Hello, everybody,
and welcome to early morning coffee at CERN.
I'm Steven Goldfarb and we have a really I would say mind boggling show
for you today because we're going to be talking about quantum mechanics.
And that's a world that boggles I think everybody's mind.
I think as Richard Feynman said, if it doesn't, then you don't get it.
Right.
So, so I have here with me, Giulia Negro from the CMS experiment.
Giulia, you work, for Purdue University.
In beautiful Lafayette, Indiana.
By the way,
for my French friends, it doesn't mean that the university has been lost.
Okay.
It is actually a beautiful place.
Lafayette, Indiana.
And also, with me, to talk about quantum entanglement of top quarks
I have one of my colleagues from the ATLAS Experiment, Yoav Afik.
Yoav.
And he is from
as you can see from his cup, the Enrico Fermi Institute,
at the University of Chicago in Chicago, Illinois.
So actually, your institutes are not far from each other,
but you guys are both far from your institutes
We are both based here at CERN, actually.
I beat you on that, though, because I actually work
for the University of Melbourne in Australia.
Okay, so this is a very typical thing, here at CERN.
So we're here actually today we're going to be celebrating
the International Day of Quantum Science and Technology
as declared by the United Nations.
UNESCO sponsors these special events.
And we'll be talking
about this really nice measurement that you guys have made,
about the quantum entanglement of two top quarks
in the Large Hadron Collider, both at ATLAS and at CMS.
And I have to say, first of all, this is a measurement that was not previewed.
We have these these books when we start these experiments.
The yellow books, right?
And I think they were not actually yellow for the LHC.
But back in the day they were yellow
they would they would sort of list all the different types of physics
and things that we might discover, things and measurements, we might make.
And you guys went beyond that. Right.
So we're going to talk about that.
But first I'd like to to get into quantum mechanics if we can.
A little bit.
Maybe you each have an opinion, but usually when UNESCO
does something like this, it's because it's 100 years since something.
So Giulia,
What what do you what would you say happened a hundred years ago
That launched this?
Okay.
Well, 100 years ago, there were a lot of, discoveries.
There was, process.
There were a lot of physicists,
that had different ideas.
Yeah.
For example, Heisenberg with his matrix mechanics,
and Schrödinger with the wave mechanics.
So that was, what, 1925?
Yes. It's the centennial this year,
that's why it's considered the beginning, but,
yeah, I think it's more of a process
than a specific year of when this all started. That makes a lot of sense.
Yuav, you have an opinion on this?
Yeah. I see it as a process.
I think what Giulia mentioned is where let's say the formalism
has been designed for quantum mechanics, but in fact,
I think it started
even earlier when Einstein introduced the photoelectric effect.
And so he didn't have the exact formalism
of the quantum mechanics, of course, as we know it today.
But he was able to show that light comes in quanta, which is exactly,
you know, what is quantum mechanics is all about - discrete quantities.
And I think at the time, I mean, he wasn't always a big fan, right?
of quantum mechanics, he thought that there was something strange
about not being deterministic.
- I think it was hard for him that there are many concepts
that weren't so let's say intuitive in quantum mechanics.
I mean, as you mentioned before, right?
Feynman said, if you think you understand quantum mechanics means you understood nothing.
Nothing. Yeah. And then we understand nothing.
We're willing to say that as scientists. It's one of the things you find out
when you're a scientist, especially at CERN, is how much you don't know
Most of the universe we don't know.
But we've learned a lot, and we're making a lot of progress, which is important.
So our director, Chetna,
always reminds me that there's a guy named (Satyendra) Bose
who was very important in all of this as well.
And I think it is interesting because some of
the concepts of quantum mechanics
came about somewhat by mistake.
I think, you know, you mentioned,
the photoelectric effect E equals Hv.
And so there were quanta in, the energy,
of a photon, which depends on its frequency.
So you have to use these exact discreet
quanta.
There were also,
even just before that, that the equation came from Max Planck, hence
the Planck constant.
When he was trying to understand (black body) radiation.
And he found that using that mathematics, using
this idea of these various frequencies,
you got
solutions to the problems that they were looking at.
So the at the time, they were trying to measure the different discrete spectra
of electrons
orbiting around the proton.
And, and that's kind of cool, I think, because we're experimentalists.
Yeah.
You know, experiment led and
they couldn't come up with a solution using classical mechanics.
And so we got to this really, really cool world of quantum mechanics.
Bose, he also made a mistake which was in counting, in the classroom.
And he was talking about counting different states, we have two particles
and they can be one spin up and one spin down.
