ana d. becerril, alfredo estrade and sean mcdaniel- ultra high energy cosmic rays

8/3/2019 Ana D. Becerril, Alfredo Estrade and Sean McDaniel- Ultra High Energy Cosmic Rays

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Ultra High Energy Cosmic Rays

Ana D. Becerril

Alfredo Estrade

Sean McDaniel

Physics and Astronomy Department, and

National Superconducting Cyclotron Laboratory, Michigan State University.

East Lansing, MI.

ABSTRACT

Among the major puzzles that remain to challenge the basis of particle astrophysics are

the source and composition of cosmic rays. Of particular interest is the high energy end

of the cosmic ray spectrum, for which no satisfactory explanation has been found so far.These ultra high energy particles are believed to be the products of the most violent

acceleration and spallation processes of interstellar matter, such as supernova explosions,

active galactic nuclei, large scale galactic wind termination shocks, pulsars, magnetars,

colliding galaxies, gamma-ray bursts, etc. In this work we give an overview of ourpresent knowledge of the astrophysical origins of the (currently known) most energetic

cosmic rays. This paper is organized as follows: in the fist part we present the basicobservational features of the cosmic ray energy spectrum. Also, we discuss the energy

losses during their propagation through the interestelar medium and the Greisen-

Zaptsepin-Kuzmin cutoff problem. In the second part of this work we present the most

remarkable features of the various astrophysical scenarios able to accelerate particles upto such high energies. Finally, in the third section we give a brief description of the

experimental techniques used in the detection of these ultra high energy cosmic rays.


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1 OBSERVATIONS

Cosmic rays were first observed by Victor Franz Hess in 1912. He showed with ground

based and balloon-borne detectors that the intensity of the ionizing radiation did notchange significantly between day and night. As a consequence, the Sun was discarded as

the source of this radiation and the question of its origin remained unanswered. Today,almost one hundred years later the question of the origin of the cosmic radiation remains

a mystery (Kappeler 1998).

Hess was awarded the Nobel Prize for his discovery in 1936. Cosmic Rays (CR) have

been widely observed since then and have been found to come from sources inside and

outside of our Galaxy. Most cosmic rays are atomic nuclei, but there are also high energyelectrons, positrons and other subatomic particles, all these traveling at essentially the

speed of light and striking the Earth from all directions.

Between the decades of 1930 and 1950, before particle accelerators on Earth reached

very high energies, cosmic rays served as a source of particles for high energy physicsinvestigations (with energies around 10

15eV) and led to the discovery of subatomic

particles such as the positron and the muon. Although these applications continue, the

main focus of cosmic ray research has now been directed towards astrophysical

investigations of where CR originate, how they get accelerated to such high velocities,what role they play in the dynamics of the Galaxy, and what their composition tells us

about matter from outside the solar system.

In 1963 John Linsley published the detection of a cosmic ray with an energy of 10 20 eV

(Linsley 1963, Stanev 2004). Three years later, after the discovery of the microwavebackground, Greisen in the US, almost simultaneously with Zatsepin and Kuzmin in theURSS, studied the propagation of ultra high energy particles in extragalactic space. They

calculated the energy loss distance of nucleons interacting in the microwave background

and arrived to the conclusion that it is shorter than the distances to powerful galaxies.The cosmic ray spectrum should then have an end around an energy of 5 x 1019 eV, this

limit is now known as the GZK cutoff (see section 1.3).

There has been a long debate since then on where the ultra high energy cosmic rays

(those with energy 1018

eV) particles come from. Many controversies have arisen

about both the experimental results and the theoretical interpretations. Ultra high energy

cosmic rays (UHECR) will be the main subject of the following sections.


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1.1 The cosmic ray spectrum

The cosmic ray spectrum (shown in Fig. 1) is fairly well described by a broken power

law E- across an energy range that spans over 11 orders of magnitude. The cosmic ray

spectrum steepens around 3 x 1015

eV (at the knee), going from a power law index of

2.7 to 3.2. Later, it steepens further to3.3

E above ~1017.7

eV (the dip) andflattens back to 7.2E around 3 x 10

19eV (the ankle) (Torres 2004, Nagano 2000).

