6.13 The discovery of the Higgs boson (optional)

[MUSIC]

[MUSIC] >> In this module,

we are discussing the properties of electroweak interactions.

And in this video we will visit the ATLAS experiment,

whose visitor center is behind me.

ATLAS is an experiment installed at the Large Hadron Collider LHC of CERN.

In video 3.2a

we have already introduced the full range of CERN’s accelerators.

And in video 3.10a, we have discussed the properties of the ATLAS detector.

Today we will talk with one of the active participants

in the discovery of the Higgs boson, who is Sandrine Laplace.

Follow me please.

>> I will quickly present to you our interview partner of today,

who is Sandrine Laplace.

Sandrine obtained her PhD in 2003

at the Laboratoire d’Accélérateur Linéaire LAL in Orsay, close to Paris.

on a subject of CP violation.

Then she joined the Laboratoire d’Annecy de la physique des particules, LAPP,

close to here, about 50 km from Geneva,

to join the ATLAS experiment,

to which LAPP contributes in a major way.

Then in 2011, if I remember correctly,

she joined the Laboratoire de physique nucléaire et des hautes énergies,

LPNHE in Paris, where she still works today,

always on the ATLAS experiment, which is our subject today.

So, Sandrine, do you still remember when you first heard about

the Higgs mechanism and the Higgs boson?

>> Yes, I do remember.

It was at the university, during a course.

>> I imagine, and I hope so!

>> Well, a course with was called DEA at the time,

in the fifth year in France.

Indeed in a field theory course, that is where I think I first

heard about the Higgs mechanism and the Higgs boson.

>> And what did you think at that time

about this somewhat complex mechanism?

>> Indeed, at the time, it seemed complex to me.

As a student, one has a sort of intuition of what

for example the mass of a particle could be.

And then one day, someone explains that it is more complicated than that.

>> In fact, that mass does not exist.

>> Yes, so I remember that I found this pretty complicated

for a property that seemed

simple to me.

And in addition, it was very theoretical at the time,

since it was in the context of a course.

It became somewhat more concrete, when during my PhD,

while I worked on a totally different subject,

in the year 2000… >> Indeed.

>> at LAPP there was some excitement about a possible signal.

>> Yes, a possible indirect signal of the Higgs mechanism.

In fact it was very indirectly, through radiative corrections,

that one found that the existence of this effect was not totally excluded,

that it was even relatively probable,

but not directly accessible to that generation of experiments.

>> And that was one reason why we built the LHC.

Did this also count among your

motivations to join the ATLAS experiment at LHC, at the time?

>> Yes, absolutely. I did not, at that time

consciously tell myself that

I will join ATLAS for this very reason.

But, once I arrived in the ATLAS collaboration,

it appeared perfectly natural to work on this subject, indeed.

>> So what was your role during the construction of ATLAS?

>> So I joined ATLAS in 2003.

In 2003, the detector was almost completely constructed.

So, I arrived at LAPP Annecy, which was involved in the construction

of a sub-detector, which is called the electromagnetic calorimeter,

and which serves to measure the energy of electrons and photons.

So, quite naturally, I joined this activity, but at that moment

the detector construction was already finished.

We exposed modules to particle beams at CERN

to test the final modules, determine their performance.

So, I participated in that, it was in 2004-2005.

And then the calorimeter was lowered down into the cavern,

and the electronics was pugged in, I think this was in 2005-2006.

And from then on, until the start of the LHC,

in 2009, we were able to study the electromagnetic calorimeter, to make sure

that it worked well in the cavern, that it was ready for the first collisions.

So, I have participated a lot in this commissioning of the detector, as we call it.

>> And this detector has evidently played a major role

throughout the history of this discovery.

So when did you decide to get involved in the data analysis

which led to the discovery of this particle?

>> Well, early on, because working on the electromagnetic

calorimeter, it was natural to

look at a physics reaction that involved this detector.

So, I got interested quickly in the search for the Higgs boson in the channel

where it decays into two photons.

And which was actually one of the two channels where we first saw

the Higgs boson.

One of the discovery channels.

