Physics World. Hello, and welcome to the Physics
World Stories Podcast. As a layman, I would
now say,
I think we have it.
You agree?
Yeah.
That was Rolf Dieter Heuer,
former director general of CERN, 10 years ago
at the announcement of the discovery
of the Higgs boson.
In this episode of the podcast, we'll hear
about the excitement at CERN at the time
of their discovery. We'll hear about the research
that's been happening looking into the Higgs since
that discovery,
and we'll look forward to what might come
in particle physics. The Higgs boson is a
fundamental particle associated with the Higgs field,
a field that gives mass to other fundamental
particles such as electrons
and quarks.
It's often known as the God particle because
for a generation of scientists, it had to
be taken on faith that it even existed.
The Large Hadron Collider,
a particle accelerator at CERN in Switzerland, was
designed at least in significant part to see
if that particle could be discovered. In a
huge loop under Switzerland
and into France, the Large Hadron Collider accelerates
protons to near the speed of light and
collides them. Around the ring, there are individual
experiments
like ATLAS and the CMS experiment,
with teams of scientists working
to study
the aftermath of those collisions.
Working at CMS at the time is Achintya
Rao. I used to work as a science
communicator for the CMS Collaboration between
2,010
and 2018, and I was at CERN helping
with the communications when we announced the discovery
of the Higgs boson. I'm guessing you didn't
have any insight into that before the announcement.
The LHC had been collecting data from 2,010
onwards.
2010 was just the beginning of the operations
of this machine, so there was not a
tremendous amount of data that were that were
delivered to the collaborations, to the to the
experimental teams.
2011, however, there was a lot of data
delivered that was enough to begin to make
your first
real inroads into determining whether the Higgs boson
exists
or if it doesn't.
And that indeed was the promise of the
LHC. It would either find the Higgs or
rule out its existence entirely.
Now
in 2,011,
all of the data that had been analyzed
gave a very narrow window in which
neither CMS nor Atlas could rule out the
existence of the Higgs.
The data showed us little a slight bump
in in the mass distribution
that had a significance of about 3 sigma
on both CMS and ATLAS. And what this
meant was that
there either was something there that was causing
this bump
or it was a totally random statistical fluctuation
that, emerged in both datasets. And this is,
of course, very likely and has happened in
the past and since as well. Just a
quick aside, because I know that for many
of you, the idea of Sigma will be
something that is just part of your parlance.
But when they plot the results on a
graph, it would normally,
in a normal distribution,
be a bell curve with the highest point
of the curve in the middle and tapering
off on either side. And the deviation
or sigma is how far any given point
plotted on that graph is away from the
average. One standard deviation
or one sigma
plotted either above or below the average value
would include 68%
of all the data points. For 2 sigma,
it's 95%.
For 3 sigma,
it's 99.7%.
In other words, the higher the sigma value,
the less likely
the discovery
is to be accidental.
Back to Archintia. So in 2011, once the
experimental teams realized that there was this bump,
what they decided in 2012, once fresh data
from the LHC began to arrive, was to
perform what is known as a blind analysis.
And this means that, essentially, you ignore
the regions
that you could not rule out where there
might be something,
and you only analyze the data on either
sides of that band.
So you analyze data at a higher mass
and at a lower mass, and you tweak
your analysis. You perfect your analysis. You're you
have a great deal of confidence in the
methods that you applied. And then once you're
satisfied that your analysis methods and protocols are
sound, are robust,
you then perform what is known as an
unblinding.
You apply that analysis
to the data
in the region that you had not excluded,
and then you see whether there's something there
or
whether your signal disappears.
And the reason scientists do it in this
way at at, particle accelerators is essentially to
avoid biasing yourself. Because if you if you
have this notion that there is something there
on a subconscious level
I'm not saying people do this, but you
want to rule out the possibility that there
is any subconscious tweaking of your analysis to
enhance
what is potentially a signal.
So the blind analysis is a process where
you do not look at the sort of
the hot region, the the region where there's
interesting data. You don't analyze it. You analyze
everything else, and then you apply your analysis
to this region.
And
in June
2012,
so almost exactly 10 years ago at the
time of our speaking,
CMS had an unblinding
of the data from the diphoton
channel.
So this is
data that have been collected from proton collisions
in which
pairs of photons, 2 photons, have emerged
that can be said to have emerged from
the same collision of 2 protons within the
Large Hadron Collider.
So pairs of photons are produced, and so
this is the diphoton channel, which is one
of the channels for looking for the Higgs,
one of the what is known as
a high resolution channel because you can really
find a very sharp peak
in your data.
So in June, in, I think, it was
15th June, CMS performed an unblinding on 14th
June, and then they revealed the results this
unblinding internally within the collaboration.
And that was a really cool moment. So
what happens is small teams
perform the analysis, and you select our priority,
whichever analysis is going to be finally used.
And then when you do an unblinding, it
doesn't matter if you get another,
analysis that is even better. You use the
one that you our priority chose. That's how
you prevent yourself from being biased once again.
So
the analysis was done, and the unblinding
was then revealed to the CMS collaboration on
15th June. You could sort of feel the
excitement in the room. So we were there.
