Translator: Maria Boura Reviewer: Denise RQ
My name is James, and I'm an experimental particle physicist.
That means that I get together
with a few thousands of my closest colleagues,
and we take the smallest possible things in nature,
and we accelerate them up to the highest possible speeds,
and we slam them into each other, to see what happens.
The reason we do this is because we're looking for things
that mankind has never seen before, at the very fundamental physical scales.
I'm here to share with you what something called a dark photon is,
but to do so, I need to set a little bit of historical context
- that was an unsuccessful clip; there we go -
I need to set a little bit of historical context.
To flashback to 1894,
when the eminent physicist Albert Michelson said the following --
he stood in front of an audience at the University of Chicago,
and he said,
"The more important fundamental laws and facts of physical science
have all been discovered, and these are now so firmly established,
the possibility of their ever being supplanted
in consequence of new discoveries is exceedingly remote."
A couple of decades previous, electricity and magnetism had been shown
to be two parts of the same force: electromagnetism.
This was considered such a gigantic breakthrough
there was a prevalent attitude amongst a lot of physicists
that this was pretty much it, and the rest were some minor details.
Sometimes I wonder if people like this
- Michelson didn't need to; he was about to get the Nobel -
say things like this with such definitive authority
just so they cab ensure their place in history as grand historical straight men,
so we can look back 100 years later and marvel
at how completely wrong they were.
A different Albert -
1905, Special Relativity; 1915, General Relativity,
and then, in the early decades of the 1900s, Quantum Mechanics.
Then the need to unify quantum mechanics with relativity
led to Quantum Field Theory.
Any one of these things by themselves
required a complete paradigm shift in our understanding of nature
at its very basic scales
that it's hard to imagine how Michelson could have been more wrong
in his pronouncement.
This quantum field theory that we came up with
was the language that allowed us to understand --
had this amazing interplay between theory and experiment
in physics in the 20th century,
that culminated in this thing that we call the Standard Model of particle physics.
It's essentially a list
of all the fundamental particles we know and the ways they interact.
It's nicely summarized in this diagram from the movie Particle Fever.
It doesn't have any significance beyond it,
it's a just a really nice way to put it down on a slide.
Essentially, you have two basic classes of particles,
you have the outer ring which is matter particles,
and they are quarks and the electron, and you have this inner ring
which is populated by the so-called force carrying particles, or gauge bosons.
It's an almost shockingly successful experimental theory --
so much so, that it earns that name, the "Standard Model,"
capital S, capital M.
Maybe a few ears may have perked up when I said that word "boson."
Anyone here heard of the Higgs boson?
For those of you who haven't heard of the Higgs boson,
perhaps you know it by its more sensationalist name,
the 'Kanye particle' -- sorry, the God particle.
(Laughter)
Physicists don't care much for that name
because it obscures the truly awesome nature of this particle,
but, nonetheless, in July of 2012, two of the collaborations,
two of the experiments at the Large Hadron Collider
- ATLAS, the one that I work on - and CMS, a complimentary experiment
at the Large Hadron Collider at CERN, near Geneva,
announced the discovery of this brand new particle,
the Higgs boson.
Its discovery is an amazing triumph,
it was the culmination of decades of work by thousands of physicists,
and it really was a fantastic triumph.
It was the last remaining piece of the Standard Model puzzle
to be plugged in.
So, you might think
that once this was plugged in, we all turned to each other and said,
"Wow, isn't this great? Finally.
120 years later, we've finally reached Michelson's dream of end of physics."
Is this what we said to each other?
Absolutely not.
We know for a fact the Standard Model is not the complete picture.
It is amazingly successful,
but it cannot be the complete picture of nature at its fundamental scales.
Also we've learned our historical lesson not to say things like that.
We also know this interplay between theory and experiment,
and these amazing paradigm shifts that happen in the 20th century,
also went over to other related fields of physics,
so we know for a fact the Standard Model is incomplete.
But it's so experimentally successful
that any new thing that comes has to build upon it,
it still has to be true at the end of the day.
The barrier that we face now in particle physics
is not the same one that was in the 1890s,
the one that Michelson s positing,
the question as to whether we should even bother
to keep searching for a new physics.
The barrier we face now is slightly different,
it's the question of how to keep searching for a new physics.
I'll give you a flavor of what I mean by showing you an aerial view
of the Large Hadron Collider.
