Measurement of jets and photons with the ALICE experiment at the LHC

Transcription

1 Measurement of jets and photons with the ALICE experiment at the LHC Lucia Leardini on behalf of the ALICE Collaboration Physikalisches Institut, Heidelberg University 53 rd Rencontres de Moriond QCD and High Energy Interactions La huile, March 3 rd 8

2 Jets and photons as Quark-Gluon Plasma probes A A collisions provide necessary conditions of temperature and energy density for QGP formation jets and photons are complementary probes pp collisions no QGP formation = vacuum scenario used as baseline to highlight in-medium effects p Pb collisions used to disentangle initial, final and cold nuclear matter (CNM) effects hypothesis of QGP droplet (?) Pb Pb collisions partons scatter in QGP and lose energy: high p hadron yields will appear depleted, suppressed asymmetry of jet energy and azimuthal distribution photons originate from same environment as jet partons: interaction of quarks and gluons in QGP (soft) and initial hard scattering photons are unmodified by the medium: can access early collision stages Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

3 Jet measurements Int. J. Mod. Phys. A9 (4) 4344 jets in ALICE are reconstructed with charged tracks (IS+PC) and EM calorimeters ALI-PUB-77 Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

4 Jet physics High energy partons fragment after hard scattering collimated jet of particles In ALICE: jet reconstruction using FastJet (JHEP 84 (8) 63) anti-k for signal jets: sequential clustering of candidate objects within a given distance k for average background estimation resolution R =. and.4 strong jet yields suppression already observed challenge is in isolating signals among local fluctuations and over large underlying background advantage of using jets instead of hadron yields: jets allow to capture the full parton shower results shown are for fully corrected jet yields PLB 746 (5) models with parton fragmentation and strongly interaction in QGP Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 3

5 Jet mass in Pb Pb PLB 776 (8) Directly related to leading parton virtuality, measure of jet broadening: soft splitting: broader jet and larger mass hard fragmentation: jet is collimated, smaller mass hints of jet collimation, consistent with other jet shape results competing effects: energy loss and medium response (= excited medium shows up in jet, seen as correlated background) models with quenching do not describe data ) (GeV/c % Pb Pb 6 PYHIA Perugia 4 JEWEL+PYHIA recoil on ALICE Q PYHIA , ch jet ch jet M 8 Charged jets, anti k R =.4, η <.5 jet Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 4

6 Subjettiness in Pb Pb relates to jet substructure: τ N = N i= p,imin( R i,,..., R i,n ) R N i= p,i search for semi-hard splittings: rare, resulting from jet scattering in medium (vs quark or gluon) proof we are probing partonic d.o.f. in strongly coupled medium signature = enhancement of -pronged jets (τ /τ ) but comparing to pp (PYHIA), no modification has been observed ALI-PREL-5649 Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 5

7 Semi-inclusive hadron-jet coincidence in p Pb semi-inclusive = no need to relate event activity to collision geometry hadron triggered recoil jet distribution = unbiased observable jet quenching may transport jet energy outside of its cone results in suppression of recoil in central collisions ratio consistent with unity jet quenching limit set at.4 GeV/c in p Pb, vs 8 GeV/c limit measured in Pb Pb 5 % VA recoil / % VA recoil ALICE p Pb s NN = 5. ev {,5} {6,7} Anti k charged jets, R =.4.43 < y * <.36;.3 < y * <.97 jet π ϕ <.6 Syst. uncert..4 GeV/c spectrum jet shift ch arxiv:7563,jet Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 6 JHEP 9 (5) 7

8 Int. J. Mod. Phys. A 9 (4) 4344 Y (cm) SSD η < SDD 4 via calorimeters (EMCal, PHOS) via conversions in detector material (IS and PC) Photon measurements 5 SPD ALI PUB 766 PC inner field cage vessel PC Rods PC drift gas PC inner containment vessel 5 5 X (cm) Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 7 3

