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

Direct Photon Puzzle: RHIC & LHC

Direct Photon Puzzle in QM15 Chun Shen, QM2015

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.

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

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

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

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)

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]

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

Intermediate result: γexp Pt range [GeV/c] Nincl Npi0 γexp 0-1 2.90E+07 7884 3.68E+03 0.2-0.8 2.16E+07 6354 3.40E+03 0.4-0.8 3.80E+06 1108 3.43E+03

How to recognize a direct photon 𝛾 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….

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

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

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 = 0.01353 Μω/Μπ0 = 0.00084 ΜΔ/Μπ0 = 3/2 Μ Σ0= 0,0077

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!

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

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

Intermediate result: γsim Pt [GeV/c] PLUTO (0-loop) HSD 0-1 1.007 1.018 0.2-0.8 1.017 1.226 0.4-0.8 1.038 1.646 Significant difference between PLUTO and HSD in γsim as expected

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…

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.

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 0.2-0.8 7.73E-04 3.46E-06 4.46E-03 0.4-0.8 9.20E-05 3.80E-07 4.11E-03 Benefit: fast, easy, input ready Drawback: no pair reconstruction efficiency, rough approximation of conversion prob.

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 0.2-0.8 14.9 12.4 0.4-0.8 13.6 8.6 Too large Rγ! Result is implausible...

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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

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= -0.31+-0.06 Φ [degrees]

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

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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.