the AMS on the International Space Station Zuhao LI / IHEP, CAS On - - PowerPoint PPT Presentation

Precision Measurement of Positron Fraction by the AMS on the International Space Station

5m x 4m x 3m 7.5 tons

Zuhao LI / IHEP, CAS On behalf of the AMS Collaboration ICRC 2017 14, July, 2017

Tracker

1

2

7-8 3-4 9 5-6

Transition Radiation Detector (TRD) Identify e+, e- Silicon Tracker Z, P or R=P/Z Electromagnetic Calorimeter (ECAL) E of e+, e- Ring Imaging Cherenkov (RICH) Z, E Time of Flight (TOF) Z, E

AMS: A TeV precision, multipurpose, magnetic spectrometer

Magnet ±Z

Z and P, E or R are measured independently by Tracker, ECAL, TOF and RICH

Transition Radiation

2

3 Proton rejection at 90% e+ efficiency

Typically, 1 in 1,000 protons may be misidentified as a positron

ISS Data

One of 20 layers

radiator

e± p

Rigidity (GV)

Transition Radiation Detector

TRD estimator = -ln(Pe/(Pe+Pp))

20 layers: fleece radiator and proportional tubes

Protons

Electrons Measurement with 1 of the 20 TRD Layers

Transition Radiation

ISS Data

3

90% 70% 80%

1 5 6 3 4 7 8 9 2

Silicon Tracker

9 planes, 200,000 channels The coordinate resolution is 10 μm. Maximum Detectable Rigidity (MDR) for Z=1 particles is 2.0 TV

4

Electromagnetic Calorimeter

Provides a precision, 17 X0, TeV, 3-dimensional measurement of the directions and energies of electrons and positrons, seperate e± from protons

Typically, 1 in 10,000 protons may be misidentified as a positron

Proton rejection at 90% e+ efficiency

Momentum (GeV/c)

Test beam result

Boosted Decision Tree (BDT):

3D shower shape

protons electrons

εe = 90%

ECAL estimator

Probability

5

In 6 years on ISS, AMS has collected over 100 billion cosmic rays. Search for Dark Matter is one of the main physics topic of AMS . 100 billion events collected 08/05/2017

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AMS

p, He + ISM  e+,, p + …

Dark Matter: 

Dark Matter () annihilations

 +   e+, p + …

        

p, He e+, p

ISM Collision of Cosmic Rays with the Interstellar Media will produce e+, p…

The excess of e+, p from Dark Matter () annihilations can be measured by AMS

• M. Turner and F. Wilczek, Phys. Rev. D42 (1990) 1001; J. Ellis 26th ICRC (1999)

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m=800 GeV m=400 GeV

e± energy [GeV] e+ /(e+ + e-)

• 1. The energy at

which it begins to increase.

• 2. The rate of increase with energy
• 3. The existence of sharp structures.
• 4. The energy beyond which it

ceases to increase.

• 6. The rate at

which it falls beyond the turning point.

• 5. Isotropy.

Dark Matter model based on I. Cholis et al., JCAP 12 (2009) 007.

Positron Fraction[e+/(e++e-)]

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Analysis: 2D fit to measure Ne± and Np

After an efficient ECAL selection to remove the majority of protons, 2D reference spectra for the signal and the background are fitted to data in the [TRD estimator-log(E/|P|)] plane. 83.2-100 GeV p e± This method combines information from  TRD  ECAL  Tracker It provides better statistical accuracy compared to cut-based analysis

log(E/|P|)

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Fit results:

The TRD Estimator shows clear separation between protons and positrons with a small charge confusion background

TRD Estimator

(83.2-100 GeV)

e−e+ p e+

Events

χ2/d.f.=0.60

Data Fit Positrons Protons Charge Confusion(e-e+)

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Fit results:

The ECAL energy and Tracker momentum matching (E/P) quantifies the small charge confusion in the signal region.

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Systematic errors

• 1. Charge confusion (e-e+) is the dominating source of systematic uncertainty

at high energies Two sources: 1) large angle scattering and 2) production of secondary tracks along the path of the primary track. Both are well reproduced by the Monte Carlo. The small difference is taken as a systematic error.

Extensive check for systematic errors on the positron fraction measurement

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Systematic errors

• 2. Selection dependence

The measurement is stable over wide variations of the cuts in the ECAL shower Shape, E/p matching, etc. For each energy bin, over 1,000 sets of cuts(trials) were analyszed.

173 – 206 GeV Width = 0.006

Positron Fraction Positron Fraction Number of Positron Number of Trials

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Systematic errors

3) σacc: Acceptance asymmetry Due to known minute tracker asymmetry (negligible for all energy bin) 4) σmig: Absolute energy scale and bin-to-bin migration negligible above 5 GeV 5) σref: Reference spectra because definition of the reference spectra is based on pure samples of electrons and protons of finite statistics.

