We critically review the common belief that ultrahigh energy cosmic rays are
protons or atomic nuclei with masses not exceeding that of iron. We find that
heavier nuclei are indeed possible, and discuss possible sources and
acceleration mechanisms for such primaries. We also show detailed simulations
of extensive air showers produced by ``superheavy'' nuclei, and discuss
prospects for their detection in future experiments.Comment: Talk to be presented at the International Symposium on Very High
  Energy Cosmic Ray Interactions X

Anchordoqui

Anders

Bhattacharjee

Bird

Burbidge

Elbert

Engelbracht

Fletcher

Forbes

Glasmacher

Greisen

Heckman

J.D. Swain

L.A. Anchordoqui

Lagage

Langer

M.T. Dova

Mazzali

Muxlow

Paglione

Rieke

S. Reucroft

Sciutto

Smith

Stecker

T. Paul

T.P. McCauley

Ulvestad

Yoshida

Zatsepin

English

arXiv

We critically review the common belief that ultrahigh energy cosmic rays are protons or atomic nuclei with masses not exceeding that of iron. We find that heavier nuclei are indeed possible, and discuss possible sources and acceleration mechanisms for such primaries. We also show detailed simulations of extensive air showers produced by “superheavy” nuclei, and discuss prospects for their detection in future experiments.Facultad de Ciencias Exacta

