>> 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.
>> 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.