Antares in support of

the design study of km3net

Aagje E. Hendriks

August, 2008

Abstract

Neutrino astronomy is attractive, because neutrinos point straightback at their source in the Universe. Antares is a neutrino telescope,completed May, 2008. It is a prototype for the 30 times larger future de-tector KM3NeT. In this Bachelor project the configuration of Antares isvirtually modified using the 10-line data and ignoring data from storeys inthe muon reconstruction program ScanFit. Three configuration changesare made: The removal of even/odd floors, the removal of two lines in themiddle of the detector, and the reconstruction with data only from theupper or lower half of the Antares detector.

In general the conclusion is that the results of the different config-urations are well explained by theory. For the design of KM3NeT theconclusion is that the detector should be as deep in the sea as possible,to minimize the background signals. Doubling the horizontal distancebetween strings compared to Antares causes too much loss of verticalmuons. A vertical distance between optical modules two times largerthan in Antares causes too much loss of horizontal muons.

Bachelor project, University of Amsterdam, NikhefSupervised by Salvatore Mangano

Contents

1 Author’s contribution 5

2 Introduction 7

3 Neutrino astronomy 9

3.1 Cosmic rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.1 What are they made of? . . . . . . . . . . . . . . . . . . . 93.1.2 Cosmic ray sources . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Gamma rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.3 Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 The Antares neutrino detector 12

4.1 Detector layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.2 How to detect a neutrino . . . . . . . . . . . . . . . . . . . . . . 13

4.2.1 Cherenkov radiation . . . . . . . . . . . . . . . . . . . . . 134.3 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.4 From GRB to data and back . . . . . . . . . . . . . . . . . . . . 14

5 Design studies for KM3NeT 16

5.1 KM3NeT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.2 Antares as a sample . . . . . . . . . . . . . . . . . . . . . . . . . 165.3 The muon reconstruction program . . . . . . . . . . . . . . . . . 18

6 Results of changing the Antares configuration 19

6.1 Number of reconstructed tracks . . . . . . . . . . . . . . . . . . . 196.2 The half height detector . . . . . . . . . . . . . . . . . . . . . . . 20

6.2.1 Muons in water . . . . . . . . . . . . . . . . . . . . . . . . 206.2.2 Track length . . . . . . . . . . . . . . . . . . . . . . . . . 21

6.3 The configuration without line 3 and 4 . . . . . . . . . . . . . . . 226.4 The configuration without even or odd floors . . . . . . . . . . . 23

7 Conclusion and summary 25

8 References 27

9 Samenvatting 29

9.1 Kaarsrechte lijnen en kegels van licht . . . . . . . . . . . . . . . . 299.2 Detectoren en opstellingen (het project) . . . . . . . . . . . . . . 309.3 Conclusie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

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1 Author’s contribution

1. Studied: Ronald Bruijn. The Antares Neutrino Telescope: PerformanceStudies and Analysis of First Data. PhD thesis, Nikhef, Amsterdam, 2008.

2. Learned to use the Root analysis package.

3. Developed an algorithm for the analysis described in this paper, based onScanFit.

4. Acquired knowledge about LATEX to write the paper.

5

2 Introduction

Neutrinos are particles with no electric charge and a small interaction crosssection. This means that a detected neutrino at the Earth points straight backto its origin, and a neutrino is able to travel far through the Universe. Thenewest neutrino detector is Antares 1, completed the 30th of May, 2008. Thedetector consists of 12 lines, anchored at the bottom of the Mediterranean Sea.Antares is a prototype for KM3NeT 2, a future neutrino telescope with a volumeof at least one cubic kilometer and due to be ready in 2015.

In this project Antares data are used to test different detector configurations,and find out what is the best design for KM3NeT. The paper has been writtenfor my Bachelor project, and is based on data from the partially finished Antaresdetector (10 out of 12 lines).

The paper starts with an introduction in neutrino astronomy and an ex-planation why neutrinos should be detected, in chapter three. In the fourthchapter more specific information will be given about the Antares neutrino de-tector, what it looks like and how the neutrinos are detected. Thereafter, theobjectives of the project of this paper will be explained in the fifth chapter, thatalso contains an introduction of KM3NeT. The sixth chapter contains resultsand partial conclusions, that will be finished in chapter seven: conclusions andsummary.

1Antares is short for Astronomy with a Neutrino Telescope and Abyss Environmental

RESearch, where the abyss environmental research is the biological deep-sea research that is

also done with Antares.2The KM3 in KM3NeT is due to the scale of the telescope, while NeT is short for Neutrino

Telescope.

