Welcome to the ATLAS detector at CERN.
Well that's just a painting of it, the
real one is deep underground. It's one of
the two main experiments here at the LHC
that's looking for all sorts of new
physics. In 2012 it found the Higgs boson
in a theory that was so elegant and
beautiful. But the real way that particle
physics works is not as easy as
just looking at a Feynman diagram,
looking at an event in a display and
saying there we go we found it. In fact
particle physicists what they really do
is play an elaborate game of dice. To
explain what I mean by that, let's forget
that we're at CERN and instead pretend
that we're at a casino, a casino run by
a rather unscrupulous owner. Now there is
a hypothesis that this casino owner is
introducing unfair dice, but can we work
out if there are unfair dice within the
casino? By counting the number of times
that a certain result comes up on the
dice, we can work out the likelihood that
the dice were fair or otherwise. Now
obviously if this were a fair dice you'd
expect the probability of any given
number to turn up would be 1 over 6.
Roll the dice 60 times and you'd expect 10 of
each number and 600 times and then
that's 100. But of course that's only the
average of what you'd expect,  so how far away
from that average do you have to go for
us to start to get a bit worried. It's
actually quite easy to work out the
probability, we can draw up the
distribution. As you can see for 60 rolls
the distribution peaks at 10 but it has
some width to it,
so for instance we shouldn't be too
worried if we got 20 sixes out instead of
just 10 as expected. But as we up the
number of rolls, the distribution gets a
lot slimmer we can use the width of
those distributions as measured in
standard deviations as a sort of measure
of the likelihood of getting this result
out purely by chance. Particle physicists
often take five sigma to be the
benchmark for a discovery, that's a
probability of 1 in one million, seven
hundred and forty four thousand, two
hundred and seventy eight of being
purely down to chance. So to find out if
there are any unfair dice within this
pile, we need to roll all of these dice a
lot and then work out the chance of this
being different from what's fair.
By rolling the dice lots of times we can
experimentally measure the probabilities
associated with each number on the die,
that's what you can see in the top panel
in blue and we compare that with what's
expected for a fair dice - a value of a
sixth. But we also show the one and three
sigma levels around that as the colored
bands. So for evidence of loaded dice
we're looking for a bump that pops above
the green. Or more precisely we look for
the significance of any differences
between the data and what's expected for
fair dice. That's what's shown in the bottom
panel as measured in sigma. We do a
fairly straightforward comparison in the
blue known as the local significance but
remember we've got six different numbers
to play with so actually the global
significance is a little bit less and
that's what's given in red. But as you
can see this is pretty slow going, so you
know what i'm going to employ a little
bit of help.
As we pool all the data together we see
a lot of fluctuation, some potential
results appearing and then disappearing,
but finally we can see that there are
some dice weighted to the number six.
Only a tenth of them were unfair. So what
has all this dice throwing got to do with
particle physics? Well you can think of
the weighted dice as being the signal,
the thing that we're actually looking
for, and all of the ordinary dice being a
background. This is exactly what particle
physicists do when they're looking for a
new particle. If they have an idea of how
it can decay into known particles, they
can look for that channel - that's the
signal that they're after. But of course
there are lots of different ways that
you can produce those same particles, so
you have a background going on as well -
processes that you're not particularly
interested in. It's separating out the
signal from the background that is the
key thing to do if you want to prove
there's a new particle. Right through
that glass is the ATLAS control center,
that is where it all happens. They can
see all of the 40 million collisions
that are happening every second but they
can't tell what each collision is. It's
only by processing it with the
statistics that you can actually tell
whether you've detected a particle, so
while Einstein famously said God doesn't
play dice -
it's for definite that particle
physicists do! Thank you so much for
joining me at the ATLAS detector for
this video. If you did like it please do
give it a thumbs up and you can subscribe
for more physics as well and I'll see
you see.
