Hello and welcome to this live physics
talk from the ATLAS collaboration!
We are thrilled to have locked Dr Laura
Jeanty from the University of Oregon
with us and she will be giving us a
presentation on physics through the
looking-glass so we're gonna dive right
in and we're all looking forward very
much to this talk.
Hello I'm Laura Jeanty.
Thank you very much for joining today.
This talk is associated to the LHCP
conference, which is ongoing and was
supposed to be held in Paris. Instead
physicists are connecting to this
conference from their homes all across
the globe and since I'm connecting from
my home in Oregon I wanted to show you
what Oregon looks like so you can
envision me while I'm giving this talk.
This is a picture I took from my porch
yesterday about the same time. As you can
see it's basically the same as being in Paris.
I'd like to start this talk with a
puzzle, this puzzle was coined the Ozma
problem in 1964 by Martin Gardner.
This puzzle involves aliens so the question
is if we could communicate with aliens
but we couldn't see any objects in
common with them could we tell them what
we mean by left or right? Is there any
fundamental experiment we could use as a
benchmark to define the difference? Up
and down is easy because we can tell
them that gravity always attracts things
down and gravity behaves the same way on
their planet so they could do that
experiment and have a benchmark for down.
But is there any physics that can
tell left from right? This question is
the same as asking whether there's a
symmetry between left and right at a
basic level? Gardener thought a lot
about symmetry but thinking about
symmetry is of course much, much more
ancient than the 60s and
we have been thinking about symmetry as
humanity in many different ways
throughout time and throughout cultures.
One way that we have been thinking about
symmetry and appreciating it is art.
So throughout time and cultures we have
used symmetry as an element to make art
more beautiful all three of these
examples derive some of their beauty
from symmetry. Here what symmetry means
is that if you rotate the image or you
shift it you get back an image that
looks the same or at least similar.
Symmetry is important in art, but it's
also important in philosophy, it's
important in biology, it's important in
physics. And in this talk I'm going to
walk you through what the idea of
symmetry means from particle physics
point of view. I'll share with you some
of the questions we're still trying to answer.
And then at the LHCP conference
that's happening now, the ATLAS
experiment has released some new results
that shed a little bit of further light
on these questions and I'll end with a
brief look at some of these results.
OK, to start this story we're gonna go
back to 1931. Let's start with a quick
look at symmetry in the evolution of
particle physics. Way back in 1931 this
was our picture of the fundamental
particles that exist, we have the
electron the photon and the proton and
with these fundamental particles you can
build up atoms and that explained much
of what we observe. Now what
distinguishes these particles from each
other? Well there were two properties
that were known about then that
distinguish them: one was the electric
charge so the electron is negative and
the proton is positive the photon has no
electric charge it's neutral; the second
was the mass the electron has a mass
this is 511 keV, this is like 10 to the
minus 28 grams, it's tiny, the point is
that the proton is much, much heavier
than the electron and the photon is
massless. So this was our picture of the
world at the fundamental level in 1931
and this was rapidly changed in 1932 by
an experiment done by Carl Anderson.
So Carl Anderson took a cloud chamber which
can look at particle tracks and he
looked at cosmic rays. Cosmic rays are
particles that are constantly raining
down on us from the atmosphere and from
outer space. He saw this track in the
picture which looks just like an
electron it looked like it had the same
mass as an electron but the cloud
chamber was in a magnetic field and the
electron was bending the wrong way, it
would only bend that way if it had
positive charge but the electron
remember has negative charge. This was a
discovery of the positron the anti-electron.
This wasn't a total surprise,
even though it's crazy to think that
antimatter actually exists, Paul Dirac
had actually discovered or predicted a
symmetry of the underlying mathematics
that describes how fundamental particles
behave a few years before this and no
one really knew what to make of that
symmetry that existed in the math until
Andersen discovered that that symmetry
actually corresponded to a real
physical particle that's produced by
interactions sent to us from outer space.
And this was the discovery of antimatter.
Because it fit into this symmetric framework
of the mathematics, the discovery
of the positron of the antimatter
electron led overnight to a doubling of
the number of particles in our model.
So we then knew that for every particle it
had an exact opposite where all of the
properties were the same, except for the
charge so you have a negative electron
and a positively charged positron, you
have a positive proton and a negatively
charged antiproton which was discovered
in the 50s now the photon because it has
no charge it actually is its own
antiparticle so there's only one version
of the photon. So this symmetry, this
is a real symmetry of nature at this
antimatter reflection and the discovery
of the positron confirmed that this
antimatter symmetry is actually
real in nature. Now I'm gonna skip over a
huge, huge amount of work essentially
corresponding to a century of
measurements and theoretical
developments to show you what our view
today is of the fundamental building
blocks of the Universe.
And that's called the Standard Model of particle physics
so this is a nice compact picture which
shows you at the most fundamental level
the most elemental particles that we
think you can get if you break stuff up
apart as much as you can.