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3 Neutrino astronomy

Astronomy has been intriguing for many centuries. Ptolemaeus (87-150) wasa Greek mathematician who speculated about an all inclusive system of theUniverse. With Joannes Kepler and Galileo Galilei in the early 17th centurythis interpretation was turned around, and since then physicists were tryingto explain the Universe through experiment. In this vision Galilei created thefirst telescope in 1609 [1]. But direct observations with a telescope do notsatisfy anymore, and nowadays researchers found a way to observe the Universeindirectly: via particles coming from far unknown places. This chapter explainswhat kind of particles are able to reach the Earth and what they can tell usabout the Universe [2].

3.1 Cosmic rays

In 1936 Victor Hess (1883-1964) received the Nobel Prize for his work on cosmicrays [3]. In a balloon he discovered that if he went higher, he detected moreradiation. He concluded that the radiation did not come from the Earth’smaterial or the Sun. This meant that the radiation came from outer space andthe phenomena were named ‘cosmic rays’. Hess did not know the exact originof these cosmic rays and neither their composition.

3.1.1 What are they made of?

Cosmic rays have been examined during the years and it became clear that therays are spread over a very broad energy spectrum: from 1010 to 1020 eV.

Figure 1 shows the energy spectrum of cosmic rays. At higher energies, theparticle flux is smaller. Cosmic radiation turns out to consist for 90 percentof protons. The other 10 percent consists of heavier elements or electrons. Forhigh-energy cosmic rays of more than 1015 eV the composition is not determinedyet. This point is given by the ‘knee’ in figure 1.

3.1.2 Cosmic ray sources

To find more information about the cosmic rays, scientists are trying to trackdown their origin. Especially the rays with high energies are interesting, becausethey are thought to be accelerated in cosmic accelerators like black holes.

For cosmic rays it is not easy to find their origin, because they are electricallycharged. The particles will be deflected by inter-galactic magnetic fields. Onlythe highest energy protons of more than 50 ∗ 1018 eV are able to point back attheir source.

If the proton has an energy of more than 6 ∗ 1019 eV, it can interact withthe cosmic background radiation, produce a pion and lose energy:

p + γcmb → π + pslower.

This phenomenon is called the GZK cut-off. The result is that the interactionlength of the cosmic rays with more than 6∗1019 eV is relatively small. Particlesfrom farther away than 100 Mpc cannot reach the Earth anymore.

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Figure 1: The cosmic ray spectrum, with data from different experiments. Atthe points ‘knee’ and ‘ankle’ the slope of the graph changes. The figure is from[4].

3.2 Gamma rays

Other particles that reach the Earth and provide information about cosmicevents are gamma rays. An important source is the Sun, with photons thathave energies up to 1012 eV. Photons with higher energies are not easy to traceback, due to different interactions. If a photon reacts with the cosmic microwavebackground radiation, it produces an electron-positron pair: γ+γcmb → e++e−.Photons can interact also with infrared light or radio-waves.

3.3 Neutrinos

Neutrinos are particles postulated in 1930 by Wolfgang Pauli, and experimen-tally supported about ten years later. Neutrinos are electrically neutral andtherefore are not affected by magnetic fields. They also have a very small inter-action cross section. This means that they are not likely to interact with anotherparticle on their way to the Earth. Instead of looking to charged cosmic particlesor gamma rays, new detectors are now trying to find these promising neutrinos.

Neutrinos are created in interactions in the Universe. If a proton interactswith a cosmic microwave background photon, a π-particle comes out, see section3.1.2. This particle decays into a muon and a neutrino; the muon decays againinto two neutrinos. The neutrinos produced in such interactions are calledcosmogenic. The following formulas show this process for a π+ (with X as restparticles).

p + γcmb → π+ + X,

π+→ µ+ + νµ,

µ+→ e+ + νµ + νe.

Another neutrino source is the interaction of two protons. This interactionproduces a pion too, that again decays into neutrinos. This interaction can

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happen during a cosmic shower: an interaction of a cosmic ray with the Earth’satmosphere. In this shower atmospheric neutrinos are produced. If the protonscollide outside the atmosphere, the neutrinos are cosmic.

Neutrino sources in the Universe produce these cosmic neutrinos throughpion decay. An example is a Gamma Ray Burst (GRB). This is a light flash(if the produced pion is neutral, it decays into photons), part of them made atthe creation of a black hole. The photons are measurable, but the detection ofneutrinos would help to understand the details of several processes. Because aneutrino only interacts via the weak interaction, the particles can come fromthe core of the Sun for example. In this way neutrinos are able to give exclusiveinformation about events like Active Galactic Nuclei, Supernovae or even darkmatter.

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4 The Antares neutrino detector

The Antares collaboration has constructed a detector to look for neutrinos. Thisdetector has been worked on for about ten years and this May 30, 2008, the de-tector was completed. This chapter contains a description of the location andlayout of the Antares neutrino detector, the result of the effort the collabora-tion’s members made. The chapter continues with the methods the detectoruses to measure a neutrino, and describes how it is possible to distinguish theneutrino from other particles.