Anchordoqui, Luis Alfredo

Dova, María Teresa

McCauley, Thomas

Paul, Thomas

Reucroft, Stephen

Swain, John

El Servicio de Difusión de la Creación Intelectual

SUPPLEMENTS 
ELSEVIER Nuclear Physics B (Proc. Suppl.) 97 (2001) 203-206 www.elscvicr.tiocatc/npe 
A pot of gold at the end of the cosmic “raynbow”? 
L. A. AnchordoquY , M. T. Dovabt, T. P. McCauleyat, T. Paulas, S. Reucroft?q, and J. D. SwainaIl 
aDepartment of Physics, Northeastern University, Boston, MA 02115, USA 
bDepartamento de Fisica, UNLP, CC 67 (1900) La Plata, Argentina 
We critically review the common belief that ultrahigh energy cosmic rays are protons or atomic nuclei with 
masses not exceeding that of iron. We fmd that heavier nuclei are indeed possible, and discuss possible sources and 
acceleration mechanisms for such primaries. We also show detailed simulations of extensive air showers produced 
by “superheavy” nuclei, and discuss prospects for their detection in future experiments. 
The unambiguous detection of cosmic rays 
(CRs) with energies above 102’ eV (see [l] for a 
survey and bibliography on the subject) is a fact 
of outstanding astrophysical interest. As shown 
in the pioneering works of Greisen, Zatsepin, and 
Kuzmin [2], the possible sources and the accel- 
erating mechanisms are constrained by the ob- 
served particle spectra due to the interaction with 
the universal radiation and magnetic fields on the 
way to the observer. The low flux of particles 
at the end of the spectrum (the typical rate of 
CRS above 102’ eV is one event/km2/century) 
puts strong demands on the collection power of 
the experiments, such as can only be achieved by 
extended air shower detection arrays at ground 
level. This indirect method of detection bears a 
number of serious difficulties in determining the 
energy, mass and/or arrival direction of the pri- 
mary particles. Astrophysical mechanisms to ac- 
celerate particles to energies of up to 1021-22 eV 
have been identified, but they require exceptional 
sites [3]. Essentially, the mechanisms rely on a net 
transfer of macroscopic kinetic energy of moving 
magnetized plasma to individual charged parti- 
cles due to repeated collisionless scatterings with 
randomly moving inhomogeneities of the turbu- 
lent magnetic field, or with shocks in the medium. 
*doquiOhepmail.physics.neu.edu 
tdovaQvenus.fisica.unlp. edu.ar 
tmccauleyOhepmail.physics.neu.edu 
Stom.paul@hepmail.physics.neu.edu 
‘stephen.reucroftQcern.ch 
Iljohn.swainOcern.ch 
Since the high-energy cutoff of shock acceleration 
increases with the charge number of the nucleus 
2, heavy ions would be nice candidates for ultra 
high-energy CRs. In this talk we review the cur- 
rent status of superheavy-nuclei as primaries of 
the cosmic radiation. 
It has been generally thought that 5sFe is a 
significant end product of stellar evolution and 
higher mass nuclei are rare in the cosmic radia- 
tion. Strictly speaking, the atomic abundances of 
middle-weight (60 5 A < 100) and heavy-weight 
(A > 100) elements are approximately 3 and 5 or- 
ders of magnitude lower, respectively, than that 
of the iron group [4]. The explosion of the H-rich 
envelopes of type II supernovae has long been held 
responsible for the synthesis of the stable super- 
heavy nuclides. There is observational evidence 
that these nuclides are synthesized in a variety of 
chemically peculiar Red Giants [5] and in special 
objects like FG Sagittae [6] or SN1987A [7]. Con- 
sequently, starbursts (astrophysical environments 
which comprise a considerable population of 0 
and Red Giant stars [8], with a supernovae rate 
as high as 0.2 - 0.3 yr-’ [9]) appear as the natural 
sources able to produce relativistic super-heavy 
nuclei. Very recently, we have presented a com- 
prehensive study of a possible nearby superheavy- 
nucleus-Zevatron [lo]. We have shown that it is 
likely that nuclei heavier than iron with energies 
above a few PeV can escape from the dense core of 
a nearby starburst galaxy (like M82, NGC 253), 
and eventually be re-accelerated to superhigh en- 
ergies (E 2 102’ eV) at the terminal shocks of 
0920-5632/01/% - see front matter 0 2001 Elsevier Science B.V All rights reserved. 
PI1 SO920-5632(01)01264-6 
204 L.A. Anchordoqui et al. /Nuclear Physics B (Proc. Suppl.) 97 (2001) 203-206 
galactic superwinds generated by the starburst.’ 
Specifically, ions are diffusively accelerated up to 
a few PeV at single supernova shock waves in the 
nuclear region of the galaxy without suffering sig- 
nificant spallation reactions [12]. The cosmic ray 
outflow is convection dominated, and the typical 
residence time of the nuclei in the star-burst re- 
sults in t N 1 x 1011 s. Thus, the total path trav- 
eled is substantially shorter than the mean free 
path (which scales as Ae213) of a super-heavy nu- 
cleus (for details see [13]). Those which are able 
to escape from the central region without suffer- 
ing catastrophic interactions could be eventually 
re-accelerated to superhigh energies at the termi- 
nal shocks of galactic superwinds generated by 
the starburst. In terms of parameters that can 
be determined from observations the maximum 
energy of the nuclei reads (again the reader is re- 
ferred to [13]), 
The predicted kinetic energy and mass fluxes of 
the star-burst of NGC 253 derived from the mea- 
sured IR luminosity are &.,,, M 2 x 1O42 erg s-l 
and b M 1.2 Ma yr-’ , respectively [14]. The 
starburst age is estimated from numerical models 
that use theoretical evolutionary tracks for indi- 
vidual stars and make sums over the entire stellar 
population at each time in order to produce the 
galaxy luminosity as a function of time [15]. Fit- 
ting the observational data these models provide 
a range of suitable ages for the starburst phase 
that, in the case of NGC 253, goes from 5 x 10’ 
to 1.6 x lo* yr (also valid for M82) [15]. These 
models must assume a given initial mass func- 
tion (IMF), which usually is taken to be a power- 
law with a variety of slopes. Recent studies have 
shown that the same IMF can account for the 
properties of both NGC 253 and M82 [IS]. Here 
we shall assume a conservative age r = 50 Myr. 
Finally, the radio and y-ray emission from NGC 
253 are well matched by models with B N 50pG 
[17]. All in all, the balance between the energy 
