exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

In figure 4 we plot the differential limits and the
corresponding integral limits assuming a flux proportional
to E -2 . Also shown is the background contribution
expected to originate from interactions of
CR protons with the CMB. The neutrino background
predicted is smaller if the highest energy
CRs are heavy nuclei. shower is directly measured
by the FD elongation rate.
UHECR composition
Fig 3 : Upper limits on the photon fraction in the
integral cosmic-ray flux for different experiments:
AGASA (A1,A2), AGASA-Yakutsk (AY), Yakutsk
(Y), Haverah Park (HP). In black the limits from
Auger surface detector (Auger SD) and in blue the
limits above 2, 3, 5, and 10 EeV derived through
the hybrid events (Auger HYB). The shaded region
shows the expected GZK photon fraction. Lines
indicate predictions from top-down models
They severely constrain the family of “top-down”
models for CR origin, which predict photon contributions
of up to 50% to the observed CR flux.
At a given energy, the average position of the
depth of shower maximum X max and its fluctuations
depend on the composition of the primary CR. The
depth of shower maximum reflects the depth of the
first interaction. Proton showers penetrate deeper
into the atmosphere, have larger values of X max
and wider distributions than heavy nuclei. Hybrid
events have been used to measure X max [7]. The
information from the SD allows to constrain the
geometry of the EAS. The longitudinal development
of the electromagnetic part of the shower is
directly measured by the FD.
In figure 5 we show the mean value of X max . The
change of < X max > per decade of energy is called
the elongation rate.
Neutrinos
A highly inclined shower initiated by a neutrino
would have a significant electromagnetic component
leading to a broad time structure of detected
signals, in contrast to showers induced by protons
or nuclei in the upper atmosphere. Earthskimming
tau neutrinos can also lead to showers with similar
properties, induced by emerging tau leptons produced
in the Earth that decay just above the array.
A search has been made for such young inclined
showers and no candidates have been found [5,6].
Fig 5 : compared with simulations based on
different hadronic interaction models
Fig 4 : Differential and integrated upper limits for a
diffuse flux of down-going all-flavors neutrinos and
up-going tau neutrinos. Limits from other experiments
are also plotted, as well as backgrounds
expected from the GZK effect..
The data show a larger elongation rate below
10 18.25 eV and a smaller elongation rate above. If
the properties of hadronic interactions do not
change significantly over less than two orders of
magnitude in primary energy, this change would
imply a change in composition, becoming heavier
above this energy. In figure 6 we observe a decrease
of the fluctuations of X max with energy. Assuming
again that the hadronic interaction properties
do not change much within the observed energy
range, these decreasing fluctuations could be
explained by an increasing average mass of the
primary particles. The interpretation of these measurements
requires comparison to air shower simu-
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