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

Transcription

1 Neutral meson and direct photon measurement in pp and Pb Pb collisions at midrapidity with the ALICE experiment at the LHC Lucia Leardini Physikalisches Institut Heidelberg on behalf of the ALICE Collaboration International Nuclear Physics Conference Adelaide, Sept. -6, 06 HGS-HIRe Helmholtz Graduate School for Hadron and Ion Research INPC 06 Neutral mesons and direct photons with the ALICE experiment

2 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) INPC 06 Neutral mesons and direct photons with the ALICE experiment

3 Motivations Neutral mesons as probes of the QGP: in pp collisions, allow the study of particle production and constraint of fragmentation functions in Pb Pb collisions, give 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 do not interact with the medium, thus carry unmodified information about early stages of the collisions photons are emitted throughout collision evolution hard to measure above a large electromagnetic background INPC 06 Neutral mesons and direct photons with the ALICE experiment 3

4 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 4

5 Photon detection with the ALICE experiment (RUN ) PHOS calorimeter: - PbWO 4 crystals - η < 0.3, 60 < φ < 30 EMCal calorimeter: - Pb/scintillator sampling calorimeter - η < 0.7, 80 < φ < 80 Photon Conversion Method (PCM): - IS and PC - η < 0.9, 0 < φ < conversion in detector material: X /X 0 = (.4±0.5)%( η <0.9, R<80 cm) conv. probability 8% Photon candidates are extracted from V 0 (neutral secondary vertex particles) sample INPC 06 Neutral mesons and direct photons with the ALICE experiment 5

6 π 0 and η in pp collisions INPC 06 Neutral mesons and direct photons with the ALICE experiment 6

7 Neutral meson 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 O π 0 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) measurements help constrain models good description at intermediate p, larger discrepancy towards higher p η compared to NLO pqcd by W. Vogelsang (PDF: CEQ6M5, FF: AES) reference at.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 ALI PREL 6590 η at 7 ev, PLB 77 (0) 6 3 η, s = 0.9 ev 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 Oη η, s = 0.9 ev syst, stat (*) arxiv: INPC 06 Neutral mesons and direct photons with the ALICE experiment 7

8 π 0 and η in Pb Pb collisions INPC 06 Neutral mesons and direct photons with the ALICE experiment 8

9 π 0 from 0 data (PCM + PHOS) (GeV/c) N d dy dp p N ev π ALI PUB 8690 ALICE, EPJC 74 (04) % 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 pp s =.76 ev sallis fit power law fit p (GeV/c) not enough statistics for η measurement in 0 data d N/dp dy AA R AA (p ) = AA d σ/dp dy pp AA : nuclear overlap function, related to the pp cross section via AA = N coll /σ pp inel suppression due to interaction with medium is larger for more central collisions R AA Pb Pb s NN =.76 ev 0 5% π % π % π ALI PUB 8794 INPC 06 Neutral mesons and direct photons with the ALICE experiment 9

10 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 WA98 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 region accessible with 0 Pb Pb data 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment

11 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 O 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% Oη 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment

12 η/π 0 ratio in Pb Pb 0 data (PCM + EMCal) η/π 0 measured in Pb Pb collisions in two centrality classes compared to other ALICE results: η/π 0 in pp at 7 ev and to K ± /π ± in 0-% Pb Pb at.76 ev comparison could highlight differences due to the presence of a hot and dense medium and from particle flow (expected to be similar for η and kaons) cannot differentiate with current uncertainties Particle ratio Pb Pb, s NN =.76 ev η /π 0, 0 % K ± /π ±, 0 % η/π 0, 0 50% PLB 736 (04) 96 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 p (GeV/c) INPC 06 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 compared to other ALICE results: η/π 0 in pp at 7 ev and to K ± /π ± in 0-% Pb Pb at.76 ev comparison could highlight differences due to the presence of a hot and dense medium and from particle flow (expected to be similar for η and kaons) cannot differentiate with current uncertainties pqcd NLO calculation for 0-% cent. class agrees within uncertainties p region 4 6 GeV/c sensitive to initial transport coefficient ( = parameter describing energy loss in medium) Particle ratio Pb Pb, s NN =.76 ev η /π 0, 0 % K ± /π ±, 0 % η/π 0, 0 50% PLB 736 (04) 96 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) INPC 06 Neutral mesons and direct photons with the ALICE experiment

14 Direct photons in Pb Pb collisions INPC 06 Neutral mesons and direct photons with the ALICE experiment 3

15 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 ) /γ source decay.76a ev Pb+Pb@LHC PRC 94, (06) hermal (FIC, σ=0.4 fm) prompt photons Compton+annihilation Fragmentation p (GeV/c) 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 ALI PUB INPC 06 Neutral mesons and direct photons with the ALICE experiment 4 PLB 754 (06) 35-48

