(new age music playing)
- It was a difficult task
thinking about what to talk about
because they stressed we
should think about what
could the next breakthrough
in the next five years be.
And I thought about it, and then I thought
yeah I wanna speak after the LHC Talk
because that's where I
think we're gonna have
a breakthrough in the next five years.
Oops.
Well, so much for the breakthrough.
(audience chuckles)
New Searches for Dark Matter
in Particle Collisions.
I'm gonna predict a very very
definite idea for we
might discover at the LHC.
What we might discover is
not just a new particle
but a new particle whose decay
is going to tell us what the
origin of dark matter was
and how it was produced
in the early universe.
That's a pretty definite prediction
and in order to understand and
to evaluate that prediction
you need to know where
we are in what I call
The Cycle, The Discovery Cycle.
It's not always true but it's often true
that discoveries in
physics go through a cycle.
Where the first, it starts
with asking questions,
basic curiosity.
Framing how to get experimental
sets setup to get your data.
Once you've got the data
you look at the data,
you look for patterns,
and patterns emerge,
and if you're lucky a real
theory comes out of that.
And then on to the next cycle.
So it often, but not always, but it often
goes in these cycles.
A quicker question then is what is the
time scale for these cycle?
And that depends on the sort of phenomenon
that you're interested in.
If you're in some very
very restricted phenomenon
it could happen really quickly.
But if you're interested
in the really big stuff,
the really big cycles,
you know, the time scale
can be a century.
It's not nice to say that but it's true.
And so I though it would be fun
to look at a half century
of particle physics
in the era of '27 to '77.
Roughly, the dates are a bit fuzzy.
But the questions appeared right
at the late 20s here, when we needed
to understand what was the
description of particles
and their interactions.
Such that they could be consistent
with the recently
discovered quantum mechanics
and also special relativity.
That was the driving thing one had to do.
And it took a 50 year cycle to get there.
The data poured in, in
the 30s, 40s, 50s, 60s,
every decade all the way up into the 70s.
The data was really pouring in.
Starting with neutrons, muons, began with,
with particular particles but ended
with symmetries and interactions.
Along the way there were,
plenty of patterns emerged.
Here's a pattern from the
early 60s of the kaons
and pions forming this
hexagonal structure.
It's pretty obvious there's
a pattern behind there.
And that pattern was what
led people to introduce
quarks as the constituents
of all of hadrons.
Theory was much harder.
The first part of the
theory came in the late 40s,
the quantum version of electromagnetism.
Here there's an electron
interacting with a photon.
And it was a real struggle.
The 30s and the early 40s
were a real real struggle.
And the struggle continued
in the 50s and 60s.
And the real theory didn't emerge until
the 70s, the theory of
the strong interactions
and the electroweak unification.
So this is the sort of pattern,
and it can take half a century or so,
but the key point to
take away from this is
that by the late 70s there was a theory.
There was a theory of particle physics.
And it's immensely important, to actually
the difference between having a theory
or not having a theory.
So, from the late 70s we had a theory.
Here's the Standard Model.
The theory, you can think
of as a bunch of particles.
The up down and electrons,
the familiar ones
of ordinary matter.
But there's three generations.
There's a pattern for you right there
that we don't understand.
In fact, I don't understand
the pattern of these symbols.
(audience laughs)
I have to construct my own
theory for that, I think.
But anyway, these
particles are going along,
as they're traveling they
interact with these force fields.
That's the photon, the Z the W.
These are the particles
of the Standard Model,
the gluons and finally the Higgs boson
of just a few years ago.
And the interactions are parameterized
if you go deep down into this vertex,
there's a whole bunch of quantum numbers,
and symmetries, and 18 free parameters
that have to be obtained from data.
But this is what we call a Standard Model.
The electroweak unification part of it
was finally deemed to be correct in '77
and it only took two years for these
gentlemen to win the Nobel Prize.
Precisely that year, one of them, Glashow,
a very bold fellow
decided that he would make
predictions about the
future of particle physics.
And he says, "We live
in interesting times.
"We have a theory at
last, of strong, weak,
"and electromagnetic interactions.
"Many new accelerators are abuilding
to test our theory."
So, let me tell you a little bit about
the predictions that he
made back then in 1979.
The first predictions had to do with
the matter and the force fields.
And this hasn't come out at
all which is a real shame
so I'll have to just tell you about
what's supposed to be in here.
This was the late 20th century version
of what happened to the
Mendeleev table in the 1860s.
He came up with the Mendeleev table and
you had to discover the
elements that fit in the blanks.
And here, the blanks
were the top cork here.
And here the blanks were the W and the Z.
And here the blanks were the Higgs boson
In a sense one might say
this was no big deal,
making these predictions, because once you
knew the theory they pretty
much had to be there.
That was true for the W and the Z.
It wasn't quite true for the Higgs,
could've been something else.
But you get the picture.
And in fact, these particles of course,
the discovery took ages.
But they came.
But next we come to a bolder statement,
and this is very bold given the
exclamations marks in there.
(chuckles)
Was proton decay.
So what this should be is the proton
decaying to a positron on a Pi zero.