4.1 Detector layout

The Antares neutrino detector is located at the bottom the Mediterranean Sea,40 kilometers off the shore nearby Toulon, France. The detector consists of 12lines that each have a buoy on top to keep it vertical. In figure 2 is visible thatthe lines have a length of 450 meter, where 350 meter is used for detection. Eachline has 25 storeys, so 300 storeys are used in total. A storey contains 3 PhotoMultiplier Tubes (PMTs), which detect light signals. Each line is connectedwith a junction box. After passing this junction box the data is transported toshore by an electro-optical cable. On shore is a data station that collects thecomplete data set. Figure 3 shows a floor plan of the 12 lines.

Figure 2: Schematic figure of the Antares detector from [7].

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Figure 3: Floor plan of the 12 lines of Antares.

4.2 How to detect a neutrino

A neutrino has a very small interaction cross section. This is good news: ittravels far and has the possibility to bring information about the early Universe.For its detection however it is a huge disadvantage. A particle that does notinteract and has no charge, is not detectable.

But, of course there is more to it. With a detector volume like that of Antaresit is possible to detect a few neutrinos a day, even when the interactions havea very small cross section. Antares detects neutrinos up to an energy of 1016

eV. The cross section of the interaction depends on this neutrino energy, anddiffers from 10−36 to 10−33 cm2 for a muon neutrino. Mostly, muon neutrinosare detected with Antares, but is it also possible to detect tau and electronneutrinos. The trick of this incidental interaction is that a muon (or electronor tau lepton) is electrically charged. And an electrically charged particle, isvisible for a detector.

4.2.1 Cherenkov radiation

How to detect a charged particle in water is described by Pavel Cherenkov, aRussian physicist that received for this the Nobel prize in 1958 [3]. A chargedparticle that propagates through water meets the electric field of this water.Normally the field of the particle and that of water interfere destructively. How-ever, the muon travels faster than the speed of light in water (this is about 75percent of the speed of light in vacuum). Now the interference is constructive,and the muon emits a cone of blue light. In figure 4 this is illustrated.

The photons are spread with the angle θC , the Cherenkov angle. This angleis defined in the following way. The speed of light in water is the speed of lightin vacuum divided by the diffraction index of water: cw = c/n. Now the lengthof the hypotenuse in the triangle in figure 4 is the distance the muon travels:xµ = v ∗ t, where v is the muon velocity. The adjacent side is the track ofthe photon: xγ = cw ∗ t = t ∗ c/n. With trigonometry the definition of theCherenkov angle becomes (where β is v/c)

cos θC =xγ

xµ

=tc

n

1

tv=

1

βn.

In the situation where Antares detects muons, this results in a Cherenkov angleof about 42 degrees.

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Figure 4: Cherenkov effect.

4.3 Background

Antares detects light. It is located in the sea, so it is possible that for examplethe light of a glowing fish is measured, instead of that from a muon. Thisbackground light is relatively easy to recognize, because a glowing fish looksvery different in the detector from a muon track.

More disturbing background are muons that are not induced by a cosmicneutrino. The tracks look the same, but these are not the tracks Antares islooking for. A ‘wrong’ muon can be created via several processes, amongstthem cosmic rays creating a shower in the Earth’s atmosphere. To distinguishthese muons from the ones we are interested in, something very clever hasbeen figured out. Neutrinos are the only particles that can travel through theEarth. So muons that come from below the detector, can only be created inan interaction that involves a neutrino. That is why the PMTs from Antares(section 4.1) are looking downwards. However, it is possible that an atmosphericmuon is misreconstructed as up going. This is background for the up goingneutrino signals. The angular distribution in figure 5 shows that there are aboutfive orders more atmospheric muons than upward going muons from neutrinointeractions.

In section 3.3 has been explained that three kinds of neutrinos exist: cos-mogenic, atmospheric and cosmic. The most interesting ones are the cosmicneutrinos, because they can tell something about unknown sources. If a fewneutrinos from the same direction are detected in a short time period, it is likelythat they are cosmic. Atmospheric neutrinos created in air showers, are comingfrom all directions (figure 5). They are another background signal, besides themisreconstructed atmospheric muons.

4.4 From GRB to data and back

To summarize, it is clarifying to follow the complete path of a neutrino and itsdescendants.

First, a neutrino is created, for example in a Gamma Ray Burst. It has highenergy, and goes right through anything that is on its way. But then it arrivesat the Earth, close to Honolulu (its purpose is to be recognized as a neutrinoby Antares, so its arrival must be on the other side of the Earth). First theneutrino is not affected, but when it reaches the sea bottom on the other side

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Figure 5: The muon flux with respect to the zenith angle of their direction.The right side shows down going muons, the left side up going. Atmosphericmuons are depicted by the solid line, while the dashed line tells that muons dueto neutrinos are coming from all directions.

something happens. It bumps despite everything into an Earth particle -whata coincidence- and interacts. The interaction creates a muon, which continuesin almost the same direction. Because the muon is charged and travels withmore than the speed of light in water, it starts to emit Cherenkov light. Thislight is caught by nothing less than the Antares detector, that is located there.The PMTs detect its light (a hit!), the electronics transform it into digital dataand these data are collected at the shore station. At this moment the Antaresphysicists are able to reconstruct a track from this data, and they will see fromwhich direction the original neutrino came.

