Photon and neutral meson production in pp and PbPb collisions at ALICE

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1 Photon and neutral meson production in pp and PbPb collisions at ALICE Dieter Roehrich University of Bergen, Norway for the ALICE Collaboration Nuclear collisions at the LHC Photons as a probe for the QGP The ALICE experiment Photon detection First results: p T spectra and nuclear modification factor Outlook

2 Heavy Ion Collisions at the LHC Ultrarelativistic heavy ion collisions at the LHC e.g. Pb+ Pb at s = 5.5 TeV per nucleon pair ALICE experiment - dedicated detector for nuclear collisions central barrel spectrometer: -0.9 < η < 0.9 tracking and particle identification in full azimuth forward muon spectrometer: - 4 < η < -2.4 charged particle multiplicity: - 3 < η < 5.4 Data sets p+p at 0.9, 2.36, 2.76 and 7 TeV Pb+Pb at 2.76 TeV/n

3 Photon sources in nuclear collisions Direct photons prompt photons produced directly in hard scatterings of partons of colliding nuclei fragmentation photons thermal photons from thermalized matter (QGP and hadron gas) hard medium-induced photons from jet-photon conversions and bremsstrahlung of partons in the medium Decay photons Decay of neutral mesons 0,,,... Rate Hadron Gas Thermal QGP Thermal Pre-Equilibrium Thermal? Jet Re-interaction? LO E g

4 Photons a probe for QGP (1) Prompt photons production in hard processes well understood colorless probe no interaction with medium -> calibration tool for scaling NN to AA via the number of binary collisions Nuclear Modifaction factor R AA R AA (p T ) N binary d 2 N AA / dp T d d 2 N pp / dp T d = QCD medium / QCD vacuum Yen-Jie Lee (CMS), QM2011

5 Nuclear Modication Factor R AA A+A p+p R AA (p T ) N binary d 2 N AA / dp T d d 2 N pp / dp T d n x m N binary varies with impact parameter b

6 Photons a probe for QGP (2) Thermal photon production thermometer for QGP PHENIX -> T ave = stat 19 syst MeV (Au+Au at RHIC) PRL104,132301(2010), arxiv:

7 Photons a probe for QGP (3) Prompt g hadron/jet azimuthal correlations g provides an estimate for the energy of the recoil jet study interaction of partons with medium Neutral mesons reconstructed from decay photons transverse momentum spectra of 0 and test of NLO pqcd in pp nuclear modification factor study suppression of 0 and in A+A collisions

8 ALICE Collaboration: > 1000 Members > 100 Institutes > 30 countries Detector: Size: 16 x 26 meters 8 8 Weight: 10,000 tons

9 Photon detectors Two complementary methods Electromagnetic Calorimeters PHOton Spectrometer PHOS PbO 4 W crystals high resolution (spatial and energy) small acceptance (100 x 0.24 in ) PbSc calorimeter EMCAL larger acceptance (120 x 1.4 in ) π 0 and η reconstruction from g conversions central barrel tracking system: ITS, TPC, TRD large acceptance (360 x 1.8 in ) low conversion probability (~8%) good resolution at low p T 9

10 ALICE 10

11 PHOS detector, readout and trigger PbO 4 W- crystal calorimeter for photons, neutral mesons, 1 to ~100 GeV Array of crystals + APD + preamp + trigger logic + readout DAQ L0/L1 trigger tasks shower finder energy sum implementation FPGA VHDL firmware L0/L1 trigger energy TOF 11

12 PHOS energy resolution

13 PHOS invariant mass distributions (1) Reconstruction of 0 0 gg M gg = 13

14 PHOS invariant mass distributions (2) 14

15 Photon conversions π 0 γγ e + e - e + e --

16 Photon conversion vertices ALICE material budget (11.4% X 0 up to middle of TPC) agrees within +3.4%/-6% with its implementation in GEANT

17 Photon conversion performance (1)

18 Photon conversion performance (2) Precise 0 measurement 0.5 < p T < 10 GeV/c

19 Results Cross sections for 0 and production pp collisions comparison to NLO pqcd -> test of gluon fragmentation functions central PbPb collisions comparison to pp via Nuclear Modification Factor -> suppression at high p T -> study of parton energy loss in a dense, opaque partonic medium

20 Invariant 0 cross section in pp collisions at 0.9, 2.76 and 7 TeV (1) Comparison of conversion method with PHOS

21 Invariant 0 cross section in pp collisions at 0.9, 2.76 and 7 TeV (2) Parametrization of combined data set

22 Invariant 0 cross section in pp collisions at 0.9, 2.76 and 7 TeV (3) Comparison to NLO pqcd

23 Invariant cross section in pp collisions at 0.9, 2.76 and 7 TeV (1) Parametrization of combined data set

24 Invariant cross section in pp collisions at 0.9, 2.76 and 7 TeV (2) Comparison to NLO pqcd AESSS FF s: Phys. Rev. D83, (2011)

25 / 0 ratio in pp collisions at 7 TeV Comparison to NLO pqcd

26 Invariant 0 cross section in PbPb collisions at s NN = 2.76 TeV

27 0 RAA in PbPb at s NN = 2.76 TeV (1) peripheral (60-80%) collisions

28 0 RAA in PbPb at s NN = 2.76 TeV (2) semi-central (40-60%) collisions

29 0 RAA in PbPb at s NN = 2.76 TeV (3) semi-central (20-40%) collisions

30 0 RAA in PbPb at s NN = 2.76 TeV (4) central (0-20%) collisions

31 0 RAA in PbPb collisions at s NN = 2.76 TeV

32 R AA in PbPb collisions at s NN = 2.76 TeV R AA for identified particles in central collisions

33 Summary pp collisions at 0.9, 2.76 and 7 TeV NLO pqcd with DSS FF overestimates neutral pion and eta p T spectra at 2.76 and 7 TeV PbPb collisions strong suppression of neutral pion production suppression is consistent with the result from charged pions results on identified particles will allow to disentangle the interplay between quark and gluon energy loss, and recombination mechanisms at intermediate p T

34 Outlook (1) Direct photons in pp Direct photons in PbPb... 34

35 Outlook (2) Photon-hadron correlations in pp - di-jet structure trigger + Next steps isolated clusters PbPb