This was just an internal meeting. Everyone was
not everyone was in the room because out
of about 2 and a half 1000 scientists,
3,000 scientists, only a couple of 100 were
there.
The rest of them were watching via webcast
or connected via video conferencing.
That was when we saw that in this
particular channel,
the significance
of the excess
had breached 4 Sigma.
And when you see more than 4 Sigma
in one channel, you know, you have a
feeling that when you combine it with the
other channels that are that are key to
discovering a new particle that it might be
there. So that was the moment where we
felt a real sense of excitement.
I was recording that meeting. When this perhaps
it's it's a little faster.
So please, everybody,
get ready
for the next 15 minutes.
So all this exciting and
So what we had decided within the CMS
collaboration was that these conversations,
these,
exchanges, these internal discussions were
ultimately of historic value. And even though in
the moment they couldn't be shared with the
world, we should still have a record of
it that we can then put out subsequently.
So we have on record
the, young researcher Mingming Yang announcing to the
collaboration after she had sort of prepared the
presentation and stuff that this is what CMS
sees.
This
black one, okay, this is the combined. You
can see it's beyond which line. Okay. 1,
2, 3, and 4. The
The break the break
is, is for the and then the Magenta
is for the 18.
On
the 28th
June,
just about a week before the announcement,
another and, another
presentation was made to the CMS collaboration.
And this was by another young researcher, Andre
David,
who
showed the results of the same analysis in
the difoton channel, but with adding
an additional 40% data that had been collected
in those previous 2 weeks, preceding 2 weeks.
And, we've gone through all of the needed
steps in order to reopen the box. This
means that we've resynchronized among the different frameworks.
We've recalculated
all the efficiency data to Monte Carlo
correction factors,
and we've rederied the energy scale and the
resolution,
corrections
from, the data.
We've also rerun, the algorithm that chooses the
background model for the FITs in the mass
distribution,
and we've gone through the full and blinding
procedure
actually just some 16 hours ago. These results
were again presented internally, and that was when
the signal, the excess in the difocal channel
had a significance of 4.1
sigma
internally. We knew. And this is across 3
different analysis performed between dependent frameworks.
So I personally think that this begs for
interpretation.
Thank you.
We knew there was something there.
The other channels were also happening at around
the same time. There was the Higgs decaying
into,
2 zed bosons
and
the the other channels in which the Higgs
transforms are much lower in resolution likelihood.
So it was the Higgs to zed zed
and Higgs to 2 photons channels that were
the most interesting, most valuable ones at
the time. But this is all internal to
the CMS experiment at this point? We had
no idea what ATLAS had, which is the
other general purpose experiment at at CERN, at
the Large Hadron Collider.
Now
the reason there are 2 experiments in the
first place that are designed completely differently using
very different technologies operated independently by 2 different
teams
is because we only have 1 Large Hadron
Collider. There is no way that someone else
can go and replicate your work and find
another
way of verifying your results. So you have
2 teams that essentially are there to counterbalance
one another. If one sees it and the
other doesn't, then you need to know whether
it's because of the limitation in the technology
or if it's really an anomaly that one
has seen, and it's not a real manifestation
of what nature has to show you.
So we didn't know what Atlas had to
say. We only knew what what, you know,
the researchers on CMS, who I was working
with, what they had to say. I imagine
that was a particularly exciting time to be
around. The day before the announcements on the
3rd July, I was up
until very late at night, in my office,
actually, in in my boss's office at the
time, where we were putting together the website
for the CMS collaboration,
where we had an article that had
essentially an explanation of what was going to
be presented the next day, which would have
only been made public once the presentation was
made. So we had an article that described
what the collaboration had seen. We had information
that explained things like what is the statistical
significance, what does 5 sigma mean, how do
you collect data from proton collisions, what do
the scientists look for to determine that you
have seen something new, why is that important,
and so forth.
So we had been tweaking
the website. We had been getting new information
with fine grain numbers that had to change.
And what we had also been doing is
we were trying to get those translated into
about 20 languages
because we had an inkling that there was
gonna be interest around the world and that
not everyone might have
the expertise to
understand the scientific seminar on the day itself.
And so we prepared these in in about
20 languages. Sort of many people around the
world could could read,
in their own language, in the language that
they prefer and didn't have to rely on
an English or French version exclusively.
So I was up late into the night,
far later than I usually work, and I
left for home a little past midnight. I
took the last tram home.
It was a very quiet night. There was
just,
I don't know, this this sense
of calm outside
soon.
And as I was walking to the tram
stop, I remember seeing a few
students, young researchers camped outside the main auditorium
where the conference would be where the seminar
would be given from.
And they had camped out the previous night
because they knew that there were so few
seats, so few free seats available that if
they had showed up the next day and
started queuing, they would not have managed to
get in. So they just stayed out all
night falling asleep outside the door of the
main auditorium
in order to guarantee that they get a
seat.
So I walked past them. I got onto
the tram. I went home, slept for far
too few hours,
and took the tram back at 6 or
so in the morning to come back on
on-site at 7.