9 ) Neutral mesons in pp and p Pb pp: arxiv: p Pb: arxiv:875 c 3 (pb GeV d 3 σ d p 3 E ALI PUB 3595 pp, s = 8 ev ALICE norm. unc..6% π η (x ) CM fit sallis fit PYHIA 8., Monash 3 PYHIA 8., une 4C π pqcd NLO PDF: MSW, FF: DSS4 η pqcd NLO PDF: CEQ6M5, FF: AESSS η π (c/gev) N d dy dp p N ev. π ALI PUB 433 π EPOS3 VISHNU DPMJet HIJING γ CGC MV NLO: EPPS6, DSS4 NLO: nceq, DSS4 sallis fit η EPOS3 VISHNU DPMJet HIJING NLO: nceq, AESSS sallis fit ALICE p Pb, NSD, s NN = 5. ev p (GeV/c) large p range covered for both mesons in pp and p Pb various theory models (NLO pqcd, hydro,...more in back up) agree with π but overestimate η ALICE measurement can be used to update η FF and improve model descriptions Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 8 η π

10 Neutral mesons in Pb Pb arxiv: (GeV/c) η N π, d dy dp p N ev π ALI-PUB large p coverage for both mesons nuclear modification factor R AA (p ) = η π Pb Pb, =.76 ev s NN η, % π, % η, 5% π, 5% SHM - PRC 9, 496 (4) % 5% NEQ EQ NEQ EQ PRC 85, 6497 () EPOS % EPOS 5% 4 - d N/dp dy AA AA d σ/dp dy pp (QGP/vacuum) is used to highlight effect of parton energy loss in QCD medium R AA π ALICE % Pb Pb () % =.76 ev ALI-PUB s NN π PHENIX % Au Au s = GeV = 6.4 GeV NN s NN = 39 GeV s NN π WA98 3% Pb Pb = 7.3 GeV s NN π R AA η ALICE % Pb Pb () =.76 ev s NN η PHENIX % Au Au = GeV s NN ALI-PUB-436 larger hadron suppression with increasing energy higher energy density of the medium Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 9 η

11 Direct photons in Pb Pb γ created during entire space time evolution after collision and leave medium unaffected ideal probe, but large background from decay photons direct photons = all the photons not coming from hadron decays hermal Photons emitted from thermalised medium dominant at low p comparison to models employing mainly hydro Prompt Photons from initial hard scattering dominant at high p described by NLO pqcd: test binary scaling pp A A pre-equilibrium photons, jet-medium interactions (only Pb Pb, scattering hard vs thermalized partons, in-medium γ bremsstrahlung emitted by quarks) γ direct = γ inc γ decay = ( γ decay γ inc ) γ inc = ( Rγ ) γ inc Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

12 Direct photons in Pb Pb γ created during entire space time evolution after collision and leave medium unaffected ideal probe, but large background from decay photons direct photons = all the photons not coming from hadron decays hermal Photons emitted from thermalised medium dominant at low p comparison to models employing mainly hydro Prompt Photons from initial hard scattering dominant at high p described by NLO pqcd: test binary scaling pp A A pre-equilibrium photons, jet-medium interactions (only Pb Pb, scattering hard vs thermalized partons, in-medium γ bremsstrahlung emitted by quarks) γ direct = γ inc γ decay = ( γ decay γ inc ) γ inc = ( Rγ ) γ inc Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

13 Direct photons in Pb Pb γ created during entire space time evolution after collision and leave medium unaffected ideal probe, but large background from decay photons direct photons = all the photons not coming from hadron decays hermal Photons emitted from thermalised medium dominant at low p comparison to models employing mainly hydro Prompt Photons from initial hard scattering dominant at high p described by NLO pqcd: test binary scaling pp A A pre-equilibrium photons, jet-medium interactions (only Pb Pb, scattering hard vs thermalized partons, in-medium γ bremsstrahlung emitted by quarks) γ direct = γ inc γ decay = ( γ decay γ inc ) γ inc = ( Rγ ) γ inc Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

