In this paper we report on the observation of the anisotropy of cosmic ray
arrival direction at different angular scales with ARGO-YBJ. Evidence of new
few-degree excesses throughout the sky region 195$^{\circ}\leq$ R.A. $\leq$
315$^{\circ}$ is presented for the first time. We report also on the
measurement of the light-component (p+He) spectrum of primary cosmic rays in
the range 5 - 200 TeV.Comment: Invited talk to the 3rd Galileo - Xu Guangqi meeting, October 11-15,
  2011 Beijing (China

for the ARGO-YBJ Collaboration()

G. DI SCIASCIOINFN - Sezione Roma Tor Vergata, Italy

English

arXiv

G. Di Sciascio

Crossref

EPJ Web of Conferences

Measurement of Cosmic Ray Spectrum and Anisotropy with ARGO-YBJ

The combined measurement of the cosmic ray (CR) energy spectrum and anisotropy in their arrival direction distribution needs the knowledge of the elemental composition of the radiation to discriminate between different origin and propagation models. Important information on the CR mass composition can be obtained studying the EAS muon content through the measurement of the CR rate at different zenith angles.
In this paper we report on the observation of the anisotropy of galactic CRs at different angular scales with the ARGO-YBJ experiment. We report also on the study of the primary CR rate for different zenith angles. The light component (p+He) has been selected and its energy spectrum measured in the energy range (5 – 200) TeV for quasi-vertical events. With this analysis for the first time a ground-based measurement of the CR spectrum overlaps data obtained with direct methods for more than one energy decade, thus providing a solid anchorage to the CR spectrum measurements carried out by EAS arrays in the knee region.
Finally, a preliminary study of the non-attenuated shower component at a zenith angle θ > 70° (through the observation of the so-called horizantal air showers) is presented

EDP Sciences OAI-PMH repository (1.2.0)

The combined measurement of the cosmic ray (CR) energy spectrum and anisotropy in their arrival direction distribution needs the knowledge of the elemental composition of the radiation to discriminate between different origin and propagation models. Important information on the CR mass composition can be obtained studying the EAS muon content through the measurement of the CR rate at different zenith angles. In this paper we report on the observation of the anisotropy of galactic CRs at different angular scales with the ARGO-YBJ experiment. We report also on the study of the primary CR rate for different zenith angles. The light component (p+He) has been selected and its energy spectrum measured in the energy range (5 – 200) TeV for quasi-vertical events. With this analysis for the first time a ground-based measurement of the CR spectrum overlaps data obtained with direct methods for more than one energy decade, thus providing a solid anchorage to the CR spectrum measurements carried out by EAS arrays in the knee region. Finally, a preliminary study of the non-attenuated shower component at a zenith angle θ > 70° (through the observation of the so-called horizantal air showers) is presented

Di Sciascio G.

