Post on 13-Jun-2020
Distinctive Properties of Cosmic Positrons and Electrons Measured by AMS on ISS
Cheng ZHANG / IHEP on behalf of AMS Collaboration
Daejeon, Korea 28 August 2018
Electrons and Positrons in the Cosmos
AMS
p, He
𝒆+
𝒆+, 𝒆−
𝒆−
2
• Electrons are produced and accelerated in SNR together with proton, Helium. They are primary cosmic rays that travel through the galaxy and detected by AMS.
• These particles interact with the interstellar matter and produce secondary source of anti-particle: positrons etc. They are much less abundant in astrophysics process.
• New physics sources like Dark Matter produce both particles and antiparticles.
Measuring antiparticles are much more sensitive to Dark Matter
M. Turner and F. Wilczek, Phys. Rev. D42 (1990) 1001; J. Ellis, 26th ICRC (1999)
Trac
ker
1
2
7-8
3-4
9
5-6
TRD: Identify e+, e-, Z
Silicon Tracker: Z, P
ECAL: E of e+, e-RICH: Z, E
TOF: Z, EParticles and nuclei are defined
by their charge (Z) and energy (E or P)
Z and Pare measured independently by the
Tracker, RICH, TOF and ECAL
AMS: a unique TeV precision, magnetic spectrometer in space
Magnet: ±Z
3
0 0.5 1 1.5 2 2.5 3
tracker/PECALE
0
0.02
0.04
0.06
No
rma
lize
d e
ntr
ies
Protons
Electrons
1
5
6
3
4
7
8
9
2
TRD
ECAL
RICH
MA
GN
ET
TOF
TOF
Silicon Tracker and Magnet
1.4 kG
L1 to L9: 3 m level arm;single point resolution 10 μm;
Maximum Detectable Rigidity(MDR) 2.0 TV for Z=1 4
Unique feature of AMS
5Momentum [GeV/c]1 10 100 1000
Pro
ton
Re
jecti
on
1
10
210
310
410
510
pro
ton
Re
ject
ion
• Proton rejection 103 to 104
with TRD
• Proton rejection is above 104 with ECAL and tracker
• TRD and ECAL is separated by magnet, they have independent proton rejection
Positron identification in AMS
Calibration of the AMS Detector
2000 positions
Test beam at CERN SPS: p, e±, π±, 10−400 GeV
Computer simulation:Interactions, Materials, Electronics
12,000 CPU cores at CERN
AMS27 km
7 km
6
The measurement of electrons and positrons in AMS
Primary cosmic ray particle:
• E>1.2∙max cutoff
TOF:
• Down-going particle β>0.8
• Charge |Z|=1 particle
TRD:
• Provide proton rejection
tracker and magnet:
• Provide accurate momentum measurement
• Charge |Z|=1 particle
ECAL:
• Provide accurate energy measurement.
• Provide proton rejection with 3D shower
shape
7
A 960 GeV positron
TRD
TOF
TOF
Tracker
ECAL
RICH
1-0.5-
00.5
1
CCL0.5
1
TRDL
0
20
40Ev
en
ts
Data
1-0.5-
00.5
1
CCL0.5
1
TRDL
0
20
40Ev
en
ts
Fit
signal+e
p background
background-e
Analysis method to determine the number of 𝐞+
8
• ECAL selection to remove bulk of the proton background.
