The progress of technology in the last century has allowed human to build incredible machines
and to do incredible things.
It has allowed us to go to the Moon, for example, or to produce nuclear energy on Earth.
But one of the most remarkable machine ever built by human is the LHC, the Large Hadron
Collider.
Want to know more about it and how it works?
Watch this video and I will tell you!
The Large Hadron Collider, commonly known as the LHC, is a gigantic particle accelerator
built under the CERN, the largest laboratory of particle physics in the world, located
in the suburb of Geneva, at the border between France and Switzerland.
The LHC is an incredible machine, and with its ring of 27 kilometres of circumference,
it is actually the largest collider in the world.
But what is a collider, actually?
In order to investigate in deep the fundamental structure of nature, physicist have to recreate
conditions which are similar to those of the Big Bang.
One way of doing it is to accelerate elementary particles to extremely high speeds (almost
at the speed of light), and then make them clashing together: this is exactly what a
collider does.
By analysing the outcome of these collisions, scientists are then able to find new particles
and to improve our understanding of the Universe.
LHC has a circular shape.
In its 27-kilometers ring, there are two beams of high-energy protons, travelling in opposite
directions.
In the collider, these particles are accelerated up to a speed of 99.999999% of the speed of
light.
How is this done?
The main method used to accelerate particles is to use the so-called “radiofrequency
cavities”, or “RF cavities”.
In simple terms, these are tubes placed all along the collider, containing an electric
field which changes direction periodically.
There are 16 radiofrequency cavities at the LHC.
When a particle with electric charge is immersed in an electric field, it is accelerated by
it, and the direction depends on the sign of the charge: if the particle has positive
charge, such as a proton, it is accelerated in the same direction as the field.
If it has negative charge, such as an electron, it will go in the opposite direction.
If the particle has no charge, this method wouldn’t work, since the particle would
not feel the effect of the electric field.
When protons enter these cavities, an electric field is turned on, accelerating the particles
in the desired direction.
Each time protons enter a new RF cavity, they gain a bit more of energy thanks to the electric
field.
Eventually, after they have completed thousands of revolutions and entered these cavities
thousands time, they reach a final speed which is almost the same as the speed of light.
That is not enough, however, to make a collider.
In fact, particles tend to go straight normally, not in a circle.
Without any additional system, these protons would quickly crash against the walls of the
ring.
So, how to keep them in a circular trajectory?
This is done by using magnetic fields.
Contrary to electric fields, in fact, magnetic fields do not accelerate particles with electric
charge; instead, they just deflect them, so that they change direction.
Here, strong magnets are used to deflect the protons and keep them along the circular ring
of LHC.
However, in order to be able to create magnetic fields strong enough to deflect these high
energetic protons, scientists need to use special magnets in the LHC.
These magnets operate in the so-called “superconducting state”, a particular state in which they
have almost zero loss of electrical power.
This can only be achieved by keeping them at very low temperatures: in this case, about
degrees!
Yes, you have read it well.
This is why they must be constantly refrigerated using liquid helium, which allows to reach
such extreme temperatures.
The magnetic fields produced by these strong magnets have an intensity of 8 Tesla: to give
you an idea of how strong this is, the Earth’s magnetic field has an intensity of 0.00004
Tesla!
Thanks to the thousands of magnets placed all along the ring, scientists are able to
keep the proton beams along their circular path.
Similar magnets are also used to “squeeze” the bunches of protons and keep them collimated.
Once protons are accelerated at the desired speed, then it’s time to make them colliding!
The collisions between the proton beams occur at four designated points along the ring.
When the two beams collide, the protons “break apart” and hundreds of new particles are
produced.
In order to detect and identify them, and to discover new things about nature, scientists
built huge detectors at these points of collision.
The main four detectors are ATLAS, CMS, ALICE and LHCb.
ATLAS and CMS are the biggest ones, and they have multiple purposes.
One of the biggest achievement of these two experiments is the discovery of the Higgs
boson in 2012, the particle that gives mass to all the others.
The ALICE experiment aims to study a particular form of matter, called “quark-gluon plasma”,
while the LHCb experiment was built to investigate why the amount of matter in the Universe is
much larger than the amount of antimatter.
However, don’t think that the task of these experiments is that easy!
In fact, the amount of data they need to record (and then analyse) is impressive.
Every second, in fact, there are about 1 billion collisions per second at LHC.
And in each of these collisions, hundreds of particles are produced.