With the advent of large detectors, it is now established that particles are accelerated or

produced in the universe with energy near 1021

eV.

Fig.1. Cosmic ray energy spectrum


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There is, however, uncertainty on the shape of the spectrum beyond 5x1019 eV, either a

mild cutoff as observed in the data collected by HiRes, or a continuation, as reported by

AGASA (Biermann 2003). According to the analysis made by Bahcall and Waxman(Bahcall 2003) of the available data from AGASA, Flys Eye, Havera Park, HiRes and

Yakutsk experiments there is general agreement in the description of the CR spectrum for

energies between 2x10

18

eV and 10

20

eV. However, for energies above 10

19

eV HiRes,Flys Eye and Yakutsk all show evidence for a turnover, which is statistically significant

and is consistent with what a simple model that includes the GZK effect would predict.

In contrast, the fluxes reported by AGASA are higher than the fluxes measured in theother experiments. These observations suggested the idea that measurements of ultra

high energy cosmic rays do not show evidence for a GZK effect. Therefore they

developed a model with two components: one with galactic origin and another with extra-galactic origin which led them to the conclusion that there is no need for new physics to

account for the observed events with energies above 1020 eV, except for the AGASA

data. They found the data from Flys Eye, HiRes and Yakutsk to be very convincing

evidence that the expected GZK suppression has been observed.

Fig.2. Currently available data on the highest energy cosmic rays. (from Bahcall et al.). As can be seen,

data from AGASA does not confirm observations by other experiments above 1020

eV. This suggests that

the GZK cutoff is not observed.

It is also important to mention that the flux of cosmic rays goes down from 104 m-2s-1 at

~1019

eV to 10-2

km-2

yr-1

at ~1020

eV. Therefore, any theory of cosmic ray origins must

account for this shape. A successful model must produce the right numbers of particles asa function of energy.


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1.2 Distribution of arrival directions

Since cosmic rays are electrically charged they are deflected by magnetic fields, and their

directions are therefore randomized, making it impossible to tell where they originated(Cronin 2004). However, cosmic rays can be traced by the electromagnetic radiation they

produce. Supernova remnants such as the Crab Nebula are known to be a source ofcosmic rays from the radio synchrotron radiation emitted by electrons spiraling in the

magnetic fields of the remnant. In addition, observations of high energy (10 - 1000 MeV)

gamma rays resulting from cosmic ray collisions with interstellar gas show that mostcosmic rays are confined to the disk of the Galaxy, presumably by its magnetic field.

Cosmic rays are also affected by the interplanetary magnetic field embedded in the solarwind, and therefore have difficulty reaching the inner solar system. Spacecraft venturing

out towards the boundary of the solar system have found that the intensity of galactic

comic rays increases proportionally with the distance from the Sun. As solar activityvaries over the 11 year solar cycle the intensity of cosmic rays at Earth also varies.

In the case of UHECR there are again discrepancies in the experimental results obtainedby the current experiments with the highest statistics: the AGASA collaboration reports

small scale anisotropy, which is not confirmed by the HiRes collaboration. They doagree, however in the large scale isotropy of UHECR (Stanev 2004).

1.3 GZK cutoff

If we adopt the idea that the arrival directions of these particles do not seem to show any

significant anisotropy it is a puzzle how these particles can reach the Earth traveling largedistances without being subject to the Greisen-Zatsepin-Kuzmin (GZK) cutoff, expected

to occur between 3 and 5 x 1019 eV for protons (Grieder 2001).

This cutoff is due to interactions of the cosmic rays with the cosmic microwave

background radiation (CMBR) that degrade the energy of the particles via photo-pion

production and fragment nuclei. The mean free path for photo-pion production is

estimated to be 6 Mpc.

A consequence of the GZK cutoff is that the sources of the most energetic cosmic rayswould have to lie within 50 Mpc around the Earth to avoid the cutoff. This, in turn,

implies that there should be anisotropies, which so far have not been detected. For UHEcosmic rays from larger distances a pileup in the spectral region just around or below theexpected cutoff could occur, and nuclei would not arrive as such because of dissociation

(Grieder 2001).

