1. ALICE EMCal Physics and Functional Requirements Overview

2. Outline Jets in heavy ion collisions ALICE EMCal overview Physics reach of EMCal Major EMCal requirements

3. Jets in heavy ion collisions

4. Jet quenching at RHIC part I: Binary collision scaling p+p Hadrons are suppressed, photons are not

5. Jet quenching at RHIC part II: recoil hadron suppression cos(  ) p T assoc > 0.15 GeV STAR, Phys Rev Lett 95, 152301 4< p T trig < 6 GeV STAR, Phys Rev Lett 91, 072304 p T assoc > 2 GeV Recoil jet is softened and broadened trigger recoil ?

6. Jet Quenching in ALICE Jet quenching: marked softening and broadening of jet structure, correlations RHIC data: strong modification of flavor composition in jet fragmentation How does (soft) medium respond to energy loss? LHC: need measurements over broad kinematic range (few hundred MeV  50 - 100 GeV), extensive PID ALICE: Optimized for heavy ion environment Superb tracking and PID (100 MeV  50+ GeV) Missing from ALICE baseline: large acceptance EM calorimeter for jet triggering and reconstruction

7. ALICE EMCal 10+1/2+1/2=11 super-modules 8 SM from US 3 SM from Europe

8. ALICE EMCal Lead-scintillator sampling calorimeter |  |<0.7,  ~110 o Shashlik geometry, APD photosensor ~13K towers (  x  ~0.014x0.014) Resolution ~ 10%/Sqrt(E)

9. Major physics capabilities of EMCal The EMCal significantly extends the scope of the ALICE experiment for jet quenching measurements in heavy ion collisions: 1.Fast trigger: yields to tape enhanced by factor ~10-100 2.Improved jet reconstruction 3.Good   discrimination 4.Good electron/hadron discrimination

10. EMCal Physics reach Criteria: 10 4 /year in minbias Pb+Pb: Statistical reach: inclusive jets: E T ~200 GeV dijets: E T ~170 GeV   : p T ~75 GeV inclusive  : p T ~45 GeV inclusive e: p T ~30 GeV

11. Jets reconstruction in heavy ion collisions Heavy ions: large background from underlying event Control background by limiting jet cone radius R~0.3-0.4, track p T cut  measure a fraction of partonic energy Cone radius R=sqrt(  2 +  2 ) pTpT

12. How to reconstruct jets in HI environment: Optimal cone size Jets reconstructed from charged particles: Need reduced cone sizes and transverse momentum cut ! Energy contained in sub-cone R Background: E ~ R 2 1.5 TeV in cone of R = 1 85% of jet energy Jets can be reconstructed using reduced cone size

13. Areal coverage [super modules] Relative Jet Acceptance Versus Module Count and Jet Radius Good acceptance for all relevant jet cone radii

14. Main EMCal physics requirements Acceptance (already discussed) Trigger Tower granularity Energy resolution

15. Jet trigger in p+p p+p in ALICE: L~ 5x10 30 /cm 2 /s ~ 200 kHz ALICE DAQ limits recording to 200 Hz  L1 rejection ~2000-4000 for p+p Sharp threshold at required L1 rejection Jet Patch 0.25x0.25 Trigger efficiency Jet energy (GeV)

16. Jet trigger in Pb+Pb good efficiency above ~75 GeV large patch needed for unbiased quenched-jet trigger L1 rejection ~10 High Level Trigger rejection ~20 Level 1 trigger efficiency in Pb+Pb

17. Very small trigger bias versus quenching physics model. Trigger efficiency does not depend strongly on the details of fragmentation softening Trigger bias versus physics …..

18. Tower granularity:    discrimination  +jet: calibration of jet energy  precise measurement of modified fragmentation function X.-N. Wang et al., PRL 77, 231 (1996)   measured in EMCal (factor 8 larger acceptance than PHOS) fragmentation function from inclusive measurements of recoil in TPC ALICE kinematic reach extended to p T  ~30-40 GeV/c

19.   discrimination in EMCal single-cluster efficiency   ratio High p T : use shower shape to discriminate one shower from two merged showers Discrimination in Pb+Pb where cross section is large (~30 GeV/c)  drives tower granularity

20. Energy resolution EMCal resolution: modest requirement < (15/  E+2)% much better than achieved jet energy resolution ~25% More stringent physics requirement: hadron rejection for electron PID Significant electron yield to p T ~25 GeV/c with e/  ~0.01 EMCal provides electron trigger Dominant contribution from heavy quark jets (estimate E T jet to 50 GeV)  basic test of energy loss: color-charge dependence (Wiedemann et al)

21. Electron/hadron discrimination Geant simulation with all ALICE materials Based on E/p from EMCal/tracking and shower-shape e h E/p 1/pion efficiency 10 3 electron efficiency 20 GeV Rejection >10 2 is sufficient for robust electron PID

22. Material upstream of EMCal

23. Summary ALICE is the primary heavy ion experiment at the LHC capabilities addressing all aspects of heavy ion physics EMCal+ALICE: Enables unique measurements of jet quenching at the LHC high sensitivity to quenching over broad kinematic range (~200 GeV) Trigger enhancement factors ~10-100 Unbiased jet reconstruction Excellent measurements of high p T    and electrons