And that's when
we sort of settled into the seminar itself.
At that seminar was Joe Incandela,
the CMS spokesperson presenting the experiment's findings.
What you see here is now the p
values. This is the probability that the background
could fluctuate
to this extent. And you see combining the
2 years, we're below we're beyond 4 sigma,
about 4.1 standard deviations. If you look at
the p values,
well,
our expected significance,
for standard model Higgs at 125.5
is 3.8 sigma, and what we're seeing is
3.2.
If we combine
the z z and gamma gamma,
this is what we get. They they line
up extremely well, and in the region of
125 GeV,
they combine to give us a an ex
a combined significance of 5 standard deviations.
Give me an idea of how secret that
that is. I mean, like, would Peter Higgs
be invited
on the basis of one of those, or
does he know that both of them have
found it?
So,
who knew that both experiments have seen it?
So the protocol that had been established internally
at some was that if
one of the teams sees something
beyond a certain significance that sort of guarantees
discovery in quotes,
they would approach the director general of CERN
and say, this is what we have. And
then the director general would invite the other
team
and
say, this is what your opposing team has,
the other collaboration has.
What do you have? And if they were
sort of in the same space of tweaking
the analysis, then they would be given, like,
a day or 2 to tweak it and
and sort of come up with it because
there is no way you can make drastic
changes and invent the presence of a new
particle, at the very last minute. So that
was a protocol established, and what happened was
in the end, both teams saw something at
around the same time. They went to the
director general at almost the same time to
say this is what we have.
So the director general obviously knew what both
teams had and the spokespersons
of both the experimental collaborations.
So Joe and Candela from CMS and Fabiola
Gianotti from ATLAS, they knew what the other
had, but they kept it to themselves.
So
and, oh, okay. And the director of research
at ZOEN also knew. So there were maybe
4 people who knew that there was something
there, but
no one knew the specific numbers. No one
knew the exact significance. No one knew whether
it had breached 5 sigma or not. It
could have been about 4.6, 4.7,
4.95.
No one knew, exactly what it would be.
But
the closer you got to the date as
the analysis kept moving forward, you knew something
was gonna happen.
What happened with inviting Peter Higgs, as I
found out very recently when I spoke with
James Gillies, who was the head of communications
at CERN at the time, was that
CERN didn't initially extend invitations to
to any of the any of the theorists.
What happened was Carl Hagen, who was one
of the 6 people who
did
the foundational theoretical work in 1964,
he got in touch with James Gillies because,
as you know, the story of the Higgs
mechanism and the Higgs boson is is a
is a
complicated one. There were lots of actors involved
in it, lots of different people who were
doing work at around the same time. And
the story goes,
to put it very
simply, that
Francois Angler and Robert Braut put out one
paper, Peter Higgs put out another paper, and,
Hagen, Guralnick, and Kibble put out a third
paper. And the reason we call it the
Higgs boson was because Peter Higgs' paper was
the first and only one to mention the
existence of of the particle itself associated with
this field.
But the mechanism itself is commonly referred to
as the
Higgs mechanism, although
Hagen, Goranlik, and Kibble argue that they should
also be included in that.
It's a complicated story, obviously, and everyone wants
to,
wants to
take take, you know, get get their get
given their due credit.
So Hagen got in touch with James Gillies.
Karl Hagen got in touch with James Gillies,
the head of SON's communications, and said, look.
We are hearing rumblings.
We we expect that if we were to
arrive to CERN that we would be given
hospitality
no less than what would be afforded to
the other theorists who are involved in the
work at the time.
He said we would all like to come.
So in the end, it was Hagen and
Guralnik who came and and Kibble stayed in
London where SDFC had organized a separate event.
So they showed up. And once it was
clear that they were gonna come, then CERN
decided to reach out to,
Francois Anglais and, and Peter Higgs and say,
we would like to invite you to come
to CERN.
Robert Brout, of course, was the other theorist,
had passed away in 2,011.
Now,
of course, people knew that something was going
to happen, and the reason for that is
that the seminar was supposed to be
a regular talk given at the International Conference
on High Energy Physics or ICHEP.
It was supposed to be, like, a status
update on where the collaborations are in the
search for the Higgs boson. It had been
penciled in to take place at this conference
for months months, and the conference was taking
place in Melbourne, Australia.
And
once
it was clear that what would be presented,
if not discovery, would be pretty close to
discovery, it was decided
that because
CERN is funded by
largely European member states. At that time, the
expansion of CERN beyond Europe hadn't, reached the
heights it has today.
So largely funded by European member states, and
and it and the director general of CERN
told me recently that it was felt that
CERN owed it to its sponsors,
to the council members,
the member states of CERN,
to have any discovery announcement take place
at home at CERN in Geneva.
So, obviously, people began to say, hang on
a minute. Why are they not having this
talk at Melbourne,
iCHAP, you know, as it was supposed to
be?
The only reason they do that is if
they have something big to say.
