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The importance of knowing the primary mass – and how little we really know

The importance of knowing the primary mass – and how little we really know. Alan Watson University of Leeds a.a.watson@leeds.ac.uk. Pylos: 7 September 2004. Key Questions about UHECR. Energy Spectrum above 10 19 eV? Arrival Direction distribution? Mass Composition?

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The importance of knowing the primary mass – and how little we really know

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  1. The importance of knowing the primary mass – andhow little we really know Alan Watson University of Leeds a.a.watson@leeds.ac.uk Pylos: 7 September 2004

  2. Key Questions about UHECR • Energy Spectrum above 1019 eV? • Arrival Direction distribution? • Mass Composition? • Aim of talk is to show where I think that we have got to in trying to answer the fundamental question of what is the mass at the highest energies. • Life may be less simple than some theorists seem to think!

  3. Question of Mass Composition “We remain with the dilemma: protons versus heavy nuclei. A clear cut decision cannot be reached yet. I believe that up to the highest energies the protons are the most abundant in the primary cosmic rays. However, I must confess that a leak proof test of the protonic nature of the primaries at the highest energies does not exist. This is a very important problem. Experimentally it is quite a difficult problem.” “Fere libenter homines id, quod volunt, credunt!” “Men wish to believe only what they prefer” Thanks to Francesco Ronga G Cocconi: Fifth International Cosmic Ray Conference, Guanajuato, Mexico, 1955

  4. Corrections necessary to determine energy from fluorescence ~ 5% The energy estimates are HIGHER if Fe is assumed Song et al Astroparticle Physics 2000

  5. For S(600), the energy estimates are LOWER if iron is assumed S0 = 50 vem 1.04 1.13 1.09 1.13 From Takeda et al Astroparticle Physics 2003

  6. Mass Composition (i): Xmax with energy Elongation Rate (Linsley 1977, Linsley and Watson 1981) dXmax/ dlog E < 2.3X0 g cm-2/decade from Heitler modelXmax = ln (Eo/c)/ ln 2 extended to baryonic primaries: dXmax/ dlog E = 2.3X0 (1 - Bn - B) where Bn = d ln(n)/ d ln E and B = (-N/X0)(d ln N/d ln E)

  7. Composition from depth of maximum (i) Model dependent AND < 1019.25 eV Abbasi et al: astro-ph/0407622

  8. Some personal comments on the recent HiRes Composition Paper • Abbasi et al (astro-ph/0407622) • Selection of events: • χ2 per dof < 20 • 2 measures of Xmax within 500 g cm-2 • Measurements within 400 g cm-2 for global fit to 2 eyes • But resolution of Xmax claimed as 30 g cm-2 from Monte Carlo • BUT surely the resolution will depend on the distance from the Eyes (apparently not considered) • Periods of calibrated and uncalibrated atmosphere (419 and 134 events) put together • - would have been interesting to have seen these groups apart

  9. HiRes Composition from Xmax fluctuations (ii) p BUT diurnal and seasonal atmospheric changes likely to be very important Solid lines: data Models are Sibyll and QGSjet Fe

  10. Astroparticle Physics in press; also data shown at ICRC2003

  11. “Standard” Atmospheres can bias composition inferences M. Risse et al ICRC03

  12. From L Perrone (Auger group): Catania CRIS meeting

  13. Mass Composition (iii): muons Muon Content of Showers:- N(>1 GeV) = AB(E/A)p (depends on mass/nucleon) N(>1 GeV) = 2.8A(E/A)0.86 ~ A0.14 So, more muons in Fe showers Muons are about 10% of total number of particles Used successfully at lower energies (KASCADE) VERY expensive - especially at high energies - conclusions derived are rather model dependent

  14. Results from the AGASA array Claim: Consistent with proton dominant component Kenji Shinosaki: 129 events > 1019 eV 1 0 Log(Muon density@1000m[m–2]) −1 −2 19 19.5 20 20.5 Log(Energy [eV])

  15. Model dependence of muon signals Sibyll 1.7: Sibyll 2.1: QGSjet98 1: 1.17:1:45 Important to recall that we do not know the correct model to use. LHC CMS energy corresponds to ~ 1017 eV

  16. From Ralph Engel’s presentation in Leeds, July 2004

  17. (i) QGSjet AGASA data: a second look (ii) (i) (ii) Plots by Maria Marchesini

  18. Mass Composition (iv): Using the lateral distribution (r)~ r –(+ r/4000) circa 1978: Feynman Scaling Primary Uranium?!

  19. Sample LDF compared with new model: QGSjet’98

  20. Distribution of lateral distribution Haverah Park data: Ave et al. 2003

  21. Estimate of Mass Composition QGSjet models (’98, dotted line and ’01, solid line). First 3 points: trigger bias The fraction of protons (Fp) as a function of energy for two QGSjet models (’98, dotted line and ’01, solid line). The three low energy points correspond to a range in which there is a well-understood trigger bias that favours steep showers [24].

  22. Lateral distribution data from Volcano Ranch interpreted by Dova et al (2004) Astropart Phys (in press)

  23. Comparisons from Dova et al (2004) Astropart Phys

  24. Are results consistent between different methods applied by same experimental group? An extreme situation HiRes/MIA data: Abu-Zayyad et al: PRL 84 4276 2000

  25. Ideas to explain the Enigma Decay of super heavy relics from early Universe (or top down mechanisms) Wimpzillas/Cryptons/Vortons New properties of old particles? Breakdown of Lorentz Invariance? • or is it ‘simple’? • Are the UHE cosmic rays iron nuclei? • Are magnetic field strengths really well known?

  26. Potential of the Auger Observatory • Directions   • Energy • Mass  - photons - neutrinos  K-H Kampert’s talk - protons or iron? HARDER: will use Xmax , LDF, FADC traces, Radius of curvature…

  27. Mass information from study of Inclined Showers

  28. M. Ave: 80°, proton at 1019 eV Details in Ave, Vazquez and Zas, Astroparticle Physics

  29. Ave et al. PRL 85 2244 2000

  30. Haverah Park: Photon limit at 1019 eV < 40% (@95% CL) AGASA: muon poor events Gamma-ray fraction upper limits (@90%CL) 34% (>1019eV)(g/p<0.45) 56% (>1019.5eV)(g/p<1.27) 60° < θ < 80° Ave, Hinton, Vazquez, aaw, and Zas PRL 85 244 2000

  31. An Elegant Mass Determination Method • Zatsepin Effect Zatsepin 1951 Zatsepin and Gerasimova 1960 Solar Magnetic Field Important Medina Tanco and Watson (1998) “..events from this very beautiful idea are too infrequent to be of use in any real experiment…”

  32. Typical scale is ~ 1000 km

  33. Conclusions Beware: the experimentalists are still some way from AGREED statements about the mass composition above 1017 eV - after one studies the differences between different experiments - and even the different conclusions from within the same experiment. From Auger, we will get neutrino and photon limits (signals?) more readily than baryonic masses - but we have many tools in our armoury and should succeed in getting the latter, when we fully understand the showers and our hybrid detector. (Recall: ground breaking was only 5 years ago). Personal view: assume 100% protons above 1019 eV at your own risk!

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