Hi and welcome to a very special ATLAS tour.
Special in two ways.
It's a 360° tour, so you can feel free to
look around a little bit while I'll
guide you through the cavern and to the
ATLAS experiment and also because we're
going see some special places that you
wouldn't even be able to see when taking
one of the regular tours here, coming to
CERN. You can already see here in the
background
our beautiful mural painting of the
ATLAS detector. Just note that even its
big already, it's maybe only half the
size of what the actual experiment will
have to offer, but you'll see that later
downstairs. So let's have a look ...
So here we are in the ATLAS building
just in front of one of the elevators
and the first layer of security checks,
which I'll go through and I'll meet you
on the other side. So here we are just in
front of the elevator that can take us
down to the ATLAS experiment and you can
already see here a schematic drawing of
of the detector and a few things that we
are about to see underground. So just
to give you a bit of an idea and a bit
of a perspective. The whole thing, the
experiment, is about 45 meters long it's
about 25 meters high and 25 meters deep.
It's more or less symmetric forward and
backward. The protons or particles, in
general the hadrons, from the Large
Hadron Collider, they enter from the left
and they enter from the right and they
collide in the very centre of the
detector and then the detector is
essentially some sort of camera, you
could say, that tries to take a picture
and tries to record and measure
everything that leaves the
interaction point. So particles that are
produced, they will stream out of the
detector and we try to measure
everything that is produced in these
collisions with our detector. By the
different colours you can also see that
our detector the ATLAS experiment is
made of different layers, different types
of detectors, that all have their own
purpose. They measure different
quantities, they target different types
or categories of particles and we have
at the very centre we have tracking
detectors. So they are trying to
reconstruct where the collisions
happen. They are trying to reconstruct where
particles – at least charged particles –
went, so they track the particle as it
traverses through the detector. Then here,
in green and in orange, we have so-called
calorimeters: an electromagnetic and a
hadronic calorimeter that measure the
energy of particles by essentially
stopping them. And one of them, the inner
one, the electromagnetic targets
electrons and photons, whereas the other
one targets hadrons so particles like the
proton or the neutron that you might
know. Then further out in blue we have
the muon spectrometer which is in
principle another tracking detector that
we use to measure the momentum of muons
again. Muons being essentially the only
Standard Model particle that is
detectable in our detector that leaves
the calorimeters. And we measure momentum
of those again out here. To measure
momentum in general, both inside and out
here, you need magnetic fields. We have a
so-called solenoid magnetic field in the
centre and the very characteristic
toroidal magnetic field in the muon
spectrometer. So what you see in grey here
is superconducting magnets that provide
a magnetic field to bend the charged
muons,
so that we can measure their momentum
and identify them in the muon
spectrometer. And then here you see a
beam-pipe shielding, which is essentially
just concrete that protects the
detectors from radiation that happens
close to the beam and you'll see some of
these components on our way down
to the experiment. So we'll get the lift
and go down about 92 meters underground
to have a look at the experiment. So
we'll take a lift down outside the the
actual cavern. There's an extra cavern that
houses computing and cooling equipment
and then we'll take our way into the
cavern and we'll end up roughly around
here, what is in this picture between the
two big wheels that you see here. Only
that the left of the two big wheels will
be moved towards the right one. This will
be removed and this and this part will
be taken out so that scientists and
engineers have a chance to get to the
inside of the detector to do repairs and
upgrades of the whole system. Let's take
the elevator and have a look down. So
let's take the lift down to about 92
meters under ground. We're going to be
roughly at the base level of the cavern.
So as I said, the experiment is about 25
meters high, so the beam pipe is roughly
in the middle, about 12 meters above us
at so-called level 4, which we're going to
pass by later on. The LHC in general is
not horizontal, completely horizontal, in
the earth. It's slightly higher towards the Jura (mountains)
and slightly lower towards the Lake
Geneva.
You can also see that in the
experimental setup that there is this
slight sort of tilt in
the LHC, the 27-kilometre ring, that provides
the protons for the collisions and that
we want to observe in ATLAS as I showed
you on the schematic at the surface. So
now we're at about minus 92 meters. Let's
have a look. Here's another level of
security and I'm going to see you on the
other side.
So now we are at the base of the
experiment. If you look over here, you can
already see sort of a glimpse of it.
