Secondary particles generated in propagation neutrinos gamma rays

Transcription

1 th INT, Seattle, 20 Feb 2008 Ultra High Energy Extragalactic Cosmic Rays: Propagation Todor Stanev Bartol Research Institute Dept Physics and Astronomy University of Delaware Energy loss processes protons nuclei gamma rays spectrum evolution in propagation Magnetic fields extragalactic galactic Secondary particles generated in propagation neutrinos gamma rays

2 The main energy loss process of the ultra high energy protons in the Universe is photoproduction interactions in the microwave background. The photoprocustion cross section is very well measured in accelerator experiments. Photoproduction cross section in the mirror system, i.e. photons interacting on target protons.

3 The UHE protons energy is so high that they can interact on photons from the microwave background and produce secondary pions. The threshold is when the center of mass energy exceeds the proton mass + the pion mass: Eε(1 cosθ) > (mp+mπ)2 For the average microwave background photon the threshold is at 1020 ev. Averaged over the photon spectrum and direction the threshold is a factor of 2 lower. The inelasticity of the proton in these interactions is important energy dependent parameter. At threshold protons lose less than 20% of their energy. At very high energy the loss can reach 50%.

4 The energy loss length of protons in the microwave background is about 14 Mpc above 4x1020 ev, about a factor of 4 above the proton interaction length. The arow in the left upper side of the graph shows the energy loss length in the BH electron-positron pair production. The proton energy loss is the ratio of electron to proton mass. The cross section is high. Secondary gamma rays and neutrinos (cosmogenic neutrinos) are produced in propagation. The mark at 4,000 Mpc shows the energy loss to the expansion of the Universe for H0=75 km/mpc/s. Neutron decay length is also shown with dashed line.

5 Bethe-Heitler pair production Example: the spectrum of positrons generated by interactions of protons in the microwave background radiation. Electron spectrum does not change linearly with proton energy. The process has the characteristic feature that while the cross section increases with the photon energy in the proton frame p ) the inelasticity decreases. As a result the energy loss length for protons has a minimum at about ev and increases at higher proton energy. Another feature is the small dependence of Ee on the proton energy.

6 Other photon fields: infrared radiation Interactions in the MBR dominate the proton energy loss above 30 EeV. Below that energy protons interact in the IRB. At higher redshift MBR dominates at lower energy because of its faster cosmological evolution. R a t i o of the total mean free path to that in the IRB Ratio

7 Photodisintegration Heavy nuclei lose energy in photodisintegration on all photon fields. A beam of heavy nuclei injected at large distance from us changes its composition in propagation. At distances larger than the energy loss distance it is a purely proton beam after the secondary neutrons decay. From: Bertone et al Energy loss time for He, C, Si, and Fe nuclei. (1 Mpc = 1014 s)

8 Calculations done by two groups that afterward joined: Chicago + Paris: ParisChicago From: Khan et al, 2003 The whole disintegration chain is followed in the calculations.

9 Energy loss length vs energy for different nuclei (Allard et al, 2005)

10 Energy loss lengths of protons in the microwave background and of gamma rays in MBR and radio background. The figure gives an idea for the ratio of the electromagnetic and hadronic cross sections in astrophysics. Electrons (like the gamma rays) always interact faster, but the rates depend on the specific seed photon field.

11 The energy spectra of UHECR (extragallactic) at Earth depend on: - Acceleration spectra at the sources. - Spatial distribution of the sources (nearby ones). - Cosmological evolution of the sources (may eliminate some of the models). - Chemical composition of UHECRs at their sources (see Allard et al, = ). - Average extragalactic magnetic field (high fields may create cut-offs we can see only a small fraction of the Universe, low fields would enhance source recognition. How do we deal with these parameters is the key for determination of the transition region starting from the hughest energies.

12 Evolution of the cosmic ray spectrum in propagation on different distances. The solid line shows the injection spectrum. The top panel is without cosmological evolution of the cosmic ray sources and the lower one is with (1+z)3 evolution up to z = 1.7. There is a pile-up of protons just below 1020 ev. Integration over source distance will produce the expected energy distribution for homogeneous isotropic source distribution.

