Observation of the suppression of the flux of cosmic rays above 4x10(19) eV

TLDR: The energy spectrum of cosmic rays above 2.5 x 10;{18} eV, derived from 20,000 events recorded at the Pierre Auger Observatory, is described and the hypothesis of a single power law is rejected with a significance greater than 6 standard deviations.

Abstract: The energy spectrum of cosmic rays above 2.5 x 10;{18} eV, derived from 20,000 events recorded at the Pierre Auger Observatory, is described. The spectral index gamma of the particle flux, J proportional, variantE;{-gamma}, at energies between 4 x 10;{18} eV and 4 x 10;{19} eV is 2.69+/-0.02(stat)+/-0.06(syst), steepening to 4.2+/-0.4(stat)+/-0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz'min.

Figures

FIG. 1 (color online). Correlation between lgS38 and lgEFD for the 661 hybrid events used in the fit. The full line is the best fit to the data. The fractional differences between the two energy estimators are inset.

FIG. 2. Upper panel: The differential flux J as a function of energy, with statistical uncertainties. Data are listed at [32]. Lower Panel: The fractional differences between Auger and HiRes I data [3] compared with a spectrum with an index of 2.69.

Full text

PUBLISHED VERSION
Abraham, J.;...; Clay, Roger William; Colombo, E.;...; Dawson, Bruce Robert; de Almeida,
R. M.; ... et al.; Pierre Auger Collaboration
Observation of the suppression of the flux of cosmic rays above 4×1019 eV Physical
Review Letters, 2008; 101(6):061101
©2008 American Physical Society
http://link.aps.org/doi/10.1103/PhysRevLett.101.061101
http://link.aps.org/doi/10.1103/PhysRevD.62.093023
http://hdl.handle.net/2440/47607
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10
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May 2013

