Neutral meson and direct photon measurement in pp and Pb Pb collisions at midrapidity with the ALICE experiment at the LHC

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1 Neutral meson and direct photon measurement in pp and Pb Pb collisions at midrapidity with the ALICE experiment at the LHC Lucia Leardini PI Heidelberg on behalf of the ALICE Collaboration 54th International Winter Meeting on Nuclear Physics Bormio, 5-9 January 06 HGS-HIRe Helmholtz Graduate School for Hadron and Ion Research 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment

2 Motivations Neutral mesons as probes of the Quark Gluon Plasma (QGP) allow in pp collisions, the study of particle production and constraint of fragmentation functions in Pb Pb collisions, insights on the bulk properties of the medium, collective effects and particle energy loss in both cases, the comparison to models is used to test predictions and improve the theoretical description π 0 and η are input for background estimates to direct photons Direct photons are photons not coming from particle decays do not interact with the medium, thus carry unmodified information about early stages of the collisions hard to measure above a large electromagnetic decay background 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment

3 he ALICE experiment Central barrel: η < 0.9 V0: multiplicity estimation IS: vertex finding and tracking PC: tracking and particle identification PHOS and EMCal: calorimetry 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 3

4 Photon detection with the ALICE experiment (RUN ) PHOS calorimeter: - PbWO 4 crystals - 3 modules at 4.6 m from IP - η < 0.3, 60 < φ < 30 EMCal calorimeter: - 77 sampling layers,.4 mm Pb and.7 mm scintillator - modules at 4.4 m from IP - η < 0.7, 80 < φ < 80 Photon Conversion Method (PCM): - IS and PC - η < 0.9, 0 < φ < conversion in detector material: conv. probability 8% X /X 0 = (.4±0.5)%( η <0.9, R<80 cm) Photon candidates are extracted from V 0 s (neutral secondary vertex particles) sample 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 4

5 Invariant mass reconstruction Photon candidates are combined into pairs invariant mass calculated in p bins: M γγ = E γ E γ ( cos θ ) pp Events / MeV/c PCM π 0 : 0.6<p <0.8 GeV/c Events / 8 MeV/c η:.4<p <.8 GeV/c m γγ (GeV/c ) Events / MeV/c pp, EMCAL s=7 ev 5.0<p <7.0 GeV/c Events / 4 MeV/c PHOS π 0 :.0<p <. GeV/c Events / MeV/c η: 4.0<p <6.0 GeV/c m γγ (GeV/c ) 0. Pb-Pb 0 ALI PUB 87 Events / 5 MeV/c PCM η γγ Pb Pb, s NN =.76 ev 0 % γγ < p < 3 GeV/c (GeV/c ) m γγ (GeV/c ) ALI PERF ALI PUB 875 PCM EMCal PHOS ALICE performance Signal (GeV/c ) M γγ 40 Events / MeV/c EMCal η γγ Pb Pb, s NN =.76 ev 0 % γγ < p < 4 GeV/c ALICE performance (GeV/c ) M γγ 0 m γγ (GeV/c ) Histo: signal + background Points: signal after background subtraction Line: fit (for mass and width) m m 3%(PCM) 5%(PHOS) 7%(EMCal) at p = 3 GeV/c in pp collisions 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 5

6 π 0 and η in pp collisions 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 6

7 Neutral mesons invariant cross section (PCM + PHOS) ) 3 c (pb GeV pp π 0 X σ 3 d E 3 ) dp Data / Fit Data / Fit Data / Fit Data / Fit 4 ALICE Preliminary ALI PREL ALICE pp ALICE pp ALICE pp ALICE pp PYHIA8.85 4C PYHIA8.85 Monash NLO pqcd + MSW + DSS4 (µ NLO pqcd + MSW + DSS4 (µ sallis fit s = 8 ev s = 7 ev (PLB 77 (0)) s =.76 ev (EPJ74 38 (04)) 3 s = 0.9 ev (PLB 77 (0)) = p /) = p ) π 0 compared to PYHIA8 and NLO pqcd (PRD 9 (05) 04035) models give good description at intermediate p but a larger discrepancy towards higher p η compared to NLO pqcd by W. Vogelsang (PDF: CEQ6M5, FF: AES) reference at s =.76 ev is used to calculate nuclear modification factor (R AA ) in Pb-Pb at the same center of mass energy c 3 (pb GeV 3 σ dp d E NLO fit NLO fit NLO fit x σ pp PCM: PHOS: x combined Spec. fit combined NLO µ= 0.5 p NLO µ= p NLO µ= p η γ η γ x uncertainty 3 η, s = 7 ev e + e 7 ev ± ev ± 7% 0.9 ev + 5.0%, 3.9% 3 η, s =.76 ev 3 η, s = 0.9 ev ALI PREL 6590 PLB 77 (0) 6 γ γ η, s = 7 ev (*) syst, stat e + e η, s =.76 ev syst, stat ALICE Preliminary pp, s = 0.9 &.76 & 7 ev η, s = 0.9 ev syst, stat (*) arxiv: th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 7