So this is a nice picture it's hiding a lot
of the details of the model but it does
correspond to the fundamental elements
that we believe are the building blocks
of the Universe. I'd like to point out
the Higgs boson
in the upper right corner in yellow. So
the Higgs boson is the new kid on the
block it was discovered in 2012 by us
the ATLAS and CMS Collaborations and it
completes this very nice table and
makes the Standard Model internally
consistent which is a real triumph of
the model. So this is one way of looking
at the Standard Model, there's another
way of representing these particles and
that's via plushy toys so this is the
only show-and-tell component
of the talk. I actually wanted to show
you a Higgs boson, I have a Higgs boson but
it turns out it's in my office and like
many people I haven't been to my office
in months so I'm I was reduced to
finding what particles I had at home!
So, I have a gluon, the gluon is massless
so you can see it's quite light. I also
have a W boson, and the W boson is not
massless it's pretty heavy and you can
see that it behaves differently. OK,
that's the only show-and-tell.
So let's go back to this this table of
the Standard Model of particle physics
so what distinguishes these particles?
Every electron is exactly like every
other electron, but the electron is
different than all the other particles
in this table, and what distinguishes
them, the properties that make
an electron different from the Higgs
boson, those properties are their
charge, the particles charge, their mass
and their spin.
Spin is essentially a quantum mechanical version
of angular momentum, so you can think of it
like the particles intrinsic
angular momentum. And these are the three
distinguishing features of the particles
in our model. So now I'm going to walk
you through just a few of these
particles and explain what their role is
in the world. So first let's start with the
photon, so the photon is the particle of
light and all electromagnetic radiation.
As such as the carrier of the
electromagnetic force, and this governs
all electric and magnetic interactions
and phenomena, so we interact with
gazillions of photons every day.
There are other forces in the Standard Model.
So another force is the strong force, the
strong force is what holds together
protons and neutrons to form the nucleus
of atoms, and the strong force is carried
by the gluon that's this guy.
The third force in the Standard Model
is the weak force and the weak force is carried by
the W and Z bosons, there's the W.
So the weak force drives the
interactions that allow the Sun to shine,
so it's also fundamental in our solar
system and in our everyday life. Now what
about all the other particles in this table?
Well in purple you have the quarks
and in green you have the leptons so the
everyday quarks that we interact with
are the up quark and the down quark. And up
and down quarks in various combinations
are held together by gluons to make
protons and neutrons and other more
exotic things like pions.
The most famous of the leptons is the
electron and the electron protons and
neutrons these are held together by
gluons and photons that builds up an
atom so together these things give us
the but you have you up quarks the down
course the electrons photons and gluons
those give us the atoms the molecules
and the chocolate bars of our lives okay
so that was a very quick overview of
some of the particles we return to some
of the other ones later but let's return
to the question that I posed at the
beginning how symmetric is the standard
model what does it mean for the model to
be symmetric and to answer this question
we want to talk not just about the
particles in the model but the laws that
actually govern their behavior so let's
pause for a moment and define what
symmetry means for physics so remember
that in art what symmetry means is that
you can provide you can perform some
sort of a rotation or a transformation
and you get back the same image the
exact same concept applies in physics
where you take a law a physical law that
describes how things behave you apply
some sort of transformation and then you
asked does that physical law still hold
is that still a valid description of the
way that things behave and if it's true
if you provide you perform a
transformation on a physical law and you
get back a good physical law then that
physical law is considered symmetric
under that transformation so that's an
apt
that description but let's take a look
at a more concrete example so let's say
that I had a mirror that reflected
charge so if you take an electron and
you reflect it through this mirror you
get positive electron take the negative
charge and you flip it to the to the
positive charge but also do vice versa
so you reflect my electron through the
mirror and I actually gather a positron
which is the same in every way as the
electron except it has the opposite
charge now we know that this particle
exists but we can ask not only about the
particle but also about the way it
interacts so we could say do the laws of
electricity and magnetism behave the
same for a positive electron as for a
negative electron and the answer is yes
the mirror image of electricity
magnetism is still the way that
electricity magnetism behaves so any
equations that you remember from high
school physics about how electricity and
magnetism behaves those are still valid
for a positive electron this is also
true for the strong force let's look at
another type of mirror first so another
mirror is actually corresponds to a real
physical mirror this is called the
parody mirror and a parody nerve again
is like the mirror that reflects your
image it flips left from right
so if you can imagine holding up a clock
to a mirror the image of the clock is
backwards it looks like it's going the
other way but the laws of physics are
still the same and what I mean by the
laws of physics is that even though it
looks like the hand of the clock is
moving the opposite direction after
12:00 o'clock comes 1 o'clock and after
one o'clock comes two o'clock and after
two o'clock comes three o'clock and so
if you were to step through this looking
glass you would still see that time is
moving forward and in that sense you
know a clock is parity symmetric it
respects there's no difference between
left and right with our clock so it's
this parity symmetry the symmetry
