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The importance of knowing the primary mass – and how little we really know. Alan Watson University of Leeds [email protected] 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

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

Alan Watson

University of Leeds

[email protected]

Pylos: 7 September 2004

key questions about uhecr
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!

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


Corrections necessary to determine energy from fluorescence

~ 5%

The energy estimates are HIGHER if Fe is assumed

Song et al Astroparticle Physics 2000


For S(600), the energy estimates are LOWER if iron is assumed

S0 = 50 vem





From Takeda et al Astroparticle Physics 2003

mass composition i x max with energy
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)

composition from depth of maximum i
Composition from depth of maximum (i)

Model dependent AND

< 1019.25 eV

Abbasi et al: astro-ph/0407622


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
hires composition from x max fluctuations ii
HiRes Composition from Xmax fluctuations (ii)


BUT diurnal and seasonal atmospheric changes

likely to be very important

Solid lines: data

Models are Sibyll and QGSjet



“Standard” Atmospheres

can bias composition inferences

M. Risse et al ICRC03

mass composition iii muons
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


Results from the AGASA array

Claim: Consistent with proton dominant component

Kenji Shinosaki: 129 events > 1019 eV



Log(Muon [email protected][m–2])







Log(Energy [eV])

model dependence of muon signals
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


From Ralph Engel’s

presentation in Leeds,

July 2004




AGASA data: a second look




Plots by Maria Marchesini

mass composition iv using the lateral distribution
Mass Composition (iv): Using the lateral distribution

(r)~ r –(+ r/4000)

circa 1978:

Feynman Scaling

Primary Uranium?!

distribution of lateral distribution
Distribution of lateral distribution

Haverah Park data: Ave et al. 2003

estimate of mass composition
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].


Lateral distribution data from Volcano Ranch interpreted by Dova et al (2004)

Astropart Phys (in press)


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

ideas to explain the enigma

Ideas to explain the Enigma

Decay of super heavy relics from early Universe (or top down mechanisms)


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
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…


M. Ave: 80°, proton at 1019 eV

Details in Ave, Vazquez and Zas, Astroparticle Physics


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

an elegant mass determination method
An Elegant Mass Determination Method
  • Zatsepin Effect

Zatsepin 1951

Zatsepin and Gerasimova 1960

Solar Magnetic Field Important

Medina Tanco and Watson (1998)

“ from this very beautiful idea are too infrequent to be of use in any real experiment…”


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!