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5 Design studies for KM3NeT

The Antares neutrino telescope is not the end goal of the neutrino researchgroups. It is a prototype, like NEMO [5] and NESTOR [6], for the reallylarge telescope KM3NeT. KM3NeT is going to have the size of at least 1 cubickilometer, and will have 300-500 lines instead of the 12 lines Antares consists of.The objective of this project is to help providing KM3NeT with the best design.This is done using the data of the Antares detector, by virtually changing itsconfiguration. In this chapter it is explained what this objective means exactlyand what methods are used in the project.

5.1 KM3NeT

Different options for the configuration of KM3NeT are studied. Questions like‘what is the optimal horizontal distance between strings’ and ‘what is the bestvertical distance between modules’, have to be answered. The idea of the designis to detect as much neutrinos as possible, for the given amount of moneythat can be used to construct the detector. In the conceptual design reportof KM3NeT [8] are three different designs proposed, which are shown here infigure 6. It is not decided yet which design is going to be used, because theyall have their own advantages. The decision will be taken in 2009. The fullKM3NeT detector is planned to be finished in 2015.

Figure 6: Three optional configurations for KM3NeT from [8].

5.2 Antares as a sample

To provide KM3NeT with the best design, Antares data are used in this project.Although the construction of the full detector is finished, this project is based on10-line data (the 10 lines started to take data in December 2007). Further, onlydown going muons are used, and not muons from neutrino candidate events.This is because there are much more of these atmospheric muons (see section4.3), and there is not enough data available yet from neutrinos. The method of

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this project is to change the configuration of Antares, by ignoring part of thesignals and study the result on the reconstruction of the muons. How has itchanged? And what does this mean for the design of KM3NeT?

Three different configuration changes have been studied. In the first config-uration the vertical distance between storeys has been doubled. This is done byremoving the even floors (and for comparison this is done also with odd floors).Figure 7 shows an example of these floors. The second configuration simulatesthe situation where the lines are further apart from each other, by removing thelines in the middle of Antares. Figure 8 shows the new appearance, where line3 and 4 have been removed. The last configuration splits the detector in half,to study the effect of reducing the length of the strings. The first half is thelower half configuration, with the lower 12 floors. The upper half configurationconsists of the remaining 13 floors.

The new configurations are not made by really changing the Antares setup.The method is to ignore information from certain storeys, when a muon re-construction program is used. The next section describes the reconstructionprogram used in this project.

Figure 7: Schematic figure of the odd and even floors in Antares.

Figure 8: Antares without the two lines in the middle.

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5.3 The muon reconstruction program

To find the muon tracks out of the raw data a program is needed. The programthat is used here, Ronald Bruijn has described in his PhD thesis published inMarch 2008. He named his code ScanFit [7].

To reconstruct a track, first a program running at the computers in the datastation at the shore near Antares selects hits that could be a track. It takes allthe hits that surround this possible track, and names these an event. ScanFitanalyzes this event by scanning the parameter space, and selecting the mostlikely connection between the hits. This is its reconstructed muon track.

ScanFit needs five parameters to define a track, what implies that at leastfive hits must exist for one track. The direction of a track is defined by thezenith angle θ, and the azimuth angle φ. The left side of figure 9 shows thesetwo angles. ScanFit takes this muon direction and rotates the whole coordinatesystem, in such a way that the the muon direction becomes the new z-axis: z ′.This is shown at the right side in figure 9. All this is defined by the rotationmatrix R [7], with ~x′ = R~x and

R =

cos θ cosφ cos θ sin φ − sin θ− sinφ cosφ 0

sin θ cosφ sin θ sin φ cos θ

.

Figure 9: On the left is the coordinate system with a muon track, on the rightthe rotated system, with z′ parallel to the track. Figure from [7].

In figure 9 on the right side the muon crosses the plane z′ = 0 in point(x′, y′) = (a, b). To summarize: there are two angles (θ and φ) for the directionand two coordinates (a and b) for the position. The fifth parameter ScanFitneeds is the time t0 at which the muon is at point (a, b).

For this project the results of ScanFit are analyzed. The program that is usedfor this is Root [9]. This program, originally made by CERN, is constructed towork with large sets of data and therefore it is particularly practical for particlephysics. Root has been used to generate the histograms in this paper.

