Einführung in die Teilchenphysik...Teilchenphysik als verhältnismäßig junger Zweig der Physik...

Einführung in die Teilchenphysik

Ein Stipendiumsbericht für das Fach Physik von...

Inhaltsverzeichnis

1 Einleitung 4

2 Introduction to Particle Physics 52.1 The Standard Model of Particle Physics - Particles and their interactions . . . . . . . . 52.2 Dark matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.3 Practical Particle Physics - Motion of Particles in Fields . . . . . . . . . . . . . . . . . 21

3 Das CERN - Der Weg eines Protons durch den Beschleunigerkom plex 253.1 Beschleuniger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2 Detektoren . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.3 Datenanalyse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Résumé 31

5 Quellennachweis 32

3

1 Einleitung

Als naturwissenschaftlich interessierte Schüler hatten wir schon länger überlegt, eine große und be-deutende Forschungseinrichtung kennen zu lernen, um dort vielleicht sogar ein kleines Projekt durch-zuführen. Auf diese Weise erhofften wir uns, einen fundierten Einblick in das Denken und Arbeiteneines Naturwissenschaftlers zu erhalten. Besonderen Reiz hatte für uns dabei die moderne Physik,die in der Schule bisher nicht oder nur unzureichend behandelt wurde. Sehr bald bot sich speziell dieTeilchenphysik als verhältnismäßig junger Zweig der Physik an.

Nicht nur durch die mediale Präsenz, sondern auch durch die spannenden Forschungsinhalte wur-den wir auf das CERN aufmerksam. Mit dem CERN fiel die Wahl auf eine der wenigen führendenForschungseinrichtungen auf dem Gebiet der Teilchenphysik.

Dankbar nahmen wir deshalb das Angebot der Studienstiftung wahr, sich eine Reise mit Bildungs-charakter finanzieren zu lassen, und bewarben uns.

4

2 Introduction to Particle Physics

Probably to the reader’s general astonishment, the theoretical chapter of our report is in English. Inaddition to the physics content, our project at CERN also represented a language challenge. Englishis the language of choice in the world of international physics, so our project was also conducted inEnglish. In order to emphasise the linguistic aspect of our project, this chapter remains in its originallanguage.

2.1 The Standard Model of Particle Physics - Particles and th eirinteractions

2.1.1 Radioactivity

In lightweight nuclei the amount of protons and neutrons is nearly balanced, but heavier nuclei tendto have a few more neutrons than protons. Stable nuclei are mostly located in the, so called, valley ofstability. Instable nuclei are radioactive and decay until they become stable. There are three differentways of decay, explained below with the example of a nucleus, C, containing a total number of Anucleuons and Z protons.

α-decay

When nuclei decay through α-decay they emit doubly positively charged helium nuclei.

AZC→ A−4

Z−2C+ 42He (2.1)

β-decay

In β−-decay, a neutron in the nucleus decays to a proton and an electron. The latter is emitted.

AZC→ A

Z+1C+e− (2.2)

The emitted particle in β+-decay is a positron. It is the product of the decay of a proton to a neutronand a positron.

AZC→ A

Z−1C+e+ (2.3)

5

Bericht der Studienstiftung - CERN 2009

γ-decay

When nuclei decay via γ-decay, short-wavelength radiation is emitted.

AZC→ A

ZC+ γ (2.4)

This corresponds to the emission of a particle, the photon, which is a particle without rest mass movingat the speed of light. According to E = mc2, it possesses a relativistic mass. Electromagnetic radiationhas a wavelength and a frequency. If multiplied, the result is the speed of light (3 ·108 m

s ).

f ·λ = c (2.5)

The energy of a single photon is the product of its frequency and the PLANCK’s constant h.

E = f ·h (2.6)

UNITS:[ f ] = 1Hz= s−1

[λ] = m

[E] = 1eV(= 1,602·10−19J)

where eV is a unit of energy used in particle physics and corresponds to the energy gained by a freeelectron in a potential of one Volt.

6

2.1 The Standard Model of Particle Physics - Particles and their interactions

2.1.2 Elementary Particles - The Particle Zoo

Abbildung 2.1: The Standard Model.

Elementary particles cannot be dividedany further and are pointlike. They arethe building blocks of all matter and al-so the foundation of the Standard Modelof particle physics. This very succesfulmodel (see Fig. 2.1) comprises all par-ticles of matter (leptons and quarks), allparticles of antimatter and all bosons.Non-elementary particles, such as ha-drons, are composed of two or threequarks. Depending on the number ha-drons consist of, they are called mesons(2 quarks) or baryons (3 quarks). Usual-ly hadrons decay in fractions of a se-cond, the only stable hadrons are theproton (uud) and the neutron (ddu). The Standard Model reveals certain symmetries concerning thecharge of the particles. Electron-like particles e, µ and τ (charge −1) are paired with neutrios νe, νµ

and ντ (charge 0); quarks d, s and b (charge -1/3) are paired with quarks u, c and t (charge +2/3).But the fact that the large differences in mass between the particles cannot yet be explained is one anindication that there is unknown physics beyond the Standard Model. Therefore, the Standard Modelis fairly likely to be extended in the future, which will hopefully happen with the data from the LHC.

Leptons

Electron e− In 1897 JOSEPH JOHN THOMSON discovered the electron in his cathode ray experi-ment, see Fig. 2.2. He observed that the cathode rays were influenced by an electric field and conclu-ded that they must carry charge. Whereas THOMSON only succeeded in calculating the electron’s ratiogiven by e

me, ROBERT MILLIKAN finally measured the exact charge in the oil droplet experiment.

Positron e+ The positron was discovered in 1932 by CARL ANDERSON. His analysis of a largenumber of cloud chamber pictures revealed tracks of a light, positively charged particle. The chargewas determined by including a lead plate in the cloud chamber and applying a magnetic field insideit. The magnetic field bent the particle tracks and the plate resulted in a loss of momentum that wasvisible in the track’s curvature. The difference in curvature before and after the plate allowed to deter-mine the direction the particle traveled in. Then from the direction of curvature the positron could bedistinguished from an electron. Furthermore, measuring the energy lost in the lead plate allowed todistinguish the positron from the proton.With the positron the first antiparticle was discovered. The positron is the antiparticle of the electronas it carries the same mass, but the opposite charge.

7


Abbildung 2.2: The cathode ray experiment.

Muon µ− In 1947 a negatively charged particle a lot heavier than the electron, the muon, wasdetected. 25 years before, VICTOR HESS had observed that ionising radiation measured on the earth’ssurface increased with altitude, which contradicted the widespread assumption that the radiation wascaused by radioactive materials in the ground. Hence, the radiation had to be cosmic. It took sometime until the muon was identified. Its properties are different from those of the electron. Due to itsmass that is 200 times higher than the electron’s it is able to travel through a lot of matter, such asseveral metres of lead, without being scattered.