PRL113,121101(2014)

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6.8 million e± events Selected from 25 billion events collected during the first 18 months of operations: May 19, 2011 to December 10, 2012 Selected by APS as a highlight of the Year 2013 PRL110， 141102(2013)

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10.9 million e± events Selected from the sample of 41 billion events PRL113， 121101(2014) Two papers had been cited ~1000 times

PRL113， 121101(2014)

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Positron Fraction

Positron fraction begin to increase at 7.8 GeV

PRL113， 121101(2014)

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The positron fraction rise slope decreases with energy, the maximum is reached at 275±32GeV

PRL113， 121101(2014)

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Examples of Theoretical Models for positrons

From Dark Matter

1)

• J. Kopp, Phys. Rev. D 88, 076013 (2013);

2)

• L. Feng, R.Z. Yang, H.N. He, T.K. Dong, Y.Z. Fan and J. Chang Phys.Lett. B728 (2014) 250

3)

• M. Cirelli, M. Kadastik, M. Raidal and A. Strumia ,Nucl.Phys. B873 (2013) 530

4)

• M. Ibe, S. Iwamoto, T. Moroi and N. Yokozaki, JHEP 1308 (2013) 029

5)

• Y. Kajiyama and H. Okada, Eur.Phys.J. C74 (2014) 2722

6) K.R. Dienes and J. Kumar, Phys.Rev. D88 (2013) 10, 103509 7)

• L. Bergstrom, T. Bringmann, I. Cholis, D. Hooper and C. Weniger, PRL 111 (2013) 171101

8)

• K. Kohri and N. Sahu, Phys.Rev. D88 (2013) 10, 103001

9)

• P. S. Bhupal Dev, D. Kumar Ghosh, N. Okada and I. Saha, Phys.Rev. D89 (2014) 095001

10)

• A. Ibarra, A.S. Lamperstorfer and J. Silk, Phys.Rev. D89 (2014) 063539

11)

• Y. Zhao and K.M. Zurek, JHEP 1407 (2014) 017

12)

• C. H. Chen, C. W. Chiang, and T. Nomura, Phys. Lett. B 747, 495 (2015)

13)

• H. B. Jin, Y. L. Wu, and Y.-F. Zhou, Phys.Rev. D92, 055027 (2015)

14) M-Y. Cui, Q. Yuan, Y-L.S. Tsai and Y-Z. Fan, arXiv:1610.03840 (2016) 15)

• A. Cuoco, M. Krämer and M. Korsmeier, arXiv:1610.03071 (2016)

…….

From Astrophysical Sources

1)

• T. Linden and S. Profumo, Astrophys.J. 772 (2013) 18

2)

• P. Mertsch and S. Sarkar, Phys.Rev. D 90 (2014) 061301

3)

• I. Cholis and D. Hooper, Phys.Rev. D88 (2013) 023013

4)

• A. Erlykin and A.W. Wolfendale, Astropart.Phys. 49 (2013) 23

5) P.F. Yin, Z.H. Yu, Q. Yuan and X.J. Bi, Phys.Rev. D88 (2013) 2, 023001 6) A.D. Erlykin and A.W. Wolfendale, Astropart.Phys. 50-52 (2013) 47 7)

• E. Amato, Int.J.Mod.Phys.Conf.Ser. 28 (2014) 1460160

8)

• P. Blasi, Braz.J.Phys. 44 (2014) 426

9)

• D. Gaggero, D. Grasso, L. Maccione, G. DiBernardo and C Evoli, Phys.Rev. D89 (2014) 083007

10)

• M. DiMauro, F. Donato, N. Fornengo, R. Lineros and A. Vittino, JCAP 1404 (2014) 006

11)

• K. Kohri, K. Ioka, Y. Fujita, and R. Yamazaki, Prog. Theor. Exp. Phys. 2016, 021E01 (2016)

……

From Secondary Production

1) R.Cowsik, B.Burch, and T.Madziwa-Nussinov, Ap.J. 786 (2014) 124 2)

• K. Blum, B. Katz and E. Waxman, Phys.Rev.Lett. 111 (2013) 211101

3)

• R. Kappl and M. W. Winkler, J. Cosmol. Astropart. Phys. 09 (2014) 051

4) G.Giesen, M.Boudaud, Y.Gènolini, V.Poulin, M.Cirelli, P.Salati and P.D.Serpico, JCAP09 (2015) 023; 5) C.Evoli, D.Gaggero and D.Grasso, JCAP 12 (2015) 039. 6) R.Kappl, A.Reinertand, and M.W.Winkler, arXiv:1506.04145 (2015) …… 19

Models to explain the AMS Positron Fraction Measurements

• R. Cowsik et al., Ap. J. 786 (2014) 124,

(pink band) explaining that the AMS positron fraction (gray circles) above 10 GV is due to propagation effects. However, this requires a specific energy dependence of the B/C ratio

The AMS Boron-to-Carbon (B/C) flux ratio

Cowsik (2014) 11 million nuclei

PRL 117, 231102 (2016)

Some models are constrained by complementary measurement

• f AMS.

