The God particle, right? Oh no, oh no! I'm thinking about the number of stars in the galaxy.
There's dark energy. I've got the dark energy, I've got it right in my coffee cup
[Music]
Hello everybody and welcome to Early Morning Coffee at CERN. My name is Steven Goldfarb and my name is Joni Pham and we have a really nice show for you today.
It's on a topic which is perhaps one of our favourites here at CERN, that's the discovery of the Higgs boson
and believe it or not, it's been about 12 years now since that discovery. With us to talk about the discovery are a
couple people who were there at the time working on the physics, so we have Heather Grey next to
me who is working on the ATLAS experiment. You're from UC Berkeley? Yes, that's right. And Andre David
who's from the CMS experiment, the other experiment that was also working on the discovery of the Higgs boson.
Both of them announced together another the discovery back then, July 4th it was.
Andre you're a CERN physicist. Yeah, that's right. So, let's get started right away.
I think it'd be nice to hear from you maybe a brief introduction of yourself Heather.
What brought you here to CERN and what have you been working on? Sure, happy to do that.
So I think the first time I came to CERN was, gosh, almost 20 years ago now. You were a kid. I was a kid, yeah.
I was a CERN summer student and so I'm from South Africa and back
in the day they had money to bring one summer student from the whole of Africa to CERN
for the summer programme and sort of a month or two before the programme was going to start they didn't
have anybody, so my professor at the time came to me, and said, "Would you like to go CERN?"
I said, "hmm, okay!" So they took only the very best volunteers is what you're saying. They took "a" volunteer at the time
who happened to be the very best, of course. No but it's actually quite funny because before that I thought I was
going to be a theoretical physicist as do many when they're young, but somehow being at CERN for
the summer and seeing all the experiments and already seeing experimental physics, I was an
experimentalist and that was done for after that. Well that's great, and you've stuck with it.
You're associate professor. Yeah I'm an associate professor at UC Berkeley.
I actually have a sort of joint appointment which means I'm half at UC Berkeley and half at Lawrence Berkeley
lab, which is great because it means I only have to teach half a load compared to everybody
else and I get to come visit CERN and do things like that. I'm sure you love teaching though but you
love the experiments as well, right? I like both of them. Excellent. But no it gives extra flexibility
Andre. Well, I hail from Portugal and I was a student, I was working on theoretical
nuclear physics and microtechnology, so making chips and then somebody told me, "Andre, you're
not really good at this theoretical stuff but I know of someone who has something going on
at CERN." And I didn't really know what CERN was, and I came here for two weeks at the end of the year
2000 and I did get hooked. I mean just seeing all of the kinds of things that are done here.
So I've been here for the last 24 years working in different types of experiments and you were also
a kid when you came here, I guess. We were students at some point.
So another question for you, Andre, on July 4th 2012 where were you?
It depends on the time. I think that when when the day started I was still here in building 40
where we have a lot of our offices, trying to make sure that all the last dots, you know all
the i's were dotted, and the t's were crossed on the slide deck that was going to be shown and
then I went for a walk to look at the people who were by the seminar room, the main auditorium here at CERN.
There was already a big queue and people were playing cards. This was like a sort
of a wild camp going on and then during the day after taking a shower, I came back and I was in the main auditorium.
For the announcement. Just so that everybody knows, it was a very exciting time
we knew something was going to be shown, right? It was the result of our searches and
we've been searching for the Higgs boson for many years actually before the LHC.
I was tricked, okay, I wasn't here. I was in Australia for the conference where it
was originally going to be announced and the CERN Council in their wisdom decided
that it should be presented at CERN. Were you here? No. Where were you?
It was quite amusing I was actually at, there's an annual meeting of Nobel laureates in Lindau in Germany
where they get together, you know, a large number of Nobel laureates and also some young students.
So I was there at the time. Being a Nobel laureate you were. Obviously. No, I was
the young student of course. But it was quite funny because it was the middle of the programme
of the meeting and we sort of said, "We think, we should broadcast the seminar." We went to
the organizers and they said no no no the other Noble laureate said no they refused to cancel their talks.
Oh yeah. So, me and a couple others about 10 of us initially we set up a sort of bootleg
setup, with you know, a bunch of computers and things because the connection kept dropping because so
many people were connected to what was going on at CERN. So when one died we switched to the next one
and we kind of got a growing following including a number of Nobel Laureates watching our bootleg. I guess so.