Observation of the Suppression of the Flux of Cosmic Rays above 4  10
19
eV
J. Abraham,
1
P. Abreu,
2
M. Aglietta,
3
C. Aguirre,
4
D. Allard,
5
I. Allekotte,
6
J. Allen,
7
P. Allison,
8
J. Alvarez-Mun
˜
iz,
9
M. Ambrosio,
10
L. Anchordoqui,
11,12
S. Andringa,
2
A. Anzalone,
13
C. Aramo,
10
S. Argiro
`
,
14
K. Arisaka,
15
E. Armengaud,
5
F. Arneodo,
16
F. Arqueros,
17
T. Asch,
18
H. Asorey,
19
P. Assis,
2
B. S. Atulugama,
20
J. Aublin,
21
M. Ave,
22
G. Avila,
23
T. Ba
¨
cker,
24
D. Badagnani,
25
A. F. Barbosa,
26
D. Barnhill,
15
S. L. C. Barroso,
27
B. Baughman,
8
P. Bauleo,
28
J. J. Beatty,
8
T. Beau,
5
B. R. Becker,
29
K. H. Becker,
30
J. A. Bellido,
20
S. BenZvi,
31
C. Berat,
32
T. Bergmann,
33
P. Bernardini,
34
X. Bertou,
19
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35
P. Billoir,
21
O. Blanch-Bigas,
21
F. Blanco,
17
P. Blasi,
36,37,38
C. Bleve,
39
H. Blu
¨
mer,
33,40
M. Boha
´
c
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ova
´
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41
C. Bonifazi,
21,26
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3
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28
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44
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J. Lozano Bahilo,
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M. C. Maccarone,
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C. Meurer,
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G. Miele,
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T. Schmidt,
33
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83
P. Schova
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40
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25
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A. Segreto,
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24
(The Pierre Auger Collaboration)
1
Universidad Tecnolo
´
gica Nacional, FR-Mendoza, Argentina
2
LIP and Instituto Superior Te
´
cnico, Lisboa, Portugal
3
Istituto di Fisica dello Spazio Interplanetario (INAF), Universita
`
di Torino and Sezione INFN, Torino, Italy
4
Universidad Catolica de Bolivia, La Paz, Bolivia
5
Laboratoire AstroParticule et Cosmologie, Universite
´
Paris 7, IN2P3/CNRS, Paris, France
6
Centro Ato
´
mico Bariloche, Comision Nacional de Energı
´
a Ato
´
mica and Instituto Balseiro (CNEA-UNC),
San Carlos de Bariloche, Argentina
7
New York University, New York, New York, USA
8
Ohio State University, Columbus, Ohio, USA
9
Universidad de Santiago de Compostela, Spain
10
Sezione INFN di Napoli, Napoli, Italy
11
University of Wisconsin, Milwaukee, Wisconsin, USA
12
Northeastern University, Boston, Massachusetts, USA
13
Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo (INAF), Palermo, Italy
14
Universita
`
di Torino and Sezione INFN, Torino, Italy
15
University of California, Los Angeles, California, USA
16
INFN, Laboratori Nazionali del Gran Sasso, Assergi (L’Aquila), Italy
17
Universidad Complutense de Madrid, Madrid, Spain
18
Forschungszentrum Karlsruhe, Institut fu
¨
r Prozessdatenverarbeitung und Elektronik, Germany
19
Centro Ato
´
mico Bariloche, Comisio
´
n Nacional de Energı
´
a Ato
´
mica, San Carlos de Bariloche, Argentina
20
Pennsylvania State University, University Park, Pennsylvania, USA
21
Laboratoire de Physique Nucle
´
aire et de Hautes Energies, Universite
´
s Paris 6 & 7, IN2P3/CNRS, Paris Cedex 05, France
22
University of Chicago, Enrico Fermi Institute, Chicago, Illinois, USA
23
Pierre Auger Southern Observatory and Comisio
´
n Nacional de Energı
´
a Ato
´
mica, Malargu
¨
e, Argentina
24
Universita
¨
t Siegen, Siegen, Germany
25
IFLP, Universidad Nacional de La Plata and CONICET, La Plata, Argentina
26
Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, RJ, Brazil
27
Universidade Estadual do Sudoeste da Bahia, Vitoria da Conquista, BA, Brazil
28
Colorado State University, Fort Collins, Colorado, USA
29
University of New Mexico, Albuquerque, New Mexico, USA
30
Bergische Universita
¨
t Wuppertal, Wuppertal, Germany
31
University of Wisconsin, Madison, Wisconsin, USA
32
Laboratoire de Physique Subatomique et de Cosmologie, IN2P3/CNRS, Universite
´
Grenoble 1 et INPG, Grenoble, France
33
Universita
¨
t Karlsruhe (TH), Institut fu
¨
r Experimentelle Kernphysik (IEKP), Karlsruhe, Germany
34
Dipartimento di Fisica dell’Universita
`
del Salento and Sezione INFN, Lecce, Italy
35
Max-Planck-Institut fu
¨
r Radioastronomie, Bonn, Germany
36
Fermilab, Batavia, Illinois, USA
37
Universita
`
dell’Aquila and INFN, L’Aquila, Italy
38
Osservatorio Astrofisico di Arcetri, Florence, Italy
39
School of Physics and Astronomy, University of Leeds, United Kingdom