And you can try to count probabilities.
And normally you say, okay, you can have them both up and both down.
Or there's two different ways it can be up and down.
And if you have something like electrons, where that's the correct way to count
because you can differentiate between the two particles.
But Bose those just said, okay, there's two up,
two down, and then there's one with one up and one down.
So three possibilities.
It turned out that worked for bosons like photons.
And then because they're called bosons right.
Because of Bose.
So it was a really good contribution.
He realised it was a mistake, but then then he started using it.
He said, this works.
And he tried to publish.
And due to various biases
around the world, maybe just the bias of this we have never done before
or could have been cultural knows, it wasn't accepted.
But then he sent it to Einstein.
and he said it was good, and it got accepted.
So we always have to fight our biases, both from past,
what we thought was true.
And we have to draft those.
But also there's still cultural biases.
We always get.
Having a place like CERN is a great place to battle that.
Right?
Because you know,
when you get when you have lunch, you know, who knows who will be sitting next to you.
So, okay, so no one asks the question.
Quantum mechanics.
It's a weird world, right?
It's not like the world that we live in.
The macroscopic.
Giulia, tell me:
What is your favourite effect?
But of course, it's the quantum entanglement.
We are speaking about that today.
And you have
to have to explain this because it's really difficult to comprehend.
And and I suppose because she said that, you Yoav?
which one? She took entanglement from me,
so I would
say Bell inequality
or the violation of Bell inequality.
And I can explain, shortly, what this all means.
Yeah, you should explain, because Bell is from CERN.
So as we spoke before about Einstein.
So, Einstein, not only Einstein, but
he had a bit of a problem with entanglement, right?
So let me let me try and explain the concept of entanglement.
So entanglement tells you ... let's say that you have two particles.
You cannot describe them
within quantum mechanics independently from each other.
What does it mean.
Let's say that we have two particles, two electrons, for example,
in two parts of the universe. These electrons,
they have a property which is called spin, which we can measure.
So it turns out that if we measure,
if they are coming
from the same source which we call as singlet,
and we measure the spin of one of the electrons
on one side of the universe, we would immediately know the outcome
on the other one.
Immediately? So not with time for it to go around the WWW?
And this is mind blowing, right.
And it can tell you, for example, it implies that the information
travels faster than the speed of light, which contradicts,
Einstein's theory of relativity.
So this is why, in the 30's, Einstein
together with, with Podolsky and Rosen came up with the
famous paper of the EPR paradox, the Einstein, Podolsky and Rosen paradox,
which basically claims that quantum mechanics is an incomplete theory.
We have a set of hidden variables within the theory, which tells us
before the measurement itself, the outcome of the measurement of one electron and the other one.
And we need more variables
in order to describe this theory.
Okay.
Now, in the 60's came John Stewart Bell,
and showed that if these theories exist,
it means that they have to fulfil
this, very famous Bell inequality.
So you can actually exclude these theories
with additional local hidden variables
as claimed by Einstein, Podolsky and Rosen.
by breaking the Bell inequality.
So it so it's broken then.
It has been measured to be violated
many times already. Yes.
And there was,
there was a Nobel Prize recently for that a couple of years ago, yes.
If I remember somewhere around here, 2022,
the winners of the 2022 Nobel Prize were Aspect, Clauser and Zeilinger
Exactly, Aspect, Clauser and Zeilinger.
Very good.
Yes, exactly. For testing this concept.
And that's important, I think, Bell was at CERN actually.
Yes he was.
So we have we have some smart people here,
On occasion, there are some smart people here.
So as long as we're talking about
quantum entanglement here,
you guys both independently did measurements of quantum entanglement.
Of course.
That's why we have different experiments, right?
You know, and CMS and Atlas are beautiful experiments.
They both enormous.
They both have a lot of people, like 5000 or 6000 people on CMS, right?
5000 or 6000 on ATLAS.
There's 3000 authors, I think
Yeah, almost
You know that that means that I've written less.
I've been on ATLAS with you.
Even before you,
Yeah, because I think, I've been around since 1998
and I still haven't written a total of one paper.
Right.
Because if you take and divide by 3000, we have to write more papers, I think.
So you've independently done this, I think, the story,
of how you came up with the idea of looking
for quantum entanglement,
in the LHC, because just like I said, this is never too far before.
I think you have the story. - We came up with the idea.
So together with, Juan Ramón Muñoz de Nova
when I was in my Ph.D., we were in the same institute.
He's actually coming from the field of condensed matter physics.