So
I guess everyone speculate. Everyone had a hunch,
and people on CMS obviously know what they
had to say. People on Atlas probably knew
what they had to say. And even if
you aren't telling someone
what you have, if you're walking past your
friends who work on the other side of
the ring, of the LHC ring when you're
going for lunch and you see them grinning
ear to ear,
you know something, sir. You don't have to
exchange words to know that, you know, there's
excitement on the horizon. I can imagine, but
it's this was a long time before I
was doing this podcast, and I'd have loved
to have been in that room. But were
the press invited?
The seminar took place in CERN's main auditorium,
and next to it is the council chamber,
which is where
the council members usually meet for their discussions.
And we had set up, that room. CERN
communications team had set up that room for
the press, essentially. Because,
again, like Carl Hagen, the
press had invited themselves
over.
SON had just issued a statement saying, we're
going to provide an update on where we
are in the search for the Higgs boson
on the 4th July at 9 AM. Also,
9 AM, unusual.
Sun seminars don't take place that early in
the day. They usually take place towards the
end of the day, and the reason was
it was still part of ICHIP.
And because that's in Australia, you didn't want
to do it after they've gone to bed.
So 9 AM. So Sun says, this is
happening. There's gonna be an update. That's it.
Journalists start ringing in and saying,
what's going on again?
Do we?
What are you gonna say? Can you tell
us? And,
again, I'm quoting the words of the director
general who said, you would regret it if
you weren't there.
That's all. They didn't say anything more. They
didn't say, oh, yes. Please come. We're gonna
announce the discovery.
It was implied that
it was something you wouldn't want to miss.
So, of course, journalists showed up. They showed
up from all over the world, from the
US,
across Europe.
So a separate room had been prepared to
have the press conference where we had a
lot of communicators present as well to help
with any issues. And
one of the things that Sun had realized
very early on was, you know, the previous
year,
December 2011,
there had been a similar seminar, a typical
end of year seminar announcing, you know, where
we are at the in in our physics
research
at the at the LHC and CMS and
ATLAS presented where they were in the search
for the Higgs boson. And
at that time, both collaborations had, as I
said, seen about 3 sigma.
And
this was supposed to be just
a normal seminar webcast to the community so
that everyone
in the particle physics world can follow what's
going on. And SIRNS webcast
broke
because there were tens of thousands of people
watching this when they heard it was happening
expecting a discovery.
And this was,
a huge number of people, which is obviously
much more than someone had ever expected. So
they knew that there was an interest happening.
So they prepared,
lay explanations,
live blogging,
lots of other things on the same day
to explain to people what was going on.
But to answer your question about the atmosphere,
we were buzzing.
I walked in that morning.
I saw,
by the time that I got in at
about 7 o'clock, that there had already been
a snake of a queue that went
from the auditorium,
went past the landing, down the stairs, went
in front of the post office and the
bank, the kiosk, skirted into the restaurant,
went past the coffee machines, went into the
extension of the restaurant, poured out into the
garden.
And
this was amazing because the people knew they
couldn't get in. They couldn't get in, and
they were grinning ear to ear. They were
smiling. They were jubilant.
They just knew it was a special day.
It didn't matter if they couldn't get in.
They just wanted to feel like they were
connected somehow with the room
as part of this queue.
They were connected with the the room that
the seminar was where the seminar was gonna
happen.
And
most of them, I mean, most of them
didn't make it in the end to the
room. They had to be dispersed. They were
sent to various auxiliary conference rooms scattered around
this the campus where a retransmission was taking
place so they could all watch the proceedings
in in on big screens in other rooms
around around CERN.
We went into the,
into the council chamber where the press conference
was taking place, and we were just
buzzing.
I mean, there is no other way to
describe it. It was
so
incredible. And I remember
our queue was when
the CMS spokesperson,
Joe and Candela, whose
spokespersons are sort of elected representatives who
lead the research team. So it's a democratic
process where new people are elected to the
role to lead the team. So you don't
have 1 PI. You have rolling
leaders of the team of the entire research
collaboration.
So Joe and Candela was gonna be presenting
the CMS results. He presented the CMS results
that day. And
our cue was once he shows the money
plot,
when he says we have found a new
particle with the significance of
5 Sigma,
was when we were gonna make that article
public that we had prepared the previous night.
So we were there waiting, and, you know,
when everything happened, we went in we were
logged into the back end of the website
already waiting. We flipped the switch, flipped it
to published, saved it, started sharing it on
social media, started monitoring the buzz on social
media.
And
Joe sat down, and our attention immediately turned
to Fabiola's presentation
because then we wanted to know what Atlas
has because we didn't know what Atlas has.
Representing Atlas at the meeting was Fabiola Gianotti,
who is now CERN's director general. We have
presented preliminary results on searches for a Standard
Model X boson using the full data sample
recorded so far for x 2 gamma gamma
and x 2 4 leptons and the 2,011
results for the other channels.
I think these results represent an impressive accomplishment
of the experiment in all its component. I
would like just to mention that less than
1 week after
we recorded the latest data, so on the
25th
July or July, 1st plot containing the full
statistics of the 2012 data first results on
egg searches were already available
with a fraction of good quality data certified
at the level of 90% of the delivered
luminosity. So for me, this is an impressive,
remarkable, achievable of education and highly skilled work
of the collaboration.
