the fermi large area telescope as a high-energy electron...

The Fermi LargeArea Telescopeas a high-energy

electrondetector

Luca BaldiniINFN–Pisa

[email protected]

on behalf of the Fermi-LATcollaboration

RICAP, May 13, 2009

The Large Area Telescope

Large Area telescope

I Overall modular design.

I 4× 4 array of identical towers (each one including a tracker and a calorimeter module).I Tracker surrounded by and Anti-Coincidence Detector (ACD)

Luca Baldini (INFN) RICAP, May 13, 2009 2 / 24

The Large Area Telescope

Large Area telescope

I Overall modular design.

I 4× 4 array of identical towers (each one including a tracker and a calorimeter module).I Tracker surrounded by and Anti-Coincidence Detector (ACD)

Tracker

I Silicon strip detectors,W conversion foils; 1.5radiation lengthson-axis.

I 10k sensors, 80 m2 ofsilicon active area, 1Mreadout channels.

I High-precision tracking,short dead time.

Anti-CoincidenceDetector

I Segmented (89 tiles) asto minimize self-veto athigh energy.

I 0.9997 averagedetection efficiency.

Calorimeter

I 1536 CsI(Tl) crystal; 8.6 radiationlengths on-axis.

I Hodoscopic, 3D shower profilereconstruction for leakage correction.


Trigger and filter

I Five hardware trigger primitives (at the tower level).I TKR: three x-y tracker planes hit in a row.I CAL LO: single log with more than 100 MeV.I CAL HI: single log with more than 1 GeV.I ROI: MIP signal in a ACD tiles close to a triggering tower.I CNO: heavy ion signal in the ACD.

I Upon L1 trigger the entire detector is read out.I Need onboard filtering to fit the data volume within the

allocated bandwidth.I GAMMA: rough onboard photon selection.

I All events with raw energy greater than 20 GeV downlinked.I Primary source of high-energy e+ e−.

I HIP: heavy ions for CAL calibration.I DGN: prescaled (×250) unbiased sample of all trigger types.

I Source of low-energy e+ e−, decent statistics up to 100 GeV.

I MIP: straight tracks for alignment (only in dedicated runs).


Fermi sensitivity to cosmic-ray electrons. . . where ”electrons” really means ”electrons and positrons”

sr)⋅2Peak geometric factor (m-210 -110 1 10

Obs

erva

tion

time

(s)

610

710

810

S. Tori

i et al.

2001

PPB-B

ETS

Fermi

(6 mon

ths)

Fermi

(5 year

s)

J. Chan

g et al

. 2008

ATIC

T. Koba

yashi e

t al. 19

99

Emuls

ion ch

amber

s

Gf = Aeff × Ω (after the selection cuts)Nobs = Flux× Gf × Tobs


Fermi sensitivity to cosmic-ray electrons. . . where ”electrons” really means ”electrons and positrons”

sr)⋅2Peak geometric factor (m-210 -110 1 10

Obs

erva

tion

time

(s)

610

710

810

S. Tori

i et al.

2001

PPB-B

ETS

Fermi

(6 mon

ths)

Fermi

(5 year

s)

J. Chan

g et al

. 2008

ATIC

T. Koba

yashi e

t al. 19

99

Emuls

ion ch

amber

s

s)⋅ sr ⋅2 Geo. Factor (m×Obs. Time

410 510 610 710 810 910 1010

Electrons above 100 GeV210 310 410 510 610 710

PPB-B

ETS

Fermi

(6 mon

ths)

Fermi

(5 year

s)

ATIC

Emuls

ion ch

amber

s

Gf = Aeff × Ω (after the selection cuts)Nobs = Flux× Gf × Tobs


Event topology

Candidate electron475 GeV raw energy, 834 GeV reconstructed

I Well defined (not fully contained) symmetricshower in the calorimeter.

I Clean main track with extra clusters close to thetrack (note backsplash from the calorimeter).

I Relatively few ACD tile hits, mainly inconjunction with the track.

Candidate hadron823 GeV raw energy, 1 TeV reconstructed

I Large and asymmetric shower profile in thecalorimeter.

I Small number of extra clusters around maintrack, many clusters away from the track.

I Different backsplash topology, large energydeposit per ACD tile.

Tranverse shower size: 23.2 mmFractional extra clusters: 1.48Average ACD tile energy: 2.46 MeVEnergy reconstruction quality: 0.73

Tranverse shower size: 34.4 mmFractional extra clusters: 0.17Average ACD tile energy: 10.2 MeVEnergy reconstruction quality: 0.15


Event selection

Energy (MeV)510 610

Had

ron

reje

ctio

n po

wer

1

10

210

310

410

qualityquality + calquality + cal + tkrquality + cal + tkr + acdquality + cal + tkr + acd + CT

I Few hundreds top level quantities describing the eventtopology in our summary n-tuples (≈few tens used).

I Developed for the γ analysis, appropriate for electrons.I Three main steps, in which all the subsystems contribute.