8 π 0 and η in Pb Pb collisions 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 8

9 Quark-Gluon Plasma (QGP) in A A collisions QGP forms in A A collisions: hot and dense medium, strongly interacting collective expansion deconfined quarks and gluons, interact with medium parton interactions result in energy loss (jet quenching): can be highlighted through the comparison with pp collisions (vacuum scenario) 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 9

10 π 0 from 0 data (PCM + PHOS) (GeV/c) N d dy dp p N ev π 4 3 ALICE, EPJC 74 (04) 38 pp s =.76 ev sallis fit power law fit pp reference measurement at.76 ev is used to calculate the nuclear modification factor d N/dp dy AA R AA (p ) = AA d σ/dp dy pp suppression due to interaction with medium is larger for more central collisions ALI PUB % Pb Pb 5 % Pb Pb 0% Pb Pb 0 40% Pb Pb 40 60% Pb Pb 60 80% Pb Pb fits to Pb Pb 7 s NN =.76 ev 5 s NN =.76 ev 3 s NN =.76 ev s NN =.76 ev s NN =.76 ev 0 s NN =.76 ev p (GeV/c) not enough statistics for η measurement in 0 data R AA Pb Pb s NN =.76 ev 0 5% π % π % π ALI PUB th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment

11 Neutral pion R AA collision energy dependence ALICE π 0 R AA in 0-% central collisions compared with results from PHENIX (PRL 9 (0) 530 and PRL (008) 330) and SPS results (PRL 0 (008) 430) R AA supression stronger for higher collisions energy: decrease due to higher energy density dominates over increase expected from harder initial parton spectra maximum value of the ratio also shifts towards lower p going to higher energy at high p, R AA is expected to rise due to a flatter spectra in A A than in pp R AA.0 π 0 ALICE 0 % Pb Pb =.76 ev s NN π 0 PHENIX 0 % Au Au s = 00 GeV s NN = 6.4 GeV NN = 39 GeV s NN π 0 WA98 0 3% Pb Pb = 7.3 GeV (GeV/c) ALI PUB 8798 EPJC 74 (04) 38 s NN p 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment

12 Neutral mesons from 0 data (PCM + EMCal) increased luminosity Pb Pb run in 0, times more statistics combined PCM and EMCal measurement N π 0 d (GeV/c) dy dp p N ev π 3 3 x 4 ALICE Preliminary π 0 γγ Pb Pb, s NN =.76 ev 0 % 0 50% π 0 N η d (GeV/c) dy dp p N ev π 3 x 4 ALICE Preliminary η γγ Pb Pb, s NN =.76 ev 0 % 0 50% η pp, s =.76 ev EPJC 74 (04) 38 pqcd NLO µ = p PDF: MSW, FF: DSS4 PRD scaled by <N > coll ALI PREL pp, s =.76 ev JPG 38 (0) 4076 pqcd NLO µ = p PDF: MSW, FF: DSS07 PRD scaled by <N > coll ALI PREL π 0 consistent with 0 data, measurement extended to 0 GeV/c first η measurement in Pb Pb at the LHC 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment

13 η/π 0 ratio in Pb Pb 0 data (PCM + EMCal) η/π 0 measured in Pb Pb collisions in two centrality classes Particle ratio Pb Pb, s NN =.76 ev η /π 0, 0 % K ± /π ±, 0 % η/π 0, 0 50% PLB 736 (04) 96 compared to other results from ALICE: η/π 0 shows behaviour similar to K ± /π ± result at same energy and centrality (0-%) with current uncertainties, no differences are observed between η/π 0 ratio measured in Pb Pb and pp collisions ALICE Preliminary ALI PREL η/π Pb Pb, s NN =.76 ev 0 % 0 50% pp, p s = 7 ev PLB 77 (0) 6 (GeV/c) ALI PREL ALICE Preliminary 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 3 p (GeV/c)

14 η/π 0 ratio in Pb Pb 0 data (PCM + EMCal) η/π 0 measured in Pb Pb collisions in two centrality classes Particle ratio Pb Pb, s NN =.76 ev η /π 0, 0 % K ± /π ±, 0 % η/π 0, 0 50% PLB 736 (04) 96 compared to other results from ALICE: η/π 0 shows behaviour similar to K ± /π ± result at same energy and centrality (0-%) with current uncertainties, no differences are observed between η/π 0 ratio measured in Pb Pb and pp collisions pqcd NLO calculation for 0-% cent. class agrees within uncertainties p region 4 6 GeV/c sensitive to transport coefficient ( = parameter describing energy loss in medium) ALICE Preliminary ALI PREL η/π Pb Pb, s NN 0 % 0 50% =.76 ev NLO DCZW (τ 0 0 % p = 0.6 fm) p (GeV/c) ALI PREL arxiv: ALICE Preliminary (GeV/c) 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 3

15 Direct photons in Pb Pb collisions 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 4

16 γ Direct photons in Pb Pb collisions direct photons = photons not coming from particle decays emitted during all stages of the system evolution, can be classified in thermal photons: dominant at low p, coming from thermal radiation of QGP and hadron gas prompt photons: dominant at high p, from initial hard scattering from jet-medium interaction: hard partonic scattering, in-medium bremsstrahlung extraction of direct photon measurement: γ direct = γ inc γ decay = ( R γ ) γ inc with R γ = γ inc π 0 / γ decay π 0 param dn/d p dy (GeV - c 3 ) A ev Pb+Pb@LHC arxiv: hermal (FIC, σ=0.4 fm) prompt photons Compton+annihilation Fragmentation p (GeV/c) arxiv: ALI PUB th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 5 /γ source decay π γγ (e + e γ) η ργ (ωγ, γγ) η γγ (π + π γ,e + e γ,π 0 γγ) φ ηγ (π 0 γ, ωγ) ω π 0 γ (ηγ) ρ 0 π + π γ (π 0 γ, ηγ ) 0 0% Pb Pb s NN =.76 ev Monte Carlo simulation

17 Direct photons in 0 Pb Pb data (PCM + PHOS) inclusive photon spectra measured with combined PCM + PHOS (arxiv: ) in 3 centrality classes with 0 Pb Pb data R γ excess at high p for all centralities in agreement with NLO/JEPHOX pqcd above 5 GeV/c at low p, 0% excess in 0-0% and 9% in 0-40% due to thermal radiation of the medium R γ arxiv: % Pb Pb s NN =.76 ev ALICE NLO pqcd PDF: CEQ6M5 FF: GRV JEPHOX PDF: C, FF: BFG JEPHOX npdf: EPS09, FF: BFG (all scaled by N coll ) 0 40% Pb Pb s NN =.76 ev ALICE NLO pqcd PDF: CEQ6M5 FF: GRV JEPHOX PDF: C, FF: BFG JEPHOX npdf: EPS09, FF: BFG (all scaled by N coll ) 40 80% Pb Pb s NN =.76 ev ALICE NLO pqcd PDF: CEQ6M5 FF: GRV JEPHOX PDF: C, FF: BFG JEPHOX npdf: EPS09, FF: BFG (all scaled by N coll ) ALI PUB th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 6

18 Direct photons in 0 Pb Pb data (PCM + PHOS) direct photon spectra measured in 3 centrality classes in the range 0.9< p <4 GeV/c at low p, upper limits with 90% CL given for more peripheral collisions comparison with pqcd NLO and JEPHOX shows again good agreement above 5 GeV/c and excess yields for 0-0% and 0-40% central collisions c ) (GeV N γ dir d dy dp p N ev. π ALI PUB arxiv: Pb Pb s NN =.76 ev 0 0% ALICE PDF: CEQ6M5, FF: GRV 0 40% ALICE (n)pdf: CEQ6.M/EPS09, 40 80% ALICE FF: BFG JEPHOX PDF: C, FF: BFG npdf: EPS09, FF: BFG (all scaled by N coll ) x x 0 x 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 7