However, with the current available observational data it is still difficult to be conclusive

on whether the GZK cutoff becomes effective or not (Bahcall 2003) (Grieder 2001).


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1.4 Chemical composition of cosmic rays

About 89% of the cosmic rays are protons, 10% helium, and about 1% heavier elements.

Heavier elements (such as carbon, oxygen, magnesium, silicon, and iron) are present inabout the same relative abundances as in the solar system, but there are important

differences in elemental and isotopic composition that provide information on the originand history of galactic cosmic rays. For example there is a significant overabundance of

the rare elements Li, Be, and B produced when heavier cosmic rays such as carbon,

nitrogen, and oxygen fragment into lighter nuclei during collisions with the interstellargas. The isotope 22Ne is also overabundant, showing differences between the

nucleosynthesis of cosmic rays and solar system material. Electrons constitute about 1%

of galactic cosmic rays. It is not known why electrons are apparently less efficiently

accelerated than nuclei.

Primary cosmic ray electrons, protons, and nuclei can produce secondary photons andneutrinos in or near the sources, which then propagate over large distances through the

universe undeflected by the magnetic fields that obscure the origins of their charged

progenitors. Photons are produced by electrons, protons, or nuclei, while neutrinos areproduced only by the decays of protons and nuclei. The proportions of the various types

of particles, secondary and primary, thus reflect the nature of their sources.

Recent experimental results provide clues that the mass of the primary cosmic rays at the

high energy end of the spectrum becomes lighter, which means that the flux of heavy

galactic cosmic rays is decreasing and the new cosmic ray component that is responsible

for the change of the spectral index at the ankle consists of protons and possibly He

nuclei. The composition is proton dominated, but a more precise prediction will dependon the hadronic interaction model used (see section 3).

2. PRODUCTION AND ACCELERATION MECHANISMS

2.1 Current theories

Mechanisms for producing ultra-high energy cosmic rays (UHECRs), as stated in theintroduction, can be broken down into two large categories: top-down (TD)

mechanisms, and bottom-up (BU) mechanisms. These crude names are fairlydescriptive of the physical processes they label. In the top-down mechanism, particleswith enormous energy are instantly created from the decay of a super-massive non-

relativistic particle, and then, as the particles further decay and propagate through the

scattered substances and fields that fill interstellar space, they lose energy. In the

bottom-down mechanism, a particle starts with very little kinetic energy, and then isaccelerated to the appropriate energies in some violent astrophysical scenario.


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2.2 Top-down theories

Top down theories are attractive. They involve exotic new physics that cleanly avoid thepitfalls of the bottom-up theories. The decay of a massive X-particle, with a mass close to

the Grand Unified Theory (GUT) scale (point at which the gauge coupling strength of the

hypercharge, weak force, and quantum chromodynamics converge), would instantly givethe secondary particles the necessary kinetic energy.

Remaining consistent with the standard model should not be a grave concern; we knowthe standard model is incorrect. It wonderfully describes the world up to 1 TeV, but we

know that we cannot extend its predictive powers to extreme high energies, like the

reduced Planck scale where gravitational effects cannot be ignored. There is the un-dismissible hierarchy problem which has been covered extensively in the literature

[cite!]. It is completely natural that there should be new particles associated with such a

drastically higher energy range and associated with the inevitable new physics construct.

The same story has occurred several times as new accelerators have unveiled new

particles and theorists have aggregated these discoveries to an existing framework (likethe standard model).

The most alluring add-on to the standard model is the Minimalist Supersymmetric Model

(MSSM), which solves both the hierarchy problem and unifies all gauge couplings at the

Grand Unified Theory (GUT) scale. Every particle is associated with a much moremassive supersymmetric particle. The X-particle is believed to a light member of the

MSSM.

The X-particle decay seeds a parton shower. Its first primary decay is assumed to be twoor more members of the MSSM with very high virtuality (a particle with virtuality exists

outside its mass shell, the surface which defines all valid classical solutions of its

equation of motion). The secondary particles continue to decay into superparticles withmuch lower virtuality until the ~1 TeV scale is reached. At this point the decay products

move on-shell and can decay to standard model particles. The showers are described by

fragmentation functions and splitting functions, and complex computer codes are neededto describe their entire development. This is one of the main weaknesses of top-down

models, their reliance on delicate computer models. See Fig. 3 for a proposed decay

scheme.


