Ce cours vous introduit à la physique subatomique, c'est à dire à la physique du noyau et à celle des particules élémentaires. Plus spécifiquement les questions adressées sont les suivantes : - Quels sont les concepts de la physique des particules et comment sont-ils implémentés? - Quelles sont les propriétés du noyau atomique et comment peut on les utiliser? - Comment accélérer et détecter des particules et mesurer leurs propriétés? - Qu’est-ce qu’on apprend à partir des réactions de particules à haute énergie et leurs désintégrations? - Comment fonctionnent les interactions électromagnétiques et comment peut-on les mettre à contribution? - Comment fonctionnent les interactions fortes et pourquoi sont-elles difficiles à comprendre? - Comment fonctionnent les interactions faibles et pourquoi sont-elles spéciales? - Quelle est la masse des objets au niveau subatomique, et comment y intervient le Higgs? - Que peut-on apprendre de la physique des particules concernant l’astrophysique et l’Univers tout entier? Le cours est structuré en sept modules. Suivant le premier module qui introduit notre sujet, les modules 2 (Physique nucléaire) et 3 (Accélérateurs et détecteurs) dépendent peu du reste du cours et peuvent être étudiés séparément. Les modules 4 à 7 approfondissent les notions de la matière et des forces élémentaires.
3.3 Eléments du LHC
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En provenance du cours de University of Geneva
Physique des particules - une introduction
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À partir de la leçon
Accélérateurs et détecteurs
Dans ce module, on va toucher les bases de la physique des accélérateurs et des méthodes de détection pour les particules. On passera en revue les principes de l'accélération et focalisation de particules chargées par les champs électromagnétiques. On verra comment ces principes sont appliqués dans les accélérateurs et anneau de stockage modernes. Vous serez introduit aux processus physiques qui permettent de détecter les différents types de particules et comment ils sont utilisé dans des détecteurs modernes.
Rencontrer les enseignants
Martin PohlProfesseur ordinaire
Département de physique nucléaire et corpusculaire
Mercedes PanicciaCollaboratrice scientifique
Département de Physique Nucléaire et Corpusculaire
[MUSIC]
>> During this third module, we are discussing techniques
for the acceleration and detection of elementary particles.
For this video, we are invited to hall SM18 of CERN, a hall where
the magnets of the Large Hadron Collider have been tested in the past and
spare magnets are still being checked out.
A part of this hall has been transformed into an exhibition.
At the end of this module, you will know how to describe the elements
of an accelerator and know how they look like in real life.
So, let us meet today’s host.
For this video, we are accompanied by
Tobias Golling, who is one of my colleagues,
professor at University of Geneva, who dedicates 100% of his
research to the analysis of data from the ATLAS experiment at the LHC.
So, Tobis,
can you let us know what exactly you are doing in this domain?
>> Yes, with pleasure.
In short, I try to find new phenomena in the data,
with my group.
Let me put this in context. The Standard Model is a great success,
it describes all visible matter, all forces, well almost all forces.
>> Except one.
>> Yes, except one.
The Higgs mechanism describes how all
particules acquire a mass, and the Higgs boson has been discovered
at CERN in 2012, so all of this is very, very well set.
But, there are weaknesses of the model.
For example, it cannot explain dark matter and it cannot explain
gravity, that his the missing force, and there are other phenomena, like, for example,
the famous hierarchy problem, i.e.
>> the difference in scale…
>> exactly, between the Higgs mass and the scale of gravity.
So, there are open questions. What are we to do?
We can ask our theorist colleagues,
and they formulate hypotheses, that there are particles,
that one must introduce particles beyond the Standard Model,
and the hypothesis is that one can create these new
particle in our collisions. So I dedicate
my research to finding new particles,
like particles predicted by supersymmetry, or theories with extra dimensions.
>> So, for this, the predicted cross sections
are very, very small, are they not?
>> They are of the order of, or smaller than the cross sections
characterising the Standard Model, and thus
we evidently need an high performance accelerator, a collider.
>> So, what are the properties of the LHC,
which are crucial for your research?
>> We expect that these particles have a very large mass,
and that their production rate is very low.
So, the two most important parameters of the LHC for this
are the energy and the luminosity.
Higher energy allows us to produce more massive particles.
>> And a higher luminosity…
>> It means more data, a higher production rate,
more events, which can be observed.
>> The LHC is a sort of double accelerator and
storage ring, where the protons circulate in two directions.
So, in principle, it is two accelerators in one, isn’t it?
>> Exactly. We have the principle of two-in-one, for all the magnets of the LHC
there are two coils.
>> Why does one need two accelerators?
>> Because there are two proton beams in opposite directions.
So, one needs two magnetic fields.
>> One beam circulates in one direction. >> Yes.
>> And the other in the opposite direction.
And they meet at the interaction points.
>> Yes.
We need the same Lorenz force towards the center of the orbit,
so we need one magnetic field pointing upwards, the other one downwards.
And for this one uses these dipole magnets.
>> So here, we are almost sitting on one of these dipole magnets,
Can you explain the elements of these magnets a little?
>> Exactly.
So here, one sees the two rings, the two vacuum tubes,
and one sees two coils, one and the other, which are exactly the same,
the only difference is that the current is reversed.
And here is the common yoke for the magnetic fields
and the cryostat is also in common.
>> Ok.
So, in one of these coils, there is an upward pointing magnetic field,
and in the other, the same dipole field, but pointing downward.