People felt there was something exciting going on. Joni, where were you? So, 12 years ago, I was
still an undergraduate student at the University of Melbourne and at that time when the announcement
was made, it was made almost like simultaneously, right? Like one at the auditorium here at CERN
and the other at the International Conference of High–energy physics (ICHEP) in Melbourne and the University
of Melbourne was one of the organizers, so I was begging my physics professor
to somehow sneak me in the conference. That's where I saw you the first time. You looked a
lot younger than you are now. I was younger, you you were the same age, I think, but I was younger.
That's true. I said I got tricked, right, and that trick happened because
I guess it was in December of 2011, we had a hint that there was something in our data and
I got very excited by this hint. We always get tricked by three sigma events but in this case
things look lined up, it looked like we might have something and so I booked my flight right away.
I knew that the next major conference, there was going to be a presentation and that was at
Melbourne, the ICHEP, that was organized there. So we were far away, but fortunately, we
had this nice webcast. It was done in both places simultaneously and we could watch it.
Was it in the middle of the night in Melbourne that you were doing it? No actually we managed it so that
it was done early here. It was done in the morning and it was
late there. It was like 5 o' clock or something like that in the evening. It wasn't wasn't too
unreasonable. It's not like the watch parties that I've seen from the US where they had pyjama-like
things to watch. Yeah they weren't so happy that we arranged it to be in the morning here
because in the States it was whatever 2 in the morning or whenever that they had
to watch it but yeah there were people in their pyjamas watching from Fermilab and other places in the US.
It was a very exciting time which brings a natural question, why do you think
this so exciting for us? What what was so important about the discovery of this particular particle?
We've discovered particles before. Why was the Higgs boson an important discovery for our field.
You mean, the God particle, right? Oh no, oh no. Okay that was Chetna, our producer in the back here
asking if it's called the God particle. So this terrible name, name we don't like because we
didn't really call it that, it came about because of a book that was put out, which is a
good book, by the way, if you want to read more about the Higgs boson. Leon Lederman and Dick Teresi
they wrote this book together and the story that I heard and it was only sort
of secondhand, someone who knew Leon Lederman, who told me this, so I hope I got this right.
Leon Lederman had just given a lecture to his students and he was talking about symmetry. Symmetries are
extremely important in physics, right? We look for those all the time because we seem to find
bit by bit that some forces which we think were separate are actually two sides of the same coin.
Electricity and magnetism. Electromagnetism and the weak nuclear force, that turns out you can make a
theory that combines them and so he was showing this to his class and he said it could be maybe we
can dream that one day all of the forces will come together and maybe there'll be just one particle
and an explanation that describes the movements of this particle and we could call it, and he said,
"Well let's call it the God particle." So he made this lecture to his class and Dick was in
the room and saw this and then they went together and they were talking about this manuscript, this
book that they were about to publish and Dick said, "Look they've asked us to give a working
title to this book," and Dick said, "I propose that we call it the God particle."
And Leon Lederman said no absolutely not and Dick said now hold on now every single book
that I've ever written in the past whenever they ask us for a title to propose a title for it they
never accept my working title and so if we call it this, it's guaranteed it won't be called that.
Unfortunately this time the publishers decided to call it the God particle and to which Leon
Lederman said it would have been much better to call it the goddamn particle because we've
been searching for it, it's really hard to predict where to find. We had no idea what its mass would be.
So there you go, Chetna. I hope you're happy. We've laid that to rest, no one is
ever going to call it that again for eternity okay but let's move on. This particular particle which
got this horrible nickname, the Higgs boson, why is it so important to to our current theory?
So we have this model which describes all the particles and their interactions, the Standard
Model and this is something that was come up with by a bunch of theorists half a century ago
a little bit longer well there were various pieces that came along and in this theory
provides a description of everything we know so all the particles all their interactions
however it has one problem which would be that certain particles don't have any masses.
So this is a problem because we have nice experimental measurements which showed
us that the particles do have mass. And so one way to think about the Higgs boson
and indeed the Higgs mechanism which is kind of how it does it is it's a mathematical trick.
It basically allows us to preserve the symmetries of the theory which you need the particles to be
massless yet for them at the same time to have mass as we know experimentally to do it and
so it's really critical in this Standard Model because it resolves this apparent contradiction between
you know what the theory says we need it to have and then what we saw in the experiment.
And even inside the theory, it's completely unique it's the only elementary particle that is
a scaler. What's that? I knew you're going to go there.