I Basic quality cuts (requiring ACD signal to remove gammas)I Event topology in the tracker, calorimeter and ACD.I Classification tree analysis:


Monte Carlo validation with beam tests


Monte Carlo validation with flight dataShower transverse size above 150 GeV

Ent

ries/

bin

0

500

1000

1500 cut va

luee+ e-

hadrons

Shower tranverse size (mm)10 15 20 25 30 35 40 45 50

Ent

ries/

bin

0

500

1000

1500Flight data

Monte Carlo

Ent

ries/

bin

0

500

1000

cut va

luee+ e-

hadrons

Shower tranverse size (mm)10 15 20 25 30 35 40 45 50

Ent

ries/

bin

0

500

1000

Flight data

Monte Carlo

I Data/Monte Carlo comparison routinely performed for:I all variables involved in the selection;I at different stages of the selection.

I Overall good agreement, residual discrepancies propagated tothe spectrum and included into the systematics (more later).


Monte Carlo validation with flight dataCT combined electron probability above 150 GeV

Ent

ries/

bin

0

1000

2000 cut va

lue e+ e-

hadrons

Combined CT electron probability0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Ent

ries/

bin

0

1000

2000

Flight data

Monte Carlo

Ent

ries/

bin

0

200

400

600

800

cut va

lue e+ e-

hadrons

Combined CT electron probability0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Ent

ries/

bin

0

200

400

600

800 Flight data

Monte Carlo

I Two different CT ensembles (based on TKR and CAL).I Each one providing an event based electron probability.

I Combined with the general (energy-dependent) scheme

pcomb = k√

ptkr · pcal/(log E − log E0)


Energy resolution: prelude

Total traversed X00 5 10 15 20 25 300

100

200

300

310×

I The LAT calorimeter is 8.6 X0 deep on axis.I The tracker adds up 1.5 X0 of material on axis.

I 1.5 X0 of finely segmented active material, providing additionalrejection power.

I The LAT is a wide-field-of-view instrument.I Candidate electrons traverse ≈ 12.5 X0 on average.


Energy resolution

Energy (GeV) 10 210 310

E/E

∆F

ull w

idth

0

0.2

0.4

0.6

LAT 95%LAT 68%

) °Beam Test MC 68% (60) °Beam Test Data 68% (60

) °Beam Test MC 68% (0) °Beam Test Data 68% (0

LAT 95%

LAT 68%

)°Beam Test 68% (0

)°Beam Test 68% (60

I Validated with the Calibration Unit beam test up to 280 GeV.I Excellent agreement over the whole (energy, angle, position)

phase space.I We have a solid ground in extrapolating to 1 TeV.

I Our energy dispersion is adequate for the measurement.


Sources of systematic errors

I Uncertainty in our knowledge of the geometry factor.I Data/Monte Carlo agreement extensively studied for each

single variable involved in the selection (bin by bin).I All the residual discrepancies mapped and propagated to the

actual spectrum.I Ranging from a few % to ' 20% depending on energy.

I Normalization of the primary proton spectrum.I Affecting the electron spectrum through the subtraction of the

residual hadron contamination

I LAT absolute calibration of the energy scaleI Unlike the other terms does not introduce energy-dependent

modifications of the spectrum.I From beam test data, calibration and flight data, the

systematic uncertainty on the absolute energy is (+5%, -10%)


Evaluation of the systematic uncertainties

Selection variable (a. u)

p. d

. f

− e+e had

rons

Data1c2c3c


p. d

. f

− e+e had

rons

Monte Carlo1c2c3c

Energy (GeV)

210 310

Eve

nt r

ate

(a. u

.)

1c2c

3c −

Energy (GeV)210 310

Bac

kgro

und

rate

(a.

u.)

1c2c

3c

/

Energy (GeV)210 310

Geo

met

ry fa

ctor

(a.

u.)

1c2c

3c

=

Energy (GeV)210 310

J (

a. u

.)× 3

E

3, c

2, c1c

Evaluating the systematics

I If the data/MC agreement was perfect, theactual spectrum would not depend on thecut values.




p. d

. f

− e+e had

rons

Data1c2c3c


p. d

. f

− e+e had

rons

Monte Carlo1c2c3c

Energy (GeV)

210 310

Eve

nt r

ate

(a. u

.)

1c2c

3c −

Energy (GeV)210 310

Bac

kgro

und

rate

(a.

u.)

1c2c

3c

/

Energy (GeV)210 310

Geo

met

ry fa

ctor

(a.

u.)

1c2c

3c =

Energy (GeV)210 310

J (

a. u

.)× 3

E 1c2c3c


I In real life data/MC discrepancies introducesuch a dependence.




p. d

. f

− e+e had

rons

Monte Carlo1c2c3c

Energy (GeV)210 310

J (

a. u

.)× 3

E 1c2c3c


I The induced variations in the spectrumeffectively map the data/MC discrepancies.

