Michael Büker: Yes, alright, thank you very much, okay. I’m glad
that you all found your way here and it’s been mentioned already,
this is Comic Sans, which as you know is the official type-font
of awesome particle physics stuff. *laughter*
But in the interest of our mental sanity, I will keep it to other fonts.
So from here on Comic Sans is just a bad memory.
Okay, two things: First the title, Breaking Baryons,
which of course is an allusion to Breaking Bad, was inspired
by the wonderful talk from last year which was called “How I Met Your Pointer”.
And which was also very successful and you can check out that talk,
I got the link there. And this talk goes especially well
with another talk that we’ll have tomorrow by a real particle physicist,
at least a bit more than myself.
And it’s called “Desperately Seeking SUSY” which is about particle theories
and the real cutting edge physical questions. This is going to be
happening tomorrow. Allright, so we’re going to start out with my talk
and I’m going to be talking about the questions of “what are we doing?”,
“why?” and “what kind of stuff do we use?”. And I’m gonna spend some time
on explaining this last part especially. What is it that we do
and how does this work? So, what we do is we give a very high energy
to small particles which we call accelerating.
But from a certain level of energy this doesn’t really make sense,
because we don’t actually make them go faster. Once they reach the speed of light
they can’t go any faster. We just turn up the energy and the speed
doesn’t really change. This is technically useful but it also gives rise
to doubts about the term accelerating, but anyway, we just call it ‘accelerate’.
There’s 2 basic types of devices that you see there, you have storage rings,
which are the circular facilities that most of you know. And then there is
linear accelerators which are in comparison very boring, so I’m
not going to be talking about them a lot. We make the particles collide
which is the reason for giving them high energies, we want them to smash head-on.
And then this last part which is about the most difficult thing is we just
see what happens. Which is not at all as easy as it might sound.
So why are we doing this? You all know this formula but I’m going to try
and put it in terms which are a little bit closer to our hearts,
as we are here at Congress. I might postulate that
parts, like electrical parts, building parts, are actually the same as a device.
Now this is not quite wrong but it doesn’t feel exactly right, either.
I mean, if you have some parts and then build a device from it, it’s not the same.
It’s made from the same thing but you do require a certain amount of conversion.
You have a building process, you have specific rules how you can assemble
the parts to make a device and if you do it wrong it will not work.
And this is actually pretty similar to the notion of energy being equivalent to mass,
because energy can be converted into mass but it’s not at all easy and it follows
a lot of very strict rules. But we can use this principle
when we analyze how particle reactions are used to take a look at
what mass and what energy forms there are. Now suppose we are
thinking about a device which is very, very rare,
such as a toaster that runs Net-BSD. *laughter*
Now as you can see from the photo and the fact that you see a photo,
I’m not making this shit up. There is a toaster that runs Net-BSD but
that’s beside the point. Now if we are particle physicists and we want
to research this question, we know that parts are the same as a device,
so if we just get enough parts and do the right kind of things to them,
there might just turn out, out of nowhere a toaster that runs Net BSD.
So let’s give it a try. We produce collisions with technical parts
and if we do enough of it, and if we do it right, then there is going to be
this result. Now from these pictures you can see, that doesn’t seem
to make a lot of sense. You will not get a toaster from colliding vehicles.
*laughter* But as particle physics go,
this is the best we can do. We just smash stuff into each other
and we hope that some other stuff comes out which is more interesting.
And that’s what we do. So to put it in the technical terms,
we use storage rings which are this one circular kind of accelerator
to produce collisions. Lots of them with high energy.
And then we put some enormous experimental devices there
and we use them to analyze what happens. Now first let’s talk about
these storage rings. This schematic view is what a storage ring is
mostly made of, and you can see right away, that it’s not actually a circle.
And this is true for any storage ring. If you look at them closely they are
not a perfect circle, you always have acceleration parts which are
not actually curved. So we have the 2 basic elements
of a curved part which is just “the curve” and then you have a straight part
which is there for acceleration. Now you have this separation, it would be nicer
to have a ring but it’s much more easy this way. You have the acceleration
where it is straight and because it is straight you don’t need to worry about
making the particles go on a curved path. So you can just leave out
the magnetic fields. We need magnetic fields
to keep them on a curve, but we need electrical fields to accelerate them.
Now we could try and assemble these into one kind of device. A device
that uses an electric field to accelerate the particles and at the same time
uses a magnetic field to keep them on a curved path. Now this is the first thing
that was tried. These kinds of accelerators where called cyclotrons,
but they were very inefficient, you couldn’t go to high energies, it was
very difficult. So the evolution went to
this way where you just physically separate the 2 tasks.
You have a straight part for acceleration, you have a curved part for the curve
and then that’s much more easy. Okay, so let’s take a look at the
acceleration part of things. You may know computer games
where you go racing about and then you have some kind of arrows
on the ground and if you go over them in the right direction they make you faster.
This is a kind of booster if you will.
If you happen to go around the wrong way and you go onto these arrows,
they will slow you down, which makes sense because you’re going the wrong way,
you shouldn’t be trying that. And this is
the same effect we can think of when we think about what an electrical field does
to a charged particle. If a charged particle moves through an electrical field
in the ‘right’ direction so to speak it will speed the particle up,
taking energy from the field and to the particle making it go faster. But if you
go the wrong way, then this particle will slow down and it will
give off energy. If we where to try and…
let’s say we have a level editor, right? And we can edit this level
where this little vehicle is going and we want to make it go really fast.
So what do we do? We just take this acceleration path, we just take
these arrows and we put them in a long line. Let’s put 4, 5, 10 of them
in a row, so if we go over them we’ll be really fast at the end.
Now suppose the level editor does not allow this. It’s just
by the rules of the game it’s not possible to put a bunch of arrows in a row.
Which sucks, because then we can’t really make them go really fast.
But then we just ask an engineer who’s got this shit together.
And what is he going to suggest? You know what he’s going to suggest.
Can I hear it? Come on, “inverse the polarity”, that’s what he always does!
*laughter and applause*
So we inverse the polarity. And we are going to make our track look like this.