Electron-Neutrino νe Energy and momentum conservation did not seem to be valid for β-decay,so a new particle called electron-neutrino (neutrino it.: small chargeless) carrying the missing energyand momentum was postulated by WOLFGANG PAULI. With this new particle β-decay could now befully described in a new equation by ENRICO FERMI in 1934:

n→ p+e− + νe (2.7)

Neutrinos are able to fly through a light-year of lead without being scattered. Consequently they interactextremely rarely and are hard to detect. Nevertheless, REINES and COWEN succeeded in provdingevidence of this particle via inverse decay in a nuclear reactor according to the following equation:

νe+ p→ n+e+ (2.8)

Muon-Neutrino νµ The muon-neutrino was discovered in 1962 by LEON MAX LEDERMAN, MELVIN

SCHWARTZ und JACK STEINBERGER. Using the method of inverse decay that had already proved theexistence of the electron-neutrino, they detected the muon-neutrino by colliding two hadrons beamsproducing muons and muon-neutrinos and passed the beam through a thick concrete wall. Since allparticles except the neutrinos were stopped by the wall, only the neutrinos could enter the detectorand produce a few muons through inverse decay, which could be detected directly.

8


Tau τ The missing energy and momentum in the decay

e+ +e− → e+µ (2.9)

indicated the presence of neutrinos. The most convenient explanation

e+ +e− → τ+ + τ− → e+µ+νe+νµ+2ντ (2.10)

contained a new particle, the tau, that was discovered between 1974 and 1977 by MARTIN PEARL andhis colleagues.

Tau-Neutrino ντ The corresponding neutrino of the tau was detected via inverse decay in 2000in the DONUT experiment. Although the principle that was applied to provide evidence for this particlewas similar to the methods used for the detection of the other neutrinos, this particular experiment wasmore difficult as the τ that is involved in the process is hard to detect itself.

Hadrons

The quark model The part of the Standard Model that explains the inner structure of all hadrons wasdeveloped in 1964. According to the model, hadrons are either mesons, consisting of a quark-antiquarkpair, or baryons with three quarks or three antiquarks. Decay properties of hadrons in acceleratorsindicate their quark content. The model comprises six quarks and their antiquarks that form the buildingblocks for all known hadrons.

Abbildung 2.3: Inner structure of a proton.

Proton p The proton, carrying a po-sitive charge that neutralises the nega-tive charge of the electron, has beenknown for a long time. More detailedstudies on this particle revealed an in-

ner structure of two up-quarks and one down-quark (uud), as shown in Fig. 2.3. Consequently, theproton’s antiparticle consists of two anti-up-quarks and one anti-down-quark (uud) and is oppositelycharged. The antiproton was detected in 1955 in Berkely by smashing highly energetic protons into aconcrete wall. From measurements of speed and momentum the antiproton’s mass could be calculatedwith the expected result that proton and antiproton have exactly the same mass.

Neutron n Predicted by LORD ERNEST RUTHERFORD in 1920, the neutron was discovered 12years later by SIR JAMES CHADWICK by bombarding beryllium nuclei with α-particles. The subsequentchain reaction produced carbon and a new particle that CHADWICK collided with other nuclei. Protonswere freed in these secondary collisions and were made visible in an ionisation chamber. As CHAD-WICK knew that the discovered particle had transferred its momentum to the proton, he calculated theneutron’s mass which was very close to the proton’s mass.

9


Abbildung 2.4: Particle and antiparticle annhihilate under emission of a boson. Vice versa, particle andantiparticle are created by a boson (pair creation).

2.1.3 Fundamental interactions

There are four fundamental interactions, listed in Table 2.1, that describe the forces, which occurbetween the elementary particles of our universe. Three of them, namely the electromagnetic, weakand strong interaction, are described by the Standard Model. Only the gravitational force could not beincorporated into the Standard Model yet. The fact, that these forces are indeed fundamental and notcomposed of other forces, makes them scientifically valuable. As a result, each other known force canbe regarded as the product of them.

For the depiction of the interaction in particular, the Standard Model assigns to each of the interac-tions a transmitter-particle. This particle carries the corresponding force from one particle to another.Through the exchange of this, so called, boson fundamental interactions become possible betweenobjects without any physical contact. This way not only the transfer of energy, but also of momentumis ensured.

Boson-exchanges can either occur in form of annihilation or pair creation, see Fig. 2.4. The firstdecay takes part if a particle collides with its antiparticle and consequently they annihilate. As a result,a boson is created. Vice versa, a boson also can create particles.

In every decay energy and momentum conservation have to be maintained. Hence, the mass of theend products has to less or equal to the energy carried by the boson. Moreover the sums of momentum,angular momentum and electric charge (or an equivalent charge) have to be each the same after theinteraction as before.

Even though science made great progress in this field during the last few decades, there still remain

Interaction Field’s source Relative Strength Range Boson

Strong Colour charge 1 Short GluonElectromagnetic Electrical charge 10−2 Infinite Photon

Weak Weak charge 10−5 Short W+,W−

,Z0

Gravity Mass 10−37 Infinite graviton (?)

Tabelle 2.1: The fundamental interactions.

10


some unsolved problems, which also attract the interest of particle physicists at the LHC. There is, forexample, the search for the transmitter-particle of the gravitational force, which has not been discover-ed yet. Another aim of today’s physics is the unification of the interactions into one theory or one setof equations. As mentioned, the Standard Model only implicates three out of the four forces and doesnot offer an ideal solution.

Gravitational interaction

Gravitational interactions can be observed between all massive bodies. It is the weakest force and,in contrast to the other forces, always attractive and never repulsive. According to NEWTON’s Law ofgravitation, the gravitational force derives from the product of the mass of the participating objectsdivided by the square of distance between them and multiplied by the gravitational constant, G:

F = G ·M ·m

r2 (2.11)

The hypothetical gravitational quantum, the so called graviton, is often used to illustrate certain physicalprocesses but has not been found yet. Here the equivalent to the electric charge would be the mass.Though it is a practical concept, it does not deliver a satisfactory quantum theory of gravitationalinteraction.

Weak interaction

The weak interaction is 103 times weaker than the electromagnetic interaction. Its transmitter-particlesare the, so called, W−, W+ and Z bosons. Because of the large mass of these particles, processes,which contain weak interaction, are slowed down. In general, weak interactions can take part betweencertain types of particles.

Abbildung 2.5: FEYNMAN diagrammfor β−-decay mediated by weakinteraction.

β-decay occurs because of a weak interaction. In the followi-ng, it will be used as an example to illustrate how weak inter-actions work. The transformation of an unstable atomic nucleusinto a nucleus with the same number of nucleons but differentnumber of protons is defined as β-decay. The change can eithertake place, such that a neutron converts into a proton emittingan electron and an antielectron-neutrino,

10n→ 1

1p+e− + νe (2.12)

or such that a proton converts into a neutron emitting a positronand an electron-neutrino

11p→ 1

0n+e+ +νe (2.13)

The emitted electrons or positrons are known under the name of β-particles, which form the β-radiation. In particular, an exchange of charge occurs from the uncharged neutron into a positivelycharged proton and a negatively charged electron in the first instance and from a positively charged

11


proton into an uncharged neutron and a positively charged positron in the second instance. This ex-change of charge is one effect of the weak interaction. However, it should be noted that the overallcharge is the same before and after the interaction. The first interaction is depicted in the diagram inFig. 2.5, which is called a FEYNMAN diagram after the famous scientist who first drew such a picture ofthe underlying interaction. To explain the FEYNMAN diagram:

• Time is running upwards along the y-axis.

