1. Status of the "direct" photon reconstruction
Outline
Introduction
Photon and neutral pion reconstruction
Cocktails and loops
(Neutral pion flow)
Summary

2. Direct Photon Puzzle: RHIC & LHC

3. Direct Photon Puzzle in QM15
Chun Shen, QM2015

4. Direct Photon Puzzle for HADES
Reconstruct γdir yield and v2
Examine the role of baryonic resonances
Start with focusing on Delta baryonic resonance:
Δ→Νγ (BR~1%)
Create cocktails with different Δ contribution.
The medium created in HADES AuAu collisions is ideal for this case.

5. Extracting direct photons
Statistical method
Important tool: direct photon excess ratio
𝑅 𝛾 𝑝 𝑡 = 𝐶∙ 𝑁 𝑖𝑛𝑐 𝑁 𝑡𝑎𝑔 𝜋 0 𝑒𝑥𝑝 𝑁 ℎ𝑎𝑑 𝑁 𝜋 0 𝑠𝑖𝑚
Flow:
yield:
𝑅 𝛾 >1 : presence of direct photon signal
γexp
γsim

6. Reconstruction strategy
Hades has no calorimeter (yet)
indirect 𝛾-reconstruction via conversion.
Reconstruct 𝛾→ 𝑒 + + 𝑒 −
Use Target + RICH as converter
Reconstruct 𝑒 + + 𝑒 − with MDC
and ToF/RPC
MDC inner segment sharing

7. Data analysis: Event selection
Generation 8
Full statistics
2.7x109 events remaining for analysis
PT3
MetaPileUp

8. Lepton identification
1) Beta vs Mom all selected leptons
2) Beta vs Mom leptons used for photons (θe+e- ≤ 2°)
3) Beta vs Mom leptons used for π0 (after selection cuts)
(1)
(3)
(2)

9. Inclusive photons: 2.9·107 Still large invariant masses for photons.
Contamination from
charged pions and/or Dalitz decays?
Needs simulation to understand and correct!
Apply additional mass cut (not done is this study)
Me+e- [MeV/c2]

10. 𝜋 0 →𝛾+𝛾→2 ( 𝑒 + + 𝑒 − ) - Result
π0 analysis code from C.Behnke
Cuts:
1) β-mom selection
2) θe+e- ≤ 2°
3) θγ : [10°,40°]
Background fit : polynomial
Signal fit: Gauss
μ = (135.4±0.1) MeV/c2 ,
σ = (4.49±0.13) MeV/c2
About 4000 neutral pions within 2σ

11. Intermediate result: γexp
Pt range [GeV/c]
Nincl
Npi0
γexp
0-1
2.90E+07
7884
3.68E+03
2.16E+07
6354
3.40E+03
3.80E+06
1108
3.43E+03

12. How to recognize a direct photon𝛾
Need to distinguish
direct photons from
hadronic sources.
𝝅 𝟎 𝛾 𝛾
Compare with known hadronic sources (cocktail)
Consider remaining photons as direct
But first, let‘s talk about the N-Δ-π loop….

13. The N-π-∆ loop Considered as important source of 𝛾 Absent in PLUTO
Jan-Hendrick Otto, Bachelor thesis (2015), JLU Giessen
π and N form a ∆
∆ decays in the medium (small propability for photons)
If the ∆ decays in N and π a new ∆ is formed before the π can decay
this loop is repeated a few times (around 10 expected)
Considered as important source of 𝛾
Absent in PLUTO
Present in HSD

14. PLUTO cocktail π0 T1=45 MeV, T2=85 MeV (prelim. HADES data, TAPS data)
Δ (140 MeV) , no loop
ω (110 MeV )
η (90 MeV) Mη ≈ MK
Σ0 (95 MeV) MΣ0 ≈ MΛ
use preliminary temperatures from K and Λ
extract T of ω and ∆ from the T for π and p from preliminary HADES data.
Jan-Hendrick Otto, Bachelor thesis (2015), JLU Giessen

15. Weighting factors – rel. multiplicities
Only relative multiplicities (wrt to π0) are needed
use mt-scaling for the η- and ω-meson
Clebsch-Gordan-Coefficients (CGC) for the ∆
Λ-Σ scaling for the neutral Σ-Baryon
Jan-Hendrick Otto, Bachelor thesis (2015), JLU Giessen
Resulting rel. multiplicities:
Mη/Mπ0 =
Μω/Μπ0 =
ΜΔ/Μπ0 = 3/2
Μ Σ0= 0,0077

16. Cocktails: pt of decay photons
PLUTO
HSD
E.Bratkovskaya,
Private communication
Jan-Hendrick Otto, Bachelor thesis (2015), JLU Giessen
Note the large difference on the yield of photons from the Δ resonance!