So this already tells us something about the interdisciplinary nature of this idea.
And we were friends.
And we met for coffee breaks at the TECHNION where I did my PhD.
And we chatted.
Also, sometimes about physics, of course.
Yeah, yeah.
Well, more than sometimes. Yeah.
And then he asked me
if I think that we can measure entanglement at the LHC.
I said, okay, that's a very interesting question.
And I started to,
to do some
research and to see the possibilities.
Then I saw this,
very unique particle,
which is the top quark, which I assume we will discuss about soon.
Yeah.
And this seemed to me also by the previous measurements
that I saw were done using top quarks as the perfect system.
to try and measure quantum entanglement at the LHC
And that is what we did.
Someone had thought about that idea
that you could use the top quark.
So we thought about it together, you know, Juan and myself together.
And this led to a paper published about a year after we chatted about it.
This was basically the baseline
for both of the measurements.
And you were a student at the time? - Yes, I was a PhD student back then.
That's actually our workforce.
People don't realise that.
But like more than a third of the authors on our experiments are PhD students.
and they're also the people you'll find on shifts a lot.
Postdocs are also okay, The rest of us
are kind of useless.
but we have fun anyhow!
So you asked about it.
Maybe Giulia can explain a little bit more
Why is the top quark
something that we can look at
in this case.
So the top quark is very special because,
a difference with respect to the other quarks is that
it decays before it can hadronise.
So the information, for example, the spin information, of the
the top quark is transferred, to its decay products.
Interesting that you bring up hadronise, so quarks are very special.
Yes, they are fundamental particles
They're fundamental,
when we talk about
doing particle physics at CERN,
So we talked about looking at the elementary
or fundamental particles those things you can't cut a top quark in half.
Right.
So we have 6 of these quarks.
Two of them are stable because they're really light.
The up and the down. The other ones, they appear for short times.
And then I guess the the more massive they are,
the quicker they decay. Indeed,
the top quark is the one with the highest mass
it is 40 times, larger than the bottom quark.
It's about 180 times the mass of the proton.
And a proton is made up
of quarks.
Right. So yeah.
The top quark is super massive.
It's the most massive of any of the, any of the elementary particles.
Even the bosons, like
the W and the Z, even the Higgs even more massive than the Higgs.
So it doesn't have a chance
like the other quarks have this wonderful life because
as soon as they're born, they find a partner or a couple partners, right.
And they hang out with them,
and it takes a huge amount of effort to pull them apart.
It's the strong nuclear force.
And when you do when you finally get these two
quark and antiquark and you pull them apart, suddenly out of, out of nowhere
out of the vacuum, a couple other quarks appear.
they can't get to, they get divorced and they immediately got new partners.
But the poor top quark is all alone.
and it disintegrates, it transforms
to other
I don't like using decays,
I agree, my wife tells me that more massive you get
the quicker you'll decay,
And I agree that like,
you know, I don't like this word because when you think of decays,
you think of the breaking into parts
and the top quark doesn't break into parts, it transforms into energy.
Plus the
W and the bottom quark
Exactly.
So you have these decay products, you know,
and then you can look at
their energy, their momentum, their angles
and then you can
figure things out from that?
In our case for the entanglement,
we measured that the angular correlation between these decay products,
So in this case with the leptons, that decay from the top quark.
Okay.
So you have you have a couple leptons.
It's complicated when they decay right?
Top quarks have a few possibilities to decay, we look specifically at,
we need to say that the top quarks are created in a pair of the top quark and its antimatter counterpart, the antitop
And between these two, we actually measured the entanglement
the spin entanglement between the top of the antitop.
Both of them can decay,
so they can
have a few possibilities to decay.
One of the possibilities to decay to final states with charged leptons.
So, for example, electron or positron
or the heavier twin of the electron
not exactly the twin,
the muon or the antimuon
why we do this?
Because the in our detectors, we measure
the charged leptons in very good precision.
And we need to have the tracks being measured very precisely.
Because, as Giulia said before,
in the end, we need to measure the angular separations between one charged lepton from one top,
to the other charged lepton from the antitop.
And this way we can deduce something about their spin correlations
and about the entanglement between the top and anti top.
So it tells you that they are correlated. Yes.
That's interesting.
And you came up with this over coffee,
during a conference in, more or less.
Yeah, a regular coffee break in the office.
Oh, yeah. Okay.
Coffee is very important, by the way.
So Chetna wants to know.
What exactly does it mean?
So under certain circumstances,
they're entangled you measure that they are entangled
So how are they entangled, what is the meaning behind this?