14 Double ratio R γ in Pb Pb PLB 754 (6) R γ % Pb Pb s NN =.76 ev ALICE NLO pqcd PDF: CEQ6M5 FF: GRV JEPHOX PDF: C, FF: BFG JEPHOX npdf: EPS9, FF: BFG (all scaled by N coll ) 4% Pb Pb s NN =.76 ev ALICE NLO pqcd PDF: CEQ6M5 FF: GRV JEPHOX PDF: C, FF: BFG JEPHOX npdf: EPS9, FF: BFG (all scaled by N coll ) 4 8% Pb Pb s NN =.76 ev ALICE NLO pqcd PDF: CEQ6M5 FF: GRV JEPHOX PDF: C, FF: BFG JEPHOX npdf: EPS9, FF: BFG (all scaled by N coll ) ALI PUB R γ γ incl γ cocktail decay = + γ direct γ cocktail decay advantage of R γ : common uncertainties cancel out excess at low p due to thermal photons = emission from thermalised medium,.6σ significance in.9 < p <. GeV/c for -% central collisions excess at high p (> 3 GeV/c) due to prompt photons = initial hard scattering, in good agreement with NLO pqcd calculations if R γ consistent with unity, upper limits are estimated (9% CL) Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

15 Neutral meson and direct photon R AA direct photon R AA obtained using NLO pqcd calculation as pp reference R AA γ pp reference: dir (n)pdf: CEQ6.M/EPS9 FF: BFG ALICE p Pb, NSD, % π s NN = 5. ev ALICE Pb Pb s NN =.76 ev % γ dir % π η ALI DER 437 PLB 754 (6) arxiv:875 arxiv: η comparison to neutral meson R AA in Pb Pb and p Pb collisions in Pb Pb collisions: π and η R AA in Pb Pb show expected suppression due to jet quenching at low p, direct photon excess from thermal emission well visible for p > GeV/c, γ dir R AA consistent with unity: EM probe unaffected by QCD medium in p Pb collisions: π and η R AA in p Pb collisions consistent with unity above GeV/c confirms that high p hadron yields suppression is final state effect Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

16 Summary = studies of hard probes in p Pb (jets and high p hadrons) show no signals of medium induced effects observations disentangle initial and final state effects and set limits for jet quenching in small systems = studies of hard and electromagnetic probes in Pb Pb......at low p (non perturbative) regime: measurement of direct photon excess in most central Pb Pb collisions (.6σ in.9 < p <. GeV/c) emission from thermalised medium, emission temperature and time (from direct photon flow, soon to be published!) provide further information on QGP characteristics...at high p (perturbative) regime: strong suppression of jet and neutral meson yields parton energy loss in QGP jet shape studies (jet mass and subjettiness) show preference for collimated jets and no hint of -pronged jets enhancement Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 3

17 Back up Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 4

18 ALICE A Large Ion Collider Experiment EMCal Central barrel: η <.9, < φ < 36 Inner racking System: silicon detectors for vertex finding, tracking Pb-scintillator calorimeter, η <.7, 8 < φ < 8 ime Projection Chamber: tracking, particle ID PbWO 4 crystals η <.3, 6 < φ < 3 PHOS VA, 34 cm from IP (.8 < η < 5.) VC, 9 cm from IP (-3.7 < η < -.7) used for multiplicity estimation Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 5

19 Centrality in ALICE geometrical overlap of colliding nuclei modelled by Glauber Monte Carlo: superposition of independent binary nucleon-nucleon collisions % central peripheral % Events (arb. units) 3 4 ALICE Pb Pb at s NN =.76 ev Data NBD Glauber fit P µ,k x [f N + ( f)n ] part coll f =.8, µ = 9.3, k = % 7 8% 6 7% % 4 5% 3 4% 3% % ALI PUB VZERO amplitude (arb. units) PRC 88 (3) 4499 Lucia Leardini Jets and photons with ALICE at the LHC Moriond % 5%