So we have an arrow which gives us a boost in the right direction and then there’s
an arrow in the wrong direction. If we go over the track in this way,
we’ll speed up and slow down and speed up and slow down. And in the end
we won’t win anything. But here is where Geordi comes into play, because
we’ll be switching polarities at just the right moment and if we switch polarities
at the precise moment that we are in between two of these fields,
then the next one will be an accelerating field. And it goes on and on like this,
we always switch the direction of the arrows at the right moment
when we are in between the two. And from the point of view of the vehicle
it will look like there is an accelerating field followed by an accelerating field,
followed by an accelerating field. Which is the same as we tried to build
but which the game, or in the case of real accelerators the universe
just wouldn’t allow. So we’re tricking the universe by using Geordi’s tip
and inversing the polarity at just the right moments. And this is what is done
in particle accelerators and this is called Radio Frequency Acceleration.
Now this kind of device that you see there is the device that is used
for this actual process in actual accelerators. It’s about as big
as a human child, but it weighs a bit more, it weighs
several hundred kilograms. And in contrast to a child
it’s made of a metal called Niobium. Now Niobium is a rare metal,
but it’s not super rare, and it fulfills
3 basic requirements that we have for these devices.
It’s ductile, which means you can easily shape it, because you see
that this shape is really weird, you got these kind of cone things going on,
and they must be very precise. If these cones on the inside of the cavity
are off by just micrometers the whole thing won’t work. So you need a metal
which can be formed very well.
Then you must be able to make it superconductive, to cool it down
to a temperature where it will lose its electrical resistance.
The electrical resistance will go down to almost zero, some nano-Ohms
is what’s left. So that’s the second requirement for this metal,
and the third one is: it shouldn’t be ‘super’ expensive. I guess
you could use platinum or something but then you couldn’t pay for the accelerator
and as we are going to see, the accelerator is expensive enough as it is.
So Niobium is what is used for this kind of device and
as I said, we cool it down to about 4 Kelvins, which is -269°C
or 4°C above absolute Zero. And at this temperature,
the electrical resistance of the metal is almost zero which we need
for the high frequency fields that we put in.
What we used to cool these things is liquid helium, so when they’re in use
inside the accelerator they’re not naked, exposed like you see here,
they are enclosed by huge tanks which are super tight and must
hold on to large pressures and be super temperature efficient,
very well insulating because these must keep
the liquid helium inside. But on the outside there is the tunnel
of the accelerator and that’s where people walk around. Not while the accelerator is
running, but people walk around to do maintenance and stuff. So you must have
a temperature differential between room temperature next to the accelerator
and 4 Kelvin inside the tank where this cavity is sitting.
So you have a temperature difference of 300 degrees, which this tank
around the cavity must keep. So that’s a very hard job, actually cooling
is one of the more difficult things
from an engineering point of view. The thing which feeds the fields
– the actual changing electrical fields are polarity switched –
into these cavities are called klystrons. There’s a picture of a klystron,
it’s the longish device sitting on the bottom. And they’re usually about
as big as a refrigerator or two. And these klystrons produce
radio waves not very much unlike that
which you hear in your car when you just turn on the radio. It’s not modulated
in the same way, so there’s no sound information encoded,
but it’s extremely strong. You can see on the bottom
that one of these klystrons as it is in use at the LHC has a transmitting power
of 300 Kilowatts. Now if you think of the transmitting power of the Fernsehturm
like the Hertz-Turm which is right next - no, that way -
which is right next to the conference center, or even the Fernsehturm in Berlin.
It has about half the transmitting power of one of these klystrons.
Now for the LHC accelerator 16 of them are used.
So that’s a lot of transmitting power. And because the power is so high
we don’t actually use cables. Usually you transfer your…
when you have some oscillator and you’re checking out some signals,
you just put cables between your source and your device.
This is not what’s used here, because cables get way too complicated
when you have these high energies.
So what is used, is waveguides and that is what you can see on the top there
in this picture. It looks like an air duct, it looks like there’s some
sort of air conditioning system and the air moves through. That’s not what it is.
It is a waveguide which is designed to have the radio waves inside
radiate in a certain direction. Think of a series of mirrors,
long rectangular mirrors and you put them all with the mirroring area inside.
So you have a tube which is mirroring inside. And then at one side
you shine in a bright light. Now the light can’t escape anywhere and it
always hits the mirrors so it goes on in a straight path.
You’ve built yourself a waveguide for light. Now this here,
this clunky looking metal part is a waveguide
but for high frequency, high energy radio waves which are fed into the cavities.
And that’s how acceleration happens. Now let’s talk about the curves.
This is where it gets less fidgety and more… boom!
So these devices you see here, there’s 2 devices sitting next to each other,
identical devices. These are the cryo-dipoles.
Again, they have the word “cryo” in them because they are also cooled
by liquid helium down to a temperature of about -270°C.
They’re 40 meters long, they weigh 35 tons and each of these babies
costs about half a million Swiss Francs.
And as you can see one line above that, there’s 1200 of these curve dipoles
in the LHC. So there you have a cost of 1.5 to 2 billion dollars
in the curve magnets alone. We’re not talking acceleration,
we’re not talking about power use, we are not talking about the helium that you need
for cooling or the power that you need for cooling. It’s just building these things,
just building the curve, 27 kilometers.
And that’s what you have there as a cost. Now what they do is, they make
a huge magnetic field, because in a magnetic field a charged particle
will go on a curve. That’s what we want, right? But
to make these particles with a very high energy and keep them on a tight curve…
now in particle physics’ terms let’s say that 27 kilometers
to go around one way is a tight curve.
We need a current of 12,000 amps. Which is a large current
that goes through these dipoles. Which is the reason why we have them
superconductingly cooled, because otherwise you put 12,000 amps
through a piece of metal and it just melts away. You don’t get a magnetic field,
maybe for a microsecond or 2. But you want to sustain a stable field
of 8.5 Tesla to make these protons go around on a curve.
So, yeah, that’s a big thing. There’s also niobium in there,
not the big clunky parts like the cavity we saw, but thin niobium wires,
actually half niobium, half titanium most of the time. But since
there are so many magnets and it’s so long a curve, there is 600 tons
of atomic niobium in this entire accelerator thing.