13 Because of the strong proton energy loss high redshifts z do not contribute much to the highest energy cosmic rays. Here is an example with the W&B parameters. Contribution of cosmic ray sources at different redshift to the observed flux. The solid black line is the flux in case of isotropic distribution of the cosmic ray sources and a flat injection spectrum for cosmological evolution (1+z)3.

14 Chemical composition at the sources One possibility is that extragalactic cosmic rays are not protons only they have mixed composition and lose energy on propagation mostly on spallation in photon fields. This changes the story quite a bit: the dip at 1019 ev becomes shallower being filled with spallation nucleons. The injection spectrum for no cosmological evolution becomes flatter. From: Allard, Parizot & Olinto, 2005 The spectra generated with mixed composition bend almost at the same position as protons spectra do. Can one alter that?

15 The spectral shape after propagation of the mixed composition models depends not only on the injection spectrum but also on the initial cosmic ray composition. It could easily be made to fit observations. (Bertone et al, 2002)

16 The cosmic ray spectrum measured by the Auger group can also be fitted with at least thre e different models as shown below (Yamamoto for Auger): either a steep ( =2.55) without evolution or a flat ( =2.3) with a strong cosmological evolution ( (1+z)5 ) if the UHECR were protons. If they were nuclei the acceleration spectrum is still flat - = 2.2. We cannot distinguish between these models from the shape of the measured spectrum.

17 While the proton spectra integrated over redshift always maintain power law shape, the photon spectra after propagation have very complicated shape. There is a minimum due to the interactions in the microwave background. 16 From: Protheroe&Stanev (1996) Top-down models can be restricted with the amount of GeV gamma rays, as done here for X-particle mass of 1016 GeV and 1 ng magnetic field.

18 Fraction of gamma rays in HE cosmic rays (Auger Coll. 2007) Current limits are strict 2 % at 1019 ev and 5% at 2x1019 ev. The fraction of gamma rays has been limited by several air shower experiments using the shower profile, the muon content and the angular distribution of the detected air showers to several % of the total flux. The statistics is, however, too low to do that at the highest energies. So... for the origin of UHECR top/down models are still possible.

19 Galactic and extragalactic magnetic fields Magnetic fields are important for charged particle propagation for two main reasons: -- scattering in the magnetic fields can hide the cosmic ray sources -- it could restrict the distance cosmic rays can propagate. Heavy nuclei expirience more scattering since they have lower rigidity (momentum/charge) Our believe is that extragalactic magnetic fields follow the distribution of the matter density and only a tiny fraction of the Universe has significant fields. The average field strength may reach 1 ng with an average coherence length of 1 Mpc (average distance btw galaxies).

20 Although not very well known, we have approximate models for the regular magnetic field and some knowledge of the random fields. The angular distance of the Auger UHECR with the correlating AGN is the same order of magnitude as in some of these models.

21 The propagation of UHECR is influenced very strongly by magnetic fields, both galactic and extragalactic. - galactic magnetic fields deflect UHECR protons only a few degrees away from the direction of their sources. In case of heavy nuclei the deflection could be stronger, as 1018 V particles gyrate along the magnetic field lines. - random extragalactic fields, if they have strengths of ng on Mpc scale, can impose a relatively small `horizon'. Protons of energy below 1020 ev scatter and loose energy so much that they can not reach us in Hubble time. This may restrict the region from which this particles can reach us. Points are results of Monte Carlo propagation. The heavy grey line shows propagation time (analytic estimate, Achterberg) exceeding Hubble time.

22 The worst case for cosmic ray astronomy would be if they were organized large scale (> 1 Mpc) extragalactic magnetic fields, even in the ngauss range. The example above shows how particle scattering gets organized in they presence 10 ng field in this case. Auger results do not show the existence of such fields.

