Table Of ContentDifferent approaches to determine the composition of
the ultra-high energy cosmic rays in the Pierre Auger
Observatory
Miguel Blanco Otano
To cite this version:
Miguel Blanco Otano. Different approaches to determine the composition of the ultra-high energy
cosmic rays in the Pierre Auger Observatory. High Energy Physics - Experiment [hep-ex]. Université
Pierre et Marie Curie - Paris VI, 2014. English. NNT: 2014PA066643. tel-01150598
HAL Id: tel-01150598
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Th`ese de doctorat
de l’Universit´e Pierre et Marie Curie (UPMC)
Laboratoire de Physique Nucl´eaire et des Hautes E´nergies (LPNHE)
present´e par
Miguel Blanco Otano
Pour obtenir le grade de
DOCTEUR DE`S SCIENCES DE L’UNIVERSITE´ PIERRE ET MARIE CURIE (UPMC)
Di↵erent approaches to determine
the composition of the ultra-high energy cosmic rays
in the Pierre Auger Observatory
Directeur de th`ese: Antoine Letessier-Selvon
Codirectrice de th`ese: Piera Luisa Ghia
Membres du jury de soutenance:
Corinne Berat (rapporteuse)
Sergio Navas Concha (rapporteur)
Jean-Pierre Lees (examinateur)
Jean-Paul Tavernet (examinateur)
Antoine Letessier-Selvon (invit´e, directeur de th`ese)
Edition for the PhD thesis defense (12/12/2014). Paris. Friday 5th December, 2014.
Para Helia.
Para Abel.
I
II
Acknowledgements
This PhD thesis is the final stage of a process that has known many di↵erent stages. It is
impossible to thank as they deserve to all the people that have carried me here, but it is
worthy to try.
Je veux remercier Antoine Letessier-Selvon qui m’a donn´e l’opportunit´e de connaˆıtre en
profondeur une profession int´eressante, libre, et qui nous permet de maintenir la tˆete active.
Sans peut ˆetre mˆeme le savoir, il m’a appris aussi la passion avec laquelle on doit aimer ce
travail. Je ne suis probablement pas le doctorant qui est arriv´e le plus loin, mais peut ˆetre
celui qui a parcouru le plus de chemin.
Un grand merci `a Pierre Billoir. J’ai appris de chaque pas qu’il fait, de chacun de ses
raisonnements, de chaque code, de chaque conversation `a la cantine, et de sa pens´ee fraiche,
intuitive, et courageuse. Merci aussi a` Piera Luisa Ghia, pour le dernier sprint qu’elle a courru
avec moi pour arriver `a temps et am´eliorer ce manuscrit.
Merci a` mes collegues Mariangela, Ioana et Romain avec lesquels j’ai ´et´e ravi de travailler et
de qui j’ai appris tous les jours. Merci `a Moritz, Lorenzo, Julien et Carla de faire partie de
cette belle famille. Et merci aussi `a Jules et Tania pour votre soutien ces derniers mois.
Es fundamental agradecer a mis padres el apoyo y empuje que de ellos he recibido en cada
momento, ayud´andome a mantener el objetivo claro y la bru´jula siempre bien calibrada,
durante estos u´ltimos tres an˜os, pero sobre todo tambi´en durante los 30 previos.
Por u´ltimo, unas gracias infinitas a mi editora personal en este trabajo, compan˜era y, sobre
todo, amiga. Sin ella y sus enormes sacrificios, estos tres an˜os en el exilio se hubieran hecho
infinitamente m´as dif´ıciles. Espero poder devolverte todo esto algu´n d´ıa. Gracias Noelia de
todo coraz´on.
In any case, even if this is the final stage of a period in my life, it is just the beginning a
new period. I am sure that I will widely benefit from the many lessons I am taking from here.
Nothing less than to thank again those that have been by my side in these three years, far or
near, at the o�ce or at home, in France, Spain, or Argentina.
Miguel Blanco Otano.
Par´ıs, 29 de octubre de 2014.