And this was a fourth of the world production of niobium
which comes mostly from Brazil by the way. This was a fourth of the world production
of niobium for 5 years. So that’s where it all went.
It just went into the accelerator. And now if we have this running,
we have it up, we have it cooled, we have a large current going, we got our nice
big magnetic fields. And there is energy stored.
I mean we put in a lot of power and the magnetic fields are up and they’re stable
and that means that there’s magnetic energy stored in this. And the amount
of energy that is stored in the curve magnets alone of the LHC when it’s running
is 11 gigajoules. Sounds like a lot,
let’s compare it to something: If we have an absurdly long freight train
with let’s say 15,000 tons. I hear that normal freight trains in Germany
or England have about 5000 tons. So let’s take a big freight train
and multiply it by 3. If this freight train goes at 150 km/h,
then the kinetic energy, the movement energy of this train
is equivalent to the magnetic energy that is stored in the LHC.
And that is why we don’t want any problem with the cooling.
*laughter*
Because if we get a problem with the cooling, bad things happen.
This is a photograph of what at CERN at the LHC they just call “the incident”.
*laughter*
Which was a tiny mishap that happened just a few weeks
after the LHC was taken into operation for the first time in 2008.
And it shut the machine down for about 8 months.
So that was a bad thing. It’s a funny story when they where
constructing these magnets; now what you see here is the connection
between 2 of these magnets. I told you that each of them weighs 35 tons.
So here you have a connection between 2 parts that are 35 tons in weight each.
And they’re shifted by almost half a meter. So it takes a bit of boom.
So what happened was: the cooling broke down and the helium escaped and
the sheer force of the helium expanding, because if you have liquid helium
and it instantly evaporates into gaseous helium then the volume multiplies
by a very large amount. And what they had was…
what I hear is that the tunnel of the LHC, which has a diameter of about
let’s say 6 or 7 meters was filled with nothing but helium
which pushed away the air for about 100 meters
around this incident. So the helium evaporated, it pushed everything away,
it made everything really cold, some cables broke and some metal broke.
And the funny thing now is, the engineers that built the LHC,
before they did that, visited Hamburg. Because here there is
a particle accelerator which is not quite as large. The LHC
has 27 kilometers; here in Hamburg we have a particle accelerator called HERA
which had 6.5 kilometers. So it’s the same ballpark, it’s not as big.
And in HERA they had a safety system against these kinds of cryo failures,
they’re called quenches. They had a protection system,
which protects this exact part. Now we’re talking about “Yeah,
how should we build this? Should we have a quench-protection
at the connection between the dipoles?” And the HERA people in Hamburg said:
“Well we have it, it’s a good thing, you shouldn’t leave it out,
if you build the LHC.” Well, they left it out. *laughter*
They ran out of time, they ran out of money, the LHC project was under pressure.
Because they had promised to build a big machine by that time and
they weren’t really finished, so they cut some edges. Well this was
the edge they cut and it cost them 8 months of operation. Which says
that they really should have listened to the people of Hamburg. Okay, so,
in summary of the operations of a storage ring we can just say this:
They get perfectly timed kicks with our polarity switching
at just the right moment by radio waves generated in these large klystrons
from the funny looking metal tubes that we called cavities.
And some big-ass superconducting magnets keep them on a curve
when they are not being accelerated. Now the trick is, one of these kicks
like moving through the cavity once, may not give you all the energy you want,
in fact it doesn’t. But if you make them go round in the ring,
they come by every couple of nanoseconds. So you just have them
run through your acceleration all the time. Which is the big difference
between the storage ring and a linear accelerator. A linear accelerator
is basically a one shot operation but here, you just give them an energy kick
every time they come around, which is often, we’re going to see that.
So that’s the summary of what the storage rings do. Now,
the machine layout, if you look at a research center
which has a bunch of accelerators, it almost always goes like this:
You have some old, small storage rings and then they built
newer ones which were bigger. So this is just
a historical development, first you build small machines, then
techniques get better, engineering gets better, you build bigger machines. But
you can actually use that, it’s very useful because the older machines,
you can use as pre-accelerators. For a variety of reasons it’s useful
to not put in your particles with an energy of zero and then
have them accelerated up to the energy you want. You want to pre-accelerate them,
make them a little faster at a time. That’s what you do, you just
take the old accelerators. And if we look at the accelerator layout
of some real world research centers, you can actually see this. On the left
you have CERN in Geneva and on the right you have DESY here in Hamburg.
And you can see that there are smaller accelerators, which are the older ones,
and you have bigger accelerators which are connected to them.
And that’s this layout of the machines. Okay, now let’s talk about collisions.
This is a nice picture of a collision. It’s not actually a proton collision
but a heavy-ion collision, which they do part of the time in the LHC.
They are extremely hard to produce, we’re going to see that, but still we make
an awful lot of them. So let’s see, first of all
let’s talk about what the beam looks like, because we’re going to be colliding beams.
So what are these beams? Is it a continuous stream of particles?
Well it’s not. Because the acceleration that we use, these radio frequency,
polarity shifting mechanisms, they make the particles into bunches.
So you don’t have a continuous stream, you have separate bunches.
But how large are these bunches? Is there one particle per bunch?
You’ve got a particle, you wait a while, there’s another particle?
Well, it’s not like that. Because if it were like that,
if we had single particles coming after one another, it would be impossible
to hit them. You have to aim the beams very precisely.
I mean, think about it. One comes around 27 kilometers around the ring.
The other comes around 27 kilometers going the other way.
And now you want them to hit. You have to align your magnets very precisely.
You can think of it like this: You have a guy in Munich
and you have a guy in Hamburg and they each have a rifle. And the bullets
of the rifle are let’s say one centimeter in size. So the guy in Hamburg
shoots in the air and the guy in Munich shoots in the air, and they are supposed
to make the bullets hit in the middle, over, let’s say Frankfurt.
Which they’re not going to manage. And which is actually way too simple.
Because if the bullet is really one centimeter in size,
then the equivalent distance that the two shooters should be away from each other,
if we want to make it the same difficulty as these protons,
would not be between Hamburg and Munich. It would be from here to fucking Mars.