So, this is a 27-km round tunnel, on the border of France and Switzerland,
100 meters under the ground.
But to make it more local,
this is what it would look like around Lower Manhattan.
So, at the LHC, we collide two beams of protons
at the highest energies mankind has ever used
in collider experiments: 13 teraelectronvolts.
So, this gives you a sense of the scale.
We have these two beams,
and the place where we focus them together and collide them,
we've built a big detector there.
So, this gives you a sense of the scope of some of these experiments.
To get up to higher energies where the new revolutionary physics
may be hiding, we need to build bigger machines.
So, this is next generation of machines;
they are planned to be something like a 100-km round
and collide particles at about 100 TeV.
Is that it? Is that the only direction we can go?
It absolutely has to happen, this is being planned.
This is absolutely the thing that needs to be done,
but is that the only direction we can go?
The answer is no.
This is where the 'dark photon' comes in,
this gamma is the symbol we often use for a photon,
and the 'D' just means dark.
So this is where a dark photon comes in.
But before we talk about the dark photon,
we should familiarize ourselves with the regular photon.
The regular photon you know quite well.
As you may have heard, depending on how you look at it,
light behaves as either a wave or a particle;
it has the properties of both.
The photon is the particle manifestation of light.
And so, when your camera takes a photo
- please, if you have cameras, take them out
and take a photo of me posing like this;
There you go, someone is taking a photo -
your camera with that photo is collecting a few billion photons.
When you're standing outside at noon, the square meter around you
is absorbing something like 1,000 billion billion photons a second.
Very bright and coherent photons make up lasers.
Some of the dimmest photons we know
are those that have been traveling for 14 billion years
from a fraction of a second after the Big Bang,
and are just now arriving to be collected and studied by astronomers.
The most energetic photons we know of
come from astrophysical sources like hyper-novae.
And the least energetic that we know of
are the ones that carry the soft rock hits of the 70s via AM radio.
So you're very familiar with regular photons;
or at least you think you are, because, as particle physicists,
we have a slightly different way of thinking about photons.
In addition to all these things,
it's also a special type of force-carrying particle.
To give you an appreciation of what I mean by that,
I have to take a brief foray
into collider physics and quantum field theory,
but it won't be painful,
because you already know what a collision is.
You already know what macroscopic collisions are.
If you take two billiard balls, they smack together and fly apart; great.
In physics, we like to keep track of things as they evolve in time.
So we're going to look at this same collision
on a billiard table, as it evolves in time.
So, at some point in time, the balls are spaced far apart,
they get closer together, they smack together and fly apart.
Then we draw lines to this,
and this gives us a trajectory of this collision
as it happened in space-time.
But these billiard balls are moving slow, and they're pretty large,
so if we want to go to the very smallest fundamental scales,
the smallest uncuttable particle scales,
and if we want to go up to the highest speeds possible,
- almost the speed of light - then the rules change
because quantum field theory takes over,
and you can't think of collisions in the same way.
So, instead of billiard balls,
let's collide another particle you know quite well,
and that we use quite a bit in collider physics, the electron.
The electron and its anti-particle, the positron, as time goes on,
get closer together, something happens,
and then an electron and a positron come out of the other side.
That 'something' can be a collision.
it can take a few forms,
one of them is they can pass right by each other,
that's not very interesting.
One thing they can do
is they can actually annihilate and create a new particle,
and according to the very strict rules of quantum field theory,
this particle can then split into other particles.
Another thing they can do is they can get close enough to each other
so they can feel each other, and by 'feel' I mean they feel a force.
Much like one planet can feel the gravitational force of another one
as they go around each other,
this electron and positron can feel the electromagnetic force between them.
In the particle level, this means they're exchanging another type of particle,
a special type of particle, a force-carrying particle.
For electromagnetism, this is the photon.
Each one of the forces has a force-carrying particle
associated with it.
But is that all of them? No.
This development of the Standard Model in the 20th century led us to understand
there were other fundamental forces that we had missed before,
and one of them is the strong force.
This is the one that holds quarks bounded to protons and neutrons
and hence, keeps your body from flying apart,
and keeps the sidewalk from dissolving underneath your feet,
and its force carrier is called the gluon -- appropriate name.
One of the other forces we found is the weak force,
You probably best know is responsible for radioactive decay,
and it's an oddball,
because it has three different force-carrying particles
associated with it, the W plus, W minus, and Z bosons.
But is that all of them?
We don't know.