With means that we are deviating the two beam in the same direction,
even though they have opposite directions, simply
applying Lorentz’ law, which gives the centripetal force,
which we need, q(v x B) in this case.
>> Exactly,
with the right hand rule.
>> Mh mh.
The two coils are in the same cryostat. Why?
Why does one not simply build two separate magnets?
>> One can reduce the cost this way,
having a common cryostat for the two coils.
>> So, for the enormous energies of the
LHC, one needs superconducting coils.
>> Exactly.
>> Ok.
And because of that, all are in one cryostat.
So, the coils themselves are made of copper, aren’t they?
>> Maybe a few numbers about the magnetic system,
which show that it is really very important,
even essential for the LHC.
Of the 27 km of circumference, 85% are
occupied by magnets, the magnet system… >> Ok.
>> …and 75% by dipoles, like this one.
>> So this is really the heart of the matter.
Magnets are practically everywhere.
>> There are magnets everywhere, right.
There are more than 1000, more than 1200 elements like this.
Each one weighs 30 tons,
has 15m length and a maximum magnetic field of 8.3 Tesla.
>> So what we see here is just a small segment of a dipole,
which in reality is much longer.
>> Ok.
So this makes a gigantic cryogenic system,
so, practically all the ring is at cryogenic temperature,
all around its length of 27km.
>> Yes.
So, later we will see the superconducting cables, there are 1200 tons of
superconducting cables and it needs 130 tons of superfluid helium
to cool down all this mass to 1.9 Kelvin.
>> Wow.
>> So I propose to move on to see
how these coils look and
also the focalising elements of the LHC.
Here we are in front of one of the quadruples of the LHC, which serves to focalise the two beams.
So, the principle of this focalisation has been explained in video 3.2,
but why is it important to focalise the beams
of the accelerator, Tobias?
>> So, here we come back to the luminosity, which we have already mentioned.
The luminosity is inversely proportional
to the common surface of the two proton beams,
and that is why one must focalise them, to reduce this surface.
And here we see very well
a section of a quadrupole magnet, which is responsible for the focalisation.
One sees very well the four poles, here two north poles and two south poles, and then
here are two version of this magnet, one is responsible for
horizontal focalisation, the other one for the vertical one.
One alternates the two…
>> Puts them in chains.
>> …to have an optimum focalisation.
>> So if we look at the coils, the wires go effectively
in this direction, don’t they?
>> Exactly. >> So here we have in fact
a section of such a coil. Can you
explain the ingredients of such a coil?
>> Yes.
All coils are composed of superconducting cables, they are very important,
as we said, there are 1200 tons of these cables.
They are in fact composed of these strands, with a diameter of roughly 1mm.
And these strands are themselves composed of
filaments, which are ten times finer than a human hair.
>> That means about 10 micrometers in diameter?
>> Something like that.
>> What is the material?
>> It is niobium-titanium.
And there is copper layer around.
>> And all of this is normally imbedded in a sort of aluminum stabiliser,
that is why there is this gray material.
>> Yes, yes.
>> OK. So this serves to wind the coils of the active part
of the magnets.
But also to transport current around the accelerator, doesn’t it?
>> Exactly.
We have these cables all around the accelerator.
If one takes the total length of the filaments,
one arrives at an astronomical scale.
One can go five times to the sun an back with this.
>> It is really incredible, all this effort
which is made to produce the particles
you are searching for, isn’t it?
>> In fact it serves this purpose.
>> So, we still need to discuss an element of the accelerator
which we have not covered yet.
These are the accelerating elements themselves.
So I propose that we move on to see what they look like in reality.
In this small video animation we see a cryostat which contains four
superconducting elements, i.e. radio frequency resonators.
The radio frequency wave enters through these wave guides
and establishes an electric field with the right polarity, such that
the beam is accelerated in its direction of motion.
So here is one of these elements.
Tobias, can you please explain these ingredients of this
radio frequency cavity?
>> Of course. Such a cavity has
as you said, the principle purpose to accelerate protons
from an energy of 450 GeV,
the energy with which they enter the LHC,
up to 7 TeV, which is their maximum energy.
Here we see the wave guides, the radio frequency
generated by the Klystron enter here and leaves there.
For the LHC, they have a frequency of 400 MHz.
>> OK.
So inside, a stationary wave is formed.
And when the polarity is right at the right time of arrival of the protons,
we will have a net acceleration in the direction of the electric field.
So, how does one synchronise this wave with the beam passage?
>> It is very important to have mono-energetic beams
So we must equalise the proton energy.
So when the proton with the right energy
enters at the right moment, there will be no acceleration
if we are already at the maximum energy level.
However, if the protons arrive too early or too late,
i.e. when their energy is a little too large or too small, an acceleration is felt
the protons are decelerated or accelerated to approach the ideal energy.
This happens automatically, when the protons enter in the increasing
phase of the electric field, as we have seen in video 3.2.
So, this accelerating cavity, the resonator and all what is around,
is made of copper and is emerged in a cryostat, which holds
it at liquid helium temperature, such that the copper is superconducting.
>> So why is it important that this structure
also is superconducting?
>> Yes, it is very important that there is a minimum
ohmic resistance, such that the power
of the wave is not attenuated.
>> Ok, this means that the radio frequency wave
arrives without loss from the klystron to the beam.
>> Exactly.
>> Ok.
So, thank you very much, Tobias, for this visit,
and for this demonstration of the LHC elements.
This concludes this video.
In the next video,
we will start to discuss particle detection.
[MUSIC]
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