It means it has no notion of direction, it just permeates, there's a field that permeates the
whole of of the universe and different particles interact differently with it but it's the only
particle in the theory that we have right now with these properties. There could be others but right
now this is what is needed for the theory to work but the theory, I mean, it's very beautiful
the mathematical edifice for which this Higgs boson was created and then a little later came an idea
of hey since we already have this thing in and this is Steven Weinberg, why don't we just write
a few more terms and insert a lot of arbitrary parameters to give masses to other particles like
the leptons (like the electron, the muon) and the quarks and so you end up having like this one
single actor playing two very different roles in one single theory and that's really really strange
how the two things are mixed together so the Higgs participates in that beautiful symmetry breaking
that Heather was talking about and then it also participates in a, you get a coupling (the strength of interaction between particles), you get a coupling,
you get a mass, you get a mass, which has a very ad hoc feeling. Yeah, it's funny you
mentioned that, so that messy part of the Standard Model, that's the part that I study because it
bothers me a lot, right? I really don't like the fact that for all these individual particles
we have these different parameters and so if I want to take a bet of you know where should we
go and have a look for something that might be different, it's there and so where my research
is trying to go through and measure some of the more tricky ones in that area. In particular
I'm looking to see if the Higgs boson interacts with the charm quarks. Okay. We've been seeing
that it interacts with the more massive quarks or the more massive elementary particles more
strongly because.. that's exactly what comes out of the theory is that
if you interact more strongly with the Higgs boson you end up appearing as having a larger mass.
Okay. Or the other way around, yeah okay it it does become circular but that's okay.
I agree wait wait wait because this is important because it's
like when you measure the mass of a particle you're measuring the mass of the particle
that's the thing that you measure experimentally. Then you take that mass and you can infer what
is the coupling, which the coupling is a theory parameter so you can go, you go that way, it's a good
point. So the coupling is how strongly it interacts with other things. So it's interesting.
I should actually step back a second and say you know we've referred to this as
the Higgs boson all the way through this. So it was proposed by Peter Higgs, there were others
involved, right? In fact there was a long list of people. This is my second least favourite question
about Higgs boson. You got the first one correct. I'm going for it. I'll get them all.
You got the god question first. Yeah exactly. But I took that over, okay,
but now who would like to? I mean there is a list of people. No that's why it was
something I think probably there's a long list of people and I think probably the reason for
that is that it was a natural thought to have at the time in terms of the theoretical development
and that's why it turned out that a number of people came up with it independently this is
at least my understanding of it. Yeah so part of the math was actually taken from
solid state physics because there you actually have things condensating together
and behaving as one and the idea here was to do some of that let's say mathematical machinery but
for the elementary particles and you do have people who contributed before Peter Higgs but then
Peter Higgs crucially got his paper rejected and then he added a paragraph and it's that paragraph which
is absolutely crucial because it's a paragraph that says and this particle will have a mass
and so that's the thing that actually makes it distinctive and I think that's the
reason why we call it the Higgs boson and not the Brout-Englert-Higgs boson but we do call it the
Brout-Englert-Higgs mechanism because it's the underlying symmetry breaking mechanism that
Heather was mentioning that one does involve a bit more but then it was not over there because
once you start adding these ingredients but you still need to make a recipe that does
not kill you when you try to eat it so there were other ingredients that were contributed by other
people like for instance making sure that you can still have massive particles but a photon that is
pretty massless from what we can observe so getting all of those things cooked together in a
way that you can make reasonable predictions took the effort of many people and taking hints
from many different places. And then you mentioned before right the fermionic side, right, so we were
talking about the Higgs being a boson. Particles can be two types, they can be bosons and fermions but
that came kind of separately and that was largely I believe from Steven Weinberg.
He had a very nice paper. Two and a half pages long. I know I mean you look back at these papers even
Peter Higgs' paper, right, I think it's four pages or something, few pages to do it and we need 15
pages for labelling the number of the physicists, the names of the physicists who were involved.
Wait wait wait wait, please do not mix theoretical work with experimental work because
one of the things that I really admire about theorists is their ability to just go walk in
the Alps and come back and write four pages with a completely new idea that then we have to go and
build a detector and get thousands of people together to build these detectors to disprove
that idea but there's a very different exercise of working together as experimentalists.
Don't forget when they took that walk it was because they'd read our papers from before that showed
them that we had found some patterns or some particle or something. Sometimes. Sometimes they
just go off in their own directions you. Since you mentioned the Higgs paper in 1964
and as far as I remember he didn't propose like any experiment that can verify his
theory so why did the scientific community take him seriously and decided to spend like
billions of euro on building the detectors to search for the boson. Well I don't think they did
immediately to do it right. I mean this was in the 60s which is now 60 years ago to do it.
There was another paper I think it came out in the ' 80s if I remember correctly,
the one of John Ellis and Mary K to do it which is called on the experimental profile
of the Higgs boson or something like that. It has this beautiful coda. Exactly and the coda basically in
this paper I think it was the 80s I could be slightly wrong about it.The phenomenological profile.