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There are two ways in which the X-particles could be produced. First, they could be

produced from topological defects left over from a cosmological phase transition in the

universes early genesis, when the temperature was close to the GUT scale. The timedependent motion of the topological defects would continuously generate short-lived

particles with masses equal to the temperature at which the phase transition took place.

There are a several exotic possibilities for these defects: cosmic strings, magnetic

monopoles, and domain walls.

Interestingly, the one of the reasons that the inflation theory was proposed, beyond the

fundamental goal of explaining how a flat, homogenous, and curved universe could haveevolved from the spherical inhomogeneous universe that the big bang dictates, was to

reduce the number of remnant, nasty topological defects. Despite their suppression,

however, it is postulated the enough defects remain to be observationally important, i.e.

create the necessary flux of UHECRs.

Second, the particles could be produced in the early universe due to an unknown

symmetry. They must have a lifetime that is comparable to the lifetime of the knownuniverse. The methods are exotic and bizarre and so are quoted in full: gravitationalproduction through the effect of the expansion of the background metric on the vacuum

fluctuations of the X-particle field, and creation during reheating at the end of inflationif the X-particle field couples to the inflaton field. (Sigl 2000) The inflaton field is the

scalar field responsible for early rapid acceleration of the universe between 10-35 and 10-34

seconds. The particles created in these processes, called WIMPZILLAs, would also

contribute to the present dark matter mass; the particles are alternatively called

Superheavy Dark Matter (SDM).

2.3 Bottom-up theories

One of the best clues to the origins of UHECRs come from their direction of arrival. If

the data is highly anisotropic we can trace back to possible galactic sources. Detection ofanisotropy would also support the entire umbrella of bottom-up theories. X-particle

decay-product arrival directions should be largely uncorrelated with astrophysical

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objects. To this end, analysis of AGASA data has shown a statistically significant arrival

anisotropy for very high energy events, and has shown them to lie in the galactic plane,

with the excess residing either in the Galactic Center (GC) or the Cygnus region. Lowerenergy cosmic rays reveal no anisotropy and there is still a lack of data to ascribe an

isotropy claim about EHECRs ( > 1019.6

eV). There are conflicting interpretations of the

data, however. New data from the Flys Eye reports no anisotropy, and re-analysis of theAGASA data by other teams has only produced watered-down revisions of the original

assertion. Thus, the question remains open: no source of UHECRs has ever been

confirmed.

At their core, bottom up theories are just an extension of conventional acceleration

mechanisms. It is quite natural, then to start with the classical theories how the lessextreme brethren of UHECRs are accelerated.

In 1949, Enrico Fermi proposed that cosmic rays were accelerated by a moving magnetic

field. The head-on reflection from this mirror would increase a particles energy. Fermi

showed that on-average a particle would have more head-on collisions than head-tailcollisions (approaching the magnetic mirror from the back) and so the particle would

accordingly gain energy. The energy gain per reflection is proportional to the velocity ofthe mirror squared, and therefore it is called second-order Fermi acceleration.

Later, it was shown that supernova shock waves could accelerate particles in a similarway. A particle can repeatedly move back through the shock front, bounce off magnetic

fields behind the shock front, gaining energy, cross the shock front, and again be

rescattered by magnetic inhomogeneties. Unlike the second-order acceleration, the

motion is not random. A particles continual trip back and forth across the shock waveefficiently funnels energy to the particle. The dependence on the shock wave velocity is

linear, and so this modification to Fermis initial theory is called first-order Fermi

acceleration.

Fermi first-order acceleration requires a strong magnetic field to contain the particles

within the acceleration region. A general criteria, called Hillas criterion provides anupper bound for a particles energy, given the sources magnetic field.

There are several exotic sources that satisfy the Hillas criterion and could accelerate

particles to UHECR energies. These sources are,1. Neutron stars. There are two subcategories.

a. Magnetohydronamic acceleration of iron nuclei pulsars

b. Magnetars: neutron stars with exceptionally strong surface dipole fields of10

15Gauss.

