the higgs boson - astroparticle...

The Higgs boson

Jennifer Arnesson

[email protected]

SA104X Degree Project in Engineering Physics, First level

Supervisor: Jonas Strandberg

Department of Physics

School of Engineering Sciences

Royal Institute of Technology (KTH)

Stockholm, Sweden, 2012

TRITA-FYS-2012;43

Abstract In this essay the standard model of particle physics and the Higgs boson are discussed. It also contains

information about CERN, the particle accelerator LHC and the ATLAS detector, all used in the search for

the Higgs boson. Results on the Higgs boson searches from ATLAS are presented using three different

data volumes at different times during 2011. These results will also be simplified by plots showing the

excluded masses from the Higgs boson searches. The results are also used for searching for a

dependency between different results and datasets and to use that to extrapolate to what we can expect

to see this year. It is found that the dependency is linear with the size of the dataset and fits very well to

the data points. By this linear fit a prediction of the needed luminosity for the exclusion of the Higgs

mass GeV has been made.

Contents 1 Introduction ................................................................................................................................................ 1

1.1 Background .......................................................................................................................................... 1

1.2 Scope ................................................................................................................................................... 1

1.3 Objective.............................................................................................................................................. 1

2 Theory ......................................................................................................................................................... 2

2.1The standard model ............................................................................................................................. 2

2.1.1 Quarks and leptons....................................................................................................................... 2

2.1.2 The forces ..................................................................................................................................... 5

2.1.3 Gauge bosons and the weak interaction ...................................................................................... 5

2.2 The Higgs boson .................................................................................................................................. 7

2.2.1 The Higgs boson ........................................................................................................................... 7

2.2.2 Finding the Higgs boson ............................................................................................................... 7

2.2.3 Cross section ................................................................................................................................. 8

2.2.4 Branching ratio ............................................................................................................................. 9

3 CERN ......................................................................................................................................................... 11

3.1 Background ........................................................................................................................................ 11

3.1.1 LHC .............................................................................................................................................. 11

3.1.2 ATLAS .......................................................................................................................................... 11

3.1.3 How to find particles .................................................................................................................. 13

3.2 Latest results ..................................................................................................................................... 13

4 Methods ................................................................................................................................................... 15

4.1 Methods ............................................................................................................................................ 15

5 Results and Discussion.............................................................................................................................. 16

5.1 Results and Discussion ...................................................................................................................... 16

6 Summary and Conclusion ......................................................................................................................... 21

6.1 Summary and Conclusion .................................................................................................................. 21

Bibliography ................................................................................................................................................. 22

1

1 Introduction

1.1 Background In the 1960s, physicists realized that two of the fundamental forces, the electromagnetic and the weak

interaction, are the same force at high energies. This is part of the foundation of the standard model, but

to make it work mathematically all particles which mediate forces have to be massless. From

experiments it is well known that they are not. To solve this dilemma the Higgs mechanism was

introduced. The Higgs mechanism describes how all particles get their mass and the theory also solves

the mathematical problems of the standard model. It also predicts a new particle, called the Higgs

boson. The problem is that the Higgs boson never has been found, which means that the theory has not

been verified experimentally.

As the Large Hadron Collider, LHC, at the European Organization for Nuclear Research, CERN, was taken

in operation 2009 the final search for the Higgs boson began. The mass of the Higgs boson is unknown,

but is estimated to be GeV. In order to prove the existence of the Higgs bosons all these masses

has to be studied. Some of the masses were excluded using earlier accelerators, both at CERN and in the

US. These early accelerators were too small to create energies needed for the creation of Higgs bosons at

higher masses. LHC is more powerful and during last year there have been major improvements

regarding the search for the Higgs boson.

Last year, 2011, three results with increasing datasets were presented from LHC and the ATLAS detector.

Many possible Higgs masses were able to be excluded. One possible Higgs mass in particular has shown

to be interesting, the mass of GeV. It is likely that this is the mass of the Higgs boson, but further

investigations are needed. It is very possible that we during this year will find out if the Higgs boson

exists or not [1].

1.2 Scope This project will include the results from last year from the ATLAS detector at CERN, published in August,

September and December 2011 [2][3][4].

1.3 Objective The purpose of this project is to clearly present which Higgs masses has been excluded from the search

for the Higgs particle. Also a dependency between the results and the different datasets used in the

search will be looked for. Hopefully this will make an estimation of which dataset is needed to exclude

specific Higgs masses possible.