Yes that's the one. It basically says we do not encourage
large experimental searches for such a boson but the paper basically is out, you know, here's
the Higgs, here's how you might look for it and things like that. If it has this mass it looks
like this if it has that mass it looks like that. That was quite useful and they discouraged
us simply because it's so hard, right, it's not easy to wasn't an easy task to find. It was hard and it
the mass was unknown, which is something Andre, I think, already mentioned which meant that it's not
like you could build an experiment to go find this particle of a specific mass you had to be able to
cover this entire range and if it was infinite you would maybe not find it at all to do it
but then subsequently we added extra constraints which able to sort of narrow down the range where
we were doing. So I think what the answer I'm trying to give is it was a bit of a process.
So it wasn't like you know Peter Higgs had this idea and they said okay let's go and do a 60-year programme
to find the Higgs boson to do it. They started actually by exploring the Standard Model because
there were a number of these particles they had to discover along the way and then as things were
discovered how to search for it became more clear and then the decision was taken and when you
come back to the billions so one of the things that the LHC had going for it is that you either
find the Higgs boson or we would be operating, we would be doing collisions at such an energy that
the theory becomes invalid and then something else has to come up. So out of these two outcomes we did
find something that looks like a Higgs boson which is great so we now have a new tool with which to
hammer nature and get the truth out but this was called a no-lose theorem and that
was critical for actually going and spending the money but I don't think Peter Higgs even thought
about spending the money. So back in the 1960s they were trying to figure out how to make the
math work. How can you make the observations, how can you reconcile the observations with what
the theory is saying because in the beginning before Brout, Englert, HIggs
all of these people got the math worked out, the theory just said all particles are massless and then
it was clear that they weren't because we had experimental evidence that they weren't.
So they figured out that problem and then there were other problems with the theory and then at some
point we reached the point where we could build an accelerator that would make collisions that
if there is no Higgs boson, then the calculations start giving answers like infinity and infinity
is a very large number. Yeah, I mean I actually really like that we were very lucky to have this
no-lose theorem. It's very rare that you can build an experiment like that but it was even I mean
when you say the theory breaks, it was quantum mechanics that would break, right, so it's not just
you know the Higgs would not be there or something, it would be that something very fundamental in
our understanding of physics would have been wrong. And that would have been great.
This is something that, well, it would have been a challenging time. No, no, no, so I agree with
what you're saying but some people panic when there's a crisis,
some people panic, some people calm down. I don't know how to put it.
So when things break it's an actual opportunity to go and try and understand
why doesn't it work because the measurements don't lie, the measurements are statements about nature.
Then how can we explain the measurements if we no longer have a way of explaining them, you have to
find a different way of explaining them. This could be like Michelson–Morley and not finding
the ether and then you have someone like Einstein come along and say of course there's no ether.
Here's another solution. We do like it when our physical theories
are broken and when we find something wrong with them and we spend a lot of time trying to find
what's wrong with our theories, right? Were either of you part of the LHC bet about the Higgs boson?
No, I heard about it. Was it a bet about whether or not or was it a bet about it was? So it was
a pool and I think it was 10 Francs or something like that and basically you could
bet on do we find the Higgs boson on or not and if so what mass it would have. Who won?
I don't actually know that question. We have to look that up. I know who did the poll, so I can
write to him and ask him. We're going to give it a challenge to our audience they can they
can put that into our comments on our YouTube channel if you find out who won the bet
of the pool as you say of the Higgs mass that'd be interesting to know. I bet on no Higgs boson.
Oh, you bet on no Higgs boson, yeah, really? That's what I wanted. That's interesting since you brought
up like the topic about the mass of the Higgs boson and like we didn't know prior like
what was the mass of Higgs boson and it made it so hard that we call it the goddamn particle
but I wonder because like I think that we already have like the Standard Model which is a
very good theory like the best theory we have so far and the data from the LEP (the Large
Electron-Positron collider) so we already had the constraint on the mass of the Higgs like before
the Large Hadron Collider come into play so why was it still very hard and why was it like searching for
the needle in the hay? Oh gosh, there's a lot of different questions all in one.
So maybe I'll go back to what we knew about it right so LEP which was the Large
Electron-Positron collider, it actually found something that some people thought might have
been a Higgs boson right at the end, so there was all sorts of excitement. There was a
little excess to do it and should we extend it should they shut down in the LHC etc.