Examples 1: Modified propagation of cosmic rays

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Models explain the AMS Positron Fraction Measurements

• P. Mertsch and S. Sarkar, Phys.Rev. D

90 (2014) 061301(R) Subir Sarkar: AMS Days@CERN, April 2015

positron fraction B/C Subir Sarkar: AMS days@CERN, April 2015: Not able to fit simultaneously the positron and B/C. Some models are constrained by complementary measurement

• f AMS.

Examples 2: Supernova Remnants

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Models to explain the Positron Fraction Measurements

Examples: Pulsars

The rate of falls predicted by pulsars model and dark matter model are different.

Positron Fraction

• M. DiMauro, F. Donato, N. Fornengo, R.

Lineros, A. Vittino, JCAP 1404 (2014) 006

E(GeV)

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• AMS-02

Models to explain the Positron Fraction Measurements

Examples: Dark Matter model with intermediate state

M.Cirelli, M.Kadastik, M.Raidal and A.Strumia, Nucl.Phys. B873 (2013) 530

The accuracy of the measurement in the last bin is limited by statistics. To understand the

• rigin of the

positron excess, we need more data.

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• AMS 2013



PAMELA 2009

Comparison of the latest results of positron fraction measurement with a Dark Matter model

Positron Fraction

e± energy [GeV]

M = 1 TeV

Model based on

• J. Kopp, Phys. Rev. D 88 (2013) 076013

AMS 2016

17 million events Preliminary Data. Please refer to the AMS forthcoming publication in PRL.

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To collect data up to 2024, we should be able to understand the origin of the positron excess

Positron fraction

M = 1 TeV

AMS 2024

Pulsars

E(GeV)

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The latest data collected by AMS in the first 6 years of data taking is being analyzed, the new data will provide more information of the positron fraction. Comprehensive measurements of AMS provide more insight

• n the origin of the unexpected e+ excess.

In the foreseeable decades, AMS is a unique experiment which could measure cosmic ray positron to 1 TeV. By collecting data through 2024, we should be able to determine the

• rigin of the positron excess.

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backups

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Event selection

• DAQ:
• livetime >50%(no SAA)
• Geomagnetic cutoff:

E>1.2 max cutoff

• TRACKER:
• Track quality
• geometrical match with ECAL

shower

• TRD:
• at lease 15 hits
• TOF：
• downgoing particle,
• β>0.8, 0.8<Z<1.4
• ECAL:
• shower axis within the

fiducial ECAL volume

• electromagnetic shape of the

shower

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Reject charge confused events

MC e- 500-700GeV

Charge confusion estimator ΛCC to reject charge confused events (e-e+)

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2D fit to measure Ne± and Np

Electron TRD likelihood

Normalized Events

Tracker CC BDT

Normalized Events

e± e-

e-e+ p

• The number of positrons and electrons are determined from a

template fit in TRD - Charge Confusion Estimator 2D phase space

• The e+ and proton template are obtained from high purity e-,

proton data

• Charge confusion studied using e- test beam and MC

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Example of fit

(149-170GeV)

Data Fit

e+ p e-->e+

• The number of positrons and electrons are determined from a template fit in

TRD - Charge Confusion Estimator 2D phase space

• The e+ and proton template are obtained from high purity e-, proton data
• Charge confusion studied using e- test beam and MC

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(d)

Pulsar Model based on D. Hooper, P. Blasi & P. D. Serpico, JCAP 0901 (2009); K. Iota, PTP 123-4 (2010) 743

Pulsars

Isotropy Current value

Anisotropy of e+/e-

The fluctuations of the positron ratio e+/e− are isotropic 16 < E [GeV] < 350.

Significance

Galactic coordinates (b,l)

Data taking to 2024 will allow to explore anisotropies of 1%

C1 is the dipole moment The anisotropy in galactic coordinates

Anisotropy

Astrophysical point sources like pulsars will imprint a higher level of anisotropy on the arrival directions of energetic positrons than a smooth dark matter halo.

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Alternative Models to explain the AMS Positron Flux and Positron Fraction Measurements

• Modified Propagation of Cosmic Rays
• Supernova Remnants
• Pulsars

Examples:

The AMS Antiproton-to-Proton ratio

The excess of antiprotons

• bserved by AMS

cannot come from pulsars.

Presented by Weiwei XU

• AMS

Dark matter

Momentum [GeV]

Positron Fraction

M = 1 TeV

AMS 2024

Pulsars

By 2024, AMS will distinguish Dark Matter from Pulsars

Donato et al., PRL102, 071301 (2009);mχ = 1 TeV 33