And he made this prediction
for a very very good reasons.
And the experimentalists
ran out and looked for it.
And they pushed the limit on the lifetime
by six orders of magnitude.
And so far no success.
So even though what looked like the best
prediction for the next
era, the next cycle,
beyond the Standard Model cycle,
even the best predictions go awry.
And that's what we have to expect
once we're swimming without
the real theory anymore.
So, what can we do with our theory?
The good news is we can calculate stuff
in particles collisions.
The reasons why early on I thought that
they'ed discovered the Higgs boson,
even before they were willing
to say they'ed discovered it,
was we can compute how many
events they should get.
And it worked out right.
So it was just very very plausible
that the discovery was there.
The bad news is that we can't compute
what the universe contains.
And if we had a real
theory we should, we should
be able to compute what
the universe contains.
Well, we can get the photons right.
That's the big success.
But when it comes to electrons and protons
we're really really way off.
We've got theories but
we just can't do it.
Worse, we don't know what most of the
matter in the universe is.
The so called Dark Matter makes up
most of the matter in the universe.
And whatever we think
about it we know that its
not a particle of the Standard Model.
So, here are my predictions.
My prediction is that next breakthrough
in Particle Physics will in data.
It will not be in patterns and theory.
Because even though this beyond
the Standard Model cycle has been going
for nearly 40 years now, I don't think
we're at the level of being to extract
the next level of theory.
I think we need this continuum cycle
of more and more data.
We've had some hints,
we've had dark energy,
we've had neutrinos,
and they're fantastic.
We need more, okay.
So that's the first thing.
The second thing is that I'm gonna guess
that the next breakthrough will tell us
something about this Dark Matter.
That I'm much much less sure about.
And, this I'm gonna say is that
I don't think it's gonna be our favorite
candidate of Dark Matter.
I think it'll be some huge big surprise.
What probabilities do I subscribe to this?
(audience laughs)
Yeah, well I really think
it's gonna come from data.
Is it gonna be Dark Matter?
I really don't have a clue.
But I think it will.
If it's Dark Matter is it gonna be
some non-standard theory?
Yeah, I think probably so.
But I'm gonna go further and say that
in the next five years there's a very
specific new long-lived particle
that will be discovered at the LHC.
And that its decays will tell us exactly
how the Dark Matter was
made in the early universe.
I really don't dare to
predict the probability
that this is right.
(audience laughs)
Okay, so the early universe.
So the early universe is a hot gas
we've got particle colliding.
All the red dots here are
Standard Model particles.
Everything's in thermal equilibrium
at high temperature tea.
The great thing about thermal equilibrium
is that it races initial conditions
and you can hope to do a calculation
of somewhat low temperatures.
What is the Dark Matter?
What is the reaction that determines
the Dark Matter abundance?
These are the key questions
that we need to answer.
And I'm gonna talk about
two very general mechanisms.
One's called Freeze-Out
and probably the majority
of the people in the
audience know all about it.
But I'm gonna go through it anyway
because I want to contrast
it with Freeze-In,
which you probably don't
know so much about.
Freeze-Out, we've had a 30 year job
of trying to find direct detection
of the Freeze-Out particle.
And maybe it will.
That's also a great possibility
for the next five years.
But I'm gonna go for
this freezing candidate.
Okay, so Freeze-Out, at high temperatures
the Standard Model and dark paricles
are scarring off each other.
They're in thermal equilibrium,
below the temperature
of the Dark Matter mass.
The Dark Matter particles
in blue annihilate away.
And the Dark Matter abundance is
determined by this cross section.
Here's a cartoon of how it goes.
I'm gonna give you some time sequences
or temperature sequences.
Here's the Dark Matter
on the Standard Model
particles in thermal equilibrium.
On the vertical axes here is the
ratio of the number of
Dark Matter particles
to the number of Standard Model particles.
And on the horizontal axes is temperature.
And we're at very high temperature.
And at very high temperature these
are comparable number densities.
Now as the universe
evolves, the first thing
that happens is that it's expanding
the numbers just dilute.
The ratio of the number density
of Darks to Standards stays constant.
Notice the temperatures dropping.
But as the temperature drops down towards
the Dark Matter mass,
we're not quite there yet,
but we're nearly there.
Now we go to temperatures
below the Dark Matter mass.
And the number of Dark
Matter particles starts to
precipitously drop because
of this annihilation process.
At some point, the Dark Matter particles,
the blue particles are so dilute
they can't find each other.
And at that point we
say you Freeze-Out and
you get a Freeze-Out relic abundance.
And the great thing about this is that
we can compute the relic abundance
in terms of that
annihilation cross section.
And Vallarta showed a plot
very similar to this one.
Here's a cross section of the function
of the mass of the Dark Matter particle.
And the direct detection searches were
Galactic Dark Matter
particles scatters of a
Standard Model particles
of these curves here.
And here's some CMS searches, these
horizontal ones from LHC.
So let me go on to Freeze-In.
Freeze-In is exactly the opposite
of what I've described for Freeze-Out.