40
Forschungszentrum Karlsruhe, Institut fu
¨
r Kernphysik, Karlsruhe, Germany
41
Institute of Physics of the Academy of Sciences of the Czech Republic, Prague, Czech Republic
42
Colorado State University, Pueblo, Colorado, USA
PRL 101, 061101 (2008)
PHYSICAL REVIEW LETTERS
week ending
8 AUGUST 2008
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43
Universidad de Granada & C.A.F.P.E., Granada, Spain
44
Case Western Reserve University, Cleveland, Ohio, USA
45
University of Minnesota, Minneapolis, Minnesota, USA
46
Universita
`
di Milano and Sezione INFN, Milan, Italy
47
Universita
`
di Catania and Sezione INFN, Catania, Italy
48
Universidade de Sao Paulo, Instituto de Fisica, Sao Paulo, SP, Brazil
49
Universidade Estadual de Campinas, IFGW, Campinas, SP, Brazil
50
Michigan Technological University, Houghton, Michigan, USA
51
Institute of Integrated Information Systems, University of Leeds, United Kingdom
52
University of Adelaide, Adelaide, S.A., Australia
53
Laboratorio Tandar, Centro Ato
´
mico Constituyentes, CNEA, Buenos Aires, Argentina
54
University of Pennsylvania, Philadelphia, Pennsylvania, USA
55
Pierre Auger Southern Observatory, Malargu
¨
e, Argentina
56
IMAPP, Radboud University, Nijmegen, Netherlands
57
NIKHEF, Amsterdam, Netherlands
58
Laboratoire de l’Acce
´
le
´
rateur Line
´
aire, Universite
´
Paris-Sud, IN2P3/CNRS, Orsay, France
59
Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Michoacan, Mexico
60
Laboratory for Astroparticle Physics, University of Nova Gorica, Slovenia
61
Universidad Tecnolo
´
gica Nacional, FR-Mendoza and Fundacio
´
n Universidad Tecnolo
´
gica Nacional, Argentina
62
Universidade Federal do Rio de Janeiro, Instituto de Fı
´
sica, Rio de Janeiro, RJ, Brazil
63
Universidad de Alcala
´
, Alcala
´
de Henares (Madrid), Spain
64
Institut de Physique Nucle
´
aire, Universite
´
Paris- Sud, IN2P3/CNRS, Orsay, France
65
Universita
`
di Napoli ‘‘Federico II’’ and Sezione INFN, Napoli, Italy
66
Universita
`
di Roma II ‘‘Tor Vergata’’ and Sezione INFN, Roma, Italy
67
RWTH Aachen University, III. Physikalisches Institut A, Aachen, Germany
68
Institute for Nuclear Science and Technology, Hanoi, Vietnam
69
Universidad Nacional Autonoma de Mexico, Mexico, D.F., Mexico
70
Louisiana State University, Baton Rouge, Louisiana, USA
71
University of Hawaii, Honolulu, Hawaii, USA
72
Centro Ato
´
mico Constituyentes, Comisio
´
n Nacional de Energı
´
a Ato
´
mica and CONICET, Argentina
73
ASTRON, Dwingeloo, Netherlands
74
J. Stefan Institute, Ljubljana, Slovenia
75
Pontifı
´
cia Universidade Cato
´
lica, Rio de Janeiro, RJ, Brazil
76
University of Ło
´
dz
´
,Ło
´
dz, Poland
77
Departamento de Fı
´
sica, Centro Ato
´
mico Bariloche, Comisio
´
n Nacional de Energı
´
a Ato
´
mica and CONICET, Argentina
78
Universidade Federal Fluminense, Instituto de Fisica, Nitero
´
i, RJ, Brazil
79
Centro de Investigacio
´
n y de Estudios Avanzados del IPN (CINVESTAV), Me
´
xico, D.F., Mexico
80
Institute of Nuclear Physics PAN, Krakow, Poland
81
Departamento de Fı
´
sica, FCEyN, Universidad de Buenos Aires y CONICET, Argentina
82
Universidade Estadual de Feira de Santana, Brazil
83
Kernfysisch Versneller Instituut, University of Groningen, Groningen, Netherlands
84
Universidade Federal do ABC, Santo Andre
´
, SP, Brazil
85
Beneme
´
rita Universidad Auto
´
noma de Puebla, Puebla, Mexico
86
Centro Ato
´
mico Constituyentes, Comisio
´
n Nacional de Energı
´
a Ato
´
mica and UTN-FRBA, Argentina
87
Southern University, Baton Rouge, Louisiana, USA
88
University of Utah, Salt Lake City, Utah, USA
89
Charles University, Institute of Particle & Nuclear Physics, Prague, Czech Republic
90
Centro de Investigaciones en La
´
seres y Aplicaciones, CITEFA and CONICET, Argentina
91
Instituto de Fı
´
sica Corpuscular, CSIC-Universitat de Vale
`
ncia, Valencia, Spain
92
Universidade Federal da Bahia, Salvador, BA, Brazil
93
Dipartimento di Ingegneria dell’Innovazione dell’Universita
`
del Salento and Sezione INFN, Lecce, Italy
94
Universita
`
di Genova and Sezione INFN, Genova, Italy
95
Instituto de Astronomı
´
ayFı
´
sica del Espacio (CONICET), Buenos Aires, Argentina
96
Colorado School of Mines, Golden, Colorado, USA
97
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, United Kingdom
98
University of Nebraska, Lincoln, Nebraska, USA
99
Argonne National Laboratory, Argonne, Illinois, USA
100
Universidad Mayor de San Andre
´
s, Bolivia
101
Departamento de Fı
´
sica, Universidad Nacional de La Plata and Fundacio
´
n Universidad Tecnolo
´
gica Nacional, Argentina
(Received 11 April 2008; published 4 August 2008)
PRL 101, 061101 (2008)
PHYSICAL REVIEW LETTERS
week ending
8 AUGUST 2008
061101-3