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2 Theory

2.1The standard model In the standard model of particle physics, particles are divided into three different groups; leptons and

quarks which are fermions with spin-

and gauge bosons which are spin- particles that mediate the

forces in nature. Excluding gravity, the standard model explains how these three groups of particles

interact and what attributes they have.

2.1.1 Quarks and leptons

Quarks and leptons are the elementary particles that build up matter. This means that they are the

smallest building blocks that have been discovered so far.

There exist six different types of quarks, called different flavors, which are divided into three generations

depending on how they occur in pairs, see table 1. The generations have similar properties but increasing

mass. In contrast to the quarks in the first generation, which are stable, quarks in the second and third

generation are heavier and are therefore unstable and decay.

First Second Third

– up – charmed –bottom

–down –strange –top Table 1: The three generations with belonging quarks

All generations consist of one quark with charge

and one with the charge

. All quarks have

antiquarks with the same attributes except the charge, which for antiquarks have the same absolute

value but opposite sign. In addition to the mentioned properties in table 2 quarks have one further

attribute, color, either red, green or blue. The color is the charge of the strong force, which only affects

colored particles. Quarks cannot be found alone, as the strong force behaves in a way which makes it

impossible to have free colored particles. The quarks can however be found forming composite particles

which are colorless.

Name Symbol Charge Generation Mass

Down First Up First Strange Second Charmed Second Bottom Third Top Third

Table 2: The six different quarks and their attributes, the charge in units of e and the approximate masses in GeV/c².

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Figure 1: A neutron consist of

two down quarks and one up quark [5]

Figure 2: A proton consist of

two up quarks and one down quark [6]

As shown in figure 1 and figure 2 both protons and neutrons consist of quarks from the first generation. The only differences between the two are that the proton consists of two up quarks and one down quark while the neutron consists of two down quarks and one up quark. This means that the total electric

charge for the proton is

e and for the neutron

e. Particles which are made

up by three quarks, like the neutron and the proton, are called baryons. The three quarks that form a baryon have different colors which result in that the baryon becomes colorless or white. Mesons are particles that consist of one quark and one antiquark. Antiquarks have the colors antigreen, antiblue or antired. A meson is like a baryon colorless or white, as it consist of one quark and one antiquark with opposite colors. However mesons are unstable [7]. The other group of fermions in the standard model is leptons. There are six different leptons that are known today, the best known is the electron. Like the quarks they occur in pairs of two and the pairs are again called generations, see table 3. As for quarks, the properties are similar for all generations but the masses increase with every generation. In a pair, one lepton is charged and the other one is neutral.

First Second Third

– electron neutrino – mu-neutrino –tau-neutrino

–electron –muon –tauon Table 3: The three generations at its leptons

Leptons have antiparticles called antileptons, the difference between them is the sign of the charge. As

shown in table 4 the electron, muon and tauon are charged while the other three leptons are neutral.

The neutral leptons ( , and ) have very small masses, approximately zero [7][8].

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Name Symbol Charge Generation Mass

Electron neutrino First Electron First Mu-neutrino Second

Muon Second Tau-neutrino Third Tauon Third

Table 4: The six different leptons, their symbol and charges in units of e

Figure 3: An illustration of the standard model and the three generations,

these are approximate masses [9]

Figure 3 illustrates the standard model. It includes the six quarks and the six leptons where the three first

columns in the figure represent the three generations. The fourth column includes the four gauge

bosons , , and . The bosons are mediating different forces, discussed further in section 2.1.2

below.

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2.1.2 The forces

There are three fundamental forces in nature that we know of; gravity, the electroweak and the strong

interactions. Often four fundamental forces are mentioned, where the electroweak is separated into

two; the electromagnetic and the weak interaction. However the electromagnetic force and the weak

interaction have been shown to be the same force at high energies. Gravity is not included in the

standard model and will therefore not be discussed further.

Gauge bosons are spin- particles which act as carriers of the different forces. The photons mediate the

electromagnetic force and the bosons , and are carriers of the weak interaction. As the

notation of the bosons reveal, the -boson is neutral while the -boson can be either positively or

negatively charged. The gluon is the force carrier for the strong interaction and like the photon the

gluon is massless.

The strong force binds the quarks, by color, into larger particles like protons and neutrons. The gluon

itself has color which means it can interact with itself in contrast to the photon and the electromagnetic

force as the photon not is electrically charged. The reason for the more complex properties of the strong

force is that the gluon has color [7][10].

