A new measurement of the Cosmic-Ray Electron and Positron Spectrum with the Large Area Telescope Alberto Manfreda Universit` a and INFN–Pisa [email protected] on behalf of the Fermi-LAT collaboration Pisa, October 17, 2016
A new measurementof the Cosmic-Ray
Electron andPositron Spectrum
with the LargeArea Telescope
Alberto ManfredaUniversita and INFN–Pisa
on behalf of the Fermi-LATcollaboration
Pisa, October 17, 2016
More than an update
Previous Fermi measurement of the spectrum from 7 GeV to 1 TeVpublished in 2010 [Ackermann et al., Phys. Rev. D 82, 092004]
The analysis is new in many respectsI Almost seven times the amount of data availableI Completely revised event reconstruction provided by the Fermi
collaboration (Pass 8)I Upper energy range extended to 2 TeVI Improved selection algorithmsI Better understanding of the environment in which the LAT
operatesI Better modelling of the systematic uncertaintiesI New detailed study of the geomagnetic field effects in the GeV
energy rangeAlberto Manfreda (UNIPI) Pisa, October 17, 2016 2 / 14
Fermi LAT
I Launched by NASA on June 2008 at 565 km altitude, 25.6◦ inclination
I Large field of view (20% of the sky), full sky observed every ∼ 3 h
I Designed to detect EM showers, naturally including e+ and e−
Tracker/Converter (TKR)I 18 x-y planes of silicon
strip detectorsI ∼ 10 k sensors,
∼ 80 m2 of siliconactive area, 1M readoutchannels
Anti-Coincidence Detector (ACD)I Segmented (89 tiles) as to
minimize self-veto at high energyI 0.9997 average MIP detection
efficiency
Calorimeter (CAL)I 1536 CsI(Tl) crystal (96 per tower); 8.6
radiation lengths on-axisI Hodoscopic, 3D shower profile
reconstruction for leakage correction
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 3 / 14
Datasets
Two slightly different analyses, for two different datasets:I High Energy (HE): 42 GeV - 2 TeV using events from the
standard on-board filterAccept all events with more than 20 GeV released in the CAL(fully efficient for electrons above ∼ 40 GeV)
I Low Energy (LE): 7 GeV - 70 GeV using events from thediagnostic (DGN) filterUnbiased sample of all triggers, pre-scaled by a factor 250
Overall live time is 4.68 years - outside SAA and in standard opera-tion modesField of view reduced to ≤ 60◦ from the instrument zenith (avoidEarth limb photons contamination).
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 4 / 14
CRE selection
Energy [GeV]210 310
sr]
2Ac
cept
ance
[m
0
1
2
3
4
5
PRELIMINARY
Trigger & FilterTrack found & energy
Minimal PSF quality
in the CAL0> 8 X
Alpha removal
I Event-quality cuts (good direction and energy reconstruction)I Removal of particles with Z > 1, tagged by ionization in ACD
and TKRI Boosted Decision Trees (BDTs) selection, for proton rejectionI LE: additional orbital selection for geomagnetic field effects
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 5 / 14
Proton removal selection
I Exploits differences betweenleptonic and hadronic eventsin the detector:
I Shower development inthe CAL
I Number of δ-raysproduced in the TKR
I A few variables have beenindividually rescaled, toimprove their data/MCagreement
Shower transverse size10 20 30 40 50 60 70 80 90 100
Rat
e [H
z]0
0.001
0.002
0.003
0.004
= 0.44--1.00]θ[112.2--144.5 GeV, cos
±MC r/w eMC r/w pMC sum
PRELIMINARY
Example:
I The BDT combines all the information into a single estimatorof ’leptonicity’ (PCRE ).
