Evidence of azimuthal asymmetries in the time structure and signal size have
been found in non-vertical showers at the Pierre Auger Observatory. It has been
previously shown that the asymmetry in time distributions offers a new
possibility for the determination of the mass composition. New studies have
demonstrated that the dependence of the asymmetry parameter in the rise-time
and fall-time distributions with sec(theta) shows a clear peak. Both, the
position of the peak, X_asymax, and the size of the asymmetry at X_asymax are
sensitive to primary mass composition and have a small dependence on energy. In
this paper a study of the discriminating power of the new observables to
separate primary species is presented.Comment: To be published in the Proceedings of 29th International Cosmic Ray
  Conference (ICRC 2005), Pune, India, August 3-10, 200

Arqueros, F.

Dova, M. T.

Garcia-Pinto, D.

Mancenido, M.

Mariazzi, A. G.

English

arXiv

Evidence of azimuthal asymmetries in the time structure and signal size have been found in non-vertical showers at the Pierre Auger Observatory. It has been previously shown that the asymmetry in time distributions offers a new possibility for the determination of the mass composition. New studies have demonstrated that the dependence of the asymmetry parameter in the rise-time and fall-time distributions with secθ shows a clear peak. Both, the position of the peak,  Xasymax, and the size of the asymmetry at   Xasymax are sensitive to primary mass composition and have a small dependence on energy. In this paper a study of the discriminating power of the new observables to separate primary species is presented.Facultad de Ciencias Exacta

Dova, María Teresa

Manceñido, Mónica E.

Mariazzi, Analisa Gabriela

Arqueros, Fernando

García-Pinto, Diego

El Servicio de Difusión de la Creación Intelectual

29th International Cosmic Ray Conference Pune (2005) 7, 58
Hinting at primary composition using asymmetries in time distributions
Dova, M. T.
 
, Mancenido, M
 
, Mariazzi, A.G.
 
, Arqueros, F.

and Garcia-Pinto, D.

