Slide 1

High Energy Cosmic Rays
The Primary Particle Types
Paul Sommers for Alan Watson
Epiphany Conference, Cracow
January 10, 2004
High Energy Cosmic Rays
The Primary Particle Types
Paul Sommers for Alan Watson
Epiphany Conference, Cracow
January 10, 2004
See astro-ph/0312475
UHE cosmic ray spectrum and anisotropy are uncertain
Importance of composition understanding
Proton dominance is a questionable assumption
Xmax analyses are so far not conclusive
Muon data do not support change to light composition
LDF studies at HP suggest heavy composition
Rise time studies at HP suggest heavy composition
Photons are small fraction of total, based on HP rate at large zenith
angles and AGASA muon density distribution
Cronin “shape parameter”
Air shower methods: Xmax and muon production
Hybrid composition sensitivity
AGASA and HiRes energy spectra plotted by Doug
Bergman (Columbia University)
Whilst detailed knowledge of the shape of the energy
spectrum is still lacking, it is clear that events above
1020 eV do exist. Evidence for clustering of the
directions of some of the highest energy events
remains controversial. Clearly, more data are needed
and these will come from the southern branch of the
Pierre Auger Observatory in the next few years. What
is evident is that our knowledge of the mass
composition of cosmic rays is deficient at all energies
above 1018 eV. It must be improved if we are to
discover the origin of the highest energy cosmic rays.
--Alan Watson abstract (Sorrento Conference, 9/03)
Xmax data compared to expectations using various models. The
predictions of the five modifications of QGSJET from which this diagram
is taken, lie below the dashed line that indicates the predictions of
HiRes Xmax data for E>1018 eV (solid lines). Dashed lines in the
upper plot show predictions for proton primaries by QGSJET and
Sibyll models. Predictions for iron primaries are shown in the lower
AGASA 2-component composition fit
14% iron at E = 1019 eV
30% iron for E > 3x1019 eV
AGASA muon density at 1000m from cores. Left: the dotted lines are
predictions for iron nuclei, dashed lines for protons and solid lines for
photons. Right: shaded histogram represents data, and line histograms are
expectations for photons, protons, and iron (rightmost).
HP measurements of LDF steepness parameter h compared with
predictins of QGSJET98 model assuming different mass mixtures.
The lower set of plots illustrates insensitivity of the mass mixture to
Risetime analysis near 1019 eV
“Recently, an analysis of 100 events has shown that the
magnitude of the risetime is indicative of a large
fraction (~80%) iron nuclei at ~1019 eV.”
Watson’s Sorrento Conclusions
The question of spectral shape of the UHECRs
remains uncertain and, along with the issue of the
clustering of the arrival directions, may only be
resolved by the operation of the Pierre Auger
Observatory. To make full use of this forthcoming
information, it is necessary to improve our
knowledge of the mass of the cosmic rays above 1019
eV. Such evidence as there is does not support the
common assumption that all of these cosmic rays are
protons: there may be a substantial fraction of iron
nuclei present. Photons do not appear to dominate
at the highest energies.
Superposition Model
Air shower by cosmic ray of energy E and mass A develops like a
superposition of A proton showers each with energy E/A.
* An iron shower is like a sum of 56 subshowers. Fluctuations in the
subshowers average out, so iron showers are much more predictable than
proton showers.
* Let “elongation rate” ER be the change in mean Xmax per energy
decade for proton showers. Then
==> Xmax(P;E)-Xmax(Fe;E) = 1.75*ER ~ 100 g/cm^2
* Let β=dln(Nμ)/dln(E) for protons (so Nμ~Eβ).
Then Nμ(A;E) = A x Nμ(P;E/A) = A1-β x Nμ(P;E) .
In particular, Nμ(Fe;E) = 1.3 x Nμ(P;E)
(since A=56 and β~.93 at EHE energies)
[ER ~ 55 g/cm^2]
separation as a
function of
Zenith Angle.
B.E. Fick & P. Sommes
Reports that say that something hasn't happened are always
interesting to me, because as we know, there are known knowns;
there are things we know we know. We also know there are known
unknowns; that is to say we know there are some things we do
not know. But there are also unknown unknowns - - the ones we
don't know we don't know. And if one looks throughout the history of
our country and other free countries, it is the latter category
that tend to be the difficult ones.
Donald H. Rumsfeld,
Department of Defense news briefing, February 12, 2002
You're thinking of Europe as Germany and France, I don't. I think
that's old Europe. You look at vast numbers of other countries in
Europe. They're not with France and Germany on this. They're with
the United States.
Secretary Donald H. Rumsfeld,
State Department, Washington, 22 January 2003
Longitudinal Profile at Various Core Distances
70% of maximum

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