The energy spectrum of cosmic rays above 2:5  10
18
eV, derived from 20 000 events recorded at the
Pierre Auger Observatory, is described. The spectral index  of the particle flux, J / E

, at energies
between 4  10
18
eV and 4  10
19
eV is 2:69  0:02stat0:06syst, steepening to 4:2  0:4stat
0:06syst at higher energies. The hypothesis of a single power law is rejected with a significance greater
than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and
Kuz’min.
DOI: 10.1103/PhysRevLett.101.061101 PACS numbers: 98.70.Sa, 95.85.Ry, 96.50.sb, 96.50.sd
We report a measurement of the energy spectrum of
cosmic rays showing that the flux is strongly suppressed
above 4  10
19
eV. This is in accord with the 1966 pre-
diction of Greisen [1] and of Zatsepin and Kuz’min [2]
(GZK) that the spectrum should steepen around 5 
10
19
eV as cosmic rays from cosmologically distant
sources suffer energy losses when propagating through
the cosmic microwave radiation. With an exposure twice
that of HiRes [3] and 4 times that of AGASA [4], our
evidence supports the recent report of the former.
The Pierre Auger Observatory, located near Malargu
¨
e
(Argentina) at 1400 m a.s.l., is used to measure the prop-
erties of extensive air showers (EAS) produced by the
highest-energy cosmic rays. At ground level, the elec-
trons, photons, and muons of EAS can be detected using
instruments deployed in a large surface array. Additionally,
as EAS move through the atmosphere, ultraviolet light is
emitted from nitrogen excited by charged particles. This
fluorescence light is proportional to the energy deposited
by the shower along its path [5]. The Observatory uses
1600 water-Cherenkov detectors, each containing 12
tonnes of water, viewed by three 9
00
photomultipliers, to
detect the photons and charged particles. The surface
detectors are laid out over 3000 km
2
on a triangular grid
of 1.5 km spacing and is overlooked by 4 fluorescence
detectors. Each fluorescence detector (FD), located on
the perimeter of the area, houses 6 telescopes. EAS de-
tected by both types of detector are hybrid events and
play a key role in the analysis. The field of view of each
telescope is 30

in azimuth, and 1.5

–30

in elevation.
Light is focused on a camera containing 440 hexagonal
pixels, of 18 cm
2
, at the focus of a 11 m
2
mirror. The
design and status of the Observatory are described in
[6,7]. Between 1 Jan 2004 and 31 Aug 2007, the numbers
of telescopes increased from 6 to 24 and of surface detec-
tors from 154 to 1388. The analysis of data from this period
is described.
A cosmic ray of 10
19
eV arriving vertically typically
produces signals in 8 surface detectors. Using relative
timing, the direction of such an event is reconstructed
with an angular accuracy of about 1

[8]. Signals are
quantified in terms of the response of a surface detector
(SD) to a muon travelling vertically and centrally through
it (a vertical equivalent muon or VEM). Calibration of each
SD is carried out continuously with 2% accuracy [9]. The
signals are fitted in each event to find the VEM size at
1000 m, S1000 [10]. The uncertainty in every S1000 is
found, accounting for statistical fluctuations of the signals,
systematic uncertainties in the assumption of the falloff of
signal with distance and the shower-to-shower fluctuations
[8]. Above 10
19
eV, the uncertainty in S1000 is about
10%.
The longitudinal development of EAS in the atmosphere
is measured using the fluorescence detectors. The light
produced is detected as a line of illuminated pixels in
one or more FT cameras. The positions of these pixels
and the arrival time of the light determine the shower
direction. The signal, after correcting for attenuation due
to Rayleigh and aerosol scattering, is proportional to the
number of fluorescence photons emitted in the field of view
of the pixel. Cherenkov light produced at angles close to
the shower axis can be scattered towards the pixels: this
contamination is accounted for [11]. A Gaisser-Hillas
function [12] is used to reconstruct the shower profile
which provides a measurement of the energy of the EAS
deposited in the atmosphere. To derive the primary energy,
an estimate of the missing energy carried into the ground
by muons and neutrinos must be made based on assump-
tions about the mass of cosmic rays and of the appropriate
hadronic model. For a primary beam that is a 50=50
mixture of protons and iron, simulations of showers with
the
QGSJET01 model indicate a correction of 10% [13]. The
systematic uncertainty is 4% [14].
Detailed understanding of the fluorescence emission is
needed for accurate energy determination. The absolute
fluorescence yield in air at 293 K and 1013 h Pa from the
337 nm band is 5:05  0:71 photons=MeV of energy de-
posited [15]. The wavelength and pressure dependence of
the yield adopted follow [16]. Systematic uncertainties in
the FD energy measurement have been estimated.
Measurements, made in combination with the fluorescence
detectors, are used to measure the quality and transmission
properties of the atmosphere. In particular, the vertical
aerosol optical depth (VAOD) profile [17] is found every
15 min by observing the light scattered from a centrally
located laser of an energy equivalent to a few 10
19
eV at
355 nm [18] yielding an hourly average. The average
correction to E
FD
from the VAOD measurement is 5%
at 3  10
18
eV rising to 18% at 5  10
19
eV, reflecting
the increase of the average distance of such events from an
FD. The absolute calibration of the telescopes is measured
every few months and is constantly adjusted using relative
calibrations [19]. The largest uncertainties are in the abso-
lute fluorescence yield (14%), the absolute calibration of
PRL 101, 061101 (2008)
PHYSICAL REVIEW LETTERS
week ending
8 AUGUST 2008
061101-4