(a) Dpto.de Fisica,Universidad Nacional de La Plata, C.C.67, 1900 La Plata,Argentina.
(b) Facultad de Ciencias Fisicas, Universidad Complutense, E-28040 Madrid, Spain
(c) Now at School of Physics and Astronomy, University of Leeds,Leeds LS2 9JT,UK
Presenter: Dova, Maria-Teresa (dova@fisica.unlp.edu.ar), arg-dova-MT-abs1-he14-oral
Evidence of azimuthal asymmetries in the time structure and signal size have been found in non-vertical show-
ers at the Pierre Auger Observatory. It has been previously shown that the asymmetry in time distributions
offers a new possibility for the determination of the mass composition. New studies have demonstrated that
the dependence of the asymmetry parameter in the rise-time and fall-time distributions with 	
 shows a clear
peak. Both, the position of the peak,  , and the size of the asymmetry at  are sensitive to
primary mass composition and have a small dependence on energy. In this paper a study of the discriminating
power of the new observables to separate primary species is presented.
1. Introduction
As it is well known, the circular symmetry observed in the signals collected by the surface detectors is broken
in the case of inclined showers. Evidence of the azimuthal asymmetries in the signal size were first observed
at Haverah Park [1] and the first observations of asymmetries in the time structure of the ground detector
signals were found in the Pierre Auger Observatory [2]. The azimuthal dependence arises mainly due to the
different paths traveled by the particles at different azimuth angles. In ground array experiments the analysis
is done projecting the collected signals and time distributions at ground level into the shower plane neglecting
the shower evolution. This results in an azimuth angle dependence in the slant depth, as it was proposed in [3].
The observed asymmetry in the time distributions offers a new possibility for primary composition determi-
nation, because its magnitude is strongly dependent on the muon to electromagnetic ratio at ground. We have
done a preliminary study on the information in time distributions using simulations of proton and iron initiated
showers with the aim of estimating the sensitivity of the Pierre Auger Observatory for hadronic primary dis-
crimination [2]. The following observables were analyzed:“rise-time” (defined as the time between the 10%
and 50% of the integrated signal) and “fall-time” (time between the 50% and 90% of the integrated signal).
These variables were studied as a function of the azimuth angle in the shower plane at fixed core ranges and
zenith angles, for showers initiated by proton and iron primaries.
In this paper we describe a new observable related to the asymmetry factor of the time structure of the signal
in the water-Cˇerenkov detectors of the Pierre Auger Observatory useful to discriminate composition.
2. Asymmetries as an indication of shower evolution and mass composition
Most of the observables related to composition are like a snapshot of the shower development and then, are
correlated with   and the atmospheric depth. The time distribution of the signals contains implicitly the
information of the shower development. Therefore, it is natural to expect a dependence of the mean values of
“rise-time” and “fall-time”, and the corresponding asymmetries observed, with the atmospheric depth.
If we call  the atmospheric slant depth, ffflfiffi! "$# &%')(+*-,.-*- with */ along the shower axis, then, ff+(102,3fi
4ff5768
9(:<;>=<?A@&BDC
E6F-4G02,HfiIJK;ML)JN(+0/, . We can treat this dependence in a first approximation using a
Taylor expansion in slant depth around fl4ff576O
 keeping only the first term, considering that the azimuth angle
correction is small compared to the slant depth. This is equivalent to using a linear function in cosine of the
6 M. T. Dova et al.
 [deg]ζ
-150 -100 -50 0 50 100 150
/r 
[n
s/m
]
50
-
t
90t
0.2
0.4
0.6
0.8
1
1.2
1.4
Figure 1. Fall-time as a function of P for 100 EeV for pro-
ton ( Q ) and iron ( R ) for 25, 45 and 60 from top to bottom.
 [deg]ζ
-150 -100 -50 0 50 100 150
/r 
[n
s/m
]
10
-
t
50t
0.1
0.2
0.3
0.4
0.5
Figure 2. Rise-time as a function of P for 100 EeV for
proton ( Q ) and iron ( R ) for 25, 45 and 60 from top to bottom
azimuth angle in the shower plane, 0 (defined with 0)fiTS in the incoming direction of the shower) to describe
asymmetries in data.
In vertical showers, a generic time distribution UV(XWZYJJ, will be function of  , the atmospheric depth along the
shower axis, and W , the core distance perpendicular to the shower axis. For inclined showers, UV(XWZYJJ,\[
UV(XWZYJ(10]YJ
-,J, . where  is the atmospheric depth along the shower axis and W the core distance in the shower
plane. Performing a Taylor expansion around   , we obtain:
UV('WZY&02,^fi_UV(XWZYJJ,(:^;
`9a
F2bcU
`ed+fg


h
%Xi8=I6F/4G0<;>jkjljm, (1)
If UV('WZY&02,fi>no;qpr6F/4G0 , the asymmetry factor s