In Freeze-In, the Dark Matter particles
interact with Standard Model particles
only very very weakly.
At high temperature there was no
Dark Matter particles around.
There was just no Dark Matter at all
because there's been no time for them to
be produced by collisions
of Standard Model particles.
However, occasionally
Dark Matter particles
are produced, and they're produced
because there's two new particles.
Not only is there a Dark Matter particle,
but there's a Mother
particle that I call M.
And the Dark Matter particles comes
from the decay of a Mother particle.
There could be hundreds
of models for this,
as well as there a hundred
models for Freeze-Out.
But I want to give you
the generic mechanism.
So the Dark Matter abundance is determined
by the life time, that's
what this weird thing means,
of this Mother particle decaying.
So let's go through a similar sequence
of temperature slides to show
how the Dark Matter appears.
At very high temperatures,
we're starting off with very
very high temperature, there
is no Dark Matter particles.
There's no blue particles there.
There's Mother particles and there's
Standard Model particles.
But as the universe evolves
and the temperature drops,
a few Mother particles decay and they make
the Dark Matter particles here and here.
And you see that the number density
of Dark Matter particles is growing.
It's growing in towards equilibrium.
Here we go.
There's dilution but we're getting a
relatively more and more
Dark Matter particles.
Until eventually the
production of them stops.
And the reason it stops is that the
Mother particles have all annihilated away
and there's no more Mother
particles to decay to them.
So this is a totally
different generic mechanism
for making Dark Matter
in the early universe.
Here's Freeze-Out and Freeze-In compared
as a function of temperature.
The Freeze-Out you start
with a huge abundance
and it drops off.
The Freeze-In you start with none
and it comes in and levels off.
Okay, we can't understand any of that
so we better not talk about it.
So the key question is,
can we measure this at the LHC?
Can we actually make a
Mother particle and watch
it decay to Dark plus Standard at the LHC?
And if so, could we measure enough
in order to be able to reconstruct
what happened in the early universe?
So, here we make a Mother
particle at the LHC.
It goes out through the detector.
Maybe it stops in the Hadron Calorimeter.
Maybe it lives a second.
Maybe it lives a month.
But eventually it decays.
We can't see the Dark Matter
particle coming out here.
But we can see the Standard
Model stuff coming out here.
The possibility of Long
Lived Stop particles
has been searched for,
and the exciting thing
is that there are limits
from CMS and from ATLAS
that show that you can put
limits on Stop particles.
Over 12 orders of magnitude in a lifetime.
So, you've really got a very very
powerful probe for looking for long-lived
new massive particles at the LHC.
It's not just STOP particles.
It could be that the particles
instead of going all the way out here
before they decay, it could be that they
decay much much earlier on.
I don't know what happened here.
But they decay much earlier on
and give you leptons or jets.
Or they may decay right close to the
interactions vertex and you may get
what are called displaced vertices.
But each of these three
detection mechanisms
gives a lifetime or a distance,
the distance lifetime between less than
a centimeter all the way up to kilometers.
And we can test, we can experimentally
test these lifetimes.
So everything that's colored here
we can actually see.
So what is the prediction from Freeze-In?
Here's the key slide, and I've only got,
I think another one after this.
Here's the key slide which gives you
the prediction for,
lets suppose the Mother
particle weighs 300 GeVs, so we can
make it at LHC, then this red curve here
gives you the predicted lifetime
as a function of the Dark Matter mass
in order that it gives you the right
Dark Matter abundance.
Okay.
And you can see that if
the mass is a few KeV
the lifetime is short, and we'll
see it as a displaced vertex.
If the mass of the Dark Matter particle
is 100 GeV, it lives
longer and we'll see it
as a stopped particle
that lives about a month
or so in the detector before it decays.
But we can see all of that, okay.
This formula which is totally unreadable,
tells you how to relate the,
how these quantities are related.
But the key thing is
that if you can measure
the lifetime of the Mother particle,
the mass of the Mother particle,
and the missing mass,
the Dark Matter particle,
then you can compute how much Dark Matter
there should be in the universe.
And if it comes out right you've resolved
the puzzle of the origin of Dark Matter
purely by looking at LHC physics.
So I started off talking about
the half century from
the Quantum Mechanics
to the Standard Model, and what a great
half century it was.
We're now into the next cycle
A beyond the Standard Model cycle.
We're about 40 years into it.
I think that we're still at the level
of framing the questions, thinking about
the puzzles, and looking for new data
that is gonna guide us.
That's not to say that we
haven't made progress in theory.
Of course we've made progress in theory.
We've got many many ideas over these
decades that are really
a tremendous progress.
But we don't know how
it all fits together.
We don't know which theory's right.
We may not even have anywhere
near the right theory.
So, the questions.
Here are some of the questions.
Quantum theory of gravity?
Why is the Standard Model what it is?
What's the origin of mass hierarchies?
What's the origin of the small numbers?
Well, I've placed my bets on
the contents of the universe.
I think the next breakthough's gonna be,
we're gonna be able to figure out
what the contents of the universe is.
Thanks very much.
(audience clapping)
(new age music playing)