We have looked for a standard model exposon
over the mass region 100 10 to 600
GeV in 12 channels.
We have excluded a 99% confidence level at
the region up to 523
GeV, expect except a window. In that window,
we observe a
an excess with a local significant of 5.06
6 5.0 sigma at a mass of 126.5
GeV.
The excess is driven by x 2 gamma
gamma and 2 4 leptons is in agreement
with that expected from a standard model x
with the fitted signal strength of,
1.2 plus or minus 0.3 of the standard
model expectation. That was when we knew that
it that it exists.
It was a
just
moment
of
elation.
Thank
you.
So after the
seminar, we had a press conference
where the spokespersons
of CMS and Atlas, as well as the
director of research for CERN and the director
general of CERN
and the head of the accelerator program, all
came and spoke to the journalists. And Peter
Higgs and Francois Engler were also there, and
they answered all of the the questions that
they had, and our senior researchers and and
the scientists who were at the forefront of
the research were all gathered there having interviews
in their own languages.
And by the time this was done, it
was around lunchtime,
and we decided to wander one floor down
and go into Sun's main restaurant.
And I can tell you that I don't
remember what I ate. I don't remember anything
about that meal. I have no recollection of
it. I'd remember sitting in the extended area
of the restaurant
just smiling at my friends and colleagues, just
going like, yeah. This has happened. This is
real.
And,
it was amazing. I don't think anyone properly
went back to work. People went back to
their offices eventually after a couple of hours.
But I don't know how many people actually
went back to work that day because it
was a day of celebration for the entire
community and indeed for for anyone who's interested
in in pure science.
And, of course,
you don't really begrudge them,
the fact that they didn't go back and
work the day because a lot of the
people who are doing the research were working
day in and day out, endless hours, getting
fresh data,
every day and trying to make sure that
it was in good enough shape to present
this discovery. Because one one of the funny
things in which particle physics works is that
ICHIP, the conference,
was in the calendar
months ago, months before the event. And so
everything that had to happen had to happen
on that day or up to that day.
So there were quite a few long nights
that people spent working on this,
putting their heart and soul into it, and
in the end, it was all worth it.
You can read Asuncia's feature, a day in
physics like no other, in the Physics World
magazine. One of those researchers playing a key
role who ended up in that conference room
10 years ago is Christina Botter. Currently, I'm
an assistant professor at the University of Zurich,
but, yeah, my,
particle physics story,
started in Torino, at the University of Torino
in Italy,
where I did my PhD
at the time. So it was between 2009
and 2011,011.
So just before,
the Higgs boson discovery,
the group of particle physicists,
at the University of Torino were was working
on
reconstructing nuance. Nuance are a specific type of
elementary particle,
in one of the big detectors of particles,
installed at the Large Hadron Collider at CERN.
This experiment is called CMS.
So I work in, with the CMS collaboration
since
2009.
Just a quick aside, but the sound that
you might be hearing is Christina's jewelry as
she spoke.
But I hope it's not too distracting, and
in fact, it sort of conveys her passion
for the project even more. And just while
I'm here, for those of you who aren't
part of the particle physics community,
the standard model of particle physics is the
framework through which physicists
have understood
the fundamental forces of nature over the past
half a century. It was developed by theorists
and experimentalists,
and it's helped predict the existence of fundamental
particles and how they interact with each other.
Let's get back to Christina.
So I got involved,
in
the analysis that turned out to be,
one of the most sensitive analysis to the
presence of,
the Higgs boson in the data that in
the collision data that we were recording,
throughout 2,011,
12, 13,
because I I started to work on the
search for the Higgs boson where the Higgs
boson decays, the Higgs boson is not a
stable particle. So it immediately decays into something
else, something that you can detect in our
detectors.
And the channel where the Higgs boson decay
into 4 mules
is one of the most easily distinguishable with
respect to all the other events that of
particles that are produced
in
such the soft frequent collisions of the large
other collider. So
so, yes, I had this
incredible choice to be there at that time,
working on that specific analysis,
and it was an an intense period and
I feel very emotional to talk about it,
of course, because,
yeah, we were working day and night because
data were coming in, every day, and there
was a period where we thought that it
was not going to be so
quick that this if
the eggs would have really existed, It would
have taken much more time to discover it.
But at some point, we understood that actually
our analysis works more sensitive with respect to
what we thought, at the beginning, and so
things just got very fast.
And,
yeah. So the for for a few years,
for 2010,
2011,
we excluded
the presence of the Higgs boson in a
certain ranges of masses. So we didn't know,
which was the mass of the Higgs boson.
It's not predicted by theory. Theory predicts all
its behavior once the mass is measured and
it's known. So we had to search for
very wide ranges of possible masses.
And, of course, with the mass hypothesis, it
comes the rate of events that we,
expect,
in in our collisions, and so we started
to exclude
certain ranges of masses. And then
at the end of my PhD, in my
thesis, there was like this,
plots that shows where the Higgs boson mass
was excluded apart from a very tiny window
around the actual mass of the Higgs boson,
which is 125
giga electron bolt, which is
how we measure the mass.
