The positron fraction

Anna S. LamperstorferTechnische Universität München

Joint Astroparticle Physics Seminar10 January 2014

Outline

● Current measurements of the positron fraction● Cosmic rays in one slide ● Calculation of the positron fraction

– Production of electrons and positrons

– Propagation

– Solar modulation

● Possible explanations of the excess– Production inside cosmic ray sources

– Pulsars

– Dark matter annihilations or decays

● Conclusions

Measurements of the positron fraction

Cosmic rays in one slide

● CR: mainly protons and Helium, 1% electrons

● Ejected from supernova remnants (SNR)

● Fermi acceleration in SNR

arXiv:0607109

arXiv:1202.0466

Outline

● Current measurements of the positron fraction● Cosmic rays in one slide ● Calculation of the positron fraction

– Production of electrons and positrons

– Propagation

– Solar modulation

● Possible explanations of the excess– Production inside cosmic ray sources

– Pulsars

– Dark matter annihilations or decays

● Conclusions

Electron and positron production

High energy(primary)

cosmic rays

Interstellar medium

Spallations electrons and

positrons

electrons Primary electrons from

supernova remnants

Secondary electrons and positrons

Secondary positrons

● Main production channels (in pp collisions):

● Required quantities: – Spallation cross sections

– Primary cosmic ray fluxes

– Density and composition of the interstellar medium

Propagation of chargedcosmic rays

arXiv:0212111

Diffusion loss equation for electrons and positrons

Stationary case

Diffusion coefficient

Source term

Energy losses

● Semianalytical solution● Numerical codes (DRAGON, GALPROP)

Solar modulation

● Deflection of interstellar cosmic ray flux in the magnetic field of the sun

● Variation within the 11 year solar cycle● Simplest approach: force field approximation

Solar modulation parameter

Force field approximation for solar modulation

Gast, Schael 2009

Minimum

Maximum

Solar modulation

● Deflection of interstellar cosmic ray flux in the magnetic field of the sun

● Variation within the 11 year solar cycle● Simplest approach: force field approximation

● Possible charge sign dependence → numerical codes: HELIOPROP

Solar modulation parameter

Calculated positron flux

Positron excess

arXiv:0810.4995

Outline

● Current measurements● Cosmic rays in one slide ● Calculation of the positron fraction

– Production of electrons and positrons

– Propagation

– Solar modulation

● Possible explanations of the excess– Production inside cosmic ray sources

– Pulsars

– Dark matter annihilations or decays

● Conclusions

Interpretation 1:Production inside SNR

● Stationary case

● Good fit can only be achieved with extreme parameters

● Incompatible with B/C ratio

● Time dependent SNR simulation: contribution only a few percent

arXiv:1312.2953

arXiv:1312.2953

Interpretation 2: Pulsars

● Pulsar: rapidly rotating neutron star● Surrounded by pulsar wind nebula and

the supernova remnant envelope● Electrons and positrons are created in an

electromagnetic cascade in the pulsar magnetosphere

● Nearby pulsars: Monogem, Geminga ...● Distant pulsars: listed in catalogs

Interpretation 2: Pulsars

A single nearby pulsar fits nicely positron fraction and electron plus positron total flux.Source term for pulsars:

arXiv:1304.1791

Can we probe the pulsar origin?A nearby pulsar gives rise to an anisotropy in cosmic ray positrons → could be probed with CTA

arXiv:1304.1792

Interpretation 2: Pulsars

Contributions from all pulsars lead to a nice fit, too.Source term for pulsars:

arXiv:1304.4128

Interpretation 2: Pulsars

● Known astrophysical objects can fit lepton data

but:● Pulsar parameters not precisely known

– Spin down luminosity

– Energy output in electron positron pairs

– Injection spectrum

– Cut-off energy

● Pulsar catalogs incomplete

Interpretation 3: Dark Matter

Many dark matter candidates can accommodate the positron fraction.

arXiv:1312.7841

Checks for thermal WIMP interpretation

BUT models must be consistent with:● Electron flux● Antiproton flux● Gamma Rays● Unitarity● Thermal cross section● Anisotropy

Electron plus positron flux

arXiv:1312.7841

Antiproton constraints:

arXiv:1312.7841

Gamma-ray and unitarity constraints

arXiv:1312.7841

Annihilation into intermediate states

arXiv:1304.1840

● Better fit to positron fraction● In agreement with antiprotons due to kinematics● In agreement with gamma ray constraints● Required cross sections:

Thermal relic?Boost factors!● Thermal relic: ● For fit to positron fraction required: ● Enhancement of cross section through boost

factors– Dark matter substructure

– Sommerfeld enhancement

arXiv:0802.0360

arXiv:0611370

Interpretation 3: Dark Matter

Constraints are weaker or do not apply for decaying dark matter.

arXiv:1307.6434

Conclusions

● Secondary production inside SNR disfavored by B/C

● DM and pulsars can still accommodate the rising positron fraction

● With increasing amount of precise data, the allowed parameter space shrinks