There actually could be other fundamental forces out there,
but, because so many people have been working on this for so long,
so many clever people, much cleverer than I,
so many experiments performed,
we have narrowed down the possible way that we could have missed new forces,
and hence, new force-carrying particles
to a very few.
One way is if the particles are super massive.
What I mean by mass is not the typical
massive bouncer at a bar type colloquial sense.
Mass, at the particle level, is an intrinsic property of a particle.
If you remember, E equals m times c squared,
this indicates an equivalence between energy and mass,
and so we often express the masses of fundamental particles
in terms of energy.
So, you'll see this GeV here.
So the proton - it's not a force carrier, but a particle you it know quite well -
has a mass of about one GeV;
the newly discovered Higgs particle has a mass of about 125 times that,
and then, the most massive fundamental particle we know of
is the top quark, and it has a mass of about 172 times that;
for reference, the W and Z force carriers have masses closer to the Higgs,
and the electron has a super small mass.
One way we could have missed new force-carrying particles
is if they are just super massive.
We haven't built a large enough collider with enough energy to get up there.
We have to build a larger collider
to get to higher energies and find new particles --
and that costs money.
That's not where you go and get the money.
It costs money to get up there.
And so, is that the only way? No.
One of the very, very few ways we could have missed physical forces
is if they have a very small mass, smaller than the mass of the proton,
and if they interact very, very slightly with our everyday electromagnetic world,
our everyday world.
You might think doesn't make any sense because I just said
that to get to the higher masses we need more money,
so why haven't we covered the broke end of the spectrum already?
The answer comes from that very very slightly interacting thing.
What I mean by that is that the most important force,
in terms of physics,
in terms of experiment is electromagnetism.
Because it's the only one we can do anything directly with,
in terms of instrumentation.
If some new physics effect doesn't eventually result
in electromagnetic interaction, we won't know about it.
So, if nature had, for instance, conspired to hide some new force from us,
a new force-carrying particle that had a very, very small mass,
and it also interacted really slightly with our everyday electromagnetic world,
so slightly we've never seen it before,
it would have escaped the detection of all the previous experiments.
This, by contrast to the regular photon, we call it the dark photon.
So this would be a new force-carrier.
You might be thinking, "That's all well, but this is like a stab in the dark.
Do you have any external motivation for this beyond just, 'It might exist.'?
A lot of things 'might' exist."
That's a good question, I like the way you think.
Yes, there is a lot of motivation.
I will to focus on this one thing.
It's because this 'dark' actually has something to do with dark matter.
We all like to look at Hubble photographs;
so take your favorite spiral galaxy, count up all the stuff you can see.
That gives you the amount of luminous matter in the galaxy.
Pull out your favorite textbook on gravity,
and put that number into the equation
that tells you how fast a star should be moving
as a function of how far away from the center of the galaxy it is.
You'll get this prediction.
Now go to your local astronomer
and get her to measure those speeds of those particles;
they're totally off from the prediction.
This means there has to be more stuff than what we can see,
and if it's not luminous, it's dark.
That's where this word 'dark matter' comes from.
Now go to your local astrophysicist and ask her to show you
the energy budget of the universe
- that's the breakdown of all the stuff that exists in the universe -
she'll tell there's over five times as much dark matter
as there is regular matter.
Now go to your local particle theorist and ask her what dark matter is;
she'll tell you, "We don't know. We have a lot of ideas though."
On the experimental end,
there are a lot of people looking for dark matter directly,
not necessarily collider physicists like me,
but other direct detection dark matter experiments,
and they have seen some weird things.
I don't have time to go into it here, but I'll just tell you,
you get the dark matter experimentalists into the room with the particle theorists,
you get them to talk together and compare notes and work it out.
Eventually, the particle theorist will turn to me, and she'll say,
"You know, if there were a new force-carrying particle
that had a mass a little less than the mass of the proton,
and if it interacted really, really slightly with our everyday world,
this would explain a lot of the dark matter craziness
the dark matter experimentalists are seeing."
And then, as a collider physicist, she'll go, "That's a dark photon.
I might be able to create that in a collider experiment."
Partially because of its relationship to dark matter,
partially for other reasons I don't have time to go into,
and partially just because this field of dark photon searching
has heated up quite a bit in the last few years.
This is the state of things as of 2008,
and that white space - completely unexplored.
That's exactly where a dark photon should live
if it's going to help explain these dark matter anomalies.