So, yeah, I ended my PhD with the
in this moment where
it was almost
going to be discovered, and then I started
a fellowship
at CERN,
and
it got crazy. In the 1st year, it
was indeed,
the the LSCI,
the large other collider really worked
well in a sense that provided us, a
lot of data in the 1st months of,
2012,
and in July,
in 10 exactly 10 years ago.
This is when we had enough data to
claim the discovery, to claim it without any
scientific doubt, let's say. So were you sort
of one of the first people to see
this data? Yeah. Because, I mean, of course,
the Higgs was searched in many different decay
channels as I said before, and ours,
so
I was working with the Torino, the university
group that was working together with the,
few people in the US and few people
in,
in
Paris.
We're working night and day altogether.
So
but, yes, indeed, I was really running the
analysis myself. So,
and that decay channel is so easy because
you can really reconstruct the mass of the
decay of the Higgs boson through the masses
of the decay particles while reconstructing our detector.
So the environment mass plots was showing an
excess of events
around the 125
GBI possibility. So the analysis was blind, meaning
that you don't
you design your strategy
and,
only after your strategy is well,
defined and approved by the whole collaboration,
you run the analysis strategy on the data,
which means
particular
set of selections,
of criteria that you apply in order to
reject other events that,
are supposed to be background events.
And so there was that specific moment when
we decided that the full collaboration give us
the approval,
to unblinded, and
I I remember very well that night, but
everything happened very late at night.
We decided to unblind with the full data
set collected up to
June 2012.
Yeah, it was it was there. It was
really visible.
And the day after we we announced it,
we showed it to all CMS.
And in parallel, everything was happening in very
same way in the ATLAS collaboration. So the
other experiments, we were not talking to each
other, not to
bias each other because it's very easy to
be biased, of course, if you,
if you're searching for something that you really
don't know where it is.
Yeah. So
it's true that our team really saw it,
in our office at, for the first time.
Amazing. Amazing. And and that day of the
announcement
to
the world,
you were actually in the room. Yeah. Yeah.
I was in the room because it was
very nice because we had, like,
until
very late in night and day before, we
helped. So there was each team. So I
was the full lactam team. Okay? The person
that worked for the full app. But the,
the statistical significance of the discovery was obtained
combining different channels. So there was each team
helping the spoke person of our collaborator join
Candela at the time, the the person that
give the speech on the 4th July.
And,
and all of us that, were
contacts for these, analysis
channels,
had a seat
reserve in the in the room, and you
were very young at the time. We're so
happy. We're all the,
super
experts of
the the machine,
the detector,
the theories, and then there was
a bunch of,
yeah, very young,
just graduated students,
in the front line. So still now, I
mean, I I sometimes,
watch again
the,
the recording of that day.
It fits but, also, it fits the motion
that there was 2 experiments completely.
They are different, and they were doing they
had the same goal, and they arrived on
the same day
with the same announcements.
It felt such it felt really that science
worked so well. Yeah.
Yeah. All the scientific method that we we
put in place,
years years of work,
yeah, came together in a very nicely way.
It's just it's just an incredible moment, wasn't
it? Particle physics that, you know, even people
who had never been interested, never even thought
about particle physics, this captured their imagination. Yeah.
I think it's all those boys now to
this to the fact that there was this
theorist, not the only one, so more than
one, but Higgs is the,
Peter Higgs is the one that gave the
name to the to the Higgs boson.
Anyhow,
there was this moment
so many years before
where mathematically
you predicted the
the the fact that if you want to
explain the masses of other particles, you need
this
mathematical trick. It's exactly that, that you need
to put it in the equations,
to make everything works and that predict the
existence of the new,
field that and quantum excitation of fields are
particles or a a new particle. And then
from that moment onwards, there was such a
huge experimental effort.
Previous accelerators,
the lap accelerator, the Tevatron in Chicago, and
then,
the LAC.
Thousands of people working on it. And when
you see our detectors, it's like
1,000,000,000 of cables. Okay? Millions of cables.
Everything works
to
together to detect
an invisible particles.
So many cables everywhere, and they're just here
because we wanna find out how everything works.
I really love it.
Okay. So that was 10 years ago.
What's happened since? Do we do is are
we still learning about this this Higgs boson?
Oh, yeah. The Higgs boson is a the
it has a few peculiar,
properties.
It's the only,
particle only elementary particle, at least
particle that we believe is elementary as of,
as what we know today,
that is a scalar, meaning that,
is a spin, which is a quantum property
of the particle is 0,
and because of this reason, this I don't
discuss the the the mathematics behind, but because
of this reason
in,
in quantum field theory,
it's very difficult to explain naturally
why this particle would have the mass that
it has. So the fact that the mass
of Higgs is so light is very difficult
to explain, it means that the theory
is very much fine tuned. So this is
a very mathematical
and awesome, somehow also philosophical concept,
but it
once you discover the Higgs boson every theory
was,
betting on the existence of
other particles.