2.1.3 Gauge bosons and the weak interaction

The weak interaction can turn quarks and leptons into other types of quarks and leptons. In general the

interactions have these restrictions;

1. When leptons interact, the total number of leptons must be conserved within each generation.

2. During quark interaction, the total number of quarks from any generation must be the same

before and after (quarks minus antiquarks).

3. The interaction must be physically possible regarding mass and energy. A lighter particle cannot

turn into a heavier, unless energy is supplied.

Only and can transform quarks or leptons, never the boson. The total charge has to be

preserved through the interaction [7].

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Figure 4: Feynman diagram of a quark interaction where a proton decays into a neutron.

The number of quarks is the same before and after the interaction.

In figure 4 can we see a proton transform to a neutron and a boson via the weak force. The

boson then decays into a positron and an electron neutrino. Important to notice is that the charge is the

same before and after the transformation. Also the number of quarks and leptons are preserved through

the transformation, one quark in the beginning and end and zero leptons at the beginning and zero at

the end because the positron and an electron neutrino cancel each other out.

Figure 5: Feynman diagram of lepton interaction where the generation is

conserved as the tauon partly decays into a tau-neutrino

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When a lepton transform it always decays to a neutrino in the same generation and a boson. The

boson can decay to lighter leptons or to quarks from the first generation, this is illustrated in figure 5.

2.2 The Higgs boson

2.2.1 The Higgs boson

Naively, for the standard model to be accurate the bosons , and have to be massless. The

photon is in fact masless but the and bosons are heavy particles with the masses:

⁄ and ⁄

To give the bosons their masses and not to spoil predictability of the theory the Higgs mechanism is

introduced. The Higgs mechanism itself introduces a field called the Higgs field. This field has two

complex components and thus four degrees of freedom. By mixing with the Higgs field, the bosons

and get their masses. They each absorb one of the degrees of freedom from the Higgs field. This

leaves one degree of freedom for the field which manifests itself as the spin- Higgs boson. Finding the

Higgs boson by experiments would prove that the Higgs mechanism is true and the correct theory of how

the bosons get their mass.

The fermions are also required to be massless in the naive version of the standard model, but they can

also be shown to acquire mass through the interaction with the Higgs field via the Higgs boson. The

stronger interaction between the particles and the Higgs boson, the more mass the particles get. The

mass is proportional to the interaction strength.

When including the Higgs boson in calculations, the solutions to equations gives reasonable results and

does not result in unphysicalities such as probabilities larger than one. This is a good indicator that the

Higgs boson in fact does exist, though today we do not have any experimental proof of its existence

[7][10].

2.2.2 Finding the Higgs boson

To experimentally prove the existence of the Higgs boson, we first have to know what to look for. The

attributes and the behavior of the Higgs boson are well known from the standard model; only the Higgs

bosons mass is unknown and is a free parameter of the theory. This means every possible mass has to be

tested against the data and excluded if no evidence exists.

First the Higgs boson has to be created, this is discussed further in section 2.2.3.The Higgs boson is an

unstable particle and decays rapidly, making it impossible to see or measure directly. Instead the decay

products from it have to be detected, for further discussion of the Higgs boson decay see section 2.2.4.

8

The number of Higgs bosons that are created, , is given by the equation

is the probability to create the particle.

is the luminosity, which is the size of the dataset i.e. a measurement of the number of collisions

that take place.

Where m² .

The number of Higgs boson that are detectable, , is given by the equation:

is the efficiency of the detector and the analysis selectors for identifying a Higgs event.

is the branching ratio of the particular decay of the Higgs boson that is studied.

2.2.3 Cross section

By colliding two protons, a number of reactions can take place that result in the creation of Higgs bosons

(if the theory of the Higgs boson is correct). In figure 6 the possible production mechanisms are shown.

Here stands for gluon, for quark (the , , or quark), for the bottom quark, for the top quark,

for the boson, for the boson and is the Higgs particle. At high energies, the proton includes

all of these particles, most of them as virtual particles. The probability of which reaction will take place

change depending on the mass of the Higgs boson, the x-axis in figure 6. is actually a cross section for

the collision between protons, but here it can be interpreted as the probability for a certain reaction to

take place.

