Implication of AMS-02 positron fraction

measurement

Qiang Yuan

(yuanq@ihep.ac.cn)

Institute of High Energy Physics, Chinese Academy of Sciences

Collaborated with Xiaojun Bi, Guo-Ming Chen, Yi-Qing

Guo, Su-Jie Lin and Xinmin Zhang

2013-05-09@KIAA, Multi-messenger workshop

AMS02: Phys. Rev. Lett., 2013, 110, 141102

• Introduction to cosmic rays

• Standard model of Galactic cosmic rays

• Implication of AMS-02 result

• Conclusion

Outline

100 year of discovery

Discovered by V.

Hess (and some

other scientists)

~1912

V. Hess won Nobel

Prize in Physics in

1936

• 1930s-1950s

1932: positron discovered by C. Anderson

1936: muon discovered by C. Anderson and S. Neddermeyer

1946: kaon discovered by G. Rochester and C. Butler

1947: pion discovered by C. Powell

1949: mu-atom discovered by W. Chang (张文裕)

• Nowadays

Connection with dark matter searches

Two golden ages of cosmic ray study

Detection of particle dark matter

Indirect detection of dark matter

Deep extragalactic

space and early

Universe

Sun

Galaxy

Cluster

Baltz et al. 2008

Better to search for

DM signal in anti-

particles (secondary)

due to lower

background

gamma-rays and

neutrinos are also

good due to the

simple propagation

Summary of the measurements of cosmic rays

Galactic cosmic rays: general picture

A. W. Strong

Two unknowns: source and propagation

Disentangle the source and propagation information

• Secondary/primary ratio: grammage

• Unstable/stable of secondary particles: residual time

Propagation

parameters

Source parameters

e+, pbar,

PAMELA observation of the positron fraction and pbar/p ratio

Positron has an “excess”, but no excess from antiprotons!

2009, Nature, 458, 607 2010, PRL, 105, 121101

Several measurements of the electron+positron spectra

ATIC, 2008, Nature, 456, 362

Fermi, 2010, PRD, 82, 092004

HESS, 2009, A&A, 508, 561

• ATIC shows a “peak”

• Fermi shows a smooth “bump”

• ATIC/HESS shows a cutoff

To explain simultaneously positron and electron data,

we need exotic electron/positron sources.

Solar modulation of

low energy part

Dark Matter

Annihilation or decay

Leptonic, quark or gauge

bosonic final states

Smooth or subhalo

…

Many can work but some

general conclusions:

TeV scale DM

Lepton dominated

Large annihilation or

decay rate

Models

Astrophysical

Pulsar, SNR, GRB

Various populations of

SNRs

SNR+PWN+SNR/MC

Hadronic or leptonic

Single or population

Burst or continuous

injection

…

CosRayMC: MCMC fitting tool of cosmic ray propagation

(Implement the GALPROP code with MCMC sampling)

• Necessary when large amount data are available

• Better and easier to constrain the parameters

• Study the global feature of the model with less bias

• Full investigation of the high-dimensional correlated parameter space

Liu, J. et al., 2010,

2012a, 2012b

Whatever the real physics is, it is possible to parameterize

the exotic source component and fit the model parameters

from the data

• Pulsar like scenario

• Dark matter scenario

Almost indistinguishable between pulsars and dark matter models in

the PAMELA era

Liu, J. et al., 2012b

What can AMS-02 precise data can tell us?

We do the same global fit with pulsar and dark matter

scenarios, based on the currently available high quality data

(AMS-02 e+/(e+e-), PAMELA e-, Fermi-LAT e+e-)

Determining the propagation parameters

Pulsars as the extra sources of e+e-

DM annihilation to muon pair

DM annihilation to tauon pair

Summary of goodness of fitting

• It is difficult to fit simultaneously the AMS-02, PAMELA

and Fermi-LAT data, which means there might be

intrinsic discrepancy of the measurements

• Pulsars seem to be better to fit the data than DM

scenarios

Comparison of different data sets

We do “observe” tension between PAMELA and Fermi

data at low energies, but not significant enough. AMS-02

data makes it more significant!

N. Mori, TeVPA 2012

Why pulsars can be better?

The positron

spectrum from DM

is generally too

hard while the

current AMS-02

data requires

softer spectrum

Another difficulty for DM models: constraints from gamma-

rays and/or antiprotons

Pulsars as natural explanation (based on the ATNF pulsar

catalog)

Yin et al., arXiv:1304.4128

arXiv:1304.1997

arXiv:1304.1840

arXiv:1304.2800

Solutions

AMS-02 measurement of the e+/e- spectra

Spectral hardening of the primary electron spectrum (like

what was observed for cosmic ray nuclei)

Feng et al. (2013), Cholis & Hooper (2013), Yuan & Bi (2013)

PAMELA, 2011, Science

Yuan & Bi, arXiv:1304.2687

Asymmetric decaying dark matter

Feng & Kang,

arXiv:1304.7492

Additional pure e- source (Vela SNR)

Gaggero et al., arXiv:1304.6718

Additional signature at ~100 GeV?

Y.-Z. Fan et al.

Conclusion

• There might be tension between AMS02/PAMELA and

Fermi-LAT data (under the current theoretical frame)

• Pulsar scenario can basically fit the data

• Dark matter scenario fits worse than the pulsar

scenario, because the positron spectrum from DM is in

general too hard

• Dark matter scenario will further suffer from strong

constraints from gamma-rays and antiprotons

• Systematic study on-going

谢谢

Charge-sign dependent solar modulation

Maccione, 2013, PRL