• Arrows correspond to particles (antiparticle-arrows point in the opposite direction).

• Dashed lines correspond to bosons.

• At each interaction vertex energy and momentum are conserved.

Thus the FEYNMAN diagram illustrates the decay of a neutron into a W−, which later decays intoan electron and an electron neutrino, and a proton. A more detailed FEYNMAN diagram highlights theinteraction at the fundamental level. As mentioned in section 2.1.2, a neutron consists of one up-quarkand two down-quarks and similarly the proton consists of two up-quarks and a down-quark. Therefore,a down-quark has to change into an up-quark. As the down-quark has a charge of −1

3 and the up-quark

+23, the W−-boson has to carry away a charge of −1, so that total electric charge before and after the

interaction are the same. The neutral electron-neutrino and the electron with an electric charge of −1are the decay products of the W−. Electric charge is also conserved.

Electromagnetic interaction

The electromagnetic interaction is responsible for the repulsive and attractive forces that dominate theatomic and molecular structure such as chemical reactions and all electromagnetic phenomena. If itacts repulsively or attractively depends on the charge of the participating particles. But in both casesa photon is the boson that is exchanged between the two particles, as shown in Fig. 2.6. The classicaldescription of the electromagnetic interaction is as a field of force, where COULOMB’s Law describesthe force between two pointlike charged particles. The electromagnetic force is proportional to theproduct of the charges (Q1 and Q2) and inversely proportional to the square of the distance betweenthem. Multiplied with a constant including the permittivity of the medium it results in COULOMB’s law:

F =1

4πε0·Q1 ·Q2

r2 (2.14)

Strong interaction

The strong interaction (also strong force, strong nuclear force) is stronger than the electromagneticinteraction - hence, its name. It only appears between quarks and is responsible for the great stabilityof the nucleus inside atoms. The strong interaction has to counteract the repulsive force betweenprotons. Taking place inside the nucleus, it works at very short range. The strong force is due to theemission and absorption of gluons. They can carry energy and momentum but also color charge. So, ifa quark emits a gluon it can change its color charge. Electrons, in contrast, do not change their electric

12


Abbildung 2.6: Exchange of a virtual photon for repulsive force.

charge when they emit a photon. Moreover a gluon itself is also able to emit a gluon. The gluon’s mainproperty is to glue quarks together inside the hadrons.

The interaction takes part between a blue and a green color charged particles. In the Feynmandiagram these two particles can be found in the lower corners. The starting point is the green colorcharged particle. By emitting a green-antiblue-gluon its color charge changes to blue (a gluon is ableto carry two charges). Subsequently, the green-antiblue-gluon moves over from the green color chargeto the blue particle whose color charge changes to green. Hence there are two vertices where colorcharge is conserved. Following the interaction step by step, it becomes clear that color charge isconserved at both vertices: The green color charge of the first particle in the interaction chain is carriedaway by the gluon. Despite the fact that it had no blue property before, it converts into a blue colorcharged particle. Therefore the gluon also removes an antiblue charge (blue and antiblue sum tozero). Absorbing the gluon, the blue charge of the blue particle gets neutralized with the antiblue colorcharge of the gluon and eventually adopts the green color charge.

2.1.4 Conservation Laws

The fundamental interactions are such that certain quantities are conserved. As a consequence, con-servation laws can be formulated that can help us analysis the large number of particles and possibleinteractions between them. Beside the already mentioned conservation laws of energy, electric chargeand momentum other laws have been revealed in years of intensive research.

The achievement of these laws is to allow us to make predictions about what interactions can appearin nature.

Baryon Number

The reaction

p+n 6→ p+ p+ p (2.15)

does never occur in nature, in spite of all known conservation laws being fulfilled. Hence, physicistsinvented a new hypothetical quantity called baryon number. From nuclear physics the number of nucle-ons is known to remain constant. Principally, the baryon number represents a generalisation of this law.

Whereas all baryons are assigned the number B = 1, all antibaryons in turn receive B = −1. If weanalyse the decay above, evidently, the baryon number is not conserved.

B = 1+1 = 2 (2.16)

13


B′ = 1+1+(−1) = 1 (2.17)

B 6= B′ (2.18)

In contrast, the following decay satisfies the conservation law:

p+ p → p+ p+ p+ p (2.19)

B = 1+1 = 2 (2.20)

B′ = 1+1+(−1)+1 = 2 (2.21)

B = B′ (2.22)

So, to explain interactions, the conservation of baryon number has proven to be a powerful law ofphysics.

Lepton Number

Particulary useful in weak interactions, the lepton number describes processes such as the β-decay.Usually an electron or a positron and a neutrino or an antineutrino is emitted. Comparable decays alsooccur with muons. However, decays as follows are never observed:

n 6→ p+e− + νµ (2.23)

n 6→ p+e− + νe+νe (2.24)

The lepton number conservation provides the explanation. The electron and its corresponding neutrinoare assigned the number Le = 1, the positron and the antielectron-neutrino Le =−1. All other particlesparticipating in the decay receive Le = 0.

In order to fully describe decays with muons or taus, additionally, the muon number Lµ and the taunumber Lτ need to be defined.

Lepton number, analogously to the baryon number, permits predictions for decays. Similarly to B,opposite values are assigned to Le, Lµ and Lτ for antimatter.

2.2 Dark matter

2.2.1 The missing mass problem

The first evidence for the existence of dark matter was discovered by the Swiss astrophysicist FRITZ

ZWICKY in 1933, during his observation of the cluster of galaxies known as the Coma cluster. At firsthe determined the entire mass of the cluster by observing and measuring the velocity of the galaxiesfurthest from the centre. For comparison he also determined the cluster’s mass from its total luminosity,a scientific measure of brightness, and made an astonishing discovery: his second calculation offered avalue 400 times smaller than the first. Hence there had to be a 400 times larger amount of matter, whichwas not visible at all, otherwise the cluster would break apart, because there would not be enoughgravitational pull to keep the galaxies together in a cluster. This missing mass problem remains, to thisday, a major unsolved mystery in astrophysics. It has been confirmed by many different astrophysicalobservations, which we will now discuss.