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6 Results of changing the Antares configuration

It is time to show the results of this project. First the number of tracks ScanFitreconstructs with different configurations will be presented. Then follow sectionsabout the half height detector and about the configuration without the twomiddle lines. The last section is about the zenith angle. There are also someconclusions in this chapter, which will be summarized in the next and lastchapter.

6.1 Number of reconstructed tracks

Figure 10: The number of tracks reconstructed with different configurations.

Figure 10 gives the number of tracks reconstructed with the different config-uration changes applied to Antares. The input is always the same 5000 events.With the full 10-line detector ScanFit reconstructs 3128 muon tracks. This isshown in the first bar of figure 10. The figure summarizes the results of thedifferent configurations that have been made.

With only the even (505 tracks) or odd (516) floors there are a lot less tracksreconstructed than with the whole detector. This makes sense, since ScanFitneeds at least 5 hits to reconstruct a track. In the configuration with line 3 and4 missing, 1897 tracks are reconstructed. The last two bars show the result ofthe configuration with only half the size of the detector. In the configurationwith only the upper half 1270 tracks are reconstructed; with only a detector inthe lower half ScanFit reconstructs 914 tracks.

There are to say a few things about the results of figure 10. There are 13odd floors and 12 even floors. This could mean that the detector with onlyodd floors reconstructs more tracks than the only even-floor detector. Figure10 shows that this is indeed the case, but the difference is small. The effect

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of the extra floor in the ‘odd’ configuration is not significantly visible in thisresult. The same thing can be said about the half sized detectors (‘upper half’and ‘lower half’). The lower half consists of floors 1-12, while the upper halfcontains floors 13-25. Since the upper half has an extra floor, more hits shouldbe recorded and more tracks be reconstructed. This is indeed the case: 1270against 914. But this difference is much to big to be attributed only to thisextra floor. In the next section there will be a closer look at this half sizeddetector.

6.2 The half height detector

The first part of this section is about the reason why with the upper half moretracks are reconstructed than with the lower half, and in the second part thetrack length in the half detector is calculated.

6.2.1 Muons in water

With the upper half of the detector more tracks are reconstructed than withthe lower half. To see why, the behaviour of muons in water must be studied.Several experiments did this, in figure 11 their results are combined. If a muontravels through water, it loses energy [4]. This is due to ionization, radiativeprocesses, nuclear interactions and the production of electron-positron pairs. Itmeans that the muon decays or slows down. If the muon travels too slow toproduce Cherenkov light (if it is slower than the speed of light in water) it isnot visible anymore. The reconstructed track stops.

The results of figure 10 show that the upper half of the detector reconstructs1270 tracks, and the lower half 914 tracks. These were all down going muons, soit is possible that their track stopped halfway the detector. The muons comingfrom above are not from the neutrinos Antares is looking for, it is looking forupcoming neutrinos. The down going muons can however be reconstructedwrongly as up going, so there should be as less as possible. Because the numberof tracks in the upper half of the detector is higher than in the lower half, onecan conclude that a detector that is deeper in the ocean has to deal with lessbackground from atmospheric muons.

Figure 11: The muon intensity with respect to the depth.

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6.2.2 Track length

A histogram of the track length is created with Root. This is done for the halfheight detector, and the mean track length of the different configurations is putin a bar chart. This chart, in figure 12, shows that while the detector is cut inhalf, the track length is not. That is because the tracks should have at leastfive hits. If a muon goes trough three quarters of the whole detector, it canbe that its track is only recognized in one half, because there are not enoughhits in the other half. With the whole detector a track that is not very longin comparison to the rest can be reconstructed, while with a half detector thetrack goes through its complete volume.

A second thing figure 12 shows is that the tracks in the upper half arelonger. If a muon enters from above and slows down in the detector until itis not measurable anymore, the tracks in the upper half should be longer onaverage. This is indeed the case, but not with sufficient significance. Anotherreason for the small difference in track length can be that the upper half hasmore floors (13 instead of the 12 from the lower half).

Figure 12: The mean track length in the whole and half detector.

Before moving on there should be an explanation how the track length isdefined in this paper. For this figure 13 is needed. If a muon µ travels throughwater it sends out a photon γ, with a Cherenkov angle θC of about 42 degrees.The point where this photon is emitted is named z′

γ here. If this point is foundfor each hit of the track, the first one is subtracted from the last one. This givesthe track length.

To find z′γ the following should be done. The detector knows the position ofthe track, (a, b), in the rotated system. The position of the PMT is also known,but should be put first in the same coordinate system, with the rotation matrixR from subsection 5.3. The position of the rotated PMT is now (x′

j , y′

j , z′

j).With this the distance between the track and the PMT can be calculated in2D: rj =

√

(a − x′

j)2 + (b − y′

j)2. Trigonometry shows that rj/ tan θC is the

distance between z′

j and z′γ . Because z′

j is known, in this way z′

γ is found. In aformula this looks like

z′γ = z′j −

√

(a − x′

j)2 + (b − y′

j)2

tan θC

.