So, as Yoav said before, that
means that you cannot describe, the spin state of one quark independently from the other
in quantum mechanics,
we use, just for example, the wave function
to describe the state of a given particle or a given system.
we can also use what, what is called the density matrix,
which gives us the density to be in some specific states.
And the idea behind entanglement is that if
we have this density matrix which describes the quantum state of the system,
and we have defined this for the top and the antitop.
for the system of the two,
we cannot describe this just by a combination
of the individual spin density matrix of each one of the top
or the antitop.
This tells us that the state is not separable.
We cannot describe the state of the top and antitop independently from the other.
You know, if you're listening
and you don't completely get this it's normal, right?
This is a world that we don't live in.
It's like the electrons that were mentioned before, right?
Because we know the outcome of the measurement of
one electron
after we did the measurement on the other one, it tells us
that basically we cannot describe the quantum state
of both of the electrons, sorry, as an independent combination
of the states of both of them,
just because the correlations look so strong, there's so much affect each other.
And so you have to describe the system as a whole.
So I understand this
was not simple to do, there were some hallenges in doing the analysis.
Where to start?
Well, one challenge is that we had to deal
with the modelling of the t-tbar
production threshold region
So, close to
two times the mass of the top quark
And
this region is not very well modelled yet,
So we had to come up with a
simplified model, and then we introduced this
pseudoscalar resonance, that is a bound state of a top quark and a top antiquark,
So this is called toponium.
and then this improves
our modelling of this region.
Including this was not an easy process
and, in our measurements
we managed to do it.
So in ATLAS they came out first but they didn't include this in the model.
Yeah, we were able to show that our measurement
is not affected by this effect.
But nevertheless, this is actually a super interesting point
because this is a very cool effect.
The idea behind this is that
close to the production threshold where the top and anti-top are slow,
there are some non-relativistic effects that enters
this is the so-called toponium.
It's funny, this is not included by default
in our Monte Carlo simulations.
It's a non-perturbative effect.
Actually, the tools we used and that were used in CMS,
let's say give a better and better
grasp of this specific effect
and perhaps also the possibility to measure it
and to develop some observables which are more sensitive to this state
because this state brings an enhancement
of the more entangled events to a cross section.
So it basically means that we should see more entangled events than
than expected from Monte Carlo simulations that we use.
So you mentioned a few things in there.
I'm not sure if we're going to go into details, but now,
QCD is very complicated.
So you you have these different realms, right?
the perturbative and the non perturbative, which has to do
with relativity effects? Yes.
The nice thing is that actually these measurement of entanglement, beyond being
these measurements of entanglement, beyond
being just, you know, being a very nice
and cool interdisciplinary thing to measure, they actually brought a lot of benefits
to high-energy physics, to measure processes of high-energy physics.
So, so tell me a little bit more about this. What are the benefits that come from this.
So besides the
possibility to have a better grasp of toponym,
mentioned before by Giulia,
So for example, if you look at the measurement by CMS,
you can see that the data agrees better with the existence of this toponium.
Which is quite nice, because if we look at our
resolution in the detector to reconstruct the invariant mass
of the top and the anti top,
We can never see this because it's
a lot more narrow than our t-tbar resolution.
But with the spin correlation effects and entanglement observables,
it's possible to see some signs of this,
It also gave a push to theorists, to investigate a bit more.
And this also came up in a few other measurements
by CMS, these effects,
In addition, I have to say that
there are many new techniques to search for physics
beyond the Standard Model that has been developed based on the quantum observables
that we thought about and that we looked at in our measurements.
Physics beyond the Standard Model
is something we all want because this is what makes our careers here
as experimentalists is forever
trying to find out what's wrong with theory.
We love theorists. We love theorists.
But they only explain 5% of the universe.
Or less, we don't even know gravity.
Okay, so there are a lot of things, a lot of big questions out there.
And so we're always looking to see where the model can break.
And that gives us hints.
And so to see this it gives us some more tools.
I should mention toponium is only one of the oniums right?
There are others that have been measured
bottomonium, charmonium,
meaning that you have a charm quark and an anti charm,
bottom, or anti bottom, but these we can measure more directly.
Yes, but
with the top it's not so easy.
Also,
it's not exactly the same thing because the tops really decay before
they can form like a particle a bound state, a meson,
So maybe toponium is a bit of a misleading name.
So they haven't really formed,
They exchange a few gluons before they decay.
Gluons being the carriers of the strong nuclear force,
like photons, we have photons coming through us right now.
That's what's inside these protons.