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Measurement of Cosmic Ray Spectrum and Anisotropy with ARGO-YBJ
G. Di Sciascio1,a on behalf of the ARGO-YBJ Collaboration
1INFN, Sezione di Roma Tor Vergata, Viale della Ricerca Scientifica 1, Roma, Italy I-00133
Abstract. The combined measurement of the cosmic ray (CR) energy spectrum and anisotropy in their arrival
direction distribution needs the knowledge of the elemental composition of the radiation to discriminate between
different origin and propagation models. Important information on the CR mass composition can be obtained
studying the EAS muon content through the measurement of the CR rate at different zenith angles.
In this paper we report on the observation of the anisotropy of galactic CRs at different angular scales with the
ARGO-YBJ experiment. We report also on the study of the primary CR rate for different zenith angles. The
light component (p+He) has been selected and its energy spectrum measured in the energy range (5 - 200) TeV
for quasi-vertical events. With this analysis for the first time a ground-based measurement of the CR spectrum
overlaps data obtained with direct methods for more than one energy decade, thus providing a solid anchorage
to the CR spectrum measurements carried out by EAS arrays in the knee region.
Finally, a preliminary study of the non-attenuated shower component at a zenith angleθ > 70◦ (through the
observation of the so-called horizantal air showers) is presented.
1 Introduction
The understanding of CRs origin at any energy is made
difficult by the poor knowledge of the elemental composi-
tion of the radiation. The determination of the CR arrival
direction does not depend on knowledge of the mass of the
primary particle, however, the use of combined data on the
energy spectrum and arrival direction distribution requires
the knowledge of the primary mass distribution to discrim-
inate between different origin and propagation models.
Inclined showers (θ > 60◦) induced by very high-
energy CRs are mainly produced by secondary muons,
in contrast to the vertical ones dominated by photons and
electrons stemming fromπ0 decays. Measurements of the
CR rate at different zenith angles gives information on the
relative number of muons in a shower, which is dependent
on the CR elemental composition, thus providing an im-
portant tool to probe the CR mass distribution.
As CRs are mostly charged nuclei, their arrival direc-
tion is deflected and highly isotropized by the action of
galactic magnetic field (GMF) they propagate through be-
fore reaching the Earth atmosphere. However, different
experiments [1–4] observed an energy-dependent"large
scale" anisotropy (LSA) in the sidereal time frame with an
amplitude of about 10−4 - 10−3, suggesting the existence of
two distint broad regions, one showing an excess of CRs
(called "tail-in"), distributed around 40◦ to 90◦ in Right
Ascension (R.A.). The other a deficit (the so-called "loss
cone"), distributed around 150◦ to 240◦ in R.A..
In the last years the Milagro [5] experiment reported
evidence of the existence of a medium angular scale
ae-mail: disciascio@roma2.infn.it
anisotropy (MSA) contained in the tail-in region. The ob-
servation of similar small scale anisotropies has been re-
cently claimed by the Icecube experiment in the Southern
hemisphere [4].
So far, no theory of CRs in the Galaxy exists which
is able to explain the origin of these different anisotropies
leaving the standard model of CRs and that of the local
GMF unchanged at the same time. A joint analysis of
concurrent data recorded by different experiments in both
hemispheres, as well as a measurement of energy spec-
trum and elemental composition of the anisotropy regions,
should be a high priority to clarify the observations.
The ARGO-YBJ experiment, located at the YangBa-
Jing Cosmic Ray Laboratory (Tibet, P.R. China, 4300
m a.s.l., 606 g/cm2), is an air shower array able to de-
tect the cosmic radiation with an energy threshold of a
few hundred GeV. The full detector is in stable data tak-
ing since November 2007 with a duty cycle larger than
85%. The trigger rate is 3.6 kHz. The detector charac-
teristics are described in [6–8]. Details on the analysis
procedure (e.g., reconstruction algorithms, data selection,
background evaluation, systematic errors) are discussed in
[9, 10]. The performance (angular resolution, pointing ac-
curacy, energy scale calibration) and the operation stability
are monitored on a monthly basis by observing the Moon
shadow, i.e., the deficit of CR detected in its direction [10].
The last results obtained by ARGO-YBJ are summarized
in [11].
In this paper the observation of CR anisotropy at differ-
ent angular scales with ARGO-YBJ is reported for differ-
ent primary energies. We report also on the measurement
of the primary CR spectrum for different zenith angles.
 This  is  an  Open Access  article distributed under the terms of the Creative Commons Attribution License 2 0 , which . permits unrestricted  use,  distributi
and reproduction in any medium, provided the original work is properly cited. 
on,
EPJ Web of Conferences
DOI: 10.1051/
C© Owned by the authors, published by EDP Sciences, 2013
,
epjconf 201/
04004 (2013)
35204004
52
Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20125204004
Figure 1. Large scale CR anisotropy observed by ARGO-YBJ
as a function of the energy. The color scale gives the relative CR
intensity.
2 Cosmic Ray Anisotropy
To study the anisotropy at different angular scales the
isotropic background of CRs has been estimated with two