19 Comparison direct photon spectra PHENIX - ALICE ALICE results compared with PHENIX direct photon measurement in Au Au at 00 GeV (PRL4 (0)330, PRC9/6 (05) ) c ) (GeV N γ dir d dy dp p π N ev. ALICE 0 0% Pb Pb s NN A exp( p / eff ) stat eff = 304 ± PHENIX 0 0% Au Au s NN A exp( p / eff ) stat eff = 39 ± 5 =.76 ev ± sys 40 MeV = 0. ev ± 7 sys MeV - c ) (GeV N γ dir d dy dp p π N ev. ALICE 0-40% Pb-Pb A exp(-p / eff ) stat eff = 407 ± 6 PHENIX 0-40% Au-Au A exp(-p / eff ) eff = 60 ± 33 stat s NN ± s NN ± =.76 ev sys 96 MeV = 0. ev sys 8 MeV ALI PUB ALI-PUB exponential fit to low p excess: inverse slope parameter larger at higher collision energy and consistent in both centrality classes 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 8

20 Direct photon spectra comparison to models Several models, all assume QGP formation and include pqcd photons at high p have different space-time evolution treatment: Paquet et al.: + viscous hydro with IP-GLASMA initial conditions, τ 0 = 0.4 fm/c, 0 0% init = 385 MeV Linnyk et al.: off-shell transport, microscopic description of evolution v. Hees et al.: ideal hydro with initial flow, τ 0 = 0. fm/c, = 68 MeV 0 0% init Chatterjee et al.: + hydro, fluctuating initial conditions, τ 0 = 0.4 fm/c, 0 0% init 740 MeV c ) (GeV N γ dir d dy dp p N ev. π arxiv: Pb Pb s NN =.76 ev A exp( p / eff ) 0 0% ALICE 0 0% 0 40% ALICE 0 40% 40 80% ALICE x x 3 Paquet et al. 0 x arxiv: Linnyk et al. 4 arxiv: Chatterjee et al. v. Hees et al. PRC 85(0) 0649 NPA 933(05) 56 + JHEP 305(03) ALI PUB th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 9

21 Summary Neutral mesons and direct photons are measured in ALICE with independent methods (calorimeters, EMCal and PHOS, and photon conversions, PCM) results in pp neutral pion and η meson cross sections measured at several collision energies with combined PCM and PHOS analysis comparison with PYHIA and NLO pqcd calculations: - describe well intermediate p region (below 5 GeV/c) - predict harder spectra at high p 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 0

22 Summary results in Pb Pb at.76 ev neutral mesons measurements: - π 0 measured with PCM + PHOS (0 data) and PCM + EMCal (0 data) and η measured with PCM + EMCal (0 data) - π 0 R AA has larger suppression in more central collisions and its magnitude scales with the collision energy - with current uncertainties, no clear dependence of η/π 0 on collision system, mass or s quark content observed direct photon measurement: - inclusive and direct photon spectra measured in 3 centrality bins - below p = 3 GeV/c, direct photon excess observed for 0-0% and 0-40% thermal radiation of the medium - photon spectrum above 5 GeV/c in agreement with NLO pqcd 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment

23 Back up 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment

24 Luminosity 0 vs 0 with large statistics collected in 0 measurement of differential invariant cross section is possible 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 3

25 Neutral pion peak position and width ALICE performance paper: Int. J. Mod. Phys. A 9 (04) pp Pb Pb ) peak width (MeV/c PCM (FWHM/.36) PHOS (σ) EMCal (σ) data MC pp s = 7 ev ) peak width (MeV/c PCM (FWHM/.36) PHOS (σ) EMCal (σ) data MC Pb Pb s NN =.76 ev centrality 0 % 5 5 ) peak position (MeV/c a) PCM PHOS EMCal data MC ) peak position (MeV/c a) PCM PHOS EMCal data MC 30 b) 5 ALI PUB b) 5 ALI PUB th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 4