For example, if the Higgs boson is light the reaction where two quarks transforms into one boson and

one Higgs boson is more likely to occur than it would be if the Higgs boson was heavy. However the

reaction where two gluons becomes one Higgs boson is the most probable for all Higgs masses [11].

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Figure 6: The different reactions for creating a Higgs boson that can take place by colliding two protons, the probability differs

depending on the mass of the Higgs boson [12]

2.2.4 Branching ratio

A Higgs boson is not a stable particle, it decays rapidly. Therefore the actual Higgs boson cannot be

detected, instead its decay products are searched for. In figure 7 the possible decays of a Higgs boson are

illustrated. The branching ratio, the y-axis, is the probability that a certain decay will occur. What decay is

most probable differs depending on the mass of the Higgs boson. For a light Higgs boson the most likely

decay is a bottom quark and an anti-bottom quark, while a massive Higgs boson is most likely to decay

into two bosons [13].

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Figure 7: The probabilities of different decay modes of the Higgs boson at different masses [14]

In the search for the Higgs boson different hypothetical masses are tested in order to find it. As

mentioned, the behavior of the Higgs boson such as how it decays for certain masses is well known. For

every mass tested the mass can either be exclude or tested further. For example:

When trying to find the Higgs boson different hypothetical masses are tested. If for example the

hypothetical mass of GeV is tested, its most probable decay is illustrated in figure 7. In this case the

most probable decay products for the Higgs boson will be two bosons. When colliding proton beams

at LHC Higgs bosons are created. It is then possible to study the number of bosons pairs that are

found. Thereafter, that number of events is compared to

1. The number of bosons pairs expected if the Higgs boson does not exists at this mass,

this is what is called the number of background events.

2. The number of bosons pairs that would be created if the Higgs boson does exist at this mass,

events from both background and signal.

If the experimental data is very similar to point 1 above, then it is likely that the Higgs boson does not

have the investigated mass. To be able to exclude the mass of GeV it has to have a confidence level

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(C.L.) of 95%, which means that it is 5% chance or less that the Higgs boson exist at that mass. If the

confidence level is smaller than 95% the mass cannot be excluded. By excluding masses with confidence

level of 95% the search after the Higgs boson can be narrowed to a smaller range of possible masses.

If the decays instead are similar or close to point 2 in the example above, the mass has to be investigated

further [13].

3 CERN

3.1 Background CERN is the largest physics laboratory in the world, that focuses on particle physics. It was founded in

1954 and is located in Switzerland and spans over the border to France. At CERN, particle accelerators

and detectors are used to study particles and attempts to create those wanted to study more closely,

like the Higgs boson [15].

3.1.1 LHC

LHC is a circular particle accelerator with a circumference of meter and located meters

underground. Two beams containing either protons or lead ions are traveling through the accelerator in

opposite directions. At full power the particles can travel at speed of light. The maximum

energy for a traveling proton is TeV which gives an energy of TeV if two protons collide. For all

current data the energies of each beam is TeV and this gives a total energy of TeV when to particles

collide.

LHC is the largest accelerator in the world and its main components are the magnets. Some of the

magnets are used to bend the beams so they travel in a circular orbit through the accelerator. Others are

used to squeeze the beams together and make them collide at four specific locations. These locations

are where the four different detectors ATLAS, CMS, ALICE and LHCb are placed [16][17][18].

3.1.2 ATLAS

One of the detectors at CERN is ATLAS. It is a general-purpose detector which means it will be used to

investigate a broad spectrum of particle physics, including attempts to find the Higgs boson. It is

meters long, meters high, meters wide and weighs tonnes. It is the largest detector ever

built.

The ATLAS detector mainly consists of four components; the inner detector, the calorimeter, the muon

spectrum and the magnet system. The energies carried by both charged and neutral particles are

measured by the calorimeter, the yellow part in figure 8. Muons cannot be stopped and are the only

detectable particles that pass through all calorimeter absorbers, nevertheless they can be detected by

the muon spectrum that surrounds the calorimeter. The magnets in the magnet system are used to bend

the charged particles so measurements of the momentum can be done.

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Figure 8: The ATLAS detector [19]

The inner detector is the grey part in the middle of figure 8 and is shown in greater detail in figure 9. It

has three sections, the pixel detector, the semiconductor tracker (SCT) and the transition radiation

tracker (TRT). The pixel detector takes measurement as close to the interactions points as possible. It

makes it possible to detect short lived particles, like B-Hadrons. Momentum, impact parameter and

vertex position are further measured by the SCT system. In the barrel SCT, coordinates are given from

eight layers of silicon microstrip detectors. TRT can detect whether a particles passing through it by

ionization of the gas inside the straw tubes [20][21].