well-known methods: the equi-zenith angle method [12]
and the direct integration method [13]. The equi-zenith
angle method, used to study the LSA, is able to elimi-
nate various spurious eff cts caused by instrumental and
environmental variations, such as changes in pressure and
temperature that are hard to control and tend to introduce
systematic errors in the measurement. The direct inte-
gration method, based on time-average, relies on the as-
sumption that the local distribution of the incoming CRs is
slowly varying and the time-averaged signal may be used
as a good estimation of the background content. Time-
averaging methods (TAMs) act eff ctively as a high-pass
filter, not allowing to inspect features larger than the time
over which the background is computed (i.e., 15◦/hour×∆t
in R.A.) [14]. The time interval used to compute the aver-
age spans∆t = 3 hours and makes us confident the results
are reliable for structures up to≈35◦ wide.
2.1 Large Scale Anisotropy
The observation of the CR large scale anisotropy by
ARGO-YBJ is shown in Fig. 1 as a function of the pri-
mary energy up to about 25 TeV.
The so-called‘tail-in’ and‘loss-cone’ regions, corre-
lated to an enhancement and a deficit of CRs, respectively,
are clearly visible with a statistical significance greater
than 20 s.d.. The tail-in broad structure appears to break
energy [eV]
1110 1210 1310 1410 1510
1
A
-610
-510
-410
-310
Norikura1973 Musala1975 Baksan1981 Morello1983
Norikura1989 EasTop1995 EasTop1996 Tibet1999
Tibet2005 Milagro2009 EasTop2009 Baksan2009
Utah1981 Ottawa1981 Holborn1983 Bolivia1985
Misato1985 Budapest1985 Hobart1985 Yakutsk1985
London1985 Socorro1985 HongKong1987 Baksan1987
Artyomovsk1990q Sakashita1990 Utah1991 Liapootah1995
Matsushiro1995 Poatina1995 Kamiokande1997 Macro2003
SuperKamiokande2007 IceCube2010 IceCube2012
ARGO-YBJ (this work)
Figure 2. Amplitude of the first harmonic as a function of the
energy, compared with other measurements.
Figure 3. Phase of the first harmonic as a function of the energy,
compared with other measurements.
up into smaller spots with increasing energy. In order to
quantify the scale of the anisotropy we studied the 1-D
R.A. projections integrating the sky maps inside a dec-
lination band given by the field of view of the detector.
Therefore, we fitted the R.A. profiles with the first two
harmonics. The resulting amplitude and phase of the first
harmonic are plotted in Fig. 2 and Fig. 3 where are com-
pared to a full compilation of measurements as a function
of the energy. The ARGO-YBJ results are in agreement
with those of other experiments, suggesting a decrease of
the anisotropy first harmonic amplitude at energies above
10 TeV.
2.2 Medium Scale Anisotropy
Fig. 4 shows the ARGO-YBJ sky map in equatorial coor-
dinates containing about 2·1011 events reconstructed with
a zenith angle≤50◦. The zenith cut selects the declina-
tion regionδ ∼ -20◦÷ 80◦. According to simulations, the
median energy of the isotropic CR proton flux is E50p ≈1.8
TeV (mode energy≈0.7 TeV).
The most evident features are observed by ARGO-YBJ
around the positionsα ∼ 120◦, δ ∼ 40◦ andα ∼ 60◦, δ ∼
-5◦, positionally coincident with the excesses detected by
Milagro [5]. These regions, named “1” and “2”, are ob-
served with a statistical significance of about 14 s.d.. The
deficit regions parallel to the excesses are due to a known
04004-p.2
EPJ Web of Conferences
ISVHECRI 2012
Figure 4. Medium scale CR anisotropy observed by ARGO-
YBJ. The color scale gives the statistical significance of the ob-
servation in standard deviations.
multiplicity
210 310
re
la
tiv
e 
ex
ce
ss
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
-310× 0.66 1.4 3.5 7.3 20
p
50E
region 1
region 2
Figure 5. Size spectrum of the regions 1 and 2. The vertical
axis represents the relative excess (Ev-Bg)/ . The upper scale
shows the corresponding proton median energy.
effect of the analysis, that uses also the excess events to
evaluate the background, overestimating this latter [14].
The left side of the sky map is full of few-degree ex-
cesses not compatible with random fluctuations (the statis-
tical significance is more than 6 s.d.). The observation of
these structures is reported here for the first time and to-
gether with that of regions 1 and 2 it may open the way to
an interesting study of the TeV CR sky.
To figure out the energy spectrum of the excesses, data
have been divided into five independent shower multiplic-
ity sets. The number of events collected within each re-
gion are computed for the event map (Ev) as well as for
the background one (Bg). The relative excess (Ev-Bg)/
is computed for each multiplicity interval. The result is
shown in the Fig. 5. Region 1 seems to have spectrum
harder than isotropic CRs and a cutoff around 600 shower
particles (proton median energy E50p = 8 TeV). On the other
hand, the excess hosted in region 2 is less intense and
seems to have a spectrum more similar to that of isotropic
CRs. As a reference value, the upper horizontal scale re-
ports the median energy of isotropic CR protons for each
multiplicity interval obtained via MC simulation.
2.3 The Compton-Getting effect
The origin of CR anisotropies is still unknown there-
fore, the observation of an expected anisotropy is im-
Figure 6. One-dimensional projection in right ascension of the
two-dimensional CR map in local solar time. The red line shows
the best-fit to ARGO-YBJ data (crosses).
Energy (GeV)
10
210
3
10 410
5
10
6
10
1
.7
5
 (
G
e
V
)
-1
-s
-s
r)
2
 (
m
2
.7
5
 E
×
 F
lu
x
 