26 ) Neutral pion spectra in pp collisions (PCM + PHOS) With additional constraint given by 8 ev results, large improvement of NLO pqcd calculations c 3 (pb GeV d 3 σ d p 3 E NLO fit NLO fit NLO fit x x x σ pp combined Spec. fit combined NLO µ= 0.5 p NLO µ= p NLO µ= p NLO µ= p uncertainty 7 ev ± 3.5%.76 ev ± 7% 0.9 ev + 5.0%, 3.9% (BKK) 3 π 0, s = 7 ev 3 π 0, s =.76 ev 3 π 0, s = 0.9 ev ALI DER 8357 π 0, s = 7 ev (*) syst., stat. ALICE π 0, pp π 0, s =.76 ev (**) syst., stat. (*) PLB 77 (0) 6 7 (**) arxiv: s = 0.9,.76, 7 ev π 0, s = 0.9 ev (*) syst., stat. ) 3 c (pb GeV pp π 0 X σ 3 d E dp Data / Fit Data / Fit Data / Fit Data / Fit 4 ALICE Preliminary ALI PREL ALICE pp ALICE pp ALICE pp ALICE pp PYHIA8.85 4C PYHIA8.85 Monash NLO pqcd + MSW + DSS4 (µ NLO pqcd + MSW + DSS4 (µ sallis fit s = 8 ev s = 7 ev (PLB 77 (0)) s =.76 ev (EPJ74 38 (04)) 3 s = 0.9 ev (PLB 77 (0)) 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 5 p = p /) = p ) (GeV/c)

27 0 0 η/π 0 ratio in s =.76 and 7 ev (PCM + PHOS) η/π PLB 77 (0) ALICE, s = 7 ev 0. NLO µ = 0.5 p NLO µ = p NLO µ = p ALI PUB 6666 η/π π 0 and η γ γ e + e e + e ALICE Preliminary pp, s =.76 ev η/π 0 ratio compared with pqcd NLO theory by W. Vogelsang η, PDF: CEQ6M5, FF: AES π 0, PDF: CEQ6M5, FF: DSS at both 7 and.76 ev an increasing trend can be observed up to GeV/c above GeV/c the ratio flattens, as the NLO calculations suggest 0. ALICE (PCM), NLO µ = 0.5 p NLO µ = p NLO µ = p s =.76 ev ALI PREL th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 6

28 DCZW prediction NLO pqcd theoretical prediction of η/π 0 ratio in Pb Pb collisions at.76 ev according to DCZW (PLB 750 (05) ) τ 0 is the initial time of the QGP medium ˆq 0 initial values of the jet transport parameter the larger ˆq 0 is, the stronger the jet-medium interaction will be 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 7

29 0 0 Assumptions for decay photon cocktail η/π ALICE, pp s = 7 ev Phys.Lett. B77 (0) 6 7 % of decay photons ω/π.4. ALICE preliminary, pp s = 7 ev -3% of decay photons Cocktail Calculations 0. η from pp s = 7 ev as input η from m scaled π % Pb Pb s NN =.76 ev, η from K scaled s ALI DER Cocktail Calculations ω from m scaled π ALI PREL 6834 η & ω meson only measured in pp, ϕ meson measured in pp & 0-% Pb Pb collisions m scaling overestimates yield at low p consistently for all 3 mesons Systematic uncertainties on cocktail 5-% φ/π 0.5 φ/(0.5 (π + π + )) ALICE preliminary pp s = 7 ev φ/(0.5 (π + π + )) ALICE, % Pb Pb arxiv: s NN =.76 ev Cocktail Calculations % of decay photons 0 φ/π, φ from m scaled π ALI PREL th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 8

30 Direct Photon Flow ν direct γ n = Rγ νinc n γ νn decay γ R γ R γ νn inc γ : weighted inclusive photon ν n due to extra photons compared to background νn decay γ : calculated decay photon ν n from cocktail calculation 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 9