Other particles
probably heavier for sure because we haven't seen
them before,
that could explain
why the mass of the Higgs is,
that is at that value,
and also could explain also other things that
we don't haven't understood yet, like what is
the particle content if it has a particle
content of, dark matter. K? That is
the matter of that is supposed to be
there in the universe, and we don't know
what it's made of. So for this reason,
LAC was
ready to,
was supposed to discover new states at energies
around the, the mass of the particle bakes,
and we didn't so far.
So this is one aspect of the story,
that we didn't so far, which is an
pretty much important piece of information. So,
they are not just out there with the
probability to be produced the,
of the value that we were expecting.
So there's something that we are not understanding,
and, it's fundamental.
It's
it's extremely interesting,
and
it this is why we really need to
add more data in front of us to
understand where things are,
hiding.
If things are really happening and if there
are new things at the energy scale that
we are exploring,
which is what many people believe for many
theoretical reasons.
On the other side, there is the Higgs
boson that
couples to anything that has mass. So any
new part any particle that has mass gets
the mass from its interaction with the Higgs
boson.
So it's not,
strange to think that if there would be
something new,
it would couple to the Higgs,
changing its properties lightly. So,
we have started to measure the properties of
the Higgs boson. So as I said before,
once you know the mass of it, you
the theory predict the probability that it would
decay into the 4 muons as I said
before,
into 2 photos, into 2,
bottom quarks, and so on and so forth.
So we go and measure the rate that
the,
how many events we saw,
given the total amount of Higgs boson events
produced
in our collisions,
that would decay in one particular channel.
But you need to know these ratios,
with
a very good precision in order to be
sure that,
the Higgs is behaving
exactly
as the theory,
is predicting.
So
after the discovery, it started a long, long
path toward the precise measurements of the property
of the Higgs boson. So if you take
the standard model of particle physics, all the
other particles since we are analyzing and measuring
their properties
much longer,
they are
better measured, okay, better understood.
The Higgs is still,
the
the uncertainty
on which we go with which we know
the property of the Higgs are about 10
to 20%,
which are huge uncertainties with respect to how
we know the other particles that are known
with the per mille uncertainty.
So there's still space.
All what we have measured
gave us the confidence that this is really
the Higgs boson because it's behaving
really has the theory is predicting, but there's
still space for,
misbehaviors
or something that is not really standard model
like. And these things takes time
because these measurements are very complex. You have
to really,
especially at a at
a we collide protons.
So the we our collision are characterized by
an hadronic environments. There's a lot of stuff
happening in our collision because of the constituents
of the protons. The protons are not elementary
particles.
And then we collide 10 to the 11
protons with 10 to the 11 protons. So
when we smash them,
there can be multiple collisions happening together. So
it's very,
difficult to
precisely
reconstruct,
each single event.
So you need a lot of data
and a lot of advanced,
techniques to analyze them
in order to really measure precisely the properties
of these new particles.
And here we are. We are at the
10 to 20% precision.
At the beginning, it was like, yeah, not
even
None of these properties were accessible at the
time of the discovery. We just saw that
there was a new particles,
with a certain mass,
but now we are really
getting everything looks
consistent with the theory as of today. There's
still
from 10% to 1 per mil, there's still
a long path
to go. Is there anything
that will sort
of excite the world the way that the
Higgs boson did? You know, this the Higgs
boson was not was exciting for
for us, for, like, for everybody that Adam
see that what didn't go through a discovery
before
for the
public outside because of the,
the story, not the drop to the discovery,
but was not so exciting for
the theories per se because it was just
confirming,
a piece that was missing
that the theory was predicting
and it fit with the the the
the the mode, the standard model of particle
physics. The the only theory we have in
our heads to predict,
fundamental phenomena,
but it doesn't,
bring us
anything new on what we don't know because
the standard model is not,
explaining many things.
We are just scratching the surface.
So all the things that we haven't understood
yet
are still we don't have any so the
we are not getting more clues with the
discovery of the Higgs boson. So something that
would be totally unpredicted
would be
even
more exciting than the Higgs boson. So if
we would discover,
a violation of the laws that the standard
model,
that up to to now we we thought
they are conserved in nature
or a new particles that is not predicted
by the standard model that
yeah.
That is what we are all
dreaming of because
we
something new we know that this is not
a fundamental theory because it has so many
things that it's not
covering,
but we just don't know at which energy
scale these new things will
manifest themselves.
So
it could be at this accelerator.
At the energy scale, we are probing with
this collision,
or we need a much more powerful accelerator.
So we it's but this would be much
more exciting because it's something that
unexpected.
It would be
there was a moment a few years ago
when,
both ATLAS and CMS started thinking that there
could have been,
a new state at 750
GeV
that would decay to 2 photons, then it
was never confirmed.
It didn't reach the statistical significance,
that we needed to claim a discovery.
But there was a hint of something, and
both experiments were seeing it. So it was,
wow.
It's not the statistical fluctuation, then it was
a statistical fluctuation. It went away. But
you can't imagine how many
theory paper,
happened the day after
when we had we had for this first
claim about this potential hint of new particle
because,
yeah, in this moment, theories also need
experimental
inputs to understand what is the fundamental
theory,
at least more fundamental than the standard model.
