-
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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