1. Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos
Dan Hooper
Particle Astrophysics Center
Fermi National Accelerator Laboratory
Aspen Workshop on
Cosmic Rays
April 2007

2. The Origin of the Highest Energy Cosmic Rays
The cosmic ray spectrum has been measured to extend to at least ~1020 eV
The origin of these extremely high energy particles remains unknown
Attenuation of UHECRs by the CMB (the GZK cutoff) requires sources within ~ Mpc
Few astrophysical accelerators potentially capable to producing such high energy events - none are known within the GZK radius
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

3. The Composition of the Highest Energy Cosmic Rays
Current observations are unable to determine whether the UHECR spectrum is dominated by protons or nuclei
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

4. The Composition of the Highest Energy Cosmic Rays
There are, however, a number of arguments favoring nuclei:
-CR data can be interpreted as marginally favoring significant nuclei composition
-Magnetic fields effect nuclei more strongly, helping to explain the lack of identified UHECR point sources
-Hillas criterion for maximum energy produced in a cosmic ray accelerator scales with electric charge, Z
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

5. The Composition of the Highest Energy Cosmic Rays
There are, however, a number of arguments favoring nuclei:
-CR data can be interpreted as marginally favoring significant nuclei composition
-Magnetic fields effect nuclei more strongly, helping to explain the lack of identified UHECR point sources
-Hillas criterion for maximum energy produced in a cosmic ray accelerator scales with electric charge, Z
The composition of the UHECR spectrum has significant implications for neutrino astronomy
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

6. Protons as UHE Cosmic Rays
Protons interact with CMB photons through several channels:
Catastrophic processes above ~ eV: p + CMB  p + 0 , n + +, and multi-pion production
Continuous energy losses from p + CMB  p + e+ + e-
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

7. Nuclei as UHE Cosmic Rays
Nuclei undergo photodisintegration via interactions with CMB and CIRB photons: ie. Fe56  Mn55 + p, Mn55  Mn54 + n, etc.
Leads to energy loss rates comparable to UHE protons
Hooper, S. Sarkar, A. Taylor, Astropart. Phys., astro-ph/
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

8. Nuclei as UHE Cosmic Rays
Leads to a mixed cosmic ray composition (various nuclei species plus protons) at Earth, which varies with energy
Hooper, S. Sarkar, A. Taylor, Astropart. Phys., astro-ph/
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

9. Cosmogenic Neutrinos
In either case (protons or nuclei UHECRs) UHE neutrinos are produced as a biproduct of cosmic ray propagation
For example:
 p e e
p + CMB  n + +
 e+ e  
Fe56 + CMB/CIRB  Mn55 + p
Neutrinos!
Mn55 + CMB/CIRB  Mn54 + n
… etc.
 p e e
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

10. Cosmogenic Neutrinos
Proton cosmic rays generate a two-component cosmogenic neutrino spectrum
Often thought of as a guaranteed flux of UHE neutrinos
Neutron Decay
Pion Decay
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

11. Cosmogenic Neutrinos
Anticipated to generate a potentially observable rate of UHE neutrinos in several near future experiments, including IceCube, Anita, Rice, and the Pierre Auger Observatory
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

12. Tools of the Trade: IceCube
Successor to AMANDA
Full Cubic Kilometer Instrumented Volume
22 (of 80) strings currently deployed (13 this season)
Sensitive to:
Muon tracks (above ~100 GeV), EM/hadronic showers (above a few TeV), Tau-unique events (above ~1 PeV)
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

13. Tools of the Trade: Radio Techniques
RICE
Array of radio antennas co-deployed with AMANDA
Effective Volume of ~1 km3 at 100 PeV; several km3 at 10 EeV
Limits on diffuse neutrino flux in 200 PeV-200 EeV range
of 6 x10-7 GeV cm-2 s-1 sr-1
Radio codeployments with IceCube promising
ANITA
Balloon-based radio antennas
ANITA-lite limit on diffuse flux
above ~EeV of ~10-6 GeV/cm2 s1 sr1
36 day ANITA flight ended Jan. 20
sensitivity of ~10-8 GeV/cm2 s sr
observe the first UHE neutrino?
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

14. UHECR Experiments as Neutrino Detectors
The Pierre Auger Observatory
Southern cite currently under construction in Argentina
First data released in 2005 (no neutrino data yet)
Sensitive above 108 GeV, 3000 km2 surface area
Neutrino ID possible for quasi-horizontal showers and Earth-skimming, tau-induced showers
AGASA experiment places limits on
UHE neutrino fluxes
EUSO/OWL
Satellite/space station based
Enormous aperture
Future uncertain
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