2. Active galactic nuclei like radio galaxies: Usually elliptical galaxies that emitradio waves from synchrotron processes.

3. Remnants of quasars: Accreting, massive black holes, which are powerful sources

of electromagnetic energy.4. Starbursts: Regions of space with an abnormally high rate of star formation.

Emax ~ 2cZeBrL


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5. Luminous infrared galaxies: Galaxies that emit a large proportion of their

luminosity in the infrared region. This could be due to starburst formation.

6. Gamma ray bursts: Ultra-bright and ultra-short objects formed by either thecollision of two orbiting neutron stars, or more commonly, the core collapse of a

Wolf-Rayet start into a black hole.

The bottom-up community has one final monkey to remove from its back, and that is the

Griesen-Zapepin-Kuzmin GZK cutoff, the photoproduction of pions when nucleons

interact with photons of the Cosmic Microwave Background (CMB). Somehow theseexceptionally energetic particles have to reach earth without severely degrading in

energy.

The problem of the GZK cutoff can be avoided by introducing particles that either do not

interact with the CMB or interact strongly at a energy beyond the maximum energy of

particles currently observed. It is important to remember that these new particles are not

the primary accelerated particles. The primary particles remain protons, accelerated in

powerful AGCs or other objects described above, which somehow transfer their energy tothese new mediating particles.

The first and obvious candidate is a neutrino. Neutrinos are well known and exist

comfortably in the standard-model rubric, but their cross section with nucleons is too

small to consider them as carriers. New physics must be introduced. There are twoproposals that do not violate unitarity. Sigl [cite] cites them as an increase in the number

of degrees of freedom beyond the electroweak scale, and a broken SU(3) gauge

symmetry dual to the unbroken SU(3) color gauge group of strong interaction is

introduced as the generation symmetry such that the three generations of leptons andquarks represent the quantum numbers of this generation symmetry.

Supersymmetric particles have also been proposed as carriers. The lightest possiblesupersymmetric particle allowed is the gluino (supersymmetric partner of the gluon), but

more massive combinations of light gluinos with quarks and gluons (R-hadrons) have

been proposed. Tuneable mass theories allow the gluino-gluon bound state gg called a

gluballino Ro and an isotriplet g (uu dd)8called .

UHECR data naturally places constraints on the possibilities. The hypothetical processes

that would produce massive symmetric particles would also produce gamma rays and

neutrinos, both of which can be observed on earth. Air showers also provide clues aboutthe impinging particles composition. So far, the signature is most characteristic of high-

energy protons, not the more exotic supersymmetric particles listed above, and forces anupper bound on the primary rest mass of d50 GeV. The newly commissioned Pierre

Auger should further lower the bound to d10 GeV. Combined with astrophysics and high

energy accelerator data, these constraints leave very play in the possible mass range, and

they necessitate the addition even more exotic physics mechanisms (e.g. modification ofU(1)x gauge symmetry, etc).


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Additional physics beyond the standard model could also allow traditional primary

particles to reach earth without eliminating the GZK cutoff. A violation of Lorenz

invariance, too small to be detected, could produce the desired kinematics, as could achange to the generalized Lorenz transformations, an introduction of deformed

relativistic kinematics, or a violation of the principle of equivalence.

2.4 Unresolved problems

Each of the possible acceleration mechanisms has its problems. Top-down theories arehighly model dependent and they absolutely require the introduction of new physics. For

a particle accelerated in a bottom-up mechanism, even if it is originally imparted the

needed energy, it is unlikely that it will travel away from source with a large enoughfraction of that initial energy. Furthermore, likely sources (such as radio galaxies) lie too

far away ( 100 Mpc). Nucleons above d70 EeV are predicted to lose energy to the

GZK effect. The mean free path for this interaction is only a few Mpc. Lastly, UHECRs

should not be significantly bent by galactic magnetic fields, and so their detection

direction should reveal their birthplace, but so far no one has unambiguously observed acorresponding possible acceleration source.

Lucky, all of these models are constrained by current experiments, and as newtechnologies mature, and new detector arrays come online (e.g. Pierre Auger), theseconstraints will continue to narrow the field of possibilities.