410
5
10
p
He
ARGO - YBJ (p + He)
MACRO & EAS-TOP (p + He)
AMS (p, He)
BESS (p, He)
CAPRICE (P, He)CREAM (p, He)
Horandel (2003) CREAM (p + He)
JACEE (p + He) RUNJOB (p + He)
Figure 7. Light component (p+He) energy spectrum of primary
CRs measured by ARGO-YBJ compared with other experimen-
tal results.
portant to check the reconstruction algorithms, the back-
ground calculation and the stability of the detector per-
formance. A well-known expected anisotropy is the so-
called Compton-Getting (CG) eff ct, a dipole anisotropy
in the local solar frame, due to the Earth’s motion around
the Sun [16]. A significant signal compatible with CG is
seen by ARGO-YBJ in solar time above∼ 8 TeV to avoid
additional effects due to heliospheric magnetic field and
solar activity. In fact, we found that including lower en-
ergy events results in much larger modulation amplitudes
than those obtained when these events were excluded. Fig.
6 shows the solar variations observed by ARGO-YBJ to-
gether with the sinusoidal curve best fitted to the data. The
fair agreement between data and calculations (φ = 6:00 hr,
A = 9.7 · 10−5) make us confident about the capability of
ARGO-YBJ in detecting anisotropies at a level of 10−4.
3 CR primary spectrum
3.1 Light component (p+He) spectrum of CRs
Requiring quasi-vertical showers (θ < 30◦) and applying a
selection criterion based on the particle density, a sample
of events mainly induced by protons and helium nuclei,
with shower core inside a fiducial area (with radius∼28
m), has been selected. The contamination by heavier nu-
clei is found negligible. An unfolding technique based on
the Bayesian approach has been applied to the strip multi-
plicity distribution in order to obtain the differential energy
04004-p.3
-1θsec
0 0.5 1 1.5 2 2.5 3
-1
 s
r
-1
E
ve
nt
s 
s
-110
1
10
210
310
Figure 8. The zenith angle distribution of EAS measured with
ARGO-YBJ. The best fit out to∼60◦ is also shown.
)°Phi (
50 100 150 200 250 300 350
)°
A
ng
le
 (
0
1
2
3
4
5
6
7
8
9
10
0
200
400
600
800
1000
1200
1400
1600
1800Data with Th>80 deg
Profile of the mountain
Figure 9. Azimuthal distribution of showers with a reconstructed
zenith angle> 80◦ (red dashed line) compared to the mountain
profile seen by ARGO-YBJ (black continuous line).
spectrum of the light component (p+ He nuclei) in the en-
ergy range (5 - 200) TeV [17]. The spectrum measured
by ARGO-YBJ is compared with other experimental re-
sults in Fig. 7. Systematic eff cts due to different hadronic
models (Corsika 6.710 with QGSJet-II and SYBILL) and
to the selection criteria do not exceed 10%. The ARGO-
YBJ data agree remarkably well with the values obtained
by adding up the p and He fluxes measured by CREAM
both concerning the total intensities and the spectral in-
dex [18]. The value of the spectral index of the power-law
fit to the ARGO-YBJ data is -2.61±0.04, which should be
compared withγp = -2.66±0.02 andγHe = -2.58±0.02 ob-
tained by CREAM. The present analysis does not allow
the determination of the individual p and He contribution
to the measured flux, but the ARGO-YBJ data clearly ex-
clude the RUNJOB results [19]. We emphasize that for the
first time direct and ground-based measurements overlap
for a wide energy range thus making possible the cross-
calibration of the different experimental techniques.
3.2 Horizontal Air Showers
At zenith anglesθ > 60◦ an excess of events (the so-
called HAS, horizontal air showers) is observed above the
rate of EAS as expected from the exponential absoprtion
(with ΛEAS ≈ 220 g/cm2) of the air shower electromag-
-1θsec
0.5 1 1.5 2
 %
/m
ba
r
β
-1.4
-1.2
-1
-0.8
-0.6
-0.4
Figure 10. The barometric coefficient for different zenith angles
as measured by ARGO-YBJ.
Figure 11. Events observed by ARGO-YBJ with a reconstructed
zenith angleθ > 70◦. Only showers with more than 500 fired
strips on the central carpet are shown. The pixels represent 4×4
pads (about 2×2 m2).
netic component in the large atmospheric depth (see Fig.
8), which implies a decrease of the EAS counting rate with
Λc ≈ 130 g/cm2.
The physical nature of these showers is confirmed by
the absence of events from the direction of the sky shaded
by the mountains around the ARGO-YBJ detector, as can
be seen in the Fig. 9 where the shower rate as a function
of the reconstructed azimuthal angle is compared to the
shadow angle due to the surrounding mountains. The ex-
pected anti-correlation is clearly visible and the mountain
rofile is reproduced quite well.
Moreover, the dependence of the barometric effect on
the zenith angle, shown in Fig. 10, clearly shows a de-
viation from the secθ behaviour for secθ > 2. In fact,
the barometric coefficient β = 1n
dn
dx (n = counting rate,
x = atmospheric pressure) is related to zenith angle as:
β(θ) = β(0◦)secθ. This can be explained by the presence
of a "non-attenuated" EAS component that dominates for
angles larger than 70◦. Due to the small ARGO-YBJ effec-
tive area at large zenith angles, we expect that the observed
04004-p.4
EPJ Web of Conferences
ISVHECRI 2012
Differential strip multiplicity
310 410
-1 s
 N
-1
 s
r
-1
E
ve
nt
s 
s
-910
-810
-710
-610
-510
-410
-310
-210
-110
1
0<Th<15
15<Th<30
30<Th<40
40<Th<50
50<Th<60
60<Th<65
65<Th<70
70<Th<75
75<Th<80
80<Th<90
Figure 12. Differential strip spectra measured by ARGO-YBJ
for different zenith angles.
HAS are due to high energy single muons which interact
through bremmstrahlung (which dominate 10:1) or deep
inelastic scattering and initiate showers at the appropriate
depth (few hundreds g/cm2) for detection, as shown in Fig.
11 where some typical events observed by ARGO-YBJ are
displayed. The characteristic elliptical shape of the show-
ers, well contained in the central carpet, is clearly visi-
ble. Such showers are essentially electromagnetic, since
the remnant muons from the initial showers are dispersed
over a very large area.
)°Zenith angle (
0 10 20 30 40 50 60 70 80 90
γ
S
pe
ct
ra
l i
nd
ex
 