*laughter and applause* I calculated that shit.
*applause*
We don’t even have rifles on Mars anyway. *laughter*
So what we got is, we got large bunches, very large bunches.
And in fact there’s 10^11 protons per bunch, which is
100 Billion. This is where I called Sagan “ you going Millions of Millions“
Okay, so you got 100 Billion protons in one bunch.
And the bunches go by one after the other. Now, if you stand next to the LHC
and you were capable of observing these bunches, you would see one fly by
every 25 nanoseconds. So you go “there’s a bunch, now it’s 25 nanoseconds,
there is the next one”. And there’s about 7.5 meters between the bunches.
Now, 7.5 meters corresponds to 25 nanoseconds, you see that
the speed is very big and indeed it’s almost the speed of light.
Which is just, we accelerate them and at some point they just go
with the speed of light and we just push up the energy, we don’t make them
go any faster actually. And if you were to identify the bunches,
which actually you can, you would see that there are 2800 bunches
going by; and then when you have number 2809,
that’s actually the first one that you counted which has come round again.
Per direction! So in total we have over 5000 bunches
of 100 Billion protons each. So that’s the beam we are dealing with.
Oh, and a funny thing: you get charged particles moving, it’s actually a current,
right? In a wire you have a current running through it,
there’s electrons moving or holes moving and you get a current. If you were
to measure the current of the LHC, it would be 0.6 milliamps,
which is a small current, but we’re doing collisions anyway
and not power transmission, so that’s fine. *laughter*
This is a diagram of what the actual interaction point geometry looks like.
You get the beams from different directions, think of it like the top one
coming from the right, the bottom one coming from the left;
and they are kicked into intersecting paths by magnets. You have
very complicated, very precise magnetic fields aligning them,
so that they intersect. And it’s actually a bit of a trying-out game.
I’ve heard this from accelerator operators.
You shift the position of the beams relative to each other by small amounts
and you just see where the collisions happen. You go like: “Ah yeah, okay,
there’s lots of collisions, ah, now they’re gone, I’m going back.”
And you do it like that. You can save the settings and load them and calculate them
but it’s actually easier to just try it out.
If we think of how much stuff we’ve got going on: you got a packet,
a bunch of 100 Billion protons coming one way,
you got another packet of 100 Billion protons coming the other way.
Now the interaction point area is as small as the cross section of a human hair.
You can see that, it’s one hundredth of a square millimeter.
Now how many collisions do you think we have? We’ve got…
Audience: Three! *Michael laughs*
Michael: …it’s actually not that bad. We got about 20 in the LHC.
And the funny thing is, people consider this a bit too much.
The effect is called pile-up. And the bad thing about pile-up is you’ve got
beams intersecting, you’ve got bunches ‘crossing’ – that’s what we call it.
And there’s not just one collision which you can analyze, there is a bunch of them,
around 20. And that makes that more difficult for the experiments,
we’re going to see why. Well, and if we have 20 collisions every bunch crossing
and the bunches come by every 25 nanoseconds, that gives us a total
of 600 Million collisions per second. Per interaction point.
Which we don’t have just one of. We have 4 experiments, each experiment
has its own interaction point. So in total, we have about 2 Billion
proton-proton collisions happening every second when the LHC is running.
Now let’s look at experiments. *laughs*
Yeah, this is a photograph of one part of the ATLAS experiment being transported.
And as for the scale of this thing, well, in the physics community, we call this
a huge device. *laughter*
I have a diagram of the experiment where this is built in and
you’re going to recognize the part which is the one I’ve circled there.
So the real thing is even bigger. And down at the very bottom,
just to the center of the experiment, there’s people.
Which if I check it like this, they’re about 15 pixels high.
So that’s the scale of the experiment.
The experiment has the interaction point at the center, so you got a beam line
coming in from the left, you got the other beam line coming in from the right.
And in the very core of the experiment is where the interactions,
where the collisions happen. And then you got the experiment in layers,
like an onion, going around them in a symmetrical way.
Inside you have a huge magnetic field which is almost as big
as the curve magnets we were talking about when I was describing the storage ring.
This is about 4 Teslas, so it’s also a very big field.
But now we got a 4 Tesla field not just over the beam pipe
which is about 5 centimeters in diameter, but through the entire experiment;
and this thing is like 20-25 meters. So you’ve got a 4 Tesla field
which should span more than 20 meters.
And, just for shits and giggles, it’s got 3000 kilometers of cables.
Which is a lot; and if you just pull some random plug
and don’t tell anyone which one it was you’re making a lot of enemies.
So the innermost thing is what we call the inner tracking. It is located
just centimeters off the beam line, it’s supposed to be very very close to
where the actual interactions happen.
And this thing is made to leave the particles undisturbed, they should just
fly trough this inner tracking detector. And the detector will tell us
where they were, but not actually stop them or deflect them.
This gives us precise location data, as to how many particles there were,
what way they were flying, and, from the curve,
what momentum they have. Outside of that we’ve got calorimeters.
Now these are supposed to be stopping the particles. A particle goes through
the inner tracking without being disturbed but in the calorimeter it should stop.
And it should deposit all its energy there and which is why we have to put around it
the inner tracking. You see, if we put the calorimeter inside, it stops the particle,
outside of that nothing happens. So we have the calorimeters outside of that.
And then we got these funny wing things going on. That’s the muon detectors.
They are there for one special sort of particle.
Out of the… 50, let’s say 60 – depends on the way you count –
elementary particles that we have. These large parts are
just for the muons. Because the muons have the property,
the tendency to go through all sorts of matter undisturbed. So you just need to
throw a huge amount of matter in the way of these muons, like:
“let’s have a brick wall and then another one”. And then you
may be able to stop the muons, or just measure them.
This is to give you an idea of the complexity of the instrument
on the inside. This is the inner tracking detector, it’s called a pixel detector;
and you see guys walking around in protective suits. That is not for fun
or just for the photo, this is a very, very precise instrument. But it’s sitting
inside this huge experiment which – again,
I calculated that shit – is about as large as a space shuttle
and weighs as much as the Eiffel Tower. And inside
they’ve got electronics, almost a ton of electronics which is so precise
that it makes your smartphone look like a rock. So there you go,
it’s a very, very complicated sort of experiment. Let’s talk about triggering,
because as I said there’s 600 Million events happening inside this.