16 Direct photons in 0 Pb Pb data (PCM + PHOS) inclusive photon spectra measured with combined PCM + PHOS (PLB 754 (06) 35-48) in 3 centrality classes with 0 Pb Pb data R γ excess at high p for all centralities R γ PLB 754 (06) % Pb Pb s NN =.76 ev ALICE 0 40% Pb Pb s NN =.76 ev ALICE % Pb Pb s NN =.76 ev ALICE.5.0 ALI PUB INPC 06 Neutral mesons and direct photons with the ALICE experiment 5

17 Direct photons in 0 Pb Pb data (PCM + PHOS) inclusive photon spectra measured with combined PCM + PHOS (PLB 754 (06) 35-48) in 3 centrality classes with 0 Pb Pb data R γ excess at high p for all centralities at low p, 0% excess in 0-0% and 9% in 0-40% due to thermal radiation of the medium R γ PLB 754 (06) % Pb Pb s NN =.76 ev ALICE 0 40% Pb Pb s NN =.76 ev ALICE 40 80% Pb Pb s NN =.76 ev ALICE.0 ALI PUB INPC 06 Neutral mesons and direct photons with the ALICE experiment 5

18 Direct photons in 0 Pb Pb data (PCM + PHOS) inclusive photon spectra measured with combined PCM + PHOS (PLB 754 (06) 35-48) in 3 centrality classes with 0 Pb Pb data R γ excess at high p for all centralities at low p, 0% excess in 0-0% and 9% in 0-40% due to thermal radiation of the medium in agreement with NLO/JEPHOX pqcd above 5 GeV/c R γ PLB 754 (06) % 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 5

19 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 PLB 754 (06) 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 6

20 Comparison direct photon spectra ALICE - PHENIX 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 7

21 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 PLB 754 (06) 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 8

22 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 9

23 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 0

24 Outlook Many papers on track for publication: Neutral mesons analysis in pp at 8 ev with combined PCM, EMCal and PHOS analyses Neutral mesons analysis in p Pb at 5.0 ev with combined PCM, EMCal and PHOS analyses (not discuss in this presentation) Neutral mesons analysis in Pb Pb at.76 ev with combined PCM, EMCal and PHOS analyses Updates on the direct photon analysis: additional inputs from measured particle spectra in comparison to the previous cocktail simulation Ongoing LHC RUN analyses: neutral mesons and direct photons in pp at 3 ev and 5 ev neutral mesons and direct photons in Pb Pb at 5 ev ongoing studies to improve material budget estimation will help reduce the related systematic uncertainty INPC 06 Neutral mesons and direct photons with the ALICE experiment

25 Back up INPC 06 Neutral mesons and direct photons with the ALICE experiment

26 Luminosity 0 vs 0 with large statistics collected in 0 measurement of differential invariant cross section is possible INPC 06 Neutral mesons and direct photons with the ALICE experiment 3

27 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 pp, s=7 ev PCM π 0 : 0.6<p <0.8 GeV/c 6 η:.4<p <.8 GeV/c PHOS EMCAL 5.0<p <7.0 GeV/c π 0 η π 0 π Events / 8 MeV/c m γγ (GeV/c ) Events / MeV/c Events / 4 MeV/c 4 3 π 0 :.0<p <. GeV/c Events / MeV/c η: 4.0<p <6.0 GeV/c m γγ (GeV/c ) η 0. 0 ALI PUB (GeV/c ) 0 ALI PERF m γγ (GeV/c ) 0 ALI PUB PCM EMCal PHOS m γγ (GeV/c ) Events / 5 MeV/c Pb-Pb 0.0 PCM η γγ Pb Pb, s NN =.76 ev 0 % γγ < p < 3 GeV/c ALICE performance Signal 40 Events / MeV/c EMCal η γγ Pb Pb, s NN =.76 ev 0 % γγ < p < 4 GeV/c ALICE performance Histo: signal + background Points: signal after background subtraction Line: fit (for mass and width) (GeV/c ) M γγ (GeV/c ) M γγ INPC 06 Neutral mesons and direct photons with the ALICE experiment 4

28 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 7666 INPC 06 Neutral mesons and direct photons with the ALICE experiment 5

29 ) Neutral pion spectra in pp collisions (PCM + PHOS) he NLO pqcd calculations are repeated considering the additional constraint given by 8 ev results large improvement 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)) p = p /) = p ) (GeV/c) INPC 06 Neutral mesons and direct photons with the ALICE experiment 6

30 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 6673 INPC 06 Neutral mesons and direct photons with the ALICE experiment 7

31 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 INPC 06 Neutral mesons and direct photons with the ALICE experiment 8

32 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 689 INPC 06 Neutral mesons and direct photons with the ALICE experiment 9