-3.8
-3.6
-3.4
-3.2
-3
-2.8
-2.6
-2.4
-2.2
Figure 13. Best-fit spectral indices calculated for the spectra of
Fig. 12.
In Fig. 12 the shower rate measured by ARGO-YBJ
is shown, as a function of the fired strips number, for dif-
ferent primary zenith angles. The spectra soften with in-
creasing angle up to about 70◦, as can be appreciated in
Fig. 13 where the best-fit spectral indices are plotted. In
the zenith angle region 50◦ - 70◦ a quick transition to a
value of about -3.6, characteristic of the EAS muon com-
ponent, is observed. Detailed simulations to reproduce the
observed size spectrum of HAS are under way.
4 Conclusions
The ARGO-YBJ detector exploiting the full coverage ap-
proach and the high segmentation of the readout is imag-
ing the front of atmospheric showers with unprecedented
resolution and detail. The digital and analog readout will
allow a deep study of the CR phenomenology in the wide
TeV - PeV energy range.
In this paper the observation of CR anisotropy at dif-
ferent angular scales is reported for different primary en-
ergies. The large scale CR anisotropy has been clearly ob-
served up to about 25 TeV. The existence of different few-
degree excesses in the Northern sky is showed by ARGO-
YBJ for the first time.
We reported also on the measurement of the primary
CR rate for different zenith angles. The light component
(p+He) has been selected and its energy spectrum mea-
sured in the energy range (5 - 200) TeV for quasi-vertical
events.
A preliminary study of HAS with ARGO-YBJ is also
presented. More than 107 well-contained horizontal events
have been analyzed, thus providing a “well shielded” sam-
ple useful to study the production and interaction of high
energy CR muons and neutrinos.
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