III
IV Acknowledgements
Summary
“Millions of dollars and years of experimental physics
can ruin a theory that took a whole afternoon to develop”.
Rocky Kolb, ISAAP 2012.
The ultra-high energy cosmic rays are particles that, coming from outside the solar system,
arriveatEarthwithenergiesupto1020eV. Thestudyoftheseparticlescanhelptodisentangle
their origin, knowing more about distant objects that create or accelerate them, and the
interactions in their path to the Earth. Besides, as they arrive at Earth with enormous energy,
they are also relevant in the field of particle physics.
Cosmic rays are charged nuclei, although their by-products, like photons and neutrinos
produced in their interactions at the source or with interstellar radiation and matter, are also a
subject of study. The number of these particles arriving at Earth decreases dramatically with
the energy. At the highest energies, those of interest in this study, only 1km 2century 1 are
� �
observed. When one of these cosmic rays enters in the atmosphere of the Earth, it interacts
with the atoms or nuclei in the air generating a lot of secondary particles. These secondary
particles constitute an extensive air shower. This e↵ect blurs the characteristics of the primary
particle making di�cult the analysis to determine its energy, mass and arrival direction. At
the same time, this is the only way by which this scarce and energetic natural phenomenon
can be measured, as the atmosphere plays the role of a calorimeter and spreads the e↵ect from
a single point to a large area, allowing its detection.
To detect the extensive air showers and, finally, the ultra-high energy cosmic rays, with a
significantly large statistic, large observatories have been constructed in the last decades. The
Pierre Auger Observatory, on which this PhD thesis is focused, is the largest in the world. It
is located in Argentina, province of Mendoza, near the town of Malargu¨e, and it is placed at
1400m above sea level. It was completed in 2008 but the data collection started in 2004. It is
the first observatory combining two techniques for measuring cosmic rays, collecting data both
with a surface detector array and a fluorescence detector. The former is an array of water
Cherenkov detectors spread over a flat surface of 3000km2 and the latter are 4 buildings at
the edges of the array, facing the interior, with 6 fluorescence telescopes in each one. These
two techniques measure the two profiles of the extensive air showers: the lateral profile, that
samples the shower at a determined height, and the longitudinal profile, that tracks the
development of the shower through the atmosphere. Di↵erent techniques of detection are
explored. These techniques allow to determine di↵erent characteristics of the extensive air
shower and, consequently, of the primary cosmic ray that enters in the atmosphere. Despite
the great evolution in the last century concerning these techniques, the complete description
V
VI Summary
of the arriving particles has not yet been achieved. The composition of the primary particle is
currently based on measurements of the longitudinal profile, and the atmospheric depth of
maximum (X ) observed in this profile. These measurements are done with the fluorescence
max
technique, that needs special luminosity conditions that reduce the exposure time to about
10% of the time. In a full duty cycle, the mass of the primary particle (composition) is di�cult
to infer. Other methods, based on the measure of X or related observables, like the muonic
max
µ
atmospheric depth of maximum (X ), by alternative methods, or the measure of the muon
max
number (also related with the mass of the primary) are not, up to now, developed enough
to resolve the question. Furthermore, the step from the measured X to the mass of the
max
primary is based on hadronic interaction models at energies higher than those reached in
accelerator experiments, like the Large Hadron Collider (LHC).
The measurements done by the Pierre Auger Observatory and similar observatories have
shed some light on some of the questions mentioned above. A suppression in the cosmic rays
spectrum has been found at the energy of 4 1019eV. Nevertheless, an agreement in the
⇥
interpretation of this suppression has not yet been achieved. Although interactions of the
travelling particles with the cosmic microwave background has been proposed as a cause, other
interpretations, like the exhaustion of sources, is still possible. Other features in the energy
spectrum have been observed, and di↵erent interpretations are also in dispute. In addition,
di↵erent studies have been carried out searching for anisotropy in the arrival directions at
di↵erent scales, but still no statistically significant evidence has been found. The composition
measurementsdonebyAuger, basedonthemeasureoftheX bythefluorescencetelescopes,
max
reveal a heavy component in the ultra-high energy part of the spectrum, that is in some
tension with the composition previously measured by other observatories.