But in the end after the analysis you know they put a constraint I think it was 114 GeV if I
remember correctly so that was a lower bound and that was super clear. But then what you were talking
about the Standard Model this is more complex. So what this was is like taking all the different
parameters in the Standard Model doing a global fit and saying are they all consistent and
with that you could put a kind of you know this is around where the Higgs should be. Also, this is
sort of more at the same time as the LHC I would say the Tevatron started to give some bounds as well
in the mid kind of range on the Higgs boson so we had that as well. But I think one of the
reason why we still weren't sure about the mass is that it's still possible you could change
the Standard Model a little bit and then that whole fit wouldn't make sense right so you still needed to
look over that full range at least that's how I kind of thought about it.
I know that's not exactly your question I'm answering, the first half. It's a good point
though because we spend a lot of effort up to this day still measuring the standard model parameters
as precisely as we possibly can actually for some of us at least hoping there'll be disagreement at
some point because there's many things that are not in the Standard Model that are not explained
by the Standard Model. So it's just because the Standard Model says so it doesn't mean it happens
in nature like that. I mean for all the successes of the Standard Model it explains what 5% of the universe.
I mean it's sometimes hard to hold this thought in my mind wow it's a beautiful theory
I work with it every day it makes predictions I can go and measure things and
at the same time it's only 1/20th of everything that's out there. You're referring to the fact
that there's a lot of the universe, about a quarter is dark matter, which we have no clue what
it is and then about 2/3rd is dark energy and there's dark matter dark energy. I've got dark energy, I've got it right
in my in coffee cup dark Energy and that's that's like a huge portion, the amount of energy,
whatever is making our universe expand by a lot more and more, right?
We have even less of a clue.
If a physicist calls it dark it's just a measure of the ignorance. Exactly.
I do want to mention and oh by the way of the 5% we don't
get gravity too right so this is even of that we aren't we don't have a complete understanding
but I I did want to mention a few names because we've said their names several times
Robert Brout and François Englert didn't say their full names. They along with, it is interesting the story that
you said when Peter Higgs had first put his paper out and then it got rejected simply you know
for by the CERN referees I think it was. I don't know exactly there's a whole history
of that in a book that I cannot recall the name right now but there's a whole explanation of how
it happened and of course humans were involved in that. We're stuck with humans I think
for now until whales can help us out with our papers.
But also I mean François Englert and Robert Brout they then got their their paper published first but then
then came in the Higgs paper and the reason we call the Higgs boson of course is because he included the
fact that there was a particle associated with the field which everybody knew there's a field
there's a particle there's a particle there's a field and I can feel I can feel a question
coming from Chetna about the Higgs "boson". Do you have a question about the Higgs boson, Chetna?
Yeah, enough of the Higgs, let's talk about what's boson where did the term come from? The boson.
So that's a very important point and this was actually brought up by, I do remember when there was the discovery,
there were some complaints that came mainly out of India asking like how come we're not talking about Bose.
when bosons get together they distribute themselves in a certain way and when fermions named after Enrico Fermi
they don't get together so they distribute themselves in a different way. So bosons tend to be the force carrying
particles call them. Excitations of field. They're all excitations of field in quantum field theory yet.
We're not going to go into quantum field theory yet but anyway yeah there's
different kinds of statistics that was there was Bose-Einstein statistics, Fermi-Dirac statistics.
So boson was very important. In my response there were some people who complained they said how
come Higgs is capitalised and boson is not capitalised and my claim is that it's even
a greater honour to have your name used with a small letter without the capital letter that means
it's common and bosons are very important and common throughout our field.
Bosons are integral, these photons heating up our room here and giving us light, they are bosons
and boson is not capitalised for that and same with fermions so there's Bose-Einstein statistics
which describe the bosons and there's Fermi-Dirac statistics which describe the fermions which tend to be the sort of
matter particles as opposed to the force carrying particles (bosons). So we got the boson point in and
Chetna is happy about that. I did want to mention there's a few things that when you guys want
to get more information there's some nice books out there we already mentioned the God particle
book of course. I really liked this very short book, easy to read, by Lisa Randall called
The Higgs Discovery simply. There's also a new one that came out by Matt Strassler.
He's a theorist and he wrote a book that's called Waves In An Impossible Sea.
So I actually interviewed Matt in Berkeley recently about his book
and what's interesting about this book is he doesn't like the way we explain
the Higgs Boson and the Higgs mechanism to the public. He thinks that we use metaphors that are wrong.
So the basic idea of the book, he's right of course, is to actually explain it from
scratch how things really work without, you know, using any metaphors and things like that.
So he's deeply critical of all the metaphors we've been using today. Okay, Matt, I will read your book.