>> But evidently, this discovery did not happen in one day.

It was a long sequence of events.

Can you recall for us how

you lived this race for the discovery?

>> Indeed, this happened progressively.

We started to take data in 2010.

But very little.

It became interesting in 2011,

when we really arrived at an energy of 7 TeV in the center-of-mass frame.

That is when we started to accumulate data.

The way we worked was

that we regularly watched, every few weeks,

the distribution of photon pairs we were able to reconstruct.

>> The invariant mass distribution.

>> Yes, the invariant mass distribution, indeed,

of photon pairs.

And then, we waited to see if photons could come from the Higgs boson,

that their mass starts to

peak around the Higgs boson mass.

And that started to be the case very gently.

That is to say, it really happened gradually.

At the beginning, we did not have much data,

we saw fluctuations everywhere.

So, we did not know too much.

And then, at a certain moment, there was one of these fluctuations, which started

to somehow deepen.

>> And which stayed always in the same place.

>> Indeed, week after week, it was always there, larger and larger,

in a regular manner.

And then, at a certain moment, we learned that our colleagues from the same experiment,

who worked on another Higgs decay channel, the one into leptons,

saw a fluctuation, which started to grow at the same place like ours.

So, at this moment, we started to…

>> You started to believe it.

>> …yes, to believe it.

Exactly. And in December 2011,

there has been a seminar.

>> Here.

>> Yes, here at CERN, where for the first time

we announced that we started to see the premise of the Higgs boson.

The evidence was too weak at the time to really announce a discovery.

But there was serious premise, which was confirmed on July 4, 2012

when we really announced the discovery, because then it was…

>> Time to celebrate.

>> Yes, right. >> Above all here at CERN,

where there was a grandiose meeting with the forthcoming Nobel laureates

for this mechanism, with Peter Higgs and François Englert.

>> François.

François who?

>> Englert.

Yes, Englert, that’s it.

So with the data accumulated until today,

we are relatively sure that we really deal with a Higgs boson, aren’t we?

How do we know that it is really a Higgs boson and not something else?

>> Well, we study its properties.

The Higgs boson

was predicted by the Standard Model since 1964.

>> Yes, right. >> And we knew…

>> So,

everything was known except its mass, wasn’t it?

>> Exactly, we knew everything about it, except its mass.

And the fact that it existed.

So, once we found that it exists and that its mass was measured…

>> …125 Gigaelectronvolts,

just to quote it…

>> 125 yes. So, from the moment when one knew the mass, everything

was predicted, like all its couplings with other particles.

So, by measuring the number of times when it decays into

two photons, into four leptons, or into other final states,

one verifies if this corresponds exactly to the prediction of the Standard Model.

So, today this is the case,

but still with a large experimental uncertainty.

That is to say that we measure these couplings to roughly 15% accuracy.

>> OK.

>> So, there is still some margin.

>> But at this precision, it is compatible with this hypothesis

that it is a Higgs boson. >> Yes exactly.

We have also verified the quantum numbers of the Higgs boson, its spin, its charge,

its parity, etc.

>> With charge zero and spin zero.

>> Right. >> Very probably spin 0,

I believe that spin 1 is practically excluded, isn’t it?

>> Spin 1 is already excluded by the simple fact that we observe

that the Higgs decays into two photons.

>> Yes! In fact, that’s true.

>> And afterwards we have excluded spin 2, which is really exotic.

That way we have excluded that it could be a graviton, instead of a Higgs boson.

Here we are.

Rather easily too.

>> OK. So what does it change.

There has been a lot of noise made around the discovery.

What does it change for a physicist like you?

Did this, the proof that this effect exists,

did this change your paradigms?

>> Well yes.

This means that what has been explained to me at university, that the mass of elementary

particles, that it is not an intrinsic property, but that

particles are massless, but they acquire a sort of effective mass,

because when moving, they are sort of slowed down

by the Higgs field, which fills the vacuum,

this is something completely counterintuitive, you learn it at university,

but the day that you really see this mechanism at work

that is when the revolution, for me at least, really happened in my brain.