The orange structure you see here right
next to me is in fact not part of the
experiment. It's a support structure to
hold one of the end-caps of the toroid
magnets, that we'll see a bit better later
on, that has been taken out of its design
position and moved to the side so that
access to the inside of the detector is
possible. But you can already see here
that building such an experiment at this
complexity and size requires a lot of
planning and thinking. There's hundreds
of cables, of pipes, tubes, data going
in, data going out, so ... many, many
things you have to take into
consideration when you design such an
experiment and we'll see a few of these
things later, while we explore the cavern
together.
So let's have a look!
Now we're at the far end towards the
A side of the experiment, the
anti-clockwise. So this way we're looking
towards LHCb and towards Lake
Geneva, whereas the other side is the
clockwise side that points towards
ALICE and the Jura mountains. We see
already sort of one of these big wheels
that I've shown you on the drawing at
the beginning on the other side of the
lift. That sits here, actually both of
them put together, and then this big
structure here is holding the end-cap of
the toroid magnet as you can see already
there. But let's go further up
to have a bit of a better view.
So this is level four. You can see the
beam pipe there now. sort of roughly centred
on this level and you can also see
sort of the structure of these
detectives that sit here at the very end
detecting muons that go towards the
centre. So it's a big wheel that is sort
of symmetric, it has a symmetry in it,
and points sort of towards the detector,
trying to cover as much in terms of
angle as possible and getting close as
possible to the beam pipe. Let's go to
level six which gives us the chance to
actually go around the whole experiment
together and get a view.
And now we are at level six which gives us
the chance to actually go around the whole
experiment. Let's have a look here
first, as I said this is part of the muon
spectrometer at the very end of ATLAS.
Same way on the other side as we're going to
see later. You can see very nicely
this type of detector which is a
so-called monitored drift tube. It's
essentially a tube with a wire inside
and a gas mixture that charged
particles, so muons, that go through that
gas ionise and then you get a readout
signal at one of the wires. It's a fairly
simple sort of setup, but there's another
nice thing you can see here. It's that you
have sort of different layers that
always overlap each other a little bit.
Which is one of sort of the principles in
designing such an experiment, that you
don't want to give any particle that
leaves the collision the chance to leave
the detector unseen. So you build
detectors that overlap slightly. There's
other reasons to do that, to have these
overlaps, where one of these is to sort
of be as efficient as possible in
reconstructing particles and not leave
them any chance to leave the detector, if
it's particles that we can see.
Obviously there's particles like
neutrinos, that only interact through weak
interaction that we are not able to see at
all in the detector. Only
through indirect measurements we can get
an idea of that. But then let's take the
chance and to go around to the
experiment to give you an idea of
the size of ATLAS.
So here we are at the A side of ATLAS.
You can see as I mentioned at the
schematic drawing, one of the big wheels
that's usually sitting at the end of the
structure here [right] has been moved to the left.
So you see two support structures here,
which is the two big wheels. Then you see
in the background the silver part is the
end-cap of the toroid magnet which has
been moved out and to the side, so that
we can get, for example, this piece, the
big sort of red'ish kind of element here,
with a lot of cables around, which is the
end-cap of the calorimeters. So the
detectors that measure the energy of
for example electrons and protons and so
forth. And you can also see these
yellow bits, they're not part of the
detector. it's platforms that have been
moved in to allow people to work on the
various components and to get access to
the inner parts of the detector, which
you can not entirely see. Another thing
that's very characteristic to see here
and in general for ATLAS, is the
toroid magnets that I mentioned before.
It's these silvery structure with
the red [orange] bands around it. There's eight of
these. This is the barrel toroids. The
end-cap also has an eightfold symmetry,
there's one at each side. But these barrel
toroid magnets are essentially what
gives ATLAS this very characteristic
shape and also this very characteristic
toroidal magnetic field that bends the
muons in the muon spectrometer, to allow for the
second step of momentum measurement.
You see also here these big sort of
silver boxes with the black sort of
layer in between, this is again parts of
the muon spectrometer. As you can see on
the left, we go back to the wheel quickly,
there's different types of detectors so
there's not just these tubes that I
mentioned before, the monitor drift tubes.
There's other types of detectors both
in the end-caps and in the inner part
the barrel part. And that is because we
have detectors for the muon
spectrometer that can a) be very precise
in the measurement, but are a little bit
slower in giving sort of a feedback; and b)
we have detectors that are really
fast and we need that for the so-called
trigger system. Which is a two-level
system that we have in ATLAS: our first
level is implemented in hardware on the
detectors essentially, and the second
level is on computers close to here,
basically in a room next to the cavern.