14

2.2 Dark matter

2.2.2 Evidence no. 1 - Galactic Rotation Curves

Velocity curve predicted by N EWTONian Laws Due to NEWTON’s law of universal gravitation it wasalready possible in the 17th century to predict the velocity of every object in the universe orbitinganother one, such as planets orbiting stars or galaxies in a galaxy cluster. NEWTON stated in hisfundamental formula of classical physics that the gravitational force F , with which two objects of massm1 and m2 attract each other, is proportional to the product of their masses and inversely proportionalto the square of their distance of separation r . This is encapsulated in the following equation:

F = G ·m1 ·m2

r2 (2.25)

where G is NEWTON’s constant. Hence one can calculate the velocity of a planet with mass mp

that is orbiting a star with mass ms on a circular path of radius r . By equating the centripetal andgravitational forces,

F =m1 ·v2

r= G ·

m1 ·m2

r2 (2.26)

the planet’s velocity turns out to be inversely proportional to radius r . The relation between theplanet’s velocity and the radius of the orbit is given by:

v =

√

G ·m2

r(2.27)

According to this, planets further from a star should have a lower speed. Indeed, this is what weobserve in our own solar system.

Similarly, stars on the outer edge of a galaxy should behave in the same way, assuming that the onlysource of gravitational attraction is the visible matter, made up of other stars within the galaxy. This istrue because most of a galaxy’s visible mass is situated close to its centre.

The relationship between the speed of stars and their distance from the centre of the galaxy, aspredicted by NEWTON’s laws, is plotted as a dashed line in Fig. 2.7. This plot is referred to as a galacticrotation curve.

Observational velocity curve Given this predicted curve, surprise was understandable when in the1960s and 70s Vera Rubin managed to detect the velocity curve of spiral galaxies.

The experimentally measured rotation curve, compared with the theoretical prediction from NEWTON

can be seen in Fig. 2.7. From this observation it was discovered that the speed of stars does not de-crease with radius but instead remains constant with increasing distance to the galactic centre. Stars,which are further away, orbit as fast as those close to the centre of the galaxy. This is entirely againstNEWTON’s predictions. Two different theories have been proposed to explain this phenomenon: Eitherthe NEWTONIAN Law does not apply at the scale of a galaxy or there has to exist something else besidethe visible matter. These two possibilities are discussed in Sections 2.2.4 and 2.2.5, respectively.

However, the measurement of galactic rotation curves is interesting in itself and is discussed in thenext section.

15


Abbildung 2.7: Rotation curve.

2.2.3 Excursion: Physical means for the measurement of velo city curves

Energy levels of Atoms

In 1913 NIELS BOHR made an important step towards establishing the atom model as we know it today.BOHR proposed that electrons orbit the nucleus on certain circular paths in the atom, just like the earthdoes in the solar system with the sun at the centre. In this case the centripetal force experienced by theelectron comes from the attraction between their negative charges and the positively charged protons.

Having detailed these ideas, he finally worked out the atom model, which states that electrons canonly orbit on defined circular paths and have correspondingly discrete energy values. These are calledenergy levels. The higher a level is, the higher the energy of the electron has to be.

If an electron absorbs energy it can change to a higher a level. The electron can move back to thelower level but it has to emit the energy in form of an electromagnetic wave. In particular, the frequencyf of the emitted electromagnetic wave multiplied with PLANCK’s constant h (6,626·10−34Js) equalsthe energy difference of the two levels.

E2−E1 = h f2,1 (2.28)

Later analyses showed that each element has characteristic energy levels. The energy of theselevels is measured in the unit called electronvolts, which is the amount of kinetic energy gained by anelectron in an potential difference of 1V .

The diagram in Fig. 2.8 shows the energy levels of hydrogen. Normally hydrogren’s only electronis situated in the ground state. Following a collision, for example, the atom absorbs energy and theelectron skips to a higher energy level. Because this state is relatively unstable, the electron quicklyreturns to the ground state after emitting a photon. The energy levels are important because, combinedwith the Doppler Effect, they allow the speed of a star to be measured. This is an important ingredientin galactic rotation curves.

16

2.2 Dark matter

Abbildung 2.8: Energy levels of hydrogen.

Doppler Effect

The Doppler Effect is an everyday phenomenon which probably everyone has experienced at someti-me. It occurs whenever an observer and some wave source are in relative motion. The common resultis an observed frequency, which differs from the actual one produced by the source.

This can be illustrated by a police car, shown in Fig. 2.9, passing a pedestrian at high speed: Thepedestrian first hears a very high frequency siren approaching from far away. As it comes closer, thefrequency of the siren decreases, reaching its actual frequency exactly at the moment when it is next tothe pedestrian and continues to decrease afterwards. The cause of this effect is that the waves emittedin front of the moving source bunch up, producing a higher frequency, whereas the waves behind thesource are rarefied which reduces the frequency and makes the sound appear to be deeper.

Abbildung 2.9: Doppler Effect.

As can be seen from the dia-gram, the higher the speed ofthe police car the higher the fre-quency observed by the pedestri-an when the police car approa-ches. The same process can al-so be observed with light-emittingsources in motion. This is an es-sential tool of astronomers andcosmologists:

The comparison of definedspectral lines of elements and the spectral lines of stars showed that there exist certain stars who-se wavelengths are longer than the experimentally defined spectral lines and also stars with shorterwavelenghts. This discrepancy becomes noticeable in form of a red shift (increased wavelength) or ablue shift (decreased wavelength). In short: by comparing the defined spectral lines of elements with

17


the spectral lines of stars, one can measure the Doppler shift and determine the star’s velocity. Usingthis technique one can measure the velocity of stars within a galaxy and hence determine the galacticrotation curve.

2.2.4 Modified Newtonian Dyanmics

One of the solutions to the galaxy rotation problem is the theory of Modified Newtonian Dynamics. Thetheory, proposed by MILGROM in 1983, states that the NEWTONian Laws are only defined for largegravitational acceleration, but not for as low ones as in the observed galaxies. Instead the followingformula was suggested for these cases:

F = G ·m1 ·m2

r(2.29)

Using this formula the velocity becomes independent of the radius, which can be seen by substitutingthe modified gravitational force in the calculation in Section 2.2.2:

F =m1 ·v2

r= G ·

m1 ·m2

r(2.30)

v =√

G ·m2 (2.31)

As can be see, the velocity is independent of radius in this case.

2.2.5 Dark Matter

The alternative solution is the existence of a, so called, halo of dark matter, in which the visible galaxyresides. Due to the additional mass of the dark matter the gravitational force exerted upon stars incre-ases. As a result, stars at the edge of the galaxy move faster than expected according to the visiblemass alone. Today, the theory of dark matter is more popular amongst physicists than the ModifiedNEWTONIAN Dynamics. The main reason for this is the existence of other phenomena that indicate thepresence of dark matter within most galaxies, as discussed in the next section.

2.2.6 Evidence no. 2 - Gravitational lensing

Gravitational lensing As many revolutionary things in modern physics the discovery of another evi-dence for the existence of dark matter began with EINSTEIN’s thoughts about a physics problem. At thebeginning of the 20th century, the famous physicist received a letter from a Czech engineer. This lettercontained a question relating to EINSTEIN’s previous calculations of the bending force that stars canexert on light, because massive objects are able to warp adjacent space-time. This means an observercould see a beam of light coming from behind such an object, because the path of the light is bent.