31 Inclusive photon ν analysis method Initial azimuthal asymmetry in coordinate space in non-central A+A asymmetry in momentum space ( + s ) NN n ν n cos(n(φ Ψ RP n )) dn dφ = π ν given by photon production with respect to event plane: ν = cos((φ Ψ RP )) event plane angle determined by using the VZERO detector: - VZEROA:.8 < η < 5. - VZEROC: 3.7 < η <.7 reaction plane resolution obtained by the three sub-event method resolution correction for EP: ν = ν EP cos(ψ EP ΨRP ) = ν raw resolution φ ALI-PERF-4366 Resolution φ V0A 3-subevent V0C 3-subevent φ JPCS 446 (03) 008 Pb-Pb, s NN =.76 ev 7/07/ ALI PERF 3433 Centrality (%) 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 30

32 Cocktail simulation of decay photon ν Decay photon ν : ν of mesons scales with KE : KE = m m = p + m m ν π0 ν π± (m π0 m π± ) ν of various mesons (X) calculated via KE (quark number) scaling from ν π± ) ( (KE X + mπ± ) (m π± ) ν X (p X ) = ν π± v Pb-Pb s NN =.76 ev -0% v {SP, η >} π K 0 Ks v {EP, η >} JPCS 446 (03) 008 π Phys. Lett. B 79, 8 Decay photon ν from different mesons obtained from cocktail calculation ALI-PERF th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 3

33 γ Comparison of inclusive and decay ν JPCS 446 (03) 008 Above 3 GeV/c inclusive photons significantly smaller than decay photons Direct photon ν contribution with ν direct < ν decay Below 3 GeV/c consistent within uncertainties Either contribution of direct photons with similar ν or no direct photons v 0.0 ALICE preliminary γ,decay v 0 40% Pb Pb, s NN =.76 ev γ,incl 0.8 v V0 event plane ALI PREL th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 3

34 Inclusive photon ν 3 First measurement of inclusive photon ν 3 at LHC Above 3 GeV/c inclusive photons consistently smaller than decay photons, with large statistical uncertainties Direct photon ν 3 contribution with ν3 direct < ν decay 3 as expected for prompt photons v 3 γ ALICE preliminary 0 40% Pb Pb, s NN =.76 ev V0 event plane γ,decay v 3 γ,incl v 3 Below 3 GeV/c mostly consistent within uncertainties Either contribution of direct photons with similar ν 3 or no direct photons ALI PREL th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 33

35 Comparison with RHIC - PHENIX Direct Photon ν QM 0, E. Kistenev Direct photon ν has been measured at RHIC with the Phenix experiment at snn = 00 GeV Direct photon ν comparable to that of other hadrons heory not able to reproduce large ν at low p Origin of the photon puzzle 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 34

36 Direct Photon ν 0-40% Direct photon ν : ν direct γ = Rγ νinc γ ν decay γ R γ Large direct photon ν for p < 3 GeV/c measured R γ ν inc γ : weighted inclusive photon ν due to extra photons compared to background ν decay γ : calculated decay photon ν from cocktail calculation J.Phys.Conf.Ser. 446 (03) 008 Magnitude of ν comparable to hadrons Result points to late production times of direct photons after flow is established ALI-PREL th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 35

37 Direct photon yield and flow Central points for direct photon yield and ν underestimated by most theoretical calculations by factors of - No significant deviation beyond σ c ) (GeV dn p dp dy N ev π 3 Nucl.Phys. A (03) 573c-576c 0 40% Pb Pb, s NN =.76 ev ALICE preliminary Direct Photon NLO Vogelsang et al. for µ = 0.5 to p (scaled pp) Phys.Rev. D50 (994) extrapolation of NLO to low p using a c π (+x/b) γ,dir v JPCS 446 (03) 008 ALICE preliminary 0 40% Pb Pb, s NN =.76 ev V0 event plane hermal Shen et al. arxiv:308. hermal Holopainen et al. Phys.Rev. C84 (0) ALI PREL ALICE preliminary thermal(shen et al.) NLO (Vogelsang) + thermal(shen et al.) thermal(holopainen et al.) NLO (Vogelsang) + thermal(holopainen et al.) ALI PREL 7595 Both measurements are coupled via R γ, critical assessment of uncertainties and their correlations needed heory curves composed out of different sources, experimentally not possible to distinguish those 54th International Winter Meeting on Nuclear Physics Neutral mesons and direct photons with the ALICE experiment 36