17. Cocktails
Jan-Hendrick Otto, Bachelor thesis (2015), JLU Giessen

18. Including photons from loops in the PLUTO data
Jan-Hendrick Otto, Bachelor thesis (2015), JLU Giessen
Observable seems to be sensitive to the number of N-π-Δ loops
HADES data is important to compare and extract experimental Nloops

19. Intermediate result: γsim
Pt [GeV/c]
PLUTO
(0-loop)
HSD
0-1
1.007
1.018
1.017
1.226
1.038
1.646
Significant difference between
PLUTO and HSD in γsim as expected

20. Correction Data Cocktail Hades acceptance Full 4𝜋
Count all reconstructed photons
Divide by photons from pi0
𝑅 𝛾 ( 𝑝 𝑡 )= 𝑪∙ 𝑁 𝑖𝑛𝑐 𝑁 𝑡𝑎𝑔 𝜋 0 𝑒𝑥𝑝 𝑁 ℎ𝑎𝑑 𝑁 𝜋 0 𝑠𝑖𝑚
Cocktail
Full 4𝜋
Count photons from hadronic decays
Divide by photons from 𝝅 𝟎
Efficiencies cancel out.
but…
Need further correction C…

21. Correction Normalized with 𝜋 0 →𝛾+𝛾→ 𝑒 ± + 𝑒 ±
Reconstruction probability (assume 𝑝𝑎𝑖𝑟 not correlated):
𝑃≈ 𝑎 2 ⋅ 𝑐 2 ⋅ 𝑒 2
pair reconstruction efficiency
conversion probability
acceptance probability
Reconstruction probability (assume 𝛾 not correlated):
𝑃≈𝑎⋅𝑐⋅𝑒 => Correction Factor 𝑪≈𝑎⋅𝑐⋅𝑒
𝑅 𝛾 ( 𝑝 𝑡 )= 𝐶∙ 𝑁 𝑖𝑛𝑐 𝑁 𝑡𝑎𝑔 𝜋 0 𝑒𝑥𝑝 𝑁 ℎ𝑎𝑑 𝑁 𝜋 0 𝑠𝑖𝑚
Badly controlled factors
do not cancel.
Try simulation.

22. Correction estimation: 1st iteration
Simulation of 100M π0→γγ.
Generated with PLUTO (T = 80MeV)
Use “mapped” conversion probability for decay
HGeant simulation to define acceptance of leptons
Count fully reconstructed pions and fully reconstructed single photons
Input from C.Behnke
Pt [GeV/c]
“singles”
Nπ0
correction
0-1
1.86E-03
6.88E-06
3.69E-03
7.73E-04
3.46E-06
4.46E-03
9.20E-05
3.80E-07
4.11E-03
Benefit: fast, easy, input ready
Drawback: no pair reconstruction efficiency, rough approximation of conversion prob.

23. Rγ Results Too large Rγ! Result is implausible... Pt [GeV/c] PLUTO
𝑅 𝛾 ( 𝑝 𝑡 )= 𝐶∙ 𝑁 𝑖𝑛𝑐 𝑁 𝑡𝑎𝑔 𝜋 0 𝑒𝑥𝑝 𝑁 ℎ𝑎𝑑 𝑁 𝜋 0 𝑠𝑖𝑚
Pt [GeV/c]
PLUTO
(0-loop)
HSD
0-1
13.5
13.3
14.9
12.4
13.6
8.6
Too large Rγ!
Result is implausible...

24. (...

25. π0 yield in 18 Δφ bins Pt range: 0-1 GeV/c Bin : Δ(φ-Ψ)= 20 degrees
Note: fixed mean and sigma parameter for Gauss fit, taken from inclusive analysis
μ = (135.4±0.1) MeV/c2 ,
σ = (4.49±0.13) MeV/c2

26. π0 v2 coefficient Fit function:
par[0]*(1+2*par[1]*Cos(ϕ) + 2*Cos(2ϕ) )
Pt: 0-1 GeV/c
Y-Ycm window: [-0.5,0.5]
Centrality: exclude 15% most central
Event plane resolution: 0.54
Result:
v2=
Φ [degrees]

27. Comparison to existing data: TAPS
L. B. Venema et al. Phys. Rev. Lett. 71, 835 – Published 9 August 1993
“Azimuthal Asymmetry of Neutral Pion Emission in Au+Au Reactions at 1GeV/Nucleon”
rms pt
<pt>
S2_corr,
Y window
err S2
136
533
-0.309
0.066
The HADES data seem to be in good comparison with TAPS

28. ...)

29. Conclusion and outlook
Search for direct photons is being carried out.
Improved cocktail simulations available to estimate background.
Computation of 𝑅 𝛾 provides so far implausible results.
Next steps:
Correction factor – try to control acceptance, conversion and reconstruction efficiencies and correlations: better precision of simulation
Include missing contributions from other hadrons
Eliminate contamination in single photon spectra
Input is welcome!
… to be continued.