The engineer’s request to EINSTEIN was to develop a method, with which the lensing effect couldbe estimated. And indeed EINSTEIN started calculations for the effect. Though these calculations we-re published in a science magazine in 1936, he thought this work would not be valuable at all. FRITZ

18

2.2 Dark matter

Abbildung 2.10: The curved lines are lights fromdistant galaxies behind the cluster of gala-xies.

Abbildung 2.11: Gravitational lensing.

ZWICKY was the one who realized the actual value of EINSTEIN’s work one year later, discovering mas-sive galaxies as giant gravitational lenses. Not much later galaxies were already used as gravitationaltelescopes, making the objects behind appear brighter and larger, like artificial telescopes do.

Gravitational lensing as an evidence for dark matter Figures 2.10 and 2.11 illustrate gravitationallensing. The curved lines in Fig. 2.10 are light from distant galaxies behind the cluster of galaxies. Thelight can be seen because it has been curved towards us as predicted by EINSTEIN’s formula. Usingthis formula, the necessary mass to produce this bending effect can be calculated. These calculationsled to an astonishing conclusion: According to the gravitational lensing effect, there had to be moremass, than was visible. This was another evidence in favour of dark matter over Modified NEWTONIAN

Dynamics.

2.2.7 Evidence no. 3 - Galaxy collisions

Abbildung 2.12: MACS J0025.4-1222.

Today, the bullet cluster 1E 0657-56 is supposed to be themajor evidence for the presence of dark matter. It develo-ped in the course of a collision between galaxies. It canbe shown that its form cannot be described with alternati-ve theories to the dark matter theory such as the ModifiedNEWTONIAN Dynamics. Calculating the centre of the to-tal mass from gravitational lensing and comparing it to thecentre of the visible matter, it turns out that there is a largedistance between these two points. This can only be ex-plained by the presence of dark matter. According to thistheory there were three processes happening during thecollision: The interaction between the stars, the hot gas

19


and the dark matter of the two colliding galaxies.

Stars, for example, were hardly affected by the collision, they only deccelerated a little because ofgravitational pull of the other stars. The hot gas clouds, on the contrary, lost most of their speed dueto electromagnetic interactions. Dark matter finally, can be assumed through the gravitational lensing.Light coming from objects behind the cluster are bent. And the way they are bent is a definite evidencefor the dark matter theory and against all alternative ones: The lensing can be observed close to thetwo galaxies in two areas, which are unconnected. If there was no dark matter, the gravitational lensingwould orientate at the visible matter, causing a different lensing.

On the image of MACS J0025.4-1222, another bullet cluster, by the Hubble Telescope the interga-lactic gas is coloured red and the blue coloured area contains the dark matter and within it the mass,which the gravitational lensing is due to.

It is clearly visible, how the dark matter could cross the collision without any disruption, in contrastto the deccelerated intergalactic gas, which stayed in between.

2.2.8 Dark Matter today

Today the dark matter theory is widely regarded as proven. Scientists consider dark matter as a formof matter that has mass but does not interact electromagnetically. This means it does not emit anylight. These characteristics make it invisible and just indirectly detectable. The most recent researchconsiders the universe to consist to 73 per cent of dark energy and to 23 per cent of dark matter. Theremaining 5 per cent is ‘normal’ matter, this means matter as well as antimatter.

Scientists see an evidence for Dark Energy amongst others in the accelerated expansion of universe.Dark Energy is a topic for itself and is not discussed here.

2.2.9 Significance in particle physics

Some of the readers might be surprised to find a whole chapter about astronomy in a particle physicsproject. Astronomy is studying the largest scales in nature, whereas particle physics is studying thesmallest. These are not only two different but completely opposite aspects of physics. However, theirresearch has many things in common and sometimes concern the same or very similar problems.Only the methods differ. The hunt for dark matter is a great example for the close interconnections inphysics. After astronomy has worked on solving the problem of the missing mass for almost a century,now particle physicists seized their chance and joined the search with the construction of the LHC. Itis one of the main aims of the LHC experiments to create and observe dark matter. This search hasbeen entirely motivated by the observations in astronomy. In particle physics it is also not possible todetect dark matter directly. However, using momentum and energy conservation and analysing particlecollisions carefully the amount of dark that emerges can be measured. Maybe these experiments willlead one day to the solution of the great question, of what 95 per cent of our universe consists of . . .

20

2.3 Practical Particle Physics - Motion of Particles in Fields

2.3 Practical Particle Physics - Motion of Particles in Fiel ds

2.3.1 Acceleration of charged Particles in Fields

In an electric field a force F that is given by

F = Q ·E (2.32)

with Q being charge and E the electric field is exerted on charged particles. This force leads to anacceleration. The voltage is defined by

V =WQ

. (2.33)

Rearranging this equation the energy that is gained is given by

W = Q ·V. (2.34)

The energy that an electrons gains in traversing a voltage of 1V is 1eV (electron volt).

1eV = 1,6 ·10−19J. (2.35)

In the LHC protons may attain an energy up to 7TeV. 1TeV is 1012eV. Given that the rest mass of aproton is mp = 1,67·10−27kg the rest mass energy of a proton can be calculated.

E0 = m0 ·c2 (2.36)

E0 = 1,67·10−27kg·3 ·108(m

s

)2(2.37)

E0 = 1,503·10−10J (2.38)

E0 = 0,94GeV (2.39)

In special relativity the energy E of a particle travelling at speed v in given by

E = m· γ ·c2 (2.40)

with γ given by

γ =1

√

1−(

vc

)2(2.41)

where c is the speed of light. Therefore the fraction of the speed of light a proton with an energy of7TeV will travel at may be calculated.

γ =E

mc2 (2.42)

γ =1,12·10−6J

1,67·10−27kg·3 ·(

108 ms

)2 (2.43)

21


γ = 7451,76 (2.44)vc

=√

1− γ−2 (2.45)

vc

=√

1−1064,5−2 (2.46)

vc

= 0,999999 (2.47)

At the LHC protons are accelerated at almost light speed. For the following calculation light speed willbe used as an an approximation of the proton’s speed. The sector in which particles are acceleratedis quite small in size compared to the whole accelerator. Charged particles just receive the energygradually by passing it many times.

2.3.2 The L ORENTZ force

At the LHC protons are accelerated in a circle. Hence a centripetal force FZ must be exerted uponeach proton, where

FZ =mv2

R(2.48)

FZ =mγv2

R(whereγ is required by special relativity) (2.49)

FZ =mγc2

R(where v= c for a relativistic particle) (2.50)

FZ =ER

. (2.51)

and E is the proton’s energy.The LORENTZ force FL acting upon a particle with charge Q and velocity v⊥, which is perpendicular

to a magnetic field B, is given by

FL = Q ·v⊥ ·B. (2.52)

The left (right) hand rule can be applied to find out the direction of the force on a negatively (posi-tively) charged particle. The thumb, the index finger and the middle finger are at right angles, wherebythe thumb represents the movement of the charged particle, the index finger stands for the directionof the magnetic field. As a result the middle finger gives the direction, in which the charged particle isdeflected.