• For each bin, fit templates to positive data sample in (𝚲𝐓𝐑𝐃 − 𝚲𝐂𝐂) plane
• Positron signal template from data using electrons
• Proton background template from proton data
• Charge confusion electron template from electron MC
149 < E < 170 GeV
Τ𝝌𝟐 𝒅𝒇 = 𝟑𝟒𝟐. 𝟖/𝟒𝟗𝟕
p 𝐞+
𝐞−
𝐞+p
2 30
0.1
0.2
Re
lati
ve e
rro
r
Charge confusion
Electron template
Proton template
Template fluctuation
Acceptance
1 102
103
10
Energy [GeV]
0
0.2
0.4
0.6
Rela
tiv
e e
rro
r
Statistical error
Systematic error
Total error
9
With 28.1 million electrons and 1.9 million positrons,
the study of systematic errors is crucial
1. Charge confusion
2. Template selection
3. Template statistical fluctuation
4. Acceptance(cancelled for positron fraction analysis)
1) Data/MC efficiency correction
2) ECAL selection efficiency
Statistical errors dominates above 60 GeV for positron flux
Po
sitr
on
flu
x re
lati
ve e
rro
r
Total error
Statistical error
Systematic error
1 10 100 1000
Energy [GeV]
electron
PAMELA
Fermi-LAT
MASS
CAPRICE
HEAT
positron
PAMELA
Fermi-LAT
MASS
CAPRICE
HEAT
0
50
100
150
200
250]2
GeV
-1sr
-1s
-2 [
m3
E-
eF
0
5
10
15
20
25 ]2
GeV
-1sr
-1s
-2 [
m3
E+
eF
Positron and electron fluxes before AMS
10These are very difficult measurements
1 10 100 1000
Energy [GeV]0
50
100
150
200
250]2
GeV
-1sr
-1s
-2 [
m3
E-
eF
0
5
10
15
20
25 ]2
GeV
-1sr
-1s
-2 [
m3
E+
eF
AMS positron and electron fluxes
11
1.9 million 𝐞+
28.1 million 𝐞−
Preliminary data. Please refer to the forthcoming publication in PRL
The magnitude and energy dependence are distinctly different between positrons and electrons
1 10 2103
10
Energy [GeV]0
50
100
150
200
250]2
Ge
v-1
s-1
sr
-2 [
m3
E×
-e
F
The origin of cosmic electrons
12The AMS accurate data can not be explained by current models
Preliminary data. Please refer to the forthcoming publication in PRL
• AMS: 28.1 million 𝐞−
The origin of cosmic positrons
13
The AMS positron flux exceeds the prediction from collision of cosmic rays, requiring a new source of high energy positrons
⦁ 1.9 million positrons
𝚽𝐞+𝐄𝟑[𝐦
−𝟐𝐬𝐫
−𝟏𝐬−𝟏𝐆𝐞𝐕𝟐]
Preliminary data. Please refer to the forthcoming publication in PRL
Models to explain the AMS Positron Flux
14
AMS data appears to be in excellent agreement with the predictions from a 1.2 TeV Dark Matter model (J. Kopp, Phys. Rev. D 88, 076013 (2013))
⦁ 1.9 million positrons
1.2 TeVDark Matter + Collision of Cosmic Rays
𝚽𝐞+𝐄𝟑[𝐦
−𝟐𝐬𝐫
−𝟏𝐬−
𝟏𝐆𝐞𝐕𝟐]
Preliminary data. Please refer to the forthcoming publication in PRL
1) Particle origin: Dark Matter
2) Modified Propagation of Cosmic Rays
3) Astrophysics origin: Pulsars, SNRs
1 10 100 1000
Energy [GeV]0
0.05
0.1
0.15
0.2
Po
sit
ron
fra
cti
on
⦁ 1.9 million positrons
1.2 TeVDark Matter + Collision of Cosmic Rays
Dark Matter model based on J. Kopp, Phys. Rev. D 88, 076013 (2013).
Positron excess also can be expressed in terms of the positron fraction
𝜱𝒆+
𝜱𝒆+ +𝜱𝒆−
Po
sitr
on
fra
ctio
n =
e+ /
(e+
+ e
– )
15
R. Cowsik et al., Ap. J. 786 (2014) 124, (pink band)Explaining the AMS positron fraction(grey circle)
as propagation effects.
