1. 1 ,  , (and  measurements in ALICE

2. 2  ’s sensitive only to production processes, do not interact, sensitive to: initial parton distributions: Intrinsic k T, k T Broadening, Shadowing, Anti-shadowing, Saturation, … some final state effects (parton/hadron rescatterings): Thermal, Jet/Parton Radiation or fragmentation after Eloss,… “The virtue of   and  Measurements” To properly deduce the properties of the produced QCD matter we must separately distinguish initial state effects from final state effects. Experimental virtues (calorimeter measurement): Measure  and   in same detector Identified particles to very high p T   ’s abundantly produced   ’s produced in final state (fragmentation or recombination): initial parton distributions: as for  large final state effects: Rescattering (low p T ), Absorption, k T Broadening, Jet/Parton Energy Loss,…

3. 3 Centrality Dependence of High p T Suppression PHENIX nucl-ex/0503003 Trend of  yield with centrality (impact parameter) follows expected binary collision scaling behavior (=constant) and agrees with pQCD (=1). Direct  are not suppressed - strong evidence that hadron suppression is due to final state, i.e. parton energy loss.

4. 4 Direct Photons and   Production for Central Au+Au PHENIX nucl-ex/0503003 High p T  yield consistent with binary scaled pQCD in contrast to factor of 5 suppression of   & hadron yields. Direct  are not suppressed - strong evidence that hadron suppression is due to final state, i.e. parton energy loss. Errors don’t preclude a thermal  contribution at low p T

5. 5 Centrality Dependence R AA for   suppression increases with increasing nuclear overlap volume.  suppression same as   - behaviour agrees with “meson-like” expectation PHENIX preliminary

6. 6 inclusive jets 10 Hz @ 50 GeV few x 10 4 /year for E T >150 GeV E FS = 250 GeV (PHOS 80 GeV) Extend measurements to much higher p T From Peter Jacobs

7. 7 Photons (all in fact): Continum Spectrum with Many Sources Turbide, Rapp, Gale EE Rate Hadron Gas Thermal T f QGP Thermal T i Pre-Equilibrium Jet Re-interaction pQCD Prompt Final-state photons are the sum of emissions from the entire history of a nuclear collision. +Weak+EM decay  ’s (    ) = Bkgd

8. 8 Resolution: PHOS vs EMCal - PHOS wins Due to much better energy resolution, lower occupancies, and lower tower thresholds PHOS will be the preferred detector for low pT photon measurements. PHOS: 3.5%/sqrt(E) EMCal: 12%/sqrt(E) or worse?

9. 9 Tower granularity  0 separation - PHOS wins  0  opening angle   shower shape discrimination Heather Gray, LBNL/Cape Town      rejection for p T <~30 GeV/c At high p T merging of showers of photon pairs from   decays become indistinguishable from single photons - become a background. Use EMCal vs PHOS comparisons to determine (PHOS rate limited) Other “tricks” like tagging conversion rate As p T increases  ’s dominate preliminary

10. 10 So…Why Large EMCal in ALICE? Measure neutral energy of jets –Improves jet energy resolution, ~30% -> ~15% –PHOS is too small to contain jet Extend PHOS measurements ,  0, and e +/- to higher p T Increase acceptance for  (EMCal/PHOS) + jet(TPC) Allow back-to-back coincidences –  (PHOS) + jet (TPC+EMCal) –  (EMCal) + leading  0 (PHOS) –  (EMCal/PHOS) + e +/- from D,B decay (heavy q jet) (PHOS/EMCal)

11. 11 ALICE EMCal is large enough for jet measurements PROBLEM: Underlying event non-jet energy: Radius Cone Energy (GeV) 0.7 750 0.5 380 0.3 140 0.1 15 Jet Energy fraction within cone radius 22 0  /2  3  /2 TPC PHOS EMCal  Jet R=0.3,0.5 0.9 -0.9  0  Use “small” jet-cone, R~0.3

12. 12 Direct  /  0 Ratio Ratio  /  0 provides figure of merit for direct gamma measurement:   /  0 ~ 10% easy ~ 3-5% limit  0 suppression by factor of ~5-10 (jet quenching) would allow direct photon measurement down to a few GeV/c Expectations for  0 suppression at high pT at LHC?

13. 13 Empirical Investigation of Energy Loss Observe  0 spectra in p+p have power-law form: If we interpret suppression as a p T shift from Energy Loss: where then The Energy Loss, or p T Shift S loss, can then be extracted from R AA as

14. 14 Suppression / Energy Loss vs Centrality Gyulassy, Vitev, Wang: 1D expansion, simple geometric scaling  Large gluon density dN g /dy~1000, Large energy density.  If behaviour extrapolates to LHC, expect even larger p T independent suppression

15. 15 Further Illumination with Direct Photons High p T direct  produced in hard q-g Compton-like scatterings. Sensitive to PDFs, (especially gluons). In the HI case,  from initial hard scattering escape. Can be used to tag recoiling jet to study jet energy loss (“Jet Quenching”), which may produce additional Brems.  ’s. Leading Particle Direct  Hadrons g q Brems.  Thermal  Additional thermal  ’s produced in scatterings in QGP or hadronic phase of collision. If ”Gluon Saturation” occurs, initial  (and  0 ) will be suppressed. With parton Eloss, the fragmentation contribution could also be suppressed.

16. 16 Investigate   yields vs Reaction Plane (Pathlength)  = 0°  = 90° Measure Reaction Plane using BBCs Measure Yields vs. orientation w.r.t. RP  Expect largest E-loss or suppression at  = 90 in “ long” direction Interplay between flow (v 2 ) and Supression? Both give expectation of in-plane enhancement. Long path, large Eloss

17. 17   and inclusive  v 2 analysis   show large v 2 to high p T - effect of suppression? (more later) Inclusive  show similar large v 2 Use   measurement as input to MC to predict decay  v 2 - if no direct  signal, or v 2 (direct  ) = v 2 (bgd  ), then should have v 2 (bgd  ) = v 2 (incl  ) Use Direct  measurement R=  Total /  Decay then v 2 (direct  ) = ( R*v 2 (incl  ) - v 2 (bgd  ) ) / (R-1) Or, assume v 2 (direct  ) = 0 to extract R. Gives agreement with measurement of R=0 in this p T range. (Proof of principle at this point) p T (GeV/c) PHENIX preliminary

18. 18 ALICE studies using  Measurements Measure direct  in p+p: –Extract fragmentation contribution by analysis with/without isolation cuts Measure direct  in A+A: –Search for effects of gluon saturation Thermal region (PHOS): –Other techniques -  HBT, conversions Measure direct  vs Reaction Plane in A+A: –q+g Compton isotropic, Thermal flows, Fragmentation enhanced or suppressed with pathlength? Measure  +jet : –The  measures p T of the recoiling parton - measure modified recoil Fragmentation Function to extract parton energy loss directly.