between right and left that
fascinated Martin Gardner and LEDs him
and others to formulate the asthma
problem okay so let's ask not just about
clocks but about maybe more fundamental
interactions so what about our electrons
now when you reflect an electron through
a parody mirror you get back an electron
it still has the same charge but because
electrons have spin you can sing of spin
something that actually has a handedness
when you reflect something that's
spinning you actually see that it's
spinning the opposite direction on the
other side we call this handedness
because if you think about your hands
when you reflect your left hand in a
mirror you get back your right hand and
there's no way you know these are these
are fundamentally different you can't
flip one to get the other so the same
way when you flip a left-handed electron
through a mirror you get back a
right-handed electron that's spinning
the opposite direction now it turns out
that the the laws of electricity and
magnetism are the same for left-handed
electrons as for right-handed electrons
so this force is considered parry
symmetric and this is also true of the
strong interaction in fact this seems
somewhat natural right it seems somewhat
bizarre to even ask whether or not
nature cares about left and right but
you can ask the question of the weak
force which was discovered you no later
than D than the other forces and this
was this was asked there seemed to be no
solution to the asthma problem until the
question about parody was asked is the
weak force also symmetric under a mirror
reflection and this question was
answered in 1956 by an experiment led by
a professor at Columbia madam Liew so
madam will led an experiment that looked
at the decay of cold cobalt nuclei
cobalt-60 nuclei decays via the weak
force and it emits an electron but
cobalt also has a spin and that spin
gives the cobalt a direction so she put
a bunch of coal but nuclei into a
magnetic field so that the spins were
all pointing in wonder
now if this if this experiment had to
look like its mirror image
then you would expect that the electrons
would be admitted the same amount up and
down because the spin of the magnetic
the orientation of the magnetic field is
opposite if you look in the mirror so
that the particles that are spinning one
direction when you look in the mirror
they're spending the opposite direction
and if you if the electrons were emitted
only up or only down that doesn't look
like the nearer reflection because
mirrors don't reflect up and down they
only reflect left and right what Madame
goo found when she actually performed
this experiment is in fact that the
electrons have a preferred direction
they she found that they do fly off in
one direction they are aligned
preferentially in one way with the
orientation of the spin of the kobolds
and this result was completely
unexpected it revolutionized our
understanding of the world she found
that the mirror image of this experiment
doesn't exist so here if you reflect
this image through the parody mirror
then what you find is that the cobalt
are spinning in the opposite direction
and therefore they must still emit
aligned with the according to the weak
force aligned with that direction and
therefore they would admit the electrons
going down but this doesn't look like
the mirror image because up and down are
not reflected and so what she found was
that in fact parody is not respected by
the weak force you or Alice or me we
could step through the list
looking-glass and perform this
experiment and depending on which way we
saw the electrons go we would know
whether or not we were in the real world
or we were in the reflection
world and this was the solution to the
Ozma problem you could tell an alien to
perform this experiment and depending on
which direction the electrons are
emitted relative that gives you a
direction you can say the cobalt nuclear
spinning left or right relative to that
fundamental direction so as I said this
results was very unexpected and was the
cornerstone of our modern understanding
of particle physics I'd like to take a
step back now and and just comment that
the appreciation of symmetries that are
broken that are carefully constructed
asymmetries this is not just a human
trait we are used to this construction
of a symmetries in the art world and in
many cases it can be more beautiful than
something that's perfectly symmetric and
it turns out that nature is both
powerfully symmetric and asymmetric in
fact the weak force is maximally
asymmetric it's so extreme that there
are some particles as far as we know
that only exist in a left-handed state
so it turns out that nature actually
prefers left-handed particles okay I'd
like to restore a little bit of your
sense of symmetry by making a slightly
more complicated mirror so this mirror
reflects both parity and charge so here
when you reflect a left-handed electron
through a mirror that inverts both
parity and charge you get back a
right-handed positron this charge parity
mirror is actually a antimatter mirror
and it restores some sense of order to
our understanding of the world it turns
out that antimatter cobalt spits out
anti electrons in the opposite direction
and so the if you flip the madam Liew
experiment the cobalt nuclei decaying
through it an antimatter mirror you
actually get back up a valid description
at that decay it does look like its
reflection so together charge and parity
are are the antimatter mirror now let's
go back to our table of particles so you
may have objected that my my fundamental
table didn't have the positron or any of
the intimate particles and that's true
that would have been a good objection
so in fact we actually have twice as
many particles then I showed in the
first table so there is a doubling of
the particles that corresponds to two
antimatter and as I mentioned before
what distinguishes the symmetry that
distinguishes matter from antimatter is
the charge so for every particle in this
model there is an antimatter equivalent
so we know in the particle content
itself there is the symmetry but it
turns out and this result was also
completely unexpected that actually
antimatter does not behave identically
in all cases to matter so certain
particles in certain circumstances
behave very slightly differently from
normal matter in weak interactions
this was also unexpected but has become