A system that has to decide whether a
collision is of interest and whether we
want to keep an event or whether we
throw it away. And we actually throw away
a lot, because a lot of the collision
that happen at the LHC are not
necessarily of interest and we're not
able to store all the collisions. So we
have to do a sort of a very fast and a
very hard decision on which events we
want to safe and keep for data analysis
in the end. Then another thing that I
mentioned, the LHC is slightly tilted if
you look very closely down there on this
orange support structure. So at the
very top of it you see sort of this
pattern of orange feet and black holes
and if you look very closely you see
that the holes, and the feeds also, they
increase in size from the left to the
right and this is essentially to correct
for this slight tilt that we have
to deal with. And another thing you can nicely
see here again, as I mentioned before,
there's a lot of things you have to think
about in designing such an experiment.
Like how do you get the voltage in. I
mean how do you get sort of .. how are you able
to steer your experiment, your
detector. So you have to send data and
you have to get voltage and current in
to have the detectors operate. You have
to get data out. Both physics data and just
conditions data, telling you what
state the detector is in, what temperature it's
at and so forth. Talking about
temperature: some other detectors,
certainly the magnets, the magnets which
are superconducting, they have to be
cooled, the magnets essentially to minus
272 Kelvin [degrees Celsius] about. Some of the detectors
operate at room temperature others, like
the tracking detectors at the very
centre, they operate at temperatures
maybe around 10 degrees below zero in
Celsius. So a lot of cooling liquid, for
example, has to go in there. There's vacuum
pipes. So all these things have to be
taken into account not just because you
have to get the stuff in and out but
also because every piece of material
that you put in the detector, that is not
the detecting element itself, essentially
also those, have an influence on the
measurements you take. Because the
particles that are created in the
collisions they can interact with with
the material, they can lose energy by
doing that, and then what you measure in
the end might be sort of washed out due
to that. We have to take that into
account also when you for example
simulate events. An essential part of
doing a physics analysis is also [to]
simulate events: so generate sort of non-real
events, but as close as possible to
real events, in the computer.
Both for processes we know well and for
processes we think nature might look
like. We have to do that and also for
these simulations you have to take into
account this extra material and you have
to simulate the interaction or the
possible interaction of particles with
that material. All right. This is a
first look at this side of the detector,
so let's try and have a look at the
other side of the detector.
And also here in between, so now we're
roughly at the middle of the detector in
terms of length along the beam-pipe of
the LHC, you see there's a lot of
infrastructure. Lots, tons, hundreds
thousands of cables that go in and out
of the detector, to be able to operate
such a complex machine in the end.
So now it's the other side .. of the
detector. Clockwise in the LHC and ..
you can see it looks slightly different
on this side. And that is because here the
end-cap of the calorimeter is still
sort of more or less in place. You can see
sort of the silvery wheel in the end,
with the boxes at the side. That's the
end-cap that on the other side [side A] we've
moved out of of the detector. Still you
have these access platforms to be
able to do maintenance and make it
possible for the team to upgrade and
repair the detector where needed. What you
see here in the middle is the so-called
small wheel, which is the first sort of
layer, first component, slightly smaller
,yet still about nine meters in diameter,
of the muon spectrometer. So it's to some
extent similar to the big wheels, but
it sits much closer [to the collisions]. This is actually
one of the components that within the
next months or years we want to replace
by so-called new small wheels that
will allow us to do more precise
measurements to improve the quality of
the trigger that I mentioned before, to
do this fast selection of events to
basically select less wrong signals
so-called fake signals that in the end turn
out to be not what we want.
Yeah. And then you also see this
blue bit in the middle there, in the
centre of the big wheel, which is the
beam-pipe shielding. So in the centre of
that you have the beam pipe, that at this
stage is already just one pipe
essentially. Whereas for most
parts of the LHC you actually have two
beam pipes where the two beams [going] in
opposite directory are separated. Here
close to the experiment, in general for
the four experiments the two beams are
guided into one pipe and then brought
into collision at the very centre of the
detector.
If you look careful to the top you also
get an idea of one of the access shafts.
This is actually the smaller of the two
that we have. So essentially above each
side of the experiment there is one of
these shafts to be able to lower
things like the support structure in
orange that you see down there, where
again you also see sort of these pillars
the change in height, and things like the
nacelle to be able to work on these
experiments and to perform repairs and
upgrades. We are going to see, when we go to the other
side, we're going to have a look at the shaft on
the on the other side as well. Okay.