That’s 40 Million bunch crossings. Now: how are we going to analyze this?
Is there a guy writing everything down? Obviously not.
So this experiment with all the tracking and the calorimeters and the muons
and everything has about 100 Million electronic channels.
And one channel could be the measurement of a voltage, or a temperature
or a magnetic field or whatever. So we’ve got 100 Million different values,
so to speak. And that makes about 1.5 Megabytes per crossing,
per every event readout. Which gives us – multiplied by 40 Million –
gives us about 60 terabytes of raw data per second.
That’s bad. I looked it up, I guess
the best RAM you can do is about 1 terabyte per second or something.
So we’re obviously not going to tackle this by just putting in fast hardware,
because it’s not going to be fast enough. Plus,
the reconstruction of an event is done by about 5 Million lines of C++ code.
Programmed by some 2000-3000 developers around the world.
It simulates for one crossing 30 Million objects, which is
the protons and other stuff flying around.
And it is allocated to take 15 seconds of one core’s computing time.
To calculate it all, you would need about 600 million cores.
That’s not happening. I mean, even if we took over the NSA
*laughter* and used all of their data-centers
for LHC calculations, it still wouldn’t be enough. So we have to do something
about this huge mass of data. And what we do is, we put in triggers.
The trigger is supposed to reduce the number of events that we look at.
The first level trigger looks at every collision that happens.
And it’s got 25 nanoseconds of time to decide:
Is this an interesting collision? Is it not an interesting collision?
We tell it to eliminate 99.7% of all collisions.
So only every 400th collision is allowed for this trigger to go:
“Oh, yeah, okay that looks interesting, let’s give it to Level 2 trigger”.
So then we end up with about 100,000 events per second. Which get us
down to 150 Gigabytes per second. Now we could handle this from the data flow,
but still we can’t simulate it. So we’ve got another level trigger.
This is where the two experiments at the LHC differ:
the CMS experiment has just a Level 2 trigger; does it all there.
The ATLAS experiment goes the more traditional way, it has a Level 2 trigger
and a Level 3 trigger. In the end these combined have about 10 microseconds
of time, which is a bit more and it gives them a chance to look at the events
more closely. Not just, let’s say: “Was it a collision of 2 protons
or of 3 protons?”; “Were there 5 muons coming out of it
or 3 electrons and 2 muons?” This is the sort of thing they’re looking at.
And certain combinations the triggers will find interesting or not.
Let’s say 5 muons, I don’t give a shit about that. “3 muons and 2 electrons?
Allright, I want to analyze it”. So that’s what the trigger does.
Now this Level 2 and 3 trigger, again, have to kick out about
99.9% of the events. They’re supposed to leave us with
about 150 events per second. Which gives a data volume of a measly
300 Megabytes per second and that’s something we can handle. We push it
to computers all around the world. And then we get the simulations going.
This is a display, this is what you see in the media.
If you take one of these events – just one of the interesting events which
actually reach the computers – because those 40 million bunch crossings… well,
most of them don’t reach the computers, they get kicked out by the triggers.
But out of the remaining 100 or 200 events per second, let’s say this is one.
It’s an actual event and it’s been calculated into a nice picture here.
Now, normally they don’t do that, it’s analyzed automatically by code
and it’s analyzed by the physics data. And they only make these pretty pictures
if they want to show something to the press. To the left you have
what’s called a Feynman Diagraph. That’s just a fancy physical way
of saying what’s happening there. And it involves the letter H on the left side,
which means there’s a Higgs involved. Which is why this event was particularly
interesting to the people analyzing the data at the LHC.
And you see a bunch of tracks, you see the yellow tracks all curled up inside,
that’s a bunch of protons hitting each other. The interesting thing is
what happens for example above there with the blue brick kind of things.
There’s a red line going through these bricks. This indicates a muon.
A muon which was created in this event there in the center.
And it went out and the bricks symbolize the way
the reaction was seen by the experiment.
There was actually just a bunch of bricks lighting up. You got, I don’t know,
500 bricks around it and brick 237 says: “Whoop, there was a signal”.
And they go: “Allright, may have been a muon moving through the detector”.
When you put it all together you get an event display like this. Okay,
so we got to have computers analyzing this. And with all the 4 experiments
running at the LHC, which is not just CMS and ATLAS I mentioned but also
LHCb and ALICE, they produce about 25 Petabytes of data per year.
And this cannot be stored at CERN alone. It is transferred to data centers
around the world by what is called the LHC Optical Private Network.
They’ve got a network of fibers going from CERN to other data-centers in the world.
And it consists of 11 dedicated 10-Gigabit-per-second lines
going from CERN outwards. If we combine this, it gives us a little over
100 Gigabits of data throughput, which is about
the bandwidth that this congress has.
Which is nice, but here it’s dedicated to science data and not just porn
and cat pictures. *laughter and applause*
*applause*
From there it’s distributed outwards from these 11 locations to about
170 data centers in all the world. And the nice thing is,
this data, these 25 Petabytes per year, is available
to all the scientists working with it. There’s about… well,
everybody can look at it, but there’s about 3000 people in the world
knowing what it means. So all these people have free access to the data,
you and I would have free access to the data, just thinking it’s cool to have
a bit of LHC data on your harddrive maybe. *laughter*
All in all, we have 250,000 cores dedicated to this task,
which is formidable. And about 100 Petabytes of storage
which is actually funny, because 25 Petabytes of data are accumulated
per year and the LHC has been running for about 4 years.
So you can see that they buy the storage as the machine runs. Because
100 Petabytes, okay, that’s what we have so far. If we want to keep it running,
we need to buy more disks. Right! Now,
what does the philosoraptor say about the triggers?
If the triggers are supposed to eliminate those events which are irrelevant,
which is not interesting, well, who tells them what’s irrelevant?
