ALICE EMCal Physics and Functional Requirements Overview
Jan 15, 2016
Outline
• Jets in heavy ion collisions• ALICE EMCal overview• Physics reach of EMCal• Major EMCal requirements
Jet quenching at RHIC part I:
ησηddpdT
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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
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
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30
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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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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