Another thing that's very essential if
you have lots of people working down
here, is ventilation. So you'll see that
also in the other shaft that we're going
to see in a moment. A lot of air has to
be cycled through so that people can
work here and in cases of emergency that
fresh air can be brought in and and
polluted air can be brought out.
So now we're still on the clockwise
side in terms of the LHC, but we are on
the other side of the experiment where
we came into the cavern. And also on this
side you can see this end-cap toroid
that has been taken out of the sort of
central structure and moved to the side
to make room to access the inner part, to
get things like the new small wheel or
potentially also the end cap of the
calorimeter out of the detector. And now
we're going to go sort of down this hall
a little bit to give you an idea, as I
mentioned before, there's a lot of
cooling infrastructure that also has to
be in place to allow for the operation
of the experiment.
So here you get a good idea of, as I said,
all the support [infra[structure and the ways
you have to sort of take into account
that some of the parts are movable. So
you also have to adjust for that. So it's
not just pipes, it's also sort of
flexible elements in it. There is ... As I
said you have to get a lot of cooling
liquid in and out, you have to get
electricity connected and all these
things. Things that you have to take into
account when building an experiment like this. Let's have a look to
the other side again, to basically close
our .. our circle.
So here we are again, on the other side
you see the toroid end-cap magnet on this
side and two small [big] wheels. Now that we
completed sort of our little tour around
experiment, you maybe also got an idea of
the size. As I said it's about 46
metres in length here, about 25 metres in
diameter so in that direction [perpendicular to the beam pipe].
And next we're going to see another special place
you wouldn't be able to see on a regular
tour, that will give us a bit of an idea
of the height. So we go up to almost the
the last floor [inside the cavern].
Here if you look in that direction, which
I'm pointing here, you see sort of what I
meant before: lots of ventilation. We'll
see it better from the top again as well.
[Air] that has to be brought
into the cavern and since we also have to
climatise the cavern that it's almost
at a constant temperature all year long,
so that some parts of the detector can
operate at their sort of ideal
operational temperature. Let's have a
look .. three more floors.
So now we're almost at the top. Level 11 .. I'm
a little bit out of breath, but it gives
you may be a good idea of the size. If
you look down there, this little
platform. That's where we have been and
looked at the inside of the detector.
Then we took the tour to the other side
of the cavern over there and back to here,
and then came up to level 11. so it not
just gives you a good idea .. of the
size and a bit of time to look around
also gives me some time to also say a
few other words. Obviously, such a big
experiment also needs a lot of people to
be able to run it, even to build it at
the beginning. So ATLAS is a
collaboration with about five
thousand members from all over the world, from
many, many different countries and many
different institutions that are a part
of this collaboration. That together make
it possible to build such a thing and
to operate it. So when the LHC is running,
which will hopefully happen again in
2021, ... we couldn't be here, that's one
thing, and we would basically be trying
to operate both the LHC and the ATLAS
experiment 24/7. But you essentially want
to take any collision you can get your
hands on, if it's good collisions. So the
LHC is trying to run as much as possible
and we're trying to be as efficient as
possible in recording the data. Having
the system up and ready at all times if
possible and in the recent past we had
more than 95% of the system up and
running for most of the components it
goes even closer to 100
percent. Which is with about .. well more
than 100 million channels in the system.
So 100 million independent readout
elements, that can give you data and that
you have to put together to reconstruct
the collisions that happen every 25
nanoseconds. So in principle every second
we have 40 million collisions .. that
happen. And that is a rate that even
particles leaving the collision point at
the speed of light, which most Standard
Model particles would do, you will have
three, about three, sort of
collision events in the detector at the
same time, because the particles created
in one collision leaving sort of the
detector towards the outside they won't
reach the outside, the muons spectrometer
for example in case of a muon, they won't
reach it before the next collision or
the second-to-next collision already
happened. So it's also in terms of
readout and understanding and
disentangling sort of all the things
that happen at the same time it's quite
the challenge, both on the hardware and
on the software side, and the
reconstruction algorithms that try to
make sort of sense out of the individual
measurements you have in the detectors.
Another thing that makes this
complicated is that with the bunches of
protons, so it's not continuous beams in
the LHC it's bunches roughly the size of
a few centimetres that with millions,
hundreds thousands of millions of
protons in each bunch, pass through
each other. And typically there's maybe
one interesting collision, so one that
has, for example, a high momentum transfer
so that a new particle, that there's enough
energy in the collision, that we can
create a new particle. One might be
interesting, but there's lots of others
happening at the same time that are not
necessarily interesting for the physics
that we're interested in, because it's
low momentum, low energy transfer.