The di↵erent interpretations for the suppression, the other features in the spectrum, and a
better evaluation of the origin needs more precise measurements of the composition of the
cosmic rays. The short duty cycle of the fluorescence technique is the main limitation.
Motivation
The main motivation of this PhD thesis is to improve the capabilities to determine the
mass composition of the ultra-high energy cosmic rays in the Pierre Auger Observatory,
and other cosmic rays observatories. Very important results and unique information about
ultra-high energy cosmic rays have been published by Auger in the last decade, but a correct
interpretation of these results needs for a measurement of the composition of the detected
cosmic rays. We certainly consider that an improvement in the composition identification
capabilities of the Pierre Auger Observatory is achievable, so three di↵erent approaches to
this challenge are proposed in this PhD thesis. They do not cover all the possibilities for the
future of the Pierre Auger Observatory and other cosmic ray experiments, but they cover the
three main ways this challenge can be faced.
New analysis
The first approach is to use the Auger data collected so far to make new analysis that can
yield new results. Horizontal air showers are composed mostly by muons, as other components
of the shower, like the electromagnetic one, are absorbed in the atmosphere. Muons, in their
Summary VII
trajectory to the ground, are deviated by the magnetic field of the Earth. This deflection
is reflected in the shape of the ground footprint of the shower and it can be measured. The
measured deflection is found to be related with di↵erent characteristics of the extensive air
µ
shower, like the X and the transverse momentum of the muons in their development in the
max
atmosphere. It is possible, then, to construct an estimator to obtain, with this alternative
µ
method, another measure of X . Besides, the hadronic interaction models can be checked
max
and compared with the data, by analysing the transverse momentum.
This new analysis is detailed in chapter 4.
Detector improvement
The second approach is to improve the actual observatory, by adding a detector that can
obtain information about the longitudinal profile of the extensive air shower with a 100% duty
cycle. This can give the Pierre Auger Observatory (and similar observatories) the capability
to measure mass sensitive parameters for the whole data set. The detection of the radio
emission produced in the extensive air showers has been suggested as a possible technique.
Di↵erent processes of emission are described, and some of them are explored. Among them,
the detection of the Molecular Bremsstrahlung Radiation (MBR) emission is found as the
most promising technique. Di↵erent tests have been and are being done by the EASIER group,
in which part of this PhD thesis is framed. The Extensive Air Shower Identification using
Electron Radiometer (EASIER) project aims to detect the radio emissions of the extensive air
showers by installing antennae in the water Cherenkov detectors of the surface detector array
of the Pierre Auger Observatory.
The possibilities of the radio detection technique are discussed in chapter 5.
A new detector
ThethirdapproachistoreconsiderthePierreAugerObservatoryfromthescratch, bychanging
the used detector in the surface array. A modification to the surface detector array is proposed,
by dividing the water Cherenkov detectors into two di↵erent water volumes. This new detector
obtains di↵erent responses in each volume to di↵erent components of the extensive air shower:
electromagnetic and muonic. The access to the muonic component is another way to access
the composition of the primary particle, and di↵erent observables can be obtained to this
purpose. The performance of this new detector and a preliminary idea of its achievements in
the discrimination between di↵erent primaries are described.
This new detector is described in chapter 6.
Outlook
ThemainpurposeofthisPhDthesiswastoexplorenewhelpfulwaystoidentifythecomposition
of the cosmic rays at the highest energies. The results shown in this work prove the validity of
the methods and the detectors proposed, and they do not have the intention to establish any
new description of the characteristics of the ultra-high energy cosmic rays. Nevertheless, we
believe that a future application of the described methods and the development on large scale
Description:Merci `a mes collegues Mariangela, Ioana et Romain avec lesquels j'ai été ravi de travailler et de qui j'ai appris tous les jours. Merci `a Moritz, Lorenzo