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Figure 13: Schematic view of a muon passing an optical module. Figure from[7].

6.3 The configuration without line 3 and 4

In figure 8 the configuration of Antares is shown with the two middle linesmissing. These lines were removed to study the effect of a larger horizontaldistance between strings. In figure 14 is shown a histogram we made of theposition of the reconstructed tracks. For the position of the track is takenthe coordinate (a, b), the point where the track is closest to the middle of thedetector. The points a and b are the parameters explained in subsection 5.3.The histogram shows that the detector indeed measures less tracks in the middlewhere these two lines are missing. Because there are only eight strings left, itis more difficult to reconstruct decent tracks in this configuration.

In the last section of this chapter we discuss the distribution of the zenithangle, θ. With a histogram made using the program Root it is possible to showthe distribution of the zenith angle of the reconstructed tracks. The definitionof the zenith angle is such that tracks with an angle of 180 degrees are comingfrom above, at 90 degrees they are parallel to the horizon and at 0 degrees theycome from below. Figure 15 shows the distribution for the full detector, and forthe detector with line 3 and 4 missing.

While the histogram for the detector with line 3 and 4 missing containsless reconstructed tracks than that for the full detector, it also shows that themean zenith angle is a little bit smaller. This means that relatively more muonsare coming in horizontally. This makes sense because two vertical lines wereremoved, leaving a hole in the middle of the detector. However, the differencein zenith angle between the two distributions is too small to draw statisticallycorrect conclusions.

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Figure 14: On the left the positions of the tracks in the 10-line detector, on theright the positions without line 3 and 4.

Figure 15: This figure shows two histograms. The upper one is the zenithangle distribution of tracks measured with the 10-line detector, with 3128 tracksreconstructed. The lower one is the distribution measured with the detectorwithout line 3 and 4.

6.4 The configuration without even or odd floors

In the third configuration the even or odd floors are removed, to study the effectof a larger vertical distance between storeys. If the even floors are removed, 516tracks are reconstructed, if the odd floors are removed 505 tracks are recon-structed. A histogram of the distribution of the zenith angle is made. Figure 16

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shows the mean zenith angle for the different configurations. Also a distribu-tion of the zenith angle is made with two floors removed (so only the storeys onfloors 1,4,7,10,13 etc. are taken). In this way the vertical distance between thestoreys is made three times bigger. This resulted in an average zenith angle of141.0 degrees, which is higher than the value for the full detector. This makessense, because more horizontal floors were removed. But also in this case thereis not enough statistics, although the uncertainty on the values for the meanzenith angle has not been calculated.

Figure 16: The mean zenith angle in the different configurations.

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7 Conclusion and summary

The interest in the Universe is of all times. The methods to observe the Universehowever, are very different than before. The Antares telescope detects neutrinos,that provide information about the Universe indirectly. Neutrinos have zerocharge and a very small interaction cross section, and therefore they are notdeflected by magnetic fields and they almost never interact with other particles.This is why neutrinos are a better probe into the Universe than protons orphotons.

When a neutrino interacts with a nucleon in the rock of a seabed, a muonis created, that travels in almost the same direction as the original neutrino.This muon is electrically charged, and if this muon travels faster than the speedof light in water it emits Cherenkov light. The Antares detector measures thislight with Photo Multiplier Tubes (PMTs). The detector consists of 12 lines,that are anchored at the bottom of the Mediterranean Sea near Toulon, France.Each line has 25 storeys with 3 PMTs each, that are looking downwards toreduce background of down going atmospheric muons: only neutrinos are ableto travel right through the Earth and create upward going muons in the detector.Antares was completed on May 30, 2008.

KM3NeT is new generation neutrino telescope of at least one cubic kilometer,due to be ready in 2015. In 2009 the final design of this large detector is going tobe decided. The objective of this project is to use the data measured by Antares,to help with tracking down the best design. The method of this Bachelor projectis to change the configuration of Antares in three different ways. The mainquestion is what happens with such a configuration change, and how this canhelp with designing KM3NeT. The data used in this project are collected with10 lines of Antares and only down going muons are used. Configuration changesare done with a muon reconstruction program called ScanFit.

The first configuration change is the removal of the data from the storeys atthe even and odd floors, to simulate that the storeys are vertically further apart.About 1/6th of the tracks are reconstructed, and the average zenith angle ofthe detector increases, but not significantly. In the second configuration the twomiddle lines are removed to simulate a larger distance between strings. A bitmore than half of the tracks measured with the whole detector is reconstructed,with a zenith angle that is smaller, but again not significantly. In the thirdconfiguration the detector is cut in half, to simulate a detector with half theheight of Antares. In the upper half a lot more muons are reconstructed thanin the lower half. This shows that the down going muons are slowed down byenergy-loss, or interact. In both circumstances they are not detectable anymore,and not reconstructed in the lower half. The average track length in both halfdetectors is about 3/4th of that measured with whole detector.