20 Subjettiness relates to jet substructure: τ N = Ni= p,i Min( R i,,..., R i,n ) R Ni= p,i search for semi-hard splittings: rare, resulting from jet scattering in medium (vs quark or gluon) proof we are probing partonic d.o.f. in strongly coupled medium signature = enhancement of -pronged jets but comparing to pp, no modification has been observed pp Pb Pb ALI-PREL-565 ALI-PREL-5649 Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 7

21 Photon detection in ALICE Photon Conversion Method (PCM): conversion in detector material: X X = (.4±.5)% for η <.9, R conv < 8 cm) P conv 8% daughter tracks (e ± ) reconstructed with IS and PC conversion point treated as a secondary vertex Calorimetry (EMCal and PHOS): calorimeter cells with deposited energy above certain threshold are grouped in clusters = γ candidates threshold level can be adjusted for triggering Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 8

22 Neutral pion peak position and width Peak width (MeV/c ) Peak position (MeV/c ) ALICE performance pp, s = 8 ev π γγ Data DataMCMC PCM PCM EMC EMC PHOS pp Peak width (MeV/c ) Peak position (MeV/c ) a) a) b) ALICE p Pb, s NN = 5. ev η γγ b) Data MC PCM PCM EMC EMC ) peak width (MeV/c ) peak position (MeV/c PCM (FWHM/.36) PHOS (σ) EMCal (σ) a) b) ALI PUB 7666 PCM PHOS EMCal data MC Pb Pb data MC Pb Pb s NN =.76 ev centrality % Int. J. Mod. Phys. A 9 (4) 4344 Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 9

23 Neutral meson reconstruction Reconstructed via -photon invariant mass: M γγ = E γ E γ ( cos θ ) Counts ALICE performance pp, s = 8 ev MB trigger PCM π :.5 GeV/c < p <.6 GeV/c Raw real events Mixed event BG Remain. BG BG subtracted Fit 3 PCM Counts ALICE performance Pb Pb, s NN =.76 ev -% PCM η:. GeV/c < p < 3. GeV/c Raw real events Mixed event BG Signal after BG subtraction scaled by 4 Fit η in Pb Pb at.76 ev arxiv: M γγ (GeV/c ) Counts 3. ALICE performance pp, s = 8 ev EMC L trigger.5 EMC..5 π : 9. GeV/c < p <. GeV/c Raw real events Mixed event BG Remain. BG BG subtracted Fit M η γγ (GeV/c ) Counts ALICE performance Pb Pb, s NN =.76 ev -% EMCal 5 η: GeV/c < p < 4. GeV/c Raw real events Mixed event BG Signal after BG subtraction Fit π in pp at 8 ev arxiv: M γγ (GeV/c ) EMCal M η γγ (GeV/c ) combinatorial background estimated w/ event mixing method: events w/ similar topology and multiplicity are mixed together invariant mass peak fitted with gaussian + exponential (for bremss. tail) to extract mass position and width raw yields extracted via bin counting (after subtracting residual background) Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

24 Neutral mesons invariant cross section at several s c 3 ) (pb GeV d 3 σ d p 3 E 4 ALICE 3 9 π γγ c 3 ) (pb GeV d 3 σ d p 3 E 3 9 ALICE η γγ NLO, PDF:CEQ6M5 FF:AESSS µ =.5 p µ = p µ = p η/π.8.6 pp, s = 8 ev pp, s = 7 ev pp, s =.76 ev pp, s = 9 GeV (Prelim.) ALICE Data, heory CM fit Data, heory CM fit Data, heory CM fit Data, heory CM fit pp, s = 8 ev (x ) pp, s = 7 ev (x ) pp, s =.76 ev (x) pp, s = 9 GeV CM fit sallis fit PYHIA 8., Monash 3 NLO, PDF: MSW FF: DSS Data, heory CM fit Data, heory CM fit Data, heory CM fit Data, heory sallis fit pp, pp, pp, pp, CM fit sallis fit 3 s = 8 ev (x ) s = 7 ev (x ) s =.76 ev (x) s = 9 GeV (Prelim.) PYHIA 8., Monash ALI PREL ALI PUB 3659 ALI PREL 3664 Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 Ratio ALICE, (η/π ) /(η/π data ) m.6 pp, s = 8 ev.4 pp, s = 7 ev. pp, s =.76 ev pp, s = 9 GeV (Prelim.) ALI PREL 3666 results at.9, 7 ev: PLB 77 () 6-7 results at 8 ev: arxiv:

25 Neutral mesons in pp and p Pb pp: arxiv: p Pb: arxiv:875 η/π ALICE pp, s = 8 ev ALICE pp, s =.76 ev.8 ALICE pp, s = 7 ev ALICE pp m scaled, s = 8 ev η/π. ALICE.8 p Pb, s NN = 5. ev η from m scaled π pp, s = 7 ev PHENIX d Au, s NN = GeV CERES APS s NN = 9. GeV p Au p Be.. PHENIX pp, s = GeV NA7 pp, s = 7.5 GeV ALI PUB 4384 η/π ratio used to test m -scaling: violation observed at LHC results consistent across s Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8

26 Neutral mesons in p Pb compared to theory heory, Data fit heory, Data fit 4 3 ALI PUB 4334 π γ CGC MV EPOS3 VISHNU NLO: EPPS6, DSS4 4 η NLO: nceq, AESSS 3 HIJING DPMJet ALICE p Pb, NSD, s NN = 5. ev p (GeV/c) NLO: largest systematic uncertainty given by uncertainty on the choice of scales HIJING: minijet production and multi-string phenomen. + nuclear shadowing, coarse implementation of jet quenching, Glauber model for pa and A A DPMJet: implementation of Dual Parton Model based on Gribov-Glauber approach, soft and hard scattering described together CGC MV γ : Color Glass Condensate model + MV γ as initial conditions VISHNU: 3D+ viscous hydro + hadron cascade model, Glaber for fluctuations of initial conditions EPOS3: 3D+ viscous hydro and flux tubes within Gribov-Regge multiple scattering framework Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 3

27 Neutral mesons in p Pb compared to theory NLO: largest systematic uncertainty given by uncertainty on the choice of scales HIJING: minijet production and multi-string phenomen. + nuclear shadowing, coarse implementation of jet quenching, Glauber model for pa and A A DPMJet: implementation of Dual Parton Model based on Gribov-Glauber approach, soft and hard scattering described together CGC MV γ : Color Glass Condensate model + MV γ as initial conditions VISHNU: 3D+ viscous hydro + hadron cascade model, Glaber for fluctuations of initial conditions EPOS3: 3D+ viscous hydro and flux tubes within Gribov-Regge multiple scattering framework η/π. ALICE ALI PUB 4338 /η/π η/π scaling m measured ALI PUB 4388 p Pb, s NN = 5. ev η from m scaled π EPOS3 VISHNU HIJING DPMJet ALICE p Pb, s NN = 5. ev p (GeV/c) Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 4

28 . Pb Pb, =.76 ev Neutral mesons in Pb Pb at.76 ev s NEQ SHM NN EQ SHM heory, Data Fit heory, Data Fit EPOS π, % EPOS: hydro + energy loss accounted for particle production (N)EQ: (non-)equilibrium statistical hadronisation model.5 π, 5% heory, Data Fit η, % heory, Data Fit η, 5% ALI-PUB Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 5

29 . Pb Pb, =.76 ev Neutral mesons in Pb Pb at.76 ev s NEQ SHM NN EQ SHM heory, Data Fit heory, Data Fit EPOS π, % π, 5% EPOS: hydro + energy loss accounted for particle production (N)EQ: (non-)equilibrium statistical hadronisation model NLO pqcd w/ parton energy loss heory, Data Fit heory, Data Fit η, % η, 5% ALI-PUB η/π Pb Pb, s NN =.76 ev % % NLO DCZW PLB 75 (5) 39 % EPOS PRC 85, 6497 () SHM - PRC 9, 496 (4) % NEQ % EQ ALI-PUB Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 5