Okay.
The Large Hadron Collider is being upgraded.
Yes. Right. Exactly right now,
we are restarting
run 3. So we had 2 run, run
1 and run 2,
and run 3 is,
will start now and will end in 2,025.
Things are changing every day. So or 2025.
And this will be,
will record data with the same at the
same and similar, slightly higher energy, but with
the same intensity. So,
there will be the same amount of data
more or less collected,
per year as it was,
during in the past. But then we will
start a completely new phase in,
2000 around 2029, 2,030
that will last probably 10 years,
where we will, at the end,
collect a factor 10 more data than the
one that we have collected so far. This
is a big jump. So,
in order to reach that,
the machine is going undergoing
a major upgrade
in order to run,
at what we call high luminosity.
So it will be high luminosity
LAC.
It means that what I said before, we
will be smashing even more protons together,
and so we will have even more overlapping
proton proton collisions, so even more complex
event selection,
in our detectors,
but at the end what we obtain with
this higher luminosity is
more,
collision events,
at the in in 10 years at the
factor of 10 more than what we we
are able to collect right now.
And this is useful to get to the
precision, to much higher precision of the measurements
that we wanna do and also to still
to close this chapter on is there new
physics like new particle states at the energy
scale that we are probing. So we are
not jumping in energy scale. We are not
going to much more powerful,
accelerators in terms of energy on the collision.
We are just increasing the amount of data
that we will collect
such that if,
these new particles are rare, are more rare
than what we thought,
with these
larger statistics, we can,
infer the infer their presence.
Yeah. So we are
really going in the corners
to try to understand if there are new
particles at the energy scale we are probing.
Before going to higher energy scales with new
accelerators,
the next 10 years will be about exploring
this energy scale with much more scrutiny,
than what we have
pursued up to now. If the new particles
and new physics isn't found at these energy
scales,
what kind of size
particle accelerator do we need, and are they
actually being built? In these years,
there's a lot of discussion,
in the community to understand which is the
next machine.
And I talk for CERN, of course, because
I'm,
I've been for a long time a CERN
employee, and I'm working on CERN experiment.
CERN is proposing
the future circular collider, so a circular collider,
the same as,
is happening at LAC, so a circular collider
with 2 beams of particles that collides in,
in many collision many, 4 collision points usually.
And the size,
of it is a 300 kilometer
tunnel.
The current one is 27 kilometer.
So,
yeah,
we are going,
on a different scale.
But what is I would I would like
to stress that okay.
That,
that size is needed,
for
going to a 100 TeV,
collision. Okay? And now we are at 13
TeV collision.
So, again,
a factor 10 increase in energy,
and there you really probed a new energy
scale with respect to the one that we
are probing today.
But before, we also need,
to run an intense program of precision measurement,
that can be done also at the new,
machine.
So not only going to higher energy scale,
but also,
exploit
collision of elementary particles like electron, proton, and
positron, not smashing protons, but going to really
use elementary particles
such that it's more easy,
to precisely measure the properties of the Higgs
boson or everything that we haven't measured super
precisely because history told us that
usually you can guess which is the energy
scale,
where new physics will appear
from deviation
in precision measurements at a lower energy scale.
So by now we are searching, okay, we
are in a proton proton collision machine,
but if we haven't we don't find anything,
in order to really have some guess on
the next energy scale,
usually what particle physicists need is to run
an e plus e minus machine,
something that is really elementary particles that collides,
there's much less noise in each collision, and
therefore you can measure precisely to a per
mil precision the properties of the explosion, for
example.
And there, you any deviation would point to,
would tell us more about which is the
energy scale, where new physics will appear.
So we have this handle.
Still, we need a new machine, definitely. We
can't do that in
the in the large other collider. As Christina
mentioned, she speaks for CERN.
It's worth mentioning that there are other proposed
successors to the Large Hadron Collider, including the
International Linear Collider in Japan
and the Circular Electron Positron Collider in China.
Let's just return to that conversation with Christina.
What are you gonna do anything on July
4th? Are you sort
of involved in any celebrations? Or I'll go
to the event at CERN. There will be,
like,
there is a celebration the all day.
I will be participating.
There will be talks by
the each the spoke person of our collaboration.
There will be, like,
theories that presents their their view on wall,
what we have met all we all they
have understood about the Higgs boson, thanks to
our measurements,
experimentalists that we show,
the status of all searches
today. I'm not anymore working directly on Higgs
boson properties.
I I I switch to searches for supersymmetry,
which we haven't found it,
yet
or never. I don't know. But still
so I'm not anymore
working on the exposome myself,
but I will be there, of course, because
I've been working on that for 10 years
and more. Thank you so much to Christina
Botter and Achinti Rao for talking to me.
And don't forget that you can find Achinti
Rao's feature,
a day in physics like no other in
the Physics World magazine.
And if you go to the Physics World
magazine or indeed the Physics World website, physics
world dot com, you'll find a host of
features and articles
all about the 10 years of the Higgs
boson.
We'll be back next month when we'll be
looking at something else from this wonderful world
of physics.
And thank you very much for listening.
Physics world.