I 8 BDTs used for HE analysis, optimized for different energyranges (plus 1 for LE)
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 6 / 14
BDT output
CREP-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0
Cou
nts
0
100
200
300
400
500
600
700
800
900
1000
Data
MC total
MC electron
MC protonPRELIMINARY
[866 − 1000] GeV
I Cut on PCRE optimized in each energy binI Normalization of Monte Carlo templates fitted to data for residual
background estimation
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 7 / 14
Performance
I Selection increasingly difficult at higher energyI Contamination below 20% in the whole energy range
E [GeV]10 210 310
sr]
2A
ccep
tanc
e [m
0
0.5
1
1.5
2
2.5
3
Res
idua
l con
tam
inat
ion
0
0.1
0.2
0.3
0.4
0.5
0.6LE acceptance (x 250)
HE acceptance
LE contamination
HE contamination
PRELIMINARY
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 8 / 14
LE Orbital Selection
McIlwain L and rigidity cut-off values across the LAT orbit
McIlwain L1 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Live
time
frac
tion
0
0.2
0.4
0.6
0.8
1
1.2
7.0 GeV7.8 GeV8.7 GeV9.7 GeV10.8 GeV12.0 GeV
13.3 GeV
14.8 GeV16.4 GeV
PRELIMINARY
I GeV range: shielding effect of theEarth magnetic field
I LAT orbit: 0.98 - 1.73McIlwain L, vertical rigiditycut-off from ∼ 6 GeV to∼ 14 GeV.
I Energy dependent selection: ineach bin accept only orbitalregions where the whole bin isabove the cut-off
I Optimized directly on data, byfitting the count spectrum inseveral McIlwain L bins
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 9 / 14
Geomagnetic correction
I Cut-off is smooth: residualparticle loss even after theselection
I Recovered with a tracingtechnique: simulated realistic fluxof e+ + e− in the LAT, tracedback in a model of the Earthmagnetic field
I Estimated correction factors(up to 40%) applied to theobserved flux
I Separate corrections for first yearof data (due to rocking angleincrease from 35 to 50 degree in2009)
Top view
10 GV/c e-
Top view
10 GV/c e+
Top view
25 GV/c e-
Top view
25 GV/c e+
longitude°0E
W
longitude°90WE
longitude°180W
E
longitude°270EW
Top view
(vertical cutoff)10.99 GV/c e-
longitude°0E
W
longitude°90WE
longitude°180W
E
longitude°270EW
Top view
(vertical cutoff)11.66 GV/c e+
longitude°0E
W
longitude°90WE
longitude°180W
E
longitude°270EW
forbidden
forbidden
allowed
allowed
E [GeV]8 9 10 20 30 40 50 60 70
Cor
rect
ion
fact
or
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
1.4nominal cut-offnominal cut-off + 30%
year, nominal cut-offst1 year, nominal cut-off + 30%st1
PRELIMINARY
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 10 / 14
Systematics uncertaintiesI Acceptance uncertainty:
I Studied with a scan on the selection cut
I Residual background uncertainty:I Given by MC simulation uncertainties in reproducing the most
electron-like hadronic showers.
I Energy scale bias uncertainty:I At 10 GeV given by comparison of observed CRE cut-off with
tracer predictionI At 1 TeV study of data/MC disagreement of key energy
reconstruction variables
I Individual variable calibration uncertainty (HE analysis):I Estimated by bracketing with different sets of corrections
I Geomagnetic corrections uncertainty (LE analysis):I Studied by varying the orbital selection and recomputing the
corrections
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 11 / 14
Results: LE spectrum
E [GeV]7 8 9 10 20 30 40 50 60 70
]2 G
eV-1
sr
-1 s
-2 In
tens
ity [m
× 3E
60
80
100
120
140
160
180
200
220
240
260
Fermi HE (2010)
Fermi LE (2010)
AMS-02 (2014)
Fermi Pass 8 - LE selection
PRELIMINARY
I Significant difference wrt the previous Fermi publicationI Geomagnetic correction (new)I ’Ghost’ signals effect not included in previous simulation
(corrected in Pass 8)
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 12 / 14
Results: HE spectrum
I Shaded band is sum in quadrature of systematic uncertaintiesI Energy scale uncertainty not includedI Spectrum is harder than that reported by AMS-02I Our systematic errors do not allow to confirm or exclude the cut-off
observed by H.E.S.S.Alberto Manfreda (UNIPI) Pisa, October 17, 2016 13 / 14
Conclusions
I New measurement of the CRE spectrum, pushing the FermiLAT to its limits in all metrics
I New event-level analysis (Pass 8)
I Energy range extended to 2 TeV (first direct measurementabove 1 TeV)
I Better handling of the geomagnetic effects in the evaluationof the low-energy spectrum
I Better characterization of the systematic uncertainties
I Paper submitted to the journal (stay tuned)
I CRE selection will be used for anisotropies study update,currently under publication (again, stay tuned!)
Alberto Manfreda (UNIPI) Pisa, October 17, 2016 14 / 14