Particles just barely hitting each other,
not much happening, but nevertheless you
see some sort of result of these
collisions also in the detector. So
typically, in sort of the latest data
taking run that we had we had about
thirty-four, on average, collisions
happening in one bunch crossing at the
same time. So the detectors, especially
the inner detectors as I said at the
very beginning, they are supposed to
reconstruct the tracks of charged
particles and they're also supposed to
reconstruct the vertex, so the point of
origin where these tracks came from,
where these particles were created. So
they try to reconstruct those to
disentangle sort of these individual, up
to 60 or 70 we had some time in the
LHC, collisions that happen at the same
time. Another very challenging thing
especially if you want to do sort of
very very precise measurements that in
one or another way are influenced by
these additional collisions happen at
the same time. And since I talked about
simulation earlier, also these kinds of
events which we call pile-up, so these
multiple collisions happening at the same
time, they have to be simulated as well
and to sort of get an idea to get a
better understanding and maybe develop
sort of methods to get more independent
of these additional collisions. So it's
for example in the sort of next step of
the LHC after the next run, so a few more
years into the future,
we have the high-luminosity LHC in store
and there you'll have hundreds of
collisions happening at the same time. So
that's another reason why we have to do
upgrades of the detector. This is one of
the reasons why the new small wheel, the
so-called new small wheel, comes in.
One hopefully by the end of the year and
another one in the next sort of shutdown.
Right. And then I said before, you can
also have a look up .. if you look up ..
the shaft here. This is the bigger of
the two. It's about 18 meters in diameter.
You see again lots of ventilation that has
to happen to be able to work down here.
You also see see this red sort of
installation going around. This is fire
protection, this is sort of special
devices that produce a dedicated, a special
kind of foam that sort of suffocates
fire, but you can still, so people can
still, breathe in it so you can find your
way out of the cavern without suffocation.
But yeah .. as I said, you can look up there.
You see basically the inner, the under, side
of the roof of the building that is on
top .. of the ATLAS cavern. So almost
where we've been at the start.
And everything that you see in here
essentially was lowered through one of
the two shafts. There's a few components
like the end-cap that in principle, I
mean the calorimeter end-cap or the
toroid-magnet end-cap that, could be
built on the surface and then lowered
down in one piece, but most of these
components were lowered down and then
down here, with these cranes you see here
– so the yellowish sort of a heavy weight
lifting crane, put into place. So it's a
bit like building a ship in a bottle to
build an experiments such as ATLAS.
Because you don't have much space to move
around. And as you see there it's also
not much space between the top
of the experiment and the crane itself.
So it's also from that point of view an
interesting challenge to build such an
experiment. All right. So I hope you
now have a good impression of what
ATLAS looks like and also maybe a little
bit of what it takes to build an
experiment such as ATLAS. And .. let's
slowly make our way back down to the
ground of the cavern and then back up to
the surface.
So to go down, let's take the elevator.
All right so on the way down we'll take
the elevator.
Small little elevator. As I said, it's in
total 12 levels and which make it quite
an exercise if you have to walk it
several times. But now we're going back
to the ground level, the base of the
experiment and then make our way out of
the experiment, out of the cavern, back to
the surface again.
All right, so we're going back up. So
we're still at about 92 meters under ground.
We are going back up to the surface .. and
out of the experimental hall. If
you have any questions, feel free to
contact us on social media, feel
free to write as an email. You find
all the details, all social media
channels on our web page http://atlas.cern/
And we're looking forward to
hearing from you. You can come to CERN
and visit CERN all year long. It's free
of charge.
There's lots of interesting things to
see. If we are still in the shutdown, or
again the shutdown phase, you can also
try to register for a visit to the
underground to not just have a virtual
360 tour, but an actual tour. But as I said,
today we actually were able to visit a
few places that you wouldn't be able to visit on a
regular tour. So let me take you outside
again.
So here we are again at the surface, in
front of the ATLAS building, in front of the beautiful
mural. I hope you enjoyed the tour. I hope
you learned something. I hope you got
an impression of ATLAS. I hope you had a
look around, took advantage of the 360.
You can just watch the video again and
look where you want, stop where you want
to have a look around and as mentioned
before feel free to get in touch with us!
If you have any questions on social
media we're on Twitter, Instagram, Facebook (all @ATLASexperiment) or
write an email if you have any questions
on what we're doing and why we're doing
it. And of course have a look at the
webpage [http://atlas.cern/] And follows all the
[social media] channels.
Good Bye!