depends on the core distance and the atmospheric depth, and
it is a measurement of the logarithmic rate of change of the variable considered.
Studying asymmetries in time distributions from this point of view allowed us to find a new observable in the
dependence of the asymmetry factor with depth, useful to discriminate primary composition.
For the present Monte Carlo study, we use proton and iron initiated showers generated with AIRES 2.6.0
/QGSJET01 with primary energies :7S]tJu eV and :7S/v ? eV and zenith angles between S ? and w2S ? ( 	
Hfi 1.103,
1.221, 1.414, 1.743 and 2). The detector response of the Pierre Auger Observatory was simulated (using the
official simulation and reconstruction tool, Offline v1r2) with the SDSim module in the full array configuration
and with all the parameters in their default values. The resulting data were reconstructed using the reconstruc-
tion modules. The sample used consisted in 20 showers for :7SGtu eV protons, 25 showers for :7S]tJu eV irons and
30 showers for :7S v ? eV proton and iron each, for each angle considered.
The dependence of the “rise-time” and “fall-time” with azimuth angle was fitted using the functional depen-
dence, UHfi>n<;xp	
f
N0 . In Figures 1 and 2 we show the mean value of the timing distributions divided by core
distance for proton and iron primaries as a function of 0 for the case of primary energy :7Syv ? eV. The fit was
performed for each primary species, energy and zenith angle, for all stations between z2S2S2{}|~$|DS/S2S2{
from the core. It is important to point out that the asymmetry factor is different for different primaries, as one
expects due to the greater number of muons in showers initiated by heavy nuclei than in showers initiated by
protons.
We studied then the behavior of the asymmetry factor with depth represented by 4J568
 . The corresponding dis-
tributions show a clear peak in atmospheric depth, which is in a different position for proton and iron showers.
Two examples of these distributions are presented in Figures 3 and 4, corresponding to the asymmetry factor
Primary composition using asymmetries ... 7
)θln(sec
0.1 0.2 0.3 0.4 0.5 0.6 0.7
b/
a
0.1
0.2
0.3
0.4
0.5
0.6
Figure 3. Asymmetry factor as a function of !Z'ŁN cor-
responding to fall-time, for 100 EeV iron.
)θln(sec
0.1 0.2 0.3 0.4 0.5 0.6 0.7
b/
a
0.1
0.2
0.3
0.4
0.5
0.6
Figure 4. Asymmetry factor as a function of c&'ŁN cor-
responding to fall-time, for 100 EeV proton
for the “fall-time” for :7S/v ? eV iron and proton respectively. The distribution is quite symmetric when plotted
as a function of the logarithm of 4J568
 . To determine the position of this peak we fit a normal distribution in the
logarithm of 4ff576V
 .
The location of the peak,
a
C(X4ff576O
-,  , as a function of primary energy is shown in Figure 5 for proton
and iron primaries for the “fall-time”. Clearly,
a
C(+4J568
/,fffi
a
C('-ZJ, . The error bars are the errors
from the fit. The position of the peak in the dependence of the asymmetry factor with N	
 is different for
different primaries and allows then to help separating primary species. The “fall-time” seems to be a better
discriminating parameter than the “rise-time” at all primary energies. It is worth mentioning that the value of
the asymmetry at the peak is also sensitive, although to a less extent, to the primary composition.
We also found that the difference in the peak position for proton and iron expressed in g. cm ev is of the same
order as the difference in   position for each energy bin. This is not surprising since observables related to
composition are in some way a measurement of the stage of shower development and then, directly correlated
with the shower maximum   .
3. Conclusions
Data collected by the surface detector of the Pierre Auger Observatory have proven to be extremely rich for the
inference of the characteristic of the primary particle. In particular, the observed azimuth angle asymmetries in
time distribution of the signals in showers with zenith angles lower than DS] , which is a unique feature of the
Pierre Auger Observatory.
We focused our analysis on the study of the azimuthal asymmetry of both the “rise-time” and “fall-time” that are
sensitive to the presence of the electromagnetic and muonic components of the shower and, because of this to
primary composition. From a study of the dependence of the azimuthal asymmetries in these timing variables
with the depth in the atmosphere using Monte Carlo simulations for proton and iron showers in different
energy ranges, we have found a new observable for primary mass discrimination: A clear peak appears in the
distribution of the asymmetry factor with atmospheric depth, the position of which is sensitive to primary mass.
With a high statistics sample of inclined showers we expect to be able to obtain information on the primary
composition with good precision using the method described in this work. An important feature of this study
8 M. T. Dova et al.
E (eV)10log
19 19.2 19.4 19.6 19.8 20
m
ax)θ
ln
(se
c
0.4
0.45
0.5
0.55
0.6
Figure 5. Maximum value of the asymmetry factor obtained from the the t for proton ( Q ) and iron ( R ) initiated
showers as a function of primary energy corresponding to fall-time.
is the weak dependence of the new observable with energy, requiring only the knowledge of the core position
with good resolution.
We have applied the method to signal pulse shape parameters but it could also be applied to some other pulse
shape parameters or shower observables like the shower age.
References
[1] C. D. England , PhD Thesis of the University of Leeds (1984)
[2] Pierre Auger Collaboration (M. T. Dova for the Collaboration), Proc. 28th International Cosmic Ray
Conference, Tsukuba, 369 (2003).
[3] M. T. Dova, L. N. Epele and A. G. Mariazzi, Astropart. Phys. 18, 351 (2003).


Hinting at primary composition using asymmetries in time distributions

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Hinting at primary composition using asymmetries in time distributions

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