Figure 9: The inner detector in ATLAS [22]

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3.1.3 How to find particles

To be able to track particles and to determine what type of particles they are, particles are forced to pass

through four layers of materials which slows them down and finally stops them (except muons). In the

first layer, called the tracking chambers, all charged particles leave a trace. In the electromagnetic

calorimeter, the second layer, the photons and the electrons/positrons are stopped. It is possible to

separate these particles from each other as the electron leaves a trace in the first layer in contrast to the

photon. In the third layer, the hadron colorimeter, all hadrons (particles that consist of quarks) are

stopped. Here, particles can also be separated by looking at the previous layers. The neutron leaves its

first trace in the hadron calorimeter while the charged hadron leaves traces in all previous layers. The

only visible particles reaching the muon chamber are the muons.

Figure 10: A simple figure showing the different layers that are used to track the particles [23]

3.2 Latest results When searching for the Higgs boson, the best way to search for the particle is not always to search for its

most probable decay products. This because those decay products can be very hard to separate from the

background. Instead less probable decay products are searched for as it is easier to detect and

differentiate from the background. One example of such a reaction is when the Higgs boson decays into

two photons. The other main detection modes are:

In figure 11 to figure 13 the results from three different dates during 2011 from ATLAS are shown. The

figures show the expected and observed limits on the Higgs cross section . The difference between the

measurements from the different months is the size of the dataset used i.e. luminosity. In the results

14

from August a luminosity of is used, in September and in December . These

plots are hard to interpret and will therefore be simplified in this essay.

The limit ⁄ ,the y-axis, is the limit on the production rate divided by the predicted production rate

from the standard model, as a function of the Higgs boson mass. When ⁄ , the mass can be

excluded. This means that the observed is less than or the same as the expected from the standard

model, , which means that the mass can be excluded as there is no unexpected events. If the

ratio ⁄ instead is larger than one, more data is needed to be able to conclude whether the Higgs

boson exist at this mass or not. This could also be an indicator of the Higgs boson, which means these

masses cannot be excluded. In figure 13, data from December 2011, a significant peak is shown around

GeV see the right hand plot in figure 13, where the ratio ⁄ . It is possible this is the mass

of the Higgs boson, for now at least it is the most interesting point to study further [2][3][4].

Figure 11: Results from August 2011, in the graph to the right the most interesting Higgs masses [2]

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Figure 12: Results from September 2011, in the graph to the right the most interesting Higgs masses [3]

Figure 13: Results from September 2011, in the graph to the right the most interesting Higgs masses [4]

4 Methods

4.1 Methods The numerical computing program matlab will be used to illustrate which Higgs bosons masses have

been excluded from the search. The program will also be used when searching for a dependency

between the results at different luminosities. The data is given by CERN [2][3][4].

For some specific masses the expected limit on ⁄ from the three ATLAS results, see section 3.2, will

be used. For these three results the luminosity has changed from to and finally to

16

. This gives three limits at different luminosities. By plotting these limits as a function of the

luminosity hopefully a simple dependence will be found. This will be done by using the function polyfit in

matlab, which uses the least square method. By finding the dependency it would be probable to

anticipate at which luminosity it would be possible to exclude specific masses.

The masses this will be done for are , , , , and GeV. As the latest results from

ATLAS indicates that the Higgs mass could be around GeV, the masses , and GeV are

chosen. GeV is interesting as this mass is the hardest to exclude from the search, theoretically. If a

luminosity needed for excluding this mass could be estimated, it would be possible to predict how soon

not only this mass, but all masses could be excluded.

The significance of a result is proportional to √

⁄ where is the number of signal events and is

the number of background events. By doubling the amount of data, the signal and the background

doubles and we get that

√ √

√ √

This would indicate that when plotting the dependence between the luminosity and the limit we would

expect to get a square root relationship. However this assumption can only be made when both and

are large. If the background is very small we instead get that

Which of these relationships that is most accurate is hard to predict.

The plots regarding the excluded Higgs masses need to be simple and easy to understand, a thick line will

illustrate the excluded masses and the still possible masses will be shown with a small discreet line.

Three different plots will show each of the three latest results from ATLAS. This will illustrate the

enormous progress done just during 2011.