3. EXPERIMENTAL STUDY OF UHECR

3.1 Extensive Air Showers

Even though direct detection of cosmic rays is possible, for example by high altitudeballoon experiments, most of the information we know about cosmic rays with energies

above 1014

eV comes from detecting the particle showers they produce when they strike

the upper atmosphere. The properties of these so called extensive air showers relevant toinferring the characteristics of the primary comic rays are explained in this section

(Cronin 2004).

Most of the new particles created by the interaction of the primary cosmic ray with the

atmosphere are -mesons and K-mesons, which in turn create plethora of other particles

by several different interactions. For example, neutral mesons will quickly decay intogamma rays, which in turn can create electron-positron pairs through pair production.This electrons and positrons can in eventually produce more gamma rays by

bremsstrahlung. Charged mesons will decay into a muon and a muon neutrino, but since

their half life is longer they could also interact with other nuclides in the atmosphere

(Sciutto 1999). The shower created by UHECRs can contain billions of particles atmaximum development. They travel roughly in the same direction as the progenitor

cosmic ray, and most of them (~ 80%) are located within one Moliere radius from the


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axis (about 100 meters). However, the shower front can extend up to a few kilometers

from the axis of the shower.

Figure 4. Simulation of longitudinal cosmic ray showers for different primary particles of 1019

eV (Nagano

2000).

The mass of the primary cosmic ray particle can be inferred from the depth of showermaximum Xmax, which is usually reported in units of g/cm

2 (the transversed atmospheric

thickness). Figure 4 show simulated shower profiles for Fe, proton, and gamma ray with

the same primary energy. It is clear that photons are more penetrating particles, and their

showers develop deeper in the atmosphere. Protons are about 25 g/cm2 more penetratingthan iron particles, but the larger fluctuation in their Xmax makes it harder to distinguish

both types of particles using this observable. The fluctuation can be explained by the

superposition model, which states that the properties of the shower generated by a heavynucleus of mass A is the same as the sum of the showers that would be generated by A

protons (Cronin 2004). Therefore, the Xmax for iron can be approximated by the average

of 56 proton showers, and its fluctuation is reduced. Hadronic primaries are believed tospan all the mass spectra from protons to iron, however, due to the difficulty to

distinguish them the different studies usually only consider the two extremes (proton and

Fe nuclides).

The longitudinal development maximum, Xmax, also increases with primary energy, asmore cascade generations are required to degrade the secondary particle energies.

According to the superposition principle (Nagano 2000):

B=0 for photons, and lower than 1 for hadrons (it depends on the hadronic interactions


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model used). The elongation rate (De) (Linsley 1977) is a useful quantity defined using

(the previous relationship between Xmax and energy:

It is used to study changes in the composition of primary CR with the energy (Figure 5).

Figure 5. Elongation rate studies for the variation of the UHECR composition with energy. The data hints

to a variation from medium heavy nuclides at energies around 1017

eV to more lighter component above

1018 eV.

Surface array detectors can not observe Xmax directly, and the primaries composition is

inferred from measuring the elongation rate of any other shower property that depends onXmax. One such property is the spread in time of arrival of the shower front to the surface

array. Because particles created higher in the atmosphere are expected to have similarpath lengths to the ground, a narrower spectra for the arrival times is an indication of a

higher Xmax. Alternatively the composition can be obtained from the observed lateraldistribution of the showers. Since the showers generated by gamma rays develop much

deeper, they have a steeper lateral distribution than hadronic primaries. They also have a

distinctively lower number of muons.

In the experiments with surface detectors the energy of the cosmic ray primary isobtained from the spatial distribution of particles in the shower front. As mentionedbefore, most of the particles are created close to the shower core, but for extensive air

showers a significant number of particles is detected up to a few kilometers from the axis.

In principle, the whole shower front profile can be reconstructed from a few measured

point of the array, and the total particle density can be integrated to obtain the totalenergy (some type of detectors actually measure the energy density of the front).

However, the large fluctuations predicted in Xmax would introduce a significant variation


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in the number of particles measured close to the core for a given energy, resulting in a

poor energy resolution. It was demonstrated by Hillas (Hillas 1970) that the particle

distribution away from the shower core is quite insensitive to Xmax, and can be used as anindicator of the total energy. This is illustrated in figure 6.

