The graviton is the particle we use to describe
what would be a quantum theory of gravity
and there's a long standing issue in fundamental
physics which is that we have no idea how
to link quantum mechanics which explains our
understanding of the small scale physics to
gravity which is a very -- all of the experimental
observations we have about gravity are of
large scale phenomena in the universe.
So finding the graviton would mean gluing
together those two very different scales and
those two very fundamental questions of physics.
It's quite a difficult question because as
I was saying, there is no real clue, real
hint of how to make a quantum counterpart
of the gravitational theory.
It's a spin-2 particle.
So those are very specific properties.
The spin-2 one is a very particular property
in the sense that we don't have any other
elementary particle that is spin-2.
So it makes it very particular -- very specific.
There is one big remaining piece of the puzzle
that is not there in the standard model of
particle physics which is gravity.
One of these questions, one of these problems
is what we call the hierarchy problem of the
standard model.
This hierarchy problem can be boiled down
essentially to one simple question.
So there is a huge 16 orders of magnitude
between the scale at which we need to have
the weak interaction to explain processes
to explain physics and the scale with which
we need gravity, quantum gravity to explain
physics.
So there are 16 orders of magnitude that are
not understood what so ever.
Nobody can answer the question of why are
those two scales so different.
We have models that are trying to find to
explain the presence of gravity at lower energy
scales - at energy scales we are able to probe
at the LHC.
The most popular one is of course the one
where you introduce extra dimensions.
So the basic principle here is that gravity
-- the scale at which gravity appears is not
so far away from the other fundamental interactions
of the standard model of particle physics.
But somehow it's diluted by the presence of
extra dimensions.
So gravity is allowed to go in a wider space
and therefore what we see in our boring standard
4D space is only a representation of gravity
and therefore it appears weaker to us.
Extra dimension models are trying to tackle
these problems straight away, like just trying
to address them directly by saying, by claiming
that no actually the two scales are way closer
to each other but the gravity appears weaker
in our 4D timespace because it's diluted by
the presence of extra dimensions.
We know that it's hard - we can hardly fit
any extra dimensions that are very large in
the context of including the weak interaction
in those extra dimensions.
So given this fact, given how well we know
the electroweak interaction, additional dimensions
- space dimensions will either have to be
very very tiny, very small, so it's a modification
another view of the space which includes very
tiny extra dimensions or you can make these
extra dimensions a bit larger by allowing
only the gravitational field that you didn't
observe yet to go to those.
So in most of those extra dimension models
what we happen to do is we add an extra dimension
to the current space, so typically it's usually
what we call a "warped" - sorry "wrapped"
extra dimension which is essentially a closed
one so a bit more complex topology than what
we are used to work with.
And we confine, we forbid all of the standard
model fields to go into those extra dimensions.
The only one that is allowed is the gravity
- the graviton field.
So if we find the graviton at the LHC, what
would mean for physics?
Well... it's huge, right?
It would be..
It would be a big thing.
And I guess by finding the graviton at the
LHC what you mean is finding a new fundamental
particle that would be a spin-2 particle.
Which has never been observed before.
So that would be probably one of the most
interesting scenarios we could be in.
Recently with this excess that both the ATLAS
and CMS collaborations are seeing around 750
GeV in the diphoton spectrum which is totally
unexpected, we don't really - we have no hint
what this can be.
If this is a graviton, it's probably the most
exciting situation.