It’s therefore impossible to record all the events, otherwise the CERN computing system
would run out of space pretty soon: so, these detectors need to identify only the interesting
events within this chaos, and to record those only.
Indeed, more than 99.99% of the events produced in a collision at LHC are immediately discarded,
and only the rest of them is stored for the analysis.
Even with this system, more than one million gigabyte of data is recorded on average every
day at the LHC!
But how do scientists use this data?
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So, we have seen so far how the proton beams collide in the LHC.
But how are the data generated in the collisions used?
During each collision, hundreds of particles are produced.
However, many of them are “not interesting” for scientists: in fact, their target is to
identify new particles in this mess.
Here it’s where the detectors play a very important role, because their sensitivity
allows to identify only the potentially interesting particles produced at the collision point.
Each of the four detectors consists of several sub-detectors, each of them having a different
purpose.
For example, one of the sub-systems of CMS is the “tracker”: its purpose is to track
the trajectory of the particles produced in the collision.
This tracker consists of several silicon layers, and when a charged particle hits one of these
layers, it leaves a “trace”.
By checking all the hits left by the particle on the layers of the tracker, it is possible
to reconstruct its whole trajectory from the collision point.
Another sub-detector of CMS is the “calorimeter”: its purpose is to measure the energy released
by the particles produced in the collision.
It is made of special crystals of a particular material, lead tungstate.
When a particle passes through it, this material emits a “cascade” of electrons and light.
And by measuring the amount of light produced, scientists can infer the initial energy of
the particle.
Thanks to these special sub-detectors, scientists are able to collect a lot of important information
about the particles produced in the collisions.
And then, what?
Then, they finally analyse the data.
The purpose of the analysis can vary.
For example, one of the goals can be the discovery of a new particle.
This is done by identifying the correct “signature” of the hunted particle.
In fact, each type of particle has different properties (different electric charge, different
mass), so it leaves a different signature in the sub-systems of the detector.
For example, an electron releases a lot of light in the calorimeter, while a neutron
does not leave any.
When scientists were searching for the Higgs boson, they knew exactly what was the “signature”
of this particle (because the theory predicted exactly how this particle was going to behave).
However, often it is not that easy to distinguish it from the tracks of other known particles.
For example, the Higgs boson decays very quickly (it means that it “breaks apart”, producing
new particles).
Since this process is fast and occurs very close to the collision point, when a Higgs
boson is produced, it decays even before reaching the detector.
So, how is it possible to detect it?
The trick is, when the Higgs boson decays, it produces particles that are well-known
and easily recognizable.
For instance, the Higgs boson sometimes decays into two photons.
These photons have a very clear signature, because they don’t leave any sign in the
tracker, but they release large amounts of light when they hit the crystals of the calorimeter.
This is what scientists do if they want to search for a Higgs boson that decayed into
two photons: they search for large signals produced in the calorimeter, without any hit
in the tracker.
However, as you can imagine, life is not that easy!
In fact, dozens of other photons are produced in a single proton-proton collision.
And we also have to take into account other particles that are produced in the collision
and that can also decay into two photons, because these particles can “mimic” the
signature of the Higgs boson.
So, how to distinguish them from the actual signal of a real Higgs boson?
This is done by requiring additional constraints on the signature released by the two photons.
For instance, the energy and the direction of the photons produced by the decay of a
Higgs boson are generally different from those of the photons produced directly at the point
of collision, so it is possible to use these characteristics to discriminate between photons
that are produced by the decay of a Higgs boson and those that are not.
Of course, this is just a simplification: in reality, physicists have to use very complicate
algorithms in order to run their analysis to identify something interesting.
Thanks to all these techniques, scientists are able to do important discoveries about
the Universe, even in this chaos of particles produced during the collisions.
The discovery of the Higgs boson is just one example.
The LHC has also been able to provide many additional measurements, used to test the
validity of the Standard Model, currently the most comprehensive theory of physics that
describes the particles that build up matter and the forces that act between them.
But this is not the end of the story, because LHC aims to answer some of the fundamental
questions about our Universe: what is the nature of the dark matter?
Why is there more matter than antimatter in the Universe?
Are there new forces of nature that we don’t know yet?
To answer all these questions, scientists need to collect much more data: that’s why
LHC is set to continue to operate for many years, also with some important machine upgrades
that will allow to increase the energy of the collisions and hopefully to observe new
rare processes that we haven’t seen so far.
The journey of LHC through knowledge has just begun, and we hope to see many new exciting
discoveries in the future!
"This video ends here!
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