Back in 2012
I was working as our outreach coordinator for the ATLAS experiment. Full disclosure, I'm on ATLAS and my good friend
Dave Barney was the outreach coordinator for the CMS experiment and we got together and we made a Ted Ed animation
which was a lot of fun and it was all about the fact that whenever we asked all of our
friends how to describe the Higgs boson and the Higgs field they would go through all of these different
metaphors we finally settled on one which I'm sure Matt won't like which was a cherry
being dropped into a a shake and the splash of the cherry in the shake and the splash, it's an injection
of energy into a field that splash that excitation of the field was I mean that was the best we could do.
It sounds more delicious than the one I heard of of dropping a stone on the lake.
It was fun and it's true that you know we live in the this we're you know all macroscopic
and we all move non-relativistically and so to actually even comprehend this is perhaps
beyond our imagination but I'm interested to see what how Matt made it accessible. Did he make it accessible?
I found it very interesting to read so there's a lot about relativity so
he starts with relativity right and sort of explaining you know hard it works and things
like that no equations by the way so oh that's nice yeah know. It's a fun book.
Let's move on. So we we found it, we've already declared victory, we found the Higgs boson.
So then we just turn off the Large Hadron Collider (LHC) and go home, right?
Why do you carry on? What have we been doing for the past 12 years with this Higgs boson? Has it taught
us more and what have we learned more about it? Well of course that would be great because then we could
you know just sit at home and drink coffee. Exactly. Actually no it would really suck.
The truth is of course is we found a particle consistent with the Standard Model, Higgs boson.
I believe that was the precise statement which meant actually we had a whole bunch of things we
needed to know, right? We needed to know was it the Standard Model of the Higgs boson. We needed to know what
was its mass to do it but the first one, this is the Standard model of the Higgs boson, that actually gives you
a whole bunch of other questions. Andre talked a whole lot about all the different
particles that it needs to interact with, we needed to measure each those because you know if some of
them were just not there then there was something wrong with the theory to do it. We had to
measure its mass, we talked about how we didn't know the mass at all and then people came
up with some clever things for example probing the width of the Higgs boson which was something
that was meant to be impossible at the LHC like three orders of magnitude impossible but yet
I mean there are some assumptions. Quantum mechanical trickery. I should mention
orders of magnitude we use that all the time three orders of magnitude means you have three
zeros after the one so it's a thousand times away. The Higgs boson is a thousand times narrower than the
resolution of our detectors. So our detectors measure things with only a finite resolution
and the Higgs boson width is much smaller than that so we had to resort to quantum mechanical
trickery. Ah quantum mechanical trickery yep that's that's a beautiful way to describe it. And were we able to measure that width?
Yes, well, we're at small number of significance but we're starting to probe the width
unfortunately consistent with Standard Model
but we're not done by the way so we have not measured everything yet. A big thing that's missing is one of the funny things
about the Higgs boson is that it needs to interact with itself without changing itself this comes
from some of the Higgs. HIggs boson gives mass. So we need to probe that and there's
a way to do it where you need to look for events containing two Higgs boson but they're very rare.
The di-Higgs searches. These are the di-Higgs searches so that needs to be done there also a number of
these decays of the Higgs boson just you know the particles it produces when it disappears that we
haven't been able to measure just yet. We also need to look for ones that it shouldn't
do right particles it could decay to so we're sort of I don't know we've got a basic profile of
it but there's still big pieces missing that's sort of how I would see it. So we were talking
earlier about the fermions like the quarks and the leptons so for an example of a forbidden decay is
like the Higgs boson decaying into a muon and an electron. It can go into an electron
and a positron, a muon and anti-muon, a beauty quark and an anti-beauty quark, charm quark and anti-charm quark
but mixing things around it should not be able to to mix things. So indeed one of the
biggest thrusts of let's say the Higgs physics programme is not only going and probing these
interactions if it decays into these lighter and lighter particles because those are more rarified
interactions they are not as strong. It's also this question of how does the Higgs interact with itself
and that has some interesting consequences, connections because it connects to the vacuum
structure of the universe and you know this sounds very arcane, it is a bit arcane but basically at least
for me as an experimentalist what is important is that the Standard Model says it must be exactly
like this so the Standard Model says that this potential of a vacuum energy in the universe must
have a specific very very specific shape and you do not really you cannot deviate from
it without breaking the Standard Model so if we are able to measure how the Higgs boson interacts with
itself we can check whether or not that is what is realized in nature so measuring the shape of
this potential becomes something that the Higgs boson allows us to do and so it's a big thrust
not just for the LHC so we've barely touched the surface at the LHC but for the upgrades of the LHC
that will start in 2029. Just to explain how the LHC works. We collide bunches
it's not really a continuous beam of protons but we collide bunches of of protons every 25 nanoseconds
at the different places where we have the detectors so there's four collision points and
these bunches go through each other, each bunch is a galaxy of protons, roughly 100 billion
protons so you have Andromeda, the Milky Way passing through each other every 25 nanoseconds, roughly speaking.
