400 500 600
Pairs from Data
+
FG e
◮ momentum can be reconstructed assuming production at the
nominal event vertex or a defined radius
◮ conversion pairs can be selected through their invariant mass
under hypotheses for production radius Nγ
incl.pconvae+e−εe+e−
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π0-decay photon tagging
◮ a second photon measured with very loose cuts in the
calorimeter is paired with converted photons
◮ the combinatorial background is modelled with a mixed-event
sample of uncorrelated converted and calorimeter photons Nγ
π0 = Y γ π0pconvae+e−εe+e− × εf
(a) pT,γ = 0.8 − 1.0 GeV/c (b) pT,γ = 2.0 − 2.5 GeV/c
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Tagging efficiency correction εf
,
◮ 2nd photon in acceptance → ε ◮ 2nd photon lost → f
The tagging efficiency εf is calculated in a Monte Carlo simulation.
◮ f can be calculated accurately, ε ≈ 90%
Nγ
incl.
Nγ
π0
= Y γ
incl.pconvae+e−εe+e−
Y γ
π0pconvae+e−εe+e− × εf =
Y γ
incl.
Y γ
π0εf
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Rγ in Au + Au at √sNN = 200 GeV
1.0 1.2 1.4 1.6
Rγ
0-20% (a) 20-40% (b) 1 2 3 4 1.0 1.2 1.4 1.6 40-60% (c) 1 2 3 4
pT [GeV/c]
60-92% (d)
Au+Au
√sNN = 200GeV
PRL 104, 132301 2007 2010 PH ENIX
preliminary
Figure: Rγ from virtual and real photons (red, blue) in 0-20%, 20-40%, 40-60% and 60-92% more central collisions.
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Direct photon pT spectrum
10−6 10−5 10−4 10−3 10−2 10−1 100 101 1 2π pT d2N dpTdy [(GeV/c)−2] 0-20% (a) 20-40%
Au+Au
√sNN = 200GeV (b) 1 2 3 4 10−6 10−5 10−4 10−3 10−2 10−1 100 101 40-60% (c) 1 2 3 4 5
pT [GeV/c]
60-92% (d)
TAA-scaled pp fit 2007, 2010 PRL 104, 132301 PH ENIX
preliminary
Figure: Direct photon pT spectra Yγ = (Rγ − 1)Y hadron
γ
in 0-20%, 20-40%, 40-60% and 60-92% more central collisions. A Ncoll-scaled fit a
T/b
−c to RHIC pp data is shown in green.
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Excess photon pT spectrum
10−6 10−5 10−4 10−3 10−2 10−1 100 101 1 2π pT d2N dpTdy [(GeV/c)−2] 0-20% (a)
Teff = (237±25±29)MeV/c
20-40% (b)
Au+Au
√sNN = 200GeV
Teff = (260±33±31)MeV/c
1 2 3 4 10−6 10−5 10−4 10−3 10−2 10−1 100 101 40-60% (c)
Teff = (228±28±27)MeV/c
1 2 3 4 5
pT [GeV/c]
60-92% (d)
Teff = (254±53±25)MeV/c
Ae−pT/Teff data - scaled p+ p PH ENIX
preliminary
Figure: Excess photon pT spectra after subtraction of hard-scattering component in 0-20%, 20-40%, 40-60% and 60-92% more central
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Centrality dependence of excess photon yield
101 102
Npart
10−3 10−2 10−1 100 101 dN dy
pT > 0.4GeV/c pT > 0.6GeV/c pT > 0.8GeV/c pT > 1.0GeV/c pT > 1.2GeV/c pT > 1.4GeV/c PH ENIX
preliminary 101 102
Npart
10−5 10−4 10−3 dN dy /N1.48 part
Au+Au
√sNN = 200GeV
PH ENIX
preliminary
Figure: Left: Integrated excess photon yield as a function of Glauber
part with
x = 1.48 ± 0.08(stat) ± 0.04(sys).
dN dy (pT) =
5 GeV/c
T =pT
2πp(i)
T ∆p(i) T
2πpT d2N dpTdy
T 13 / 19
We have measured Rγ and pT spectra for real photons. Real and virtual photons show similar Rγ. An excess yield of photons is seen across all centralities. No change in the shape of the photon pT spectra is seen between centralities outside uncertainties. The excess photon yield grows stronger than Npart in the pT window 0.6 − 2.0GeV/c and is described by a power law with x = 1.48 ± 0.08(stat) ± 0.04(sys).
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Characterization of excess photon pT spectra
Excess photon spectra are roughly exponential in low pT range. The shape of the spectra doesn’t change outside uncertainties across centralities.
0-20% 20-40% 40-60% 60-92% [Teff] 237 ± 25 ± 29 260 ± 33 ± 31 228 ± 28 ± 27 254 ± 53 ± 25 MeV/c
Integrated yields
To quantify the centrality-dependence of the yield we can calculate dN dy (pT) =
5 GeV/c
T =pT
2πp(i)
T ∆p(i) T
2πpT d2N dpTdy
T 16 / 19
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
pT [GeV/c]
−4 −2 2 4
(spectrum - fit)/fit
0-20% 20-40% 40-60% 60-92% PH ENIX
preliminary 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
pT [GeV/c]
−1.5 −1.0 −0.5 0.0 0.5 1.0 1.5
(spectrum - fit)/fit
0-20% 20-40% 40-60% 60-92% PH ENIX
preliminary
Figure: Normalized fit residuals for fits of the excess photon spectra to an exponential in pT = 0.6 . . . 2.0 GeV/c (left) and zoomed (right).
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To calculate Rγthe efficiency-corrected ratio needs to be scaled by the expected ratio of photons yields from hadron and π0 decays
Y γ
hadron/Y γ π0.
We implement a cocktail including
◮ π0 → γγ ◮ η → γγ, π+π−γ ◮ η′ → γγ, π+π−γ, ωγ ◮ ω → π0γ
using experimental π pT spectra and mT scaling for other mesons with experimental meson/π0 ratios.
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1.0 1.2 1.4 1.6
Rγ
0-20% (a) 20-40% (b) 1 2 3 4 1.0 1.2 1.4 1.6 40-60% (c) 1 2 3 4
pT [GeV/c]
60-92% (d)
Au+Au
√sNN = 200GeV
PRL 104, 132301 2007+2010 PH ENIX
preliminary
Figure: Rγ from virtual and real photons in 0-20%, 20-40%, 40-60% and 60-92% more central collisions.
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