Figure 6. Deviations from the average particle density as a function of the distance from the shower corefor simulations of 10

17eV primary cosmic rays.

3.2 UHECR Detectors

3.2.1 Surface Array Detectors

Large arrays of particle detectors, which could exceed a hundred station spread over areas

up to 100 km2, have been the most common method to study UHECR for the past 50

years. The first event exceeding 1020 eV was discovered in such an array, at Volcano

Ranch in 1962 (Linsley 1963). An array can be composed of one or more type of particle

detectors. The most common types are large plastic scintillators, water Cerenkovdetectors, and shielded muon detectors.

The direction of the shower axis is determined time information for the arrival of the

shower front on the surface of the array. Thus, a good timing resolution is required for thearray elements, and the shower direction is typically obtained with an uncertainty

between 1 to 5 degrees. After the information on the direction of the shower is combined

with a measurement of the particle density at each detector, the position of the showercore is obtained through function minimization algorithms.

As explained in the previous section, the energy of the cosmic ray primary can not be

precisely determined from integrating the particle number for a reconstructed showerprofile because it is subject to large fluctuations due to fluctuations in Xmax. A less

sensitive parameter, such as the particle density at 600 meters from the core ((600)) isused in stead. The relation of this parameter to the primary energy is somehow subject todependent on the model used to calculate the shower development. Besides, there is no

direct experimental way to calibrate the surface array detectors. In spite of these

difficulties, the energy resolution of the surface detector arrays is around 25% (Nagano

2000).


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3.2.2 Fluorescence Detectors

An alternative approach to study extensive air showers is to detect the fluorescent light

emitted isotropically by atmospheric nitrogen molecules as they are excited by the

shower particles. For many years the Fly's eye experiment in Utah was the mostimportant project in this field (Baltrusaitis 1985). In 1993 it was replaced by HiRes,

whose superior design allows for a higher sensitivity and a larger visibility range. The

fluorescence telescopes are composed of a mirror to collect and focus the light into thesurface of a camera built with photomultiplier tubes (PMTs). HiRes is made of two

stations, each containing 21 such telescopes, which can subdivide their field of view of

the sky in pixels of 1 by 1 degree.

Figure 7: Geometry of the detector and cosmic ray shower used to determine the trajectory. A line in the

shower detector plane is fixed by the time difference for the arrival at the telescope of light emitted from

two different positions along the shower path.

The direction of the shower is obtained by defining the shower detector plane withtelescope's position, and a couple of observed incoming directions for fluorescent

photons. The direction can be derived though trigonometric arguments from the

information of the photons arrival times (figure 6). A better alternative is to observe theeven in stereo (with a telescope in each station), since then the direction is simplydetermined by the intersection of both shower detector planes.

The fluoresce detection technique achieves a more direct measurement of the energy ofthe cosmic ray primary. The principle is somehow analogous to the calorimetric

measurements done in particle physics, using in this case the atmosphere as the detector.

However, several parameters are still necessary to convert the observed fluorescent light


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accurate position of the detectors, as well as a time stamp for each event. This

arrangement allows for each detector to work independently from the status of the rest of

the array, so the Auger observatory has been obtaining data since the beginning of itsconstruction with little down time for deploying new tanks.

There are four fluorescence detector stations planned, which will be located in the cornersof the array. Each station consists of six telescopes, which follow a design similar to Fly's

eye experiment. Each pixel (PMT) of the telescope's camera covers 1.5 degrees of sky.

Since the detectors would only operate during moonless cloudless nights, only about 10%of all the observed events are expected to be hybrid (observed both by the surface and the

fluorescence detectors)

A vertical laser station constructed at the center of the array was constructed to calibrate

the fluorescence detectors. Weather monitoring stations at each tank, and stations to

monitor the atmospheric conditions located at each fluorescent detection station will

provide very important information for the data analysis.

3.4 First results from the Pierre Auger Observatory

There have been conflictive claims about the distribution of incoming cosmic rays. In

particular, the AGASA experiment in Japan has reported an excess of 22 % above an

isotropic cosmic rays flux coming from the region near the galactic core. An investigationof the same region using the first two years of data from the Pier Auger observatory has

not found any significant excess (Abraham 2007b).