Yeah no no no I'm thinking about the number of stars in a galaxy yeah it's
roughly 100 billion and actually if Andromeda and Milky Way are going to do this
it goes slower, much more slowly and they predict
none of the stars are actually going to hit each other probably they won't even though those are
pretty dense galaxies but in our case we really put these bunches together as densely as possible
which you have to do because they don't want to be together they're protons they have charges
that make them push apart and they go through each other and as tight as we squeeze them as well as
we focus them and bring them through each other
we get actually on average about 60 will actually go through each other pass through each other and
then of that less than that will actually interact and give us something that's interesting yeah and
we call it luminosity because you make the beams as narrow as possible
because you are saying here they are as bright as possible right
so the brighter they are the more collisions you get and the more luminous the interaction region is.
So I wanted to clarify one thing we don't actually want these 60 collisions to happen, so what we want
is we would like our rare processes right like Higgs boson to happen as much as possible. On demand.
And on the background ones to stay away but so but the problem is like increasing the rate of
those processes has a side effect of having these extra collisions and it makes
our life harder because we have to actually search through those extra ones to find out the ones that we rarely want.
We get it when when something interesting, we label as "interesting"
triggers our detectors we call the system a "trigger" like taking a picture it's because something
something came out from this passing of these two bunches like a muon coming out or electron or some
energy or some balance or something that we put into our our menu of things we want to look for
and then you've got all of these other collisions that happened at the same time and then you have
to sort them out from that so that's what we're going to be going up for in four
years time or something like that. We will actually go up to, we hope we get everything on time and
in that case we can look for things which are much more rare and we can look for maybe
Higgs bosons interacting with muons for example or transforming to muons maybe we'll have that at
the end of this run. Yeah that's coming and thanks to a lot of work, I mean Heather knows
this better than anyone here, there's a lot of techniques from machine learning that have
been helping us distinguish between like what a beauty quark looks like in our detector and a charm quark
looks like in our detector and it's very titillating that we are being able to tease out
all these charming signals. Yeah no this is kind of one of the things that motivates me the best
way to get me to do something is tell me it's impossible then I like to go and solve the
problem to do it but you're right it's actually amazing just how much better things are in certain
places than we predicted right we found and I think it comes there's a whole chunk from machine
learning like you were saying but I also think that that people get clever in a wide variety
of ways and if you sort of compare you know what people thought we would do and what we actually
do we do far better than any of the predictions that we had like you know we're probably going
to see Higgs to mumu in Run 3 looks like that at least with combination between ATLAS and CMS
and then di-Higgs we are getting awfully close whereas that initially was maybe we'd
see it at the High-Luminosity LHC so it's kind of pretty cool I find this
something very exciting about how well we actually end up doing our physics yeah. So one interesting
phenomenon here is that when you turn on the detector, the detector is also partly unknown
so with time you learn all its features, the good parts, the not so great parts and you learn how to
calibrate it and you know extract more reliable information from it so yes we get smarter at
figuring out the detectors that we build. So I want to ask you one more thing before before we end this
about the future, I think they can be just predictions what you want to happen. I mean the
Higgs boson, what we've established it interacts with any elementary particle that has a mass so far
that's proposed and that's what we hope we're seeing. One thought about this dark matter for
example is that there's this stuff out there which we've seen really clearly Vera Rubin saw it, he actually
calculated first how much dark matter there was in our galaxy and since then we've mapped out this
existence of dark matter all over our universe right and one possibility is that's an
elementary particle so if it were an elementary particle does it necessarily mean that we will be
able to find it if we look hard enough through the Higgs boson on or not? So I take exception to the
notion if you'll find it if you look hard enough. The way I see things is that, if you don't look
you will not find, that's definitely the case. Now as you were pointing out the
fact that this Higgs boson speaks or talks with many different types of particles you know we've
discussed the bosons and the fermions, perhaps it also interacts with dark matter, if dark matter is
an elementary particle. The Standard Model really has no allowance for dark matter particles but
again one of the reasons why it's called a model is because it's always tentative it's something
that can be built upon and so it might be that it needs a certain extension in order to
if we find that kind of interaction of the Higgs boson with dark matter particles it will have to be extended
to account for reality. It would be great. Will it happen? I don't know, it depends on what nature has in store.
