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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?
University of Leeds
Pylos: 7 September 2004
“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
The energy estimates are HIGHER if Fe is assumed
Song et al Astroparticle Physics 2000
S0 = 50 vem
From Takeda et al Astroparticle Physics 2003
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)
Model dependent AND
< 1019.25 eV
Abbasi et al: astro-ph/0407622
BUT diurnal and seasonal atmospheric changes
likely to be very important
Solid lines: data
Models are Sibyll and QGSjet
can bias composition inferences
M. Risse et al ICRC03
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
Sibyll 1.7: Sibyll 2.1: QGSjet98
Important to recall that we do not know the correct model to use.
LHC CMS energy corresponds to ~ 1017 eV
presentation in Leeds,
AGASA data: a second look
Plots by Maria Marchesini
(r)~ r –(+ r/4000)
Haverah Park data: Ave et al. 2003
QGSjet models (’98, dotted line and ’01, solid line).
First 3 points:
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 .
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
Abu-Zayyad et al: PRL 84 4276 2000
Decay of super heavy relics from early Universe (or top down mechanisms)
New properties of old particles?
Breakdown of Lorentz Invariance?
K-H Kampert’s talk
- protons or iron?
HARDER: will use
Xmax , LDF, FADC traces,
Radius of curvature…
Details in Ave, Vazquez and Zas, Astroparticle Physics
Photon limit at 1019 eV
AGASA: muon poor events
Gamma-ray fraction upper limits (@90%CL)
60° < θ < 80°
Ave, Hinton, Vazquez, aaw, and Zas
PRL 85 244 2000
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…”
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!