15. Cosmogenic Neutrinos
Although their peak sensitivity lies at different energies, IceCube, Anita and Auger each anticipate ~1 event per year (or per flight) for a standard (proton) cosmogenic neutrino flux
F. Halzen and Hooper, PRL, astro-ph/
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

16. Cosmic Ray Nuclei and Cosmogenic Neutrinos
In the case of a cosmic ray spectrum dominated by heavy nuclei, however, the pion decay component of the cosmogenic neutrino flux is reduced
protons He O Fe
Hooper, S. Sarkar, A. Taylor, Astropart. Phys., astro-ph/
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

17. Cosmic Ray Nuclei and Cosmogenic Neutrinos
The degree of suppression depends critically on the maximum energy to which cosmic rays are accelerated
Fe56 + CMB  Mn55 + p
In order to contribute to the cosmogenic neutrino flux, photo-disassociated protons must exceed the GZK cutoff, thus the original nuclei must exceed EGZK x A
Fe, Emax=1022.5
Emax=1021.5
Hooper, S. Sarkar, A. Taylor, Astropart. Phys., astro-ph/
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

18. Extragalactic Sources of High Energy Neutrinos
Cosmic ray spectrum of protons/nuclei extends to ~1020 eV
pp, p interactions generate neutrinos from cosmic ray sources
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

19. Extragalactic Sources of High Energy Neutrinos
Cosmic ray spectrum of protons/nuclei extends to ~1020 eV
pp, p interactions generate neutrinos from cosmic ray sources
The flux of neutrinos produced in UHE/HE sources can be tied to the cosmic ray spectrum
“Waxman-Bahcall” Argument:
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

20. Extragalactic Sources of High Energy Neutrinos
Cosmic ray spectrum of protons/nuclei extends to ~1020 eV
pp, p interactions generate neutrinos from cosmic ray sources
The flux of neutrinos produced in UHE/HE sources can be tied to the cosmic ray spectrum
“Waxman-Bahcall” Argument:
Fraction of proton energy to pions
Accounts for source evolution, etc. (~1)
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

21. The Extragalactic Neutrino Flux
IceCube will reach well below the predicted levels for  ~ (ie. the Waxman-Bahcall “Flux”)
Models of gamma ray bursts, active galactic nuclei, and starburst galaxies each predict a flux of neutrinos within the reach of IceCube
IceCube (3 yrs)
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

22. The Extragalactic Neutrino Flux
IceCube will reach well below the predicted levels for  ~ (ie. the Waxman-Bahcall “Flux”)
Models of gamma ray bursts, active galactic nuclei, and starburst galaxies each predict a flux of neutrinos within the reach of IceCube
Likely to observe first cosmic high-energy neutrinos in coming years
IceCube (3 yrs)
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

23. The Extragalactic Neutrino Flux
IceCube will reach well below the predicted levels for  ~ (ie. the Waxman-Bahcall “Flux”)
Models of gamma ray bursts, active galactic nuclei, and starburst galaxies each predict a flux of neutrinos within the reach of IceCube
Likely to observe first cosmic high-energy neutrinos in coming years
Likely to be more difficult if the bulk of the UHECR spectrum consists of nuclei
IceCube (3 yrs)
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

24. Nuclei and the Extragalactic Neutrino Flux
Different classes of comic ray sources are expected to photodisintegrate accelerated nuclei to varying degrees
In the fully disintegrated limit, Waxman-Bahcall prediction is restored
Lesser disintegration reduces the expected neutrino flux
L. Anchordoqui, Hooper, S. Sarkar, A. Taylor, astro-ph/
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos

25. Nuclei and the Extragalactic Neutrino Flux
Above ~100 TeV, GRB neutrino spectrum is largely unchanged (overall rate reduced by ~20%)
For AGN, neutrino flux is reduced considerably (overall rate reduced by ~80%)
Dan Hooper - Ultrahigh Energy Cosmic Ray Nuclei and Neutrinos
Anchordoqui, Hooper, Sarkar, Taylor, astro-ph/

26. Summary
Composition of the highest energy cosmic rays is still an open question, with important implications for neutrino astronomy
The presence of heavy or intermediate mass nuclei in the UHECR spectrum can substantially reduce the expected cosmogenic neutrino flux
Nuclei accelerated in cosmic ray sources (AGN, GRB, etc.) can result in a reduced estimate for the neutrino flux as compared to the all-proton case
As the first experiments reach the sensitivity needed to observe HE/UHE neutrinos (Anita, IceCube, Auger, etc.), the composition of the cosmic ray spectrum is also being indirectly probed