Or to put it in the terms of Conspiracy-Keanu:
“What if the triggers throw away the wrong 99.something % of events?”
I mean, if I say: “If there’s an event with 5 muons going to the left,
kick it out!”. What if that’s actually something that’s very, very interesting?
How should we tell? We need to think about this very precisely.
And I’m going to tell you about an example in history where
this went terribly wrong, at least for a few years. We’re talking about
the discovery of the positron. A positron is a piece of anti-matter;
it is the anti-electron. It was theorized in 1928, when
theoretical physicist Dirac put up a bunch of equations. And he said: “Right,
there should be something which is like an electron, but has a positive charge.
Some kind of anti-matter.” Well, that’s not what he said, but that’s
what he thought. But it was only identified in 1931.
They had particle experiments back then, they were seeing tracks of particles
all the time. But they couldn’t identify the positron for 3 years,
even though it was there on paper. So what happened? Well,
you see the picture on the left. This is the actual, let’s say baby picture
of the positron. I’m going to build up a scheme on the right
to show you a bit more, to give you a better overview of
what we are actually talking about. In the middle you’ve got a metal plate.
And then there’s a track which is bending to the left, which is indicated here
by the blue line. Now if we analyze this from a physical point of view,
it tells us that the particle comes from below,
hits something in the metal plate and then continues on to the top.
So the direction of movement is from the bottom to the top.
The amount by which its curvature reduces when it hits the metal plate
tells us it has about the mass of an electron. Okay, so far so good.
But then it has a positive charge. Because we know the…
we know the orientation of the magnetic field. And that tells us: “Well,
if it bends to the left, it must be a positive particle.”
So we have a particle with the mass of an electron, but with a positive charge.
And people were like “Wat?”. *laughter*
So then someone ingenious came up and thought of a solution:
‘They developed the picture the wrong way around!?’
*laughter and applause*
*applause*
It’s what they thought. Well it’s wrong, of course, there’s such a thing as
a positron. And it’s like an electron, but it’s positively charged. But…
to put it in a kind of summary maybe: you can only discover that
which you can accept as a result. This sounds like I’m Mahatma Gandhi
or something but it’s just what we call science. *laughter*
Okay, so to recap: What have we seen, what have we talked about?
We saw from the basic principle, that if we have energy in a place,
then that can give rise to other forms of matter, which I called ‘parts = a device’.
You got your little parts, you do some stuff, out comes a device.
We have storage rings which give a lot of energy to the particles
and in which they move around in huge bunches. Billions of billions of protons
in a bunch and then colliding. Which gives in the huge experiments
that we set up an enormous amount of data ranging in the Terabytes per second
which we have to program triggers to eliminate a lot of the events
and give us a small amount of data which we can actually work with. And then
we have to pay attention to the interpretation of data, so that
we don’t get a fuck-up like with the positron. Which is a very hard job.
And I hope that I could give you a little overview of how it’s fun.
And it’s not just about building a big machine and saying:
“I’ve got the largest accelerator of them all”. It’s a collaborative effort,
it’s literally thousands of people working together and it’s not just about
two guys getting a Nobel Prize. You see this picture on the top left, that’s
about 1000 people at CERN watching the ceremony of the Nobel Prize
being awarded. Because everybody felt there’s two people getting a medal
in Sweden, but it’s actually an accomplishment… it’s actually an award for
everybody involved in this enormous thing. And that’s what’s a lot of fun about it
and I hope I could share some of this fascination with you. Thank you a lot.
*huge applause*
Before we get to Q&A, I’m going to be answering questions that you may have.
My name is Michael, I’m @emtiu on Twitter, I’ve got a DECT phone,
I talk about science, that’s what I do. I hope I do it well.
And you can see the slides and leave feedback for me please
in the event tracking system. And tomorrow, if you have the time
you should go watch the “Desperately seeking SUSY” talk which is going to be
talking about the theoretical side of particle physics. Okay, that’s it from me,
now on to you. Herald: Okay, if you have questions,
please line up, there’s a mic there and a mic there. And if you’re on the stream,
you can also use IRC and Twitter to ask questions. So
I’m going to start here, please go ahead.
Question: Thanks a lot, it was a very fascinating talk, and nice to listen to.
My question is: Did HERA ever suffer a quench event
in which the quench protection system saved the infrastructure?
Michael: No, actually it didn’t. There were tests where they provoked
a sort of quench event in order to see if the protection worked. But
even if this test would have failed it would not have been as catastrophic.
But there were failures in the operation of the HERA accelerator
and there was one cryo failure. Which is actually a funny story. Which is
where one part of the helium tubing failed
and some helium escaped from the tubing part
and went into the tunnel. Now what happened was that the air moisture,
just the water in the air froze at this point.
And the Technical Director of the HERA machine told us this: at one point
he sat there with a screwdriver and a colleague, picking off… the ice
off the machine for half the night before they could replace this broken part.
So, yeah, cryo failures are always a big pain.
Herald: Do we have questions from the internet? …Okay.
Signal Angel: We have one question that is:
“How are the particles inserted into the accelerator?”
Michael: They mostly start in linear accelerators.
Wait, we’ve got it here. So you got the series of storage rings
there at the top in the middle and you have one small line there.
That’s a linear accelerator. To get protons is actually very easy.
You buy a bottle of hydrogen which is just a simple gas you can buy.
And then you strip off the electrons. You do this by ways of exposing them
to an electric field. And what you’re left with is the core of the hydrogen atom.
And that’s a proton. Then you accelerate the proton just a little bit
into the linear accelerator and from there on it goes into the ring. So that means
basically at the start of these colliding experiments is just a bottle of helium
that somebody puts in there. And at the LHC it’s about, you know,
a gas bottle. It’s about this big and it weighs a lot. At the LHC they use up
about 2 or 3 bottles a year for all the operations, because
a bottle of hydrogen has a lot of protons in it.
Herald: You please, over there.
Question: Actually I have 2 questions: One part is,
you said there are 2 beams moving in opposite directions.
And you explained the way where you switched polarity. How can this work
with 2 beams opposing each other?
Michael: That’s a good question. Now, if I show you the picture of the cryo dipole,
you will see that these 2 beams are not actually in the same tube.
