The importance of knowing the primary mass – and how little we really know Alan Watson University of Leeds Pylos: 7 September 2004

Key Questions about UHECR Energy Spectrum above 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!

“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.” G Cocconi: Fifth International Cosmic Ray Conference, Guanajuato, Mexico, 1955 Question of Mass Composition “Fere libenter homines id, quod volunt, credunt!” “Men wish to believe only what they prefer” Thanks to Francesco Ronga

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

From Takeda et al Astroparticle Physics For S(600), the energy estimates are LOWER if iron is assumed S 0 = 50 vem

Mass Composition (i): X max with energy Elongation Rate (Linsley 1977, Linsley and Watson 1981) dX max / dlog E < 2.3X 0 g cm -2 /decade from Heitler model X max = ln (E o /  c )/ ln 2 extended to baryonic primaries: dX max / dlog E = 2.3X 0 (1 - B n - B ) where B n = d ln(n)/ d ln E and B = (- N /X 0 )(d ln N /d ln E)

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

Some personal comments on the recent HiRes Composition Paper Abbasi et al (astro-ph/ ) Selection of events: χ 2 per dof < 20 2 measures of X max within 500 g cm -2 Measurements within 400 g cm -2 for global fit to 2 eyes But resolution of X max 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

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

Astroparticle Physics in press; also data shown at ICRC2003

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

From L Perrone (Auger group): Catania CRIS meeting

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 ~ A 0.14 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

Claim: Consistent with proton dominant component Log(Energy [eV]) −2−2 −1−1 0 1 Log(Muon –2 ]) Results from the AGASA array Kenji Shinosaki: 129 events > eV

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 ~ eV

From Ralph Engel’s presentation in Leeds, July 2004

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

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

Sample LDF compared with new model: QGSjet’98

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

Estimate of Mass Composition 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]. 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]. QGSjet models (’98, dotted line and ’01, solid line). First 3 points: trigger bias

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

Comparisons from Dova et al (2004) Astropart Phys

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

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?

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

Mass information from study of Inclined Showers

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

Ave et al. PRL

Haverah Park: Photon limit at eV < 40% CL) AGASA: muon poor events Gamma-ray fraction upper limits 34% (>10 19 eV) (  /p<0.45) 56% (> eV) (  /p<1.27) 60° < θ < 80° Ave, Hinton, Vazquez, aaw, and Zas PRL

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…”

Typical scale is ~ 1000 km

Conclusions Beware: the experimentalists are still some way from AGREED statements about the mass composition above 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 eV at your own risk!