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