Figure 8. New limit set on photon fraction for UHECR set by Auger Observatory data (red). Previous

results correspond to the AGASA (A1, A2) and Haverah Park (HP) surface array detectors. Also shown are

prediction of some "top-down" models (Abraham 2007a).

As explained in section 3.2.2, fluorescent detector are in a better position to distinguish

shower produced by primary gamma rays than surface detector experiment, since Xmax is

directly observed. The Pierre Auger group used 29 event above 1019

eV recorded during


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their first two years of operation to further constrain the limit on the fraction of gamma

rays in the UHECR flux already set by surface array experiments. For each event several

photon showers were simulated, and the results for Xmax were compared to the measuredvalues obtaining a probability that the shower was originated by a photon primary

(Abraham 2007a). The results are shown on figure 8.

Auger Preliminary results have also been presented for the energy spectra of UHECR

(Lettessier-Selvon 2005), but are not significant enough to resolve the discrepancies

above GZK cutoff energies of the AGASA data with the rest of the experiments (seesection 1.3). However, the first Auger results have large uncertainty, in part because of

low statistics, but also due to systematic errors in the energy calibration. A very simple

model for the atmosphere was used to correlate the observed fluorescent light to theprimary energy. With a better treatment of this effect, as well as new laboratory

measurements of the fluorescence yield of nitrogen, they expect to attain an energy

resolution of 15%. Also, when the observatory construction is finalized, its exposure will

be seven times greater than at the time this measurement was done.

Figure 9. First UHECR spectra reported by Pierre Auger Observatory, and deviation from the best fit power

law. The points with horizontal error bars indicate the variation of the energy resolution with energy

(Letessier-Selvon 2005).

5. CONCLUSIONS

In spite of the large difficulties associated with measuring the properties of UHECR (very

low statistics data sets, significant need for simulations and poorly known models to

interpret the observables) the experimentalist ingenuity has allowed us to learn a greatdeal about them. The first UHECR was detected almost 50 years ago, but the field is still

active with important new projects and ideas being currently implemented. In particular,

there are high expectations for the new results that could come from the Pierre AugerObservatory, which could settle very important questions such as the existence of the

GZK cutoff still as inferred from the energy spectra, or the composition and distribution

of the incoming cosmic rays. Once this issues are resolved, it will be up to the theorists to

come up with the explanation for the origin and acceleration mechanisms of this


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mysterious particles, a quest that is already leading us into the exploration of exotic new

physics as described in section 2.

REFERENCES

Abraham J., et. al.Astrop. Phy. 27 (2007) 155

Abraham J., et. al.Astrop. Phy. (2007) in press, arXiv:astro-ph/0607382

Achterberg A., et alAstrop. Phy. 26 (2006) 155

Bahcall J. N., Waxman E. arXiv:hep-ph/0206217 V5 27 Feb 2003

Biermann P., Medina-Tanco G. arXiv:astro-ph/0301299 V1 15 Jan 2003

Cronin J. W. arXiv:astro-ph/0402487

Falcke H., et. al.Nature 435 (2005) 313

Grieder P. Cosmic Rays At Earth. Elsevier Science B. V. 2001.

Hillas A. M.Acta Phys. Acad. Sci. Hung. 29 (1970) 355

Hillas A. M.Nucl. Phys. B 52S (1997) 29

Kappeler, F. et. al.Annu. Rev. Nucl. Part. Sci. 1998. 48: 175-251.

Letessier-Selvon A. arXiv: astro-ph/0510627

Linsley J. Phys. Rev. Let. 10 (1963) 146

Linsley J. 15th ICRC, Plovdiv, Bulgaria 12 (1977) 89

Nagano M., Watson A. A.Rev. Mod. Phys. 72 (2000) 689

Sciutto S. J. arXiv:astro-ph/9905185

Sigl G. arXiv:astro-ph/0008364 V2 9 Dec 2000

Stanev T. arXiv:astro-ph/0411113 V1 4 Nov 2004

Torres, D. arXiv:astro-ph/0402371

ana d. becerril, alfredo estrade and sean mcdaniel- ultra high energy cosmic rays

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