It is at the moment when we discovered the Higgs

that I told myself: but, this is thus really the mechanism?!

And it was really then that I thought a bit more intensively

that this was a revolution.

>> We all shared the same thought.

To abandon this well rooted notion, that

mass is an intrinsic property, that particles are born with,

and replace it by a coupling constant to an ubiquitous field, that is a big step.

>> That is a big step.

Yeah yeah, it really is an interior revolution.

>> So, what does this have to do with the

mass of the trolley which one pushes?

>> Well, nothing! Haha, nothing at all!

>> That is where our world decouples a bit

from everyday’s world, yeah, yeah.

>> Yes, as I already explained in the previous module

the mass of ordinary matter, the gravitational mass

and the inertial mass are given essentially by the mass of the nucleus,

which has nothing to do with the mass of electrons and quarks.

So, what will follow now?

Why does one continue

when one has already completed the Standard Model?

One could stop there, no?

Why does one continue experiments like ATLAS?

>> There are plenty of good reasons to continue!

>> Give us some, please.

>> So.

We have there this Higgs mechanism, which is a complicated mechanism,

to answer the question: what is the mass of elementary particles?

We have answered this question, but at the same time, we have opened lots of new questions.

So, the existence of this field in the vacuum poses lots of questions.

It poses a lot of problems.

It is in fact funny, because one shifts the problem: one poses a

first question, one answers it, but in answering it, one puts a new question.

>> One has shifted the intrinsic mass to a coupling constant,

that one must now predict or explain. >> Exactly.

And then the mere existence of the field is problematic.

Because this field is fluctuating, it receives all kinds of

radiative corrections, and thus, it should not be the way it is.

That is already a first reason for us to think that we have not understood everything,

and that we must continue to make collisions

to understand this mechanism a little better.

So this, by the way,

is really a question linked to particle physics.

But there are plenty of other observations, which are more linked to the Universe,

like the existence of dark matter and dark energy

which correspond to no less than 95% of the contents of the Universe,

and of which we do not know the origin, and which are not at all explained

by all the particles we have seen to date at accelerators.

So we know from the start

that we must still find things that we do not know yet.

Thus, we continue to search.

>> To stay with the Higgs just a little more, we do not even know,

as far as I understand, if it is the only particle with these properties, do we?

There could be several. >> Absolutely.

In particular, to relieve the problems which the Higgs boson brings

to the Standard Model, which predicts only one,

one has invented, for example, a solution to these problems

which is called Supersymmetry.

This is one of the solutions, there are several, but in this framwork

>> It is not my favorite one, but be it.

>> It was my favourite one, I think,

before the LHC started, but by now it is in bad shape,

because we have still not seen it, but OK.

So this is one extension of the Standard Model, where

one effectively predicts other Higgs bosons.

Where there are even five,

neutral ones and charged ones.

At minimum.

So yes, we continue to search for other Higgs bosons.

>> It is for that reason that one has now increased the energy by practically a factor of 2,

where one is now. During the whole year,

you have experimented at a total energy of 13 teraelectronvolts.

Is there still more margin,

concerning either the luminosity or the energy of the accelerator?

>> So,

with the LHC energy one is practically at the maximum.

The theoretical maximum, the energy for which the LHC has been constructed

was 14 TeV, and we are now at 13 TeV, I believe

it is quite complicated to arrive at 14. >> And I believe it would not make

a big difference,

it is just 10% more. >> Right,

it will not make a large difference, we have thus arrived at the maximum

of the LHC as far as energy is concerned. In terms of luminosity, clearly,

there are improvements and there is already what we call

upgrades, foreseen in the future, to strongly increase the luminosity,

so as to accumulate 100 times more statistics, which is what will happen.

And then afterwards, to go to higher energies,

means really to construct a new accelerator

and that is for the longer term, we talk about several decades here.

There are projects on the table to do that.

>> We will certainly dedicate another MOOC to this great project,

which has a nickname, it is called FCC for the moment.

>> Yes. >> So, thank you, Sandrine,

for this interview.

It was very interesting to see how someone who has had

a rather important role in this discovery has lived it.

Thank you very much.

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