an essential component of understanding
our world the fact that the antimatter
mirror is nearly symmetric but not quite
ok so so far we've been reflecting
particles through our mirror but we can
get a little bit more ambitious we can
say what happens if we reflect the
universe through our antimatter mirror
well we get back an antimatter universe
and as I said this mirror looks almost
this antimatter universe looks almost
identical to ours there should be a
valid existence because we know that
antimatter is almost the same as matter
but when we look up into the night sky
we don't see any evidence of any matter
planets or antimatter galaxies if they
existed then there would be
characteristic signals of photons or
gamma rays where the boundary of the
antimatter galaxies met the boundary of
the matter galaxies so matter and
antimatter annihilating when they meet
and that gives out photons with very
characteristic energies and we don't see
any evidence of those photons coming
from anywhere in the night sky and this
tells us as far as we know that as far
as we can see the universe is made of
matter not antimatter in our model
and our standard model we have a small
amount of antimatter asymmetry but when
we look up into the night sky we see an
enormous amount a big one in the night
sky and the difference between this
small asymmetry in our model and this
enormous asymmetry that we observe
astronomically this is one of the major
outstanding questions in particle
physics and that question boils down to
why is the universe made of matter and
not antimatter another way of asking
this question is where did all the
antimatter go after the Big Bang this is
one of the fundamental questions that we
are trying to answer but before I take a
stab at that not that I'm going to give
you an answer but I just would like to
go back and while we're talking about
galaxies I'd like to bring up one more
mystery so let's focus on one specific
galaxy and zoom in we can measure the
speed of all of the stars in any
particular galaxy and we can calculate
how much mass that star must be orbiting
to produce that that speed that velocity
what we find is that we travel out as we
travel out in the galaxy we see that the
stars are going much faster than they
should be if all of the matter in the
galaxy or from stars or dust or other
things that interacts with light so this
dotted line here this is a velocity
curve as we move out in the galaxy that
we would expect if all of the matter
were from stars or visible matter but
what we observe is a solid line which
tells us that as we go out far in
universe there's far in a galaxy there
seems to be much more matter than can
interact with light and the difference
between these two curves is dark matter
so this brings us to another mystery
what is dark matter made of we've
carefully examined all of the particles
in the standard model and all of the
astronomical phenomena that we can see
and observe and none of these can
explain
the velocity curves of galaxies we think
that dark matter is a new type of
particle that does not interact with
light we haven't yet seen it but it may
interact with other particles and that
gives us a handle for trying to see what
it is so for both of these fundamental
questions where is the antimatter and
what is dark matter it may just be that
the new kid on the block the Higgs boson
may be the key to answering these
questions so the Higgs boson interacts
with most known standard model particles
and it could also interact with unknown
or undiscovered criticals some are
predicted some could be completely
unknown one new particle could be a
particle that could explain the
antimatter mystery we don't know what
this particle would be so we just call
it a the Higgs boson could also interact
with a particle that's dark matter and
so we have a couple of models that
predict what kind of dark matter the
Higgs boson could interact with so to
see if the Higgs boson can interact with
these new particles and if that gives us
a tool for discovering them first we
have to produce a lot of things so how
do you produce a Higgs boson well if we
travel to to the border of Switzerland
and France and we go to CERN so here in
this picture you can see the Alps and
you can see Lake Geneva and this red
circle represents the the outline of
where underground is the Large Hadron
Collider so the Large Hadron Collider is
an accelerator at CERN as its name
suggests it is large it's about 27
kilometers in circumference
now Hadron is a fancy name for proton
and so what the LHC does is it coli
accelerates two rings of protons in
opposite directions to very high
energies and then collides these protons
at four different spots around this ring
now what happens when you collide two
protons
well everything that's allowed by the
standard model and what other whatever
other new models may be there that we
hope to discover that's what happens the
collision each collision is these
collisions are fundamentally quantum
mechanical which means even though each
collision is identical in terms of two
protons going out what happens as the
results can differ so it's essentially a
probabilistic event the standard model
predicts exactly the rate of everything
that should happen we can't predict any
one collisions results but we can
predict very precisely the output of
many collisions this is exactly like
rolling dice so sometimes when you have
two collisions you produce a Higgs boson
and so here for example the two gluons
one in each of the protons interacts in
a way to give you a Higgs boson now how
often does that happen first I want to
show you to answer that question I want
to show you what else can happen when
you collide two protons so this plot
shows the rate of different particles
being produced in proton-proton
collisions as a function of the
different types of particles so if you
start in the left-hand side you can see
very often when you collide two things
two protons you get back two protons
okay a little bit less often when you
collide two protons you get a double you
goes on and as you step down these
processes become very rare you can see
the Higgs is many orders of magnitude
more rare than for example a deputy
bosom and this is part of the reason it
took us so long to discover it because
we had to first produce enough
proton-proton collisions that we had a