The observed features of the AMS data cannot be explained by propagation effects
16
Alternative Models to explain the AMS Measurements
• Modified Propagation of cosmic Rays
Examples:
[GeV/n]K
Kinetic Energy E
B/C
Flu
x R
ati
o
0.1
1 AMS
TRACERPAMELAJuliussonOrthWebberLezniakHEAO3CRNSimonMaehlAMS01ATIC02CREAM-ICowsik Model
AMS: 11 million nuclei
This requires a specific energy dependence of the B/C ratio
ruled out by AMS B/C measurement
4*10-11 2 3 4 10 20 1022*102 10
32*10
3
Alternative Models to explain the AMS MeasurementsNew Astrophysical sources (Supernova Remnants)
P. Mertsch and S. Sarkar, Phys.Rev. D 90 (2014) 061301 17
e+
e+
Energy [GeV] Kinetic Energy [GeV/n]
B/C
B/C
Model parameter tuned to fit the positron flux data
Model parameter tuned to fit the B/C data
Energy [GeV] Kinetic Energy [GeV/n]
Science 358, 911-914 (2017)
18
Energy [TeV]
3-10 2-10 1-10 1 10
]-1
sr
-1s
-2m
2J
(E)
[Ge
V3
E
3-10
2-10
1-10
1
10
210
AMS-02
=0.33(base)d
range(base)s3
AMS
HAWC3σ range
18
HAWC rules out that the positron excess is from nearby pulsars
Science 358, 911-914 (2017)
In addition, AMS Measurement of positron, electron anisotropy will distinguish and constrain Pulsar origin of high energy e±
19
Positron spectrum beyond the turning point
Combining last 3 points (E > 370 GeV), 2-sigma deviation from 𝜱 ∝ 𝑬−𝟑
⦁ 1.9 million positrons
1.2 TeVDark Matter + Collision of Cosmic Rays
2-s
igm
a
𝚽𝐞+𝐄𝟑[𝐦
−𝟐𝐬𝐫
−𝟏𝐬−𝟏𝐆𝐞𝐕𝟐]
Preliminary data. Please refer to the forthcoming publication in PRL
By 2024AMS will collect
4 million positrons1.2 TeV
Dark Matter + Collision of Cosmic Rays
5-s
igm
a
𝚽𝐞+𝐄𝟑[𝐦
−𝟐𝐬𝐫
−𝟏𝐬−𝟏𝐆𝐞𝐕𝟐]
20
Positron spectrum beyond the turning point
By 2024, we will extend the measurements to 2 TeV and reach 5 sigma significance
1 10 2103
10
Energy [GeV]0
5
10
15
20
25]-1
s-1
sr
-2 [m
3 E
+e
F
5-sigma
Conclusion
21
• The individual positron and electron fluxes are measured to 1 TeV with 28.1 million electrons and 1.9 million positrons.
• Both the amplitude and energy dependence are distinctly different between positron flux and electron flux.
• Positron flux hardens from 20 GeV and exhibits a cutoff at high energy.
• Above 370 GeV, Positron flux deviates from 𝜱 ∝ 𝑬−𝟑
with 2 sigma significance. By 2024 we will reach 5 sigma. 𝚽
𝐞+𝐄𝟑[𝐦
−𝟐𝐬𝐫
−𝟏𝐬−𝟏𝐆𝐞𝐕𝟐]
By 2024
AMS will collect4 million positrons
1.2 TeVDark Matter+ Collision ofCosmic Rays
10 100 1000
Energy [GeV]
3.5-
3-
2.5-
gS
pe
ctr
al
Ind
ex
Positrons
Electrons
𝜱𝒆+ = 𝑪𝒆+𝑬𝜸𝒆+
𝜱𝒆− = 𝑪𝒆−𝑬𝜸𝒆−
Complex energy dependence of positron and electron fluxes
The spectral indices and their energy dependence are distinctly different between positrons and electrons 23
Preliminary data. Please refer to the forthcoming publication in PRL