ALICE EMCal Physics and Functional Requirements Overview

Outline

• Jets in heavy ion collisions• ALICE EMCal overview• Physics reach of EMCal• Major EMCal requirements

Jets in heavy ion collisions

Jet quenching at RHIC part I:

ησηddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

=

Binary collision scaling

p+p

Hadrons are suppressed, photons are not

Jet quenching at RHIC part II:recoil hadron suppression

cos()

pTassoc > 0.15 GeV

STAR, Phys Rev Lett 95, 152301

4< pTtrig < 6 GeV

STAR, Phys Rev Lett 91, 072304

pTassoc > 2 GeV

Recoil jet is softened and broadened

trigger

recoil

?

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

ALICE EMCal

10+1/2+1/2=11 super-modules8 SM from US3 SM from Europe

ALICE EMCal

Lead-scintillator sampling calorimeter|η|<0.7, ~110o

Shashlik geometry, APD photosensor~13K towers (ηx~0.014x0.014)Resolution ~ 10%/Sqrt(E)

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 ~10-1002. Improved jet reconstruction3. Good discrimination4. Good electron/hadron discrimination

EMCal Physics reach

Criteria: 104/year in minbias Pb+Pb:

Statistical reach:inclusive jets: ET~200 GeV

dijets: ET~170 GeV

: pT~75 GeV

inclusive : pT~45 GeV

inclusive e: pT~30 GeV

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 pT cut measure a fraction of partonic energy

Cone radius R=sqrt(2+η2)

pT

How to reconstruct jets in HI environment: Optimal cone size

Jets reconstructed from charged particles:

Need reduced cone sizes and transverse momentum cut !

Ene

rgy

cont

aine

d in

sub

-co

ne R

Background: E ~ R2

1.5 TeV in cone of R = 1

85% of jet energy

Jets can be reconstructed using reduced cone size

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11

number of 10 degree segments

relative acceptance

R=0.2

R=0.3

R=0.4

R=0.5

Areal coverage [super modules]

Relative Jet Acceptance Versus Module Countand Jet Radius Good

acceptance for all relevant jet cone radii

Main EMCal physics requirements

• Acceptance (already discussed)• Trigger• Tower granularity• Energy resolution

Jet trigger in p+p

p+p in ALICE: L~ 5x1030/cm2/s ~ 200 kHzALICE DAQ limits recording to 200 Hz

L1 rejection ~2000-4000 for p+p

Sharp threshold at required L1 rejection

Jet Patch 0.25x0.25

Tri

gg

er

eff

icie

nc

y

Jet energy (GeV)

Jet trigger in Pb+Pb

• good efficiency above ~75 GeV• large patch needed for unbiased quenched-jet trigger

L1 rejection ~10High Level Trigger rejection ~20

Level 1 trigger efficiency in Pb+Pb

Very small trigger bias versus quenching physics model.

Trigger efficiency does not depend strongly on the details of fragmentation softening

Trigger bias versus physics …..

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 pT

~30-40 GeV/c

discrimination in EMCalsingle-cluster efficiency ratio

High pT: 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

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 pT~25 GeV/c with e/~0.01

EMCal provides electron trigger

Dominant contribution from heavy quark jets (estimate ET

jet to 50 GeV) basic test

of energy loss: color-charge dependence (Wiedemann et al)

Electron/hadron discrimination

• Geant simulation with all ALICE materials• Based on E/p from EMCal/tracking and shower-shape

e

h

E/p

1/pi

on e

ffic

ienc

y

103

electron efficiency

20 GeV

Rejection >102 is sufficient for robust electron PID

Material upstream of EMCal

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are needed to see this picture.

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 pT η and electrons