gains and synchrotron/photodisintegration losses 
71t is important to stress that M82 is positioned close to 
the arrival direction of the highest CR event detected on 
Earth. This was first pointed out in [ll]. 
[18] of a super-heavy nucleus (like gold) leads to 
a maximum energy of a few hundred EeV.s 
Let us turn now to the analysis of the energy 
loses during propagation. The energetic nucleus 
is seen to lose energy mainly as a result of its 
photodisintegration. In the universal rest frame 
(in which the microwave backgroud radiation is 
at 3K), the disintegration rate R of an extremely 
high energy nucleus with Lorentz factor F, prop- 
agating through an isotropic soft photon back- 
ground of density n is given by [19], 
2rE dE’E’cr(E’), (2) 
where primed quantities refer to the rest frame of 
the nucleus, and (T stands for the total photon ab- 
sortion cross section. Above 1020 eV, the energy 
losses are dominated by collisions with the relic 
photons. The fractional energy loss around this 
energy is R - lo-l5 s-l. With this in mind, it 
is straightforward to check that superheavy nuclei 
may reach the Earth (for details see Fig. 2 of Ref. 
[lo]). In the rest of this report we shall discuss the 
characteristics of the extensive air showers that 
these nuclei may produce after interaction with 
the atmosphere. 
Golden Shower Simulations: In order to per- 
form the simulations we shall adopt the superpo- 
sition model. This model assumes that an average 
shower produced by a nucleus with energy E and 
mass‘number A is indistinguishable from a super- 
position of A proton showers, each with energy 
E JA. We have generated several sets of lg7Au air 
shower simulations by means of the AIRES Monte 
Carlo code [20]. The sample was distributed in 
the energy range of lo’* up to 1020.5 eV. SIBYLL 
was used to reproduce hadronic collisions above 
200 GeV [22]. All shower particles with ener- 
gies above the following thresholds were tracked: 
750 keV for gammas, 900 keV for electrons and 
positrons, 10 MeV for muons, 60 MeV for mesons 
and 120 MeV for nucleons and nuclei. The par- 
ticles were injected vertically at the top of the 
atmosphere (100 km.a.s.l), and the surface de- 
sit should be stressed that for A > 140 the bulk solar- 
system abundance distribution peaks at A = 195 [21]. To 
make some estimates, we then refer our calculations to a 
gold nucleus. 
L.A. Anchordoqui et al. /Nuclear Physics B (Proc. Suppl.) 97 (2001) 203-206 205 
tector array was put at a depth of 1036 g/cm2, 
i.e., at sea level. Secondary particles of differ- 
ent types and all charged particles in individual 
showers were sorted according to their distance R 
from the shower axis. 
Figure 1. Ground lateral distributions of proton 
and ig7Au air-showers. The incident energy is 
E = 3 x 102’ eV. 
In Fig. 1 we show the lateral distributions of 
different groups of secondary particles (we have 
considered separately y e+e- and p+p-). One 
can see that the number of miens from the gold 
nucleus shower is greater than the number of 
muons from the proton shower. 
As the cascade develops in the atmosphere, it 
grows until a maximum size (number of particles) 
is reached. The location in the atmosphere where 
the cascade has developed the maximum size is 
denoted by X,,, with units of g cmm2. For cas- 
cades of a given total energy, heavier nuclei have 
smaller X,, than nucleons because the shower 
is already subdivided into A nucleons when it en- 
ters the atmosphere. At 1020 eV, the < X,,, > 
of a proton (gold) shower is x 879 g/cm2 (x 777 
g/cm2). A dust-grain has an even larger cross 
section, so it tends to interact sooner than pro- 
tons and nuclei [24]. In Fig. 2, we compare the 
longitudinal profile of showers initiated by a pro- 
ton, a gold-nucleus and a dust-grain. It is clearly 
seen how the X,, decreases when increasing the 
mass. The simulated gold shower is partially con- 
sistent with the Fly’s Eye data. Furthermore, its 
longitudinal development better reproduces the 
data than protons or dust-grains. It should be re- 
marked, however, that extensive air shower simu- 
lation depends on the hadronic interaction model 
[25]. We also point out that for the simulation, 
detector effects were not taken into account. 
150 
100 
0 
00 
Depth (g/cm’) 
Figure 2. Longitudinal development of 3 x 1020 
eV showers induced by a proton, a gold-nucleus 
and a dust-grain (log l? = 4.5), together withy the 
data of the highest event recorded by Fly’s Eye 
1231. 
Even though the superheavy nucleus hypoth- 
esis is partially supported by data from the 
CASAMIA experiment [26], more data is cer- 
tainly needed to verify this model. In order to 
significantly increase the statistics at the end of 
206 LA. Anchordoyui et al. /Nuclear Physics B (Proc. Suppl.) 97 (2001) 203-206 
the spectrum, the Southern Auger Observatory is 
currently under construction 2271. It will consist 
of a surface array which will record the lateral and 
temporal distribution of shower particles, and an 
optical air fluorescence detector, which will ob- 
serve the air shower development in the atmo- 
sphere. These two techniques provide comple- 
mentary methods of extracting the required in- 
formation from the shower to test the ideas dis- 
cussed in this paper. 
Note added in proof: After this talk was 
presented we have completed the air shower anal- 
ysis by using a more inelastic hadronic model for 
the first generation of particles [28]. Definite con- 
clusions on the nature of the highest energy Fly’s 
Eye event cannot be reached, mainly because of 
large fluctuations from model to model. How- 
ever, it should be stressed that even though the 
primary chemical composition remains hidden by 
the hadronic interaction model, it is evident that 
in both cases the shower profile is inconsistent 
with a proton, or a gamma-ray primary. 
This work was partially supported by CON- 
ICET, Fundacion Antorchas and the National 
Science Foundation. 
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A pot of gold at the end of the cosmic “raynbow”?

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A pot of gold at the end of the cosmic "raynbow"?

A pot of gold at the end of the cosmic "raynbow"?

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