In general there it can be concluded that the results of the different configu-rations are well explained by theory. For the design of KM3NeT the conclusionis that the detector should be as deep in the sea as possible, to minimize thebackground signals. Doubling the horizontal distance between strings comparedto Antares causes too much loss of vertical muons. A vertical distance betweenoptical modules two times larger than in Antares causes too much loss of hori-zontal muons. To optimize these distances, further studies are required.

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8 References

1. Floris Cohen. De herschepping van de wereld, het ontstaan van de mod-erne natuurwetenschap verklaard. Uitgeverij Bert Bakker, Amsterdam,2007.

2. Francis Halzen, Dan Hooper. High-energy Neutrino Astronomy: The Cos-mic Ray Connection. Rept.Prog.Phys. 65 (2002) 1025-1078.

3. http://nobelprize.org

4. W.M. Yao et al. Review of particle physics. J. Phys., G33:1-1232,2006.

5. www.km3net.org

6. http://www.nestor.noa.gr/

7. Ronald Bruijn. The Antares Neutrino Telescope: Performance Studiesand Analysis of First Data. PhD thesis, Nikhef, Amsterdam, 2008.

8. KM3NeT, Conceptual Design for a Deep-Sea Research Infrastructure In-corporating a Very Large Volume Neutrino Telescope in the MediterraneanSea. Made public at the VLVnT08 Workshop.

9. http://root.cern.ch

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9 Samenvatting

Neutrino’s zijn deeltjes die constant door alles heengaan dat op hun

weg komt, zo ook de aarde. De deeltjes worden opgewekt in processen

in het heelal, die niet met het blote oog te zien zijn: zelfs met de

sterkste telescoop zijn ze niet te bereiken. Als deze deeltjes nu eens

gedetecteerd zouden kunnen worden, zou dan het universum iets van

zijn ondoorgrondelijkheid verliezen? Antares, en over een paar jaar

KM3NeT, zijn de detectoren die dit waar kunnen gaan maken.

Op de bodem van de Middellandse Zee, vlakbij Toulon in Frankrijk, zijnduikboten de afgelopen jaren in de weer geweest met het opstellen van 12 lijnen.Aan deze lijnen zitten elk 25 modules. Deze modules meten neutrino’s, metbehulp van PhotoMultiplier Tubes (PMT’s: detectoren voor licht). Op 30 mei2008 is de laatste lijn Antares-detector succesvol aangesloten. Het uiteindelijkedoel van het neutrino-onderzoek is niet Antares, dat eigenlijk een prototype is,maar een veel grotere detector. Deze detector zal 300-500 lijnen hebben, en eengrootte van een km3. De detector, die in 2015 klaar moet zijn, krijgt de naamKM3NeT: een KM3 NEutrinoTelescoop.

9.1 Kaarsrechte lijnen en kegels van licht

Het meten van neutrino’s heeft een groot voordeel boven de detectie van anderedeeltjes die vanuit het heelal op de aarde terecht komen. Kosmische stralingbijvoorbeeld, dat voornamelijk bestaat uit protonen, is elektrisch geladen. Destraling wordt afgebogen door magneetvelden, en een proton zal dus nooit rechtterugverwijzen naar zijn oorspronkelijke bron. Ook een lichtdeeltje, een foton,kan worden gemeten, maar deze kan reageren met achtergronddeeltjes in hetheelal: van ver weg komt een foton niet op de aarde aan. Een neutrino is nietgeladen en reageert bijna niet met andere deeltjes. Hierdoor zijn neutrino’sperfect om meer informatie te krijgen over bijvoorbeeld de ontploffing van eenverre ster.

Doordat een neutrino niet met andere deeltjes een interactie aangaat, gaathet deeltje ook dwars door de aarde heen. Dit is goed omdat het neutrino opdeze manier heel ver kan reizen, maar het maakt het daadwerkelijke meetwerkerg lastig. Gelukkig botst een neutrino toch een paar keer per jaar tegen eenaardedeeltje. Uit deze interactie komt een muon, een deeltje met lading dat derichting van het neutrino behoudt. De lading van dit muon is essentieel voorhet detectieproces. Een geladen deeltje dat door water gaat, komt in aanrakingmet het elektrische veld van de waterkernen. Normaliter zal dit niet veel effecthebben. Als het deeltje nu snel genoeg gaat (de snelheid moet hoger zijn dan delichtsnelheid in water!) interfereren de velden op zo’n manier dat het deeltje eenkegel van blauw licht achter zich laat. Dit licht heet Cherenkovlicht, en wordtgemeten door de PMT’s van Antares. Om deze lichtsignalen te onderscheidenvan de Cherenkovkegel van andere deeltjes, is Antares naar beneden gericht.Het neutrino is namelijk het enige deeltje dat dwars door de aarde heen gaat,en zo kan zijn pad worden herkend.