Dipole magnets, which produce this LORENTZ force, FL, are used at the LHC to produce the centri-petal force, FZ, on the protons. So that FZ = FL.

With the radius of the LHC being R= 4297m, the centripetal acceleration a is given by

a =v2

r(2.53)

a =

(

3 ·108ms

)2

4297m(2.54)

a = 2,1 ·1013 ms2 (2.55)

22

2.3 Practical Particle Physics - Motion of Particles in Fields

Abbildung 2.13: Deflection of charged particles in a magnetic field.

According to Newtonian mechanics, the force FZ is given by:

FZ = m·a (2.56)

Modified by the rules of relativity, this force is instead:

FZ = m· γ ·a (2.57)

FZ = 1,67·10−27kg·7451,76·2,1 ·1013 ms2 (2.58)

FZ = 2,61·10−10N (2.59)

As already mentioned, the force to bend the path of the proton is the LORENTZ force exterted on aparticle with charge Q and speed v in a magnetic field of strength B. Only the component of the speedperpendicular to the magnetic field is affected. With the equation for the LORENTZ force the strengthof the mangetic field can be calculated using FL = FZ:

B =FZ

Q ·v⊥(2.60)

B =2,61·10−10N

1,6 ·10−19C ·3 ·108ms

(2.61)

B = 5,4T (2.62)

23


The magnetic field density B created by current I at a distance r is given by the AMPÈRE’s Law:

B =µ0 · I2π · r

(2.63)

with µ0 being the permeability of free space:

µ0 = 4π ·10−7N A−2 (2.64)

Rearranging this formula, the electric current that is able to produce this magnetic field can becalculated.

B =µ0I2πr

(2.65)

I =B ·2πr

µ0(2.66)

I =8,3T ·2π ·0,044m

4π ·10−7NA−2 (2.67)

I = 1826000A (2.68)

2.3.3 Generation of the Magnetic Field by Superconductivit y

Using the result of the previous section, the current required to bend protons in the LHC is 1826000 A.In a normal wire with a resistance of 1 Ω, the emitted energy per second given by

P = R· I2 (2.69)

would be 5 ·1012 W. In comparison, a large power station provides 1 MW. Therefore, over a millionpower stations would be required to run the LHC if it was using normal copper wires.

Therefore in the accelerator the effect of superconductivity is used, where some materials lose theirelectrical resistance at very low temperatures. So that the power lost in the wires is almost zero.

24

3 Das CERN - Der Weg eines Protons durchden Beschleunigerkomplex

Abbildung 3.1: Die CERN-Zentrale.

In seiner Funktion als internationa-les Kernforschungszentrum vereint dasCERN (Conseil Européen pour la Re-cherche Nucléaire) viele Nationalitäten.Während das CERN seine finanziellenMittel aus zwanzig Mitgliedstaaten be-zieht, so sind dennoch Wissenschaftleraus aller Welt vertreten. Unter Anwen-dung hoch entwickelter Technologie bil-det es die Wissenschaftler von morgenaus und erzielt dabei fast beiläufig nochFortschritte in Bereichen wie Informatikund Biologie.

Die folgende Darstellung ist am Wegeines Protons durch den Beschleunigerkomplex der CERN orientiert. Im Rahmen dieser Darstellungist es nicht möglich, auf alle Beschleuniger und Detektoren einzugehen. Immer wieder sollen auchunsere persönlichen Erfahrungen mit einfließen (siehe farbige Boxen).

3.1 Beschleuniger

3.1.1 Injektorkette des LHC

Die Reise jedes Protons im Beschleunigerkomplex beginnt in einer roten Wasserstoffflasche. Durchelektrische Entladung wird das Proton des Wasserstoffatoms von seinem Elektron getrennt. Die ersteStation des Protons ist der LINAC 2 (Linear Accelerator), ein Linearbeschleuniger. In einer Reihe vonAntriebsröhren erfolgt sogleich die Beschleunigung durch ein elektrisches Feld auf 4000km

s (13 der

Lichtgeschwindigkeit).

25


Die Wasserstoffflasche 18.08.2009

Heute haben wir einen Blick auf den Ursprung eines jeden Prot ons desLHC werfen können. Wir waren erstaunt, dass ein solch gigant ischerTeilchenbeschleuniger seine Protonen aus einer kleinen, h andelsüblichenWasserstoffflasche bezieht.Eine solche Wasserstoffflasche mit ihren 140g enthält genug Protonen,um den LHC für 40000 Jahre zu versorgen, weil pro Jahr nur 3,3mgWasserstoff benötigt werden. Wie wir erfuhren, verließen s ich dieMitarbeiter auf die unglaublich lange Laufzeit der Flasche . Deshalb warniemand in der Lage, eine Verbindung zwischen der Wassersto ffflascheund einem plötzlichen Ausfall des LHC herzustellen. Nach la ngerBedenkzeit kam man auf die Idee, den Druck in der Wasserstoff flasche zukontrollieren und stellte fest, dass dieser abgefallen war . Es reichte,die Flasche auszutauschen, damit der LHC wieder lief.

3.1.2 LHC (Large Hadron Collider)

In 2008 wurde nach 15 Jahren Arbeit mit dem LHC nicht nur der größte Teilchenbeschleuniger, son-dern auch die größte Maschine der Welt fertiggestellt. Der 27 Kilomenter lange Tunnel befindet sich100 Meter unter der französisch-schweizerischen Grenze in der Nähe von Genf.

Die Hauptaufgabe des LHC besteht im Beschleunigen von Teilchen, die in vier Detektoren kollidie-ren. Im Moment der Kollision werden Bedingungen geschaffen, von denen man heute ausgeht, dasssie nach dem Urknall geherrscht haben. Bevor die Teilchen die dafür erforderliche Energie erreichen,müssen sie rund 20 Minuten zirkulieren.

Protonen erreichen im LHC eine Energie von 7TeV. Wie im Abschnitt 2.3 dargelegt, werden gela-dene Teilchen durch die elektrische Kraft beschleunigt und durch die LORENTZkraft auf der Kreisbahngehalten. Für Letzteres stehen 1232 Dipolmagnete von je 15 Meter Länge und 35 Tonnen Gewicht zurVerfügung. Diese generieren das Magnetfeld mithilfe von supraleitenden Drähten. Zur Kühlung wirdflüssiges Helium verwendet.

26

3.1 Beschleuniger

Das Control-Center 19.08.09

Gleich beim Eintreten fällt uns die angenehme Arbeitsatmos phäre imGehirn des Beschleunigungssystems auf. Die Zentrale ist sichtbarin vier Sektionen gegliedert, unter anderem ist ein Bereich dem LHCzugeordnet. Unzählige Computer und Bildschirme kontrolli eren hier rundum die Uhr die Beschleunigungsvorgänge.Als unser Blick auf die Champagnerflaschen auf den einandergegenüberliegenden Schränken fällt, erklärt man uns, dass es unter denMitarbeitern üblich ist, wichtige, gelungene Experimente mit Champagnerzu feiern. Jede Champagnerflasche steht also symbolisch fü r einenMeilenstein in der Geschichte des CERN.