There we go. You see a cryo dipole and
on the inside of this blue tube, you see that there’s actually 2 lines.
You can’t see it very well but there’s 2 lines. So they are
inside the same blue tube, but then inside that is another small tube,
which has a diameter of just about a Red Bull bottle. Say 5 or 6 centimeters
in diameter. And this is where the beam happens. And they are just sitting
next to each other. So the beams are always kept separate
except from the interaction points where they should intersect.
And the acceleration happens obviously also in separate cavities.
Herald: You had a second question?
Question: The second question is: The experiments, where are they placed,
on the curve or on the acceleration part?
Michael: The interaction points are placed between the acceleration
on the straight path. Because, again, it’s much easier if you had the protons
going straight for 200m; then you can more easily aim the beam.
If they come around the curve then they have – you know they have a curve motion,
you need to cancel that. That would be much more difficult.
Herald: And the left, please.
Question: Okay, so you got yourself a nice storage ring and then
you connect it to the power plug and then your whole country
goes dark. Where does the power come from?
Michael: Well, in terms of power consumption of, let’s say
households, cities, or aluminum plants:
accelerators actually don’t use that much power. I mean
most of us don’t run an aluminum plant. So we’re not used to this
sort of power consumption. But’s it’s not actually all that big. I can tell you about
the HERA accelerator that we had here in Hamburg, which I told you is about
6.5 kilometers, not the 27, so you can sort of extrapolate from that.
It used with the cryo and the power current for the fields
and everything – it used about 30 MW. And 30 Megawatts is a lot,
but it’s not actually very much in comparison to let’s say aluminum plants,
our large factories. But in fact, the electricity cost is a big factor.
Now you see the LHC is located at the border between Switzerland and France.
It gets most of its power from France.
And you always have an annual shutdown of the machine. You always have it off about
1 or 2 months of the year. Where you do maintenance, where you replace stuff,
you check stuff. And they always take care to have this shutdown
for maintenance in winter. Because they get their power from France.
And in France many people use [electrical] power for heating.
There’s not Gas heating or Long Distance heat conducting pipes
like we have in Germany e.g. The people just use [electrical] power for heat.
And that means in winter the electricity price goes up. By a large amount. So
they make sure that the machine is off in winter when the electricity prices are up.
And it’s running in the summer where it’s not quite as bad. So it’s a factor
if you run an accelerator. And you should tell your local power company
if you’re about to switch it on! *laughter*
But actually, it won’t make the grid off, even a small country like Switzerland
break down or anything.
Herald: Do we have more questions from the internet? Internet internet, no,
no internet. Okay. Then just go ahead, Firefox Girl.
Question (male voice): So you see a lot of events. And I guess there’s many
wrong ones, too. How do you select if an event you see is really significant?
Michael: Well, you have different kinds of analysis. Like I told you there is
100 Mio. channels you can pick from.
With the simplest trigger that you have, the Level 1 trigger,
it can’t look at the data in much detail. Because it only has 25 ns.
But as you go higher up the chain, as the events get more rare,
you can look at them more closely. And what we end up in the end, these 100,
maybe 200 events per second, you can analyze them very closely. And they get…
they get a full-out computation. You can even make these pretty pictures
of some of them. And then it’s basically, well, theoretical physicists’ work,
to look at them and say: “Well, this might have been that process…”, but
still a lot of them get kicked out. When the discovery of the Higgs particle
was announced, it was ca. 1 1/2 years ago…
Well, the machine had been running for 2 1/2 years. And, like I told you,
there’s about 2 Billion proton collisions per second. Now the number of events
that were relevant to the discovery of the Higgs – the Higgs events –
it was not even 100. Out of 2 Billion per second.
For 2 1/2 years. So you have to sort out a lot. Because it’s very very, very rare.
And that’s just the work of everybody analyzing, which is why
it’s a difficult task, done by a lot of people.
Herald: The right, please.
Question: What I’m interested in: You say ‘one year of detector running’.
How much time in this year does this detector actually run…
…is it actually running?
Michael: Well, yeah, like I said, we have the accelerator off for about
1 or 2 months. Then if something goes wrong it will be off again.
But you want to keep it running for as long as possible, which…
in the real world… let’s say it’s 9 months a year. That’s about it.
Question: Straight through?
Michael: Straight through – ah, well, not in a row. But it’s always on
at least for a week. And then you get maybe a small interruption
for a day or two, but you can also have a month of straight operation sometimes.
Herald: Internet, please!
Signal Angel: Yeah, another question: what would happen if they actually find
what you are looking for? *Michael laughs*
Do we throw the LHC in the dumpster or what do we do?
Michael: That’s a good question! It would be one hell-of-a waste
of a nice-looking tunnel! *laughs* You might consider using it for
– I don’t know – maybe swimming events, or bicycle racing.
Well, but actually that’s a very good question because the tunnel
which the LHC sits in, this 27 km tunnel, it was not actually dug,
it was not actually made just for the LHC. There was another particle accelerator
inside before that. It had less energy, because it didn’t accelerate protons
but just electrons and positrons. That’s why the energy was a lot lower.
But they said: “Well, okay, we’re going to build a very large accelerator,
does anyone have a 30 km tunnel, maybe?”
and then someone came up with: “Yeah, well, we got this 27 km tunnel
where this LEP accelerator is sitting in. And when it’s done with its operations
in…” – I don’t know, by that time, let’s say in – “…10 years, we’re going
to shut it off. Why don’t we put the next large accelerator in there?” So you try
to reuse infrastructure, but of course you can’t always do that. The next big,
the next huge accelerator, if we get the money together as a science community,
because the politicians are being a bitch about it…
if we get the money it’s going to be the International Linear Collider.
And that’s supposed to have 100 km of particle tubes
and, well, you need to build a new tunnel for that, obviously.
Question: First off, couldn’t you use it in something
like material sciences, like example with DESY?
Well okay, if you are done with leptons you can still use it
for Synchrotron Laser or something like this.
Michael: That was thought of. The HERA accelerator at DESY was shut off
and people were thinking about if they could put a Synchrotron machine inside it.