5 Results and Discussion

5.1 Results and Discussion In figure 14 to figure 16, each of the graphs represents a specific Higgs mass, three data points are

plotted. These points represent the theoretical limits ⁄ at the three different dataset sizes. A line

was fitted to these data points. In contrast to what could be expected a linear dependence was found.

This can have several explanations. The search channels with the relationship can be of more

significance than those with √ . In addition, the analysis of the data and the technical aspects of

the measurements have been improved during the year which leads to a steeper relation.

Conclusions can be drawn regarding at what luminosity the mass GeV will be able to be excluded

from the search. That is when the fitted line takes the value one, this will occur at a luminosity of

17

approximately , derived from figure 14. After this summer LHC is expected to have a luminosity

at . As the mass GeV is theoretically the hardest Higgs mass to exclude, this result does not

only say the exclusion of this mass is near, but all. This is if the Higgs boson does not exist.

Figure 14: Limit on the expected ⁄ at three different times with increasing large datasets,

for the Higgs masses GeV to the left and GeV to the right

Figure 15: Limit on the expected ⁄ at three different times with increasing large datasets,

for the Higgs masses GeV to the left and GeV to the right.

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Figure16: Limit on the expected ⁄ at three different times with increasing large datasets,

for the Higgs masses GeV

In figure 17 not only the fitted line and the theoretical limits are found, but also the three observed

limits. For this mass the fitted line is very accurate. The expected limit for the dataset size already

has a value under one, this means that the mass should already have been excluded. However the mass

is not excluded yet, as the observed limits differs a lot from the expected. This means that more

reactions than expected are occurring at this mass, it indicates that this could be the Higgs boson’s mass.

19

Figure 17: Limit on both the expected and observed ⁄ at three different times with increasing large datasets,

for the Higgs mass GeV

In figure 18the results from August 2011 are shown in my own representation. The wide lines represent

the excluded masses, both the expected and the observed results are used. Figure 19 and figure 20 are

constructed in the same way. The figures show the progress done during a few months and as shown in

figure 20 there are not many possible masses left. The masses that are not excluded yet, in the gap in

figure 20, are GeV and the masses between GeV. Also masses over GeV are not

yet excluded, but Higgs masses this heavy are not likely. Therefore the most interesting masses for the

moment are around GeV. This is because the abnormalities found around this mass, both shown in

section 3.2 and figure 17.

Given the expected increase of the dataset in 2012, there are two possible scenarios of what can

happened:

All masses will be excluded, thereby discarding the theory of the Higgs mechanism.

The Higgs boson will be found around the mass GeV.

If the Higgs boson does not exist, all masses can probably be excluded by this summer. Actually proving

the existence of the Higgs boson is a lot harder than excluding masses. It will probably be known if the

Higgs boson does exist by the end of this year.

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Figure 18: Excluded Higgs masses in August 2011

Figure 19: Excluded Higgs masses in September 2011

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Figure 20: Excluded Higgs masses in December 2011

6 Summary and Conclusion

6.1 Summary and Conclusion In this project the standard model and the Higgs boson have been studied. The latest results from ATLAS

are presented and the data is used to find dependency between the luminosities used in the search for

the Higgs boson and the expected limits on ⁄ . It is found that the dependency is linear and fits very

well to the data points. By this linear fit a prediction of the needed luminosity for the exclusion of the

Higgs mass GeV has been made.

Plots have been made in order to clarify what Higgs masses still are possible and what masses have been

excluded. These plots show the progress of the exclusion of masses done last year. It also shows the few

remaining possible masses.

During this year it will be known if the theory of the Higgs mechanism is correct. If the theory is correct a

new particle will be found and an old theory will be verified. However if the theory is incorrect a new

theory has to be presented. In any case this will be an interesting year.

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[obtained 2012-04-16]

[18] CERN. http://public.web.cern.ch/public/en/LHC/Facts-en.html

[obtained 2012-04-16]

[19] CERN. http://public.web.cern.ch/public/Objects/LHC/ATLAS_3D_0912.png .


[20] CERN. http://public.web.cern.ch/public/en/LHC/ATLAS-en.html .


[21] CERN. http://atlas.ch/ .


[22] CERN. http://www.atlas.ch/images_atlas1/in_det_lab.jpg .


[23] CERN. http://www.atlas.ch/etours_exper/images/decay_chart.gif .


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