High Energy Cosmic RaysThe 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, CracowJanuary 10, 2004

See astro-ph/0312475

Overview

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 QGSJET01.

[Watson]

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 plot. [Watson]

[Risse]

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).

[Watson]

AGASA 2-component composition fit

14% iron at E = 1019 eV

30% iron for E > 3x1019 eV

HP measurements of LDF steepness parameter compared with predictins of QGSJET98 model assuming different mass mixtures. The lower set of plots illustrates insensitivity of the mass mixture to energy.

[Watson]

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(Fe;E)=Xmax(P;E/A)=Xmax(P;E)-Log(56)*ER [ER ~ 55 g/cm^2]

==> 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)

Heavy-Lightseparation as afunction ofZenith Angle.

B.E. Fick & P. Sommes

Reports that say that something hasn't happened are alwaysinteresting to me, because as we know, there are known knowns;there are things we know we know. We also know there are knownunknowns; 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 ofour country and other free countries, it is the latter categorythat 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 thinkthat's old Europe. You look at vast numbers of other countries inEurope. They're not with France and Germany on this. They're withthe United States.

Secretary Donald H. Rumsfeld, State Department, Washington, 22 January 2003

Longitudinal Profile at Various Core Distances

10m

30m

100m

300m

1000m

3km

10km

70% of maximum