And what we build, right? There are proposals out there now
which we're discussing amongst ourselves. We take it very seriously when we talk about the
future in our field because it's a big investment for all of us and it's a long term, very long-term.
The plans I've see right now are talking about something covering 70 years
to actually build the next scale so and there's different ideas out there, we've talked about
building another ring that's much larger, higher energy, order magnitude higher energy
I use the term order magnitude again that would mean one order is 10. Huge and maybe up to
you know 91 to 100 kilometers around whereas the LHC is small, 27-km around or other ideas
like a linear collider which could run at with the same energy as the mass of the Higgs Boson to produce
lots and lots of Higgs bosons. So those are possibilities. There's also this idea of a muon collider
that's out there which could possibly run at that mass. What do you think
about the future for our field? So I'm in an interesting state right now I think our field
we've had a paradigm about how we're looking for new physics that the LHC has shown us is maybe not correct, right?
So we had the Higgs boson on which we've been talking about which we found but
we also had many predictions for you know there should be new physics around the TeV scale we had
a whole range of theories like that so I actually think we're at a very interesting time is there's
a paradigm shift that's needed towards thinking I mean there are many possible options for it but
at least to me I don't see which way we're going to go so I preface that by saying and then we
talk about you know what collider should we build we talked all about the no-lose theorem
we were in this fantastic situation at the LHC right we said we had this thing we knew which
one to build and we built it and we found it to do it. We're not there now and so I
think we're in a stage where we need to go and explore as broadly as possible because right now
the theory is not clear and maybe it's a time when experiment needs to lead the way to do it.
So if it was up to me I would go to search as far as possible to do it. There are many
constraints it's a complex space and so you can't necessarily do that but that's
at least the way I think about it because I think it's unknown and we need to try and look as far
as possible and then we may find big answers that change things yeah. I agree especially because
projects take so long if you think about it the the Higgs boson was found like 50 odd years after
it was predicted. During those 50 years, theory was guiding the way right and and indeed I totally
agree with what Heather has said we are now in a data driven era if you want and in that case
the question is what kind of tools are we building to gather data and to explore nature and I don't
think there's ever a promise that can be made that oh we are going to find the but that's the
thing if if you don't look for it if you don't try you're definitely never going to find it.
I think there was a sort of a consensus in the community that the next machine should be what
we call a Higgs factory which is you make collisions and lots of Higgses come out because it has
not only this very central role regardless of it having a central role in the theory it has this
very central role it talks to many particles as you were saying, Steve. We also do not understand
how come there's only one, is there only one? Who knows if there's only one and changes in the
properties predicted by the Standard Model for this particle can tell us whether there are more Higgs bosons
if there are other interactions for instance measuring the total width is really
cool because the total width via some quantum mechanical trickery is related to all the possible
ways that the Higgs boson can decay into other particles. So if you would find something anomalous
there you know some departure with respect to Standard Model it could be because it is the
go into I don't know the dark matter particles if they are there. So all of these things, many
people think well think of the Higgs boson as a sort of object of study. I think of it as a tool
so it's a tool that we can now go and try in different types of screws and different
types of knobs and wrenches so you just try it out and see exactly how does it operate in
that particular context. Very good. Let's leave it at that. That's a very nice conclusion there.
I'm looking forward to these new accelerators wherever and whatever they they may be.
Andre David is a CERN physicist on the CMS experiment here on the Large Hadron Collider. Heather Grey is an
associate professor at UC Berkeley in Berkeley lab working on the ATLAS experiment here on
the Large Hadron Collider. Thank you both for coming here and being on Early Morning Coffee at CERN.
Okay Joni it's time for you. You have a question for the audience. So my question for you is that,
what is the other important discovery that brought a CERN physicist a Nobel prize after
the benchmark for the Higgs discovery? So we have Nobel Prize that was awarded
to Peter Higgs, François Englert. Unfortunately, Robert Brout died the year before we discovered it but
before that there had been a Nobel Prize at CERN so tell us what that Nobel Prize was and you said
to send it to CERN social media so @CERN on pretty much any social platform right with #EMC2 and
you can also put comments in our YouTube channel and you're going to be able to follow all of our
podcasts on anywhere you get your podcast. We look forward to seeing you again soon.
Next time we are going to probe into the experiment called LHCb which is a very interesting experiment because
they don't just just look at the quarks, the elementary particles but they look at various
combinations of these quarks and they found a lot so they've made a lot of discoveries a big list
of discoveries that have come from LHCb filling up our particle data group books with discoveries.
So we'll be talking about that next time so have a great time and thank you for listening.