23 Secondary particles generated in propagation Cosmogentic neutrinos are neutrinos from the propagation of extragalactic cosmic rays in the Universe. These neutrinos were first proposed and their flux was calculated in 1969 by Berezinsky & Zatsepin. An independent calculation was done by Stecker in In 1983 Hill & Schramm did another calculation and used the non-detection by Fly's Eye of neutrino induced air showers to set limits on the cosmological evolution of the cosmic rays sources. The main difference with the processes in AGN and GRB is that the main photon target is the microwave background (2.75oK) of much lower temperature than the photon emission of these sources. This raises the proton photoproduction threshold to very high energy: Actually the proton photoproduction threshold in the MBR is about ev. There is also production in the isotropic infrared/optical background. The photoproduction energy loss of the extragalactic cosmic rays cause the GZK effect the end of the cosmic ray spectrum.

24 Cosmogenic neutrinos are very sensitive to the cosmological evolution of the cosmic ray sources because of the lack of energy loss. This could be useful for establishing a model for the extragalactic cosmic rays. Note the logarithmic scale in redshift. Cosmological parameters are as in the cosmic ray example. The contribution increases until the source luminosity is significant (z = 2.7 in the W&B model). At higher redshift the production is still high because of the (1+z)3 increase of the MBR density and the lower energy threshold for photoproduction.

25 The figure shows the fluxes of electron and muon neutrinos and antineutrinos generated by proton propagation on (bottom to top) 10, 20, 50, 100 & 200 Mpc in MBR. The top of the blue band shows the proton injection spectrum (E-2 in this example). From: Engel, Seckel & Stanev, 2001 Muon neutrinos and antineutrinos are generated with a spectrum similar to the one of electron neutrinos at twice that rate. As far as neutrinos are concerned the cascade development is full after propagation on 200 Mpc. Even the highest energy protons have lost enough energy to be below threshold. Slightly more of the proton energy loss goes to cosmogenic gamma rays generated in photoproduction and in the BH pair creation.

26 Comparison of neutrino and electromagnetic fluxes generated by protons in propagation on 200 Mpc in MBR. The electromagnetic particles also have a double peaked distribution, higher energy gamma rays from neutral meson decay and electrons from neutron decay. The further development of electromagnetic cascades depends on the strength of magnetic fields electrons easily lose energy on synchrotron radiation even in 1 ng fields.

27 Muon neutrinos and antineutrinos generated in propagation from isotropically and homogeneously distributed sources to us from two of UHECR proton models. Cosmological evolution of the sources (1+z)3 is taken into account for the W&B model. In case of mixed composition only the proton component generates high energy neutrinos.

28 The same for electron neutrinos and antineutrinos

29 The interaction rate for cosmogenic neutrinos from the W&B model is higher by about a factor of 10. This is not due as much to the neutrino flux, which at maximum is only a factor of 2 different, but on the lower neutrino energy in the Berezinsky et al model. Neutrino interaction cross section still increases with energy even above 1016 ev.

30 In the mixed composition model only the electron antineutrino flux (from neutron decay, yellow symbols) is higher than that in the proton models. The muon neutrino and antineutrino fluxes (also tau neutrinos at Earth because of oscillations) are lower. Electron antineutrino flux peaks below the Glashow resonance.

31 The cosmogenic neutrino flux above 1018 is the same as the UHECR flux. The difference in the hadron and neutrino cross sections makes it so much difficult to detect. The theoretical neutrino rate (no efficiency) is about 0.3 events per km3.yr. Bigger detectors needed for detection.

32 Summary Ultrahigh energy cosmic ray spectrum is shaped by the propagation from the sources to us as a function of the cosmic ray acceleration spectrum and their chemical composition at the sources The chemical composition of UHECR is currently not obvious. Contrary to expectations the Auger results suggest a mixed nuclear composition. Data is not fully consistent the with AGN may point at protons as the highest energy cosmic rays. The possible detection of cosmogenic neutrinos (IceCube, Auger, ANITA, RICE, Moon Observations) will be very important for understanding of the origin and propagation of these exceptional particles.