measurable number of Higgs bosons this
plot is really a triumph of the standard
model so even though I'm raising
outstanding questions at the standard
model I'd like to just you know take a
moment to show you that each gray line
here is a prediction of the standard
model of how often something should
happen and each colored square is a
measurement and you can see for all of
these particles over many orders of
magnitude we find perfect agreements
between the the model
and ends the measurement and this is
this is really a triumph it's an
amazingly predictive model now I'd like
to go back just to address the question
of how often we produce Higgs bosons so
if we collided protons at the rate of
one collision per second it would take
us on average 500 years to produce one
Higgs boson now not even not even
graduate students are not patients so we
collides about a trillion protons per
second and that gives us about one Higgs
boson every two seconds so how do we
look at these Higgs bosons after we've
produced them we use the Atlas detector
to give you a sense of scale here the
photo of me from 2006 with the inner
part of the detector before it was
installed now Alice is not only a
detector it's an entire collaboration
there are over five and a half thousand
people who are part of the Atlas
collaboration of people work on
everything from building one small part
of the detector to making sure that it
runs smoothly 24 hours a day to writing
software to analyzing one small piece of
the data it's an amazing collaborative
effort and now I'm going to share with
you a few new results that the
collaboration has released this week so
the first is a search that is looking
for a new particle that could explain
where all the missing antimatter is and
why we don't see antimatter galaxies so
what we do is we take all of the
collision data and we write algorithms
to sort the collisions to sort through
them and put them into different
categories one of these and those are
the bins of this this plot one of these
categories tries to identify collisions
that looks like they produced a Higgs
boson
two came to this new particle
unfortunately or fortunately some
collisions that produce standard model
particles also look similar and that's
the the blue in this plot if you look at
the leftmost spin here this is the bin
where we would expect to see this new
particle showing up and if this new
particle are showing up we would get
additional contribution of collision
events that correspond to the size of
that red line so if this particle were
being produced in our proton-proton
collisions you'd expect to see the data
on top of the the prediction from the
standard model by about 20 events now it
turns out that the day that exactly
matches the standard model prediction
which means that in this search we did
not find any evidence of this new
particle now what about dark matter so
dark matter is even trickier because it
turns out it's invisible to our detector
so what we're searching for is we're
searching for the higgs decaying to
something invisible and it's quite a
difficult search this is a brand new
result from Atlas that puts the best yet
upper bound on how often the Higgs could
decay to something that's invisible and
it turns out that's eleven percent of
the time that you produce the Higgs
boson from that we can rule out dark
matter as a particle with a certain mass
and with certain interactions and so the
red and the blue line here are the lower
limits on two different types of dark
matter from this Atlas search which
means that all all of the mass and
interacting of dark matter above this is
experimentally ruled out now turns out
that this is a part of face face that
was theoretically preferred and so it's
incredibly interesting that this result
has actually ruled out apart a large
part of that interesting mass and
interaction characteristics of dark
matter so the mystery deepens and the
search continues we have to now continue
to challenge some of our guesses about
what dark matter could be and we have to
keep looking okay I'd like to briefly
return to another mystery because we
also have a very interesting result that
corresponds to that so I quickly went
over the fact that the strong
interaction when you flip it through the
antimatter mirror is actually symmetric
so the strong interaction is exactly the
same for antimatter
unlike the weak interaction we don't
know why the strong interaction is
symmetric across the antimatter mirror
and the weak interaction is not which
leads us
to another mystery why is the weak force
slightly asymmetric in our charge parity
near and the strong force is not and
Atlas has a new result that speaks to
this as well so in this result
Atlas actually measures a process called
light by light scattering which is when
two photons indirectly interact with
each other photons cannot directly
interact with each other but they can
exchange electrons and interact but we
could also have a new particle called an
accion that could explain why the strong
force is symmetric across the charge
partner and two photons could also
interact virtually via an axiom and so
this result this process light by light
scattering was first observed by Atlas
quite recently so it's a very new
process to be experimentally observed
and now we can measure it precisely
enough that we can actually ask whether
or not there's another particle
interacting there we did not observe
evidence for that and so this light blue
area tells us again gives us a sense of
where the axiom is not what the maxium
mass is not and so this further extends
our knowledge about what answers could
or could not be viable to the question
about parity and charge symmetry ok
before I end I'd like to to talk about a
few other types of symmetries so far
we've only been talking about the
antimatter mirror but there are other
ones so what about other symmetries now
you've no may have noticed in the table
that there were a couple of columns I
didn't talk about and these other
columns correspond to heavier versions
of the the light particles that make up
our atoms so you have up in the down
cork and then you have a heavier version
of them you have a heavier version of
the electron which is the new one and an
even heavier version of that which is
the Tau and this is a symmetry in mass
these particles are exactly the same
except for their mass and so one
question