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9.2 Detectoren en opstellingen (het project)

Dit verslag beschrijft mijn project, dat ik heb gedaan als afsluiting van de Bach-elor Natuurkunde. In het project is gekeken naar data die geproduceerd is doorAntares, toen er 10 lijnen werkzaam waren. Omdat er nog niet veel data vanneutrino’s was, is er gekeken naar signalen van andere muonen (die van bovenafkwamen). Hierbij is uitgegaan van 5000 mogelijke muonpaden. In het projecthebben we op drie verschillende manieren het programma aangepast dat de datavan Antares analyseert. Het gebruikte programma heet ScanFit. Onderzoekerszijn nu bezig met het bedenken van de opstelling van de grotere neutrinotele-scoop KM3NeT. Het doel van dit project is kijken of we hier een steentje aanbij kunnen dragen. Door de modules van Antares als het ware verder uit elkaarte zetten, geven de verschillende configuraties een voorproefje voor de opstellingvan KM3NeT.

Zoals gezegd hebben we drie verschillende configuraties geprogrammeerd.De eerste moet simuleren dat de modules in verticale richting verder van elkaarverwijderd zijn. Dit is gedaan door de modules op alle even verdiepingen uitte schakelen. Hierna is dit, om te vergelijken, ook gedaan voor oneven rijen.Uit de resultaten bleek dat er maar 1/6de van alle muonpaden werd gerecon-strueerd. Ook kwamen de muonen gemiddeld iets verticaler in, omdat er padenin horizontale richting niet gereconstrueerd konden worden (er zijn horizontaleverdiepingen verwijderd). Dit was echter een te klein verschil om conclusies uitte trekken.

Voor de tweede configuratie hebben we twee gehele lijnen weggehaald, die inhet midden stonden. Hierdoor komen de modules in horizontale richting verderuit elkaar te staan. Om te kijken of er echt minder muonen werden gemeten inhet midden, hebben we een histogram gemaakt met de plaats van elk muonpad.Het weghalen van deze lijnen bleek duidelijk zichtbaar: er bevinden zich veelminder paden in het middelste deel van Antares. Iets meer dan de helft van deoorspronkelijke paden werd gereconstrueerd, en nu kwamen ze (zoals verwacht)iets meer horizontaal in.

Als laatst is naar het verschil gekeken in de bovenste en onderste helft van dedetector. In de bovenste helft worden veel meer muonen gereconstrueerd. Ditkomt omdat het muon energie verliest en hierdoor steeds langzamer gaat. Als hetmuon nu zo langzaam gaat, dat zijn snelheid niet meer boven de lichtsnelheid inwater uitkomt, zendt het geen Cherenkovlicht meer uit. Antares kan het muondan niet meer meten, en het signaal stopt in dit geval halverwege de detector.Het kan ook zijn dat het muon een interactie aangaat, waardoor er geen padmeer kan worden gemaakt. De lengte van een gereconstrueerd pad wordt in ditverslag gemeten als het deel van het pad dat je in de detector kunt zien. Inde hele detector is dit ongeveer 220 meter. De bovenste en de onderste helftreconstrueren hier 3/4de van.

9.3 Conclusie

De installatie van Antares was spannend voor de onderzoekers: zou de detectorhelemaal goed werken als hij eenmaal klaar was? De resultaten van dit projectlaten zien dat de detector muonpaden reconstrueert op de manier zoals hetbedacht was. De verschillen die er ontstaan zijn als de opstelling iets wordtaangepast, zijn goed verklaarbaar met de theorie over deeltjes en hun interacties.

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Als hulp om de beste opstelling voor KM3NeT te vinden, moet je kijken naarhet verschil tussen de bovenste helft en onderste helft van de detector. In debovenste helft werden meer muonen gereconstrueerd. Deze muonen van bovenafzijn niet afkomstig van de neutrino’s die Antares en KM3NeT zoeken, en zijndus een achtergrondsignaal voor de neutrino’s die door de aarde heen komenen een muon produceren dat van onderaf komt. De onderste helft heeft mindervan deze achtergrondsignalen: de KM3NeT-detector zou het beste werken zodiep mogelijk in de zee. In de andere twee configuraties (als er verdiepingenweg worden gehaald, en als de twee middelste lijnen worden genegeerd) wordener te weinig muonpaden gereconstrueerd. Dit betekent dat de afstanden niettwee keer zo groot gemaakt moeten worden. Hoe groot de afstanden wel preciesmoeten zijn, moet verdere studie uitwijzen.

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