EMCal Jet Trigger Analysisfor ALICE*

Christopher Anson

Creighton University

*Supported by the U.S. DOE Office of Science

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Introduction

The goals of this study are to…• Investigate trigger properties starting with event

simulations + simple assumptions about detector.• Compare conclusions with other results starting with

advanced simulations of detector response.• Investigate the underlying physics and behavior of

triggers.

By using…• 2 million jet events with AliPythia• 1000 background events with HIJING Pb+Pb

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Outline

I. Jet Triggers with Pythia Jets

a) Leading 0 trigger

b) Cone trigger

c) Patch trigger

II. Patch Triggers with Pythia Jets and HIJING Background

a) Jet vs. background energy in patches

b) Centrality dependence

III. Patch Triggers with Rate Requirement Introduced

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Trigger Requirements

• Reduce data rate into higher level trigger• Efficient jet selection at lowest possible energy• Use most efficient patch size

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An EMC for ALICE

inclusive jets10 Hz @ 50 GeV

few x 104/year for ET>150 GeV

From Peter Jacobs

Interaction Rate ~ 4 kHz

Into High Level Trigger ~100 Hz

Data to tape rate ~ 100 Hz

Need 100% efficiency above ~ 100 GeV

And enhancement at 50 GeV

Must reduce data rate by 10-50 times

Rates at ALICE

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Leading 0 Trigger

Investigating efficiency for different cuts

Conclusions:

•Increasing cut reduces efficiency for high energy jets.

•Some higher energy jets have a low energy leading 0.

Cut Energy

•2 GeV cut

•4 GeV cut

•6 GeV cut

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Cone is around Pythia jet axis

100% e+,e-, energy

25% hadron energy

Conclusions:

•Reduced efficiency for smaller cones

•Sometimes jet energy is not centralized near Pythia defined jet axis

Cone Radius

• R = 0.05

• R = 0.10

• R = 0.20

• R = 0.30

• R = 0.40

22 ddR

Cone Trigger with Different Cone Radii

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~ 0.05 ~ 0.05

Patch Trigger Slides Across the DetectorIn reality the smallest 2x2 Tower units are ~ 0.028x0.028

Smallest patches I use are 0.05x0.05

Larger patches are built from summing the smaller patches

The patches looked at in my study are:

0.05x0.05 = 1x1

0.10x0.10 = 2x2

0.15x0.15 = 3x3

0.20x0.20 = 4x4

0.25x0.25 = 5x5

x ~ 0.15x0.15

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10 GeV cut

x = 0.15x0.15 patch

Cone has equal area

Conclusions:

•Sometimes jet energy is not centralized near Pythia defined jet axis

•Cone trigger has poor efficiency

Patch Trigger vs Cone Trigger

Trigger Type

•0.15x0.15 Patch Trigger

• Cone Trigger (Same Area)

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Patch Size Dependance

Energy is summed in dηxdφ patches

Cuts produce 50% efficiency at fixed 72 GeV to investigate behavior of trigger

Conclusion:

•Patch trigger efficiency is independent of patch size

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Patch Size Dependance

Pythia + assumption of 25% hadron energy detected

Agrees with full GEANT simulation

Cuts produce 50% efficiency at 72 GeV to investigate behavior of trigger

Conclusion:

•Patch trigger efficiency is independent of patch size

*Full GEANT simulation: Bill Mayes, Houston

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1x1 patch trigger doesn’t reduce to leading π0 trigger

Turning off the energy deposited by hadrons…

Trigger Type

•0.05x0.05 Patch Trigger

• Leading π0 Trigger

Comparing Patch Trigger and Leading 0 Trigger

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With energy in patch only due to e+,e-, and energy…

Now the two curves are similar

Conclusions:

•Hadronic energy is significant

•Efficiency increases as more hadron energy is deposited

Trigger Type

•0.05x0.05 Patch Trigger

• Leading π0 Trigger

Comparing Patch Trigger and Leading 0 Trigger

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Summary using just Pythia

• Leading 0 trigger efficiency decreases with larger cuts (6 GeV).

• Cone trigger is inefficient.- (Energy not always near jet axis).

• Patch trigger is most efficient.• Small patches as efficient as large patches.• Hadronic energy contribution enhances efficiency.

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Central HIJING Pb+Pb collisions

Background increases monotonically with patch size

Jet energy levels off with patch size

Background decreases for peripheral collisions

Conclusions:

•Background comparable to jet energy for central collisions

•Need centrality dependent trigger (Agrees with Peter Jacobs and Andre Mischke’s conclusion)

Comparing Jet Energy to Background in Patches

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Patch Trigger with Rates

The cuts here select 1/10 events (about what is needed in the Level 1 trigger)

NOTE: This is with central HIJING and should be redone with min-bias HIJING. Central HIJING may give a worst case scenario.

Conclusion:

•Only smaller patch is less efficient

•Larger patches make the trigger more robust against fluctuations

This graph is also consistent with the Full GEANT simulation done by Bill Mayes.

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Summary

• Patch trigger is most efficient trigger for the EMCal.• For p-p jets, efficiency is independent of patch size.• For p-p + background + meeting required rate,

larger patches are efficient.• (smallest patch is not).

• Centrality dependent higher level trigger is required– (due to decreasing background with decreasing centrality).

For more information refer to pages listed at

http://pdsfweb01.nersc.gov/~canson/HijingHTMLStuff.html

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Backup Slides

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Increasing the Cut on the Energy in a Cone shifts the efficiency curve downwards for smaller patch energies.

Larger cuts eliminate more low energy AND high energy jets.

Cone Radius

• R = 0.05

• R = 0.10

• R = 0.20

• R = 0.30

• R = 0.40

Cone Radius

• R = 0.05

• R = 0.10

• R = 0.20

• R = 0.30

• R = 0.40

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Patch Trigger with Rates

Only Pythia Jets

25% Hadron Energy contributed

The reduction in rate is estimated by dividing the integrated jet spectra with a cut by that without a cut

The cuts here select 1/50 of the events

Conclusion:

•Patch trigger is still efficient even for small patch sizes

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Calculation of Trigger Rate

1) Project jets above “cut Et” and count how many (integrate).

2) Project jets with higest patch Et above 0 gev and count how many.

3) Divide number in Step 1 by number in Step 2.

4) This give the number of events selected.

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Smallest towers

x ~ 0.02x0.02