Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
The Fermi LargeArea Telescopeas a high-energy
electrondetector
Luca BaldiniINFN–Pisa
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.
Luca Baldini (INFN) RICAP, May 13, 2009 2 / 24
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).
Luca Baldini (INFN) RICAP, May 13, 2009 3 / 24
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
Luca Baldini (INFN) RICAP, May 13, 2009 4 / 24
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
Luca Baldini (INFN) RICAP, May 13, 2009 4 / 24
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
Luca Baldini (INFN) RICAP, May 13, 2009 5 / 24
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:
Luca Baldini (INFN) RICAP, May 13, 2009 6 / 24
Monte Carlo validation with beam tests
Luca Baldini (INFN) RICAP, May 13, 2009 7 / 24
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).
Luca Baldini (INFN) RICAP, May 13, 2009 8 / 24
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)
Luca Baldini (INFN) RICAP, May 13, 2009 9 / 24
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.
Luca Baldini (INFN) RICAP, May 13, 2009 10 / 24
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.
Luca Baldini (INFN) RICAP, May 13, 2009 11 / 24
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%)
Luca Baldini (INFN) RICAP, May 13, 2009 12 / 24
Evaluation of the systematic uncertainties
Selection variable (a. u)
p. d
. f
− e+e had
rons
Data1c2c3c
Selection variable (a. u)
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.
Luca Baldini (INFN) RICAP, May 13, 2009 13 / 24
Evaluation of the systematic uncertainties
Selection variable (a. u)
p. d
. f
− e+e had
rons
Data1c2c3c
Selection variable (a. u)
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
Evaluating the systematics
I In real life data/MC discrepancies introducesuch a dependence.
Luca Baldini (INFN) RICAP, May 13, 2009 13 / 24
Evaluation of the systematic uncertainties
Selection variable (a. u)
p. d
. f
− e+e had
rons
Monte Carlo1c2c3c
Energy (GeV)210 310
J (
a. u
.)× 3
E 1c2c3c
Evaluating the systematics
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.
Luca Baldini (INFN) RICAP, May 13, 2009 13 / 24
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!
Luca Baldini (INFN) RICAP, May 13, 2009 14 / 24
Conclusions
Spare slides
Luca Baldini (INFN) RICAP, May 13, 2009 15 / 24
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)
Layer number0 1 2 3 4 5 6 7
En
erg
y p
eak
(MeV
)
500
1000
1500
2000
2500
Energy Profile (Beam P = 20 GeV/c, Theta = 0)
Layer number0 1 2 3 4 5 6 7
En
erg
y p
eak
(MeV
)
0
1000
2000
3000
4000
5000
6000
Energy Profile (Beam P = 50 GeV/c, Theta = 0)
Layer number0 1 2 3 4 5 6 7
En
erg
y p
eak
(MeV
)
0
2000
4000
6000
8000
10000
12000
Energy Profile (Beam P = 100 GeV/c, Theta = 0)
Layer number0 1 2 3 4 5 6 7
En
erg
y p
eak
(MeV
)
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
22000
24000
Energy Profile (Beam P = 200 GeV/c, Theta = 0)
Layer number0 1 2 3 4 5 6 7
En
erg
y p
eak
(MeV
)
0
5000
10000
15000
20000
25000
30000
Energy Profile (Beam P = 280 GeV/c, Theta = 0)
10 GeV on axis 20 GeV on axis
50 GeV on axis 100 GeV on axis
200 GeV on axis 280 GeV on axis
Luca Baldini (INFN) RICAP, May 13, 2009 16 / 24
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
Monte CarloFlight data
Measured energy: 346−−415 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
Monte CarloFlight data
Measured energy: 503−−615 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
Monte CarloFlight data
Measured energy: 772−−1000 GeV
Luca Baldini (INFN) RICAP, May 13, 2009 17 / 24
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).
Luca Baldini (INFN) RICAP, May 13, 2009 18 / 24
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.
Luca Baldini (INFN) RICAP, May 13, 2009 19 / 24
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.
Luca Baldini (INFN) RICAP, May 13, 2009 20 / 24
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.
Luca Baldini (INFN) RICAP, May 13, 2009 21 / 24
Energy resolution and spectral features
Luca Baldini (INFN) RICAP, May 13, 2009 22 / 24
Energy resolution and spectral features
Luca Baldini (INFN) RICAP, May 13, 2009 22 / 24
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.
Luca Baldini (INFN) RICAP, May 13, 2009 23 / 24
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.
Luca Baldini (INFN) RICAP, May 13, 2009 24 / 24