The gluons, great name.
Keeps it all glued.
So my next question for you is, what's next?
I mean, if you you guys learn off from each other, this is very common
for experiments. We go to conferences or we have discussions or seminars.
That's when we share information.
Of course, you know,
some information gets shared during coffee breaks,
As we've learned,
But now, I mean,
will ATLAS, for example, do a measurement with toponium
included, or,
you found that it's an effect that doesn't we don't know.
Can't say.
Oh, yeah.
We can't always say these things.
In general, how do you see us going forward with these measurements?
Well there is not only quantum entanglement that we can measure,
there are also lots of other quantum correlations,
like Bell's inequality, as we mentioned before.
So for sure, one of the next
point will be to also discover
these new quantum effects.
I want to give maybe a number.
Just to give you an idea.
Why is it interesting to do these measurements
So we mentioned before the Nobel Prize in physics.
Right.
And it was done by testing
the entanglement and the Bell inequality with photons.
What we do here at the LHC, the measurement
that both CMS and ATLAS did
with top quarks, is about 12 orders of magnitude higher in energy than
all of these extremely important laboratory experiments.
So when you go so much higher in the scale,
there is already a fundamental interest of why we do this.
And what CMS and ATLAS did,
at least to me, is a proof of concept
that we can actually perform such measurements using collider physics,
using collider experiments,
and there is so much room for doing more. So many other proposals,
to measure many other things.
A lot has come up
since then.
And we think we can say both in CMS and ATLAS we are
now working on performing these other measurements in some other parts of phase space with top quarks,
or, for example, with the Higgs boson decay.
And so there is a lot to do.
This is the first time this is being done in collider research.
It was the measurement with the highest energy ever performed.
At the LHC it was the first time.
It was done before
in colliders with mesons
at lower energies, in particular with B mesons,
but it's a different type of entanglement.
It's more flavour entanglement it's related to other properties of particles.
But never at the LHC,
never with quarks
which are fundamental particles,
It's pure quantum entanglement because it's a fundamental particle
Remember the example that I gave with
both of the electrons at other parts of the Universe.
Yeah. This was never done actually.
Yeah.
That would take some effort to go to different parts of the universe.
Well yes.
But in general between two what we call free electrons.
Yeah.
I mean it was done with a bit more complicated systems.
Here, we actually measured the entanglement between two quarks which are
let's say quasi-free particles.
So, you have theorists
coming to you, saying, I got some ideas now, right?
I guess you have both been approached by theorists
and different things in quantum mechanics that can be measured.
Also though, what we have coming up in a few years,
we're going to tear our experiments apart
and we're going to go to a High-Luminosity LHC.
So actually, we have, a year and a half or so
of running and then nothing for 3 and a half to 4 years.
And then we start up with brand new beautiful detectors
made to be able to go at a much higher rate, so we'll have much higher statistics,
will that help?
Will that be of use to you?
We will have more data so we can get measurements more precisely.
So, for example, I mean, as was mentioned before by Giulia,
we have a lot more concepts of quantum correlations that we can measure.
If you look, for example at Bell state, or Bell inequality,
if we look at the measurement,
in t-tbar, we have to go to a lot more extreme
parts of phase space, to perform these measurements.
And if we have more data in order to do it,
it means that we have more events and more statistics to actually be able to
do these measurements because,
of course, we need statistics to make
precise measurements So extreme parts of phase space.
Sounds like a great place to go.
But it is
sort of those things which are rare.
It's rare to find something that has this momentum,
this energy,
this mass, whatever.
Those are what we consider phase space.
And yeah, with more statistics.
Even if we're not going to increase energy, because the LHC won't increase
much more,
we are about as high as we can go,
having much more data is a way of also going up in energy
because we can produce those things the more rare things you will see there.
Okay.
Well, I'm looking forward to seeing new results.
I think it's going to be a lot of fun. I think especially because
you've gone into an area that we haven't been in before.
So congratulations on doing that.
So thank you.
I want you to thank Giulia Negro
From Purdue University, in the CMS experiment.
And also Yoav Afik from the ATLAS experiment and from
Enrico Fermi Institute, as shown on his cup
and you have your beautiful CMS cup here.
And I celebrate outreach.
with my IPPOG cup here.
This has been early morning coffee at CERN.
It's a podcast by the scientists of CERN
about the science of CERN.
You can find all of our episodes on CERN's YouTube channel.
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Our editor and producer is Chetna Krishna.
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My name is Steven Goldfarb, and this has been Early Morning Coffee at CERN.
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