3.1.3 AD (Antiproton Deccelerator)

Beim Antiproton-Deccelerator handelt es sich um ein System, welches in seiner Funktion einmalig amCERN ist. Im Gegensatz zu den Beschleunigern, gelingt es mit ihrer Hilfe energiearme Antimaterieund insbesondere Antiprotonen zu produzieren. Neuartig an dieser Einrichtung ist, dass es nicht län-ger eine Kette von hintereinander geschalteten Anlagen bedarf. Im Antiproton Deccelerator werden dievorher separaten Stationen gekoppelt und können in einem Zug ausgeführt werden - von der Produk-tion der Antimaterie bis zu ihrer Bereitstellung für die Experimente. Aus dem PS (Proton Synchrotron)kommend verfügt die Antimaterie eine Geschwindigkeit die annähernd derLichtgeschwindigkeit entspricht, sodass ein eingehendes Studium derselben unmöglich ist. Von denfokussierenden Magneten des AD auf einer exakten Bahn gehalten, wird der hochenergetische Strahlmit starken Magneten verlangsamt und in einen niedrigenergetischen transformiert. Die Technik coo-lingreduziert dazu die seitlichen Schwingungen. Auf diese Weise können derzeitig einer Vielzahl vonExperimenten die nützlichen gebremsten Antiatome zur Verfügung gestellt werden.

27


Antimaterie 18.08.2009

Ein Aspekt der Antimaterieforschung ist die äußerst intere ssante Frage,ob Antimaterie nach oben fällt. Schließlich fällt normale M aterie nachunten. Bemerkenswerterweise ist man diesem Problem bislan g noch nichternsthaft auf den Grund gegangen. Mit reichlich Humor speku lierendie Wissenschaftler bereits über den Ausgang des Experimen ts, wiedie Karikatur eines französischsprachigen Mitarbeiters NE W T O Netl’antipomme beweist. Sie zeigt NE W T O N, der zu Lebzeiten der Legendenach seine Studien zur Gravitation an Äpfeln durchgeführt h aben soll,wie er einen nach oben „fallenden“ Apfel beobachtet.

3.1.4 CNGS (CERN neutrinos to Gran Sasso)

Neutrinos sind leichte, neutrale Teilchen, die nur sehr wenig mit Materie wechselwirken. Diese Ei-genschaft bildet die Basis für das CNGS-Projekt, von welchem sich die Physiker einen Beitrag zurEnträtselung der zahlreichen Neutrino-Mysterien erhoffen. Zu diesem Zweck wird einProtonenstrahl vom SPS (Super Proton Synchrotron) mithilfe eines magnetischen Linsensystems inRichtung desmittelitalienischen Gran Sasso National Laboratory geschickt. Auf dem Weg dorthin finden mehrereZerfallsprozesse statt, aus denen unter anderem die rätselhaften Neutrinos resultieren. Etliche Zer-fallsprodukte werden von dem Gebirge gestoppt, bis zuletzt nur noch die Neutrinos übrig bleiben undim 732kmweit entfernten italienischen Labor zur Auswertung aufgefangen werden können.

3.2 Detektoren

3.2.1 CMS (Compact Muon Solenoid)

Der CMS-Detektor befasst sich vordergründlich mit der Suche nach bislang unerkannten Partikeln(wie beispielsweise nach dem Higgs-Boson) sowie mit offenen Fragen des Standardmodels, wie derMassendiskrepanz der einzelnen Teilchen. Der Detektor erlaubt Messungen mit noch nie da gewe-sener Präzision, wodurch die aussichtsreiche Untersuchung von Phänomenen wie Extradimensionenund Dunkler Materie ermöglicht wird. Wie schon die große Überschneidung in den untersuchten Fra-gestellungen andeutet, liegen die Ziele des CMS dicht mit denen des ATLAS-Detektors zusammen.Grund hierfür ist, dass CMS und ATLAS ein Detektorpärchen sind. Aufgabe solcher Pärchen ist es,die Messdaten des Partners mithilfe andererUntersuchungsmethoden zu überprüfen und bestenfalls zu bestätigen. Dass das CMS seinem gigan-tischen Bruder in Komplexität in kaum etwas nachsteht, bewies schon seine spektakuläre Installation.An der Oberfläche wurden die 15 Teile des gigantischen Detektors zusammengesetzt und in Schräg-lage unter die Erde hinabgelassen. Einzigartig am CMS ist das so genannte Tracking System - ein

28

3.2 Detektoren

System aus mit Silicon-Pixeln übersäten Detektorschichten - welches nicht nur der Rekonstruktionvon der Spur des durchquerenden Teilchens dient, sondern anhand der Krümmung des Bogens auchden Impuls wiederzugeben vermag.

3.2.2 ATLAS

Abbildung 3.2: Der ATLAS-Detektor.

Der ATLAS konzentriert sich auf die Suche nach dem Higgs-Boson, Extradimensionen und Dunk-ler Materie. Er besteht aus einem kreisförmigen Magnetsystem. Ein elektromagnetisches Kalorimetermisst die Energie von Elektronen, Positronen und Photonen. Zudem verfügt der Detektor über einenHadronendetektor. Passieren Hadronen den Detektor, kommt es zu einem Teilchenschauer, der in denSzintillatoren einen Lichtimpuls auslöst, dessen Intensität gemessen werden kann. Dieser wiederumlässt Rückschlüsse auf die Energie des Teilchens zu.

29


ATLAS 17.08.2009

Gleich am ersten Tag bietet sich uns die Gelegenheit einen wa hren Gigantenkennen zu lernen. Durch Zufall und spontan wird uns eine Führ ung durchden ATLAS-Detektor gewährt. Die Sicherheitsvorkehrungen sind enorm. EinTechniker aus dem Kontrollraum schaltet die Sicherheitssc hleuse für unsfrei. Mitarbeiter müssen sich hier durch einen Augenscan id entifizieren.Hat der Computer die Person erkannt, wird das Gewicht überpr üft, umzu verhindern, dass eine weitere Person unbemerkt hineinge langt.Unglücklicherweise berücksichtigt das System nicht die et was beleibterenMitarbeiter. In einem Einzelfall kam es sogar schon vor, das s der Zutrittmit der Begründung, es würden sich zwei Personen in der Schle use befinden,verweigert wurde.Aus Sicherheitsgründen trägt hier jeder Mitarbeiter ein Do simeter, um dieStrahlungsexposition abzuschätzen.Beim ATLAS selbst handelt es sich auf den ersten Blick um eine großeMaschine mit erstaunlicher Farbenvielfalt. Aufgrund sein er Bauweisekonzentriert sich der ATLAS vornehmlich auf den Nachweis sc hwererHiggs-Bosonen, die bei ihrem Zerfall theoretisch vier Myon en emittieren.Zurück im Besucherzentrum konnten wir diese indirekte Art d es Nachweises ineiner Computersimulation nachvollziehen. Entsprechend d er Realität wurdenwir auch hier nicht fündig.