But the problem there is the HERA accelerator is 25 m below the ground.
This is not enough space. With particles accelerating
you just need a small tube. But for Synchrotron experiments you need
a lot of space. So you would have to enlarge the tunnel by a lot,
and this was not worth it, in the case of the HERA accelerator. But interestingly,
one of the pre-accelerators of HERA, one that was older is now used
for Synchrotron science, which is PETRA. Which used to be just an
old pre-accelerator, and now it’s one of the world’s leading Synchrotron machines.
So, yeah, you try to reuse things because they were expensive.
Question: And may I just ask another question?
You said you get… you use just the matter
from a bottle of hydrogen or a bottle of helium.
Well, most helium or hydrogen is protons
or, in the case of helium, helium-4. But
you have a little bit helium-3 or deuterium.
And well, you are looking for interesting things you don’t expect.
So how do you differentiate if it’s really
something interesting or: “Oh, one of these damn deuterium nuclides, again!”
Michael: You don’t get wrong isotopes because you just use a mass spectrometer
to sort them out. You have a magnetic field. You know how large it is. And
the protons will go and land – let’s say – 2 micrometers next to the deuterons,
and they just sort them out.
Question: I have 2 questions. One is:
I guess you mentioned that basically once the experiment
runs at speed of light you just put more energy into it.
But what is actually the meaning of the energy that you put into it?
What does it change in the experiment? Like the Higgs was found
at a particular electron volt…
Michael: Yeah, it was found at 128 GeV. Well,
it’s more of a philosophical question. There is a way of interpreting
the equations of special relativity where you say that, when you don’t increase
the velocity you increase the mass. But that’s just a way of looking at it.
It’s more precise and it’s more simple to say: you raise the energy.
And at some low energies that means that you raise the velocity.
And at some high energies it means the velocity doesn’t change anymore.
But overall you add more energy. It’s one of the weird effects
of special relativity and there is no very nice explanation.
Question: Let’s assume there is an asteroid pointing to earth.
*Michael laughs* Could you in theory point this thing
on the asteroid and destroy it, or would it be too weak?
*laughter*
*applause*
Michael: I’m going to help you out. Because it wouldn’t actually work
because between the accelerator and the asteroid there’s the earth atmosphere.
And that would stop all the particles. But even if there were no atmosphere:
no, it would be much too weak. Well,
you’d have to keep it up for a long time at least. There was this one accident
at the HERA accelerator where the beam actually went off its ideal path
and it went some 2 or 3 cm next to where it should be.
And it hit a block of lead – just, you know, the heavy metal lead –
and the beam shot into this lead thing and the entire beam,
which was a couple of Billions of protons, was deposited into this lead
and some kilograms of lead evaporated within microseconds
and there was a hole like pushed by a pencil through these lead blocks.
So, yeah, it does break stuff apart. But even if you managed to hit the asteroid
you would make a very small hole. But you wouldn’t destroy it.
It would be a nice-looking asteroid then. *laughter*
Question: Before you turned on the LHC the popular media was very worried
that you guys were going to create any black holes.
Did you actually see any black holes passing by? *Michael laughs*
Michael: Well, there may have been some, but they were small, and
they were insignificant. The interesting thing is… sorry, I’m going to recap, yeah.
The interesting thing is that whatever we can do with the LHC – where
we make particles have large energies and then collide – is already happening!
Because out in space there is black holes with enormous magnetic fields
and electrical fields. And these black holes are able to accelerate
electrons to energies much, much higher than anything we can produce
in any accelerator. The LHC looks like a children’s toy
in comparison to the energies that a black hole acceleration can reach. And
the particles which are accelerated in these black holes hit earth all the time.
Not a lot, let’s say one of these super-energetic particles they come around
about once a year for every square kilometer of earth.
But still, they’ve been hitting us for Millions of years.
And if a high-energy particle collision of this sort were able
to produce a black hole that swallows up the earth it would be gone by now.
So: won’t happen. *applause*
Question: Maybe more interesting for this crowd: you talked about
the selection process of the events.
So I guess these parameters are also tweaked to kind of
narrow down like what a proper selection procedure.
Is there any kind of machine learning done on this to optimize?
Michael: Not that I know of. But there is a process which is called ‘Minimum Bias
Data Collection’. Where you actually bypass all the triggers
and you select a very small portion of events without any bias.
You just tell the trigger: “Take every 100 Billionth event”
and you just pass it through no matter what you think. Even if you think
it’s not interesting, pass it through. This goes into a pool of Minimum Bias Data
and these are analyzed especially in order to see the actual trigger criteria
are working well. So yeah, there is some tweaking. And
even for old machines we have data collected
and sometimes we didn’t know what we were looking for. And some 20 years later
some guy comes up and says: “Well, we had this one accelerator way back.
There may have been this and that reaction. Which we just theorize about.
So let’s look at the old data and see if we see anything of that in there
now, because it’s limited because it goes through all the filters”.
You can’t do this all the time with great success. But sometimes,
in very old data you find new discoveries. Because back then
people weren’t thinking about looking for what we are looking now.
Question: I always asked myself about repeatability of those experiments.
Seeing as the LHC is the biggest one around there, so there’s no one out there
who can actually repeat the experiment. So how do we know
that they actually exist, those particles?
Michael: That’s a very good question. I told you that there is 2 main
large experiments. Which is the CMS experiment and the ATLAS experiment.
Now these both sit at the same ring. They have some 10 km between them
because they’re on opposite ends of the ring. But still, obviously,
they’re on the same machine. But these 2 groups, the ATLAS and the CMS experiment,
operate completely separately. It’s not the same people, not the same hardware,
not the same triggers, not even the same designs.
They build everything up from scratch, separate from each other. And
it’s actually funny because when you look at a conference and here is CMS
presenting their results and here is ATLAS presenting their results,
they pretend like the other experiment is not even there.
And that’s the point of it: they’re not angry at each other. It must be
2 separate experiments because obviously you can’t build a second accelerator.
So you try to have redundancy in order
for one experiment to confirm what the other finds.
Herald: Okay. It’s midnight and we’re out of time.
So please thank our awesome speaker! *applause*
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