30 DCZW prediction for η/π ratio in Pb Pb at LHC NLO pqcd theoretical prediction of η/π ratio in Pb Pb collisions at.76 ev according to DCZW (PLB 75 (5) ) τ is the initial time of the QGP medium ˆq initial values of the jet transport parameter the larger ˆq is, the stronger the jet-medium interaction will be Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 6

31 R AA comparison to theory NLO DCZW: PLB 75 (5) WHDG: Int. J. Mod. Phys. E6 93 (7) Djordjevic et al.: PLB 734 (4) R AA Pb Pb, s NN =.76 ev π, % % NLO DCZW WHDG Djordjevic Pb Pb, s NN =.76 ev π, 5%.5.5 R AA Pb Pb, s NN =.76 ev η, % Pb Pb, s NN =.76 ev η, 5% ALI-PUB Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 7

32 Decay photon cocktail simulation Input: parametrised spectra of particles that decay into photons largest contribution from π ( 88%) and η ( %), considering only these two already gives good estimation decay /γ source γ π γγ (e + e γ) η ργ (ωγ, γγ) η γγ (π + π γ,e + e γ,π γγ) φ ηγ (π γ, ωγ) ω π γ (ηγ) ρ π + π γ (π γ, ηγ ) m-scaling approximation used when a specific particle measurement is not available (η, ω,...): m = p + m 3 4 ALI PUB % Pb Pb s NN =.76 ev Monte Carlo simulation dn p = dn dp m dm All meson and baryon spectra can be obtained via m -scaling starting from π and protons, respectively: dn dp = C(m) dnπ dp Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 8

33 R γ separate systematic uncertainties.5 % Pb Pb s NN =.76 ev ALICE Stat. Syst. A Syst. B Syst. C R γ.5.5 4% Pb Pb s NN =.76 ev ALICE Stat. Syst. A Syst. B Syst. C 4 8% Pb Pb s NN =.76 ev ALICE Stat. Syst. A Syst. B Syst. C type A: correlated point-by-point type B: correlated in p and of p -dependent magnitude type C: constant uncertainties Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 9

34 Direct photons spectra in Pb Pb PLB 754 (6) N γ d c ) (GeV dir dy dp p N ev. π 3 Pb Pb s NN =.76 ev A exp( p / eff ) % ALICE % 4% ALICE 4% 4 8% ALICE x x Paquet et al. x arxiv: Linnyk et al. 4 arxiv: Chatterjee et al. v. Hees et al. PRC 85() 649 NPA 933(5) 56 + JHEP 35(3) ALI PUB common assumption: QGP formation, direct γ = thermal + prompt γ Paquet et al.: IP-Glasma viscous hydro discrete particle production (Cooper-Frye) late stage & freeze-out with UrQMD van Hees et al.: ideal hydro + LQCD EoS, QGP phase then hadron gas with radial and elliptic flow included Chatterjee et al.: event-by-event hydro with long. boost invariance Linnyk et al.: off-shell transport approach PHSD, emphasis on γ emission from M+M & B+B bremsstr. Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 3

35 hermal γ temperature - comparison PHENIX vs ALICE ALICE results compared with PHENIX direct photon measurement in Au Au at GeV (PRL4 ()33, PRC9/6 (5) 6494) c ) (GeV N γ dir d dy dp p π N ev. ALICE % Pb Pb =.76 ev s NN A exp( p / eff ) eff = 34 ± sys ± 4 MeV stat PHENIX % Au Au =. ev s NN A exp( p / eff ) stat eff = 39 ± 5 ± 7 MeV sys - c ) (GeV N γ dir d dy dp p π N ev. ALICE -4% Pb-Pb A exp(-p / eff ) eff = 47 ± PHENIX s NN =.76 ev stat sys 6 ± 96 MeV -4% Au-Au =. ev s NN A exp(-p / eff ) eff = 6 ± 33 sys ± 8 MeV stat ALI PUB ALI-PUB exponential fit to low p excess: inverse slope parameter larger at higher collision energy and consistent in both centrality classes Lucia Leardini Jets and photons with ALICE at the LHC Moriond 8 3