that we're still trying to measure the
answer to is are some interactions
exactly symmetric across flavor across
mass so do particles with different mass
but the same other properties we call
these this is a flavor symmetry are some
interactions perfectly symmetric for
particles of different mass and Atlas
has a very interesting new result on
this and so the way that Atlas tries to
answer this question is to look at the
heavier versions of the electron there
the Tau and the muon and ask whether or
not the W boson W boson decays equally
to the Tau lepton into the the muon
lepton and Atlas has actually made a
very important measurement which
significantly improves our sensitivity
to this question and so you can see in
blue in black you have the measurement
from Atlas and then you have the
uncertainty on that measurement and you
can see that it's consistent with one
the ratio of these processes is
consistent with one and what is this
tells us is that for now
it looks like this symmetry is perfectly
respected
okay I'd like to conclude with one final
symmetry so in this table I talked about
how we have antimatter which is the
symmetry associated with charge and
different flavors or generations which
is a symmetry associated with mass and
you might ask is there a symmetry
associated with spin now we can divide
this table into particles that have
different types of spin so fermions have
one type of spin and the force carriers
have a different type of spin and those
are called bosons so is there a symmetry
between fermions and bosons the answer
is yes there is a mathematical symmetry
between these two things and it's such
an awesome symmetry that we call it
supersymmetry and so what supersymmetry
does like antimatter if it says for
every Fermi on every particle with one
type of spin there is another particle
that's identical in every way except it
has a different spin and so this is a
symmetry between fermions and bosons
that doubles again the number of
particles in our model now supersymmetry
is interesting not only from the
philosophical or the mathematical point
of view it's not just a you know you can
think it's amazingly powerful because it
is an additional symmetry of our model
that should be respected the same we
think should be respected the same way
that the other ones are but when you
double the number of particles you can
solve for free another of the other
mysteries so supersymmetry can give you
a Dark Matter candidate and it can
explain why the Higgs boson has the mass
that it does so you take a fundamental
symmetry and ask if it's there it can
actually solve other puzzles and this is
a pretty compelling reason to think that
supersymmetry might be a symmetry that
exists in nature and so the question is
does supersymmetry exist in the real
world we haven't yet found it but it
could exist in many forms and we're
looking for it everywhere that it could
be I'd like to finish with one last
result from Atlas so Atlas has released
in this conference three new results
that are searching for different
types of supersymmetry this particular
search combines two of our favorite
things supersymmetry and Higgs bosons
and so here this diagram represents two
proton-proton collisions that could
produce some type of supersymmetric
particle that would decay via a Higgs
boson to dark matter now if you look in
the right hand no spin of this plot if
this type of super symmetric process
were being produced no collisions then
the data would follow the red line what
you see is actually that the data
follows the standard model prediction
without the super symmetric prediction
so we can conclude that this particular
flavor of supersymmetry doesn't exist in
nature but there are many more that
could exist and we continue to look for
them so I'd like to end going back to my
original question how symmetric is the
universe we are still trying to answer
this question and I invite you to stay
tuned we will continue to take data
until at least 2035 and as we collect
more and more data we can make more and
more precise tests of these symmetries
and we can look for heavier and heavier
versions of new particles that might
answer some of the outstanding questions
I'd like to just say as a closing remark
that these questions aren't only driven
by curiosity about the world around us
in some sense this these questions are
the most existential questions we could
ask if the symmetry between matter and
antimatter were not slightly broken then
after the Big Bang all matter would have
annihilated with antimatter and our
galaxy or planet and us we wouldn't be
here today so our very existence is
dependent not only on the symmetries of
nature but on the a symmetries as well
thank you
thank you so much Laura for that
wonderful talk so we already have a lot
of questions that have come in so I'm
going to launch into those and we will
try and get through as many as we can in
the time and I'm just going to make sure
that your video it's for everyone to see
ok so the first question we have is in
what extent do you at les work with
other experiments at CERN do you
exchange ideas or is it like a contest
who gets the best results first yeah
that's a great question there are
probably many different ways to answer
this so we absolutely share theoretical
ideas we share tools and techniques with
our colleagues at different experiments
we do not share while we're doing
analyses the details of those searches
so part of the reason for having more
than one experiment is that you if you
do if we do discover something new we
want to make sure that we have a way to
test with another experiment that that
is a real meaningful result and so we
don't want to do things exactly the same
way because then it's not an independent
confirmation of our results and so the
fact that we don't share the details of
our analysis while we're performing them
allows us to have independent checks
there is also somewhat of a competitive
element of course we Atlas want to be
the ones to discover things and not see
us that's also part of the reason we
don't tell them what we're doing until
after we've done it but that being said
we do meet regularly to discuss you know
theoretical advances to try to make sure
that we're presenting our results in a
way that makes them comparable between
the different experiments and to you
know share the techniques that you can