3.3 Datenanalyse

Angesichts der enormen Menge an auszuwertenden Daten, erscheint es sinnvoll, dass sich das CERNum eine Speichermöglichkeit mit breit angelegten Kapazitäten bemüht - da auch zunehmend der Be-darf bei Wissenschaftlern auf der ganzen Welt entsteht, auf diese riesige Datenmenge von überall herzugreifen zu können. Zu diesem Zweck entwickelte das CERN das sogenannte Grid-System. DiesesSystem verteilt die Daten an insgesamt 11 große Computer, den Tier-1 Centern in der ganzen Welt,von wo aus eine Weitergabe an untergeordnete Knotenpunkte, den Tier-2-Centern, erfolgt. Das CERNbehält allerdings eine Backup-Kopie. Somit wird nicht nur ein unbegrenzte Speicher, sondern auch einstandortunabhängiger Zugriff gewährt. Das Grid-System bildet allerdings vielmehr die Endstation derDaten, sobald die Kollision stattfindet gelangen sie zunächst einmal ins Subdetekormemory - demersten Filter. Hier werden die Daten schon zu 99 Prozent aussortiert. Anschließend folgt eine Unter-suchung durch Computer Farms, welche die Rohdaten verarbeiten und die Bahn der Teilchen mithilfeder Energiedaten rekonstruieren. Gespeichert werden die gebündelten Datenpakete, nachdem sienummeriert und gereinigt wurden, im Computing Centre, wo 100 000 Rechner für die vorläufige Auf-bereitung bereit stehen.

30

3.3 Datenanalyse

Abbildung 3.3: Das Grid-System am CERN.

Abbildung 3.4: ... in der Sicherheitsschleuse.

31

4 Résumé

Abschließend können wir sagen, dass uns der Aufenthalt am CERN in jeder Hinsicht bereichert hat.Durch die intensive Auseinandersetzung mit der Teilchenphysik, noch dazu auf Englisch, wurde nichtnur unser naturwissenschaftliches Verständnis, sondern auch unsere sprachliche Kompetenz ent-scheidend erweitert und verbessert.

Während unseres Aufenthaltes wurden uns zahlreiche Möglichkeiten eröffnet, die sogar der Mehr-heit der CERN-Mitarbeiter vorenthalten werden, zum Beispiel die Besichtigung des ATLAS. Durch dieAtmosphäre, an der wir alltäglich teilhaben konnten, und den engen Kontakt mit renommierten Wissen-schaftlern wurde unsere Neugier geweckt, uns auch weiter eingehend mit dem Thema zu befassen.

Wir danken der Studienstiftung für die großzügige Bereitstellung der finanziellen Mittel, ohne die dasProjekt nicht möglich gewesen wäre.

Ebenso danken wir Katherine und Dean Horton für die Organisation im Vorwege, die kostenloseUnterkunft, die mühevolle Vorbereitung der Präsentationen und die Beantwortung unzähliger Fragen.

Schließlich danken wir Herrn Hauschild, der uns während seiner Führungen an seinemumfangreichen Fachwissen über die einzelnen Anlagen am CERN teilhaben ließ.

Auch Frau Müller-Dassau und Herrn Schönfelder gilt unser Dank für das Verfassen der Gutachten,Letzterem zusätzlich für die Korrektur des Berichts.

Uetersen, den 4. Mai 2010

32

5 Quellennachweis

1. Vortrag The Who is Who of the Particle World von KATHERINE HORTON von der Oxford Univer-sity

2. Vortrag A Hitchhiker’s Guide to Accelerators von DEAN HORTON von der Oxford University

3. Physik von DOUGLAS C. GIANCOLI erschienen 2006 im Verlag PEARSON STUDIUM

4. Elementarteilchen - Bausteine der Materie von HARALD FRITZSCH erschienen 2004 im VerlagBECK

5. Physik Oberstufe - Relativitätstheorie, Quanten-, Atom-, und Kernphysik von ERHARD WEIDL

erschienen 2007 im MENTOR VERLAG GMBH, MÜNCHEN

6. Physik Oberstufe - Elektrizität und Magnetismus, Ladung, Felder, Induktion, elektromagnetischeWellen von ERHARD WEIDL erschienen 2007 im MENTOR VERLAG GMBH, MÜNCHEN

7. DUDEN Abiturwissen Physik von PROF. DR. HABIL. LOTHAR MEYER ; DR. GERD-DIETRICH

SCHMIDT erschienen 2007 in BIBLIOGRAPHISCHES INSTITUT & F. A. BROCKHAUS AG, MANN-HEIM, und DUDEN PAETEC GMBH, BERLIN

8. Skurrile Quantenwelt von SILVIA ARROYO CAMEJO erschienen 2007 im FISCHER TASCHENBUCH

VERLAG

33

Abbildungsverzeichnis

2.1 The Standard Model. Eigene Quelle. . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 The cathode ray experiment. Quelle: Wikimedia Commons. . . . . . . . . . . . . . . . 82.3 Inner structure of a proton. Quelle: Wikimedia Commons. . . . . . . . . . . . . . . . . 92.4 Particle and antiparticle annhihilate under emission of a boson. Vice versa, particle and

antiparticle are created by a boson (pair creation). Quelle: Wikimedia Commons. . . . 102.5 FEYNMAN diagramm for β−-decay mediated by weak interaction. Quelle: Wikimedia

Commons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.6 Exchange of a virtual photon for repulsive force. Quelle: Wikimedia. . . . . . . . . . . 132.7 Rotation curve. Quelle: Wikimedia Commons. . . . . . . . . . . . . . . . . . . . . . . 162.8 Energy levels of hydrogen. Quelle: Wikimedia Commons. . . . . . . . . . . . . . . . . 172.9 Doppler Effect. Quelle: Wikimedia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.10 The curved lines are lights from distant galaxies behind the cluster of galaxies. Quelle:

Wikimedia Commons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.11 Gravitational lensing. Quelle: Wikimedia Commons. . . . . . . . . . . . . . . . . . . . 192.12 MACS J0025.4-1222. Quelle: Wikimedia Commons. . . . . . . . . . . . . . . . . . . 192.13 Deflection of charged particles in a magnetic field. Quelle: Wikimedia Commons. . . . 23

3.1 Die CERN-Zentrale. Eigene Quelle. . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2 Der ATLAS-Detektor. Eigene Quelle. . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.3 Das Grid-System am CERN. Eigene Quelle. . . . . . . . . . . . . . . . . . . . . . . . 303.4 ... in der Sicherheitsschleuse. Eigene Quelle. . . . . . . . . . . . . . . . . . . . . 30

Abbildung auf dem Cover. Quelle: Wikimedia Commons

34

Einführung in die Teilchenphysik...Teilchenphysik als verhältnismäßig junger Zweig der Physik...

Documents

Transcript of Einführung in die Teilchenphysik...Teilchenphysik als verhältnismäßig junger Zweig der Physik...