I Exercise carefully performed for each energybin and for each single variable involved inthe selection (fwf % to ≈ 20% dependingon energy).

I Event selection is a trade-off betweenelectron efficiency, hadron rejection andcontrol of systematic effects.


Conclusions

I The Fermi LAT Area Telescope is a powerful high-energyelectron detector

I Unprecedented geometric factor and observation time in themulti-100 GeV energy range.

I Hadron rejection power and energy reconstruction are the keyissues.

I Very detailed Monte Carlo simulation framework including allthe relevant aspects of the detector.

I Extensively validated at beam tests.I Extensively validated with flight data.I Background rejection and energy resolution under control and

adequate for the measurement.

I The Fermi LAT is able to provide a measurement of the CRelectron spectrum up to 1 TeV only limited by systematics.

I Actual results and interpretation after the coffe break!


Conclusions

Spare slides


Energy resolution: validation with beam test

Layer number0 1 2 3 4 5 6 7

En

erg

y p

eak

(MeV

)

200

400

600

800

1000

1200

Energy Profile (Beam P = 10 GeV/c, Theta = 0)


En

erg

y p

eak

(MeV

)

500

1000

1500

2000

2500



En

erg

y p

eak

(MeV

)

0

1000

2000

3000

4000

5000

6000



En

erg

y p

eak

(MeV

)

0

2000

4000

6000

8000

10000

12000



En

erg

y p

eak

(MeV

)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

22000

24000



En

erg

y p

eak

(MeV

)

0

5000

10000

15000

20000

25000

30000


10 GeV on axis 20 GeV on axis




Shower profile: Monte Carlo vs. flight dataAfter the electron selection, integrated over all angles

Layer number0 1 2 3 4 5 6 7 8

Ave

rage

laye

r en

ergy

(G

eV)

0

20

40

60

80

100

Monte CarloFlight data

Measured energy: 246−−291 GeV

Layer number0 1 2 3 4 5 6 7 8

Ave

rage

laye

r en

ergy

(G

eV)

0

20

40

60

80

100



Layer number0 1 2 3 4 5 6 7 8

Ave

rage

laye

r en

ergy

(G

eV)

0

20

40

60

80

100



Layer number0 1 2 3 4 5 6 7 8

Ave

rage

laye

r en

ergy

(G

eV)

0

20

40

60

80

100




Shower profile: flight dataAfter the electron selection, integrated over all angles

Layer number0 1 2 3 4 5 6 7 8

Ave

rage

laye

r en

ergy

(G

eV)

0

20

40

60

80

210−−246 GeV 246−−291 GeV

291−−346 GeV346−−415 GeV

415−−503 GeV

503−−615 GeV

615−−772 GeV

772−−1000 GeV

I Showers of different energies look different in the detectors(i.e. can be distinguished).

I The shower maximum at 1 TeV is at 11.5 X0 (candidateelectrons traverse ≈ 12.5 X0).


Energy resolution and spectral features

Energy (GeV)210 310

)2

GeV

-1 s

r-1

s-2

J(E

) (m

× 3

E

210

ATIC (2008)Fermi (2009)Model, no smear

)σE/E = 12% (1 ∆Model, )σE/E = 25% (1 ∆Model,

I Model adapted from Chang et al. 2008:I broken power law with Γ = −3.1 below 1 TeV, −4.5 above;I harder (Γ = −1.5) feature with break at 620 GeV.

I 12% is a conservative estimation for Fermi in the 100s GeV.


Energy reconstruction quality

Measured energy/true energy0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

200

400

600

800

301−−412 GeV

I Probability of good energy reconstruction: diagnostic outputof our energy analysis.

I A CT is trained to identify events in the core of the energydispersion.


Energy reconstruction quality

0

50

100

e+ e-

hadrons

Good energy reconstruction probability0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

50

100

Flight data

Monte Carlo

0

50

100e+ e-

hadrons

Good energy reconstruction probability0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

50

100 Flight data

Monte Carlo

I Distribution of the probability of good energy reconstructionprovided by the standard energy classification tree analysis.

I Events above 400 GeV at two different stages of the selection.


Energy resolution and spectral features


The observatory

Large Area Telescope (LAT)

I Huge field of view:

I 20% of the sky at any time;I all parts of the sky for 30 minutes

every 3 hours.

I Long observation time:

I 5 years minimum lifetime (10planned);

I 85% duty cycle (SAA).

I Great potential to tag electrons in themulti-100 GeV range:

I advanced particle detectors;I all events above 20 GeV sent to

ground.

Gamma-ray Burst Monitor (GBM)

I 12 NaI and 2 BGO detectors.

I Energy range: 8 keV–30 MeV.


The launch

Launch

I Launched on June 11, 2008 from the Kennedy Space Center.

I Launch vehicle: Delta 2920H-10.

I Circular orbit, 565 km altitude, 25.6◦ inclination.


the fermi large area telescope as a high-energy electron...

Documents