great Thanks and so the next question we
have is will Atlas find new particles to
add to the standard model yeah that's
the million-dollar question I you know
we're physicists we like to make
predictions but we can't tell you what
nature what nature has in store for us
so some of us hope that we will discover
new particles maybe some of us hope that
we won't and you know that's also an
interesting outcome that tells us that
some of the guesses we had about how
things should work are not right and
that makes the the mystery of the
standard model and these outstanding
questions even more puzzling but I would
say the majority of us hope that we do
discover new particles but you'll have
to wait until you know 2035 for the
final answer on that one um and there
are many people who have been inspired
by your talk and so one of the questions
is how can I be a particle physicist at
CERN great question I should probably
ask you take this question so one of the
ways so if you're a student you can
absolutely approach there are different
programs for joining the research at
CERN if you're a student so you could if
you have local faculty who work at CERN
you could talk to them or you could
apply to various programs that exist
throughout the world and hosted by CERN
to send students to CERN if you're not a
student you're welcome to come visit
CERN there are a lot of great ways that
you can that you can interact with
certain when you visit and you research
there I don't know Clara if you want to
add anything to that no I think that's a
good summary perhaps also since we're
all in lockdown at the moment then there
are ways to visit the the detector
remotely so you can also check out on
our YouTube channel Atlas we have a 360
degree video which is touring the Atlas
detector so if you want to see the
detector there and learn a bit more
about how that works then you can watch
that video as well okay and so the next
question we have is what does the Higgs
boson do yeah that's a good question
so the Higgs boson does a couple of
things so one of the things the Higgs
boson does is the standard model if the
standard model did not have the Higgs
boson the mathematics of the Higgs boson
rubato would not actually be internally
consistent that means that the standard
model would make a prediction for
being more than a hundred percent which
doesn't really make sense and so the
first thing that higgs-boson does is it
makes the standard model predictions
internally consistent and so that's
really a keystone reason why we believe
that we would discover it when we turn
down knowledge see how the other main
thing that the Higgs boson does is that
in its interaction with some of the
fundamental particles it gives them a
mass and so it is the reason the Higgs
boson interacting with the W boson is
the reason that the W boson is heavy the
Higgs boson does not interact with the
gluon which is why they go on its liked
great and then the next ones maybe
slightly controversial do you believe in
supersymmetry ah this is another
interesting question so here I think the
answer depends you know as a physicist
beliefs are not something that or at
least as an experimentalist beliefs are
not something that I really ask myself I
believe it it's important to go out
there and make measurements and ask good
questions about what's out there I try
not to you know stake make a prediction
about the way nature behaves because you
know Who am I to say what nature has
decided should be there I think it's
interesting to ask the question and try
to answer the question but it's not
really my place to have a strong belief
I'll have a belief once we've observed
something until then I try to remain
open okay thank you
so you mentioned during your talk that
there are these three generations of
particles do we know why there are three
generations and what are the differences
between them another great question so
in some sense we don't know why there
are three there could be you know there
could have been a universe in which
there was only one or two or three or
four but there's on the other hand
there's an interesting puzzle which is
the fact that if there were only two or
only one generations
actually you could not have
mathematically you could not have a
difference in weak interactions between
matter and antimatter and remember I
said that that that asymmetry is
essential to us being here without that
slight asymmetry then you would not have
had any matter leftover after the Big
Bang and so if there were fewer
generations if there were only two then
we would not exist and so in some sense
that's a reason why there are three is
because we wouldn't be here to ask that
question if not but that is a that is
you know that may not be a satisfying
answer to that question but it's one of
the ones that we have okay thank you
and so our next question up is are you
expecting to find more than one type of
Higgs boson good question um we're
hoping to so there are many theoretical
models that predict more than one Higgs
boson and one of those is supersymmetry
supersymmetry actually gives you five
Higgs bosons how about that and so we we
hope that we could find more Higgs
bosons but yeah it depends on what model
is out there for us to discover okay and
maybe link to this how is atlas
searching for dark matter particles so
dark matter particles would not interact
directly with our detector so we could
not directly look for a dark matter
particle being produced but what we can
do is we can look for for example we can
look at Higgs bosons and see when it
Higgs boson decays to something that we
can't see decays invisibly and we can
measure how often that happens and we
know from the standard Marlowe that that
should be a very very very very small
rate and so if we saw that the Higgs
boson decayed to something invisible a
significant fraction of the time that
would be evidence that the Higgs boson
was decaying to dark matter
okay and then linked a bit too to the
higgs-boson that we were talking about
if the Higgs boson interacts with other
particles and they gain mass then how
can the Higgs boson have its own mass
yep good question so the Higgs boson
actually so the Higgs boson gets gets
its mass in a slightly different way
than than the other particles but
interestingly the Higgs boson can
actually interact with itself so unlike
the photon the Higgs boson can actually
