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

22

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