Flash forward to today,
to six years later, this is the state of things.
Those shaded regions are exclusions, places where the dark photon can't live,
and the open lines are planned experiments to take place in the next years.
So, what changed?
Some theorists, in fact, N. Toro, P. Schuster, R. Essig, and J. Bjorken
pointed out that we can indeed look in the space for a dark photon,
and we can do so on on the cheap.
They made some calculations and made the observation
that we can use existing fixed-target facilities
built for other particle physics purposes to look for this dark photon.
So this is where the experimentalists rub our hands together and say,
"Yes, this is good, we can do this."
These experiments are everywhere;
they're in Russia, they're in Germany, they're in Italy, they're in California.
There are three experiments taking place
that are planned to happen at a facility in Virginia
called Thomas Jefferson National Accelerator Facility, or Jefferson Lab.
I'm going to focus briefly on APEX because that's the one that I work on.
So, Jefferson Lab, by contrast to the LHC,
where we collide two beams of protons at really high energies, at TeV.
At Jefferson Lab, it's one beam of electrons; you get up to the GeV scale.
That beam of electrons goes into an experimental hall.
Once it goes into there - this is what it looks like for APEX -
the beam comes in over my head, goes into this targeting closure
where you hold a chunk of metal fixed.
So what happens when the electrons encounter the chunk of metal?
I don't know, let's look at the diagram.
A chunk of metal is a dense collection of atoms, of large nuclei,
and when an electron comes in, it get close to the nucleus,
they feel each other, and they exchange a photon.
As the electron bends around this nucleus, it spits off another photon,
which splits into a couple of particles that we can detect.
Then we detect those particles,
we calculate the mass of the particle they came from, and then we bend those up.
Overwhelmingly, it's just a background noise shape,
a smooth background shape.
But occasionally, if a dark photon exists, you'll create a dark photon instead;
very, very rarely - which has an actual mass.
So, you're looking for a tiny bump on top of this smooth background thing.
We did this for the APEX experiment, in July of 2010,
where we held the test run,
and in an ideal world, we would have seen something like this.
You don't have to work too hard to convince someone it looks like a bump.
But that's a simulation, what we saw was this;
if you think it doesn't look like anything, you're right.
That was just a background spectrum.
The thing we'd been looking for would be a much subtler effect
- closer to the one at the right, than the one on the left.
We didn't see a dark photon, otherwise, you would have heard about it,
but we did rule out a small space where one can live.
We proved the feasibility of the experiment
which will happen sometime in the next couple of years.
Like I said, other experiments are looking too so the dark photon race is on.
Finally, what is a dark photon?
A dark photon would be a new force-carrying particle,
much like the regular photon,
is the force-carrying particle of electromagnetism.
It's dark because it interacts so slightly with our everyday electromagnetic world
that it's effectively been hidden from all previous experiments,
and because it could help explain some of the odd astrophysical results
related to dark matter.
Because it interacts so slightly with our everyday world,
we need to perform specialized experiments but at existing facilities
to look for a tiny dark photon signal on top of a huge background.
It's potentially revolutionary
because it would be the first unambiguous evidence
of a fundamental particle not predicted by the widely successful Standard Model
that we're now in love.
I know the crowd here a little bit, I've just spent the whole day here.
What would this do for you?
What would you get if we found a dark photon?
Would you soon be able to take dark photons with your phone?
Possibly -- probably not.
I'm the wrong person to ask,
because I'm an experimentalist not a product developmentalist.
I look for a dark photon because I'm interested in
the fundamental open questions in physics, like what is dark matter.
More importantly, I look for a dark photon because I look at this plot, and I think,
"I wonder what's there?"
Much like in hiking, you look at a hill and think,
"I wonder what could be in that next hidden valley?"
We push the limits of human knowledge because we're curious.
That being said, I do know the pushing of the limits
of human knowledge for curiosity's sake
by my particle physics forebears has led eventually to such things
as CT scans, PET scans, major advances in RF power, the World Wide Web.
I've no idea what the discovery of a dark photon will mean
beyond expanding our understanding of the universe.
The rest of it will be up to you.
At the end of the day, though, is it possible that every single one
of these experiments could come up completely empty?
Yes, absolutely.
I mean, these are experiments,
and in particle physics there is no such thing as failure.
Even if you don't find something,
you've still gained an important piece of information about nature.
The only failure is to stop searching.
Thank you.
(Applause)