interact with itself and that's a
process that we're trying to measure as
well on Atlas and CMS we haven't yet
observed that process the Higgs boson
interaction with itself but we hope to
one day have some sensitivity to that
process okay and then on slightly
different topics now can we collide two
muons since they're charged particles
excellent question yes we can we haven't
done that yet but they're ideas for a
muon Collider it's one of the the new
accelerators that people are thinking
about it's technically very challenging
for a number of reasons that I will not
get into it would give us very
interesting types of collisions um we
have already collided two electrons
together so before the LHC in the same
tunnel at CERN there was actually an
electron positron Collider and so we've
done that and it gives us interesting
collisions because muons are heavier
than electrons there would be more
energy in those collisions that could
get produce heavy particles and so that
plus a couple of other reasons we could
have very interesting collisions from
two muons colliding we haven't yet built
that Collider it's it's a pretty
technically challenging collider toodles
and then the next question follows on
from this so can photons also collide
and what exactly would happen so as I
mentioned photons cannot directly
interact with each other so you could
not in the same sense collide two
photons but photons can indirectly
interact with each other so if you had
two photons of high energy coming near
each other they could exchange for
sample electrons or maybe a taxi on and
and in that sense they could exchange
energy which is one way of having a
collision okay and which model or theory
best describes particle physics that's
an easy one
it's the standard model so we've
developed the standard model over the
last fifty to a hundred years and so far
in every test we've been able to make of
the standard model the measurement has
agreed with the standard models
prediction it is one of the most precise
theories that we have it can make
incredibly accurate predictions about
very rare or very small processes and to
every degree of accuracy that we've been
able to both make those predictions and
make those measurements we have seen
agreement
but just add you know the standard model
is predictive of every prediction it
makes these questions that I raised are
not problems with this standard model
there are things that the standard model
doesn't have an answer for so the theory
every way we've been able to test it
works but these questions are most of
them a lot of them come from either
questions of symmetry or from
astronomical observations trying to make
sense of what we observe out in the
night sky and seeing that the standard
model doesn't have an answer for those
questions great thank you so the next
question how is atlas searching for dark
matter particles and so I think I may
have addressed this question already so
we're one of the ways we're doing it is
by looking for Higgs bosons decaying to
invisible particles I can add for
completeness that that's not the only
way so Atlas is also looking maybe the
Dark Matter doesn't interact buried but
maybe it interacts with something else
so we're also looking for other
processes where you have some standard
model particle coming out and something
invisible and that would again be
evidence of dark matter so maybe to
follow up how do we measure something
that we can't detect and uh in our
detector yeah that's a great question so
we know when two protons collide that in
the initial collision energy in in the
transverse direction if this is the
longitudinal direction and this is the
transverse direction we know that energy
is conserved is the initial energy in
the transverse direction is zero and
then we can measure all of the energy in
the transverse direction of all of the
particles coming out and we sum that up
and if there's something if we don't get
zero it means that there was some energy
that we didn't observe and that that
invisible energy that missing energy
corresponds to an invisible particle
that's carrying away some energy from
the initial collision
so it's only by measuring everything
that we can infer that there was
something didn't see
great thank you so we're coming up on
the Ahlers so I've got two more
questions coming up and then if we
didn't manage to get you to your
question then please post it in the
video description afterwards and we'll
try and get to it in in the text though
we will try and make sure that all the
questions are answered later if not
right now and so the next one is is it
challenging to compute the huge amounts
of millions of collisions per se
absolutely it is this is one of the big
challenges so so just recording all of
the data so we can't actually record all
of the collisions it's too much data and
so we have to make very quick decisions
in microseconds about which collisions
are worth saving and which ones are just
two protons bouncing off of each other
and so then we we only record a fraction
of the collisions and then we have still
a huge amount of data trillions and
trillions and trillions of collisions to
analyze that it is both computationally
in terms of writing software and in
terms of designing good algorithms it is
an enormous challenge to efficiently and
sensibly sort through all of those
collisions and that's you know that's
what a large fraction of the Atlas
collaborators that's what we spend our
time
okay wonderful thanks and then the final
question is a bit of a thought
experiment and so it's what if one day
the gravitational force disappeared life
be possible yes no the answer is no so
the amazing thing about the standard
model is that you can take any one of
these properties and if you changed most
of them even slightly then life as we
know it would disappear so so everything
is very sensitive to the particle
content and exactly how these particles
interact and the same is true for
gravity so for sure biology would fail
miserably if you took away gravity okay
thank you thank you so much that's all
we have time for for the questions today
so let me thank you dr. Laura for
joining us for your wonderful talk and
for answering all of these questions
thank you very much thank you for all
the great questions thanks and thanks
everyone for joining and we'll end the
live stream here so take care bye
